Cowl Mod. Thanks Bob

[Kahuna]

It was with great dissappointment that I read Bobs results of his cowl mod . How depressing.

Bob you did a ton of work, great docs... Very sorry it did not heed the results you anticipated. Im sure we have all looked at our engine installs and seen opportunities for improvement. I once contemplated what Bob has done. I never would have done as good a job. I never did follow through with my ideas figuring that if it had significant improvement, we would see this on other piston installations. Folks like Mooney and Cirrus fight tooth and nail for every knot. Their engine installs look just like mine.

Thanks to Bob for ignoring what others are doing and trying this out. While the results are disappointing, the efforts are extrodinary. Thanks for the great pics, details, follow through, and results Bob.

Best,

[DGlaeser]

Bob
Your efforts are what experimental aviation is all about. Your design and results are valuable data. Thanks for sharing it with us.

[Ironflight]

Good work Bob! Every result of an experiment is, in a way, a positive - even when you don't get the results you expected.....it adds to the overall knowledge base, and helps everyone. I don't know how many times I've tried somethign in asimulation and the (useful) result was "well, let's make sure not to try THAT again!!"

I followed your saga with great interest - I admired the creativity. It showed great promise in the idea...

So...what are ya' gonna try next?!

Paul

[Kevin Horton]

Cooling drag is created because the air that comes into the cowling must slow down to match the aircraft's speed (using the aircraft as the frame of reference). Or, if we switch our frame of reference to the outside world, that air must go from a standing stop to the same speed as the aircraft. It takes a force to accelerate that air, and this force comes from the aircraft pushing forward on the air.

If you want to decrease cooling drag, you need to reduce the amount of air that is being used for cooling. The only way to do that is to reduce the size of the cowl inlets and/or outlets. If you reduce the amount of cooling air, the engine temperatures will increase, so there may need to be improvements in the baffling to make better use of the cooling air. Adding devices like this mod to control the air inside the cowling could help make better use of the cooling air, and thus allow the amount of cooling air flow to be reduced by other cowling mods, thus leading to a drag reduction.

Don't give up. There may be some speed to be had by playing around with cowling inlet and exit areas. But first, get a good bunch of baseline speed data using a more accurate flight test technique. The amount of inaccuracy in the U.S. Air Race, Inc technique could be greater than the speed changes you are trying to measure. I strongly recommend the four leg option on the NTPS spreadsheet. And do several runs on different days, to be sure you have a repeatable result.

Ideally, all testing would be done at the same altitude and temperature. But that is not possible in the real world, so the next best bet is to always be at the same density altitude. The U.S. Air Race, Inc handicapping test procedure tries to compensate for temperature, but they seem to assume that there is a standard lapse rate between 6,000 ft and the test altitude. The test altitude they suggest is a good first guess, but once you get there, check the temperature, and adjust the test altitude as required to get the desired density altitude.

If you want 6,000 ft density altitude, the following combinations of pressure altitude and temperature will do it (temperatures in deg C):

9,000 -26.7
8,500 -21.9
8,000 -17.1
7,500 -12.1
7,000 -7.1
6,500 -2.1
6,000 3.1
5,500 8.4
5,000 13.7
4,500 19.1
4,000 24.6
3,500 30.2

Plot those points on a graph, and draw a line through them. Take it with you when you do the testing, and find an altitude where the temperature falls on the line.

Note: engine power is not exactly proportional with density, so using density altitude to try to correct non-standard day data to predict standard day speed is not an exact science.

[BruceMe]

The factors you are working against are that the air flow inside the cowling is significantly slower than free-streem velocity. Items causing parasitic drag inside the cowling do not significantly effect overall cooling drag because of this low velocity. It was certainly a valient effort though, and very interesting.

Can we all conclude that changes to the upper deck and baffling more directly effect cooling and cooling drag than what happens after the air passes through the fins?

-Bruce

[kevinh]

Thanks Bob - great write up.

[gmcjetpilot]

Originally Posted by Kahuna

It was with great disappointment that I read Bobs results of his cowl mod . How depressing.

I am not depressed. Now we know that does not work. OK

Good for Bob, he is the man. He had an idea and excellently executed and documented it, from start to finish. When Edison was asked if he was depressed about the 1000's of failures he had finding a working light bulb, he said no. I recall his quote was to the extent of now I know 1000 things that don't work. Bob is following a great tradition like Edison and the Wright Bros.

I don't think anyone could say for certain if this would have improved speed without trying it. Now we know thanks to Bob. Hats off.

PS. I don't think he is going slower and not sure how he got his average but I would agree it did not make a large difference.

[Bob Axsom]

Sometimes you just have to try and idea to clear your mind for something better. Thanks to Doug for posting my e-mails. I got a lot of ideas from you folks before during and after this little experiment and I appreciate them. I have thought about the mod and the results and I am not quite ready to give up yet. I'll let you know what comes up. Paul good luck with Discovery tomorrow. There is a young man out there that turned 50 today - Happy Birthday Donald.

Bob Axsom

[Lorin Dueck]

All -

I am fortunate to know and work with Chris Zavatson.
He's a great person, very unassuming... and probably the most brilliant experimental airplane guy you'll ever meet.
My only regret is he's a "fast glass" person instead of a "Hershey Bar wing" advocate.
Please - Don't hold that against him.

Chris built an award wining Lancair 360 - and has spent the last several years working to improve it.
One of his most recent endeavors was aimed at cooling efficiency / performance increase.

Check out his website.... it's worth it!!
www.n91cz.com

Lorin D
9A Wings
N194LD reserved

[Lorin Dueck]

... PS Bob -

I hope you didn't take my last post the wrong way.
We appreciate the blood, sweat & tears you put into your bird.
I can't imagine how tough it must be when things don't quite turn out as we hoped.
I know I learned someting.... and that's a good thing..

So, on behalf of all of us - Thank you for all your extraordinary work and willingness to share!!

Lorin D
9A Wings