I know this is likely to be a stupid question.

One of my airplanes (a Kougar) has 3/4 to 1 degree positive wing incidence. The incidence should be zero. I did not build the airplane but apparently the horizontal stabilizer was put on with the front of the stab down relative to the wing. To compensate the prop has 4 degrees of down incidence relative to the stab.

Does anyone have a trick to correct the stab incidence without a total makeover. Is there is a way to partially correct the incidence.

Bill

Hey Bill. I haven't been at this Sport / Hobby very long, but one thing I have learned, is that there are no stupid questions. Can't say the same about some of the answers though.

You are a very quick learner

Having a bit of positive incidence on the wing can be a good thing, however you have not mentioned just how much down (negative incidence) if any does the horiz. stab. have, and as is how does the model fly ??

Bill, if the plane has been flown, what elevator trim is it showing? Are the power on and power off trim positions the same?

The four degrees down thrust was added to compensate for too much climb under full power or is that down thrust standard for the build?

If the elevator is close to neutral trim and the power on/off trim are the same... then all is well. Very often however if the elevator is not near neutral... power on/off trims will not be the same and can be made so by slight incidence adjustments. Caveat... remember always that the first step to trim is balance and incidence adjustments should not be made until the balance is correct.

Generally a wing can be shimmed slightly and in the case of your low wing Kouger, the front would be shimmed down by elongating the dowel/s holes and then adding some weather stripping or something to the forward saddle.

The Sig Kougar MK11 is an all time favourite of mine so much so that I have a NIB kit under my workbench. As I recall the build instructions required no engine down thrust and the wing cores were cut with some washout in them to overcome any tendency to tip stall. Incidences were pretty much all 0-0-0.

Negative on the stab will also be 3/4 to 1 degree relative to a neutral wing. A quick CAD sketch verified the numbers for the stab.

The model has 115 flights on it. The model flies fine but much trimming for flight is how the prop came to have 4 degrees down. None of my Kougars fly well upside down.

Thought it might be time to see if a real incidence fix could be made.

Bill

Set the engine thrust to be zero in relationship to the stab. After that if the wing has a degree or less of positive I would go out and fly it before any other adjustments. Verify the washout, I personally would not want any but the builder of the airplane may have set the wing slightly positive as a trim adjustment. FYI I set up all my aerobatic airplanes with .5 degree positive in the wing so that just the wing flys at a positive AOA and not the whole airplane.

115 flights. Trim should be 0, 0, 0. Elevator trim is neutral and power on/off is approximately the same. I don't notice any difference but may have become immune. I have balanced the airplane several times but not as a process of trimming. None of my Kougars fly inverted without lots of down stick which I thought was a normal characteristic.

Thanks for your response. Looking at wing shimming now and it calculates to be slightly over .200" at the front of the wing.

Bill

I can't recall where mine was balanced but it flew quite well inverted with only a tad of forward stick being required for level flight.

It flies fine, you say.

So, leave it alone.

Insure that you are using the correct longitudinal fuselage center line, reset your thrust line to 0 and spend a little time sanding and reshaping your wing saddle utilizing your LFCL as a reference until you achieve your goal, whatever that may be. You can trace your saddle from the fuse side and use it as a quick reference guide as well.

Bob

I assume that you are measuring the wing incidence at the side of the fuselage or wing center joint. But because of the washout in the wings, you must measure the wing incidence at the center of each wing panel, half way between the fuselage and the wingtip.

Washout increases stability when the airplane is right-side-up, but it decreases stability when inverted. That might explain the comments about inverted flight performance.

Also, the airfoil shape determines the necessary wing incidence. With the Kougar's fully symmetrical airfoil, the wing will have to have positive incidence in level flight. If the recommended setup is 0-0-0, it will fly with up elevator trim and be somewhat speed sensitive. Worse yet, add washout and a 0-0-0 set up at the wing saddle and you will need a lot of up elevator trim, which will make it very speed sensitive, which will then require a lot of down thrust to smooth it out.

It sounds like someone has taken a lot of time and trouble to try and trim your Kougar for completely neutral flight. Unfortunately, the Kougar was designed as a low wing trainer (per the Sig website) and you may never be able to get it there.

Jim

Sounds like someone was trying to make inverted flight rather easy.

Here are some aerodynamic truths:

1. The Horizontal Stabilizer IS the active incidence setter. The longitudinal axis is simply a designer's / drawer's / or whomever's working line. The stabilizer sets and holds the required aerodynamic realities.

2. In sub-sonic convergent airflow, Given a symetrical stabilizer, and a symetrical wing, any real incidence is the trim of the stabilizer to hold the wing in a flying position. There are numerous examples. If you have a lifting wing, stab. elevator trim/position loads the wing to stay where you want it. Flaps down changes the wing Angle of Attack (AOA) which changes the lift. Lift is a result of basically Air Mass Density, Square of the Airspeed (flow over/under the wing) and surface area.
Changes require additional change.
#1. 0° Stabilizer reference the longitudinal axis: Same for wing: Then the sym. wing must be forced into a lifting position (Positive angle-of-attack) by trim of the stab. Given then the stab/elevator is set to something "up"to hold the wing into a lifting position. If the engine thrust line is also 0 to the axis, and is now inline with the wing chord line there is another force to be contained by stab trim.
2. The stabilizer is nothing more or less than a tool to hold the wing in a loaded position. Engine thrust is in attempt to pull the machine in the direction of the thrust-line. Most single-engine prop airplanes have a vertical stabilizer that is offset a couple +/- degrees leading edge to left to assist with the torque problem during high power operations.

