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08-21-2007, 12:04 PM
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Banned
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Join Date: May 2006
Location: Phoenix, Az
Posts: 920
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jet thrust????
Several posts, including the last have eluded to "thrust" a proper cowling produces. From everything I have read over the years, a good cowling actually propells the aircraft forward, rather than producing drag. Think about it.......only 29% of the heat energy is converted into mechanical motion, the rest is discarded out the exhaust and cooling system.
The cowling is like a jet engine. Cool air is forced into a restricted space, large amounts of heat is added, and is allowed to escape out a restricted
exit, at high speed. I think our cowlings have been optimized by Van to take advantage of this principle, but at the expense of climb cooling. I'm starting to think a good set of cowl flaps is a good idea for those of us in the desert.
That way, the "jet thrust" could be optimized for cruise, but the cooling still be available for climb.
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08-21-2007, 12:48 PM
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Join Date: Jan 2005
Location: Ontario, Canada
Posts: 1,544
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John Huft
Thanks for your note. I had not thought of the stagnant area aft of the flap when I started this experiment and a few people have pointed that out. The difference between what I am doing and the C182 flap system is that their closed position is like my open position, in relation to the cowl shape. If I had a cut cowling and hung the flap down to get extra airflow through the cowling for climbs I am positive that I to would get a speed increase when I pulled it up even with the rest of the cowling.
To get rid of this dead air area in my current situation I would have to cut my cowl and pull the bottom of the cowling up to the floor. The trick here would be to keep the outside air attached to the flap at the hinge line. Otherwise I am going to get the same turbulent airflow. Another method would be to have a flap on the fuselage with a leading edge that would drop down to the cowling. It would be hinged a foot or two aft on the fuselage. Sealing the front of a flap system like this would be problematic and also the failure mode is in the closed position which I do not really care for.
I think that before I cut the cowling, or try other flap systems,I will extend the aft flange a bit more and focus my efforts on reducing air intake.
Even this failed experiment has taught me a lot about what is going on down there. The positive criticism and exchange of information has been most helpful.
__________________
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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08-21-2007, 02:00 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Studies in 1936 by F.W. Meredith appeared to show that jet thrust was possible but only on well designed ducts at speeds over 300 mph. In fact later experimentation showed that speeds had to be close to 400mph to realize net thrust.
The pressure losses present in typical opposed air cooled piston engined installations and the speeds our typical RVs achieve make this highly improbable.
At best like the stock P51, we can only expect to offset some cooling drag. Later studies showed that its rad design was offsetting cooling drag to the tune of 150-200 equivalent hp.
In the case of unlimited Reno racers, using spray bars (higher mass flow and possible state change), considerable jet thrust is obtained on both air and liquid cooled installations. Further theory suggests that pumping the cooling stream outlet duct with exhaust energy can produce useful thrust at much lower airspeeds. The same method is used to pump the underbody air in F1 cars. This is highly effective.
Last edited by rv6ejguy : 08-26-2007 at 10:49 AM.
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08-21-2007, 02:34 PM
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Banned
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Join Date: May 2006
Location: Phoenix, Az
Posts: 920
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Ross,
Consider this: If the exit area is nearly the same as the inlet area, and we are raising the temperature of the air 50-100 degrees instantaneously, I can't see but how there wouldn't be a net thrust. Reverse flow on in-cowl tufting experiments seem to verify the possibility of lower cowling back pressure.
Somebody on this thread mentioned 6 inches of pressure in the lower cowl area. That's got to be doing work as it exits the cowling.
Consider the Cessna 152 with a 118 hp Lycoming. 120 mph. On a good day!
RV-9, same engine 172 mph. I just don't think airframe drag can be the only
cause for this disparity. Even larger disparity between 172 and 160 hp RV-9.
One more data point. 4 cyl Egg is 15-20 mph slower than like hp Lycoming.
Cooling drag/lack of cowl thrust a likely culprit.
Just something to think about.......our experiences are at least as valid as research from 1936!!!! 
Last edited by Yukon : 08-21-2007 at 02:50 PM.
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08-21-2007, 03:22 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
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Originally Posted by Yukon
Ross,
Consider this: If the exit area is nearly the same as the inlet area, and we are raising the temperature of the air 50-100 degrees instantaneously, I can't see but how there wouldn't be a net thrust. Reverse flow on in-cowl tufting experiments seem to verify the possibility of lower cowling back pressure.
Somebody on this thread mentioned 6 inches of pressure in the lower cowl area. That's got to be doing work as it exits the cowling.
Consider the Cessna 152 with a 118 hp Lycoming. 120 mph. On a good day!
