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Cowl Inlet Area

Pounder

Pounder

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I compared a Sam James (RV4) cowl to my old stock vans cowl. The total inlet area of the SJ is about 22 sq inches as compared to my 52 square inches.

Anyone out there ever tried to build a slick cowl inlet "plug" to reduce their the engine cooling drag?

It's gotta add a hundred knots???

BP
 
Of course they have

The effect I saw was an increase in CHT in essentially direct relation to the reduction in inlet size through many incremental inlet area reductions with no increase in speed whatsoever. This cooling drag thing is not a simple area to work to achieve increased speed. The conceptual changes that intuitively seem like sure thing drag reductions and increases in speed most often do not pan out. It is well worth the effort but it is tedious work.


Bob Axsom
 
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Has probably been covered before (Search and I are yet to make friends), but what makes those Lancairs and other fast-glass thingamabobs able to get away with bugger all inlet area and tiny circular inlets? It can't be all due to the higher ram effects...Can it?
 
Do the reading.

An easy place to start is Chris Zavatson's site:

http://www.n91cz.com/

....where you'll find accounts of his own Lancair work and and a dozen or more technical papers.

NASA CR3405 is a classic. It does not offer all the answers, but you should read it, think about it, and read it again. Also pay particular attention to the Dec 2007 article Chris wrote for Sport Aviation. Two statements there should taken to heart:

(1) ...inlet size is not the main driver in determining how much air enters the cooling system. Instead, the inlet size determines where your pressure recovery takes place.

....to which I would add "and how much".

(2) The exit serves as the throttle to the cooling system.

....and its velocity is the yardstick for cooling drag.

Me? I'm just a student. It's a fascinating subject.
 
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Pressure recovery?

DanH said:
(1) ...inlet size is not the main driver in determining how much air enters the cooling system. Instead, the inlet size determines where your pressure recovery takes place.

....to which I would add "and how much".
Click to expand...

Dan,

Something I've never completely understood is what is meant by "pressure recovery". What pressure is being "recovered"? How does this compare to the ambient air pressure at a given altitude?

Would it be accurate to say that we're actually talking about pressure rise (above ambient), that takes place when the air meets the aircraft? Chris talks about this in his article on induction inlet sizing, but doesn't use the pressure recovery terminology there.
 
A common view is static airplane/dynamic air

A common view is static airplane/dynamic air when neither is completely true. It is at least interesting to sit back and visualize this large mass of air surrounding the Earth that is often moving around at fairly slow speeds and this shark of an airplane at cruise smoothly flowing through it at over 200 mph. If the cooling flow path cannot process all the air in front of the intakes there will be some of this air that never enters the intakes as the plenum air capacity and the flow restrictions saturate and the air in the system pushes back. What happens in front of the system is important and the amount of drag is controlled by the system and not just the size of the holes in front. As you can see there is some tuning in the system and I'm sure it could be modeled with an analog computer with resistors, capacitors and operational amplifiers.

In my personal experiments with many configurations I found the best so far was:

- Stock inlet openings
- A curved baffle from the the rear of the engine to the bottom of the engine to the bottom of the fuselage
- Vertical baffles attached and sealed to the curved baffle coming in from the sides of the lower cowl (sealed with red rubber edge strips) to the width of the cooling air outlet
- Horizontal baffles just below the cowl separation in the lower cowl, sealed to the engine valve covers and side plenum walls with high temp RTV and to the cowl sides and lower angles baffles with red rubber edge strips.

This configuration increased the speed by 4 kts. It was NOT arrived at with a one shot modification. Baffles tried later to eliminate back flow of cooling air out of the cowl around the crankcase/prop shaft behind the spinner, turn the air below to guide it back toward the outlet, avoid the turbulence over the nosegear support structure, etc. all reduced the speed of the airplane. I have done some experimenting with the outlet flow with nothing but losses in speed. I still think something in this area on the order of the Larry Vetterman experiment is well worth looking into.

You are asking the right questions - keep it up.

Bob Axsom
 
Alan Carroll said:
Would it be accurate to say that we're actually talking about pressure rise (above ambient), that takes place when the air meets the aircraft?
Click to expand...

Exactly, dynamic pressure, (fluid density multiplied by velocity squared) / 2. In this case pressure recovery simply means we want to capture all the dynamic pressure we can.
 
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Flow separation?

Bob Axsom said:
The effect I saw was an increase in CHT in essentially direct relation to the reduction in inlet size through many incremental inlet area reductions with no increase in speed whatsoever. This cooling drag thing is not a simple area to work to achieve increased speed. The conceptual changes that intuitively seem like sure thing drag reductions and increases in speed most often do not pan out. It is well worth the effort but it is tedious work.


Bob Axsom
Click to expand...

Bob,

Is it possible that potential gains were negated by increased turbulence behind your cowl plugs? One of the common factors in airplanes with small inlets seems to be the use of a prop extension, which provides more room to smoothly transition the inflowing air. Its more challenging to do this with the stock system because the cylinders are very close to the inlets, and even more so if round inlets positioned farther outboard are used. Perhaps this has something to do with why many fast airplanes such as the Nemesis NXT don't use round inlets?
 
Anything is possible

Alan Carroll said:
Bob,

Is it possible that potential gains were negated by increased turbulence behind your cowl plugs? One of the common factors in airplanes with small inlets seems to be the use of a prop extension, which provides more room to smoothly transition the inflowing air. Its more challenging to do this with the stock system because the cylinders are very close to the inlets, and even more so if round inlets positioned farther outboard are used. Perhaps this has something to do with why many fast airplanes such as the Nemesis NXT don't use round inlets?
Click to expand...

