Cooling drag question

I've been following the cooling drag threads for some time and am fully indoctrinated with the following blue print of cooling drag optimization:


One of the core ideas is that the air coming is high speed and that it slows down through the plenum to pass the engine at low velocity and then it speeds up at the cowl outlet.

My problem with this is that when I look at the amount of cross sectional area available through the cylinder fins, it looks to me like the area is, if anything, less than the area of the inlets. If the area was exactly the same, then the velocity through the fins would be the same as the inlet velocity, (not necessarily aircraft air speed) and there would be no need for a plenum (to slow the air down to low V/high P). Even worse there is more drag through the fins because of the greater surface area, so equal area would actually look like less area.

This observation could explain:

1) Why we can reduce the inlet size with little effect on cooling (RV-4's)
2) Atkinson reports lots of ugly airflow under the plenum
3) Washers between cylinder and baffle improve cooling in a measureable way
4) Why Axsom saw a drag decrease by cleaning up the airflow in the supposedly low-velocity region of airflow

Is my eyeball estimate of the available cylinder thru area way off, or am I missing something else?

EXCELLENT SMITHERS!

chuck said:

1) Inlets as far out as possible (circular preferable) (A)
2) Hard plenum
a) reduce leakage by baffling
b) reduce air speed in laminar fashion
c) create max pressure in upper cowl
3) Clean up flow in plenum (per Atkinson sp?) (F)
4) Use outlet area ratio per Dave Anders
5) Use exhaust augmentation (E)
6) Clean up internal airflow (per Bob Ax.)
[/CODE]


(B) My problem with this is that when I look at the amount of cross sectional area available through the cylinder fins, it looks to me like the area is, if anything, less than the area of the inlets. If the area was exactly the same, then the velocity through the fins would be the same as the inlet velocity, (not necessarily aircraft air speed) and there would be no need for a plenum (to slow the air down to low V/high P). Even worse there is more drag through the fins because of the greater surface area, so equal area would actually look like less area. (B)

This observation could explain:

1) Why we can reduce the inlet size with little effect on cooling (RV-4's) (A)
2) Atkinson reports lots of ugly airflow under the plenum (C)
3) Washers between cylinder and baffle improve cooling in a measurable way (D)
4) Why Axsom saw a drag decrease by cleaning up the airflow in the supposedly low-velocity region of airflow

(B) Is my eyeball estimate of the available cylinder thru area way off, or am I missing something else? (B)

Click to expand...

Nice summary:

A) " Why we can reduce the inlet size with little effect on cooling (RV-4's)"

I say any plane can reduce the inlet size with little effect on cooling, but it must be accompanied with cooling efficency improvements. Cooling drag reduction is using as little air as possible to get the job done. When you say little effect on cooling, I assume you mean no increase in CHT. Have you heard of problems with a Sam James Cowl and high CHT, that would not have been there any way on other RV models? A RV-8 with a stock cowl can have high CHT, and it will not get better with a SJ cowl.

Since reducing the inlet area has a great effect on reducing cooling drag on any plane I will say any plane benefits. However you must have enough cooling. I don't think the RV-6/7/8/9 run hotter than they typically do with a SJ cowl, but they do run hotter than a stock RV-4. The reason you can lose 40% inlet area is because you are using air more efficiently (see below)

The question is why is the RV-4 so efficient in its basic stock form? SEE Item -F- below. Here is one of the "secrets" of the round wide spaced inlet. Even the new stock Mooney Ad shown in the latest magazine shows a semi-round wide spaced inlets. See my diagram for explanation:



B) We can't do any thing about the fin's. First you NEED that many fins and surface area. Why? It has to do with heat transfer. You need enough surface area. You need that pressure differential between the high and low press plenum as a forcing function to get air thru those cyl fins. This pressure is the power of the cooling system, volumn the capacity. Volume is not an issue, there's plenty of that, but pressure is an issue. However in theory you're correct, wide spaced fins would allow less restriction and thus req less pressure diff (and cooling drag). The down side is the engine would burn up for lack of enough heat transfer. The engine engineers took care of the fin geom for us. You'll find different designs, Lyc, Cont, Frank, P&W and Wright but more similarities between them. We can can forget playing with fins, but it's a great question.


C) Clearly there must be some turbulent air flow under the (high pressure) plenum, but I would like to see the data or supporting argument for this theory. From the NASA/MSU/T&AM report I am not seeing it, but clearly turbulence is likely and undesirable and should be minimized. The critical area and one that needs the most focus is the inlet and diffuser TO THE plenum. When it gets into the plenum, it's slowed down and not as critical, but I guess small improvements can be made. The inlet is an area of much research and analysis. I see people carve a round hole in their cowl and say SEE! The real magic of the inlet starts at the lip, through the inlet throat (laminar) and into the diffuser. There's much work there. The plenum is just a big reservoir. Here's an excerpt showing pressure survey in upper plenum of test aircraft:



D) The reason playing with baffle to fin gap is the asymmetric fin depth front to back on Lyc engines. Since all cylinders are identical, the fins against the #2 and # 3 cyl (fnt lft, rear rt) are choked off, since the fin depth is almost zero on the side. The reason Lyc did this? Save weight and allow closer cyl spacing (I guess). See my diagram:

(I guess using the air you have better comes under the heading of cooling drag reduction.)


E) "Use exhaust augmentation " - Well understood and exploited in piston planes for 6 decades, but hard to execute in a stock RV airframe. Van could design an exhaust ramp verses the flat bottom into the belly, but this makes it complicated, heavier and more expensive. Pipes sticking out in the breeze is not ideal. To make an augmentor you would need to go externally, below the belly well past the firewall in my opinion.

Heck the 1958 Piper Apache I owned had exhaust augmentor tubes, as did Cessna 310's and many planes since than. Twin engine planes have more room to develop an augmentor, firewall to wing trailing edge. I'm not sure I'll try it myself. The reason, it adds weight and surface area, whose drag might negate the benifit. A recessed tunnel in the belly of the plane allowing the pipe to be reaccessed is ideal, but structurally not practical.


F) "Clean up internal airflow (per Bob Ax.)" - I think Bob had an idea which seemed reasonable. Why let the air float around in the accessory area when you want it to go out the exit. The results where mixed but interesting.

WHY DO RV-4's cool WAY BETTER than any other RV? That's a good question. Does any one know? No one has ever answered my question. So I came up with a theory. Instead of making that massive sealed vertical bulkhead as Bob did, why not replicate the internal dimensions of a RV-4 cowl, especially the lower cowl. Look at a RV-4 cowl compared to the RV-6/7/8/9/10 "wide cowl". The lower "wide cowl" volumn is huge compared to a narrow RV-4 cowl (hint, hint). An easy mod to the cowl may be to bond internal step/fillers to reduce the volumn, a molded form verses just dumping air into a cavernous lower cowl. This lower shape guides the air from the front of the cowl all the way to the exit. Exhaust pipes, air box may get into the way, but work around them as best you can. This also partly applies to the upper plenum and upper cowl. Instead of a big flat upper plenum, make it follow the contour similar to the RV-4. The concept in summary is replicate the RV-4 cowl internally. hmmmm

The RV-4 works. When you think about it (rationalize it), getting the entire volumn or cross section of a massive lower cowl funneled out the small exit produces needless turbulence. Control the exit flow from the front of the lower cowl all the way to exit, not just at the exit.

