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12-03-2007, 03:03 PM
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Join Date: Jan 2005
Posts: 804
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Quote:
Originally Posted by Bob Axsom
(snip). However, I have laid under the plane visualized every kind of hinge point motion direction and control mechanism I could imagine for an "A" model like mine and I have not come up with anything (snip)
I think the outlet will have to be relocated to implement a beneficial cowl flap (or flaps).
Bob Axsom
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Bob--this isn't a priority for me since my temps are good )I've actually got some leeway to start closing down inlets which will probably be a "next winter" project) but I've spent some time thinking about it.
If you size the normal center outlet for the cruise condition, why not put cowl flaps outboard of the center tunnel? One or two small flaps just in front of the firewall could give extra outlet area for climb, and close completely for cruise. This especially makes sense to me since my oil cooler is on the right side of my firewall and air leaving the cooler already has to make a turn to get out of the middle outlet.
__________________
James Freeman
RV-8 flying
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12-03-2007, 04:16 PM
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Join Date: Mar 2005
Posts: 5,685
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We are drifting here
The question of this thread is very specific so I ask that the thread focus on the non-standard tubes connecting the air inlets to the plenum and their measured effect on airplane speed exclusive of all of the other components in the engine cooling air system. No technofluff just good hard facts if there are any - which I am beginning to doubt.
Bob Axsom
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12-03-2007, 05:52 PM
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Join Date: Jan 2005
Location: Ontario, Canada
Posts: 1,544
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I currently have a cowl flap installed on my EVO1. The results have been mixed and for the most part dissapointing. The flap is about 6" wide and closes the area between the two exhaust pipes. The neutral position is the old cowl line. When I open the flap, lower from neutral, the temperatures go down as well as the airspeed, which you would expect. However when I pull the flap closed from the neutral position the temperatures go up but the speed actually goes DOWN. Not much but definately not as fast. The one good thing about this mod is that you do not have to do multiple tests you can see the results directly on the airspeed idicator as you change the position of the flap.
The interesting thing about the position of the cowl flap is that it affects the trim of the airplane. With the plane trimmed for level flight, if I close the flap from the neutral position, the nose goes down. Returning the flap to the neutral position will raise the nose to trimmed flight again. For me this means that somehow I am creating more drag, by turbulence perhaps, than I am saving from reduced airflow. The turbulence can be in two places, at the actual cowl flap area or backed up air spilling out the inlets.
For winter use the flap will be benificial and as a side benefit when you are descending pulling the flap closed helps to reduce shock cooling.
I would not install a cowl flap again. On the F1, in my opinion, if you have the inlets operating correctly, and the exit area cleaned up, the stock cowling is pretty effiecent in the stock form.
Years ago I had tried a cowl flap on my RV4 with very similiar results. I thought I was smarter now but apparently not!
__________________
Tom Martin RV1 pilot 4.6hours!
CPL & IFR rated
EVO F1 Rocket 1000 hours,
2010 SARL Rocket 100 race, average speed of 238.6 knots/274.6mph
RV4, RV7, RV10, two HRIIs and five F1 Rockets
RV14 Tail dragger
Fairlea Field
St.Thomas, Ontario Canada, CYQS
fairleafield@gmail.com
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12-04-2007, 12:19 AM
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Join Date: Sep 2006
Location: Beaver, OK
Posts: 447
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I agree
Tom, your post is correct as I found out the same thing as I tried a cowl flap which was a little different than yours on my RV-6 several years ago and it did make temp changes, but when the cowl flap was opened or closed it caused my RV to pitch up or down and I had to make small adjustments with the elevator trim, yep it was very pronoused and the speed change was nill to nothing and made no changes in airpspeed that I could see, so I went back to other thoughts and ended up with what I've got now. Thinking here, but I might be wrong, the air might have been building up inside the plenum when the cowl flap was closed and caused a reversion at the inlets like your talking about and made the airplane speed change nill to none or worse. I did all this along time ago and did not have photos or tuft work till later years after trying a few other things so I'm kinda out in the wind with what was going on a few years ago with testing things on the cowl flap idea, but later on a few years passed and I did do some tuft work with the inlets and did see reversion at the inlets and that's when I found the sweet spot and came up with the diffusers / size that I have at present that work.