I don't thing this is a straight flow problem. If the plenum is working right I don't think this is a concern but it could be. I have thought of a prop shaft extension. When I was experimenting with my plugs they were very blunt on the rear I could work on that and check it out to be sure. I did work on the front end as much as I could and I did get some light high pitch prop scuffing of a plug at one point - a prop extension would certainly give more room to work that area and I think that could be priductive. I will keep thinking about it.

Have you looked at Red Hamilton's front end on his super fast Wittman W-10 Tailwind? It kind of stands the old NACA studies on their ear. Not too surprising I guess after 60 or 70 years.

Bob Axsom
 
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Alan Carroll said:
Is it possible that potential gains were negated by increased turbulence behind your cowl plugs? One of the common factors in airplanes with small inlets seems to be the use of a prop extension, which provides more room to smoothly transition the inflowing air. Its more challenging to do this with the stock system because the cylinders are very close to the inlets?
Click to expand...

Very good Alan. Note the concept of inlet velocity ratio (Vinlet/V). An effective high velocity inlet (ie small diameter) depends on internal pressure recovery, which requires the length to fit diverging walls. A low velocity inlet (large diameter with the system exit throttled) requires little or nothing behind the open hole; shape behind the hole becomes less and less critical as the ratio decreases. Either system works, but their physical requirements are quite different.

Take a look behind the high ratio inlets on Chris Z's Lancair. Then take a look behind the low ratio inlets on a Mooney Acclaim.
 
Dan is correct

What Dan says here is exactly right.

If you make the inlets large, they capture the same flow, at lower velocity. What happens is the "recovery" takes place out in front of the intake, the streamtube diffuses without loss to adjust to the large diameter, low velocity.

The kicker is what happens to the spillage when the diffused streamtube hits the intake lips. If the intake has generous radius lips, like the stock cowl, there may be very little spillage drag. If you have sharp lips then the flow may separate and reattach later, and may have some drag associated with that bubble of trapped recirculating flow.

Traditional cowl inlets with generous intake lips will have very minor spillage drag over a wide range of operating conditions, meaning different flow rates, angles of attack, prop swirl, etc.

The small round inlets on SJ and others have relatively sharp lips, so when they are working "on design" they are fine, but if the diameter is mis-matched to the flow, they will separate. At high flows, they are at risk for separating internally (thats why the internal diffusion becomes important).

On balance, I think it is better to design for some spillage, and just manage it so that the spillage drag is very low -- which has been the experience of Bob Axsom and others that have experimented with inlet area.
 
How many Sq. Inches?

"I compared a Sam James (RV4) cowl to my old stock vans cowl. The total inlet area of the SJ is about 22 sq inches as compared to my 52 square inches. "

My SJ cowl has 4.75" diameter round cooling inlets. That makes the radius 2.375", the square of which is 5.64, making the area of 1 hole 17.72 and the area of two thus 35.44 InSq.

Are yours smaller or did we do the math differently?

BTW, The SJ cowl is meant to be used with the SJ plenum and taken together they seem to provide excellent cooling with less intake air. I will leave it to others to say how or why, but it works extremely well on mine.
 
hevansrv7a said:
"I compared a Sam James (RV4) cowl to my old stock vans cowl. The total inlet area of the SJ is about 22 sq inches as compared to my 52 square inches. "

My SJ cowl has 4.75" diameter round cooling inlets. That makes the radius 2.375", the square of which is 5.64, making the area of 1 hole 17.72 and the area of two thus 35.44 InSq.

Are yours smaller or did we do the math differently?
Click to expand...

How many square inches? :)

I figure my standard RV7 cowl inlet at 7x3.25 (2) with rounded ends coming in at 45.5" total. Actually, it really doesn't matter a whole lot. What probably matters more is exit area.

I liked an earlier analogy that the exit area is like a throttle.

Considering that the inlet area of Van's cowls appears to be the same whether it is a 0320, 0360 or IO360, the key to effective cooling has to be exit area. The 200 HP IO360 makes much more heat than the 0320 and there has to be a way to accommodate that circumstance and that is by opening exit area to get more air moving across the engine.

My totally non-scientific, non-engineered experimenting in this area has resulted in very good cooling without a noticeable loss of performance - I am not winning any races but the airplane does match Van's numbers for make, model and engine.

My calculations of the standard RV7 cowl are 45.5" of inlet and 49" of exit area. The actual exit area is less when considering exhaust pipes, so the inlet and exit areas are about equal. Not enough from what I have read concerning this matter. Exit area should be at least 114% of inlet area however than number may be subjective to how smoothly air flows through the cowl.

In any event, air flow through the cylinders is not smooth. It makes turns, is disconnected and finally finds its way out the aft cowl bottom after navigating through exhaust and intake pipes, wires, fuel lines and host of other obstructions. How much heat it carries with it is a matter of how quickly it moves on through and that is a function of a pressure differential from top to bottom cowl and atmospheric pressure.

I have found, opening exit area improves engine cooling a lot. That's not to say controlling air flow through the engine is not important but increasing exit area does make a difference. At present my exit area is 108" and maybe it is the down and dirty way to adequate cooling, but it works.
 