"This is brain surgery, not rocket science... now hand me that Icecream scoop." Homer Simpson

Very Interesting

The information about the reduction of volume in the lower cowl and devising a way to get the air directed to the outlet cleanly is very interesting and consistent with what I saw in my experiment. My first modification addressed the idea of getting the air to the outlet cleanly. Basically three surfaces: (1) a surface that starts straight down then curves back to overlap the outlet area of the lower fuselage skin (2) a verticle plane extending at and angle determined by the location of cylinder #3 from the right side of the outlet to the right side of the cowl (3) a verticle plane extending at and angle determined by the location of cylinder #4 from the left side of the outlet to the left side of the cowl. I saw a 2 kt loss in speed in a flight test.

In a subsequent experiment I further modified the system by running a baffle a little below the edge of the lower cowl from the angled baffles added in the first modification. This reduced the volume in the post cylinder area and it reduced the flow options of the air leaving the cylinder fins. The speed increased 6 kts over the first mod and 4 kts over the stock configuration. Photos and test results were added to the builder mod area of this website. These results are exactly in line with some of the information in George's post.

Today I will finish a mod to the air box that converts the top to a flat plate. No test yet of course but the results could go either way. I seems that as the flow gets cleaner the volume of air (mass) increases for greater cooling but it also produces an increase in cooling drag. Because the cooling efficiency is greater you can cut down the air mass required to flow through the system for appropriate cooling. This can be done at the inlet or outlet (I'm just at the thinking stage on this but when I have more time I plan on restricting the outlet and testing it now that I have CHT probes in all cylinders).

Bob Axsom

gmcjetpilot said:

Nice summary:

snipped
D) The reason playing with baffle to fin gap is the asymmetric fin depth front to back on Lyc engines. Since all cylinders are identical, the fins against the #2 and # 3 cyl (fnt lft, rear rt) are choked off, since the fin depth is almost zero on the side. The reason Lyc did this? Save weight and allow closer cyl spacing (I guess). See my diagram:

(I guess using the air you have better comes under the heading of cooling drag reduction.)
snipped
"This is brain surgery, not rocket science... now hand me that Icecream scoop." Homer Simpson

Click to expand...

George probably already knows this, but I'd like to clarify it for the others reading this thread. Lycoming was definitely trying to save weight here. Note that the finning is thinner on the cylinder head nearest the INTAKE port. Since this area has nice cool air flowing internally through the port, it should require less finning. As George has pointed out, the problem comes when the airframe manufacturer mounts the local baffling to tightly. It has been reported on the Matronics RV List, that adding one or two standard AN960 washers between the rear baffling & the cylinder head on cylinder #3 will make a noticeable improvement in CHT. The washers allow an increase in local airflow between the head and baffle.
Charlie Kuss

I think George is on to something

George's comments dug something out of my memory------take a look at the cowling outlet of the AR 6 in the attached link.

http://www.ar-5.com/

Scroll down towards the bottom, click on photos under AR 6.

In fact, as I have mentioned before, there is a lot to learn by looking at this guys planes.

Mike

In trying to be complete I gave too much information so I'll try a different approach to asking my question.

As I understand this is what we supposedly do with the cooling air.
1) Inlet is small cross section
2) plenum to slow down air and minimize losses
3) through fins (large cross section)
4) to outlet (small cross section) to increase speed to ambient.

This is not unlike what we do to reduce losses through an air filter.

My observation is that since the inlet cross section and the cross section through the fins is similar we really have a system that looks like a constant velocity (I'm using that term loosly) system until the point the air passes the fins, at which point the cross section gets big (entire lower cowling) and then small at the outlet.

If that is the case I would surmise that:

1) The plenum as resoivoir doesn't do what we think (e.g.Atkinson's tufted video) reverse flows, turbulence and other counter flows.
2) Increasing fin cross section (e.g. baffle washers) should increase flow.
3) Reducing the effective cross section in the lower cowl will eliminate a needless (and uncontrolled) expansion. (Bob A)
4) Cleaning up flow in the lower cowling (as opposed to reducing cross section changes) should help (e.g. rounded outlet lip on RV8's).

So I'll raise the question:

IS THE CROSS SECTION THROUGH THE COOLING FINS LARGER OR SMALLER THAN INLET CROSS SECTION?

If it is smaller then I think current plenum/cooling systems don't have the right conceptual model of the cooling flow.

Or I could be full of it

AREA IS DIFFERENT

To try answer. The area of the fin openings should not be the same as the inlets. As George alluded to the surface area of the fins produces alot of drag so even if the area was the same, the volume of air entering the inlets would be greater than what passes thru the fins.

You also have to take into account the change in direction the plenum air has to make to go thru the fins. We are not talking laminar flow here. However, turbulence is not all bad. You want the air passing thru the fins to be somewhat turbulent, because it will transfer heat better. Like stirring a pot of soup.

I also agree with George that to make an optimum augmentor per report would be excessively long and complex. But, the key word is optimum. Perfect case gives 6 inches of pressure drop. I think a compromise can be made that while not optimum, will provide enough benefit to be worthwhile. I have talked with a Long Ezy builder who's set-up is far from optimum (8 inches), yet he can actually feel the exhaust inlet draw at idle. One good thing for pushers. I would be happy with 3 inches, which I think would be enough to further decrease inlet size and still cool idle and slow (because the augmentor is really only to make up for low cooling flow when on the ground, slow, or in climb). I am going to give it try anyway.

From another source:
"S.J. Miley (Miss St) actually recommended making the high pressure plenum's cross-sectional area equal to that of the inlet(s), so the the plenum is not so much a plenum as a duct."

How that squares with plenum resevoir ideas I don't know.

I have been really brainstorming these cooling ideas and induction pressure recovery mods. I am going to try and make a scaled prototype of the induction assy first and instrument it with a water manometer to see what I can find out. I have also been thinking about ways to test and have ruled out a high pressure blower, as they are very expensive. But I was thinking about 3-4 sources of pressurized air (compressors or bottles) hooked up to a ganged together set of nozzles to blast air into the test subject opening. That should provide both high volume and pressure. If that fails plan B is mount it to the car and drive 70mph. Wonder what kind of looks I will get on that one?

Any suggestions or other ideas?

Computer time?

Almost forgot. For the working engineers out there, how difficult is it to set-up a fluid dynamics simulation of inlet shapes or plenum shapes to see what it says.

I found some of the cheaper desktop programs that were only $3,000.00 .

Fins, washers and gaps

Wade Lively: "From another source:
"S.J. Miley (Miss St) actually recommended making the high pressure plenum's cross-sectional area equal to that of the inlet(s), so the the plenum is not so much a plenum as a duct."

How that squares with plenum reservoir ideas I don't know."

Yea that is GREAT. I am not sure either. I guess practical aspects come into effect. Like room for the plugs and push rod tubes. I missed that quote from S.J. Miley, but tend to believe him. Hummm wounder if he is still around.


