[NSRCA-discussion] Trimming question

Mon Jan 30 17:15:37 AKST 2006

Bob, I like your theory.  Years ago, Don Lowe published some test reports in
RCM where he rigged an Extra to have a moveable stab.  He claimed that that
was the ultimate cure to the pitching problem - move the stab, in the
directions you indicated.  Only have tried it on two planes - a CAP, where
it did resolve the problem, but made it un-scale like, and currently a
Funtana that's still on the bench waiting for other mods.  Can't say what
bad things this does to other flight characteristics, as the CAP had a host
of other problems..which is why it is gathering dust in the corner of the
shop as I type this!  If I ever get my home-brew pattern plane done, that's
one of the adjustable features it will have - a stab I can raise or lower,
to see really what the effects are on the flight envelope.

Ted Sander

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Bob Richards
Sent: Monday, January 30, 2006 1:41 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Trimming question

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 l! ike 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 dr! ag 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 s! everely 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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