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Inlet/Outlet Ratios

hydroguy2

Well Known Member
I'm working on my cowl and baffles. I'm using a James cowl and plenum set-up. I have calculated the inlet area of 33.6sq.in. and the outlet area of ~37sq.in. So the ratio comes out to about 1 to 1.1.

RV-7build443.jpg


The 4 pipe system sticks aft of the firewall about 6". I didn't cut the lip off the cowl outlet and it extends aft about 1".
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I have read the threads discussing outlet velocity and seen the pic of tufting the area aft of the outlet.

So my question is: Is it worth the trouble to reshape the outlet to reduce the ratio a bit or do something to the corners to help streamline the exit area flow?
 
From my research, unfortunately, there is no magic inlet/outlet ratio as there too many variables in individual building design. For some 75% work for others 180+ work.

I think reworking the square outlet is well worth the effort in that is has been shown to be a flow disrupter.

Just remember that we want to speed up the exhaust air to as high as posssible, resulting in lower pressure and better transition to free stream air. Also you want to make that transition as smooth as possible, and the stock square is not the best for that.
 
Reworking the square outlet is what I am thinking about.

1. extending cowl AND at the same time tapering it towards the center.
2. adding a bump aft and above the pipes, something like L Vetterman added to his -6a.
3. cutting the pipes to get them out of the airstream

outletmods.jpg


Looking for opinions. what do you racers out there think?

I may just try to get this thing flying and then make the mods in order to evaluate them.
 
It is an area I intend to work on but nothing yet

Larry Vetterman's experiment with a boat tail fairing and outboard exhaust and cooling air outlet openings were successful. I have a different idea but at this time it is only a mental concept and it may not increase the speed at all. Tom Martin has done a lot of work in this area on his EVO Rocket and he is the fastest one out there so it is an area that is definitely worth working on.

Bob Axsom
 
Cowling Inlet/Outlet Ratio(s)

If you are really serious about this (and it appears that you are), I would suggest you study what Dave Anders did on his RV-4. I don't know if his work is readily available except by talking to him. I remember talking to him at Oshkosh several years ago and he told me the 1-to-1 ratio he read about in the literature didn't work for him.

He ended up with something like 1.33.(P.S. I just looked it up...there's an article on the Oshkosh365 web site about the TriAviathon contest...he had an inlet area of 34 sq in and an outlet area of 24.7 sq in for a an apx. 4-to-3 inlet to outlet ratio) I didn't talk to him about cooling issues and now wish I had. I do remember that he was using a semi-round shaped outlet with a 4-into-1 exhaust resulting in an exhaust "augmentor."[Not to be confused with "augmentors" used on certified twins in the 1950's; that's a different "augmentor."] If I understand that right, the engine exhaust actually helps to scavenge the cowling exhaust so that could have a considerable influence on the chosen inlet/outlet ratio. I'm really on the edge of what I remember so don't take this as fact until you talk to someone like Dave who did a lot of work in this area. I don't think Dave was the first to use an "augmentor," but he may have been the first to use the concept on an RV. No one can argue that he didn't have one of the fastest RV's in existence at that time. (Notice...he later modified his exhaust system with mufflers to compete in a noise reduction contest; the speed mods discussed here were in the 1994-1997 time frame.)

With your four exhaust pipes, your cowling inlet/exhaust ratio goal of 1-to-1 might be a better choice. Whatever you chose, I would suggest you be prepared to make several tweaks in order to optimize your design.

I remember being very impressed with Dave's study of the then existing literature relating to drag reduction of general aviation airplanes. I believe he still lives in California, but I haven't heard too much about him lately. Maybe someone else with knowledge of the details of his much modified RV-4 can contribute information about Dave Anders' cowling design.

Keep us posted on what you do. I've read that 15-20% of the total drag on a typical general aviation airplane is due to air flow through the cowling. I'd like to think that Van's stock design is much better than that. I can't recall that anyone has ever tried to quantify those losses on an RV.

Interesting stuff.
 
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Lower Cowl Baffling

I don't know what results anyone else had but after a significant amount of direct experimentation I increased the speed of my RV-6A 4 kts with baffling in the lower cowl. The baseline speed before the current configuration was achieved was 170.67 kts. Most of the modifications I did resulted in decreases in speed until I hit upon the current configuration. One could try a lot and achieve nothing if the program isn't organized and the testing isn't methodical and consistent.

