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Bob Axsom

Well Known Member
I like to be thinking ahead a little bit so while I'm nearing the end of the cooling drag experiments I'm starting to think about the induction air.

I have read about extended inlets that reach out near the trailing edge of the prop ( Roy LoPresti, two friends with competing RV-6s in an RVator several years ago, an RV-4 at LOE a couple of years ago with a cylindrical extension - painted like a gun barrel, etc.) that claim manifold pressure gains and more power/speed.

Cris Ferguson told me about a NACA development that placed the air intake imediately below the spinner as used on the P-51 and his 300 mph biplane design.

I personally wonder about an angular face in the inlet that favors the right hand prop rotation. If the head on view sees a circular opening, but the view from the right side would be an oval shaped opening and the view from the left side would show no opening at all, could the air mass be increased with minimal drag increase?

Thoughts?

Bob Axsom
 
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Thought about it, did not do that

Bob the extension of the induction was discussed in Speed with Economy, by Kent Paser. The Mustang II cowl is similar to our RV cowl with the induction scoop coming off the lower portion of the cowl (typical of our FAB updraft induction). I recall that there was no speed increase or MAP increase and it looked frankly kind of odd. His solution was 10 degrees into the advancing prop blade, 11 degree diverging interior wall of the long scoop, no more than 5/8" from prop blade and slight bell mouth with an inlet area 10% (larger) than area of the carb throat. He DID NOT get a MAP increase or top speed bump but felt he got more climb? I guess it got a little more power at the slow speed of the climb. The effect of prop blast I guess was more predominate at slow speed. Going fast, the prop blast effect I guess is nullified. Also the relative angle of air hitting the scoop is different in climb than cruise. (Of course the holy grail is variable cowl inlet and intake geometry but complication v benefit is probably not practical.)

Kent's other insight was an absolute seal between the scoop and air box is needed. Per Van's plans we use a soft baffle seal to bridge the gap between the cowl scoop and air box. Not super air tight. Kent found a removable door under the cowl was needed to conveniently make & break the duct connection. I'm adding a access door under the cowl to get at the induction coupler (clamp) between the scoop and air box. Van has suggested this in old RVators. The stock set up is OK but leaks. My comment or opinion is forget the long snorkel scoop but make sure the seal is tight.


The "P-51 Scoop" under the spinner is bad news for one reason, the air really sucks there. WE ARE NOT A P-51. Look real hard at the spinner, prop, scoop on a late model P-51. The spinner is almost as big as our whole fuselage and the blades are aerodynamic airfoil shapes right up to the spinner. Now look at your Hartzell. What do you have. The prop blade roots adjacent to the spinner have the aerodynamics of a baseball bat. Fixed props are not much better being almost like a 2-by-4 near the spinner.

We could add a BIG HUGE P-51 spinner. That would do wonders for air flow and hide the prop blade hubs. While we are at it, we should extend the prop out about 12 inches to give a real gradual transition for the cowls scoops and inlets. Unfortunately neither idea is practical, so we are stuck with fairly blunt blade parts sticking out in the breeze and short transitions to the cowl inlets, both cooling and induction. The little O200 powered Reno formula racers have foot long prop extensions. Not going to happen for us.

The best place to take advantage of boosting induction from prop thrust is about 2/3 rds out the blade, about where our scoop is now. The air near the spinner just gets beat to heck. There is not a lot of thrust or boost there. Also for an updraft induction you would now have to route it down to the updraft. That longer duct equals more loss. Also keep in mind the engine does not suck air continuously, it pulses. You can puff air into the inlet but if the induction air box volumn cannot "store" the air it will just spill out. You need a large air box for that. We just don't have a lot of room, but Van did a great job with the room he had. Bottom line I would not do the extend scoop to close the gap with the back of the prop blades.


