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  #211  
Old 04-18-2012, 07:28 PM
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NDrv8r NDrv8r is offline
 
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Location: Bismarck, ND
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Default cowl flaps

I went out to do some testing this afternoon, but it was so rough at 4000DA I couldn't even make notes on my paper, so a decent test will have to wait until a better day. The next couple don't sound so good.

I went up to 6000 and did a bit of full open/full closed, and even full closed, my chts only went up about 10 degrees. However my oil temp dropped 7 degrees with the cowl flap closed! what is up with that? airspeed went up maybe 1kt. Now I can't wait to do a decent test.
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  #212  
Old 04-18-2012, 08:08 PM
David Paule David Paule is offline
 
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Location: Boulder, CO
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Default

Can you test it partly closed? Okay, for some people that would be partly open. In the video it looked like that might be a sweet spot.

Dave
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  #213  
Old 04-18-2012, 11:00 PM
Bob Axsom Bob Axsom is offline
 
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Default It took several on and offs to get the trim right







Once I got it done I flew in very calm air at 6000 D alt as always. This configuration is exactly the same as the version tested on April 10 when the speed was 177.2. The earlier test on April 9 without the cutouts was exactly 3 knots faster at 180.2. Today's speed was 181.7 or 4.5 kts faster with the bumps than without them in the otherwise same configuration. Now this is good enough tp fly in the Llano race Saturday but if I had time to make the new cover I agree with Gary and F1 Boss, the test results indicate that it would be faster.

This test was flown with the nav antenna on, the cooling air vents closed but not covered, tie down rings on (same as the previous tests).

CHTs (1-4) 356, 373, 361, 335 (4 is not trustworthy)
After the flight the fairing was cold to the touch.
I measured the distance from the firewall to the high point of the bump and it is 6 3/8".

Bob Axsom

Last edited by Bob Axsom : 04-18-2012 at 11:02 PM.
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  #214  
Old 04-19-2012, 12:39 AM
SHIPCHIEF SHIPCHIEF is offline
 
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Default

I'm gonna guess that you should cut the cowl bottom straight across at the forward end of the exhaust cutouts.
The coanda bump should be taller, and the trailing edge should have a more gentle slope, maybe even flush to the side strake's profile. then tune the exit duct by making the trailing cover longer or shorter as tests dictate?
Very encouraging. Bob, you have grabbed the bull by the horns. You can't let go now!!
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  #215  
Old 04-19-2012, 10:38 AM
johnny stick johnny stick is offline
 
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Default Formula 1 link?

Looks like the tail end of an F1 race car. I believe the bumps and exhaust are interacting to make the airflow reattach to the lower fuselage skin. Very good work, thanks for putting the experimental back into Amatuer Built.
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  #216  
Old 04-19-2012, 11:14 AM
David Paule David Paule is offline
 
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Default

I'd concur with Shipchief and add that the bottom cover plate makes a diverging duct with the bumps in the center area. Besides slowing down the exit air in that area, it promotes some vorticity with the adjacent non-diverged duct area, which will tend to slow it further.

Dave
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  #217  
Old 04-19-2012, 11:42 AM
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rzbill rzbill is offline
 
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Default Thoughts about NACA duct geometry



Bob,
I've been watching your excellent work and thinking about how to do mine in the future. One item that has been included in my thoughts is inclusion of the 7 degree ramp angle used on the "floor" of a NACA duct. It is designed to allow velocity change (expansion) with minimal separation turbulence and I really don't want to do the math to redesign it so 7 degrees is it. My books are too dusty.

So, where to apply it?

I will start by making the assumption the cowl exit air is slower than free stream. I think the coanda bump can be doing a reasonable job of accelerating the exit air via area reduction and possibly redirecting the exit air to be more parallel to the belly (if it is not separating on the rearward expansion ramp) Needs tuft test.

SO, my second assumption is that the exit air is still slower than free steam even with the good effort.

I postulate that a cover might work better if is at a 7 degree angle to the free stream. (Narrowing the exit)
The purpose is twofold:
1) Reduce the expansion area (slowing of velocity) on the backside of the coanda bump
2) Purposely expand the freesteam and slow it in the local region of the exit to better match the two stream velocities.

Use or discard at your pleasure. You are WAY ahead of me.
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  #218  
Old 04-19-2012, 01:01 PM
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RVbySDI RVbySDI is offline
 
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Default

Quote:
Originally Posted by rzbill View Post
I postulate that a cover might work better if is at a 7 degree angle to the free stream. (Narrowing the exit)
The purpose is twofold:
1) Reduce the expansion area (slowing of velocity) on the backside of the coanda bump
2) Purposely expand the freesteam and slow it in the local region of the exit to better match the two stream velocities.
Is the "freestream" the air flowing outside the airplane or are you referring to the air flowing through the cowling exit?

Any chance you can submit some drawings of these ideas for those of us who are engineering challenged? A visual model of these ideas would go a long way for my understanding of what you are saying.
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  #219  
Old 04-19-2012, 04:37 PM
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DanH DanH is offline
 
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Default

Quote:
Originally Posted by rzbill View Post
I will start by making the assumption the cowl exit air is slower than free stream.
It is, by measurement as well as in theory.

Quote:
I think the coanda bump can be doing a reasonable job of accelerating the exit air via area reduction and possibly redirecting the exit air to be more parallel to the belly (if it is not separating on the rearward expansion ramp) Needs tuft test.
I don't see much area reduction.

Hard to tell if the exhaust jet is attaching to the curved surface, a proper Coanda Effect. Bob, are there now exhaust stains on the bumps?

If the tailpipes were pointed more directly at the curved surfaces and the belly cutouts were filled, I would then expect the exhaust flow to attach, and to entrain cooling outlet flow through a reduced area, increasing it's velocity.
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  #220  
Old 04-19-2012, 05:31 PM
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flyboy1963 flyboy1963 is offline
 
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Default re-attach flow....sounds like a job for.....

Quote:
Originally Posted by johnny stick View Post
Looks like the tail end of an F1 race car. I believe the bumps and exhaust are interacting to make the airflow reattach to the lower fuselage skin. Very good work, thanks for putting the experimental back into Amatuer Built.
I may be ill-qualified to comment, but this is the part of home-building that excites me, too!
...and if it's too much work to re-profile the exit ramp, can we cheat a bit by using vortex generators at the right spot, to keep the flow attached, with little drag penalty?
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