Lycoming curve 13242-A cooling requirements for the O-320D at 7500', +21C, calls for 1.4 lb/sec for 435F CHT; that would be about 25 cu. ft. The pressure drop will be between 3"-4" H2O, or 15-20 psf. At 200 mph that would require a minimum inlet area of 12.3 sq. in. My 125 HP requires 12 sq.in. to give CHTs of 350-380, so using that I would say that 160 HP at that altitude and temp would need about 16 sq in, 180 HP about 18 sq in, and 200 HP about 20 sq. in., so approximately 10 HP/sq. in. I calculated inlet/outlet area ratio based on the increased volume of the cooling air from the temperature rise at about 1.25, which approximates my own.
 
inlet ratio

Dave Anders has been very sucessful with his RV4 with exit considerably smaller than inlet. I don't have the exact numbers in front of me but I think the outlet is about 15% smaller. Interesting to see Bob mention Red and Marilyn Hamiltons very fast Wittman Tailwind. The cowl on this airplane, typical of many Tailwinds, does not conform to some of the "rules". The airplane does not recognise the "rules" and goes very fast. 228 in the Pagosa race, slightly faster than one 540 powered Rocket. The "standard" Tailwind cowl has a 4" prop extension.
 
Great discussion! I've been asking questions like this (of guys like Dan, Tom Martin, Mark Frederick, Bob Axsom, Larry Vetterman...anybody that will listen...) for a while.

So far I've been working on plenum leaks while I incorporate other low-hanging-fruit type speed mods...but this winter I hope to get the guts to work on exit area...means cutting painted fiberglass!!

I cool very well, even at race power settings, so I thnk I have room to work. I have (rough measure) 43 sq in of inlet, and 84 sq in of exit (big exit!), so Dan, hearing of you over-cooling at 40 sq in is encouraging (though I know you have other design goodies under the hood to help!). What is your inlet size (looking for your ratio)? I hope to also add some sort of firewall bell, or at least a curved surface at the base of the firewall at the exit...to get some of that Coanda effect going! I may also decrease the down angle of the exhaust pipes, but cut the rear of each pipe at an angle to direct the noise (thumping) down, while keeping the plume closer to the plane (in the hopes it decreases that aspect of drag).

Biggest question at this point is where (in terms of amount) to start with the exit area reduction. So many factors have interplay here. I may not be able to go as small as you Dan, due to differing flow patterns (smoothness by design in your well-designed motor and cowl). May just be trial and error, and just take it down incrimentally. Not sure if there a metric by which we can tell if we have reached max return on the reduction, or if its just go smaller, look for more speed, and go till temps get as high as is prudent to go. Thoughts? Insider information? Magical incantations? :rolleyes:

On the inlets, the discussion here has matched what many of the above named guys have said to me. Going smaller on the inlets requires a much more critical eye on the shape behind the inlets, or as F1 Boss told me, if I go smaller on my inlets, I better be ready to make some very nice divergent inlets into the plenum. Bigger inlets mean the shape is less critical (as stated by others here), but I still want to continue to find the leaks and try to smooth the flow there...and I need to get smarter on spillage...interesting points on that above.

Thanks much for the discussion!!

Cheers,
Bob

And Dan, yes...its total witchcraft! :)
 
Here's a photo of a Dave Anders exit. He is cooling an angle valve motor with it. For perspective, that tailpipe would be about 2.5 inch diameter, maybe 3. Can you say "small"?



Bob, my current ratio is 1/.63.....which means nothing. Anyway, I'll be trying a smaller exit in the future. Recall I built the cowl so I could swap exit sizes at will, with a screwdriver.
 
Ramp to block cylinder #1

I am about to add metal to block cooling air reaching cylinder #1 since it runs cooler than the other three cylinders. Some folks have metal at 90 degrees to the airflow.

Would it be better to make that air blocker ramp up to the cylinder to minimize turbulence from the vertical air block (metal)?
 
What you can not see from the Anders picture is the extensive ducting that is going on inside of the cowling. It is not a simple inlet/exit size issue, it is the whole system that works together. I have spent a lot of time smoothing the flow of the inlet air, creating the best transition from dynamic to static air region that I can. Rectangular inlets are limited in this way due to their, usually close, proximity to the cylinders. Typically the round inlets are closer to the prop which gives you more room for this smooth transition area.
But the inlets are only one area the needs to be smoothed. To get the air out of the cowling you need to clean up the exit area ahead of the opening. Your goal is to get the air smoothly flowing in the direction of the outside air. While doing this you also have to gradually reduce the internal ducting to increase the speed of the cooling air.
Once you do all of that then you can start to reduce the outlet area.
Personally, over a number of years, I have made significant gains with cooling but speed increases due to cooling drag reduction have been elusive. From this I can only deduce that the F1 rocket cowling is quite efficient in the stock form, from a speed perspective.
Keep in mind if you have success cooling your engine you are likely going to demonstrate a reduction in speed for the simple fact that you are probably flowing more air. It takes time to develop a system that works for your plane and even though you would think that all RVs would be the same, they are not when it comes to fine tuning. Some engines just run hotter then others and this might limit how much you can choke the cooling air. Chasing the cooling drag reduction target has been a very interesting and often frustrating experience.
 
cowl

A relatively easy way to experiment with cowl outlet is to build a temporary fairing on the INSIDE of the cowl outlet. This can be done with foam and fiberglas, if you don't like the results just grind off the fairing. No issues with paint. Dave Anders has a Lycon engine with, I believe, 10-1 pistons, which makes the small outlet even more amazing. Note the fairing on the bottom of the firewall in the picture, you can just see the start of the generous radius. Most probably know this but Kent Pasers book, Speed With Economy, covers most of this and more.
 
Tom Martin said:
What you can not see from the Anders picture is the extensive ducting that is going on inside of the cowling....
Click to expand...

