Van's Air Force
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Smile cowl inlet
- Thread starter rv8ch
- Start date
robertahegy
Moderator/Tech Counselor
I'm sure the original cowl inlet could be reworked to that shape. You would need to maintain the original area (or larger) to prevent restricting airflow.
Roberta
Roberta
KirkGrovesRV8
Well Known Member
Hi Mickey,
Jon Johanson's Flymore inc sells them. But I do believe you must have FI for it to work.
Best of Luck,
Kirk RV-8 Fuse
Jon Johanson's Flymore inc sells them. But I do believe you must have FI for it to work.
Best of Luck,
Kirk RV-8 Fuse
Let's see if I can slip that one by my wife!f1rocket said:Yep, sure do. Just build a F1 Rocket!!
Is that a standard cowl inlet for the F1 Rocket? I've seen them on RVs, too, so perhaps they had the Flymore product. I'm really just looking for the smile shape to glue onto the cowl. It's not for my induction system, but for my oil cooler. With the Eggenfellner Subaru that's where the oil cooler is located.
flyeyes
Well Known Member
Hi Mickey.
That is a standard Rocket part, although there are at least two types because the one Mark Frederick sold me is flat across the top. Given what you pay for shipping, you might want to roll your own. Look at James Redmon's Berkut site for details on how he made his oil cooler duct--the same principle should apply to yours.
James Freeman
That is a standard Rocket part, although there are at least two types because the one Mark Frederick sold me is flat across the top
. Given what you pay for shipping, you might want to roll your own. Look at James Redmon's Berkut site for details on how he made his oil cooler duct--the same principle should apply to yours.James Freeman
Captain_John
Well Known Member
Sam James explains how to do it in his Fiberglass Video.
Pretty cool looking, If you ask me!
CJ
Pretty cool looking, If you ask me!
CJ
gmcjetpilot
Well Known Member
Myth Buster? That is not a good location
This is one of the biggest myths around, that you can get huge boost from your intake scoop design. It is a urban legend and claims of boost are like claims of high top speeds. However it is harder to get away with an exaggeration the top speed of a RV because most people know how fast RV's go, which are fairly consistent. Any gain over ambient air pressure at RV speeds is possible but not common.
As far as intake shape, there is no magic and the claims of tremendous ram rise are questionable . Often pilots report large MAP increase mostly because their alternate source is so restrictive. It is more you are dropping MAP by going to alternate than reading your MAP increase from RAM pressure.
In actual fact you would be good just to get ambient pressure with no losses. The standard pressure @ 8,000' std day is 22.23 in-hg. Most Lycomings RV's have a hard time getting 21 inches much less +22 in-hg plus at 8,000' @ say 190 mph.
From my recall most RV's claim MAP of 21-22"-hg at 8,000', wide open throttle. I have seen claims of, 22.5", 23", 23.5 or even 24". Keep in mind the standard pressure is 22.23"-hg.
24" at 8,000' is over 1.75"-hg raise. That is more total RAM pressure than is available, which is only about 1.2"-hg at 190mph. This claim of cruise MAP (8,000' WOT) was from an O-320 powered RV. I would have to question the accuracy of their MAP gage or actual true ambient pressure (corrected for temp) or density altitude, unless he was going 385 mph, which I doubt. I can believe about 0.3"-hg with a very fast RV. However given the casual method most take MAP measurements, any claim of RAM rise has to be suspect.
For any one getting more than a true honest 22.0" or 22.5"-hg they have a great intake, very effective system with no boost but with no loss, which is real good. Boost is a myth unless you are going 250-350 mph or have a special designed intake very close to the prop (not near the hub).
If you want boost you need a turbo or super charger.
The best way to measure RAM pressure in the intake is put a pressure probe in the intake. Using a separate airspeed gage hooked to a reliable static source, you could read airspeed and convert to pressure or ram or MAP pressure. The problem is with small errors in static source when measuring small a pressure (which it will be negative to small). For accurate flight test data, static cones are pulled behind the aircraft to get accurate static pressure.
If you could recover even 1/4 of the total RAM pressure at 190mph it would be about 1/3 in-hg. Again that is a pretty good trick and has been done with special installations that place the intake snorkel real close to the prop (out from the hub), at angles (to the right 10 degrees I recall) to take advantage of the true combined relative airflow of the prop (thrust) with the fwd velocity of the aircraft. After all that the gains where small. With the stock Vans installation and the Smiley intake you are lucky not get a loss, which is way better than most that have losses.
When you start going well over 200mph, 250-350mph you can get more RAM just because there is more available. However the idea of massive boost (.5" to 1"-hg or more) is a little dubious for a RV. Van's airbox is about as good as it gets with a filter.
The snorkel extended from the cowl to with in 1/4" to 3/8? of the prop blade, positioned and angled properly looks weird and thus is not popular. Plus the gain is small, not to mention may be a little more drag. You see these induction tubes on Reno formula racers, but for every day flying the stock position and shape it very good.
If you like the-face scoop for looks (like a P-51) that is fine but doubt the claims of great gains in RAM. It may be better than a round intake in that the wide horizontal mouth may take advantage of the fact the relative air under the cowl is not perfectly straight. However if the area is greater, the drag will be greater, negating the advantage from (small) RAM rise of MAP.
Cheers George
The location near the spinner is absolutely the worst place to get induction air. Look at the shape of the prop blade near the spinner? It is a blunt club beating the air to death. The spinner is not help either with airflow any which way but straight. The further out towards the prop tips the better. Many a flight test has been done, see the book Speed with Economy by Kent Paser.osxuser said:
This is one of the biggest myths around, that you can get huge boost from your intake scoop design. It is a urban legend and claims of boost are like claims of high top speeds. However it is harder to get away with an exaggeration the top speed of a RV because most people know how fast RV's go, which are fairly consistent. Any gain over ambient air pressure at RV speeds is possible but not common.
As far as intake shape, there is no magic and the claims of tremendous ram rise are questionable . Often pilots report large MAP increase mostly because their alternate source is so restrictive. It is more you are dropping MAP by going to alternate than reading your MAP increase from RAM pressure.
In actual fact you would be good just to get ambient pressure with no losses. The standard pressure @ 8,000' std day is 22.23 in-hg. Most Lycomings RV's have a hard time getting 21 inches much less +22 in-hg plus at 8,000' @ say 190 mph.
From my recall most RV's claim MAP of 21-22"-hg at 8,000', wide open throttle. I have seen claims of, 22.5", 23", 23.5 or even 24". Keep in mind the standard pressure is 22.23"-hg.
