Hi

I have an unsolved problem with
one of my RCM (which was fixed after crash)
it needs a very large amount of down trim in order to keep
a horizontal flight.

C.G is 100% in place.
Thrust angle seems to be fine.

I suspect that the wing incidence is too high ->
too much pith-up is produced.

any other suggestions?
is 1 deg mistake with the thrust angle can
cause such a pitch-up?

It's easy to check out your theory which sounds right on, just shim up the trailing edge of the wing about 1/8 inch something like a scrap piece of plywood and see what kind of elevator trim is then required. Almost as easy would be to reset both ailerons so their neutral point is up 1/8 inch or so. There are a huge number of variables in the construction and setup of any plane such as wing incidence, horizontal tail incidence, control surface rigging, etc. During a repair/rebuild any one and even several of these variables can wind up changed from your original configuration and require you to retrim your airplane. The two items suggested should get you started in the right direction.

Steve Woodruff

That 1/8" shim sounds a little drastic. I would try a 1/32" shim first and then a 1/16" shim.

Dave Segal

If I'm assuming correctly this is a flat-bottom-airfoil trainer type aircraft.
What were the results of the crash-what was damaged?

I experimented with my trainer years ago. I took out the 3-5 degrees of engine down-thrust with shims behind the mount. It acted much the same as you're describing. All of the engine down-thrust is necessary in a trainer due to the high-lift airfoil.

If I remember correctly, the bottom of the airfoil should be parallel to the horizontal stab.

Check the alignment of the two airfoils and adjust as necessary as described above in previous posts.
If they are in good order try putting two small washers behind the engine mount's upper most mounting bolts. Between the mount and the firewall. (One washer on each side-providing it has a removable mount and not built-in rails.)

Setting the neutral point of the ailerons 'up' will compound the effect by pushing down on the trailing edge of the wing thus forcing the tail down even more! If you go this route, lower them if anything.

Good Luck and tell us what you find!
Jeff

You would think that is how it would work, but every aeroplane behaves differently to flaperon/spoileron. My Sig SE would balloon up every time I lowered flaperon, and pitch down with spoileron. Other planes behave opposite. At any rate, you should test your plane with VERY minute adjustmens to try this method. The last thing you want is an inadvertant nosedive (or balloon into a low altitude stall) upon takeoff! However, I would shim the wing for a (in my opinion) better solution.

Regards,
Philip

Your problem could also be caused by an improper thrust line. If you have to change trim every time you change throttle setting, you may have to much upthrust. If, when you rebuilt the crash, you miss aligned the firewall, this could be the problem.

I agree but it could be a combination of wing incidence and to little down thrust. If you see drastic pitch change when you change throttle settings it is most likely thrust angle.

Setting the neutral point of the ailerons 'up' will compound the effect by pushing down on the trailing edge of the wing thus forcing the tail down even more! If you go this route, lower them if anything.

Jazzy,

I don't believe you thought about the statement above or you don't know how a wing works. Following your reasoning above lowering flaps would cause the tail to pitch up and the airplane would dive, or the wing half with the aileron deflected down would pitch down and cause the plane to roll in the direction of the down going aileron and we all know that doesn't happen.

An aileron or flap going up or down changes the camber and effective angle of attack (AOA) of the wing and thus changes the lift being generated by that wing. An aileron deflected downward increases the camber on that side of the wing increasing the effective AOA and creating more lift on that wing half which results in that wing half rolling upward.

Getting back to the original question about needing more down elevator after repairing the airplane. The first part of my suggestion was to raise the trailing edge by 1/8 inch or so. On a flat bottom airfoil I designed for a modified Great Planes Trainer 60 a number of years ago I measured the change from a 1/8 inch shim and the incidence decreased by less than 2 degrees. This is for a chord of about 12 inches so for a trainer with a 10 inch chord the change would still only be about 3 degrees. This is not enough to create any drastic unexpected changes in flight characteristics, but rather a noticeable change in required elevator deflection to know if the change is moving things in the desired direction.

The second half of my suggestion was to move the ailerons up from their geometric neutral position by 1/8 inch. This would have the effect of reducing the wing camber and affective angle of attack. Again, in the case of Trainer 60 wing, the effect is about a 1 degree reduction in the effective AOA, enough to be noticeable but without unpleasant surprises.

This technique is called reflexing the trailing edge and is widely used by sailplane pilots to increase the penetration of their airplane when moving out to seek another thermal for one example. Just because this is something common to sailplanes doesn't mean it can't be useful to prop turners. In fact Dow Lowe has in numerous articles on trimming pattern planes discussed this very same technique. More on this topic can also be found in Martin Simons' book MODEL AIRCRAFT AERODYNAMICS.

Another respondent wrote about setting the bottom of a flat bottom airfoil parallel to the horizontal stab and using 3-5 degrees of engine down thrust to keep from having to re-trim the elevator for each throttle/speed change. Finding the correct amount of down thrust can be a pain. I would suggest increasing the angle of the wing bottom relative to the horizontal stab and positive from leading edge to trailing edge. Do this by either raising the trailing edge or lowering the leading edge and set the mean chord line parallel to the horizontal stab or even something close to the zero lift AOA. This will eliminate the need for down thrust and the wing will act more like a symmetric airfoil over changing speeds while the plane retains the gentle stall and flying characteristics of the flat bottom wing.