More Anders pictures. I don't recall where I got these photos or when they were taken, but:





The RV4 doesn't offer a lot of physical space for a converging exit duct, and this one isn't real long. Tom, do you know if Dave later built something more extensive? I only know of two guys who built ducts from engine to outlet, Bob and Alan. I went with the thought expressed by another writer, to wit, the air right under the cylinders is slow and turbulent and not really worth worrying about. Instead worry about it a lot as it nears the exit.

Gents, listen to Tom....exit velocity is THE end goal for drag reduction.

BTW, a point if I may. You often hear folks say they want to reduce the total volume of air passing through the system in order to reduce cooling drag. Those believers often suffer high CHTs while going only a little faster.

Cooling capacity (the quantity of heat moved by the system) is determined by total mass flow and how high you can raise the temperature of that mass as it passes the hot engine body. Drag is a function of exit velocity, not mass. For example, two systems, one flowing twice the mass of the other. If they have the same exit velocity at the same aircraft velocity they have the same drag.

Postscript:

Brain fart....it's a dumb illustration, see below
 
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Momentum loss, mass flow and velocity

DanH said:
Drag is a function of exit velocity, not mass. For example, two systems, one flowing twice the mass of the other. If they have the same exit velocity at the same aircraft velocity they have the same drag.
Click to expand...

Dan, et al
The energy extracted from the air stream due to passing that air through the cooling system is represented in terms of momentum loss.

bhpc = Wa x Vo(Vo-Vexit)/(550g)
where,
bhpc = cylinder cooling drag in hp
Wa = mass flow
Vo = TAS
Vexit = exit velocity
g = 32.2 ft/s^s
550 = (ft-lb/s)/bhp

Both mass flow and exit velocity are factors in determining drag.
 
Dan
These pictures give you a tease about what is going on in his cowlings. This alone would be a great place for builders to start. Only put things in that area that need to be there. Route all wires, hangers, heat muffs away from the exit area. Just think of the aft inside end of the cowling as if it were outside the cowling. You would never consider hanging draggy stuff on the wing or fuselage and so try to keep it from the region of the cowling where the air is starting to change back from static to dynamic pressure.
 
Don't waste time with this:

jrs14855 said:
A relatively easy way to experiment with cowl outlet is to build a temporary fairing on the INSIDE of the cowl outlet. This can be done with foam and fiberglas, if you don't like the results just grind off the fairing. No issues with paint. Dave Anders has a Lycon engine with, I believe, 10-1 pistons, which makes the small outlet even more amazing. Note the fairing on the bottom of the firewall in the picture, you can just see the start of the generous radius. Most probably know this but Kent Pasers book, Speed With Economy, covers most of this and more.
Click to expand...

This approach has been tried, and makes no sense. Closing the exit area by just obstructing the internal area but maintaining the outer cowl shape will do nothing to reduce drag, just make the engine run hotter.
I'm not talking about radiusing the bottom edge of the firewall - that always makes sense. I'm talking about making in internal plug just to reduce area. Waste of time.

You MUST make a cowl flap or a permanent reduction in exit area that achieves lower frontal area and allows external flow to align better with the belly of the fuselage. The drag you are trying to get rid of is associated with the external flow separating at the cooling exit and creating low pressure (which also helps pull the cooling flow through the cowl). By reducing that "base" area of separated flow, you reduce drag. Also, by reducing the flow angle where the flow leaves the cowl, you don't create as low a pressure.

Putting a plug inside will not prevent the outer flow from separating. You will have just as much base drag at the exit. The one exception is the transition plug idea that Larry Vetterman is now well known for. This helps fill up the volume that would have been full of dead, separated air and gets the cooling flow merging well with the outer flow. The downside is there is a lot of "scrubbing drag" from the higher velocity exit flow running around that fairing.

The "right" way to do this is just what you see on the bottom of a Cessna C-185, or a Bonanza, or for an extreme example, an F4U Corsair. Reduce the exit area by eliminating (a portion of) the angled external ramp on the cowl that increases frontal area and makes the outer flow turn away from the belly. If you want good cooling for climb, make it a movable flap. If you don't care about that, just make it permanent. If you really want to make it agressive, make a tunnel for the exhaust, and fair the rest of the cowl onto the belly ( like Dave Anders, like a Lancair, etc.)
 
cowl

1. The Anders photos appear to have white paint on the gear fairings. The airplane was flown polished for a long time, the white is fairly recent.
2. I did not mean to imply that closing the outlet via an internal fairing would make the airplane faster. This is simply a means to determine how small the outlet can be while still maintaining acceptable temps. Hopefully this would allow one(instead of several) modifications to the lower cowl. HOWEVER in taking a closer look at the Anders pictures, Dave appears to have used the firewall fairing as a means of reducing outlet area. The fairing appears to be very close to the exhaust and approx 3/4" below the bottom of the standard cowl.
3. The Paser book talks abouit an internal lower cowl ramp to orient parallel to the exhaust ramp in the forward fuselage.
The tandem airplanes don't have the space for optimum modifications. The Anders firewall fairing is open at the top, there just doesn't appear to be room for any alternative.
Keep in mind, that unless I missed something, Dave has the fastest four cylinder RV in existence by a huge margin. The airplane was optimized a LONG time ago.
 
The tandem airplanes don't have the space for optimum modifications.
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I have to scratch my head on this statement as the author already noted that Dave Anders has the fastest four cylinder RV in an RV 4, John Huft is very close to him in an RV8 and my rocket, a tandem is also doing well in the SARL races.