I don't know what it is with these VANs RV's or Rockets, but the cowl flap idea I feel is maybe more for the spam cams at lower airspeeds. If I had the time I'd like to design a means to control the air inlet area between winter and summer and not at the cowl outlet and leave that area fixed.
Anyone have any thoughts on this???
__________________
Alan (AJ) Judy
Beaver, OK in NO MANS LAND
RV-6 IO360A1B6 C/S Hartz 200HP ?
Also Fly North American NAVIONs
Race car engine builder/Machinist/Fabricator 1982--present.
Last edited by rv969wf : 12-04-2007 at 12:56 AM.
Reason: added txt...
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12-04-2007, 12:28 AM
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Join Date: Jan 2005
Posts: 4,285
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Bob is fighting "The Force"
Quote:
Originally Posted by Bob Axsom
The question of this thread is very specific so I ask that the thread focus on the non-standard tubes connecting the air inlets to the plenum and their measured effect on airplane speed exclusive of all of the other components in the engine cooling air system. No technofluff just good hard facts if there are any - which I am beginning to doubt. Bob Axsom
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Bob just do it, your throwing away 5-8mph. The article said 5 kts, which is about right. He even got 20F-30F lower CHT's. What more do you want?  You say you don't want technofluff but ignore the results. I know you're fighting the idea. What else do you need to know? It works. If you want evidence, take the article to heart and the data from Dave Anders I posted. This is the most popular mod to RV's for a reason. Nothing wrong with the original RV cowl baffle design, but it can be improved on. You can use your existing cowl and existing baffles. You're handy with fiberglass. Glass in some inlet rings, make a top for the baffle (metal or fiberglass), make a transition from the soft duct to plenum (like the article shows) and with two soft ducts/tubes & clamps you're in business. Simple right. You can always write a big check to James Aircraft, but many have modified the stock cowl and baffles with success. There is a right way and a wrong way to roll your own.
"focus on the non-standard tubes connecting the air inlets to the plenum and their measured effect on airplane speed exclusive of all of the other components in the engine cooling air system"
You know I am going to be honest with you Bob. It makes no sense to look at it that way, at least to me. Its the total package from the lip of the cowl inlet > ring throat > soft duct > into the plenum, all of which is considered the diffuser. You can't pick one component and say what affect does that have. The soft duct is a necessary evil to connect the inlet rings and plenum. Remember high velocity air need careful attention. The tube is part of the effort to flow high velocity air smoothly. It's not perfect because there are are some transition lips, but it's leak free. There is no way you can say that about a stock Van cowl to baffle interface.
"hard facts if there are any - which I am beginning to doubt."
I'm speechless, yes its true. I don't have anything to say.
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Technofluff nerd stuff warning - Do no read below unless you are using "The Force"
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The article mentioned stagnation (air momentum = zero) but skipped the external drag that happens. Big inlets means stagnation outside the cowl. When the air stops externally before going into the inlet, that air than spills out and mixes back in with the fast moving external air going around the airplane. That's not efficient and it's drag; stopping and re-accelerating air is drag.
You're cutting inlet area down by at least 45% and still getting the same cooling! IT'S LESS DRAG. However to get that you have to be very efficient (careful) with the (high velocity) air going in the cooling system (diffuser). You need smooth non-turbulent air flow down stream. The "non-standard tube" is part of that effort. The only way to do that is with careful ducting (diffusing if you will). Bigger low velocity inlets or the stock Van's inlets are fine and less critical, but they have more drag (like 30-50% more). Last and very simple to understand, absolutly and positively NO LEAKS in the high pressure part of the cooling system is a must for efficiency. You can't get better than a round tube clamped to round flanges. As well elimination of the soft baffle seals is goodness. Bob, don't fight the force, use the force, the stock soft seal design leaks at 200 mph in ways you can't see. Also a side benefit of not using the cowl to seal the top of the cooling plenum is reduced stress and maintenance on the cowl. Bottom line, How good? About 5-8 mph. I'm done.
__________________
George
Raleigh, NC Area
RV-4, RV-7, ATP, CFII, MEI, 737/757/767
2020 Dues Paid
Last edited by gmcjetpilot : 12-04-2007 at 01:01 AM.