24" at 8,000' is over 1.75"-hg raise. That is more total RAM pressure than is available, which is only about 1.2"-hg at 190mph. This claim of cruise MAP (8,000' WOT) was from an O-320 powered RV. I would have to question the accuracy of their MAP gage or actual true ambient pressure (corrected for temp) or density altitude, unless he was going 385 mph, which I doubt. I can believe about 0.3"-hg with a very fast RV. However given the casual method most take MAP measurements, any claim of RAM rise has to be suspect.
For any one getting more than a true honest 22.0" or 22.5"-hg they have a great intake, very effective system with no boost but with no loss, which is real good. Boost is a myth unless you are going 250-350 mph or have a special designed intake very close to the prop (not near the hub).
If you want boost you need a turbo or super charger.
The best way to measure RAM pressure in the intake is put a pressure probe in the intake. Using a separate airspeed gage hooked to a reliable static source, you could read airspeed and convert to pressure or ram or MAP pressure. The problem is with small errors in static source when measuring small a pressure (which it will be negative to small). For accurate flight test data, static cones are pulled behind the aircraft to get accurate static pressure.
If you could recover even 1/4 of the total RAM pressure at 190mph it would be about 1/3 in-hg. Again that is a pretty good trick and has been done with special installations that place the intake snorkel real close to the prop (out from the hub), at angles (to the right 10 degrees I recall) to take advantage of the true combined relative airflow of the prop (thrust) with the fwd velocity of the aircraft. After all that the gains where small. With the stock Vans installation and the Smiley intake you are lucky not get a loss, which is way better than most that have losses.
When you start going well over 200mph, 250-350mph you can get more RAM just because there is more available. However the idea of massive boost (.5" to 1"-hg or more) is a little dubious for a RV. Van's airbox is about as good as it gets with a filter.
The snorkel extended from the cowl to with in 1/4" to 3/8? of the prop blade, positioned and angled properly looks weird and thus is not popular. Plus the gain is small, not to mention may be a little more drag. You see these induction tubes on Reno formula racers, but for every day flying the stock position and shape it very good.
If you like the
-face scoop for looks (like a P-51) that is fine but doubt the claims of great gains in RAM. It may be better than a round intake in that the wide horizontal mouth may take advantage of the fact the relative air under the cowl is not perfectly straight. However if the area is greater, the drag will be greater, negating the advantage from (small) RAM rise of MAP.Cheers George
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James - seeing your cowl at OSH is what sparked my interest in doing this.flyeyes said:That is a standard Rocket part, although there are at least two types because the one Mark Frederick sold me is flat across the top
http://www.rv8.ch/gallery/view_photo.php?set_albumName=Oshkosh05&id=DSC03266
Thanks for the hint on the Berkut site.
gmcjetpilot
Well Known Member
Not what I said
2. NASA studies show the area around the spinner is dirty, sorry no offense just fact. Now as for the actual difference between an induction scoop next to the spinner or 2 feet away, I can't say, but neither is producing huge boost like people think. However see #1 above, going 250 mph can't be all that bad.
3. Having the scoop all tight and close to the spinner may be a little less drag, due to wetted area? As far as shape (round, rectangle or smiley) I have no research or opinion, but the smiley looks nice. I also guessed that a wide mouth or horizontal aspect ratio or semi-circle if you will, is of help when you are close to the spinner since it is shaped like the profile of the spinner?
4. Don't be so sensitive; we are talking about little bits of pressure here. The fact is the claim of any scoop in any location giving a huge boost is not support with facts. To repeat you are doing good to get with in 1/3 to 1/2 in-hg of ambient. Boost (plus pressure above ambient) not easy to do; however if you are getting well above 200mph like a rocket you start to get in the pressure recovery area.
Enjoy your F-1. You Rocket guys are real proud of your planes (and should be). If your intake scoop is so efficient how about some flight test data Mr. f1rocket (see PS below).
George
PS Thought of a test, "T" into and hook up a second MAP gage to your static system (calibrate to prime engine MAP gage). The difference between the MAP on the engine and the pressure the second MAP gage connected to static pressure will show you the "boost" you are getting. If your engine MAP is with in -0.25 to -0.5 in-hg of the 2nd pressure gage you very good.
Another method with just one MAP gage, would be note MAP on the ground with engine off. Take off and fly a low, full throttle pass over the runway and compare MAP pressures. (This is a little risky, do it at your discretion).
The last way (and not without risk also) is fly to say 8,000' do a full throttle run and note MAP. Pull up and shut engine down and note MAP passing thru 8,000'. Again difference is your boost (loss).
The best possible system is a true ram air straight into the intake (fwd facing induction) with no filter, which if perfect may yield zero loss (zero boost). Filter loss if done well (like vans airbox) can be about only 0.25 in-hg. Van's airbox operates at about -0.3 in-hg. The benefit of a filter is it smoothes out the airflow. Non-filtered Ram air set-ups can produce turbulence and internal induction losses. Also fwd facing ram air set-ups lend themselves towards scoops close to the spinner. This is not a black and white subject and there is a lot to the subject. There is no free lunch a plus here may be a negative in another area. It all balances out.
However in the past there has been a little elaboration on the benefits of one design or induction scoop shape, when there is not a lot of difference. As I said the Van's air box is about as you can get with a filter. The best would be a straight shot into the inductions (fwd facing induction) with out a filter. So a Rocket/F-1 unfiltered RAM air induction, smiley intake next to the spinner or not, is probably good. Plus it likely has less frontal area lower external drag. SO any loss from crummy airflow around the spinner is compensated by the other benefits of the design, especially on a fast plane doing over 200mph. The only way to compare is flight test. Other builders don't care as long as it looks good. Which is also a fine criterion.
1. Well when you are doing 250mph it can't be all that bad.f1rocket said:Geez George, I didn't realize my Rocket had so many problems with the air intake!
2. NASA studies show the area around the spinner is dirty, sorry no offense just fact. Now as for the actual difference between an induction scoop next to the spinner or 2 feet away, I can't say, but neither is producing huge boost like people think. However see #1 above, going 250 mph can't be all that bad.
3. Having the scoop all tight and close to the spinner may be a little less drag, due to wetted area? As far as shape (round, rectangle or smiley) I have no research or opinion, but the smiley looks nice. I also guessed that a wide mouth or horizontal aspect ratio or semi-circle if you will, is of help when you are close to the spinner since it is shaped like the profile of the spinner?
4. Don't be so sensitive; we are talking about little bits of pressure here. The fact is the claim of any scoop in any location giving a huge boost is not support with facts. To repeat you are doing good to get with in 1/3 to 1/2 in-hg of ambient. Boost (plus pressure above ambient) not easy to do; however if you are getting well above 200mph like a rocket you start to get in the pressure recovery area.