Bob, I know you have updraft but just a note on guys with forward induction set ups. It makes more sense to put the scoop under the spinner or near it. It may not be perfect but they will have a straight (very short) shot right into the throat of the throttle body and sump. There are gives and takes, pluses and minuses, pros and cons to each setup. No one installation is the best of everything for every engine induction. Van's RV fwd induction air box is very efficient, about as good as it gets.

I would not mess with making a "annular smiley chin scoop" for an updraft carb. It has been done on a RV-6 before. In fact Van's first prototype RV-6 cowl he flew for some time had a annual chin scoop. Guess what? He went back to what the RV-4 has and all RV's have. If you have Fwd induction sump its a thought. Dan C. did both Van's and an alternate air inlet right under the spinner. He found he got some boost with the chin scoop. I think the big advantage is that its unfiltered and the duct is about as long as the cardboard tube for a roll of TP, ie short.

Remember the NACA cowl, Barnard cowl, James Cowl, LoPresti Cowl which has the inlets as far outboard as possible. This gets into the clean air and take some advantage of the prop blast. RV's with the rectangular inlets that fun horizontal to the edge of the spinner have funky air flow. Air actually goes OUT the cowl scoop near the spinner. This is draggy with the slow air spilling back out into the fast airstream.

Bob WHY DON'T YOU BREAK DOWN AND PUT ROUND INLETS ON YOUR COWL AND A SEALED CLOSED UPPER PRESSURE PLENUM? You will pick up at least 5 mph (probably more). :D I know you resisted it for may be money and hassle reasons, plus the paint job re-do, but the results are verified and have real theory behind it. You can nickle and dime but you are throwing a quarter away here. You can keep your existing cowl and baffles and just modify them. The cost would be nominal. Time and effort probably significant; it will be fiberglass intensive.

With all that RAIN on your parade, I say you are the mad scientest. Try it. My conclusions may be wrong. However I'm lazy and like to steal others ideas and efforts. :D I am also a slave to fashion and both snorkel and chin scoops don't turn me on. From the data I see neither the long snorkel scoop or chin scoop will do much for me and my bottom draft O-360/RV-7. I am extending my stock cowl intake scoop a little and modifying the inlet with a round machined aluminum bell mouth, instead of Van's stock induction scoop "D" shape. I'll add the access door so I can get at the duct and clamps, after the lower cowl is in place for an air tight seal. I might angle a little to the right as well, but only if it does not look cockeyed.
 
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Bob Axsom said:
I like to be thinking ahead a little bit so while I'm nearing the end of the cooling drag experiments I'm starting to think about the induction air.

I have read about extended inlets that reach out near the trailing edge of the prop ( Roy LoPresti, two friends with competing RV-6s in an RVator several years ago, an RV-4 at LOE a couple of years ago with a cylindrical extension - painted like a gun barrel, etc.) that claim manifold pressure gains and more power/speed.

Cris Ferguson told me about a NACA development that placed the air intake imediately below the spinner as used on the P-51 and his 300 mph biplane design.

I personally wonder about an angular face in the inlet that favors the right hand prop rotation. If the head on view sees a circular opening, but the view from the right side would be an oval shaped opening and the view from the left side would show no opening at all, could the air mass be increased with minimal drag increase?

Thoughts?

Bob Axsom
Click to expand...

CFA plots on a 6A show the highest pressure on the airframe around the spinner area extending all the way down to the stock carb scoop and outwards about 3 inches from the spinner laterally. Manometer flight testing on my plane confirms this. I use an extended gun barrel type 3 inch inlet to feed one of my heat exchangers. Instrumentation showed this worked well. Tuft testing showed that it reduced turbulence considerably over a flush inlet.