Chuck Bass "If that is the case I would surmise that:
1) The plenum as reservoir doesn't do what we think (e.g.Atkinson's tufted video) "

Do you have a copy of this video I have not seen this tuff test.

Chuck Bass - "IS THE CROSS SECTION THROUGH THE COOLING FINS LARGER OR SMALLER THAN INLET CROSS SECTION?" I see you want answers! I see you are frustrated but there is no one answer. 20 ingredients going into the aerodynamic soup of calculating inlet area. There's nothing like flight test. We have good data from NASA, Barnard, Sam James and Dave Anders. Here is an example of some old NACA stuff. This looked at fins spacing and position relative to airflow: http://naca.larc.nasa.gov/reports/1939/naca-report-674/naca-report-674.pdf (Although for a radial, one conclusion was cooling improved with tighter fin spacing, until resistance increased too much. This 1939 data helped Lycoming optimized fin spacing for good cooling and reasonable air resistance. There's a usable range of fin spacing, depth, thickness and structural considerations. OLD but interesting news. If you have energy to read them, they are interesting but too technical for most non engineers. I have an engineering degree and know just enough to get in trouble.) Here is one more of 42 on cooling fins: http://naca.larc.nasa.gov/reports/1939/naca-report-676/naca-report-676.pdf

What's the area thru the fins to start with? Do you know? I don't. We are stuck with that fin area we have to provide the needed cooling. To get air down thru the fins to do the "WORK" of cooling needs differential pressure across those fins, to produce mass airflow to reject the heat, other wise CHT's are too high. Cooling is a necessary evil.

The easy answer to your question is we copy what other RV'ers have done. The inlet area has been calculated for us already for our cruise speed and engine HP.

The area of the inlet for my 180HP RV w/ aluminum inlets are about 4.6" dia at the mouth and 4" at exit. They are 1.75" deep and nest into the cowl. The nozzle/diffuser uses the A-10 or -20 NASA laminar airfoil contour. They are custom inlets, not from Sam James, which does not have the same design or internal shape. My cowl is a modified stock cowl with these aluminum inlets to a custom plenum. My total inlet area is 2 * Pi/4 * (4")^2 = 25.2 in-sq, 33 sq based on mouth. How does that relate to fin area of the O360A1A? I don't know or care as long as I go fast and stays cool.

The NASA research was NOT complete and somewhat left unfinished in the early 80's. Great data and observations where made, but they did not give design details. That's where our creativity comes in. When it comes to utilizing the NASA concepts, I think Barnard's Holy Cowl (now Sam James) interpretation is very good but not perfect. I choose to make my own cowl (from a stock one) and plenum to get it the way I wanted it.

There are some excellent books on engine cooling, but don't recall the title or author's. Than there is stacks of NASA reports, many from War Time on the subject of air cooled engines. Me personally, I copied from Dave Anders and Tracy Saylor, because they showed it was a reasonable area. Lazy but smart. Know when to copy. The racers get away with tiny inlets and fixed geometry because they are made to just go fast, not climb for 12,000 feet. So be careful who you copy and why.

The inlet is only one piece of the puzzle. The way you handle the air after that is critical.

Variable inlet geometry would be nice, but that's not really workable. Even Cessna has variable geometry in the form of cowl flaps. Why don't we? Well one it adds weight and design complexity, but it's doable. My exhaust tunnel and cowl flap would involve massive belly structure rework. It could be done, but not sure if it's practical or worth it.

Looking at some of the old classic GA planes of the past you might say, geee what where they thinking. In the last two decades the standard has changed. However they did not have 20 channel engine monitors, much less one CHT. This is where LoPresti made bread and butter in his latter years, making NASA (MSU) inspired cowl mods for older factory planes: http://www.speedmods.com/

Now the new Mooney looks like this: http://www.mooney.com/



Lycoming Cylinders
Absolutly the fins are thicker / deeper on the (hot) exhaust port side by design. Clearly this was engineered (well), but since they where designed in the 1940's / 50's something, all those guys are gone, so we can't ask. The (cool) intake side does not need the same "fin-age" (made up a word to day mom). It does save weight and allow a more compact design. I think the Lyc design is brilliant and well thought out. Also the fin area, height, thickness and spacing has been researched to death I recall, in the 30's/40's.

There's no free lunch, you will always have some cooling drag. I don't think redesign of the fins is the key. Hey that is why the Hawker Sea Fury is the fastest plane at at Reno with a BIG radial. At least the air is going in the same direction as the fin. As was pointed out the air has to turn and go down (draft) on our little Horz opposed engine.

Van's stock baffle if fitted too high and tight on #2 & #3, will choke the air off to the lower (deeper fins) on the bottom of the cylinder. You need all the air you can get there. Kent Paser ("Speed w/ Economy) played with these gaps and got dramatic results.

The washer trick, absolutly you can control the gap and increase the flow on the #2 and more important #3 jug. It is really a local airflow issue, that is all. You are bypassing air to get it where you want. However I suspect you are thinking bypassing the air around the fins (more baffle fin gap) air would speed up. It would reduce drag and cooling may increase to a point. A 1937 NACA report discusses baffle to fin gap: http://naca.larc.nasa.gov/reports/1937/naca-tn-620/naca-tn-620.pdf
(be careful how you use this radial ring cowl data; also more cooling may be more drag as well.)

The washer trick works but is a little crude. It is a good way to experiment, but a better way is have baffles fit properly without shims and washers. When you add washers you tend to get leakage out the side of the lower wrapped around portion of the baffle. Some baffles have formed flanges on the edges to control the edge leakage. You can achieve the similar effect with silicone beads.

My theory is you need to have a good cooling system to start with before you can reduce cooling drag. Every little part has to work together. One small detail deficiency can ruin the efficency of the cooling system.

Mike S - "take a look at the cowling outlet of the AR 6 in the attached link." Isn't that crazy. This is a good use for fiberglass. Every detail on these formula racers are extream. Not sure how much we can incorporate in our "daily flyer's", but agree, there are things to learn. At some point there's a balance and sacrifice for speed. These are all out racers. Cooool! (I think most formula planes are trailered to the race)

George, my suggestion about the AR 6 cowl was in support of the idea you put forth in a prior post about "Why rv 4's cool so well"

You suggested it might be that the inner cowl is tall and narrow compared to other RV's----------if I read your intent correctly.

No attempt to push fiberglass, but the SHAPE of the thing is what I find interesting. It sticks WAY DOWN below the fuse--------most folks would think "wow That must have lot of drag"--------In my case, I think Hummm, good idea, cause a low pressure area BEHIND the end of the cowl, and SUCK out the hot air.

Mike

Yes interesting I see, I think

Mike S said:

George, my suggestion about the AR 6 cowl was in support of the idea you put forth in a prior post about "Why rv 4's cool so well" Mike

Click to expand...

Yea I saw that thanks, cool little plane. I got to look at the first nemesis once with the cowl off, a similar plane. Amazing stuff.

Yea, I'm not that smart to figure out what's going on, but there is something there with the "cheek" cowls and the narrow lower cowl. The AR6 is radical looking. The AR5 is the round one right.

I'll eventually play with "filling" the inside lower sides of my RV-7 cowl and see what happens.

Good luck, and keep us posted.

