Knife edge pitch trim ?

[Dean Pappas]

Hi Ed,
If your plane pitches to the canopy equally in both knife edges and the vertical down line, then you can probably find a more aft CG and less downthrust combination that produces acceptable trim. Unfortunately, many small stab-ed airplanes behave poorly at the aft CG, or that CG sweet-spot you're looking for is very small, and tough to find.

Back in the pre-turnaround days, some of us (myself included for a while) trimmed for an equal amount of back pressure to fly upright as forward pressure to fly inverted, and used zero incidences and very little downthrust. The thing was that back then we used much less elevator throw to fly our maneuvers, and had plenty of time to lock the pitch for entry and exit lines. I find that this simply isn't practical now.

What we are aiming for is a highly refined compromise. Remember that it's only been twenty years now (yikes!) that you could fix a plane that pitches in knife edges with the radio! Back in the bronze age you really needed to trim the hard way.
Regards,
	Dean  



-----Original Message-----
From: discussion-request at nsrca.org
[mailto:discussion-request at nsrca.org]On Behalf Of EDward Skorepa
Sent: Monday, June 28, 2004 5:20 PM
To: discussion at nsrca.org
Subject: Re: Knife edge pitch trim ?


Hi Dean,
my airplanes don't do that. If I trim them for perfect hands off horizontal
flight ( I don't do that anymore) then, they will pitch to the canopy on
down lines and KE. Up lines are different. I believe it's like you've said
thrust line issue. All my pattern planes are 0-0 and CG at 6% (static
margin). Then I adjust up/down thrust so plane will descent hands off at the
same angle flying right side up - up side down. Then the amount of lite
push/pull to fly horizontally should be the same and down/up lines, KE very
close but never perfect.
Ed
----- Original Message -----
From: Dean Pappas <d.pappas at kodeos.com>
To: <discussion at nsrca.org>
Sent: Monday, June 28, 2004 2:15 PM
Subject: RE: Knife edge pitch trim ?


Ed,
That is strange isn't it! Obviously, as long as the wing is at zero angle of
attack, then it isn't producing lift. How does it stay there with some small
amount of incidence angle in the wing? The answer is in two parts: 1)
downthrust, thrustline higher than the CG and center of drag, and 2) not all
that well. This is even more true of the downline. My best trimming planes
have all had as close to 0-0 incidences as I can remember.

Nat Penton was asking a similar question, and I guess that the answer is
that we trim to approximately perfect.

Dean

-----Original Message-----
From: discussion-request at nsrca.org
[mailto:discussion-request at nsrca.org]On Behalf Of EDward Skorepa
Sent: Monday, June 28, 2004 2:59 PM
To: discussion at nsrca.org
Subject: Re: Knife edge pitch trim ?


Hi Dean,
are you trying to say, that if I trim my plane for a perfect (HANDS OFF)
horizontal flight then if I push or pull to a perfect (HANDS OFF) down or up
line  the wing is not producing any lift? If so, how that could be?
Ed
----- Original Message -----
From: Dean Pappas <d.pappas at kodeos.com>
To: <discussion at nsrca.org>
Sent: Monday, June 28, 2004 1:38 PM
Subject: RE: Knife edge pitch trim ?



Hi Ed,
Loaded means making lift. When you pull a vertical correctly, you forcibly
"drive" the plane to a vertical line. In this condition the wing makes no
lift.
If you have a plane that is trimmed for a nice straight vertical, and
over or under pull, the plane usually behaves differently, no?
Dean




-----Original Message-----
From: discussion-request at nsrca.org
[mailto:discussion-request at nsrca.org]On Behalf Of EDward Skorepa
Sent: Monday, June 28, 2004 2:19 PM
To: discussion at nsrca.org
Subject: Re: Knife edge pitch trim ?


Hi Dean,
would you mind to explain what "loaded" and "unloaded" wing means?

Thanks,
Ed
----- Original Message -----
From: Dean Pappas <d.pappas at kodeos.com>
To: <discussion at nsrca.org>
Sent: Monday, June 28, 2004 12:44 PM
Subject: RE: Knife edge pitch trim ?


Hi Paul,
The short answer is no. That wasn't very helpful, now was it!
Your conjecture about the positive incidence requiring some down trim is
good. The trim change is probably not the result of rudder displacement
making the elevatore more or less effective.

The difference between level flight and a vertical downline is that when the
wing is "loaded" the downwash that leaves the wing and strikes the stab, in
effect, produces a small bit of up trim. When the wing is unloaded, that up
trim effect goes away. In the upline, the effect of the thrustline (usually
above both the center of gravity and the center of drag) comes into play. In
the knife edge, the yaw of the plane produces small pitching effects from
different sources ranging from the size of the plane's chin, how big and how
far back the canopy is, to the distribution of the fin and rudder area and
to the effect of wing dihedral. That's why, in my opinion, the usefulness of
a trimming chart is sometimes limited. I have seen a dihedral change fix a
knife pitching problem, and a canopy placement change create one. Stab
versus elevator height has a very big effect on how much the downwash
induced trim adds into the mix, and a simple CG change has dramatic effects,
especially if the plane is borderline tail-heavy.

The result is that you start with the trim chart (it usually works) and then
experiment. The state of "proper Pattern trim" is actually a fairly tricky
thing.

Sorry but there is no "one size fits all" answer!
Regards,
Dean P.



-----Original Message-----
From: discussion-request at nsrca.org
[mailto:discussion-request at nsrca.org]On Behalf Of Paul Horan
Sent: Saturday, June 26, 2004 7:51 PM
To: discussion at nsrca.org
Subject: Knife edge pitch trim ?


    The NSRCA site's trim states that during knife edge if the model pitches
to the canopy wing then increase wing incidence.
    I assume what they are trying to accomplish is by increasing wing
incidence the elevator will carry more down.
    OK, how does elevator position tie in with rudder ?  Does rudder
deflection cause the elevator to become more effective - thus
the pitch coupling ?

Thanks,
Paul

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