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Easy wind tunnel...

pierre smith

pierre smith

Well Known Member
...for you guys wanting to reduce drag.

Years ago, I put drops of black, used oil on the leading edges of my homemade brake fairings on my Cassutt and I wondered if I'd shaped them correctly. I flew around the pattern and went wide open and then landed. Low and behold.....beautiful streaks all the way to the rear....good, laminar airflow.

So today while I was refueling my dirty Air Tractor for another load, I looked at the leading edges and saw the turbulence behind the round-head rivets and remembered reading in an EAA publication years ago that the first rivet causes as much drag as the next seven..well here it is...the leading edge is on the right side and the chemical streaks show airflow.

DSCN0342.jpg

I'm gonna do the fuel drains and aileron hinges of my -6 with a few drops of black oil and go fly....a cheap wind tunnel.

Regards,
 
Pierre, excellent photo! That is how I found out that some of the cooling air exit area has reverse flow - I put drops of oil on the belly and noted some went forward!

I guess some of the air gets scared and climbs back into the cowl.
 
Big ol' protruding head rivets...

Pierre, we use those great big headed rivets for noise abatement. We don't want you guys breaking the sound barrier going to and from the field!:D It keeps the sonic booms to a minimum (happy neighbors) and also keeps the Thrush guys from getting too jealous!

CDE
 
Secret is out

So the secret is out; what every glider-guider knows. The oil tells the whole story. If anyone can get there hands on some of the glider flight test reports from Soaring magizine, oil testing is a big part of performance maximizing. The biggest gain is to look at the transition from laminar to turbulent flow and reducing the drag from this transition. Most gliders have a step in the finish to trip this laminar flow to prevent a drag-inducing bubble, or reverse flow region. Besides keeping everything clean and dealing with the cowl exit, I believe tripping the flow results in the biggest reduction in drag. So if someone can minimize the drag from the cowl exit and trip the laminar flows properly, the airplane will have maximum performance. I suspect the only laminar flow is outside of the prop wash, but not sure. Good luck and hope to see a lot of oily planes out there.
 
And it can also be done in color...

I believe this is oil with food coloring added. Spots (dabs?) placed on the ground and flying then makes the streaks along the flow lines...

Left_Ft.jpg
 
Gil, I wonder...

....if oil paints would stay wet long enough for a takeoff and flight to WOT.

I think we're leaving a bunch of speed on the table, as Dave Anders has already proven...a 265 MPH -4, even with 230HP he has really cleaned her up.

Bob Axsom(sp?) has already picked up around 20 MPH and there's more to be had.

I'm also gonna oil up the surfaces around the gaps between the ailerons and flaps and wingtips to see what's going on there as well.

Thanks,
 
wind tunnel

...anyone ever rigged up a wind tunnel that they mounted on the back of their pickup?
I'd like to try an airfoil with flaps to figure out the best place for VG's on the -9 airfoil, among other things.
Even a 1/4 scale chunk of wing bolted to the side mirror would be interesting, no?
I guess the ulitmate rolling wind tunnel was the rig hung off the front of a truck so customers could 'fly' a BD-5 down the runway without loss of life or significant aluminum! (hard to find pics...here's one in Bud Davisson's article circa 1974!)

http://www.airbum.com/pireps/PirepBD-5.html
 
Last edited:
flyboy1963 said:
Even a 1/4 scale chunk of wing bolted to the side mirror would be interesting, no?
Click to expand...

You would end up with a very different Re number than the real thing. 1/4 scale at car speed is similar to a 1/4 scale model airplane at model airplane speed. Your 1/4 scale wing would behave very different than that of a full scale at real speed.

Wouldn't it be better just to glue or tape light threads directly on the real wing?
 
Interesting stuff, Pierre, tracking oil on an air frame to see what's going on with air flow.

A while back I had a few very thin streaks of oil coming up over the windshield and tracking them forward discovered they were coming from inside the cowl via the inlet from a leaking PSRU which meant there was air coming out of the cowl in that area. You'd think the oil would be all over the top of the engine but it wasn't, it was on the windscreen, the engine was dry.

It could be with a fixed cowl inlet, there is a point in the speed plot where internal cowl pressure is such that air can't all go aft, down and out, so pressure builds up and it burbles forward and up and out.

For a few million, maybe we could engineer a variable inlet. I wonder what that lucky family in Winner, SD is doing with the $220 million they won a couple weeks ago? :)
 
Pierre, I would not describe it as...

pierre smith said:
....if oil paints would stay wet long enough for a takeoff and flight to WOT.
....
Thanks,
Click to expand...

..."oil paint" - I believe it's mainly oil with a bit of food coloring added.

If you want, I can contact the guy who was designing the cowl in question and find the exact formula...
 
My late friend --

Dick Johnson, of glider testing fame, used the old oil he drained out of the VW Diesel he towed his glider trailer around with. Worked great and is already black. No special formula required. ;)

Don
 
Not that, Gil..

az_gila said:
..."oil paint" - I believe it's mainly oil with a bit of food coloring added.

If you want, I can contact the guy who was designing the cowl in question and find the exact formula...
Click to expand...


Oil paint that artists use to paint paintings of things.

Regards,
 
Artists oils can take several weeks (or months) to fully dry, so it would stay fluid for a test flight. it's quite thick though, so you will want to thin it (artists use linseed oil, but you can use vegetable oil for this as its cheaper). Don't use acrylic paint by mistake :)
 
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