Mike

Gentlemen:

Lots of really neat discussion here. Some of it is supported by the science. Unless you tuft an engine compartment and LOOK at the airflow in flight, all of the discussion about what happens to the airflow is speculative. BTDT.

If one does not slow the airflow AND makes it go where it NEEDS to go, the results are suboptimal. BTDT.

Unless each individual change is tested as in #1, you have no idea what the effect of that change is. BTDT.

Forget intuition. Forget Logic. None of that works in this case. Some of the best ideas we had turned out to make things worse WHEN TESTED.

Any system must SLOW down the airflow to be effective. High speed air is less effective at temperature transfer than slow moving air. To do this the encasing structure must be as large as possible. How can a plennum be as large as the cowl?

The biggest issue is not really the plenneum--I've seen some really good ones. The big issue is the metal baffle design that directs the airflow. I've seen some really nice looking ones that were wrong when it came to making the air go where it needs to go.

This whole subject is so counter-intuitive that unless I've TESTED an idea, I discount it's purported effectiveness.

Walter Atkinson

Walter, welcome back, havent seen you here in a while.

Mike

Just a Thought

You know, if you proceed with reasonable care this experimenting can be very educational. Sometimes you just have to think about the system and try what seems right to try and see what happens. As you do this with a curious mind YOU will come to know what worked and what didn't with your airplane. Even when the results are negative there are things to be learned. I am finding this part of owning an EXPERIMENTAL aircraft to be the most rewarding. I seems that others feel this way as well as I am contacted by people like Paul Lipps, Chris Zavatson and Cris Ferguson off line with encouragement and insight when they learn that I am probing to improve my airplane's performance. The risks are very real but I think the rewards are worth the exposure.

I do not believe that it is essential that you have a classic theoretical crutch or access to sophisticated test methods to improve the performance of your airplane. If you make a change that seems reasonable and the plane goes faster many people will race into the arena to explain why. You can take what makes sense set aside what doesn't and the result it will allow you to form your own ideas and experiment direction.

This kind of information is so much more important to the experimental aircraft owner than typical startup builder subjects that I hope we see more of it in the forum.

Bob Axsom

INFO please

Walter

Glad to see you back.

You have mentioned these same things before in other posts, but did not give any details. To demonstrate what you are referring to, could you please tell us exactly what you tried that did not work? What experiments you tired?

Walter Atkinson said:

Gentlemen:

Any system must SLOW down the airflow to be effective. High speed air is less effective at temperature transfer than slow moving air. To do this the encasing structure must be as large as possible. How can a plennum be as large as the cowl?

Click to expand...

You're teasing us man! When is your Atkinson cowl coming out ? Do the RV4 first!

I understand the idea behind slowing the air down, but if the area between the cylinders is smaller than the cowl inlet area this doesn't make sense. I get that this 3D Fliuid dynamics can't be understood but I think the boundry conditions at the inlet and outlet have some relevance...

I'll be out later this week and make some measurements, perhaps my eyeball is a bad estimator...

AR-5 link

I don't know if anyone read the articles on the AR-5 website. Thanks Mike!!

But there is an interesting tidbit in the Today's Pilot article about the AR-6.

" Particular attention has been paid to the engine cooling. David Hoover worked with aerodynamicist David Lednicer to manage air as it enters the cowl and decelerates over the engine, and Mike Arnold designed the exit shape. There is a seperate plenum box for each set of cylinders. Initial testing has shown that it works too well, with cylinder head temperatures all the way down to 127 deg C (260 F). Experiments to cut down on the already small intakes have now brought the temperatures up to the 200 C considered optimal."

That is now the second time I have heard that about dual plenum set-ups, being too cool. Sure seems to be a strong consideration, at least for me.

It also appears from the photos that the exit edge of the lower cowl extends past the end of the exhaust collector.

Say what?

BTDT??

BTDT - Been there, done that?

pierre smith said:

BTDT??

Click to expand...

BTDT - Been there, done that?

One BIG plenum or two separate?

RV8RIVETER said:

That is now the second time I have heard that about dual plenum set-ups, being too cool. Sure seems to be a strong consideration, at least for me.

It also appears from the photos that the exit edge of the lower cowl extends past the end of the exhaust collector.

Click to expand...

Wade again you bring something up that interests me. I discounted the separate left / right plenum a while back but may have to reconsider.

The reason I discounted the idea awhile ago in favor for one large plenum where as follows:


Just random thoughts about one large plenum or two single plenums. Me thinks that I might give the dual runners a try.

"Talk amongst yourselves."

RV8RIVETER said:

Walter
You have mentioned these same things before in other posts, but did not give any details. To demonstrate what you are referring to, could you please tell us exactly what you tried that did not work? What experiments you tired?

Click to expand...

Yes please lets see some pitcures. chuck mentioned tuff work on a plenum. That would be of interest to us. Thanks a million

2 plenums

If there is a large difference in pressure between the left and right intakes, you might find that the intake with the higher pressure is trying to push air out the other intake. That alone could make the two plenums a good idea.

I think we have an Air Head

rv8ch said:

If there is a large difference in pressure between the left and right intakes, you might find that the intake with the higher pressure is trying to push air out the other intake. That alone could make the two plenums a good idea.

Click to expand...

Hummmm good idea Miceky, are you becoming an air head? (that is a compliment btw ) Next thing you will tell us you're trading your Subaru in for a Lycoming.


Bob Axoms latest results.
"The cylinder head temperatures at the end of the run were #1 = 322, #2 = 369, #3 = 375 and #4 = 343. The Manifold Pressure during the runs was 26" the oil temp was ~200, the oil pressure was over 75 PSI."

Bob, it looks like you need to heat up #1 (raise the baffle up on the cylinder) and cool #2 and #3 (lower the baffler or increase the gap at the upper edge to increase air flow, aka washer trick). The idea is to match #4. It would be easy to get the balance better. Then you could cut your inlet down like the new Mooney's (ovation 3 or 2gx), not quite round but a rounded rectangle with the inboard edge blocked.

http://www.mooney.com/aircraft/ovation2gx/

This keeps the stock look of the RV cowl.

The Mooney Acclaim uses massive round inlets but it is a FL250 dual turbo aircraft and no doubt needs more cooling.
http://www.mooney.com/aircraft/acclaim/

(EAA mag in the last year or so with yellow one off composite fixed gear 2 seater).

Cory Bird's "symmetry"??

http://www.kitplanes.com/issues/22_01/

http://www.popsci.com/popsci/aviationspace/4d415b4a1db84010vgnvcm1000004eecbccdrcrd.html

Mike

If I could get my CHTs down to 260dF, I'd be very happy. TCM set their liquid cooled engine to run at 240dF as a matter of routine. Lycoming says anything above 150dF is OK. TCM alludes to 200dF as the low limit. Ya can't run 'em that cool no matter what you do, can you? The cooler the better. Period. Shooting for 200dC is a bad idea in my mind--based on reams of data (start with the strength of the alloy as temp goes above 380).

As for the baffle ideas that failed... there ain't enough bandwidth! Seriously. We gave a 6 hour seminar on engine baffling; there's no way to do the subject justice in an email posting.