In regards to another post that was advocating cowl flaps, I have one, and it is always closed. I have never really had to use the thing in the open position and I have tried a number of different versions on two aircraft. I really wish that someone could come out with a cowl flap design that works on our airplanes. The SX 300 aircraft have a nice shape but it would not really work well on my aircraft. Pictures of successful RV cowl flaps would be most appreciated.

It is too bad that Larry Vetterman would not comment on his latest after cowling plug. The one that I saw in Mitchell SD this year is nothing like the ones that others mention so often. It has been modified quite a bit and although it has the same shape as before it is only about one inch deep and as such, to me, appears to merely reduce the outlet opening in an efficient manner, rather then the fully closed/louvered shape that he had last year.
 
For starters, I'm cooling a bunch of horsepower with about 24 square inches inlet area. I have round inlets, diffusers and a plenum.

My exit area is huge, so I can't say I have "optimzed" it yet. This will be my winter project.

Something I think needs to be kept in mind in regards to exit air is that it only matters near the exit itself. Yes you need to accelerate it and point it the correct direction, but using a baffle from the engine all the way down is kinda pointless. Imagine your lower cowl full of water, the only "flow" that is of concern is very near or at the exit itself, all the rest is just pressure.

Just my 2 cents,
Tom
 
I-O ratios

When I see input-output ratios approaching 1 or even less than 1 the first thing that comes to my mind is that the inlets are much too large. All of the air that comes in through the inlets and passes through the cylinders gets heated. When this air is heated the volume increases. In order to get the air to pass out of the cowling at near free-stream velocity, ignoring the pressure drop through the engine, the oulet area must be at least the inlet area multiplied by the absolute inlet-outlet temperature ratio. If the inlets are sized larger than necessary for the flow and the pressure recovery occurs ahead of them, the air flow through the propeller root region is slowed down which affects the helix angle of the flow into the prop.
Here's an idea of an approach that you might try. On Tom Aberle's Phantom Reno record-setting biplane, he has individual cooling outlets immediately above each cylinder in his updraft cooling system. This air exits without having to undergo the pressure drop associated with changing directions and passing by all manner of wires and tubes in the bottom of the cowling.
If you were to wrap the cylinders with carbon fiber from the push-rod tubes on the top of the cylinder to a rectangular aperture on the bottom of the cylinder, the air would stay in intimate with the cooling fins, enhancing the heat transfer, and increasing the cooling efficiency. This efficiency increase would reduce the amount of cooling air required. You could then make a rectangular cross-section duct that met with the duct on the bottom of the cylinder and curved out to the sidewall of the cowling, then exited to the rear through a curved aperture on the sidewall; one for each cylinder. You could even gang doors at each outlet, controlled from the cockpit, to vary the outlet area. If you really want to reduce cooling drag it's time to quit experimenting with different ways of doing the same thing and try a fresh approach.
 
Agreeing w/ Ellipse

In a conversation with Dave Anders about this subject and in earlier ones with Sam James, it became apparent that:

1. The tighter the baffling around the cylinders, the better the cooling. Sam recommends a very small gap between the inter-cylinder lower baffles. Dave suggested bringing to near-zero. Yes, I know it sounds extreme.

2. The less air the better as long as it is enough. Enough is (as stated above) a function of air mass but also one of temperature rise. Slower air has more time to absorb heat.

4. Streamlining the out-flow after the cylinders does reduce drag. Many have shown this, Anders included.

August Raspet at U. of Mo., many years ago, proved that cooling drag is very significant. He did the original prop-less glides and tried blocking the cooling intakes. If zero cooling intake reduces drag by "X" then it is a reasonable inference that reduced intake under the correct circumstances can reduce drag by "X-y" where y >0<X.

Dick Martin (very fast RV8) reduced the inlets on his SJ cowl and got good cooling and increased speed. I can't recall the other examples by name, but I do recall there have been similar results reported here.

As Paul Lipps points out, the exit volume is a function of the intake volume times the absolute temperature rise (Boyle?). The exit velocity must then be a function of the exit volume and the exit area. If the exit velocity is less than the ambient stream then it will create a low pressure area which will be experienced as drag. Of course, Paul also points out that you don't want so much "choke" that you get back pressure which would be experienced as drag, too. He is also on the right track in observing that the speed of the air actually arriving at the prop is not equal to the free stream speed and that the blades must be designed for that. Read appendix "SSSS" in Jack Norris's book for more on that.
 
If the inlets are sized larger than necessary for the flow and the pressure recovery occurs ahead of them, the air flow through the propeller root region is slowed down which affects the helix angle of the flow into the prop.
Click to expand...

Interesting point and certainly true, but is it really significant? For example, a pair of 6"D inlets would be 56 sq in. A 74" prop with 13" spinner would have an active disk area of 4167 sq in. The slowed air region would be 1.34% of the prop disk.

I like to think of design choice as practical compromise. Ok, the large, low V inlet means the prop pitch is imperfect for some tiny portion of disk area. In return, I don't need a propshaft extension to allow enough length for the diverging duct necessary with internal recovery. I went with external/low V inlets because I wanted to use the BA Hartzell with a 390, a combination with a full vibration survey. A shaft extension on an RV8 would have created a CG problem.

Of course, Paul also points out that you don't want so much "choke" that you get back pressure which would be experienced as drag
Click to expand...

He did?
 
While the gist of this discussion is cooling drag, one aspect of the matter not considered so far is the effect of changing OAT. The medium of heat exchange here is plain old air and its incoming temperature is a factor just as is inlet/exit areas and internal disruption of the flow.