Enjoy your F-1. You Rocket guys are real proud of your planes (and should be). If your intake scoop is so efficient how about some flight test data Mr. f1rocket (see PS below).
George
PS Thought of a test, "T" into and hook up a second MAP gage to your static system (calibrate to prime engine MAP gage). The difference between the MAP on the engine and the pressure the second MAP gage connected to static pressure will show you the "boost" you are getting. If your engine MAP is with in -0.25 to -0.5 in-hg of the 2nd pressure gage you very good.
Another method with just one MAP gage, would be note MAP on the ground with engine off. Take off and fly a low, full throttle pass over the runway and compare MAP pressures. (This is a little risky, do it at your discretion).
The last way (and not without risk also) is fly to say 8,000' do a full throttle run and note MAP. Pull up and shut engine down and note MAP passing thru 8,000'. Again difference is your boost (loss).
The best possible system is a true ram air straight into the intake (fwd facing induction) with no filter, which if perfect may yield zero loss (zero boost). Filter loss if done well (like vans airbox) can be about only 0.25 in-hg. Van's airbox operates at about -0.3 in-hg. The benefit of a filter is it smoothes out the airflow. Non-filtered Ram air set-ups can produce turbulence and internal induction losses. Also fwd facing ram air set-ups lend themselves towards scoops close to the spinner. This is not a black and white subject and there is a lot to the subject. There is no free lunch a plus here may be a negative in another area. It all balances out.
However in the past there has been a little elaboration on the benefits of one design or induction scoop shape, when there is not a lot of difference. As I said the Van's air box is about as you can get with a filter. The best would be a straight shot into the inductions (fwd facing induction) with out a filter. So a Rocket/F-1 unfiltered RAM air induction, smiley intake next to the spinner or not, is probably good. Plus it likely has less frontal area lower external drag. SO any loss from crummy airflow around the spinner is compensated by the other benefits of the design, especially on a fast plane doing over 200mph. The only way to compare is flight test. Other builders don't care as long as it looks good. Which is also a fine criterion.
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gmcjetpilot
Well Known Member
I don't care either
Dear Randy:
Thank you, I just took your comment:
Not that you care, but others might like to know Van's Aircraft did flight test in 1991 and reported the results in the RVator to show this. The flymore.com (jon johanson) RAM air induction looks like a F-1 cowl with the smiley scoop. However the scoop inlet is down from the spinner. They claim their smiley face intake for the RV's gives 2" inches of MAP boost. http://www.flymore.com.au/ I just don't believe it, but I must admit it looks awsome.
Cheers George
Dear Randy:
Thank you, I just took your comment:
and the word "problems" as may be your feelings where hurt. Glad to hear you don't care. Since your F-1 is a great plane with out question, it is moot, like I said, when you are going 250 mph. Regardless of model of aircraft RAM air boost is a little over-rated, that's all. As far as not caring about what your intake pressure recovery is, that is too bad since the data would help others. It was not a challenge that your set-up is better or not, jsut that it would be interesting to see. Hey I may be wong, may be the spinner effect is totally off set by all the other gains? I guess we will never know, at least with your plane, fly in good health.f1rocket said:Geez George, I didn't realize my Rocket had so many problems with the air intake!
Not that you care, but others might like to know Van's Aircraft did flight test in 1991 and reported the results in the RVator to show this. The flymore.com (jon johanson) RAM air induction looks like a F-1 cowl with the smiley scoop. However the scoop inlet is down from the spinner. They claim their smiley face intake for the RV's gives 2" inches of MAP boost. http://www.flymore.com.au/ I just don't believe it, but I must admit it looks awsome.
Cheers George
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We use CMARC CFA software to plot pressure distributions on our RV for cooling system studies. This indicates that the pressure across the whole chin area is essentially the same (very high) and is a good location for maximum ram recovery.
Flight tests by one of the Mustang II owners who made major gains in speed strictly through aerodynamic cleanups and mods involving exhaust augmentation and ram tuning owners showed that a ram tube with it's inlet less than 2 inches from the prop was the best.
Induction air on the P51 is taken from the base of the spinner as well because this is typically one on the highest pressure regions on most aircraft.
At 250 mph at SL, you may recover as much as much as 1.5 inches of additional MP with a good intake design.
Flight tests by one of the Mustang II owners who made major gains in speed strictly through aerodynamic cleanups and mods involving exhaust augmentation and ram tuning owners showed that a ram tube with it's inlet less than 2 inches from the prop was the best.
Induction air on the P51 is taken from the base of the spinner as well because this is typically one on the highest pressure regions on most aircraft.
At 250 mph at SL, you may recover as much as much as 1.5 inches of additional MP with a good intake design.
gmcjetpilot
Well Known Member
Software?
Does it take into effect the prop and the blade hubs? I consultant to Pratt and Whitney, I did not do CFD (computational fluid dynamics) but understand it enough to know the results of 2D free air stream calculations are complicated, much less 3D fluid dynamics. IF you have that computational horse power to do 3D CFD, have taken the time to model it (100's of hours?), great, but it does not match the flight test data.
My gut feel and knowledge from flight test is those hubs are just blunt non-airfoils and affect the area around it to a great degree. The hub (stubs) pass right in front of the scoop. If there is great pressure there, the prop and spinner is not interfering, it is easy to prove with out a computer, by just using a few pressure taps and tubes. I guess I am saying I don?t like computer data if I can get flight test info easily. I am not flying yet and I have a vertical inductions system (Carb with Van?s Airbox), so I am unable to do it, nor do I have the motivation.
Flight test done by Van during the first RV-6 prototype, remember the one with just one annular chin scoop for cooling and intake, showed this was not a good location or effective for pressure recovery. NASA's flight test and study of cooling of horizontally opposed aircraft engine installations showed a pressure gradient that favored intake (cooling or induction) away from the hub. Flight test, tuft test with a stock RV cowl showed air going OUT the inboard part of the cowl inlets (near the spinner). This shows a negative pressure gradient or at least much less positive pressure than further outboard (away from the prop hub).
As far as 200mph and 250mph, you have almost 60% more dynamic pressure at 250 mph, so you do have more to recover. But what does that matter. To get 1.5"-hg at 250mph you are doing getting a plus 60% pressure recovery. 1.5 in-hg is something a turbo charger has a hard time doing. Again numbers get thrown around like P-51, 250mph, 1.5 in-hg, which mean nothing unless you take it in the context of something. The P-51 had a HUGE supercharger on it, so what pressure you are talking about and how is it measured is more important than just a number floating around like 1.5 in-hg. That is the exact thing I am talking about. From experience it is almost impossible to even achieve NET pressure at 200 mph much less a gain. That is the experience of Van's aircraft and NASA. If you have a some trick set-up let us all know.
