Exit Area Too Small ??????

[Yukon]

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.

 

[Tom Martin]

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.

 

[rv6ejguy]

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.

 

[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!!!!

 

[rv6ejguy]

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!!!!

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.

 

[Nuisance]

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.

 

[Nuisance]

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

 

[rv969wf]

Go for it

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

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.

 

[Tom Martin]

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.

 

[rv969wf]

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.

 

[gmcjetpilot]

Cowl flap down up speed difference? No jet thrust here?

Here are a couple of pictures of cowl flaps on my 185.

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.

John have you done any speed test with the cowl flaps, ie in level fast cruise flight. I suspect it will not make much speed change. It should a little just from the cowl flaps not hanging out in the breeze. Just curious, its not a homework assignment. I have plenty of GA plane time with cowl flaps, singles and twins and all I remember is I used it for temp control.

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.

I think I said or alluded to "thrust" My bad, I miss spoke. I was referring to exhaust adding (in theory) a small amount of thrust. To have thrust you need a change of momentum, action / reaction. The fact is air is leaving the cowl is slow, slower than the free air stream and also lower pressure (as the tuff pictures show).

Even though heat energy is added by the time air is in the lower cowl, the air has expanded in volumn and lost pressure going thru the cowl, engine fins, baffles and all the junk under there. Cooling DRAG is a way of life. It's a large part of the total airframe drag, so small improvements are very worth while.

After air goes in it actually it's expanding, losing pressure and velocity (energy) while doing the work of cooling. The only time you have a chance to acclerate the air is right at the exit, which is the firewall. Aft the firewall you are outside airframe in the slip stream. Typically we run our exhaust aft the firewall. We can hope to get some thrust out of the exhaust but to optimize you would need to make a nozzle shape. Again trade offs, more thrust by may be less HP due to more exhaust back pressure? It's always something.

We can acclerate the exit air as much as possible. One way is to reduce the outlet as Dave Anders suggested (less area w/ same flow = faster velocity). Total pressure has to be great enough to re-accelerate the air, so making it more efficient upstream (minimizing pressure loss) helps. This is where Dave Anders says you should have a smaller outlet area than larger relative to the inlet.

The second way is the exhaust augmenter (gas injector) tube. Here is an example.

It's pretty ingenious, using the fast moving exhaust to create suction and accelerate the cowl air (using the energy in the exhaust to add back to the cowl air). It does improve cooling and lowers drag. However in a twin you have room to hide this big tube, from the firewall to the wing trailing edge. With our configuration you have to cut the pipe off right at the firewall and run the augmenter tube fwd into the cowl. Not easy.

Alan and others found mixed results with their RV exhaust augmenter experiments. Not surprising, with the augmenter tube totally out in the breeze, which adds parasitic drag. Also being under foot was negative. There's little room in the RV. I think the T-34 Mentor has an augmenter. Also as Alan said the augmenter is louder, which is also a factor.

From Wikpedia:
The 310 first flew on January 3, 1953 with deliveries starting in late 1954. The sleek modern lines of the new twin were backed up by innovative features such as engine exhaust thrust augmenter tubes and the storage of all fuel in tip tanks in early models. In 1964, the engine exhaust was changed to flow under the wing instead of the augmenter tubes, which were considered to be noisy.

In the above the word thrust is used. A better word really should be injector. There have been studies (check NACA server under exhaust gas injector). The tube diameters and length need to be designed properly. They are not small dia or length.


Bottom line airframes are made with the concept of an augmenters from the drawing board. Some famous multi-engine WWII planes and Airliners have augmenter tubes. I am not going to try it based on the reports I have heard here. There is always the practical aspect, adding more weight and so on. Also cutting the pipes short affects the efficiency engine. Trade offs as usual.

Yak w/ side exhaust augmenters. Exhaust pipes end just aft of the last 'S' in the "MISS". Down side? Exhaust soot/stains.

 

[pierre smith]

Taxiing on hot days

This is a great thread with a lot of good advice. Van probably knows a lot of this and has a big industry connection base but in reality, the cooling system is a compromise between desert summers, long slow taxi times behind a bunch of airplanes (think of Sun 'n Fun) with 30 airplanes ahead of you.

Alan has very small inlets as we've seen but how does the CHT look during a midsummer lengthy taxi? Or a long hold for traffic? Remember that springtime and winter temps will cause dramatic CHT/oil variances. We have airplanes knocking around 200 MPH or better, the -6's close to redline TAS. I personally like 145-160 MPH climbs but not everybody will do this.

Nevertheless, I'm enjoying this thread and seeing careful step by step testing as Alan is doing is neat.

Regards,
Pierre

 

[gmcjetpilot]

Big oil cooler

Alan has very small inlets as we've seen but how does the CHT look during a midsummer lengthy taxi? Or a long hold for traffic? Remember that springtime and winter temps will cause dramatic CHT/oil variances. We have airplanes knocking around 200 MPH or better, the -6's close to redline TAS. I personally like 145-160 MPH climbs but not everybody will do this.

Pierre remember he has a secret weapon (that he mentioned), a huge oil cooler ducted to its own external scoop he can manually open. He has the air/oil cooled IO360.