It was interesting to hear Sonja Englert comment at OSH a couple years ago, on any given day, all else being equal, if the OAT is 30 warmer than yesterday, so will be engine temperatures.

What that means to me, just an ordinary functionary here in this matter, is that adequate cooling across the wide range of OAT's we encounter throughout the seasons requires a moveable cowl flap or a system that will over cool on cool days. A system with adequate cooling capacity in October at 60F will not work well in July in 95F+ temps or launching with a heat soaked engine on such days.

I guess the point here is, no matter how much planning goes into a system to achieve best drag reduction and yet have adequate cooling, it may not work on extreme temperature days if the system is fixed. The cooling medium, plain old air, is variable and so must be the system to accommodate this variable.

Maybe that's why moveable cowl flaps have proven effective in a wide range of applications. We lessor members of the club are stuck with a fixed system that works across a wide range of temperatures but obviously can not be optimal across the board. Considering the relative wide speed range of RV airplanes, a variable system could be beneficial operationally but maybe not be from a practical point of view considering cost and complexity.

I do know from experience as a flight engineer on a B707 years ago, if the exit area of a cooling system is trimmed, so must be the inlet area. If not, the familiar "rumble" of stalling inlet air could be felt and heard. :) Maybe not a big deal here but from a physics point of view, certainly a factor.
 
Correction re: Professor Raspet

hevansrv7a said:


August Raspet at U. of Mo.,
Click to expand...

Please, PLEASE. Dr. Raspet was not a professor at U. of Mo.!!! He was a Professor at Mississippi State College which is now known as Mississippi State University!!! We Southerners are rightly proud of the few educated folks here in the South who make major contributions to such studies as Aeronautics, and we'd rightly appreciate it if folks would provide proper attribution to the University where he did his work. He also taught a Turkey Vulture to fly in a wind tunnel in his study of wing tip vortices, but we won't go into that right now!
 
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Choke and drag

My interpretation of Paul's comments took a leap, but not so large as it looks.
If a given change slows the airplane it will be experienced as drag. If, in reality, it is lower prop performance, the airplane still slows down. My use of the term "choke" was to take a verbal shortcut. Choking cuts off air. In this case, choking the exit changes the relative sizes and would produce the back pressure (slowed intake) to which Paul referred. Long answer for a short question. I'll try to be clearer next time.

Anyhow, to reiterate, Paul is on the right track in observing that changing the net airflow through the prop disk affects prop performance just as he is correct that the prop's local pitch at the inboard ( really all) stations must be designed to the correct in-flow. That latter point is a very difficult subject and one that I have yet to fully understand. Just to illustrate the point, though, the speed of the air at the prop (tractor configuration) is less than 100% of the aircraft's TAS all the way out to the tip (97%) and typically is in the 50-60% range at the spinner for planes like ours. This is an area where Paul Lipps and Jack Norris are in at least partial agreement.

So now we have an interesting and surprising potential change in prop performance as a direct result of cooling changes. Who'da thought it?
 
DanH said:
Bob, my current ratio is 1/.63.....which means nothing. Anyway, I'll be trying a smaller exit in the future. Recall I built the cowl so I could swap exit sizes at will, with a screwdriver.
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Dan,

Concur with you that ratio, in and of itself, is meaningless. My query was strictly gathering a data point, to aid in viewing what appears to work in the field, and what does not (and just to clarify, the .63 is your exit, and you are going smaller, right?) I'm not working with a blank sheet, in that I'm looking at modifying an existing aircraft I didn't build...so I'm learning and going slow, so as to avoid unintended consequences (hmmm, wonder if I can move that gascolator out of the way...and what is it doing ther in the first place...stuff like that).

I know that there is no magic bullet, or ratio/number, that says "do this, and you will get that"...its all experimenting, and we each have, in a sense, a one-off design to try to optimize (no two engines, cowls, exhausts, etc. are exactly alike). There are just too many factors, from plugging leaks (plenum, prop shaft-to-cowl area, etc) to smoothing flow, to baffles and ducts...on and on. You and Tom (and others have been good about pointing that out...no quick fixes...its a system of systems approach. By the way Dan, how's your little baffle-like structure working to seal the air at the prop shaft...been weighing that versus Tom's fiberglass work at that area...both look innovative and effective.

I've listened a lot to Tom, and he has great advice. Have talked with Dave on his tail design (he and others have encouraged me to build an 8 tail for my S6, to get a counterbalanced rudder on board, due to the speed its running now). I hope to visit and look more at his modified tail, and now want to add a tour of his cooling mods as well...if he'll let me peek! :)

There are a lot of great posts and great info here, and I'm eatin' this up (along with Paeser's book and others y'all have recommended). You guys are an awesome braintrust, so I'm takin' notes, crawling under the hood and thinkin'. Little by little I'm learning, so thanks much!

And do my eyes deceive me, or are Howard and and Paul concurring!! :D (All in fun, the debates here are very enlightening. Good stuff!)

Finally, Dan...that removable exit area you've designed is an awesome idea...I did notice it a long time ago! Still trying to figure how to retrofit something like that. Just takes time...and a little dab of courage to cut...perhaps when flying slows in the winter...having too much fun right now, and it is a balance...maintenance man-hours to RV grin hours...now that's a ratio we all want to keep high...or is it low...oh heck, I dunno, just keep the glass half full or better, right! :D

Cheers,
Bob
 
Alan Carroll said:
Bob,

Is it possible that potential gains were negated by increased turbulence behind your cowl plugs? One of the common factors in airplanes with small inlets seems to be the use of a prop extension, which provides more room to smoothly transition the inflowing air. Its more challenging to do this with the stock system because the cylinders are very close to the inlets, and even more so if round inlets positioned farther outboard are used. Perhaps this has something to do with why many fast airplanes such as the Nemesis NXT don't use round inlets?
Click to expand...