Right below the spinner is little or no thrust. If the P-51 has its induction there, it is a good place for many reasons, including mechanical and fuselage drag. When talking about an almost trans-sonic WWII prop fighter, those designs parameters may not translate to a sub sub sonic plane. Alo look at a P-51 close. The spinner is HUGE, The prop is an airfoil shape right up to the hub and the chin scoop is really in an area where the prop is a full airfoil, not a blunt club like a Hartzell is on a general aviation aircraft. I just don't by that the area around the prop hub on a RV is good. Why do you think the Sam James, Lopresti cowls place the intakes as far outboard as possible? The round shape is not a secret, only that it is a good shape to move the required area as far out as possible while staying inside the cowl profile. This is all old news from an aero stand point, but may be I am wrong. Again it would be easy to prove on the plane.
Cheers George
Does it take into effect the prop and the blade hubs? I consultant to Pratt and Whitney, I did not do CFD (computational fluid dynamics) but understand it enough to know the results of 2D free air stream calculations are complicated, much less 3D fluid dynamics. IF you have that computational horse power to do 3D CFD, have taken the time to model it (100's of hours?), great, but it does not match the flight test data.
My gut feel and knowledge from flight test is those hubs are just blunt non-airfoils and affect the area around it to a great degree. The hub (stubs) pass right in front of the scoop. If there is great pressure there, the prop and spinner is not interfering, it is easy to prove with out a computer, by just using a few pressure taps and tubes. I guess I am saying I don?t like computer data if I can get flight test info easily. I am not flying yet and I have a vertical inductions system (Carb with Van?s Airbox), so I am unable to do it, nor do I have the motivation.
Flight test done by Van during the first RV-6 prototype, remember the one with just one annular chin scoop for cooling and intake, showed this was not a good location or effective for pressure recovery. NASA's flight test and study of cooling of horizontally opposed aircraft engine installations showed a pressure gradient that favored intake (cooling or induction) away from the hub. Flight test, tuft test with a stock RV cowl showed air going OUT the inboard part of the cowl inlets (near the spinner). This shows a negative pressure gradient or at least much less positive pressure than further outboard (away from the prop hub).
As far as 200mph and 250mph, you have almost 60% more dynamic pressure at 250 mph, so you do have more to recover. But what does that matter. To get 1.5"-hg at 250mph you are doing getting a plus 60% pressure recovery. 1.5 in-hg is something a turbo charger has a hard time doing. Again numbers get thrown around like P-51, 250mph, 1.5 in-hg, which mean nothing unless you take it in the context of something. The P-51 had a HUGE supercharger on it, so what pressure you are talking about and how is it measured is more important than just a number floating around like 1.5 in-hg. That is the exact thing I am talking about. From experience it is almost impossible to even achieve NET pressure at 200 mph much less a gain. That is the experience of Van's aircraft and NASA. If you have a some trick set-up let us all know.
Right below the spinner is little or no thrust. If the P-51 has its induction there, it is a good place for many reasons, including mechanical and fuselage drag. When talking about an almost trans-sonic WWII prop fighter, those designs parameters may not translate to a sub sub sonic plane. Alo look at a P-51 close. The spinner is HUGE, The prop is an airfoil shape right up to the hub and the chin scoop is really in an area where the prop is a full airfoil, not a blunt club like a Hartzell is on a general aviation aircraft. I just don't by that the area around the prop hub on a RV is good. Why do you think the Sam James, Lopresti cowls place the intakes as far outboard as possible? The round shape is not a secret, only that it is a good shape to move the required area as far out as possible while staying inside the cowl profile. This is all old news from an aero stand point, but may be I am wrong. Again it would be easy to prove on the plane.
Cheers George
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As there are many variables with propeller proximity to the inlet entrance and the propeller design itself, I agree that flight testing a particular combination is probably better than CFA.
In our extensive flight testing of many different configurations and positions of air scoops for induction, radiator and intercooler inlets, little if any pressure difference has been noted on any scoop facing directly into the relative wind. In other words, it mattered little where it is placed if it is a ram type scoop.
It does make a BIG difference where it is placed for a NACA type or flush inlet or an outlet.
The calculated maximum ram recovery at SL, 250 mph is actually 2.27 inches hg with a perfect duct, not counting any benefit from the prop. It has not been uncommon in our pressure testing to see ram recoveries of over 95%.
I'd agree that 2 inches hg MAP on a 200 mph RV is unlikely unless some major benefit from the prop is present. Max ram recovery in theory would be about 1.45 inches hg.
As an interesting sidenote, I saw about 1.5 inches of H2O pressure on a full throttle ground runup in our induction inlet which is in a similar location to the Rocket one. This represents only about 1/10 of an inch of hg.
In our extensive flight testing of many different configurations and positions of air scoops for induction, radiator and intercooler inlets, little if any pressure difference has been noted on any scoop facing directly into the relative wind. In other words, it mattered little where it is placed if it is a ram type scoop.
It does make a BIG difference where it is placed for a NACA type or flush inlet or an outlet.
The calculated maximum ram recovery at SL, 250 mph is actually 2.27 inches hg with a perfect duct, not counting any benefit from the prop. It has not been uncommon in our pressure testing to see ram recoveries of over 95%.
I'd agree that 2 inches hg MAP on a 200 mph RV is unlikely unless some major benefit from the prop is present. Max ram recovery in theory would be about 1.45 inches hg.
As an interesting sidenote, I saw about 1.5 inches of H2O pressure on a full throttle ground runup in our induction inlet which is in a similar location to the Rocket one. This represents only about 1/10 of an inch of hg.
gmcjetpilot
Well Known Member
don't know how but OK
Like I said I am looking at a pressure plot from the NASA study of prop thrust right now and the Cp is 0.8 at the hub (edge of spinner) and about 2/3 span the Cp is 1.6. If the prop blast is boosting you engines manifold pressure with an annular chin scoop or any scoop for that matter that is incredible, unless your engine is super charged or turbo charged. The pressure off the prop goes positive just about 2/3rds the way out to the edge of the cowl on a RV, thus the round ring inlets.
Also for annular inlets, look at this (click to enlarge)
The spinner is the size of a whole RV fuselage and no doubt the blade is airfoil shape at or near the edge of the spinner. I think this is the ideal aircooled piston plane inlet, and its all annular.