 

[RV8RIVETER]

Alan has very small inlets as we've seen but how does the CHT look during a midsummer lengthy taxi? Or a long hold for traffic? Remember that springtime and winter temps will cause dramatic CHT/oil variances. We have airplanes knocking around 200 MPH or better, the -6's close to redline TAS. I personally like 145-160 MPH climbs but not everybody will do this.
Pierre

While exhaust augmentors/ejectors provide a big benefit for reducing drag by energizing the exit flow, I think they also can play an important role here. By increasing the pressure drop across the cylinders at high AOA or taxi.

As for the thumping.... NACA report 818, which details the ejector study states that there is no performance difference with regard to straight or curved ejectors, so it may be possible to minimize the negative effects by slightly turning down the outlet. Also, while a the optimum L/Da ratio was 8, good perfromance gains were had at L/Da ratios of 6. Also the experiment was conducted with a single cylinder 85hp engine. One would think that with 4 cyl engine decreasing the time between power pulses may have a sightly different effect. I guess what I am saying is for real world practicality we may not have to use the "optimum" configuration and can arrive at a design that is both helpfull as well as easy to live with.

There is that engineering compromise again.

 

[rv969wf]

Heres the answer

This is a great thread

Alan has very small inlets as we've seen but how does the CHT look during a midsummer lengthy taxi? Or a long hold for traffic? Remember that springtime and winter temps will cause dramatic CHT/oil variances. We have airplanes knocking around 200 MPH or better, the -6's close to redline TAS. Regards,
Pierre

This is a worst-case scenario. I live in the panhandle of Oklahoma; we had one week this month where it never got under 100F during the day. I flew my ?6 one afternoon in this yucky hot air and I peeked my CHTs around the 340F-350F range at 100F ROP. In the pattern with power pulled back and slowed up the CHT?s dropped to around 325F. On the ground taxing the CHT?s drop to 280F. It doesn?t matter if I?m facing into the wind or not. Idling at 700-800 rpm and the mixture leaned for maximum idle speed, the CHTs will settle in the 270-280F range as long as you want to sit there on the ground.

The oil temp is not an issue either. I?m using a huge 11 row SW dual bypass cooler just for this reason that has the external air supply.

Coating the exhaust with a barrier will also help with under cowl temps while taxing in hot weather and I do agree that most airplanes need huge inlets or cowl flaps to keep them cool on the ground for lengthy times.

I agree with Pierre that some of the Vans airplanes have already pushed there limit and are reaching or exceeding VNE speeds. But there is always going to be someone out there that wants to go faster or is trying to outrun his buddy. I?m not an air racer, just a cruiser liking to get from point A to B in the most effective way efficiently.

Some of the things that I tried and did might not apply to everyone?s airplane, but it has been a learning experience with great return in the end.

 

[Yukon]

George,

I hear what you are saying, and I am not suggesting that the cowl provides
a majority of our motive power, but I think it is doing more than most people seem to think. These airplanes display an amazing amount of efficiency utiizing stock GA powerplants, and I am facinated about how they do it.

When I get mine flying, I really don't want to be fussing with cooling problems out here in the desert, so I will probably start out with a set of louvers. If they cost too much in speed, I suppose I will experiment with cowl flaps.

 

[rv6ejguy]

I wouldn't think you'll have cooling issues with the 235 if you're still going that route. You getting any work done on that thing with the lovely Maya around?

 

[Bill Dicus]

Augmenters

One benefit of augmenters on T-34A was experienced duriing a night landing with a folded nose gear. Couldn't stop prop before nose fell thru (even with full nose down trim to try to increase elevator effectiveness). It's amazing how far that nose comes down with no wheel up front! The only damage was to prop tips, scrapes on nose gear doors, and flattening and abrasion of lower exit area of augmentors. They probably prevented some additional damage. Bill

 

[gmcjetpilot]

I hear you Bro!

George,

I hear what you are saying, and I am not suggesting that the cowl provides
a majority of our motive power, but I think it is doing more than most people seem to think. These airplanes display an amazing amount of efficiency utiizing stock GA powerplants, and I am facinated about how they do it.

When I get mine flying, I really don't want to be fussing with cooling problems out here in the desert, so I will probably start out with a set of louvers. If they cost too much in speed, I suppose I will experiment with cowl flaps.

No I hear you Bro! Yea I think we're splitting hairs, but that is what it takes to go fast. However going fast and worrying about temps is no fun. I think you will be fine with the stock cowl.

What I'm looking for, which I think most folks do as well, are little improvements, mods that result in more efficiency with out major mod. Of course those "little mods" often take a lot of work and effort. The faster you go, each successive mod gets harder, resulting in less pay off each time. At some point people just say, good enough.

There's no doubt "Van The Man" has thought about almost everything under the sun, coming up with his combo (the total package), efficient, light, easy to build & maintain, in stock per plans config. There are great ideas and theories that work, like augmenter tubes, cowl flaps & louvers, but practical aspects (trade-offs) come into the picture. Each builder is free to config and mod as they see fit, and that's the fun part of experimentals.

We'll never have zero cooling drag or 100% cooling in extreme hot climates, but we can keep trying to shave drag and increase efficiency, while keeping the temps stay in the green. Keep up the good work!