You've got to slow the air down and make it go where it's needed. To discover what we did discover, it took a 36 probe CHT so we could see the temps around the cylinder as we made changes (wanting a round cylinder is good) and tufting so we could watch the air movement on a video display in the cockpit while we were flying.

It's amazing what one can learn with the right tools. It was amazing how much of what I had learned in A&P school about baffling and cooling was dead wrong. It's even more amazing how off-the-mark the local airport know-it-alls can be.

If someone is having a specific problem, I'll be more than happy to try and help, but to give a treatise on the matter is not possible in the bandwidth of a post.

Walter

Walter Atkinson said:

Gentlemen:

This whole subject is so counter-intuitive that unless I've TESTED an idea, I discount it's purported effectiveness.

Walter Atkinson

Click to expand...

THIS is the understatement of the year.

We're been tweaking & testing the SAME 200hp/200mph liquid cooled installation for over 3 years. It's no exaggeration to say that 90% of "intuitive" designs proved to be COMPLETELY wrong under the rigors of testing. And this is with trained aerodynamicists.

Several of the best gains were thunk by non-aero engineers.

gmcjetpilot said:

Wade again you bring something up that interests me. I discounted the separate left / right plenum a while back but may have to reconsider.

Click to expand...

Thanks George.

I too had always planned a one piece plenum. But a seed of doubt was planted after I saw that photo of Bill James Varieze and his dual upper and lower cooling plenums. Plus, the fact he made it work with NACA ducts which have relatively poor pressure recovery. The more I research and think about it, it is definetely the way I am going to go. I think they may even be easier to build than a 1 piece. I say that, because you are building 2 small plugs instead of one large one, you don't lay up as much material (I plan on carbon/glass) , and you don't have to make any special arrangements to access the oil stick. I also agree that a single tall duct directly over the cylinders provides for much better control of the air flow, not "wasting" any over the case or out the other side. Of course I am not quite dumb enough to think that exit plenums are NOT going to be a bear to build.

I just read a snipit from a report on a "roller" leading edge wing that greatly improved the airflow attachment at very large angles of attack (the leading edge of the wing is an electric powered cylinder that spins energizing the boundry layer airflow on top of the wing). That got me thinking that we could improve pressure in the inlet plenum by increasing the divergence angle and by placing very very small fins or vortex generators (say carve a small groove in the plug) to keep the airflow following or "attached" to the plenum walls with the wider divergence angle. This induced turbulence would also help improve the thermal transfer of the cooling air.

Just more rambling thoughts.

I'm OK

bumblebee said:

Several of the best gains were thunk by non-aero engineers.

Click to expand...

Yeehaw! I guess I'm OK, being a "college flunkie" !

RV8RIVETER said:

Almost forgot. For the working engineers out there, how difficult is it to set-up a fluid dynamics simulation of inlet shapes or plenum shapes to see what it says.

I found some of the cheaper desktop programs that were only $3,000.00 .

Click to expand...

Save your money: at this level, unless you're building weather models or nukes, it's easier to look at a flag out the window (or a tuft under the cowl) than to build a meaningful simulator for complex systems (engines inside cowls). Key word is "meaningful".

Test early, test often. And whenever you think you're done, test again.

testing

bumblebee said:

Test early, test often. And whenever you think you're done, test again.

Click to expand...

Sounds like the voice of experience. I'd really enjoy hearing more about the testing you have done, and what you have learned, particularly since I've installed a liquid cooled engine, and will soon start working on the cowl.

Cowls and Drag

To extend the big fin near exhaust port idea, why do we insist on forcing air down as it gets heated through the fins, instead of up from below and make some use of the hot air rises principle? If we went to updraft cooling the incoming coolest air would strike the exhaust side of the head first, and it should be more effective where it counts.

I have read some of the NACA reports, and am sure that it was found that only the first 1/16" of air adjacent to the fin was effective in cooling, thus you see the fine finning with about 1/8" gap between fins. Sorry, I don't know what the fin gap area is either!

With downdraft cooling the normal belly cowl exit area is often in a high pressure zone, especially at low speed (hi alpha), eg climb, just when you need the cooling. Extending the cowl lip back can improve this. The better option is to exit the air to a low pressure area, either on top of the cowl, or on the cowl sides above the wing LE. Look again at that Sea Fury cowl. Now when climbing, big suction over wing, big draw on the cooling air out of the cowl.

I would like to try a single pitot type inlet below the prop, with a curved dam/baffle extending from the lower accessory case to the cowl bottom. Pass the exhaust through, use a skirt around it to partially seal the hole and allow movement. Exit air on top or sides as above. Reverse cylinder baffle idea for reversed air flow. Mount the oil cooler in the dam wall and get updraft air through it again. Additional benefits would be keeping the fuel system in the cool plenum below the engine, and only one air inlet to both engine and carburettor.

Downside would be risk of getting smelly fumes or god-forbid, oil, coming back over the canopy! Plus the extra work to make it work, but we seem to go to a lot of trouble anyway, so why not?

RV8RIVETER said:

Of course I am not quite dumb enough to think that exit plenums are NOT going to be a bear to build.

Click to expand...

All things being equal, exit geometry is the key to cooling.

RV8RIVETER said:

That got me thinking that we could improve pressure in the inlet plenum by increasing the divergence angle and by placing very very small fins or vortex generators (say carve a small groove in the plug) to keep the airflow following or "attached" to the plenum walls with the wider divergence angle. This induced turbulence would also help improve the thermal transfer of the cooling air.

Click to expand...

Listen to Walter. If it sounds like a good idea, it probably isn't. Turb in diffusers is horrendously complex. Stick with smooth inlet diffusers.

rv8ch said:

Sounds like the voice of experience. I'd really enjoy hearing more about the testing you have done, and what you have learned, particularly since I've installed a liquid cooled engine, and will soon start working on the cowl.

Click to expand...

Strange as this may seem, cooling design is very similar to propeller design, i.e. more art than science and it'll fool you when you test it.

It is impossible to overstate Walter's statements re intuitive designs with regard to cooling. To the experienced eye, the Subaru cooling problems are easy to see. Even so, they will be hard to solve.

My money is on a racer solving the problem.

Liquid installation has been flying for several years. Not at liberty to discuss details or platform except to say it works very well after many failed iterations.

Suffice to say 99% of testing is tweaking inlet geometry, outlet geometry, radiator sizes, radiator solid-to-void ratios, radiator stacking, coolant flow rates, alloys, boundary layer flows, divergence angles and many other attributes.

Still use tufts and oil drops. Highly dependable.

Cooling is science.
Aviation Cooling is an art.
Aviaton Cooling with low drag over a wide speed range is black art.

Updraft cooling link

garnt.piper said:

Downside would be risk of getting smelly fumes or god-forbid, oil, coming back over the canopy! Plus the extra work to make it work, but we seem to go to a lot of trouble anyway, so why not?

Click to expand...

Grant

There is a good write-up on how Peter Garrison designed and built his Melmoth2 airplane. He has updraft cooling. Interesting reading for sure.

http://www.melmoth2.com/texts/Cooling flow.htm
http://www.melmoth2.com/

Cool(ing)

Great stuff, never seen the Melmoth 'page, have heard of the man and aircraft though. He likes gadgetry!