John Huft is a very successful racer flying an RV-8 that he built for speed and he has a website revealing all.

Bob Axsom
 
Brian,

IIRC, Sam James told me that the ratio should be 1:1. This way air is leaving the cowl just as fast as it entered it. The air enter at 170+mph, slow down so 15-17 mph as it moves through the baffle needs accelerates back up for the exit.
Sam is always willing to talk shop, pick up the phone and ask him. If you can't get a hold of him, I see him once a month. I'll print out your pictures and ask.
If you're looking for speed, I'd cut those pipes down since they're adding drag and long as there is not some unintended consequences.
 
Dave Anders

Airmans records lists Dave in Cottonwood CA. As far as I can determine Daves RV4 still holds the triaviathon record.
The true augmentor exhaust was probably designed by John Thorp. It is totally different than the four into one exhaust. The augmentor is a very large collector tube that is open on both ends. The individual pipes feed into the large tube and end well short of the aft end. The augumentors were used on most of the light twins in the fifties but fell out of favor because they are VERY noisy.
Google "Dave Anders RV4" and you will find a wealth of info on Dave and his airplane.
Another builder who has done a lot of research on cowling /cooling is the Lancair builder in the Bay area. He has written some articles in Sport Aviation.
 
I'll probably just keep these mods on the shelf and get this thing flying. Once I have a baseline, I can start fiddling again. It was wishful thinking to have someone say, "yes do this or that and things will be better"

gotta start somewhere.
 
Another builder who has done a lot of research on cowling /cooling is the Lancair builder in the Bay area. He has written some articles in Sport Aviation.

you're probably thinking of paul lipps, i believe he posts here under "elippse" or something similar
 
Another builder who has done a lot of research on cowling /cooling is the Lancair builder in the Bay area. He has written some articles in Sport Aviation.

That may have been Fred Moreno.....I copied this post on the subject a couple years ago. It is a long read but has some good stuff in it.

Recently (April, 2007) there was an excellent thread on the Lancair email list about cooling drag. I have obtained permission from the author, Fred Moreno, to post it here. Fred is a Californian Mechanical Engineer who is living in Australia, and flying a Lancair Legacy. You will see that his calculation revolve around a 230 Kt cruise speed, but the numbers are scalable to RV speeds using the square of the indicated airspeeds.

?Has anyone done any work with cooling drag??

Walter and the GAMI boys have done nice experiments and learned a lot, particularly about reversed flow in the cowl, leakage out from inlets and other neat stuff. I have spent a lot of time over the last few years trying to learn more since my original training included a lot of heat transfer, internal fluid flow and such, and I think cooling drag is about the last place to reduce drag on a Lancair IV in cruise. (Reno racing with high G turns creates different conditions and requires more mods, but I digress.)

I have corresponded off line with several of our members on this topic, and a lot of good work is being done, much of it covertly to gain some benefits in racing.

Cooling drag is at times a counter intuitive business, and like much in aviation, there is a lot of mis-information because it can be a bit complex.

Observation: if you want to wring the most out of the airplane, it will take a lot of work. I am into cowl rebuilds, cowl flaps, cooling plenums, discharge nozzles and such for more than 300 hours and I am not done. I have had to make a lot of tooling and scratch build a lot of composite parts, all time consuming. So this is a business for fanatics only. Benefits? On a Lancair IV, my current guess is plus 10-12 knots.

Perspective: One an aspirated C210 or Bonanza class airplane at 170 knot cruise, cooling drag is reported at about 7%. These planes have a flat plate drag area of about 4.5 square feet, more or less. A Lancair IV has a flat plate drag area of about 2.1-2.2 square feet, about half. I think the Legacy comes in at about 1.7-1.8 square feet. Therefore, if you use the same method of cooling as for the spam cans, cooling drag will be approximately 14-15% of total drag.

If you are turbocharged, pressurized, and at 25,000 feet, it is probably more like 20-25%, maybe more because of cool air and added flow from intercoolers. And it is harder to minimize.

Rule of thumb for small changes: a 2% reduction in drag will give about a 1% increase in speed. So if you can get that 14% of drag down to, say, 4% (probably not possible, but reach for the stars!) then the 10% drag reduction gives a 5% speed increase. For the Legacy and aspirated LIV guys (like me) who are in the 200-240 knot range at maximum cruise, 8000 feet, you could get 10-12 knots. So that brackets expectations.

Some other rules of thumb.