 

[bryanflood]

My Cowling Mod



So I wasn't going to post this b/c the it is such an early test picture, I know, it really looks ugly. The 2 "tunnels" are now slightly larger and made of stainless to avoid the burning problem. (Yeah this was STRICTLY a test part) and no it's not held on just by tape, it was riveted. There was some work done interally to to smooth the air flow a bit. My testing seemed to indicate this mod increased speed by 1-2 knots, but don't rely on my test data. I did a gage R&R on my airspeed indicator and found that even corrected true airspeed for density, and humidity was only repeatable to 2-3 knots, so my observations could be totally in the range of error. This version ran a bit hot but after baffling improvements the cruise CHT was down to 325F, still climb had to be limited b/c the exit area was so small. So as I said I opened it up a bit by making the ramp angle less severe and slightly wider,that seemed to do the trick. I don't have any pictures of the most recent test part. I got all consumed on the baffles and front end of the cowling for a while and have not made a final part. The other problem is that I want to actually fly the plane instead of work on it, so progress is slow. The other thing is I am tired of modifying fiberglass molds. On of these days I will get off my butt and complete this one.

 

[F1Boss]

Fellas:

Seems to me we are swimming upstream here: why continue to try to optimize the exit into a high pressure area?

Fact: the entire bottom of the plane is a high pressure area.

So, let's investigate channeling our exit or exits to a low pressure area.

Fact: Most of the upper surface of an airplane is a low pressure area.

If we can't re-locate our outlets to a low pressure area, let's at least move the outlets to a lower pressure area, instead of the center of the belly, where pressure would be the highest. I will submit that the cooling air exits should at least be at the outer corners of the bottom of the cowling (there IS a reason why Columbia and Cirrus do this), with the sides of the cowling being lower pressure yet, but this location can cause a bit of a mess down the side of your ship... One fella I worked with, who set many altitude and time to climb records, did try the side outlets,, but he related that at high alpha angles, the flow thru the inlets would stop, and the sides became inlets to the still open bottom outlet. Of course this didn't provide the most optimal cooling!

Shop tip: if you're going to experiment with side outlets, you're gonna have to close off the bottom, as much as practical.

Go back and look at the picture of the Yak, and maybe Rear Bear too: their outlets (and many other relatively high tech piston engine ships) are not on the bottom/high pressure area...

A fixed panel used as a side outlet (look at the Yak again, and a Sea Fury too) could have a very effective cowl flap -- easy to manufacture and actuate, and tough as nails (= very little maint issues). Rear Bear has its cowl flaps across the top of the fuselage, as does a Corsair; this is not the best placement if you like to look thru your windshield (just where is that leaking oil gonna go?), so there are practical limits to our quest to achieve lower cooling drag.

Carry on!
Mark

 

[RV8RIVETER]

So, Mark when are you going to switch the Rocket to updraft cooling?

 

[F1Boss]

So, Mark when are you going to switch the Rocket to updraft cooling?

As soon as I get your deposit check!

Seriously, the side panel idea has been kicked around here for quite a while. The exhaust would be dirt simple (3 short pipes), but would sound really weird, as you would hear only 3 cyls at a time from the ground -- kinda like a T28 or Wildcat...

I think it's too easy to ignore, as the cowl mod would take minutes, the outlet panel would be easy (some of it would have to be stainless of course), and the exhaust could be mocked up out of steel for testing purposes.

Maybe John Huft will try this idea before I can get my plane in the air??

Cheers
Mark

 

[rv6ejguy]

You might be interested to know that CFD plots on a 6A show the area just aft of the lower exit and the sides of the cowling just forward of the firewall have about the same pressure distribution (around ambient). One foot aft of the wing leading edge on the belly is actually very low pressure. One foot ahead of the windshield is also low pressure so this might give the highest Delta P for cooling outlets.

 

[rv969wf]

Has anyone ever thought of this??????

Has anyone studied how an air blow nozzle works?

I?m talking about the air blow nozzles that you use to blow parts dry with. There are many designs out there and the OSHA ones that have holes placed in the side are the ones I?m talking about. Most of them are designed so if you block the tip where the air comes out, the air can be relieved out the other side holes. Now some of the more expensive ones have somewhere around a 45 degree hole that angles towards the outlet to produce more air pressure at the nozzle exit point. The way they are designed, it allows extra air to be pulled into the mainstream to provide more output per given about of air pressure. I play around with an air nozzle that I have and it was interesting playing with it because if I blew air into one of the holes that is drilled on the side, it pulled a vacuum/suction through the inlet. Next time you?re at a store that has air nozzles, look at them and you?ll see what I?m talking about. Don?t look at the ones that have a hole 90 degrees to the air stream, as they are junk.

OK, enough about air blow nozzles. I?m not sure if it would work, but why can?t a person provide an external air source with a scoop to your cowl outlet and feed high-pressure air on each side of the cowl outlet at somewhere a 45-60 degree angle to speed up the air at the cowl outlet ? One would probably need an extended cowl aft of the firewall to do this. This could probably help with mixing the hot and cowl air and provide a little bit of thrust and it should help pull air through the cowling if it was designed right. That?s the big question, if it was built properly and tested.