RV-4
250hrs

Grant:

Up-draft systems are used on some twins. Note the oily residue on TOP of the wing. In a single that WILL be on the windscreen immediately. I'm not positive, but I think the OEMs have thought that far ahead!

Walter

Guide Vanes and separate cooling plenums

RV8RIVETER said:

Thanks George.

I too had always planned a one piece plenum. But a seed of doubt was planted after I saw that photo of Bill James Varieze and his dual upper and lower cooling plenums. Plus, the fact he made it work with NACA ducts which have relatively poor pressure recovery.

That got me thinking that we could improve pressure in the inlet plenum by increasing the divergence angle and by placing very very small fins or vortex generators (say carve a small groove in the plug) to keep the airflow following or "attached" to the plenum walls with the wider divergence angle. This induced turbulence would also help improve the thermal transfer of the cooling air.

Just more rambling thoughts.

Click to expand...

You're reading my mind, there's research on this, guide vanes in ducts like the inlet area. The rule of thumb is 7 degree dict divergence I recall, unless the surface is curved than you can go more. Putting guide vanes (small vortex like devices) in the floor and roof of the duct helped keep the flow attached and delaying turbulent flow. The center guide is aligned with the air flow, than there are several other guides on each side, progressively more angled with the side wall. The end result was you can have much large diverging ducts with out penalty. I think that's what you are saying. Any way I think the real key is in those first 10 inches of the cooling inlets. The formula racers with their long long prop extensions have an advantage of developing a nice duct (diffuser). We have limited room. Even the "long" RV cowls are less than 2" longer.

I took at look at all kind of planes yesterday, pictures on my computer, web and old classics like Tony Bingelis books. The idea of cylinders sticking out the side of the cowl with "scoops" over the cylinders is as old as the.... Piper Cub. Hey what is OLD is new again. In Tony's Firewall forward book there are a few designes for separate pressure plenums (with cylinders inside the cowl of coruse). They where made of metal and looked nice. Working around the valve tubes on the inboard side is a little challenge, but really this was standard practice at one time with the old planes, w/ cylinders sticking out in the breeze. So there must be patterns out there? Anyone know of a baffle pattern for the inner edge along the cylinder bases? They way I would do it is make the plenum out of aluminum the transition duct/diffuser out of fiberglass, which is attached to a flex hose and the cowl inlet ring.

Here are a few more examples (unknown ref/source):

(these show the lay down rectangular inlet, verses the round inlet as I plan)

(BTW: If you are the owner of either of the planes on the left, red valve cover or red park plug wires, love to hear from you, on how you made the patterns.)

PS: Navion's have updraft cooling.

Greetings

Fellow builders and cooling nuts.

Reading through the various posts on cooling drag, I have been very impressed with the amount of intelligent thought going into the discussion. I like the way all the aspects of cooling are being grappled with in a fundamental way, trying to understand the physics of what is going on.
The RV community looks like a really neat bunch, truly interested and excited about the educational aspect of experimental aviation.
Please excuse my affinity to fiberglass and epoxy and my intrusion into your world of sheet metal. The one thing we certainly have in common is our engines and the cooling thereof.

Walter Atkinson said:

The biggest issue is not really the plenneum--I've seen some really good ones. The big issue is the metal baffle design that directs the airflow. I've seen some really nice looking ones that were wrong when it came to making the air go where it needs to go.

Click to expand...

Walter,
It looks like we?ve got two separate issues mixed together here. One is how the available air is routed through the cylinder fins. This is controlled by the sheet metal baffling. The second issue is how efficiently that air was delivered to the engine. This is where hard plenums and such can have a profound impact. A wonderful NASA report, CR3405, found that a full third of the air entering a typical cooling installation is lost to leakage BEFORE it ever gets to the engine cooling fins. Even if CHTs are good, this will have a big impact on overall cooling drag. This leakage is sort of an accepted loss in most GA installations. In the experimental world, we can recover a lot of this wasted energy.

chuck said:

1) Inlets as far out as possible (circular preferable)
2) Hard plenum
a) reduce leakage by baffling
b) reduce air speed in laminar fashion
c) create max pressure in upper cowl
3) Clean up flow in plenum (per Atkinson sp?)
4) Use outlet area ratio per Dave Anders
5) Use exhaust augmentation
6) Clean up internal airflow (per Bob Ax.)

Click to expand...

This is a nice summary. I have one general caution though. You need to be careful when adopting a ?rule of thumb? number from another installation. The outlet area ratio that Dave used has built into it many assumptions about the rest of his installation. It cannot be blindly applied to any other aircraft unless the other parts of the system are functioning as well as on the original aircraft. This is particularly the case for the inlet to outlet ratio. These are typically set somewhat independently from each other; the exit being used as a throttle to the whole system and the inlet size being used to determine where the pressure recovery takes place.

parts working together

gmcjetpilot said:

Wade Lively:
Every little part has to work together. One small detail deficiency can ruin the efficency of the cooling system.

Click to expand...

Well said! One of the all time hottest running aircraft engines I have ever heard of had an absolutely beautiful (aesthetically speaking) custom plenum chamber installation. Every piece in the engine compartment was polished or painted. There were, unfortunately, one or two critical flaws in the design of the inlets. The amount of time spent on finish work has been a real disincentive to correct the fundamental cooling problems. Please test you system before painting your baffling.

Nother "Plastic" builder

N91CZ said:

Please excuse my affinity to fiberglass and epoxy and my intrusion into your world of sheet metal. The one thing we certainly have in common is our engines and the cooling thereof.

Click to expand...

Hey Chris, nice to see you over here.... These guys are a good bunch... . Oh, they'll poke ya for playing with glass ... er... .plastic airplanes, but for the most part, there is some good info shared, learned, and utilized. Welcome.

I've followed your 360 efforts along and of course am in the middle of my own Legacy. I think I'm headed down the path of a Plenum as well and the other mods that Andy Chiavetta does at Aerochia.com. I'm a NA bird, so I'll stay with the stock cowling, but it looks like Andy has the plenum, and other parts figured out for a Legacy...

So enough of that... I'll set back and learn from those that have been there, done, that, got all the t-shirts...

kudos to Walter and GAMI

Walter Atkinson said:

If I could get my CHTs down to 260dF, I'd be very happy. TCM set their liquid cooled engine to run at 240dF as a matter of routine. Lycoming says anything above 150dF is OK. TCM alludes to 200dF as the low limit. Ya can't run 'em that cool no matter what you do, can you? The cooler the better. Period. Shooting for 200dC is a bad idea in my mind--based on reams of data (start with the strength of the alloy as temp goes above 380).

As for the baffle ideas that failed... there ain't enough bandwidth! Seriously. We gave a 6 hour seminar on engine baffling; there's no way to do the subject justice in an email posting.

You've got to slow the air down and make it go where it's needed. To discover what we did discover, it took a 36 probe CHT so we could see the temps around the cylinder as we made changes (wanting a round cylinder is good) and tufting so we could watch the air movement on a video display in the cockpit while we were flying.

It's amazing what one can learn with the right tools. It was amazing how much of what I had learned in A&P school about baffling and cooling was dead wrong. It's even more amazing how off-the-mark the local airport know-it-alls can be.