Also a jet engine has to have a spinner that can take a bird strike. With out the spinner a jet engine would stop to operate due to turbulent flow. The aerodynamic guys will not allow the tolerance of a spinner to be out of spec even a little. Some "grow" with time and during overhaul they are tossed because they a few thousands out of profile! So airflow around a spinner is powerful stuff.
Cheers George
PS for the record Randy and now "Hard" Knox do not care, and thinks it "looks cool". thanks for letting us know.
If you are getting 0.11 in-hg above the ambient pressure, for example ATIS says 29.92 and you are seeing 30.03"-hg on the MAP gage engine running WOT on the ground, that is great. Got some pictures?rv6ejguy said:
Like I said I am looking at a pressure plot from the NASA study of prop thrust right now and the Cp is 0.8 at the hub (edge of spinner) and about 2/3 span the Cp is 1.6. If the prop blast is boosting you engines manifold pressure with an annular chin scoop or any scoop for that matter that is incredible, unless your engine is super charged or turbo charged. The pressure off the prop goes positive just about 2/3rds the way out to the edge of the cowl on a RV, thus the round ring inlets.
Also for annular inlets, look at this (click to enlarge)
The spinner is the size of a whole RV fuselage and no doubt the blade is airfoil shape at or near the edge of the spinner. I think this is the ideal aircooled piston plane inlet, and its all annular.
Also a jet engine has to have a spinner that can take a bird strike. With out the spinner a jet engine would stop to operate due to turbulent flow. The aerodynamic guys will not allow the tolerance of a spinner to be out of spec even a little. Some "grow" with time and during overhaul they are tossed because they a few thousands out of profile! So airflow around a spinner is powerful stuff.
Cheers George
PS for the record Randy and now "Hard" Knox do not care, and thinks it "looks cool". thanks for letting us know.
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We use Magnehelic sensitive pressure gauges for our studies which are calibrated in inches of water. For in-duct studies we use a multi faced, stopped probe to average the pressure at the gauge and cancel static effects. For dynamic pressure studies, we generally face the gauge tube straight into the wind so to speak.
Some of the Eggenfellner guys are using these gauges now for rad and cowling pressure measurements to improve cooling and reduce drag. They are far superior to liquid filled U tube manometers which I found too hard to read in rough air while flight testing. They are available from Davis Inotek Instruments in a variety of calibrations.
Some of the Eggenfellner guys are using these gauges now for rad and cowling pressure measurements to improve cooling and reduce drag. They are far superior to liquid filled U tube manometers which I found too hard to read in rough air while flight testing. They are available from Davis Inotek Instruments in a variety of calibrations.
Many different concepts are being floated on this subject now.
Drag is the big reason why round inlets, some ways away from the spinner have been shown to be better than something like the conventional inlets on an RV. There is a reason why we see this configuration on Greenameyer's Lancair. He ain't no dummy. Also look at the air exits around the exhausts and the additional cowling air exits on top of the cowling for maximum extraction. CMARC would predict these as being ideally located for their purposes.
Inlet duct pressure recovery also shows that round shapes may be slightly more effective than other shapes by reducing the possibility of forming votices in the diffuser area, also reducing internal drag of the inlet which is often overlooked.
More studies have been done if F1 car racing wind tunnels on this subject than in aircraft. The shape of the inlet and diffuser/ airbox is important to extract everything possible. It is important to realise that engines, especially 4 cylinder ones, do not intake their airflow in an even manner but rather in pulses. This start/stop flow has a marked effect on dynamics and also permits possibilities of harnessing the energy in these pulses to increase ram effect. This is also used in many racing engines and even many modern street auto engines to boost VE.
There is much more than meets the eye on the exterior as to performance here. The Aussy guys have probably flight tested their ducts and quantified their performance as they state. The proximity of the duct to the prop and being some ways from the centerline may make it possible to see 2 inches hg at 210 mph.
I would not discount this claim based on conjecture. I guess the bottom line is really a speed test to see how much faster the plane was before and after. If 2 inches were gained but the scoop offered more drag in return, the excercise might be a waste of time except for the asthetics.
Nothing like actual flight testing to prove or disprove things like this.
Annular inlets are likely very efficient. The boundary layer is very thin here, the pressure is high and the velocity is high as well. If you look at Rare Bear and the total inlet area vs, hp, this is quite impressive. The FW 190 D9 with the inline DB engine used an annular radiator setup, this was likely a better solution than some of the boundary layer sucker scoops used on the Spitfire and BF 109.
As far as turbocharger or supercharger inlets go, first rule, these must be large enough to pass the mass flow required at maximum power without restriction, however this is not that difficult to do as engine internal mass flow is relatively low in airplane terms of total mass flow around it. A 3500hp race Merlin would require something like 5300 SCFM to feed the supercharger. A duct with a inlet area area of only about 22 square inches would free stream this much air at 400 mph. The compressor will suck in what is needed until duct velocities start to reach the mach .8 range or so however we'd like to reduce the pumping work required of the compresor by ramming it in so restriction is zero or even negative.
Single stage compressors are capable these days of pressure ratio of over 4 to 1 which equates to 120 inches hg at SL. The Merlin's two stage superchargers are run in excess of 145 inches at Reno's elevation on some air craft. 1.5 inches of water here is not even noticeable! Spark plugs are optional at this manifold pressure. ADI is not!
Drag is the big reason why round inlets, some ways away from the spinner have been shown to be better than something like the conventional inlets on an RV. There is a reason why we see this configuration on Greenameyer's Lancair. He ain't no dummy. Also look at the air exits around the exhausts and the additional cowling air exits on top of the cowling for maximum extraction. CMARC would predict these as being ideally located for their purposes.
Inlet duct pressure recovery also shows that round shapes may be slightly more effective than other shapes by reducing the possibility of forming votices in the diffuser area, also reducing internal drag of the inlet which is often overlooked.
More studies have been done if F1 car racing wind tunnels on this subject than in aircraft. The shape of the inlet and diffuser/ airbox is important to extract everything possible. It is important to realise that engines, especially 4 cylinder ones, do not intake their airflow in an even manner but rather in pulses. This start/stop flow has a marked effect on dynamics and also permits possibilities of harnessing the energy in these pulses to increase ram effect. This is also used in many racing engines and even many modern street auto engines to boost VE.
There is much more than meets the eye on the exterior as to performance here. The Aussy guys have probably flight tested their ducts and quantified their performance as they state. The proximity of the duct to the prop and being some ways from the centerline may make it possible to see 2 inches hg at 210 mph.
I would not discount this claim based on conjecture. I guess the bottom line is really a speed test to see how much faster the plane was before and after. If 2 inches were gained but the scoop offered more drag in return, the excercise might be a waste of time except for the asthetics.