I sketched a couple of pictures just as ideas to help understand. The first picture is a top view. The second picture is a sketch of an air blow nozzle that has two 45degree holes on each. 3rd picture is the actual air nozzle tip.

Anyway, I?ve been thinking about this for a long time but just don?t have the time to tinker around like I used to, so I?ll leave this up to someone else. Who knows, it might work and it may not, there?s only one way to find out. Any takers out there???????

 

[gmcjetpilot]

Ross, was that from the EAA Mag article?

You might be interested to know that CFD plots on a 6A show the area just aft of the lower exit and the sides of the cowling just forward of the firewall have about the same pressure distribution (around ambient). One foot aft of the wing leading edge on the belly is actually very low pressure. One foot ahead of the windshield is also low pressure so this might give the highest Delta P for cooling outlets.

Ross is that the April 1997 article with the RV-6A CFD plots?

I remember the above article when it came out. I have been looking for a copy for awhile and Gent was nice enough to send me a scanned copy a few months ago. I enjoyed it, but was a little disappointed with the lack of belly plots.

Can't argue with your analysis of where pressure is ambient / low from the info avaiable. However I'd point out two big draw backs of the computer model/analysis, no cowl exhaust / cooling flow or prop wash affect. One good pressure plot of the belly would have been nice.

Dispite the area under the plane not being well represented with a pressure plot, I think you're right, you have to go aft of the wing leading edge to get lower pressure. Not so sure how low it really is.

The article is very cool (no pun) but lacking on belly plots and some critical detail for cooling. Prop wash and cowl exit air flow is probably not trivial for fuselage air flow, but it's a good starting place.

The impression I got from the article was the main interest was overall characteristics and wing aerodynamics. The wing of course is mostly unaffected by prop wash. His wing body fairing idea was interesting.



Upper or Side cowl flaps?

This seems to be a reoccurring theme. The oil cooler door does bulge as was mentioned. WWII fast movers had exhaust exits on side of fuselage. Hummm, I say no to full updraft cooling, as some where kidding about, but what about an upper auxiliary cowl flap?

Ross mentioned ambient pressure fwd of windscreen by a foot? It seemed like a lot was going on in the CFD plots, but some where on the top of the cowl I'm sure a reasonable pressure gradient can be found to locate the cowl flap. It's changing very fast, so flight test would be in order, to get the ideal location. Will that location be practical?

If you use a sealed plenum you could locate the flap where you like in the upper cowl. Room for a door mechanism would be limited and a challenge may be. If the ideal location is near and fwd of the firewall or over the accessory area of the engine, behind the upper plenum in the low pressure area, all the better.

The late Tony Bingelis showed some variations on the upper cowl flaps in his book's. If you introduce air out the cowl top into to slip-stream, it will add drag. This idea may be more for the hot climate folks, than a speed secret? The lower louvers idea Yukon mentioned is easier than an upper cowl flap, but the nice thing about an upper flap, on the ground after shut down, you can open the chimney during cool down. The cowl (and engine) cook after shut down.

May be the side air exits are best? The Thorp T-18 had side cowl out lets? Hummm noting new under the sun?

 

[rv6ejguy]

I sprung many years back and purchased LOFTSMAN/ CMARC/ POSTMARC to run my own CFD plots. Not cheap but fascinating. Not sure if Peter Garrison still sells this. There is probably something better/ cheaper these days.

I have side exits with entry ramps feeding them. These were highly effective to solve initial cooling problems with the Subie. I noticed no speed degradation with these.

I've tuft tested the stock 6A exit and videoed it in flight. It is a big mess.

 

[Kahuna]

There's no doubt "Van The Man" has thought about almost everything under the sun, coming up with his combo (the total package), efficient, light, easy to build & maintain, in stock per plans config.

Boy do I disagree with this. There are MANY things he has not spent the time to consider and there are thousands of these flying with incredible mods that Vans has neither contemplated, nor implemented to improve his design. Van brought a basic design to the table which is a tremendous platform to improve upon. He is spending little time doing that. Good on all you guys who are making it better.
Best,

 

[David-aviator]

Boy do I disagree with this. There are MANY things he has not spent the time to consider and there are thousands of these flying with incredible mods that Vans has neither contemplated, nor implemented to improve his design. Van brought a basic design to the table which is a tremendous platform to improve upon. He is spending little time doing that. Good on all you guys who are making it better.
Best,

Making it better must mean improved cooling and more speed.

Better cooling is good. More speed may not be. These machines have definite structural limitations and to push the speed envelope beyond what has been flight tested certainly advances the risk meter.

For sure these airplanes will come apart in smooth air long before Mach one. In turbulence or under load, sooner yet. Just how far that break up point is from the design limits is totally unknown. Cruising at 220 knots may not be a good idea at all. We know a quick stick movement can break a wing as is. If one needs to go that much faster, there are plenty of airplanes designed to do it.

...just disagreeing a little in the other direction.

 

[doneil]

CFD Plots on 6A

Ross

You mentioned "CFD plots on a 6A at 0 alpha".

Can you tell me if this information is available in the internet and where?