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12-04-2007, 02:13 AM
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Join Date: Mar 2005
Posts: 5,685
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I do not agree or disagree
Deep Breath - I have extensively modified and fine tuned the cooling system in my RV-6A and for my efforts I gained 4kts over the baseline speed of 170.67kts. Then I made new wing tips and gained 3 more knots over the demonstrated best case cooling system speed on my airplane. My current demonstrated 6,000 ft density altitude speed with everything trimmed and set for maximum speed is 177.8 kts (3-way with GPS tracks and speeds input to the NTPS spread sheet). At these speeds, on this airplane, in this configuration, I am confident that I have reached a point where further increases in true airspeed based on cooling system changes will be exceedingly small and will require very precise and correct changes. I cannot afford to expend my resources on this kind of pursuit. The idea that I can arbitrarily put a separate lid on my plenum and gain TAS with no other changes is not believable. The possibility of gaining speed by reducing plenum leakage alone is not believable. The possibility of reducing the air mass needed to cool the engine by reducing plenum leakage is believable. Expanding on that idea, tuning the inlet to take in the reduced amount of air mass to achieve the cooling of the engine has the potential for increasing the TAS if it is processed properly and external aerodynamic changes made possible by the smaller inlet requirement are incorporated. I have achieved cooling efficiency already that does not require the amount of air mass which is entering the plenum but incremental efforts to reduce the air mass flow in the system by reshaping, relocating and reducing the cross section of inlets has only been effective in reducing the air mass flow and increasing the CHTs incrementally up to the 400 degree limit in direct relation to the inlet cross sectional area without an increase in TAS. To my satisfaction the system is not performance bound by plenum leaks in its current configuration. I think there is potential for improvement at the front end by more effective closure of the front of the plenum and sealed defuser tube inputs but I'm not sure - which is why I asked the initial question. I observe that out of over 1,000 observations of this thread, no one has come forward with a test of the isolated TAS effect of sealed defuser tube input to the plenum. If you look at the round inlets to the New 230+ kt Mooneys and Columbias lack of attention to this possibility is hard to ignore. I may try the defuser tubes anyway but I will be thrilled if the TAS goes up to 178kts (0.2kt increase). I hope you can appreciate that after a couple of years of work and significant speed improvement on my airplane I am not asking for generic cooling system design philosophy but specific TAS results isolated to one system element - inlet defuser tubes. I have received some good input here from all of you and I see it as a medium risk low gain potential experiment that I may or may not conduct.
Bob Axsom
Last edited by Bob Axsom : 12-04-2007 at 06:46 PM.
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12-04-2007, 03:08 AM
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Join Date: Oct 2006
Location: Melbourne, Australia
Posts: 427
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I sat down and read NASA CR3405 on the weekend since I was bored and my empenage kit is still several thousand miles from where I was/am.
If you haven't already, I recommend you read this report in detail from start to finish, because many of the points raised here are covered and there is at least one conclusion opposite to what George has postulated.
If I may summarise my understanding of it is this:
- cooling flow leakage past (not through) the engine is the major player
- leakage at the front of the engine is a large part of it.
- throttling the cooling flow downstream is the way to go
- circular cooling air entries are generally the way to go
Interestingly, large, low velocity ratio inlets were more effective at dynamic pressure recovery than the smaller, high velocity ratio designs as we see on the James cowl etc. I think this has more to do with the ability to diffuse high velocity air over a relatively short distance.
Another interesting point was that the stagnation point of the cowl was not axial with the propellor, but below by some distance. The airflow past the cooling inlets has a significant vertical velocity component which is not reflected in many of the cowling designs I've seen. It is on Mr LoPresti's PA-28 cowl though - I've no idea how effective it would be, but it could help with packaging the diffuser along the top of the cowling.
They did mention that the smaller circular inlets were more prone to stalling on the bottom edge which could dramatically reduce the pressure recovery that was possible. Maybe this is what Mr LoPresti is attempting to avoid?
I also wonder why the edges of these sleek circular entries are always sharp edged. I understood that, kind of like a re-entrant nozzle, the edge should be as rounded as possible - kind of like the large entry nozzles that are put on turbofans when they're tested on ground-based test-rigs.