Allan:

Yes, this is in fact the case. The maximum divergence should be 7 degrees or so, and this is where a prop extension helps get things better situated for optimum plenum design. I was not willing to give up aerobatic capabilities by installing a prop extension just to get a better plenum design. The round inlets are simply the easiest to make and connect via a flexible duct to the cooling plenum. It is my understanding that at the time early measurements were taken I have been told (unconfirmed) that the software of the day could not compute shapes other than round inlets. That said it is likely other inlet shapes would be more optimum for the smooth transition.

Notwithtsanding Paul Lipps excellent observation that inlets that are reduced in size cool well because they may have been too large in the first place, I think the idea is that when compressing the air below and behind the engine when nearing the cowling outlet it leaves the cowl at an accelerated speed closer to that of the slipstream thereby reducing drag. In my experience this is why the outlet size should be less than (say 70-90%)of inlet size area. This is probably why some folks don't see any speed increase when installing a plenum; not enough attention was paid to the outlet area size and configuration. This is all a slippery slope on the RV-8, because of exhaust pulses pinging on the exit ramp and cabin floor, not to mention the dirty belly syndrome.

I would also like to say that Dave Ander's theories and practices work very well. Exit air should be "guided" into a zone of convergence, and all of the stuff on the firewall above the rounded exit lip on the RV-8 is a drag producer. It's also more difficult to get smooth airflow out of a reduced size exit with four exhaust pipes with clamps & hangers. This is where the 4 into 1 exhaust collector can be superior in terms of reduced outlet drag.

Anyway, my .02c, YMMV.

How's your own cooling plenum coming along?
 
hevansrv7a said:
....choking the exit changes the relative sizes and would produce the back pressure (slowed intake) to which Paul referred.
Click to expand...

Perhaps more accurate to say "pressure" and leave off the "back", which sounds like something bad. Developing upper plenum pressure is highly desirable....the whole point of the exercise.

So now we have an interesting and surprising potential change in prop performance as a direct result of cooling changes. Who'da thought it?
Click to expand...

I didn't, but again, you anticipate a measurable change in performance due to slowing the inflow in 1.34% of the prop disk?

...and just to clarify, the .63 is your exit, and you are going smaller, right?
Click to expand...

Bob,
0.63 is the exit side of the ratio, and yes, smaller is planned.

Just hit 14 hours, only enough for superficial observations. Quantifying propshaft seal performance will have to wait for in-cowl pressure and temperature sensing later. It isn't showing any physical distress, for example heating due to friction.

Jon, agree 100%. FWIW, I have a stainless steel ramp with a filler block sandwiched under it, silicone rubber blobs (boobs?) injected between the floor and belly skin (aft of the tapered formers, just in front of the spar), and a 4 into 1 with half the downturn angle cut off. No floor vibes.
 
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The reason for sealing around the crankshaft or extension is to prevent flow from coming forward and exiting between the spinner and cowling. This flow exits normal to the flow coming back from the spinner and results in drag and turbulence. One way of getting rid of this without the rotary seal is to create a Coanda surface around the cowling right behind the spinner which takes the air flowing out and turns it 90 degrees to the rear. This is easily done using Bondo or some such to enlarge the cowling radius by 1/4" which takes the form of a 1/4-round. The forward portion is formed so that it is in-line with the cowling surface behind the spinner, turns through a 1/4" radius to the rear, and then flows smoothly into the cowling surface behind it.
 
avpro56 said:
Allan:

Yes, this is in fact the case. The maximum divergence should be 7 degrees or so, and this is where a prop extension helps get things better situated for optimum plenum design. I was not willing to give up aerobatic capabilities by installing a prop extension just to get a better plenum design. The round inlets are simply the easiest to make and connect via a flexible duct to the cooling plenum. It is my understanding that at the time early measurements were taken I have been told (unconfirmed) that the software of the day could not compute shapes other than round inlets. That said it is likely other inlet shapes would be more optimum for the smooth transition.

Notwithtsanding Paul Lipps excellent observation that inlets that are reduced in size cool well because they may have been too large in the first place, I think the idea is that when compressing the air below and behind the engine when nearing the cowling outlet it leaves the cowl at an accelerated speed closer to that of the slipstream thereby reducing drag. In my experience this is why the outlet size should be less than (say 70-90%)of inlet size area. This is probably why some folks don't see any speed increase when installing a plenum; not enough attention was paid to the outlet area size and configuration. This is all a slippery slope on the RV-8, because of exhaust pulses pinging on the exit ramp and cabin floor, not to mention the dirty belly syndrome.

I would also like to say that Dave Ander's theories and practices work very well. Exit air should be "guided" into a zone of convergence, and all of the stuff on the firewall above the rounded exit lip on the RV-8 is a drag producer. It's also more difficult to get smooth airflow out of a reduced size exit with four exhaust pipes with clamps & hangers. This is where the 4 into 1 exhaust collector can be superior in terms of reduced outlet drag.

Anyway, my .02c, YMMV.

How's your own cooling plenum coming along?
Click to expand...

Jon - thanks for the insights. You must be doing something right because I seem to be losing ground to you in the AVC!

