[NSRCA-discussion] Trimming question

[Bob Richards]

I don't like to get involved with knife-edge trim discussions since I don't feel that I am an expert on the subject. However, I developed a theory about it years ago that seemed to fit most problems I have seen.
   
  The vertical placement of the wing and stab on the fuselage has a lot to do with it. With a top-wing and a bottom stab (such as the Ultrasticks) the tendency would be to pitch nose-up during knife edge. OTOH, a Cap 21 has a bottom wing and a top mounted stab, and I know only too well that this pitches severely to the belly in knife edge - my old Cap21 took 35% mix to fly straight.
   
  My theory is that, when the fuselage is presented to the relative wind at a given angle of attack, there is pressure built up on the side of the fuselage (has to be, to fly knife edge). This pressure will naturally wrap around the fuselage, so there will be a separation line along the fuselage where the air separates, just like at the leading edge of a wing -- some goes over, some goes under. The stab should be placed near this natural separation line. If it is far below this line, the plane will pitch to the canopy, if far above, it will pitch to the belly. This is due to the stab trapping some of the pressure.
   
  I don't think the wing incidence change can correct anything other than a very minor pitch coupling problem. For really bad situations, a change in the stab location is probably what is needed.
   
  Anyway, that is my story and I am sticking to it. :-)
   
  Bob R.
  

John Ferrell <johnferrell at earthlink.net> wrote:
  My Guess:
The sum of the drag vectors is well above the vertical CG. You MIGHT be able 
to help the condition with really big wheels generating some drag down low. 
Its anybody's guess what the turbulance might trigger.

The wing is not lifting in knife edge, but it is still draggin'.

John Ferrell
http://DixieNC.US

----- Original Message ----- 
From: "John Pavlick" 
To: "NSRCA Discussion" 
Sent: Friday, January 27, 2006 11:31 PM
Subject: [NSRCA-discussion] Trimming question


> Ok here's a good one. The owner of the L.H.S. asked me what I thought 
> about
> this and I couldn't come up with a good answer. He has a Hangar 9 Ugly
> Stik - the real big one. I can't remember exactly what it's called. Guess 
> it
> has a gas motor in it. It's big. He says he has it trimmed so it will fly
> inverted with just a very slight bit of down elevator (almost none). Ok, 
> so
> it's not nose heavy, maybe even a little tail heavy. Now, when he flies it
> knife edge (either side) it pulls severely to the canopy. I mean like it
> will fly a complete circle. It needs almost 3/4 down elevator to fly
> straight. It's beyond a mix - fix. He also says that it has a bit of roll
> couple when flying level (not knife edge) but it doesn't pitch severely up
> or down under these conditions. Any thoughts? I guess you can rule out 
> C.G.
> because if anything it should pull to the belly considering how it's 
> trimmed
> to fly level upright and inverted. I haven't flown it so I can't add too
> much more. He builds OK so I don't think there's anything way out of
> alignment, but again I haven't seen it yet so maybe there is. What should 
> I
> look for if I get my hands on it to try and help him?
>
> John Pavlick
> http://www.idseng.com
>
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