Several years ago I think I remember a picture or a drawing of the high and low pressure areas on the 6A in Kit Planes or Sport Aviation or maybe the RVator. I have spent quiet a bit of time looking for it and have not been able to find it again.

Do you or anyone else know about this? Or maybe I just dreamed it up?

Thanks

David Roberts

 

[gmcjetpilot]

EAA mag 1997 & RV's the greatest Kit plane of ALL TIME?

Ross

You mentioned "CFD plots on a 6A at 0 alpha". David Roberts

David I mentioned the reference above, April 1997 EAA article with the RV-6A CFD plots. Ross said he has his own CFD program he plays around with.

Boy do I disagree with this. There are MANY things he has not spent the time to consider and there are thousands of these flying with incredible mods that Vans has neither contemplated, nor implemented to improve his design. Van brought a basic design to the table which is a tremendous platform to improve upon. He is spending little time doing that. Good on all you guys who are making it better. Best,

Well I think we are looking at it differently.

"There are MANY things he has not spent the time to consider and there are thousands of these flying with incredible mods that Vans has neither contemplated, nor implemented to improve his design. He is spending little time doing that."

I am not challenging you but what specifically are you talking about? What mods are so great that they have changed the basic design or performance significantly?

Many mods I see builders make have nothing to do with the basic airframe kit, like engine/prop mods or the mods are trivial and have little affect on the plane. The exception, in my opinion, is the James cowl inlet/sealed plenum type set-up, which I see as worth while. Not a must but worthwhile.

Most mods are frosting on the cake. Sure builders like Dave Anders RV-4 is an example of taking a 220 mph RV and making it go +250mph. If you follow his work its beyond what most are willing to do, like using a highly modified engine with short engine/prop TBO's.

You say Van is not improving his design? Well the RV-7 and RV-8 are the improved versions of RV-6 and RV-4 respectively. That was some thing. Producing a value priced kit plane with more bang for buck is doing something. He could continually change the design but at some point you have to freeze the design.

Heck he came out with the RV-10, pretty much a success from what I hear. It uses new construction techniques (composite/metal hybrid structure) and new airfoils, plus a 260 hp engine. The RV-9? In about 10 years he has come out with the RV-8, 7, 9 and 10. How many manufactures do you know who have made as many innovation. Now he's working on the RV-12. I don't think Van's aircraft is resting on their laurels and past reputation and not innovating all the time.

Van has gone back to the RV-3 and redesigned significant parts like the main spar. He has gone back to the RV-4 and made improvements to the engine mount, longer gear and elevator. The RV-6 got top hinged rudder pedals and the RV-7 got a big rudder and so on.

Van does improve the design adding safety and efficiency when needed. He has released 3 wing tips: Horner, shear and symmetric. Clearly Pre-Punch is a quantum leap in kit value and construction. Also cowls are now epoxy/nomex. Van has made significant progress over the years in my opinion.

You could call me on the RV-6A/7A nose gear controversy. Van has changed the nose gear design recently (the new fork). Some think he should change the whole nose gear, kitandkaboodle. Another debate for another time.

Airplane designs as you know are a million compromises. I know you have done cool and extensive mods to your RV, like a 540 in a RV-8. Is it better? Sure more power is better, but in the big scheme of things a IO360 or now IO390 +200 HP RV is no slouch, even with 180HP or 160HP, you're faster than 90% fixed gear single engine cert GA planes (and many retracts). It's not that Van can't design a 540 RV-8, it's just he felt the "sweat spot" was with the 4 banger. Van is an engineer and like a Chess master thinks 20 moves ahead. Sure we see a mod as improving something, a no brainer, but he sees the affect overall. His conclusion of a mod's value may differ.

Van has been known as the "Flying Dutchman", where Dutch means cheap. Van is also a frugal designer, not one to make elaborate complicate designs when simple work's. His personality comes out in his design. I think you're addressing personality or preference more than a lack of Van's design skills or thoroughness. I still contend its hard to improve on the basic airframe design or performance, and most "Improvements" are really frosting not the meat of the design.


Not sure specifically what you are talking about, but in regards to cooling drag & cowl flaps I know Van has explored and tested them. I'm a huge proponent of the Barnard/James/Holly Cowl sealed inlet/plenum design. Why does Van not change his cowl and baffle kit? I don't know, but you can delete the cowl from your kit and buy a SJ cowl. Other factors may be involved, including inertia of manufacturing, why fix what ain't broke. To change major parts adds cost. Van's cheapness is good for us, other wise he would need to charge $40,000 like Lancair charges for their kits.

Did you know the first RV-6 prototype had an annular cowl inlet under the spinner (smiley face), verses the previous RV-4 axisymmetric dual inlets on each side of the spinner. Yep he tried to make the annular inlet work, but after much flight test he went back to the RV-4 design, as used on all current models today. Sure all his planes share common design characteristics, but so do Boeing jets. I don't think he is lazy but again why fix it what ain't broke.