Nothing like actual flight testing to prove or disprove things like this.
Annular inlets are likely very efficient. The boundary layer is very thin here, the pressure is high and the velocity is high as well. If you look at Rare Bear and the total inlet area vs, hp, this is quite impressive. The FW 190 D9 with the inline DB engine used an annular radiator setup, this was likely a better solution than some of the boundary layer sucker scoops used on the Spitfire and BF 109.
As far as turbocharger or supercharger inlets go, first rule, these must be large enough to pass the mass flow required at maximum power without restriction, however this is not that difficult to do as engine internal mass flow is relatively low in airplane terms of total mass flow around it. A 3500hp race Merlin would require something like 5300 SCFM to feed the supercharger. A duct with a inlet area area of only about 22 square inches would free stream this much air at 400 mph. The compressor will suck in what is needed until duct velocities start to reach the mach .8 range or so however we'd like to reduce the pumping work required of the compresor by ramming it in so restriction is zero or even negative.
Single stage compressors are capable these days of pressure ratio of over 4 to 1 which equates to 120 inches hg at SL. The Merlin's two stage superchargers are run in excess of 145 inches at Reno's elevation on some air craft. 1.5 inches of water here is not even noticeable! Spark plugs are optional at this manifold pressure. ADI is not!
gmcjetpilot
Well Known Member
Flymore for scoop? Gage how to use?
http://www.flymore.com.au/
OR check out team rocket
http://www.teamrocketaircraft.com/
They may sell you a scoop
Gage? http://www.davis.com/showpage.asp?L3ID=828
I guess this the gage? a differential mechanical gage. Not sure how you are using it. However it is a differential gage, so not sure how you are comparing this to total pressure (ambient air)? So you connect one side to aircraft static. I mentioned when trying to get good data, a good static reference is hard to get. Small changes in static throw your data off. In fact the prop wash over your static ports or cockpit pressure could cause a .11in-hg drop in pressure, which would show a gain in relative pressure. So if you are talking about delta pressure from the ambient pressure and that of the induction, how are you getting the static reference? Your work sounds interesting. Keep it up. As far as getting a RAM rise from the prop alone, what prop? What scoop and position of scoop? Pictures
May be some day when you have time you guys can publish a little data on your findings, I would like to see that.
Thanks George
Try this locationrv8ch said:
http://www.flymore.com.au/
OR check out team rocket
http://www.teamrocketaircraft.com/
They may sell you a scoop
COOL stuff, keep up the good flight test stuff.rv6ejguy said:
Gage? http://www.davis.com/showpage.asp?L3ID=828
I guess this the gage? a differential mechanical gage. Not sure how you are using it. However it is a differential gage, so not sure how you are comparing this to total pressure (ambient air)? So you connect one side to aircraft static. I mentioned when trying to get good data, a good static reference is hard to get. Small changes in static throw your data off. In fact the prop wash over your static ports or cockpit pressure could cause a .11in-hg drop in pressure, which would show a gain in relative pressure. So if you are talking about delta pressure from the ambient pressure and that of the induction, how are you getting the static reference? Your work sounds interesting. Keep it up. As far as getting a RAM rise from the prop alone, what prop? What scoop and position of scoop? Pictures
May be some day when you have time you guys can publish a little data on your findings, I would like to see that.
Thanks George
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This flight testing different configurations is very interesting however has the drawback that it is never done. As you test new things, it simply creates more questions that HAVE to be answered. The term obsession does come to mind. You should see my box of "20 minute" (that's how long it takes to build them) sheet metal scoops that have been used to test things. Tape 'em on and fly 'em. I have quite a pile of discarded heat exchanges as well.
I use 2 gauges (gages) on a panel. Can measure pressure or vacuum with twin ports on each and we can use one for a static reference. Also have 4 digital temperature gauges with remote probes to measure inlet/ outlet temps in various places behind heat exchangers.
The tower here is always asking what I'm up to today when they see this stuff taped all over the RV. "How's that intercooler working today?" or something like that. "Test 1 ready to roll 34". Just kidding, but I've logged a lot of short flights just to document some pressure or temperature readings.
My background includes 15 years of running a flow bench (which I built from scratch) so I'm very familiar with pressure measurement and references plus the scientific method.
I could tell you a lot of stuff about NACA ducts for instance but I'd have to shoot you as they say.
We keep the good stuff secret!
I have some of the initial testing documented on our site but there is much more that is not up there. I'd rather be flying than updating everything on the website. To much data, not enough time: http://www.sdsefi.com/rv12.htm
Next project is a new boundary layer duct for the belly intake after more testing verifies the benefits or not.
I don't pretend I'm an expert on this stuff but we are actually flight testing ideas to be applied to our next projects. I've learned an immense amount on this subject but I'm pretty sure people like DG know a LOT more than I do at this point. I'm heading to Reno this year again. Can't get enough of that stuff.
I use 2 gauges (gages) on a panel. Can measure pressure or vacuum with twin ports on each and we can use one for a static reference. Also have 4 digital temperature gauges with remote probes to measure inlet/ outlet temps in various places behind heat exchangers.
The tower here is always asking what I'm up to today when they see this stuff taped all over the RV. "How's that intercooler working today?" or something like that. "Test 1 ready to roll 34". Just kidding, but I've logged a lot of short flights just to document some pressure or temperature readings.
My background includes 15 years of running a flow bench (which I built from scratch) so I'm very familiar with pressure measurement and references plus the scientific method.
I could tell you a lot of stuff about NACA ducts for instance but I'd have to shoot you as they say.
We keep the good stuff secret!
I have some of the initial testing documented on our site but there is much more that is not up there. I'd rather be flying than updating everything on the website. To much data, not enough time: http://www.sdsefi.com/rv12.htm
Next project is a new boundary layer duct for the belly intake after more testing verifies the benefits or not.
I don't pretend I'm an expert on this stuff but we are actually flight testing ideas to be applied to our next projects. I've learned an immense amount on this subject but I'm pretty sure people like DG know a LOT more than I do at this point. I'm heading to Reno this year again. Can't get enough of that stuff.
For those interested in the maximum possible RAM air gain here is a plot, red line is for knots, blue is for mph.
A couple of easy points to remember
200knots = 2 inHg
160mph = 1inHg
Dan Checkoway did a bunch of tests with a RAM air system and was pretty methodical. He reported that best case he had a .4inHg delta in his RAM air over his filtered air. Granted that is an improvement it doesn't take into to account the drag increase. In the end it looks like a wash without going to extremes like synching the prop pulse with the intake valve opening or tuning intake lengths for pressure increases.