Many of the biplane and F1 designs at Reno use the gun barrel inlets close to the prop, a few inches from the spinner. Might be worth a try.
 
gmcjetpilot said:
Kent's other insight was an absolute seal between the scoop and air box is needed. Per Van's plans we use a soft baffle seal to bridge the gap between the cowl scoop and air box. Not super air tight. Kent found a removable door under the cowl was needed to conveniently make & break the duct connection. I'm adding a access door under the cowl to get at the induction coupler (clamp) between the scoop and air box. Van has suggested this in old RVators. The stock set up is OK but leaks. My comment or opinion is forget the long snorkel scoop but make sure the seal is tight.

A tight seal makes perfect sense in terms of induction, but I would suggest against a rigid connection. I've read several accounts of people routinely having to replace the airbox mounting plate due to cracking. There will definitely be movement between the airbox and cowl, especially during startup and shutdown. Some kind of flexible connection seems best.

My 2 cents-
Jim
Click to expand...
 
Good information all

This is going to be very interesting area to work with I'm sure. I can certainly adapt the information provided here and John Huft's information on the filter elsewhere to the task. George you have me pegged on the paint job concerns. Airplane beauty is very important to me and my wife so things that affect the paint job require a LOT of thought.

Bob Axsom
 
Following in Good Footsteps....

Bob,

I don't think I can be much help in your quest here, but I can tell you that you're following a well-trodden path.

About 20 years ago, I was working late one night in the hangar on my Grumman, and the "neighbor" from the hangar behind me stopped by with the air intake from his little formula racer, asking essentially the same question - did I have any ideas about how to make it more efficient. I don't know why he was asking me...we went to the same aeronautical engineering school (decades apart), but he'd done more real-world airplane engineering than I had by that point...I don't know if he ever figured it out before he passed away, but I think his airplane is hanging in a museum someplace out west.

That was Deke Slayton - a pretty good guy, and a down-to-earth pilot when all was said and done.

Good luck on the quest Bob! Maybe when I retire, I'll have time to follow in your footsteps...
 
WOW!!!

Paul,

You will never know how that hit me. When I got out of the Air Force I went to work for McDonnell Aircraft as a Radio, Electrical & Electronic Inspector. In April of 1959 I was assigned to the Mercury Project. Less than two years earlier I never even heard of the concept of an orbiting satellite - then the Russians launched Sputnik in 1957 and the common man's world was changed forever. My own father finally had to admit the world was not flat. Our initial attempts to duplicate the feat were dismal failures. When we started building the capsules no human had ever been there and we wanted to make sure that the astronauts were provided with the best spacecraft we could make. Many times I clocked out and came back to work on my own time which is unusual when you are an IAM Union Member. We didn't need any pep talks to motivate us and as far as we were concerned, the seven astronauts were very, very special people. One day the company gave every employee a copy of a photograph autographed by each astronaut. It was taken outside Building 102 where the original white room and first five capsules were built. They are standing in front of a mock-up of the capsule and escape tower. I still have mine. I never even saw one of them in real life, AND YOU TALKED TO THE MAN! WOW! Retirement is working out very well for me but I would trade places with you in a heartbeat - space exploration is without a doubt the most exciting thing a person can be associated with and we are just cautiously sticking our toe in the water.

Bob Axsom

See the attached Photo:
astronautsrs3.jpg
[/URL][/IMG]
 
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You know Bob, I am humbled every time that I do a "plant visit" as part of my job. I might go someplace where they are building a small component for the program - tires, brakes, an APU - and I see hundreds of people who are dedicating their time and lives (no one makes actual money building anything for NASA - the paperwork alone drives the costs beyond profitability) to make the space program work. It is humbling to realize how many people are putting in the effort so that we can go fly these things, and my teams and I are well aware of the debt we owe the American public.

It's guys like YOU that have made this program work Bob.

BTW, that picture is great - you just KNOW that Cooper is thinking about pushing Carpenter off the edge of that platform - you can see it in his eyes!

Paul
 
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OK Back on track!

I got us distracted here - sorry. Any thoughts on an intake design that extends out from the stock configuration directly toward the propeller with the leading edge cut back at a sharp angle like an insulin syringe and the opening turned to the right toward the oncoming propeller rotation?