1) Flow only the air the engine needs to stay cool. That means no leakage (and there are leaks everywhere, big and small in a stock installations) and restricting the airflow to the necessary amount in cruise. The easy and less effective way favored by most is to choke down the inlets. The more effective but much more complicated way is to use cowl flaps to throttle the exits. Also, this means don?t overcool the engine. I will let Walter speak, here, but I think that the choke in the cylinders is sized assuming that the CHT for cruise is around 350-400F. I think 380F is a good trade off number, but I may be wrong (since it happens all the time). J

2) No leakage means a top plenum, attention to leakage detail, and in particular extreme attention to detail eliminating leakage out front to behind the spinner, usually one of the largest offender areas.

3) Hard core fanatics will do the following:

Reposition the inlets outward from the spinner to get out of ?fouled? air coming from behind the prop shanks and spinner boundary layers which are thick because of centrifugal and shear effects. Legacy does this. LIV does not. Get the inner edge of the inlet at least 1.0-1.5 inches away from the spinner.
Reposition the inlets upward about 1.5 inches above the crankshaft centreline to straighten the S bend for the flow as it enters then moves up above the engine. Columbia does this as well as moving the inlets outward.
Size the inlets so that the inlet velocity is about 0.4 of the free stream velocity to get good pressure recovery IN FRONT of the inlet where it is frictionless. This velocity ratio will get you 84% of the total ram pressure before the flow enters, and then the diffuser and flow inside is less important. This means about 6 inch diameter for aspirated 550s, and 7 inches for turbo 550s.
Streamline the first few inches behind the inlet to reduce residual losses.
Use a plenum. Make its volume large to minimize internal velocity above the engine and thus minimize losses and help make the pressure distribution more uniform.
Don?t worry about the flow below the engine UNTIL you approach the exits, then worry a lot.
Have nicely contoured exhaust nozzles to accelerate the flow aft and get a bit of thrust recover to offset the momentum loss that occurred when the flow slowed in the first place.
Watch for leaks out nose gear doors, hinges, and other places where the flow can squirt out and now backward.
If you are a fanatic and use cowl flaps to control the cooling flow, you will pressurize the cowl a LOT (necessary to accelerate the flow backward) and it will try to get round like a balloon. On a Lancair IV, this means the top of the cowl will rise a lot, enough to let conventional cooling baffles above the engine to flop back under the influence of air pressure, and let air escape over the top of the baffling. This creates flapping rubber baffles against the cowl. I am told that the resulting sound is never forgotten causing immediate secretion of large amounts of bodily fluids. Fred Moreno
 
You need to search on past posts. About a year ago this topic was running wild.

You need to remember that the air heats up and therefore you need more area on the outlet. I have done extensive work on my plane and ended up with down draft cooling. This lowered the EGT's & Cht's by 50*. My CHT's run around 280-300*

Read the past posts.....
 
Lancair

The Lancair builder is Chris Zavatson. Lancair 360 MK II. He built a complete new cowl and plenum system. It was written up in Sport Aviation a while back. He is somewhere in the San Francisco bay area.
 
I have one of Chris Zavatson's cowls and diffusers on my Lancair, and a plenum I made. The cowel has many of the things Fred Moreno talks about regarding inlet size and placement. But in my case, I am working to improve the exit area due to my odd engine. But, for those considering cowl/plenum upgrade for an RV, I'm sure you will realize measurable increase in speed.
 
Let me try this: The air flow in must equal the air flow out. As the air moves through the cowling, it picks up heat, becomes less dense.

Mass Flow rate = density * area * velocity. For the velocity to remain them same, a decrease in density means there must be an offsetting increase in area.

That said, I cannot buy that the inlet to outlet ratio should be 1:1. It would have to be something greater than 1:1

Just my two cents.
 
Best Idea!

I'll probably just keep these mods on the shelf and get this thing flying. Once I have a baseline, I can start fiddling again. It was wishful thinking to have someone say, "yes do this or that and things will be better"

gotta start somewhere.

You can keep thinking about the possibilities and implement them in a controlled manner later.

Bob Axsom
 
I have one of Chris Zavatson's cowls and diffusers on my Lancair, and a plenum I made. The cowel has many of the things Fred Moreno talks about regarding inlet size and placement. But in my case, I am working to improve the exit area due to my odd engine. But, for those considering cowl/plenum upgrade for an RV, I'm sure you will realize measurable increase in speed.