Chuck
A couple of easy points to remember
200knots = 2 inHg
160mph = 1inHg
Dan Checkoway did a bunch of tests with a RAM air system and was pretty methodical. He reported that best case he had a .4inHg delta in his RAM air over his filtered air. Granted that is an improvement it doesn't take into to account the drag increase. In the end it looks like a wash without going to extremes like synching the prop pulse with the intake valve opening or tuning intake lengths for pressure increases.
Chuck
gmcjetpilot
Well Known Member
I see, cool scoops
P.S: From you site I see why you get some RAM pressure from prop thrust near the spinner. It looks like you have an electric IVO prop which has airfoil shape right up to the spinner. The Typical Hartzel or any of the other hydraulic props have the BLUNT blade shanks near the hub as I mentioned. You mentioned the P-51 has a annular scoop under the spinner; I looked at the web and the P-51's Big Ham-Std paddles have an airfoil shape right up to the huge spinner. So the insight I get from this is the dirty airflow that flight test show near the spinner is pimarily a function of prop balde design near the hub. The NASA study I ref. was on a Piper Aztec with a typical 2-blade Hartzel you would find on a RV. G
Again super work, lots of neat ideas and the cojones and the ability, skill to build it and fly it.rv6ejguy said:I don't pretend I'm an expert on this stuff but we are actually flight testing ideas to be applied to our next projects. I've learned an immense amount on this subject but I'm pretty sure people like DG know a LOT more than I do at this point. I'm heading to Reno this year again. Can't get enough of that stuff.
P.S: From you site I see why you get some RAM pressure from prop thrust near the spinner. It looks like you have an electric IVO prop which has airfoil shape right up to the spinner. The Typical Hartzel or any of the other hydraulic props have the BLUNT blade shanks near the hub as I mentioned. You mentioned the P-51 has a annular scoop under the spinner; I looked at the web and the P-51's Big Ham-Std paddles have an airfoil shape right up to the huge spinner. So the insight I get from this is the dirty airflow that flight test show near the spinner is pimarily a function of prop balde design near the hub. The NASA study I ref. was on a Piper Aztec with a typical 2-blade Hartzel you would find on a RV. G
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gmcjetpilot
Well Known Member
Filter and press drop
From test Van did found the filter cost about .25-.30 in-hg drop by itself. (Actually a factory filtered design where it is bolted to the face of the cowl, like a Cessna, can cause 1-1.5 in-hg drop.) Van also found with a Carb and vertical induction the filter airbox had benefits in improving fuel distribution between the cylinders, thought to be from smoothing the airflow into Carb.
For the Fwd Fuel injection induction the non-filtered RAM air has an advantage over filtered no doubt. Dan's finding 0.4 in-hg sounds very reasonable. From that I could conclude most of the benefit he got was from bypassing the filter. However I still doubt that even RAM (fwd induction with out a filter) is able to recover much of the dynamic pressure with the current crop of concepts (straight tube going right into the induction). The fwd induction has advantages but leaves little room from the fwd induction to make a good duct, to slow the air down.
What Dan did was just take a straight tube right into the injector. Simple, works but not much in pressure recovery. As was mentioned, 4 cylinder engines breathe in pulses. With a short straight constant area induction of little volume you have a lot of air spilling off the intake (external drag). An intake that can act as a reservoir and diffuser (convert velocity into pressure like Vans airbox do) is the way to get the energy out of the dynamic airflow going around the airplane. That is why I find claims of 2" hg-in from a scoop to be hard to swallow, unless there is a duct system designed to go with it. As was mentioned this part of the design is the part neglected. When most are only getting net pressure (no loss no gain) even without a filter, I think there is some work to do here. One thing needs to be done, a standard way of reporting the pressure in the intake (induction scoop). Reporting delta gains is useful but MAP gages are not accurate.
Like I mentioned (Van's idea actually) a few ways to determine the RAM rise is fly fast down the runway wide open, note MAP , land, shut down and note MAP. The other way was do the same thing at 8000', but shutting down the engine at 8,000 after a high power run inflight is a little too sporty for me. I like the idea RV6ejguy has, using a delta P gage. I would hook one side into the aircraft static system and run a pitot tube into the induction system right at the Carb or FI throttle body inlet. G
Good stuff Chuck. What would be interesting is what is the absolute pressure compared to the static ambient pressure or actual boost. I think this is where some people get the mis-conception that RAM air gives positive manifold pressure above ambient when they hear a 0.4 in-hg boost from filtered to non-filtered RAM.chuck said:
From test Van did found the filter cost about .25-.30 in-hg drop by itself. (Actually a factory filtered design where it is bolted to the face of the cowl, like a Cessna, can cause 1-1.5 in-hg drop.) Van also found with a Carb and vertical induction the filter airbox had benefits in improving fuel distribution between the cylinders, thought to be from smoothing the airflow into Carb.
For the Fwd Fuel injection induction the non-filtered RAM air has an advantage over filtered no doubt. Dan's finding 0.4 in-hg sounds very reasonable. From that I could conclude most of the benefit he got was from bypassing the filter. However I still doubt that even RAM (fwd induction with out a filter) is able to recover much of the dynamic pressure with the current crop of concepts (straight tube going right into the induction). The fwd induction has advantages but leaves little room from the fwd induction to make a good duct, to slow the air down.
What Dan did was just take a straight tube right into the injector. Simple, works but not much in pressure recovery. As was mentioned, 4 cylinder engines breathe in pulses. With a short straight constant area induction of little volume you have a lot of air spilling off the intake (external drag). An intake that can act as a reservoir and diffuser (convert velocity into pressure like Vans airbox do) is the way to get the energy out of the dynamic airflow going around the airplane. That is why I find claims of 2" hg-in from a scoop to be hard to swallow, unless there is a duct system designed to go with it. As was mentioned this part of the design is the part neglected. When most are only getting net pressure (no loss no gain) even without a filter, I think there is some work to do here. One thing needs to be done, a standard way of reporting the pressure in the intake (induction scoop). Reporting delta gains is useful but MAP gages are not accurate.
Like I mentioned (Van's idea actually) a few ways to determine the RAM rise is fly fast down the runway wide open, note MAP , land, shut down and note MAP. The other way was do the same thing at 8000', but shutting down the engine at 8,000 after a high power run inflight is a little too sporty for me. I like the idea RV6ejguy has, using a delta P gage. I would hook one side into the aircraft static system and run a pitot tube into the induction system right at the Carb or FI throttle body inlet. G
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osxuser
Well Known Member
Speaking of props, hartzell makes a neat one for the Piper Comanche series. Here is a terrible picture of one:
See it's got a wide chord in a little area near the shank that will increase the pressure into the intake (in theory). Also I believe McCauley props have a bit better blade design near the spinner than Hartzell. But I don't pretend to know a whole lot about this stuff.