Steve Woodruff

Ddekel, if this problem only came up after the repairs then I suspect that something in the area of the repairs is wrong and needs to be corrected. What broke in the crash?


If the model did this BEFORE the crash then you're just confusing me by mentioning the crash....

First,
Thanks for everyone.
I particularly liked sewoodruff explanation,
which seems to have some good understanding
in aerodynamics.

The A/P is a VMAR PC9 .45 size engine,
having a semi-symetric low wing.

I had several crashes and did some repairs,
I probably set the firewall and/or wing incidence
incorrect.

Please correct me if i am wrong.
Is a higher wing incidence is not the same case
as a low hor-stab incidence?

in such a case, a negative incidence angle for
the stab would create a pitch up moment,
which is actually my current problem...

anyhow,
I had already lowered the wing incidence.
next week i will make a test
flight and inform you my results !!!


B.T.W
lowering the engine inclination angle
didn't show any improvement.

While it may be true that spoilerons act differently on different aircraft, I stand behind my statement as stated given the information available at the time.
In support of Steve Woodruf's analysis:
I found out the hard way on my Hobbico Superstar 40. After a high G pull-out in a dive the wing shifted backward out of the saddle and required almost full up elevator to compensate.
In reality the entire wing incidence was changed not the effective incidence and without the increased spoileron drag.

Having dual aileron servos in my Advance 40 and my Magic I know from experience that raising both with the flap knob on my computer radio caused both aircraft to pitch up. The opposite occurred when both were lowered.
Please note that the Superstar 40 had a flat-bottom airfoil, the Advance 40 a semi-symmetrical, and the Magic, (I believe), also a semi-symmetrical airfoil. If memory serves correctly, the same was observed with my SuperChipmunk having a fully symmetrical airfoil.

Aerodynamics can be a very technical course of study but, some of it is just common sense.

No, lowering one will not cause that side to pitch down. The drag induced and the positive change in effective incidence will cause lift. But, we were not discussing just one side and this scenario is completely different than what was stated.

While it does make sense-strictly from an effective incidence aerodynamic perspective-that raising both will effectively decrease the overall incidence, the increased drag and subsequent downward force on the trailing edge of the wing will, as in the cases I've stated, cause the aircraft to pitch up.
It is possible however that a very minute change will not have the effect I described.

Steve, I don't think you looked at all of the factors involved. Disrupting an airfoil is not the same as changing it's actual incidence.

Daniel, if your horizontal stab's incidence was decreased, that would explain it. Your assumption is correct.
I know it would be easier to adjust the main wing's incidence through the use of shims but, your going to change the aircraft's intended flying attitude. You'll probably find that the aircraft will behave much differently at different speeds. While usually more difficult, changing the horizontal stab's incidence to it's original orientation will provide the best results.

If your instruction manual does not give you the engines thrust-line, wing incidence, and horizontal stab incidence in relation to the aircraft's centerline, give VMar a call and speak to a tech.

Gool Luck and let us know what you do and what happens!
Jeff

PS. Ever notice you usually need a little up elevator when using flaps and a little down when using spoilers?

I agree with jazzy
about flaps (pitch down affect)
and spoilers (pitch up affect)
but why lowering the wing incidence
does not give the same affect as using spoilers
or increasing wing incident the same affect as flaps?

using this equation can anyone explain this?

CM=CMo+CLa*a+CMdf*df

where:
CMo=CM @ CL=0
a=alfa
df=flaps deflaction

Daniel,
As far as that equation goes... ?
Simply,
The reason the effects are different is due to the orientation of the wing with respect to the aircrafts centerline.

Ever stick you hand, (held flat), out a car window? With your fingers held level, call it the centerline of the airfoil, your hand is basically a airfoil.
Tilt you hand up and your hand rises.
Tilt your hand down and it drops.

Basically,
An airfoil's lift is generated by a decrease in air pressure. Change the angle of attack-the airfoils centerline with respect to the aircrafts centerline-and you'll change the lifting properties of the airfoil.
By shimming up the trailing edge of a wing you are increasing the pressure on the top of the wing and decreasing it on the bottom. I.E. you are decreasing the lift of the top of the wing and creating 'downward lift' on the bottom.
Flaps and spoilers change the shape of the trailing edge of the airfoil and not it's centerline's angle of attack nor it's original lifting ability. These control surfaces are acting like mini-airfoils-just like your hand out the car window.

I'm not employing technical terms or describing all of the forces involved; hopefully, as was the intent, it is easily understandable.

An airfoil's centerline with respect to the aircraft's centerline:
A trainer's flat-bottom, (high lift), airfoil has it's centerline some 15 degrees positive-just a guess. Thus the reason for the engine's down-thrust.
An intermediate type aircraft's airfoil, usually semi-symmetrical which creates more lift on top than on bottom, is only a few degrees positive if at all.
An experienced type aircraft's airfoil, usually fully symmetrical which creates lift equally in both directions, is also only a few degrees positive if at all.

This is just a generalization.

problem had been solved !!!

I am keeping my prommise and let you know the results:
lowering wing incidence did some improvement,
but raising both ailirons up was the ultimate solution !!!
many Thanks to sewoodruff who gave that solution
at the beggining of this thread.