A test intake could be fabricated to fit over the stock intake ("bolt on") starting with a flat forward facing inlet that could be removed and progressively sanded and tested at ever increasing angles with GPS average speed recorded at least with no additional instrumentation cost.

Bob Axsom
 
Might be easier and quicker to test on the ground with a sensitive air pressure gauge like we use in testing. Just do a runup at full throttle and record the value after each change. You could throw in some flight tests as well to get both results. More pressure = more power.

Davis Inotek Instruments- Magnehelic DW33510 reads up to 10 inches H2O and they are around $70.
 
Take off V. Cruise

A thought for your consideration. I share a hanger with Paser's plane and have had some disucssions on the topic. Seems that the greatest advantage of placing the intake near the prop was realized during take off and climb. He also found great advantage in timing the prop so that the pressure wave corresponded with an open intake valve. Again, not much effect in cruise but substantial in climb.

The acceleration of air (relative to free stream) immediately behind the prop is greater at takeoff and climb than in cruise. Once established in cruise, the net increase in airspeed immediately behind the prop disk is just a few knots. A lot of air passing though the disk and a slight acceleration of that mass gets us the thrust. At low speed, a smaller volume of air is accelerated to much greater extent. The rotational vector of air near the prop is also likely to be less at speed.

All that being said, seems you would need to optimize for one condition or another and recognize that there is not as much to be had at cruise.

Keep looking for that free supercharger!
 
That's Why I love this site

zav6a said:
A thought for your consideration. I share a hanger with Paser's plane and have had some disucssions on the topic. Seems that the greatest advantage of placing the intake near the prop was realized during take off and climb. He also found great advantage in timing the prop so that the pressure wave corresponded with an open intake valve. Again, not much effect in cruise but substantial in climb.

The acceleration of air (relative to free stream) immediately behind the prop is greater at takeoff and climb than in cruise. Once established in cruise, the net increase in airspeed immediately behind the prop disk is just a few knots. A lot of air passing though the disk and a slight acceleration of that mass gets us the thrust. At low speed, a smaller volume of air is accelerated to much greater extent. The rotational vector of air near the prop is also likely to be less at speed.

All that being said, seems you would need to optimize for one condition or another and recognize that there is not as much to be had at cruise.

Keep looking for that free supercharger!
Click to expand...

I will zav6a, this is a valuable input. I can noodle around in the wilderness forever if left to my own devices. This makes the reason behind the effect very clear to me. Thank you very much. I have your buddy Kent's book which I bought and read several years ago. No homebuilder interested in speed or economy of operations should be without it. When I read it I was working the development of Genesis with Lockheed Martin south of Denver. I asked LMA's PM about Kent Paser and the response was very respectful - I wouldn't have expected anything less. Thanks again for the information.

Bob Axsom

P.S. I just got my copy down from the shelf and thumbed through it - I think it is time to read it again.

Bob
 
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Induction inlet

Because my prop produces thrust all the way to the spinner, inlets close-in to the spinner actually enjoy a slight velocity increase due to displacement of the air by the spinner. As far as the angling is concerned, the provides some benefits at low forward speed and high power where the air's twist behind the prop is greater, but at cruise, it's so small as to be insignificant! With my prop the air speed increase at the disk is about 5 fps to the rear and about 10 fps tangentially at a cruise of 300 fps. Arctan (10/303) is about 1.9 deg. But it's true that if your prop's shape is klunky near the root, move your inlets outboard!
 
As for shape and stuff. Jon Sharp was running a smile intake right below his spinner on the NXT for the first year, after that he converted it to a round ram, lower like you describe. That is what it currently wears if i'm not mistaken. I think that is pretty much the cats meow. I know how to make it too if your interested. I can take pictures of one that is installed on a Skybolt if I get the chance in a couple weeks when I'm at WHP again.
 
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