Plane won't fly level!
 

I have built a Top Flight plane kit, the Contender. The plane looks great but .. it would never like to fly level. Give it a bit of throttle and it would nose up and starts climbing severely. The more throttle and speed the harsher the climb. The plane was built as per specs and drawings .The C.G is spot on as indicated in the instructions book . I did try to bring the C.G. forward and backward the design point however neither would improve the climbing trend. The engine was tilted downward by placing one washers on each top side of the mount ,then two and finally three washers and that seems to have improved the situation just slightly .However the over all flight characteristics is really not good. I can not think of anything else to do. I would appreciate it if you could give me any suggestions that might help please .Thanks.

RE: Plane won't fly level!
 

All aircraft respond to thrust changes (throttle) with a change in pitch (elevator).

The one item you did not mention is are you trimming in down elevator in response and does that help? Also can you fly at a steady reasonable power setting and trim the airplane so it hold altitude?

There is always the possibility that you have a small incidence problem.

So can you trim the airplane for level flight at a fixed power setting and does that result in a lot of down elevator from a normal intrail position?

John

RE: Plane won't fly level!
 

Yes I can set the throttle to one setting and trim elevator to fly level in one wind direction but then the plane would respond negatively on the other wind direction travel.
I can trim the elevator for one throttle setting but the trim will change once the speed is reduced and that would cause serious down pitch

RE: Plane won't fly level!
 

Yes, when you change throttle settings, the plane will want to climb or fall, depending upon which way you moved the throttle. That's normal. You can't trim the plane for level flight, add throttle and expect it to remain in level flight.

As far as wind direction is concerned, if the plane is trimmed for level flight into the wind, it should remain in level flight with the wind, assuming you don't change throttle settings. The AIRSPEED of the plane remains the same at a given throttle setting regardless of the planes direction with respect to the wind.

Groundspeed will change, obviously.

RE: Plane won't fly level!
 

I think John hit it on the head there. My ARF trainer was flying just as you descrive, a slight change in throttle would result in huge pitch differences. At the last, I had a stack of four popsicle sticks under the TE of the wing and it was flying much better.

I also had a 4*60 that I broke the tail off of and when I repaired it, I screwed up the horziontal stab inicidence just a bit. I had to fly with a bit of down elevator trim, but this was sensitive to throtle.

I would get an incidence gage, or rig one up and see where you stand. I'll bet the wing is off a bit, probably to much incidence. You have a low wing, so if you were to put one popsicle stick between the wing and fuselage at the TE, your problem should be a little worse if the incidence is the cause.

One other possibility is motor down thrust, You could always add a washer or two on the top two screws on the motor mount to give a little more down thrust.


Don

Don

RE: Plane won't fly level!
 

Hi!
First of all you have to check your wing/stab and engine incident with a Robart incident meter.
The wing, stab and engine should all be set at zero degrees incident. That's the general setting on most of all airplanes.

When you have flown the plane and know that the C of G is rellativly in the right position , check for nose or tail heaviness by flying fast straight ahead and then do a sharp pylon racing turn (To the left) using just aileron and elevator (as usual). The plane should, with it's wing more or less vertical, hold it's heading without the nose pointing down. You should be able to do a smooth turn without any corrections what so ever.
A nose heavy plane will always dip it's nose slightly in the turn.

Always fly at full throttle when you trim the plane for straight and level flight! Thats the rule!

RE: Plane won't fly level!
 

I beg to differ with Jaka, most models have a built in right thrust and down thrust although a very few do not. Since the contender has its wing center of drag line near the center of of gravity, it will require less down thrust than a high wing or shoulder wing model does. From your description of the increasing pitch up with added power, I'd almost bet your problem is not enough down thrust: you may even have inadvertently built in some up thrust. I like to trim all my models out so that, with increasing throttle, only speed increases-not climb rate. This means careful adjustment of thrust line although you sometimes also have to adjust the relative incidence between wing and stab.

RE: Plane won't fly level!
 

Thanks for this valuable information. I just want to add that I did add down thrust to the engine by using 3 relatively thick washers in between mount/firewall screws. The engine looks very tilted downwards and when I trim for level flight at high throttle and bring the throttle back quickly the plane would nose up sharply and climb until the speed drops .From what I gather from the responses it looks like that it is wing stabilizer incidence problem which I don’t know how it came about because I did follow the plan correctly during construction. I have built several kits before and all fly great. Do you check the incidence of the stabilizer as well as the wing?

RE: Plane won't fly level!
 