RV-9, same engine 172 mph. I just don't think airframe drag can be the only
cause for this disparity. Even larger disparity between 172 and 160 hp RV-9.
One more data point. 4 cyl Egg is 15-20 mph slower than like hp Lycoming.
Cooling drag/lack of cowl thrust a likely culprit.
Just something to think about.......our experiences are at least as valid as research from 1936!!!! 
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I think you are forgetting the pressure (energy) loss across the cooling fins which is huge in the case of an opposed engine due to viscous effects (turbulent airflow) and turning the airflow through 180 degrees.
The 4 cyl. atmo Egg engines were not outputting anywhere close to even 160hp as they were spinning 700+ rpm below power peak. The early rad setups were terrible from a drag standpoint.
Until quite recently, few people were doing anything scientific with regards to in flight measurement of pressures and more importantly velocities in, around and out of the cowling. Measurement is simple with Magnehelic gauges and pitot tubes. Real data can be obtained and guesswork eliminated. The quantity of data on cooling both air and liquid cooled engines from the '30s and '40s dwarfs anything being done today.
Interestingly the Brits came out with this data initially but it was never properly implemented in designs such as the Spitfire which had a poor radiator setup with high losses and inlet separation. The Americans were intrigued by the possibilities and looked at applying this to the P40 first. This did not lend itself easily to using a proper duct so it was applied to the clean sheet P51 design.
CFD plots on a 6A at 0 alpha show exit air pressure aft of the lower discharge point below ambient (net drag). It is true that the CFD plots do not take into account either the restriction of the engine in the cooling airstream flow or the heat added by a running engine so this area is probably not accurately modeled. If you calculate the expansion of air by adding say 200F to it and the inlet vs. exit area and look at the high turbulence in the exit duct (A models way worse), it is hard to swallow that the stock RV setup adds any thrust.
Last edited by rv6ejguy : 08-21-2007 at 07:11 PM.
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08-21-2007, 04:03 PM
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Join Date: Feb 2006
Location: Pagosa Springs, CO
Posts: 130
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cowl flaps again
Here are a couple of pictures of cowl flaps on my 185.
First, open
Next, closed.
Finally, a view from the rear
You can see that the shape is better with the flaps closed. Ole' Clyde Cessna figured it wasn't so important at climb speed.
What is really interesting is that the area is only 41 sq. in. when closed, and even then part of it is blocked by the boost pump. hmmmmm.
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08-21-2007, 04:09 PM
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Join Date: Feb 2006
Location: Pagosa Springs, CO
Posts: 130
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back to RVs
Now, I really have to thank you all for this thread. I thought I had quite a different ratio from input to output than I actually have, so now it is time to go cut up my cowl again.
My inlets are 4" in dia. for an area of 25 sq. in.
My outlet, subtracting for the exhaust pipes, is 44 sq. in.
Note that my inlets are smaller than on my 185, and the outlet is LARGER. The 185 has an IO-550 in it!!
I need to trim the outlet on the RV! There is speed to be had here (maybe).
Stay tuned, John
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08-21-2007, 04:42 PM
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Join Date: Sep 2006
Location: Beaver, OK
Posts: 447
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Go for it
Quote:
Quote:
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Originally Posted by Nuisance
Now, I really have to thank you all for this thread. I thought I had quite a different ratio from input to output than I actually have, so now it is time to go cut up my cowl again.
My inlets are 4" in dia. for an area of 25 sq. in.
My outlet, subtracting for the exhaust pipes, is 44 sq. in.
Note that my inlets are smaller than on my 185, and the outlet is LARGER. The 185 has an IO-550 in it!!
I need to trim the outlet on the RV! There is speed to be had here (maybe).
Stay tuned, John
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Go for it John! You can do it and I bet money it'll work because I'm at 25", this is including the exhaust at the cowl outlet and my inlet diffusers vary from 2.125" up to a max of 2.625" I.D. to cool the cylinder heads and barrels. With the 2.125" round inlets I'm at around 7 square inches total up to 10" with the 2.625" for summer.
You have to remember that my oil cooler is not being fed air through the plenum or inlet diffusers. I have the capability to open my oil cooler door to provide upwards of 12.5 square inches of inlet area in the summer or for a climb out, but most of the time it is shut down roughly half of this say 6" of inlet area down to a couple of inches of area in the winter.
I'm between a 1.11 ratio in hot weather, down to a 2.5 ratio in the winter to keep heat in the engine. I know for a fact that my cowl outlet can be reduced even smaller for winter use as I'm done it by adding material to the inside of the cowl outlet.
When and if I get time, Id like to play around with a devise that is adjustable inside the cockpit to cut down the airflow at the inlet diffusers. Right now it's looking like several months as I booked with other work. I'll keep everyone posted if anything comes out of it.