Cheers,
A
Last edited by Andy_RR : 04-20-2008 at 11:48 PM.
Reason: updated URL for CR3405
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12-04-2007, 05:17 AM
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Join Date: Feb 2006
Location: Pagosa Springs, CO
Posts: 130
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Ah, the early morning musings of the sleep deprived...
What we are talking about here are small differences. Van already made all the big gains in efficiency with his excellent design. All we can do is make little bitty changes and get little bitty gains.
Bob, you are asking for irrefutable prove that you will make substantial gains before you do any work. I'm sorry, it doesn't work that way. That is why we are called experimenters.
I can't give everyone before and after numbers on my cooling plenum. I built it that way from the start, because I believed in the science. Bob used the word "believe" several times in his last post. To me, math and science is fundamental, and if it makes sense to me, it is kinda like religion. That is why I ended up being an engineer I guess.
One of my planned winter projects is to refine the airflow from the inlets into the plenum. It will be a lot of work including repainting the front of my cowl. It might not gain me anything. That is the game.
I have played this game many times in the 5 years I have been flying my airplane. During those years I have wasted time and money on things that didn't work, but I have gained speed as well.
I think this is a lot of fun, but then I am weird.
John
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12-04-2007, 05:37 AM
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Join Date: Jan 2005
Location: Ontario, Canada
Posts: 1,544
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Bob
I aggree with your thoughts on the standard inlets. I feel I have gone as far as I can with my rocket cowling to get the cooling that I reguire for my type of flying. It is my opinion that the main advantage of round inlets is not so much that they are round but that they are farther forward. This gives a much greater distance between the opening and the front of the cylinder in which to create a better diffuser. A nicely made rectangular opening would be just as easy to seal as a round opening. Mathematically I believe that a round opening gives the greatest area in relation to perimter so in that regard it is more optimal. However most round inlet installations have what I consider to be a draggy area between the inlet and the spinner. I look for direct bug splats to define draggy areas. I wonder why the round inlets could not be moved closer to the spinner. This would reduce this draggy area and probably reduce the total wetted area of the front of the cowl.
However, if you are looking for the most bang for your time spent, I do not think you will see huge increases from this work as you have already shown some significant improvements from internal sealing and baffling. Claims of huge speed increases from round inlets are only valid if the internal workings of the cowl are also done and you are already there.
I think there are more gains to be made, for your time, if you continue with your wheel pant, gear faring, experiments.
__________________
Tom Martin RV1 pilot 4.6hours!
CPL & IFR rated
EVO F1 Rocket 1000 hours,
2010 SARL Rocket 100 race, average speed of 238.6 knots/274.6mph
RV4, RV7, RV10, two HRIIs and five F1 Rockets
RV14 Tail dragger
Fairlea Field
St.Thomas, Ontario Canada, CYQS
fairleafield@gmail.com
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12-04-2007, 05:42 AM
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Join Date: Jan 2005
Location: Louisville, Ga
Posts: 7,840
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Camera aperture
Quote:
Originally Posted by rv969wf
..... If I had the time I'd like to design a means to control the air inlet area between winter and summer and not at the cowl outlet and leave that area fixed.
Anyone have any thoughts on this???
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Alan,
A 35 MM camera aperture adjustment comes to mind. If you could cut your inlet inserts in half from left to right or top to bottom, then hinge them on one side and with a cockpit controlled vernier cable to the opposite side, adjust them smaller or larger. Doing this in flight would allow you to see airspeed and oil/cht changes, no?
The Bearcats and Corsairs had these "fan" type cowl exit air flaps that overlapped each other but also, the firewall had a generous radius to ensure laminar air flow out.
Regards,
__________________
Pierre Smith
RV-10, 510 TT
RV6A (Sojourner) 180 HP, Catto 3 Bl (502Hrs), gone...and already missed
Air Tractor AT 502B PT 6-15 Sold
Air Tractor 402 PT-6-20 Sold
EAA Flight Advisor/CFI/Tech Counselor
Louisville, Ga
It's never skill or craftsmanship that completes airplanes, it's the will to do so,
Patrick Kenny, EAA 275132
Dues gladly paid!
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