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ACRO E-mail Archive Thread: [Acro] Re: Vortex generators on Akro aircraft

Thread: [Acro] Re: Vortex generators on Akro aircraft

Message: [Acro] Re: Vortex generators on Akro aircraft

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From: William Connor <william.connor at central.sun.com>

Date: Wed, 18 Apr 2001 04:09:54 UTC

Message:

  Klein Gilhousen wrote:

> Today I (finally) went to the airport to dust of the Extra and see if I 
> could still do a hammer or a spin or whatever or even land the thing.
> 
> After landing, there was a Super Decathlon sitting there getting a going 
> over by on of the local CFI guys.  Turns out the local guy who owned it 
> had just sold it to an out of towner, who had some "improvements" made 
> and the CFI had been engaged to go test fly it.  One of the more 
> peculiar things was the installation of vortex generators.  The things 
> look like little tabs sticking up into the airstream on the top of the 
> wing.  There were also some on the bottom of the horizontal stabilizer.  
> I'm reasonably familiar with the concept and had installed, under STC, a 
> set on a Cessna 421 I used to own.  Basically, they lower the stall 
> speed maybe 10% or so.  In the case of the twin, they also lower Vmc a 
> substantial amount.  But I've never seen these things on an akro plane.  
> Methinks there's probably a reason.  On the plus side, they will 
> probably lower the stall speed a significant amount, resulting in lower 
> landing speeds, less runway, etc., etc.  On the minus side, I wonder if 
> the thing will snap any more?  I wonder what happens to the spin 
> recovery?  Will it even spin?  On the negative side, g that is, there 
> aren't any on the bottom side of the wing (or the top of the stab) so 
> inverted performance probably isn't affected much.  I wonder what the 
> STC says about akro maneuvers?  Are the CFI and the new owner now test 
> pilots?

I swear I saw VGs briefly on Sean Tucker's wings, but I can't seem to 
find a photo to prove it. I would worry first off about Va and max snap 
speed. Both index off of stall speed and would be reduced accordingly by 
the same percentage that the VG's reduced stall speed. If you were to 
snap at the unadjusted book values, you could over stress the airframe. 
For example, if your stall speed is 60 and max snap is 120, you'll incur 
4 Gs plus bonus points for torsional stresses. If you reduce the stall 
speed to 50 and snap at 120, you'll incur 5.8 Gs plus bonus points - ouch.

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