Van wrote a series of articles in the RVator about cooling drag, cowl flaps and ways to improve the design for speed (in the 1993 time frame). He is a trained engineer and clearly his choices are well thought out? I know people complained about drawing inaccuracy or inconsistencies in the early days. Now that dwg and part fit is excellent. There will always be "WHY DID HE DO IT THAT WAY?" Some love Cam-Loks for the cowl verses the piano hinge. So what, its really an insignificant mod and the piano hinge has advantages, not the least of which is cost/weight.

Van is not here to defend his design. I'm sure 9 out of 10 times he would have good reasons for doing what he did. The 1 out of 10 times he may nods and says, good idea, but to get into the kit, it has to contribute a significant improvement with out adding cost, weight & build time. At air shows Van cruises the RV flight-line, when no one is watching. He appreciates SOME mods. Some mods make him shake his head. The KISS and KILL (keep it light lad) principals have their own rewards. Van's way is not the only way. I know that, and the joy of kit planes is modifing them, in part because we can. Nothing wrong with that.


Some mods have negative affect of adding weight, build time and cost.

Is there really a better kit plane on the market? Opinions will vary. With +5,000 flying, they all can't be wrong. I can say without reservation, RV's are the most popular and numerous high Perf kit planes in the sky. There really is no competition on the "total performance" horizon. Van has a fair and balanced design in my opinion even in stock form. Is it a perfect or optimal design in your opinion or mine? No but what's perfect.

We can agree GO forward and modify (carefully and safely).

 

[Nuisance]

As soon as I get your deposit check!

Seriously, the side panel idea has been kicked around here for quite a while. The exhaust would be dirt simple (3 short pipes), but would sound really weird, as you would hear only 3 cyls at a time from the ground -- kinda like a T28 or Wildcat...

I think it's too easy to ignore, as the cowl mod would take minutes, the outlet panel would be easy (some of it would have to be stainless of course), and the exhaust could be mocked up out of steel for testing purposes.

Maybe John Huft will try this idea before I can get my plane in the air??

Cheers
Mark

Mark, quit playing on the computer and get to work! I want to see that 550 powered EVO flying!

I am not so sure that the bottom of the cowl (actually the beginning of the fuselage) is such a high pressure area...the bottom of the wings, yes; the front of the cowl, yes. I think most of the fuse is a freestream pressure area. That is why we put our static pressure ports on the sides.

Meanwhile, the mechanical complexity gets harder. And, I am not sure I want to fly in that plane! It is loud enough in a big airplane!

Maybe this thread will inspire some innovators out there. And, with the new SARL race series, we can get some independent verification of all the claims.

John

 

[Mike S]

Ross

You mentioned "CFD plots on a 6A at 0 alpha".

Can you tell me if this information is available in the internet and where?

Several years ago I think I remember a picture or a drawing of the high and low pressure areas on the 6A in Kit Planes or Sport Aviation or maybe the RVator. I have spent quiet a bit of time looking for it and have not been able to find it again.

Do you or anyone else know about this? Or maybe I just dreamed it up?

Thanks

David Roberts

This was done on Steve Barnards plane, it was done in Redmond Washington, by a company called "Analytical Methods Inc.".

The author was Dave Lednicer.

Google it???

Hope this helps.

 

[RV8RIVETER]

I was kidding earlier, but there is some interesting reading to be found about a custom designed aircraft that uses updraft cooling here.
http://www.melmoth2.com/texts/Cooling flow.htm

I have not been able to find any RV plots, but my search did remind me of the above link.

Actually has a CFD plot, that while not the same demonstrates the front end pressure. The low pressure area is very near the front of the cowl as to be a difficult choice to deal with. And then the pressure builds again at the windscreen. I agree with John that the lower exit is not in a high pressure area. I guess if you really wanted to use a low pressure area you could build a duct to outlet over the top of the wing, but that is alot of fairing to hang out there or alot of fuselage modifications.

Another great thread for ideas and learning!

 

[rv6ejguy]

Ross

You mentioned "CFD plots on a 6A at 0 alpha".

Can you tell me if this information is available in the internet and where?

Several years ago I think I remember a picture or a drawing of the high and low pressure areas on the 6A in Kit Planes or Sport Aviation or maybe the RVator. I have spent quiet a bit of time looking for it and have not been able to find it again.

Do you or anyone else know about this? Or maybe I just dreamed it up?

Thanks

David Roberts

Peter Garrison ran ads in various magazines for Loftsman/ CMARC under his company AeroLogic years back. http://www.aerologic.com/. Be forewarned that the lofting program part to define the shape of an object is very complicated to use (for me at least) I ain't no engineer.

Fortunately, the RV6A had already been lofted by someone a lot smarter than I and I conned Peter into giving me those files when I purchased the programs.

Once you get the shape, the rest runs pretty easily in DOS.

Like I said, there may be better, cheaper stuff available today.

 

[doneil]

April 1997 Sport Aviation

George

Thanks for the information. This was the article I was refering to. Guess I didn't dream it up after all. I found your earlier reference and the fact there was no belly plot. I still find it useful.

To Mike and Ross thanks for the replys. Ross I can see from your reply I would not be able to use the software even if I could afford it.

David Roberts

 

[ksouthar]

Exit Area Too Small ???

