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Bob Axsom

Well Known Member
The natural thinking about air entering the inlets at 200 mph is that it it travels to the back of the plenum and impacts the flat baffle surface and deflects off in all directions and the continuing inflow of air opposes (not prevents) its coming back out the inlet. Pressure builds toward some level determined by the airplane velocity and propeller force to the extent that there is no excape path. The major intended excape path is down through the cylinder fins and baffle openings (plus blast tube holes, oil cooler hole, heater air hole, etc.). I can see that the vertical flat wall may have some optimizing characteristics and conventional wisdom would say that since "everybody does it this way" it has stood the test of time and is the best way to orient the back wall. However, in a "question everything mode", I am looking at it and wonder if anyone has tried a forward inclined back wall? and what were the results (primarily speed)?

Bob Axsom
 
Bob Axsom said:
The natural thinking about air entering the inlets at 200 mph is that it it travels to the back of the plenum and impacts the flat baffle surface and deflects off in all directions and the continuing inflow of air opposes (not prevents) its coming back out the inlet. Pressure builds toward some level determined by the airplane velocity and propeller force to the extent that there is no excape path. The major intended excape path is down through the cylinder fins and baffle openings (plus blast tube holes, oil cooler hole, heater air hole, etc.). I can see that the vertical flat wall may have some optimizing characteristics and conventional wisdom would say that since "everybody does it this way" it has stood the test of time and is the best way to orient the back wall. However, in a "question everything mode", I am looking at it and wonder if anyone has tried a forward inclined back wall? and what were the results (primarily speed)?

Bob Axsom
Click to expand...
Hey Bob,
There is only one way to find out for sure. Next project! :D
 
Convention

Bob

I think you are right, that is has pretty much always been done that way, especially since in most aircraft the firewall is verticle and flat. Most plenums have been constructed that way for ease of install. I have also not seen anything or anyone who has changed only this one parameter.

Also, if the cowling/plenum is configured correctly the air pressure is greater inside the plenum, than at the inlet, since the air has hopefully "slowed down".

I recall the VariEze pictures, I posted on your first thread, as the only "form fitted" shaped plenum. I plan on 2 seperate plenums fitted to the cylinder shapes on my -8, as the most effecient way to make use of cooling air, IMHO. Of course, no one has the time to wait on me finally getting in the air. :)
 
Wade-
What engine are you using?
I'd be interested in some in progress pics of your plenum if you're OK with that. Post them here or email me tmshort_at_gmail.com

Thomas
 
Will do

Thomas

Will post them when I am done. But I hope you are not in a hurry, I am juggling 2.5 yr old twins, a year old house, and a wife I want to stay married to. :)
 
Oops

TShort said:
Wade-
What engine are you using?
I'd be interested in some in progress pics of your plenum if you're OK with that. Post them here or email me tmshort_at_gmail.com

Thomas
Click to expand...

Oh forgot, IO-360A3B6D.
 
Bob Axsom said:
The natural thinking about air entering the inlets at 200 mph is that it it travels to the back of the plenum and impacts the flat baffle surface and deflects off in all directions and the continuing inflow of air opposes (not prevents) its coming back out the inlet. Pressure builds toward some level determined by the airplane velocity and propeller force to the extent that there is no excape path. The major intended excape path is down through the cylinder fins and baffle openings (plus blast tube holes, oil cooler hole, heater air hole, etc.). I can see that the vertical flat wall may have some optimizing characteristics and conventional wisdom would say that since "everybody does it this way" it has stood the test of time and is the best way to orient the back wall. However, in a "question everything mode", I am looking at it and wonder if anyone has tried a forward inclined back wall? and what were the results (primarily speed)?

Bob Axsom
Click to expand...

Bob, I'm no expert, but the early mooneys suffered from "cooling drag" That air entered the inlets and pressure was great enough to cause it to go back out with little if any cooling. The air leaving the inlets cause drag and you slowed down. Lopresti was the first to come up with the *SMALL* round inlets and solved the problem, later, everyone went that way. Either round, or small "slits".

I think this is all a game in "fluid dynamics". It has to do with High and Low pressures. Create a High on the top of the engine, and a low on the bottom, with ducting, cowl flaps, etc and the air moves from high to low. If you could create a large enough low on the exit, you could continue to restrict the inlets (until you restricted flow that is).

