Good ideas, guys.
A couple of things to add (perhaps food for thought):
I don't think you are going to get trans-sonic testing without a shock tube.
Supersonic aerodynamics (and testing) is a whole 'nother ballgame. I think I
got a C in that class, at best. ;-)
The subsonic testing is also going to be subject to weird environment-specific
effects that are dissimilar to what the fin will actually experience in
rq17zt eloquently touched upon the tip of the iceberg, so to speak.
Even so, this kind of test could be illuminating, at the least for trend
analysis useful in refinement of the design, and also, to discover other
"unknown unknowns" that only show up when trying different tests.
In light of the problems we've had with fins in the past, any knowledge gained
would be worth it.
[psas-airframe-boun...@lists.psas.pdx.edu] On Behalf Of rq1...@q7.com
Sent: Monday, July 27, 2009 12:26 AM
Subject: Re: [psas-airframe] Roll control-wind tunnel
A rotary test track definitely makes sense when testing performance in
an already turbulent airstream.
However, i think the results of a rotary test on our canard idea would
be hard to interpret due to the non-negligible, and mostly unknown
vorticity in the impinging stream that would be present due to repeated
passage through already disturbed air.
For our application the performance parameter that is most difficult to
predict theoretically, and therefore that is of most interest
experimentally, is the position on the leeward surface of the canard
where the (vortex) flow becomes separated. Unfortunately, the
separation position is quite sensitive to the vorticity in the impinging
stream, so to get results meaningful to the control problem from a
rotary track test we would need some method of calibrating the incident
vorticity and some theoretical way of scaling the results to zero
At this point i don't know how to do that scaling, and i expect that it
is not easy.
(But i don't _know_ that it is not easy. It _might_ be easy ;)
If it turned out that the scaling was easy, then we'd have a really
nifty idea, and possibly a significant new insight into aerodynamics.
> OK. I've been toiling with developing a way to test our fin designs
> in a high speed wind tunnel:
> 1) We could build a compressed air wind tunnel and get fast (perhaps
> supersonic) flows, but only for a brief period and not at one steady
> flow rate.
> 2) We could build a small high speed wind tunnel using a 2 stage
> centrifugal fan driven by a small block chevy (the red-neck in me
> loves this idea), or perhaps a more manageable surplus 4 cylinder
> engine from work (50 HP, $30).
> What about moving the fin in the air?
> 1) Moving the fin on a test rocket doesn't allow steady state
> testing, and instrumentation is a problem.
> 2) The fin travels a long way at 300 meters/second, so a linear
> track is out of the question.
> Finally an idea hit me: Move the fin in a circle! (Naturally, this
> idea came to me while I was working on my helicopter).
> We can remove the rotor blades from a gas powered helicopter and
> replace them with carbon fiber tubes (available at local hobby
> shops). At the end of the tubes, we attach our fin design. With the
> existing helicopter mechanics, we have precise control of the
> blade's angle of attack. I happen to have an old helicopter that I
> can donate to this cause. Naturally, this test would take place
> behind a shield and/or at a remote location.
> At typical rotor head speeds of 2000-3000 RPM, a fin mounted at a
> radius of 600 mm from the center of rotation will see airspeeds of
> around 125-190 meters/second! That's 280-425 miles/hour!
> Further, we can evaluate the lift / AOA curve using a scale under
> the helicopter.
> This could easily be expanded to a 1 meter swing radius, getting us
> into the trans-sonic speed regime. Filming the blade with a high FPS
> camera would show us shock lines if we get there.
> Fire away with thoughts, check math, etc.
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