Link to Chris' Site
 
The air flow in must equal the air flow out. As the air moves through the cowling, it picks up heat, becomes less dense. Mass Flow rate = density * area * velocity. For the velocity to remain them same, a decrease in density means there must be an offsetting increase in area. That said, I cannot buy that the inlet to outlet ratio should be 1:1. It would have to be something greater than 1:1

Ahh, but why would you want exit velocity to remain the same as inlet velocity? Perhaps you wish to ensure you have enough cooling drag? ;)

POSTSCRIPT: Hardly fair to tease Alton and leave others scratching their heads.

Here's the basis for the above. Typical inlet velocity is below freestream velocity (for example, consider the inlets identified by their velocity ratio in NASA 3405), thus an exit velocity the same as inlet velocity is a drag producer. The cooling drag Holy Grail is exit velocity equal to freestream or higher.

I have seen one exception, based on data published by Chris Zavatson in a Sport Aviation article. Here's a link: http://www.rv-8a.net/106-111_BuildingBasics Cooling Drag.pdf

Examine the charts at the end, specifically at the lower right. You'll see Chris recorded an inlet velocity higher than freestream by using a small inlet carefully shaped for internal diffusion. In this case, if Chris was able to match exit velocity to inlet velocity, he would have net exit thrust (and my personal respect ratio would go from its current 100% to around 150%). You can bet he is working on it.

Before you get all excited about such inlets, take note of the inlet illustrations on the second page (SA page 107) and think about practical considerations. See the difference in length between the inlet face and the front of the cylinder? A good internal diffuser requires long length, careful shape, and minimal surface disruption. IMO, you simply don't have enough length without a prop extension; little round inlets on a short cowl are not going to work well.

Design is the art of compromise. I wanted to use a heavy Hartzell BA and IO-390 combination on an RV8, so a propshaft extension was off my list. Without one there is not enough length for good internal diffusion, so I went with large low velocity inlets. I think Chris is right in that I'll probably have a bit more external drag. The flip side is data suggesting very good pressure recovery with typical slow inlets, and I may be able to parlay the additional pressure into high exit velocity. Like Chris I've gone to a lot of extra trouble to optimize the particular scheme I've chosen.

Which brings us back to the original thread question, inlet-outlet area ratio. Both low velocity and high velocity inlets are viable, yet they would have very different inlet-outlet area ratios.
 
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WHACK!

Ahh, but why would you want exit velocity to remain the same as inlet velocity? Perhaps you wish to ensure you have enough cooling drag? ;)

Well THAT didn't take long!:D The explanation is totally counter-intuitive. We use about .8 exit ratio on the F1 and it generally overcools. One very fast ship is closer to .6 with no cooling problems.:confused:

I'm NOT making this stuff up!

The goal, while not actually attainable, is to accelerate the exit flow to match the free stream velocity. 1:1 won't get ya there, or even close, generally speaking, as the inlet ramps cannot easily be made with the correct expansion angles (available length is too short). There are a very few exceptions to this rule -- clever folks, indeed. I do wish I was one of 'em!

Carry on!
Mark
 
Well THAT didn't take long!:D The explanation is totally counter-intuitive. We use about .8 exit ratio on the F1 and it generally overcools. One very fast ship is closer to .6 with no cooling problems.:confused:

I'm NOT making this stuff up!

The goal, while not actually attainable, is to accelerate the exit flow to match the free stream velocity. 1:1 won't get ya there, or even close, generally speaking, as the inlet ramps cannot easily be made with the correct expansion angles (available length is too short). There are a very few exceptions to this rule -- clever folks, indeed. I do wish I was one of 'em!

Carry on!
Mark

(Sir, standing at attention):)

May I point out that the question of flow was settled about 75 years ago with liquid cooling - slow the air down after it enters the heated chamber so it can do it's thing, i.e., absorb heat and then speed it up to get rid of it.

The purpose of the entire process is to remove heat be it from liquid or metal. Seems like a diffuser of sorts coming and going is the answer. There is evidence that such a device installed on the fire wall above the exit area does work better than not. Another device used even on a certified airplane is the scupper.

Seems like, also, the old saying "there's more than one way to skin a cat" may apply. I like the idea of exit area, lots of it. Granted, I am not winning any races, but I am staying up with factory performance numbers and experiencing excellent cooling which is not an unpopular objective for most builders here.
 
Worthy goal!