If someone is having a specific problem, I'll be more than happy to try and help, but to give a treatise on the matter is not possible in the bandwidth of a post.

Walter

Click to expand...

Walter

You and the GAMI folks did a fantastic job getting those big Continentals to cool like they should. I couldn?t believe the mess Beechcraft and Lancair (on the IV and ES) made of their baffle design. I was not very familiar with Continental engines. One thing that really stood out, when compared to the Lycoming, is how the pushrod tubes being underneath the cylinders creates more work to make a good fitting and functional installation. All of the sheet metal changes you guys developed where right on the mark. I would consider these changes mandatory for anyone with a big-bore Continental installation.
I know this was developed into a retrofit kit for Bonanza owners. I was a bit curious why this work was not continued to other aspects of the cooling system. You had gotten to the point of using the available air more effectively, but overall aircraft drag reductions are still possible. This may have been beyond the scope of work and I can imagine dealing with the FAA to get more than just internal baffle changes approved must be a real headache.
There has been a lot of discussion of the interaction of cooling and drag in this forum. The stock inlets on a Bonanza are not exactly a shinning example of efficiency. The low pressure region adjacent the spinner is not want you want sitting in front of your inlet. (George M., in a previous post, had a nice little pictorial diagram showing how this encourages a flow reversal in the inlet.) The video you folks took inside the engine compartment showed this effect quite nicely. Seeing that video finally got me inspired enough to stick a camera in my own cowl. As one would expect, when comparing a very large, low inlet velocity ratio inlet to a very small, very high inlet velocity ratio inlet, placed far outboard the results are quite different. You certainly don?t get air escaping out the front anymore.
A general note on testing:
The more data, you have the better. Be it visual, temperatures, pressures, or whatever. To diagnose a problem, or measure the effect of a design change you simply cannot have too much data. It is very time consuming to set up tests, but your results will be put in context and you will have an easier time interpreting the effect of your changes.

Simple experiment to lower CHT's

Today, I ran a simple experiment.

Usually at take-off power from sea level to 1000 ft., my O-320 in my 9A would spike CHT's over 410F. This is down from 435F plus before I rejetted my carb. Ground temps have been about 70F (sea level).

All my CHTs are within 5 degreees.

Reasoning that a little bit more airflow would help, I looked for a simple way to reduce exit air drag near the cowl exit by streamlining the nose gear leg weldments.

Being a cheap SOB, I just wrapped aluminum tape around pairs of braces (fore/aft) to 'streamline' them, and taped around the main strut back to the cowl support bracket.

All of this crude streamlining resulted in about a 10F drop in peak CHTs on takeoff, with an 80F ambient temperature. I never exceeded 400F.

It's a simple experiment to try, I'd appreciate feedback from anyone else who tries this. Sorry, no pics... my camera has vanished.

Vern Little

I have not tried that but ...

I have stared at the NLG strut support structure for the purpose of cleaning up the flow obstructions and I have tried some mods in the area with similar results. The result has been cooler CHT accompanied by a reduction in aircraft speed. Just the opposite of what I would have expected. I thought of streamlining the individual struts and the whole collection with one large fairing but what you tried never entered my mind before (it is there now however).

I have been toying with this simple computer anology in the back of my mind for a few days now. Say you have a computer that is multi tasking with a basic Input-process-Output flow. The inputs (I) are anologous to the inlets; the main work of the process is anologous to cooling the engine; and the output is anologous to the outlet; the data are like air, the conditions of being I/O bound and process bound relate to the air flow, the CHT is a measure of the process work, the amount of data flow is inversely related to the aircraft speed, etc, etc. It is a work in process but it gives me a sense of order and relationships and avoids confusion in the cause and effect area.

Bob Axsom

Cooling drag reduction

N91CZ said:

You need to be careful when adopting a ?rule of thumb? number from another installation. The outlet area ratio that Dave used has built into it many assumptions about the rest of his installation. It cannot be blindly applied to any other aircraft unless the other parts of the system are functioning as well as on the original aircraft.

<snip> There were, unfortunately, one or two critical flaws in the design of the inlets. The amount of time spent on finish work has been a real disincentive to correct the fundamental cooling problems. Please test you system before painting your baffling.

Click to expand...

Good stuff Chris. Welcome aboard. I enjoyed you web site. OK, back to cooling drag reduction.

This brings up variable cowl exit area or "COWL FLAPS".

Dave Anders reports the RV-6 (assume same for RV-7) has a 44 sq-in inlet and 56 sq-in exit. That is a 1.27% ratio. Dave suggest inlet: 34 sq-in, exit 24sq-in or a 76% ratio is better. As you point out Chris you can't fit one size to all installation. That is a great point.

Dave's plane has a fire breathing IO360 with ++250HP / 265 mph RV-4. Dave also uses an exhaust augmentor to increase exit velocity.

Does anyone have pictures of Dave's exhaust augmentor?

My modified inlet is 40% less than the stock cowl inlets at 33 sq-in bell mouth area, with a 25 sq-in exit throat. Running a mild O360A1A (180-190HP) the inlets are somewhat empirically validated since they are copied directly off another RV with simular area, I hope, not flying yet.

Here is tracy's plane with inlets like mine; it is a modified stock cowl:

The inlet is more than a round hole, but more a bell of a trumpet.

Chris you bring up the exit area. That is an area I have left unattended. My RV-7 has no nose gear structure to deal with. I have about 7 to 9 sq-in taken up by the exhaust so I estimate I have about 46 sq-in. Clearly I could cut that down. So right now my modified ratio is 1.40.

If I used Dave Anders 76% ratio my exit area would be a small 25 sq-in area. May be that would be fine for level flight on a standard day, but we need something that can handle a 100 degree day and climb with out melting the engine. Clearly a vairable cowl exit is ideal, but........

Traditionally RV's have used a simple FIXED cowl exit. It works well in that it's simple, light, easy to build, maintain and fly. However I'm thinking something on the firewall which can translate down, reducing the exit area about half in cruise, like a garage door. Cowl flaps on the cowl itself tend to shake, vibrate and wear. Clearly in high speed cruise we have something to gain by reducing the exit area.

I discounted cowl flaps because of the precieved difficulty fitting them. I suspect from the fact many RV's don't have a cowl flap is some indication of the difficulty. It could just be inertia, since Van's plans and stock cowl call for a fixed exit. That does not mean we can't make it better.

Cowl flaps on high performance planes have been around for a long time; I agree with you Chris that the exit acts as a throttle. Now the question is how to do it eligantly put a cowl flap on the RV air frame? Again any ideas? The RV structure and cowl construction offers challenges.

Here are some ideas
The only cowl flap I have seen was in a RV'8 with a "hot IO360 angle valve". The min area was at least as large as a stock cowl, so I assume there's no high speed cooling drag reduction. It was intended by the builder to increase exit area to solve a climb cooling issue. Don't know how long the MAC servo will last in the engine compartment.