RV9-A
Lycoming 0320-E3D (150hp)
Sam James Cowling with Glass Plenum

OAT ~ 80 degrees

CHTs - 350 +/- in climb 320 in cruise

Oil Temp 190-200 climb, cruise - doesn't seem to care... Never seen it break 200.

S/W Oil cooler mounted to the rear baffle (firewall mounting will cost you higher temps!)

Keith
N355RV

 

[gmcjetpilot]

You can make the firewall Oil Cooler mount work

RV9-A
Lycoming 0320-E3D (150hp)
Sam James Cowling with Glass Plenum

OAT ~ 80 degrees

CHTs - 350 +/- in climb 320 in cruise

Oil Temp 190-200 climb, cruise - doesn't seem to care... Never seen it break 200.

S/W Oil cooler mounted to the rear baffle (firewall mounting will cost you higher temps!)

Keith N355RV

Thanks for the info, but I'd disagree about your last comment, "firewall mounting will cost you higher temps!"

I'd say you are right if uisng Van's standard brand of coolers and firewall mount kit, which are not great. However using a SW cooler and making proper ducting of proper size (larger than 3"), makes the OT a non-issue ,even if mounted on the firewall.

If done right, than OT's can be as good or better with a firewall or engine mounted cooler. The baffle location works fine temp wise, but it has a nagging tendency to crack the baffle on many installations, at least if you don't reinforce it and brace it (a lot). Than some times the cooler cracks.

The 360's don't do as well with the cooler on the baffle, crack wise. The O-320 are better suited for baffle mounted oil coolers, I think. The 320 coolers are smaller, weigh less and the O-320 may not shake as much as 360's. Trying to support the weight of the cooler, oil lines and the oil inside them, while the engine shakes like a wet dog, is hard on the flimsy sheet metal baffle. That's my opinion any way. Just stay away from cheap coolers and Vans remote mount cooler kit. Search the forum archives for proper remote cooler design.

 

[Bill Palmer]

Firewall Mount is Okay

George,

As far as I can understand from various threads, a firewall-mounted oil cooler works fine if: (1) it is fed by a 4-inch-diameter duct and (2) it's an appropriately-sized Stewart Warner. Note: Van's RV-10 firewall-mounted oil cooler plenum is fed by a 4-inch duct. I know you have a negative opinion about the less-than-optimal angled flow inside Van's RV-10 oil cooler plenum, but Van's plenum volume appears to be adequate, if not better, than most oil cooler plenums, and plenum pressure (differential pressure across the oil cooler) and mass flow rate across the cooler are arguably more important for oil cooler efficiency than the flow path itself. Flow path seems to be somewhat important in delivering air to the plenum, however, so the 4-inch-diameter SCAT duct from the baffle to the firewall-mounted oil cooler plenum should be as straight and short as possible. Also, the downstream face of the oil cooler needs to "see" a low, or ambient, pressure area in order to achieve the maximum possible differential pressure across the oil cooler.

These are my personal observations/conclusions/opinions ("2 cents") anyway!

Best Regards,

Bill Palmer

 

[gmcjetpilot]

I agree 100%

George,

As far as I can understand from various threads, a firewall-mounted oil cooler works fine if: (1) it is fed by a 4-inch-diameter duct and (2) it's an appropriately-sized Stewart Warner. Note: Van's RV-10 firewall-mounted oil cooler plenum is fed by a 4-inch duct. I know you have a negative opinion about the less-than-optimal angled flow inside Van's RV-10 oil cooler plenum, but Van's plenum volume appears to be adequate, if not better, than most oil cooler plenums, and plenum pressure (differential pressure across the oil cooler) and mass flow rate across the cooler are arguably more important for oil cooler efficiency than the flow path itself. Flow path seems to be somewhat important in delivering air to the plenum, however, so the 4-inch-diameter SCAT duct from the baffle to the firewall-mounted oil cooler plenum should be as straight and short as possible. Also, the downstream face of the oil cooler needs to "see" a low, or ambient, pressure area in order to achieve the maximum possible differential pressure across the oil cooler.

These are my personal observations/conclusions/opinions ("2 cents") anyway!

Best Regards,

Bill Palmer

No offense about the RV-10 set up Van is selling (supplied only for the RV-10 kit I guess?). They should make it avaiable in their accessory catalog. 3" v 4" is 77% more area for the latter.

I'm talking about their standard Van's 3" remote kit not their RV-10 kit, which sounds better, just from the fact they went to 4" dia. When the duct/tube starts to get as big as the cooler, than the shape of the cooler air-box transition matters less, since there is little transition. An optimal shaped duct transition is frosting on the cake, if the duct is at least close to the **proper area.

Sure better shapes always help with flow, but if overall volume and pressures are high enough, it overcomes any little less than elegant shapes. The RV-10 kit works it works. Also the RV-10's are using larger or better SW coolers I suspect, so air is used more efficiently used.

The standard 3" remote oil cooler kit, in my opinion, is what my gripe is about, too small except for may 150hp or may be 160hp coolers. It's not only the poor shape of the cooler air-box (flat shallow square box) but other issues as well. When a less than efficient clone cooler is mated to an under-sized 3" SCAT, with hot climate and/or +180HP engine, you have an OT problem. That's a proven fact. Just trying to keep people from making the same mistake over, not trying to kill Vans remote cooler kit sales.