I've got the materials in hand to work on the plenum, but haven't done it yet (too much good fall flying weather). I hope to make this a winter project; meanwhile collecting ideas.
 
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Pics?

elippse said:
The reason for sealing around the crankshaft or extension is to prevent flow from coming forward and exiting between the spinner and cowling. This flow exits normal to the flow coming back from the spinner and results in drag and turbulence. One way of getting rid of this without the rotary seal is to create a Coanda surface around the cowling right behind the spinner which takes the air flowing out and turns it 90 degrees to the rear. This is easily done using Bondo or some such to enlarge the cowling radius by 1/4" which takes the form of a 1/4-round. The forward portion is formed so that it is in-line with the cowling surface behind the spinner, turns through a 1/4" radius to the rear, and then flows smoothly into the cowling surface behind it.
Click to expand...
Paul, do you have (or have you already posted) any pictures of the Coanda surface? Thanks in advance.
 
I am in the middle of making my sealed cooling system with augmented outlet. Trying to decide the best things to try first.

On the one hand you want only enough air on board for optimal cooling. On the other you must restrict the flow as it moves to exit or you will not accelerate it. Like a chimney or air duct the area decreases with length to lower the pressure and accelerate the air, which is what we want to do. So the outlet area controls the mass flow and should be sized for the optimal amount of air.

But that leads to what is the best size for the inlet to give the highest pressure above the cylinders with the least drag? And, air flow thru the exit is determined by exit velocity, what is the highest velocity we can reasonably expect to achieve?
 
cowling air flow

This is a fascinating thread.

In terms of air flow, does it not seem that there would be a lot of drag as the air goes around each side of a cylinder base and then the front side air pointed at the back side air before continuing on?

has anyone experimented with a splitter on the bottom of the cylinder where the 2 streams meet? I have seen the cylinder wraps and the gap sealing which look great, but man the exit air from the inter cylinder baffles must be turbulent as heck.


I know all you go fast guys are running tail draggers and probably no cabin heaters, but what happens to the air through the heat exchanger when cabin heat is not required? You still need to flow air through the heat exchanger to avoid heat damage to it and the hoses when cabin heat is not required. It seems like just dumping this into the low pressure side just reduces the pressure differential.

I have not yet started working on the cowl on my 7A, but am considering routing the heater bypass air through a 2" hose to the cowl exit to augment the flow or at least point it in the right direction when heat is not required (bypass air). Then when heat is required, the 2" tube would take up space in the cowl exit and act like a cowl flap.

Also how about the oil cooler exhaust air? Would it be best to exhaust that from a separate louver?

I have not seen any pictures of a transition curve for trike cowl exhausts. It looks like a night mare to smooth out the air around all those tubes before the cowl air exit. Any pictures?
 
Cylinder Baffle Exits

NDrv8r said:
has anyone experimented with a splitter on the bottom of the cylinder where the 2 streams meet? I have seen the cylinder wraps and the gap sealing which look great, but man the exit air from the inter cylinder baffles must be turbulent as heck.
Click to expand...

The following two papers might be of interest. Unfortuanately the best theoretical solution is not very practical given other physical constraints.

http://www.n91cz.com/Interesting_Technical_Reports/naca-tn-620.pdf
http://www.n91cz.com/Interesting_Technical_Reports/NACA_Cylinder_Baffle_3H16.pdf
 
Dan, thanks for the reply...copy all.

Paul, I'll echo Howard's request for pics of the coanda lip at the cowl opening for the prop shaft. I have seen pics of Tom Martin's cowl closure there, and Dan's seal closure there, and would love to compare those to your coanda lip. If you send me a picture, I could post a side-by-side comparison of pics...if the others would not mind. Seems a physical block to escaping air would be the most effective, but if it can be controlled aerodynamically, it could save weight, complexity, and the potential for wear at the shaft interface (which may not be a factor from Dan's initial observations...good news!)

Paul, I also enjoyed the pics of your cylinder cooling outlet ducts that you sent...still studying them...they are facinating, and would love to see them in person. You oughta post those in this thread (I can do so if posting forum pics is a pain for you...just need your green light).

Larry, I know John Huft and others (including Dan I believe) have routed oil cooler oulet air to the exit opening. Gary Wilcox has recently done a project on that as well. Something on my to do list, and I'd also love to see more pics of those installs, and assessments on its effect on performance.

On the heater bypass...I recently re-routed my ducting for fresh air and heater air (both came off the back of the plenum, which I thought was a bad idea for fresh air). I now pull them both from the right inlet (no effect on cooling so far), then split the flow and send it to opposite sides of a mixer duct on the hot side of the firewall (mixer was installed by the builder). When I did that mod, I ducted the mixer bypass air to the cowl exit area with a 1.5" scat tube, angled towards the inlet at the Paeser 10 degree down angle (or close). Cooling remains good, and speed is creeping up, but that mod was concurrent with other work, so I don't have a measurable speed delta to credit to it...I think of it as one of those small incremental changes, which very well could be zero. Should have tested it separately, but in this case, the time was not there. I have some more work to do on that ducting, as I'm not satisfied with fresh air performance down low, but I'd be interested to see what others do with that heater bypass air as well.

Cleaning up the path for the air to the exit is sounding like a priority, along with creating a directed path for it to follow and accelerate within before exiting...as seen in Dave's metal baffles down there. How to do that with a clobbered firewall...that's my engineering mystery! Especially since I'm adding smoke now! :)

Great thread...great info...thanks much for the discourse!

Cheers,
Bob
 
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