Good luck John and keep testing, maybe you can get another couple mphs with some trial and error.
__________________
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 : 08-21-2007 at 09:41 PM.
Reason: ooopps error with wording. dang it
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08-21-2007, 05:34 PM
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Join Date: Jan 2005
Location: Ontario, Canada
Posts: 1,544
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My inlets are 36.8" total. My current flap open exit measures 42" which is interesting as it is very close to John Huft's cessna 185 exit area of 41". On my plane this means I have a ratio of 1.14% inlet to exit air opening.
I have a I0-540 engine.
In cruise I am running 320 to 340 CHTs which is too cool. This outlet size is about 25% smaller than the stock outlet I started with.
When I close the flap my exit area goes down to 27" for a ratio of 73%. The temperature goes up slightly and as discussed earlier in this thread I am seeing no speed gains. Probably due to the turbulence aft of my flap door but that is just theory at this point.
Before I installed the cowl flap I had closed off a bit more of the cowling exit air for a total area of 35.5" This left a ratio of 96% and the temps were about 10 degrees higher than without this blockage. I did not notice any significant speed difference with this partial blockage. This blockage was done between the pipes with a curved insert that should not have caused an increase in turbulence.
I do not know what the ideal ratio is but I am sure that I could close my inlets and reshape them in the process.
It is important to note here that Alan Judy, John Huft, Bob Axom, and to a lesser degree myself, have paid a great deal of attention to inlet and exhaust area cleanliness. Without this work I know I would not be able to close my outlet as much as it is without running into cooling issues.
__________________
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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08-21-2007, 08:30 PM
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Join Date: Sep 2006
Location: Beaver, OK
Posts: 447
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For those of you experimenting....
This is just my opinion so take it for what it?s worth.
I think cowl flaps are great at controlling the volume or mass of air that passes through the cowling, but I think there is a better way to control the mass of air if one is trying to perfect the CHT?s with even temps and a means to control temps between winter and summer. I?m working on an idea for my ?6, but it will be down the road.
Trying to not make this a long story, but after trial and error testing various things, I found out that you can only push so much air through a given hole or gap at say 200mph. What I?m referring to is the cylinder head fin area, the cylinder barrel fins, the 1? baffle gap below the cylinder fins and the 2 ?? gap below the cylinder head fins. Exclude the oil cooler out of this for now. At a given air speed say 200mph if you had two 6? round inlet holes ?just an example? how much air is passing through the cylinder fins and how much air is packing up in front of the inlets and creating a plume or turbulence??????
What I found out with my cooling system was optimizing the inlet diffuser size/shape to match the airflow needs of the engine, forget about the cowl outlet for now, just leave it big then start downsizing it later until you find the optimum size or design.
I started out with 4 ?? inlets and I kept downsizing them a little at a time with no other changes done to the cooling system. What I found out was the CHT?S started running colder until I found the correct size that met my engines needs. Just as an example in cruise: with 4 ?? round inlets I was seeing 360F CHT?s, when I got down to 2 5/8? I was at 325F CHTs and much more even or closer together. Now when I went smaller than 2 5/8? the CHTs starting climbing back up. When I got down to 2.125? I.D. inlets, my CHTs went back up to the original 360F temp and not as even. I used a manometer while testing the various inlets sizes. No plenum pressure changes were seen between the 4 1/2" inlets and the 2 5/8", I then started seeing a pressure drop when I went smaller than 2 5/8". Sit back and think about what just happened by doing this.
All of this is about air management. After seeing and understanding race car cylinder heads, induction systems, exhausts on a flow bench over the years got me to thinking about how screwed up the cooling system is on most airplanes.
My feeling is the cooling system on an airplane is just like a race car engine. If the carb, intake manifold, cam, heads, the exhaust system are not matched, it?s not going to be optimized or perform the best with bad results. It's kinda like putting to big of a carb on a stock factory car engine, if it's to big, it looses velocity and is sluggish, with the correct size of carb, say a little smaller, the velocity is up and it's more responsive because it is matched to the air needs of that engine. Think of the cowl inlets in the same manner. Too big of a cowl inlet can be worse than one that is optimized to the correct size or design/shape.
Get the air in efficiently, direct it to where it needs to go and optimize your cowl outlet just like a tuned exhaust on a race motor. It?s all about using the air correctly.
I Hope some of this helps with some of you experimenting with ideas. Take this for what it?s worth as no two airplanes are the same. Be careful and fly safely.
__________________
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 : 08-21-2007 at 08:57 PM.
Reason: Added a couple of things.....
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