To make a LOOOONNGGG story short, try to rebuild your machine to have your thrust-line, wing center line (LE to TE) aligned with the stab line. Many of the older Pattern RC Models were designed with stab and wing in alignment. Then the engine was set at 1.5 to 2° down thrust. This meant that the engine pulled down, causing the wing to be generating some lift positive, the elevator trimmed a tad up to hold the wing in a positive Angle of Attack(AOA) to be providing lift, then when the machine is turned up-side down, the engine is pulling up while the elevator is wanting the wing to go down. The pilot's punches in a tad of down-trim, (Now UP ) to assist the wing to be in a lifting AOA.

One can fly a model which is way out of reality aerodynamic rules. The 3-d folks do it all day every day. OTOH I am a sport flier mostly as I did Pylon, competition
Fun Fly, and tried pattern but not my style. Scale (somewhat-scale that is ) is my forte. Enjoy your pleasure but a small understanding about flying machines makes things easier.

Excellent post and IMHO well worth the 3 stars rating I gave it.

Good answer Kevin!

Yeah, I'll second that Karolh, and thats a cool thing about the stars, i didn't know that.
great responce Hossfly, right on the money.

I tend to agree with you.

Usually if a plane has a neutral elevator and the trim is good for both power on and off flight... then it will fly fairly well. A neutral elevator usually means relationship of wing and stab incidence is fine. In this case, it seems someone added 4° down thrust in the engine to obtain the neutral elevator rather than adjust an incidence. Down thrust is more often a drag couple counter measure for a high wing plane or asymmetrical wing or both. This might mean then that without the 4° down thrust, that the elevator would require some down trim for power on flight and the OP's inquiry was about the incidence of the wing showed slight positive. That makes sense.

If he is then wanting to take out the down thrust and perhaps realign the spinner with the cowl better, the option would be to do so and then adjust the wing incidence to obtain neutral elevator... doing so by shimming the front of the wing down very slightly.

A word of caution... I have a P-51 with front wing bolts... and if they are not tensioned the same for each outing, the slight incidence change causes significant trim change... so make any front of wing adjustment in very small amounts. I'd use some 1/64" plywood strips forward on the saddle making adjustments by only one strip at a time. I've shimmed several wings to finalize trim and rarely used more than 1/32"

Thanks everyone for all the information. There was much that had not been considered. The comments inspired me to attempt a real fix.

The incidence was measured at the fuselage although I did know the wing had washout. Hadn't considered using the fuselage to make a template of the wing saddle but it is a very good idea. Of course I made the wrong image the first time.

I added most all of the prop down thrust in order to get the airplane to fly level power on and power off but did no checking of CG along the way.

Bill

.

Hello Bill,

The airplane will not fly level very long with the power off.
As it slows down in speed it will within seconds start a descent, the wings angle of attack will increase and soon it may stall. It depends where the CG is located.

Zor
Edited to change scconds to seconds

Getting the CG right is the first thing one should do in the entire model trimming exercise.

very informative thread. I was just jigging my plane up to check incidences when the 3 star post appeared and now all the angles i am seeing makes sense. Thank you. Phil

The information present here is appreciated and has been extremely valuable. However as a long time flyer but a poor builder correcting an incidence issue is a daunting task. Like other things the learning process is laced with danger.

1. Make the wing saddle first. I didn't.
2. Try not to loose your reference points. I lost mine.
3. I attempted to sand the saddle after each change. After a couple of days decided the approach was backwards.
4. After checking incidence and turning everything upside down right becomes left and left becomes right. I knew that but somehow forgot many times anyway. Always discovered after 30 minutes of sanding.
5. Disconnect the servo from the ailerons first and clamp ailerons in a centered position. I didn't.
6. Somehow attach the leveling shims together and to the table so he airplane will stay in place while being checked. I didn't and still look for more shims every time a wheel jumps off of a leveling stack of shims.
7. Use a single short shim (less than one inch) and check incidence. I didn't. But I am now.
8. Don't forget that a long move requires moving the dowel holes. I didn't move the holes far eought.
9. Don't forget that on a long move the nylon bolts at the rear of the wing will not screw in straight like they were in the beginning. I forgot.

This has been a good experience. A good builder would not have the problems listed above.

The forward plan is:
1. Shim
2. Check
3. Shim
4. Check
5. When close fly to see if I went the correct direction.
6. If wrong direction the airplane will break an I am done.

Bill

I am curious about how the airplane is jigged? Do you jig the airplane once or each time it is checked? If once do you work on the saddle while it is jigged?

Bill