Forming aluminum for a cooling outlet fairing

[Bob Axsom]

After a few days to apply another layer of fiberglass the the right MLG subfairing, fill it and paint it I finally got back to the outlet fairing work. As an elementary school student one of my comic book heros was Batman. In the mid 60s a popular song that Jeanine and I danced to was the Batman theme (listen to the Ventures version to get an idea). So, when I made this latest cover mod I had a feeling that destiny was being played out before my eyes.





The thought was, at the angle of attack of the airplane in flight any squared off surface perpendicular to the flight path is going to have a broad turbulent flow coming off of it and if these kinds of surfaces were brought to points like a "V" there would be less drag.

At 19:53 CDT I was ready to test the batman cover. Both runs were near identical:

180.5 kts
180.8 kts

Good but no cigar. The no cover tests with the thick fins with the platenuts hanging in the breeze are still the fastest followed closely by the thin fins and no cover which in turn is followed closely by the Batman cover. Tomorrow is the last day before going to Montana for the Big Sky Air Race on Saturday. I guess I have no choice but to remove the Batman cover and go to Montana with the thick fins and no cover. I would like to make up a set of thick fins without the platenuts hanging in the breeze and test that but I'm out of time.

Bob Axsom

[Veetail88]

Might not be faster Bob, but I'm betting your radar signature will be lower!

[Bob Axsom]

I enlarged the batman cutouts by cutting a triangular section out of the two inboard edges and rounding the fwd end of the cutout.





I made two trips around the area in normal three leg 6,000 ft dalt test fashion. The speeds were 182.6 kts and 181.5 kts respectively for an average of 182.05 kts. These rank as the fastest and the 7th fastest of the 32 test flights in this series. The average of the two test flights today is 1/2 of a tenth of a kt slower than the previous high speed in this test series.

Bon Axsom

[David Paule]

I wonder if a more parabolic shape to the cut-out, instead of straight cuts, might have a small advantage?

Dave

[Bob Axsom]

It's a thought. I suspect the improvement in speed has to do with the exhaust plume impinging on the cover based on experimental observations. If that is true the focus of each cutout should be centered on the exhaust pipe outlets (which is what I have been instinctively moving toward without considering a parabolic shape). The interaction of the cooling air flow and the exhaust flow has always been a question.

Good observation thanks. I think I will make a pattern of the current cutouts using file folder stock the recut the development cover following a more parabolic line and give it a test flight.

Bob Axsom

[David Paule]

Also, what about adding exhaust extensions? I'm thinking that they could extend the current tube another eight inches or a foot, then make a gentle bend so that they end up parallel to the airflow. That would give you some exhaust separation from the skin.

You'd no doubt need some bracing for that, both to keep the extension under control and to keep from putting too high loads on the upper parts of the pipes. And it could well be that the trade-off between the drag of all that might not be made up by the added thrust and cleaned-up exhaust.

It would separate the effects of the exhaust from the cooling air exit to some degree, but it might be to the detriment of both. As you can tell, I'm not at all sure whether this would offer an advantage or not.

This is sort of the opposite of a jet-augmented exhaust, which might be preferable if it could practically be done.

Dave

[Bob Axsom]

Originally Posted by David Paule

Also, what about adding exhaust extensions? I'm thinking that they could extend the current tube another eight inches or a foot, then make a gentle bend so that they end up parallel to the airflow. That would give you some exhaust separation from the skin.

You'd no doubt need some bracing for that, both to keep the extension under control and to keep from putting too high loads on the upper parts of the pipes. And it could well be that the trade-off between the drag of all that might not be made up by the added thrust and cleaned-up exhaust.

It would separate the effects of the exhaust from the cooling air exit to some degree, but it might be to the detriment of both. As you can tell, I'm not at all sure whether this would offer an advantage or not.

This is sort of the opposite of a jet-augmented exhaust, which might be preferable if it could practically be done.

Dave

I sawed off the exhaust to get the direction I have now and got a speed gain I see no reason to go back I think I'm guiding it in the right direction as far as these pipes are concerned. On the positive side I recut the cover outlet - not technically a parabola but in that vicinity as opposed to a blunted triangle.



I spent so much time trying to coordinate things on the computer today that I will have to wait until tomorrow to test fly the change but it is ready to go:





Bob Axsom

[David Paule]

Dang! After a career in aerospace development, that's the quickest I've seen one of my suggestions implemented in hardware. Usually it takes a couple days or more. Usually more.

I'm impressed. Fly safe!

Dave

[Bob Axsom]

I flew the test this morning when every thing was calm and the results were good. The first test was 184 kts and the second was 182 kts for an average of 183 kts. Now that I know what I'm looking at, it seems to me that the right side cutout of the cover at the forward outboard edge could be relieved a little more and I might pick up another fraction of a knot. Unfortunately I am starting to run into AVC launch schedule pressure. I am thinking about tracing the current cutout outline then cutting it once again and making one more test flight but first, breakfast!

Bob Axsom

[johnny stick]

Bob,
Seems you have almost hit the magic 185kts.Wow what a great accomplishment. What's the next mod series, wing root fairings?