Trimming Question

[george kennie]

Back in the late 60's I had a fiberglass bodied Taurus that had no roll
coupling despite 3" of dihedral under each wing tip, but pitched very
strongly to the canopy despite the fact that the stab was about 3" above
the center line of the wing. I used to trim the aircraft to fly zero
pitch knife-edge and then fly all my maneuvers holding about 1/16" up
elevator which also gave me a similar responce inverted.If I would let
go of the elevator the plane would immediately assume an angle of
descent of about 15 degrees. I could do wonderful 4-point rolls about
15' off the deck the likes of which I can't even come close to today.Of
course, we flew much closer in back then and that might have had some
advantage.Today, however, I try to trim for hands off, but I still don't
like a trim set-up where the plane has the slightest tendency to climb,
I'd rather have a very slight sink and fly with a touch of control on
the elevator.
It should be pretty obvious that if you set up with sink today on a
serious pattern ship you are not going to see very straight up and down
lines when you let off the elevator and I for one would not want to be
making an up or down line correction manually while trying to perform in
front of a panel of judges.
When I was designing my new Fresca my intent was to build a completely
neutral airplane in all axes. When I first flew it and checked it for
knife I thought for sure I had achieved my desired objective, as I had
2% up elev. with right rudder and 1% down with left. Man, I thought I
had attained genius status. Well, subsequent flights disclosed that it
had a slight pull to the canopy on very long down lines even though the
C.G. was where I had thought it should be.  So, in went 2% down elev. on
low throttle and that problem was fixed.
Then I went to the Nats and got cornered by Roy Speights and made aware
that there is more to C.G. than just a casual consideration. When I got
home and back to flying the Fresca I decided that holding 1/16" down
elevator inverted was more than I wanted. At that point the C.G. was at
30% so I decided to move it back till I had to just touch down elev. to
maintain level inverted.Eventually, I added 3&1/2 ounces under the stab
and now the down elev. felt terrific and I could now take out the down
line mix, buuuuuutttt!!!!!!!, what happened to my beautiful
knife-edge??? Now this beautiful creature of mine wants to pitch to the
belly to the degree of 8 or 10 % up trim mix. So as you can see the
design process is not fully realized (and may never be), But the old
wheels are turning and I have the next generation full 2-meter baby on
the boards and I think I know the answer (but I'm not sure). This
winter's project is, at this point in time, highly anticipated.
Still learning,
Georgie

Lance Van Nostrand wrote:

> Ed,Paul Villarrubia told me once he trims for a slight sink in upright
> level flight.  Now I understand why.  Not sure I agree, but I now
> understand the theory behind it.  Thanks.  I'll have to think on
> this.--Lance
>
>      ----- Original Message -----
>      From: Edward Skorepa
>      To: discussion at nsrca.org
>      Sent: Sunday, August 17, 2003 8:35 PM
>      Subject: Re: Trimming Question
>       "Model trimmed for hands off level flight" This is where
>      your problem started. I can't understand why somebody would
>      want  pattern airplane to fly hands off in level flight.
>      This is how I trim my Piper Cub ! During hands off level
>      flight the airplane must have some declage to overcome
>      gravity. That declage is what's causing pulls to canopy in
>      vertical dives,  climbs and also in knife edge. Personally,
>      I'm still trimming my airplanes the same way like I did in
>      seventieth. Wing - stab  0 - 0. To maintain level flight
>      either right side up or up side down will require light pull
>      -push on elevator stick. I like to have that pull - push to
>      be the same. No more problems with verticals and knife edge.
>      If so, then there is something  else wrong most likely with
>      engine's down thrust. Decreasing wing incident will not help
>      in your case. If you still will want your plane  fly hands
>      off in level flight, then you have to put in some up trim
>      and you're back where you started. My recommendation - learn
>      to fly with light pull-push or mix it out. Ed Skorepa
>
>           ----- Original Message -----
>           From: Peter Pennisi
>           To: discussion at nsrca.org
>           Sent: Sunday, August 17, 2003 4:33 PM
>           Subject: Trimming Question
>            I would like some suggestions to overcome a
>           trimming problem I have with my current F3A
>           model.Problem:The model pulls to canopy on
>           vertical dives as well as vertical climbsCurrently
>           I have an electronic mix to overcome this problem
>           but this is starting to get in the way on some
>           manoeuvres.Setup:Model trimmed for hands off level
>           flight.
>
>              * CG is about correct. Any more further back
>                model becomes nervous in wind and harder to
>                snap - difficult to control exit.
>              * Model fly's with "NO" elevator trim.
>              * Model feel's about right inverted. A small
>                amount of down elevator required to maintain
>                level.
>              * 1-2 degrees of down thrust.
>
>           Can this be overcome by changing wing incidence or
>           will screw up something else.
>
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