I doubt it has anything to do with "slants", if it did the certified guys would be doing it.

Only real way to tell is to "tuft" and camera the insides in flight.

Also, if curious, there is a NASA article on "chin scoops" that you can find in Google. While not directly related, it talks about the advantages of the "spinner" being the "inlet" ramp to the inlets. In effect you want to create "laminar flow" down the spinner and *NOT* have it detatch until it's inside the cowling/plenum. Ram air is also developed that way...
 
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Management and Optimization are Enemies

I was in the business too long to believe people in commercial business would do everything possible to optimize performance. If that were true we would have had a 237kt Mooney 20 years ago and the Mooney 301 would have gone into production. Commercial business thrives on "good enough" and profit maximization. What I was wondering is, has anyone in our little circle actually done it. I have read very high numbers quoted as the cooling drag percent of total aircraft drag I seem to recall one place said it is as high as 46%. Which a number like that it seems almost silly to focus on things like airfoiled fuel vents and drains, etc. Today I was working with my baffle in the lower cowl and I was fretting over rivet orientation,and lap joint orientation and staring my in the face is this 2.75" dia heat muff oriented blunt end to the wind. Four months ago I had intuition to go on but now I have made mods and I see the difference in performance. My intuition was OK but there was more to it than I expected - for example just cleaning up the cooling air flow path does not reduce the cooling drag - it may very well increase it as it did on my RV-6A. However, cleaning up the airflow path in combination with other flow control changes can reduce the cooling drag and increase the speed of the airplane as it did in my RV-6A. I do not expect to see Mooney RTVing between horizontal baffles and the valve covers anytime soon (if Columbia goes up to 250kts it could happen). The important thing is this every change I made in my work with the lower cowl changed the top speed of the airplane and some very clear relationships are becoming apparent to me. Now this last change I am currently working on hits three gross parameters chamber volume, flowpath cleanup and exit cross section ... It can go either way. I'm expecting more speed and higher CHTs but we shall see later this week I hope. Then maybe I'll do more than think about the plenum configuration - I'm certain there is performance to be had there. The dual plenum approach sure looks good on some of the planes I have seen them on but I once thought "T" tails and "V" tails and twin rudders looked good. How does it work under test? I also will be interested in how it works out for RV8RIVETER.

The proper scoop under the spinner is having a lot of success. A few weeks ago I was talking to Cris Ferguson speed mods etc. and he brought this up. He designed, built and tested/developed the 2nd place Gold class biplane in last years Reno race. He has the scoop on that airplane he said it is the product of an old NACA study and it is used for the induction air on the P-51. He has that biplane (owned and raced by Jeff Lo) pushing 300 mph. He also gave me some other ideas involving wing intersection fillets that are not the customary radius designs that extend along the full chord at the root. I'm keeping that one in my back pocket for a while.

OK based on the feedback I guess other than the COTS plenums and home made plenums, no one has experimented with forward slanting the rear baffle. That is good information - keeps it on the table.

Bob Axsom
 
Here ya go

Yes, I've been talking with Jeff Lo as well. Also bumped into Cris. Here is that study that he mentioned. I had found it a few months ago and referenced it in a note above. I think the pictures of the scoop on Jeffs airplane can be found at www.flying-lo.com. With it, Cris said they got almost 3" of MP at the DA of Reno. I think it was 2.63". Paul Lipps of goofy looking but *fast* propellers came by (he's also an aerodynamicist) and ran some quick calculations which came out to within 3% of the displayed results. So Cris and Jeff were pretty happy.

Anyway, have fun, it's been fun to watch...

http://naca.larc.nasa.gov/reports/1948/naca-report-920/
 
I made a copy of the NACA Report 920

I made a copy of the NACA Report 920 and I intend to read it as I work through my back log. When we flew to Alamogordo in April for the semi-annual opportunity to visit Trinity Site, I bought a copy of "The Making of the Atomic Bomb" by Richard Rhodes and I am reading it in the usual moments of solitude. I am currently on page 214 of this 866 page book but soon after that I'll read the 58 page NACA Report 920. I expect it to be very good. I notice that it is dated 1948 which was right after the aeronautical watershed year 1947 (X-1, Hughes Flying Boat, etc.). This was of course, long after the development of the P-51 ... like I said this should be interesting reading with future applications possible.

Bob Axsom
 
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