See it's got a wide chord in a little area near the shank that will increase the pressure into the intake (in theory). Also I believe McCauley props have a bit better blade design near the spinner than Hartzell. But I don't pretend to know a whole lot about this stuff.
rv8ch said:Don't tell us about it, just start selling it!
All I can say about NACA ducts is that they are not the thing to use for high mass flow and good pressure recovery. They have their place for other uses however (cabin ventilation without bug ingestion is a good one). Placement in a high pressure zone and with positive AOA is critical for best performance.
How close are you to flying yours?
NACA vents
What do you think of the NACA vent (in reverse) on the bottom of the cowl to let air out?
(click image for larger version)
Still got a few engine parts to bolt on, canopy skirt, windscreen, and cowl. That could be 3 months or 3 years! I hope it's 3 months.
What do you think of the NACA vent (in reverse) on the bottom of the cowl to let air out?
(click image for larger version)
3 months sounds good. We'll hold you to it!
I've never tested NACA ducts for exits so I wouldn't want to speculate. I have seen a couple modern F1 cars with them for this purpose. With their multi million $ a year wind tunnel budgets, I have to assume that they work or they wouldn't be on there.
I've never tested NACA ducts for exits so I wouldn't want to speculate. I have seen a couple modern F1 cars with them for this purpose. With their multi million $ a year wind tunnel budgets, I have to assume that they work or they wouldn't be on there.
rickrv8
Well Known Member
Modified F-1 Scoop on RV-8
Mickey,
Below is a photo of my RV-8 and the modified F-1 scoop I used. I have an IO360-A3B6D engine. It was a significant amount of work but I am quite happy with the results. My MPs all match those on Lycomings power charts. If you're interested, I'd be more than happy to describe how I made the modification.
Rick McBride
Mickey,
Below is a photo of my RV-8 and the modified F-1 scoop I used. I have an IO360-A3B6D engine. It was a significant amount of work but I am quite happy with the results. My MPs all match those on Lycomings power charts. If you're interested, I'd be more than happy to describe how I made the modification.
Rick McBride
NACA ducts as engine inlets?
When I was at college we had the chief engineer of a local (military) aircraft company as a visiting professor. For our design project (small light jet trainer/personal aircraft) we suggested using NACA ducts as intakes. He was very scathing. His view was that to get enough air into the engine a intake that stuck out into the airflow was essential. I got the impression that NACA dusts we good for relatively constant flow uses. As our engines are not constant flow, due to changes in throttle position, etc, I would have thought they are not that appropriate. I don't know about exits, but I know that exit area is critical to good cooling. Will a NACA duct give you enough exit area? How much drag reduction will you get? If you cannot use full throttle 'coz the engine is running too hot then you lose more than you gain. Sorry, I don't know any answers here.
Seems that the most efficient shape is a round inlet as far from the spinner as possible. However, as several have noted, the difference between best and not so good might not be much (0.5 in at best). My take would be that aesthetics would then be quite important - so smiley intakes under the spinner get my vote! The was a guy at Osh who had such an intake on his G202. I took a couple of photos - his filter was a one of a kind bolted into the cowl just behind the inlet (but no bypass for a clogged filter) - the way he arranged it might be of interest. The hole further back in the cowl is an oil cooler intake.
Pete
When I was at college we had the chief engineer of a local (military) aircraft company as a visiting professor. For our design project (small light jet trainer/personal aircraft) we suggested using NACA ducts as intakes. He was very scathing. His view was that to get enough air into the engine a intake that stuck out into the airflow was essential. I got the impression that NACA dusts we good for relatively constant flow uses. As our engines are not constant flow, due to changes in throttle position, etc, I would have thought they are not that appropriate. I don't know about exits, but I know that exit area is critical to good cooling. Will a NACA duct give you enough exit area? How much drag reduction will you get? If you cannot use full throttle 'coz the engine is running too hot then you lose more than you gain. Sorry, I don't know any answers here.
Seems that the most efficient shape is a round inlet as far from the spinner as possible. However, as several have noted, the difference between best and not so good might not be much (0.5 in at best). My take would be that aesthetics would then be quite important - so smiley intakes under the spinner get my vote! The was a guy at Osh who had such an intake on his G202. I took a couple of photos - his filter was a one of a kind bolted into the cowl just behind the inlet (but no bypass for a clogged filter) - the way he arranged it might be of interest. The hole further back in the cowl is an oil cooler intake.
Pete
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NACA ducts have certainly been used for engine cooling and induction inlets. The Lancair IV turbine, Legend turbine etc. use them to good effect. Some Canard aircraft are flying successfully with aft belly mounted NACA ducts. Note the size of these however. You need some pretty large ones for these high mass flow requirements.
It would take some serious wind tunnel or flight testing to really determine if NACA ducts are more efficient than boundary layer ram scoops in reducing drag. However on RV type aircraft, the short cowlings and engine placement would not lend themselves well to NACA duct cooling in my opinion where some serious length is required for best flow.
My thinking is that round inlets placed in the conventional locations, allowing the boundary layer air to bypass the area between the spinner base and the inlet opening are the best compromise for engine cooling.
The oil cooler duct pictured above would be more effective with a 1/2 to 3/4 boundary layer bypass standoff than sitting flush as it is here. Although harder to build, it would obtain higher pressure recovery and exhibit less drag almost certainly.
For most people, using the stock RV cowling inlets works. Reducing drag in cruise might be easier by installing a cowl flap on the belly exit to reduce mass flow and internal drag if your engine temps are on the low side in level flight.
It would take some serious wind tunnel or flight testing to really determine if NACA ducts are more efficient than boundary layer ram scoops in reducing drag. However on RV type aircraft, the short cowlings and engine placement would not lend themselves well to NACA duct cooling in my opinion where some serious length is required for best flow.
My thinking is that round inlets placed in the conventional locations, allowing the boundary layer air to bypass the area between the spinner base and the inlet opening are the best compromise for engine cooling.
The oil cooler duct pictured above would be more effective with a 1/2 to 3/4 boundary layer bypass standoff than sitting flush as it is here. Although harder to build, it would obtain higher pressure recovery and exhibit less drag almost certainly.
For most people, using the stock RV cowling inlets works. Reducing drag in cruise might be easier by installing a cowl flap on the belly exit to reduce mass flow and internal drag if your engine temps are on the low side in level flight.