** Getting the engineering data out for a cooler, any cooler (if tested and published) it tells you the kind of pressure (inches of H2O) and flow (CFM) is needed to achieve the design cooling capacity of the oil-cooler, which dictates the size (area) of hose/duct needed (for a given pressure)........ The size of tubes and fin spacing on the cooler affects what kind of air flow it needs to work best, low pressure / high flow or high pressure / lower flow. The cooler needs to be optimized for the application and installation. The SW units tend to work better in our application with the kind of pressures and flow we can supply the cooler. Example, the SW 8406R, needs more than 3" to feed it at typical differential pressures we see under the cowl. It's just better quality cooler. However just using it with 3" scat is kind of a waste and a band aid on the real issue, the area of the 3".

When you take the 3" remote kit and use a less than ideal brand of clone cooler you make the problem worse. This recent post is excellent regarding Pro/Cons of brands and models of typical coolers we use in RV's (for the smaller Lycs):

http://www.vansairforce.com/community/showthread.php?p=150794#post150794

I agree with you about the cooler exit area. An exit shield or short duct can promote or improve flow out of the cooler, no debate, but baffle mounting the cooler is not a MUST to get low OT's if done right (SW cooler, duct area sufficient and exit area is shielded). I try to keep the cooler higher up on the engine mount than down and low on the firewall where air is gathering and swirling around the cowl exit. The key is putting the cooler on something that is not shaking, i.e., the engine.

 

[gasman]

Kahuna......

Boy do I disagree with this. There are MANY things he has not spent the time to consider and there are thousands of these flying with incredible mods that Vans has neither contemplated, nor implemented to improve his design. Van brought a basic design to the table which is a tremendous platform to improve upon. He is spending little time doing that. Good on all you guys who are making it better.
Best,

You need to reat this........ http://www.vansairforce.com/community/showthread.php?t=19692&highlight=rv3

Then ask yourself.... What did Van do wrong?

 

[ksouthar]

Exit Area Too Small ???

With the Sam James Plenum, the baffle is cut very short and the plenum attaches along the upper edge (so short that shorter plugs are required to get the plenum to fit!).

Very careful plannning is required to get the oil cooler to fit in the remaining space on the back of the baffle. The baffle is reinforced (used the same reinforcement that is the Vans Baffle Kit) where the oil cooler mounts. When assembled, the whole kit is very rigid.

keith
N355RV
RV9A

 

[kentb]

I finially measured my cowl.

RV9A with IO-320, cross over Vetterman.
Inlet size = 48.75 inches
TB inlet size = 6.175 inches
hot air outlet = 41.75 inches (this has the exhaust pipe area removed)
exhaust size = 16.27 inches.

This is all stock Vans setup. I haven't modified anything yet.

It is interesting that the outlet for air is 7 inches smaller then the inlet.
The outlet for combustion is almost 3 times the inlet size.

Now if I open up the cabin air this should make up for the difference in cooling air under the cowl at about 9 inches, so the cooling area balances. But should it? With the extra heat added to the air flow shouldn't the exit air size be bigger?

I have measured my exhaust pipes where they exit the cowl and they project 6 inches behind the FW and stick down one inch below the cowl at about a 45 degree angle.

I have seen some builders have cut the ends so that they are flush with the bottom of the cowl and other so that they are pushing air directly into the slipstream.

Is there any data on which way to cut the pipes?

By the way I have no heat problem in my plane and I use the stock Vans oil cooler mounted on the baffle.

Kent

 

[gmcjetpilot]

Cut them pipes

I have seen some builders have cut the ends so that they are flush with the bottom of the cowl and other so that they are pushing air directly into the slipstream.

Is there any data on which way to cut the pipes?

By the way I have no heat problem in my plane and I use the stock Vans oil cooler mounted on the baffle.

Kent

Thanks for the data. If you plow through all the post there are some threads on exhaust. Using common sense the less in the breeze the better, less frontal area, less drag.

The general rules, the more aligned the exhaust gas exists with the free air stream the better, with some gain in residual thrust, in theory. When the exhaust gas exits into the free air stream at an angle you get "plume drag".

The down side of cutting pipes shorter is more (may be) cockpit noise and (may be) exhaust/oil belly stains?

"I have measured my exhaust pipes where they exit the cowl and they project 6 inches behind the FW and stick down one inch below the cowl at about a 45 degree angle."

Does not sound bad but the 45 degree is a little abrupt but OK. Changing that angle by just cutting may be hard, depending on where the bend radius is. 25-30 degree exit angle might be a nice compromise. You could cut the pipe exit, the very tip, at an angle.

Square cut tip is a better nozzle, meaning what ever thrust you can get, will be best left square to pipe. Of course if that "thrust" is at an extream angle down than its not much good any way. We're splitting hair and there is not much thrust to gain or lose in my opinion, at least with minor trimming.

Check the archives (exhaust, cut, angle tip drag...). I doubt you'll notice any measurable performance difference with small changes. There may be a small gain, but measuring it will be hard. Some claim they gained 2 mph by cutting their pipes? The pipes would have to be pretty bad to get 2 mph.