Hey David:

Yes, since we are footing our own maint bills, keeping the infernal combustion device at proper temps is a good thing. Trying to reduce the drag from this process is the point here, and you are spot on with your (at attention!;)) report.

Scupper outlets: I had plenty of cooling issues with the 550 until I installed that type of outlets, combined with a ramp on the firewall flanges. Now I get to experiment with sizing and interior ramp shape to see if I can maintain temp control while reducing drag. Wish me luck -- I can use it!

At ease!:D
Mark
 
Hey David:

Yes, since we are footing our own maint bills, keeping the infernal combustion device at proper temps is a good thing. Trying to reduce the drag from this process is the point here, and you are spot on with your (at attention!;)) report.

Scupper outlets: I had plenty of cooling issues with the 550 until I installed that type of outlets, combined with a ramp on the firewall flanges. Now I get to experiment with sizing and interior ramp shape to see if I can maintain temp control while reducing drag. Wish me luck -- I can use it!

At ease!:D
Mark

Have at it, Boss, and good luck. Everything can be made better.
 
Yep. I guess you are enjoying the snowfall by staying in the hangar.

I've decided to skip it for now and get this thing flying, then tinker next winter
 
Dave Anders

Here are his notes from a lecture he gave us a few years ago.

http://sacrvators.com/Aircraft Efficiency N230A.pdf


Yes, he lives in Cottonwood. Last time I saw the airplane it was in the configuration where he was trying to get it to fly slower and had removed some of the speed improvements. I have lots of pictures but have not posted them. I started to but then held off.
 
Brian,

IIRC, Sam James told me that the ratio should be 1:1. This way air is leaving the cowl just as fast as it entered it. The air enter at 170+mph, slow down so 15-17 mph as it moves through the baffle needs accelerates back up for the exit.
Sam is always willing to talk shop, pick up the phone and ask him. If you can't get a hold of him, I see him once a month. I'll print out your pictures and ask.
If you're looking for speed, I'd cut those pipes down since they're adding drag and long as there is not some unintended consequences.
Update: I talked to Sam last Sunday and have good new: my memory is not failing :) His RV-4 is set up for a 1 to 1 ratio and that's what he recommends.
 
I-O Ratio

I'm going to try this reply for the fifth time!
Before you can consider the outlet area, you must calculate the inlet area required for your engine's HP. My O-235, at 213 mph TAS, 2950 rpm, sea-level, ingests 26 cu. ft. / sec.through its two 6 sq. in. inlets. My oil cooler has a separate 2 sq. in. inlet, and takes in 4.3 cu.ft. / sec., for a combined total of 30.3 cu. ft. / sec. Since the engine is rated at 125 HP at 2800 rpm, at 2950 it's generating 132 HP, which is 4.36 HP per cu. in. / sec., or 0.23 cu. in. / sec per HP. My cooling system is very efficient, so yours may require 25% to 50% more inlet area. Using these values, you can calculate your inlet area by (0.23 X HP) / (TAS X 22 / 15). For 180 HP at 200 mph, this becomes 20 sq. in. to 30 sq. in. The engine will increase the temperature of the cooling air by 100F to 140F which will increase the outlet volume by (459.7F +59F + 100F to 140F) / (459.7F + 59F) or 19% to 27% for an O-I of 1.2 to 1.3. A 1:1 would indicate that the inlet is too large and the flow is choked, allowing some flow to spill out of the inlet. In testing I did with RVs, the OAT did not drop as much as it should have at higher altitude, in one case the temperature hardly changed at all in going from 2000' to 10,000'! We guessed that the warmed flow out of the inlet was going down the side and into the cabin air NACA inlet where the OAT sensor was mounted. It's important that the TAS used for the calculation be obtained from a calibrated pitot-static system. My tests with RVs showed that those using the domed rivet-static port had IAS errors of 10 mph to 12 mph more that actual. You can experiment (this is an experimental airplane, non?) with inlet area by closing down the inlets with foam. Be sure to use generous radii at the edges. It is best to operate the engine above 350F CHT, preferably 370F to 380F so that the piston will expand into the bore to make the engine run efficiently. Lycoming piston clearance cold is 0.018" to 0.022", as compared to my old '58 Cadillac where it was 0.0005" to 0.0015". Cadillacs actually had the pistons and cylinders graded every 0.0002", two-ten thousandths, at the factory with a letter stamped by each cylinder and on each piston indicating its value!
 
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