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Positive/hhigh pressure at rear of canopy.
http://t18.net/thorpforum/viewtopic.php?f=6&t=7625		 Page 1 of 1
Author:  flyingfool [ Thu Aug 07, 2014 12:53 pm ]
Post subject:  Positive/hhigh pressure at rear of canopy.
I've read that the canopy on the T-18 has a positive pressure behind the canopy. This explains the rear vents I see on many T-18's I've seen in the past.

To me however this suggests that there is turbulent air behind the canopy rather than laminar flow. This in turn would seem to indicate that this adds considerable drag to the airplane and could posibly reduce the vertical stabilizer/rudder effectivness.

I was wondering if anyone had ever "tuff tested" with systematically taping yarn to the top of the canopy to see what the airflow is doing?

It would seem possible that adding vortex generators to the canopy just aft of the maximum curve COULD cause the airflow to remain attached and reduce a fair amount of drag and thus increase speed?

has anyone thought of this or tried this? I know it wouldn't look too aesthetically pleasing, but it may prove to be effective. (or not).

If VG's did remove the positive pressure behind the canopy. I wonder if it would create a vacuum and thus want to rip the canopy off/open or put a lot of stress on the canopy lock mechanism.

Just a newbie thinking aloud (so to speak).
Author:  leewwalton [ Thu Aug 07, 2014 1:22 pm ]
Post subject:  Re: Positive/hhigh pressure at rear of canopy.
Tom,

There is positive pressure at the rear of the canopy and in turn a slight bit of drag. 180hp Thorps have little problem hitting the red line of 210 mph, one would argue the benefits of fresh air on the back of the neck in the summer outweighs the slight bit of drag created at the aft end of the glass. There have been fast/round back Thorps made before and the quite frankly the looks of the bubble canopy (or the lack there of in the absence) far outweigh any performance benefit (in my opinion).

Not sure about vortex generators but again the drag is minimal.

Tuft test picture attached.

Attachments:
AirProgessT18TuftTest.pdf [1008.08 KiB]
Downloaded 701 times
Author:  flyingfool [ Thu Aug 07, 2014 2:34 pm ]
Post subject:  Re: Positive/hhigh pressure at rear of canopy.
Lee, you are a "machine" when it comes to responses and providing absolute dead on detailed information quickly too I might add!

That picture of the tuft testing ends all debate. (DDo you have the whole article that I could get a copy of?)

From the looks of that black and white photo there are no tufts on the rear of the canopy that are standing straight up or going backwards. So clearly it is not an airflow issue but a pressure issue. Given that, it would seem that there would be absolutely no benefit to VG's on the canopy as it seems to show that the air remains attached to the canopy all the way back.

And I would NEVER consider getting rid of the sliding canopy. That is one of the most admired attributes that I most like about the T-18. That and the bent up wing like my 2nd favorite WWII fighter the Corsair. (my first is the P-38 Lightning)

I have a realistic chance of owning a T-18 poor mans Coursair, but there really isn't much of a chance for me to own or afford a personal P-38 type plane.
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