In another RV, I think his name is Jeff Nielsen has cowl inlet flaps. They are butterfly valves that are laptop computer controlled! They inlets look massive. Active variable cooling geometry? An area to consider may be. The question to ask, is it practical. With microprocessors and robotics why not.

cooling airflow video

Chris Zavatson mentioned a video showing the airflow pattern through an engine cowling. Does anyone know where I can find the video....I'd love to see it.

Glenn
N654RV @ OKZ

Love it

A very big welcom to Chris! Gladly welcome your interest and insight.

What do you think about two seperate plenums?

This kind of discsussion really thrills the "airplane geek" in me.

I too was thinking about a cowl flap, but decided it was too much complexity. That is what sold me on the augmentor idea. It is fixed, no moving parts, and nothing to adjust. I think it also kind of functions like a cowl flap because it helps draw more air in when moving slow, besides the extra benefit of increasing the velocity of the exit air.

George, I am glad you posted the picts of the inlet "bell". Hopefully y'all can clear up a question for me. Why do you want the inlet to narrow and if so, how much for how long? I ask this because everything I have read suggests that the highest pressure recovery over a broad speed range is a "pitot" style inlet duct with a trump shaped divergence, ala "Aerodynamics of Propulsion, by Kuchemann and Weber".

Darn, keyboard challenged

"trump shaped divergence"

Meant trumper shaped diverging duct.

NASA

RV8RIVETER said:

George, I am glad you posted the picts of the inlet "bell". Hopefully y'all can clear up a question for me. Why do you want the inlet to narrow and if so, how much for how long? I ask this because everything I have read suggests that the highest pressure recovery over a broad speed range is a "pitot" style inlet duct with a trump shaped divergence, ala "Aerodynamics of Propulsion, by Kuchemann and Weber".

Click to expand...

Great question. Actually I took that close up picture off since it looked radical compared to what I have. I don't have any pictures of my inlet rings but they are more mild than that appeared. The inlets I have are more of a 1/4 inch lip. Never the less it does neck down in the first inch. Now why? I'll take a guess and get back to you for sure. Here is my story and I am sticking to it for now.

I agree you are intuitively trying to slow air down and convert velocity to pressure with a diverging nozzle. Clearly reducing the area of the inlet down stream will increase velocity and lower pressure. I think the answer is in the NASA No. 3405 report. It has to do with local inlet effects and losses than overall airflow mechanics. Also it has to do with optimizing reduced external drag, even at the loss of some internal pressure recovery.

Keeping the air from not separating in the inlet and losses is critical. Most of the taper is in the front, where the Kutchemann A-series axisymmetric inlet contour is greatest (Fig 35). It is the continuation of the lip of the inlet if you will. Clearly you want the air to initially remain laminar as best you can. With sharp inlet radius or diverging inlet wall from the start, air flow may trip right away, therefore the initial curved (A-20) internal shape and slight necking down or taper of the inlet. It's like the upper surface of an airfoil up till you get to max thickness. The wing has some "chord", as does the inlet. Think of the inlet as a inside out wing. The key is to "capture" that air with little or no losses. Of course it does not help if all heck breaks loose 2 inches down stream. Truth be told making it perfect is pretty much impossible with all the wild air flow moving around and near the prop.

NASA research suggest using a nozzle (inlet) with an airfoil shape, so I think that is the Inspiration for the inlet design I have, which is from Dave Anders. Keep the air initially accelerating to avoid stagnation (or minimize the effect of stagnation), than diffuse. So to keep initial loss to a min at least at the lip we do this. That is my interpretation.

First the NASA report: "The inlets capture the cooling and are partially converting the dynamic pressure head to static pressure than conveys it to the upper plenum...The upper plenum serves as reservoir for engine and auxiliary cooling. (at that point) The cooling air should be at stagnation conditions with full recovery of the dynamic head."

Well this does not explain or answer your Q. How about this:

"In actuality, the pressure recovery in the upper plenum may be as low as fifty percent of the free stream dynamic pressure, due to flow losses through the inlet. Also, insufficient plenum volume results in finite velocities, consequently, the flow is not evenly distributed over the engine face nor is the transition from horizontal flow to vertical flow through the cooling passages accomplished efficiently..."

Well still nothing but it does talk to plenum volumn and inlet losses, before the plenum! Yes they found a leaky plenum produced loss, but more over the loss they looked at was the pressure loss between the (lip of the) inlet and plenum! 15% pressure recovery is lousy. Why? It has to do with stagnation point and avoiding "possible flow separation and stall". Also "inlet lip contours should be well rounded with large radii of curvature to minimize inadvertent suction peaks and following adverse pressure gradient."

What does that mean? I am not sure, but the criticality of the inlet varies for the 0.30 inlet (big dia) and 0.60 inlet (small dia). "The lip radii and inlet curvature should be maximized." I suspect that is why my small dia inlets have an internal curved surface, to avoid losses in the throat. My only concern is the disconitunity between the aluminum ring (which has a 0.10" thick edge) and the flexible duct. I guess I could file it down to a thinner or feather edge.

Hummm ....... Not only size but internal shape of the inlet important. I think I am getting close to why my inlets look like they do.

My ratio is 4.6" to 4" dia. over 1.75". Most of the taper is in the first 1/2 to 3/4th inch curved surface, like the upper surface of a wing from the leading edge to a 3rd chord. Yes the air accelerates and the pressure drops, but keep in mind we are not talking about a wing, we are talking about internal flow of an inlet. It is after the inlet we'll start to diverge and convert the dynamic into static pressure.

I agree that this is counter-intuitive, you want to slow the air down and have pressure build, but this slight taper is to control inlet losses only. If you had parallel or diverging inlet the air would separate, stagnation would occure outside the inlet and you would loose pressure recovery..... That is how I interpret the data. In my opinion many of the so called round inlets are poor with massive initial losses before they even get into the upper plenum. The NASA report details talk to the internal flow mechanics and inlet effects. That is where the pearls of wisdom are, in the detail. How you read it and turn that into a inlet design is definitely subject to interpretation. No where do they say do it this way, XYZ. They do point to problems of the inlet's they did check which where just a few. Reading between the lines is a bit of an art and science. Clearly the 0.30 inlet is best for pressure recovery but it is huge and had its own issue. The 0.60 had issues but clearly less drag. How you design an inlet and mitigate the negatives is full of compromise. The judgment to do this is a bit of an art. I think Chris said you need to test it. That is a good idea.

The trick is making a nice transition from inlet to plenum. The formula guys with a yard long prop extensions can develop a very gradual nice diverging duct. Although many formula racers use a slot inlet verses round, I bet they have some generous radii at the inlet lips. We have a very short distance to make things happen, especially on the (right) #2 jug which is forward of the #1. I doubt we relize super ideal laminar flow, but than again if you screw it up at the lip of the inlet you have blown the whole deal.

In conclusion it's local flow right at the lip and first 1-1/2 inch that's being controlled to reduce suction and a strong pressure gradient. It is all complicated due to the complex flow near the prop. Bottom line is I am not 100% sure and did not question it until you asked. I thought it matched the data and conclusions of the report. Keep in mind what works for a Vi/Vo = 0.60 inlet (small dia) is not the same as a 0.30 inlet (large dia).

I know it does not answer you question definitively, but I think a re-read of the NASA No. 3405 report might be in order. I'll get back to you.