Technically...yes, the stabilizer incidence is checked.

Practically...sometimes no, for two reasons:1) A center line down the fuselage side is used, and most times this is NOT easy to locate/identify. 2) In most kits, stabilizer installation is uncomplicated, and installed with acceptable incidence.

IMO, what counts is the relationship (of incidence) between the stabilizer, wing and engine crankshaft.

So, since your stabilizer is happily epoxied in place...put a level on it, level it, then take a good SIDEVIEW look at your plane from far enough away so that you can eyeball from the front of the spinner to the end of the elevator (making sure that you aligned the elevator with the stabilizer)

Imagine a line through the chord of the wing (center of leading edge to center of trailing edge) and extend this imaginary line aft to the stabilizer.( or, from the stabilizer chord forward to the wing...whichever works). Most likely, you can visualize the incidence relationship.

With the stabilizer still level, you can get a good idea of your engines thrust line also.

Of course this all comes from a sport flier, but I hope is is of some help.

RE: Plane won't fly level!
 

I would put the engine back with no washers, using the built in thrust. Yes, you need to check the stablizer also. You can use a couple pieces of scrap wood and clamp them over the elevator and stablizer to make sure the elevator is centered when you check the incidence. If you are using the TE of an aileron when checking, you also have to make sure that is in a netural poition.

The pitch up when you cut throttle is to much down thrust. Remember that the datum line for incidence is a line from the center of the prop to the rear that is level when the plane is in level flight. Kind of hard to check in the air, so just eyeball the level and work from there. It is the relationship between engine down, wing and stablizer that is important. If you get the plane setup up and the wing shows -10 degrees, you can just adjust your other readings by the 10 and still get the relationship. It is less prone to math errors if you get the plane realy close to level though.

By the way, what do the plans show for incidence, I pulled the manual off ToPFlights site and they don't mention it that I could find.

added: For comparison, the 4*60 shows 2 degrees down on the engine, +1/2 degree on the wing and the stab at 0

Don

RE: Plane won't fly level!
 

Don,neither the instruction book nor the drawings show any figures for the angles of incidences so I would assume that all are meant to be at a zero angle .I will get rid of the washers that were added to tilt the engine downward and check the incidences. Thanks.

RE: Plane won't fly level!
 

I definitly believe you have a problem with the decalge,(difference in angle of incidence between wing and stab). Not sure of the plane you're referring to. I do know of one named Contender a low wing fully symetrical wing designed perhaps 30 years ago if that was the one. If however it is another with a flat bottom airfoil they will definitly cause problems with major speed changes since designed to be flown at low speeds.Try to set wing, stab at zero and engine 2 degrees down thrust. when flight testing check what it does at both upright and inverted flight. Should maintain level with none or very little trim adjustments and at all speeds other than very low.

RE: Plane won't fly level!
 

Your statements here need some analysis. Your first statement in the Original Post stated that the airplane just doesn't like to fly level. Several things can cause such a situation.
1.) A CG aft of 30 percent of the mean aerodynamic chord (MAC) will start creating such. For a TF Contender, the wing is straight enough to just use the average chord of the wing (chord at midpoint of the wing panels) to obtain good information. Since I don't remember that exact distance, let's say it is 10". If the BALANCE POINT (CG) is aft of 3" back from the leading edge, you are moving back into trouble city. Any point between 2.3" and 3" will not contribute to the situation you have. Aft of 3.3" will be moving into serious instability.
2.) Incidence differential: Regardless of all the hype reference some imaginary horizontal axis line of a fuselage, in convergent airflow (sub-sonic) the true datum line that sets all incidences is the Stabilizer Chord line. It is Zero. Aerodynamically all true incidences are then relative to that specific Zero.
3.) Flight Control Sensitivity: relates to the amount of elevator movement relative to your stick movement. Fast controls, especially if a short elev. horn in relation to a long servo horn, which moves the elevator very quickly and a large amount with slight deflection of the control stick.
4.) A very sharp wing leading edge or one that varies considerably in leading edge manufacture.

With all that, the contender is, IIRC, a symmetrical wing. That means if the wing and stab line are together the incidence is zero. Given the engine is zero with these two, the the aircraft will always be required to fly with some very small amount of up (or down if inverted) elevator. Back before all the mixes, etc., pattern fliers usually used about 1 degree of down thrust, which pulled the model down, then they trimmed a tad of up-elevator to compensate. When they rolled inverted, the elevator was seeking down but the thrust was pulling up, so the differences in trim were small enough to be resolved with very little if any other trim.
Now here you have placed 3 washers under your engine front. That provides lots of down-thrust at full throttle. You are trimming the elevator to compensate with the engine's nose-down force (loading the wing to down with less lift). Of course when the nose-down thrust is removed, the up-elevator takes command and UP SHE GOES. Welcome to aerodynamics. :D
My suggestion to you is this:
1. Be certain that your model balances laterally at a point within the 25-28% of the MAC. Distribute/add/remove weight nose and tail as needed. Remember that in most airplanes it requires about 4 Oz. of weight in the nose to compensate for 1 Oz. in the tail area. (depends on tail/nose moment arms)
2. Pencil in a line along the fuselage (forget incidence meters - have been flying very well CL and RC since before 1950 without them) that parallels the bottom of the stabilizer. Check that the centerline (chord) of the wing parallels that line. That is now zero wing/tail incidence. Set the engine to same parallel or to a maximum of 1 degree negative.
3. Check for warps with resulting incidence deviations. If not able to change, adjust your wing incidence in effort to zero out the deviations, or use trim tabs to compensate. That will take a few flights. (I have one 50" bi-plane, ST. .51, that I made some serious building errors. Wing turned out with significant warps. Using some trim-tabs, fixed and adjustable, that model is stable as a rock through all speeds, T.O. cruise. and landing. It can be done) Now that is all done, I think your machine should do well.
BTW: Check that your ailerons are parallell to the wing chord, or mabe just a SLIGHT down. If UP you need to adjust. It seems that one contender version had flaps. If so either SLIGHT droop but absolutely NO UP.

Now know that any aircraft trimmed for a given speed, will climb when more power is added, and start to descend when power is reduced. Lift being a function of the square of the airspeed increases or decreases with added power providing more speed or less speed when power is reduced. The given trim will TRY to maintain the given airspeed.

Contender Traits: The Contender horizontal stab - vertical stab. has a relationship that can cause some strange items. When straight and level, apply rudder either direction, watch it yaw in the applied direction, then roll the other way. [X(] This cause many pilots to constantly hunt for a better aileron trim which is not the ailerons, but caused by using rudder in maneuvering flight. A number of models do this and it is one of the first things I look for when into the first few flights of a new model.

RE: Plane won't fly level!
 

Geeeee...and to think I had HYPE all this time and never knew it![sm=bananahead.gif]

RE: Plane won't fly level!
 

OOPS! My BAD SafeTwire. I had a brain thing when I was composing that short-story. I do apologize for those words since you used that specific which it looks like I referred to.

Actually I was thinking of all these years building from plans, kits and a number of my own designs, that thought just creeps right in when I get into basic applied aeronautics for pilots. There is a lot of hype about flight performance within these walls of toy airplanes.

While my apology to you stands, so does my statement. The line that one uses to draw a fuselage may be very straight and the way to attain the desired appearances, but the airplane never knows where that line is. The airplane feeds only on airflow and the stabilizer in conventional subsonic arcraft sets that base line.

Thanks for speaking up.

RE: Plane won't fly level!
 

Apology and statement both acknowledged Hossfly.

We wanted to help raidh as best we could, as well as others, in our own respective way, and that's what counts!

RE: Plane won't fly level!
 

Thanks Hossfly and SafeT and all. I have visually checked the datum line with a stretched string (couldn’t get hold of an incident meter) and I believe the stabilizer leading edge level is probably pointing very slightly upward. Anyway, I will try the following lines of action successively.
a. Take out two of the three washers which were placed behind the engine mount to tilt the engine downwards as the engine level was way out of line with everything else.
b. Move the C.G some 2 cm forward.
c. Place a popsicles stick or two under the trailing edge of the wing and observe the changes in fly characteristics of the plane.
d. If all of the above didn’t work I will try to lower the leading edge of the wing; I really don’t know how to do that now but maybe there is way. I will report back once I find a remedy to this plane that wont fly reasonably level when changing speed.

RE: Plane won't fly level!
 

You can also try adjusting the ailreron linkages so they both droop a little from neutral when tx stick and servo at center point. Very easy to do and may work, if not nothing lost.

RE: Plane won't fly level!
 

It's a great learning experience. Your effort and determination at this time will help you with all future builds from here on in!

RE: Plane won't fly level!
 

Very interesting thread here. Some very thoughtful replies, well stated. I think the popsicle sticks under the trailing edge of a low wing plane is going to make your situation worse, though. It seems to me you need the opposite ...

Good luck,

Phil