Hello guys....

I Built a Sig Mid-Star 40 last year and i have a big problem.
I build the plane as described in the book, with no modification at all. I adjusted both ailerons streamline with the bottom of wing before the first flight. When i took off for the first flight, the plane started to roll (i don't remember right or left) at a fast rate. I had to put the trim for ailerons fully to one side (left or right i don't remember) on remote to stop the roll. The plane has a OS 46 FX and the wing look straight. When i landed the plane, i looked at the ailerons and one side in 1/4 up and other side 1/4 inch down, and that is to make the plane fly straight !!! I know that we should put some "right thrust" on the engine and i just noticed after measuring my plane that my engine is at 0 degree, no right thrust at all. Do you guys think that it can be my problem ??
Having no right thrust would make the plane roll to the right or to the left ?? Finally, how many degrees should i put, considering that i put a bigger engine than they suggest. Should i put a little more right thrust ? If that's not the problem, what else can cause the plane to roll so fast ? ... a warped wing ?

Thanks all for your tips and help. really appreciated

Are you sure the wing is not warped? It's a wing that has a high-ish aspect ratio, and is therefore difficult to build straight.

The only vice mine has is that it takes one-and-a-half turns to come out of a spin after the controls are centered. The first time that happened was a heart stopping moment!

David C.

What do you mean by "high-ish aspect ratio" ??
This is only my second plane i built after i crashed my Pt-60 due a battery pack failure !

By high=ish, I mean the ratio of the wingspan to the chord is high.

That is, it's much longer than it is wide, which makes building it without a warp much more difficult than, say, a Somethin' Extra, which is a short stubby wing.

-DC

Bidule:

If the plane acted pretty much the same power on and power off the thrust line will make no difference in its tendency to roll.

I'm almost sure your problem, as DC said, is a warped wing.

The good part, though, if you used any form of heat shrink, whether fabric or film, you can heat the wing covering, while holding it twisted opposite to the warp, and take it out.

Might take you two or three tries, try to get half the warp heating the top, and the other half heating the bottom.

My last high aspect wing was a Goldberg "Electra," I set the washout in the wing with a heat gun, exactly as I explained here. Or I hope I've explained it.

Fix it, enjoy it.

Bill.

PS: If you want it to be aerobatic, leave the engine thrust line zero-zero. Offset thrust makes it act differently upright and inverted. wr.

PPS: DC, just noticed your sig line, "Klaatu barata nikto." Michael Rennie and Patricia Neal. I have a tape of the film. Did you ever read the story it was based on? "Farewell to the Master." wr.

WR: I thought K B N was the secret password in Army of Darkness?

It does sound like a warped wing, though if it was trimmable then the warp should be able to be fixed like WR said. I loved my Mid-Star - keep it light and it'll take care of you!

Ryan:

"KBN" was the "Magic" phrase Klaatu (Michael Rennie) taught Patricia Neal to say so the robot wouldn't kill her. This movie was, I think, 1950. Titled "The Day the Earth Stood Still."

Bill.

Hi Bill,

The Day the Earth Stood Still is definitely one of my favourite films.

I especially like the scene where General Cutler (my namesake - argh!!) has learned of the dangerous robot and sends all his troops all in different directions at once. Looks like not a single of his troops was in the right place at the time, and had to drive frantically to somewhere else, passing other troops going to where they just came from!

Yes. I think I did read Farewell to the Master (by Harry Bates, wasn't it?) although it was so long ago, I think it was written in Latin.

-David C.

When covering the wing it problay got twisted. Look at it from the leading edge with it installed on the fuse to see if on of the trailing edges is higher or twisted. Dont worry this is easy to fix with a helper and a heat gun.This is a comon problem with an open wing design.

Thanks for all your advices...
William i did not noticed any difference when playing with the throttle so i guest ym wing is the problem :-(
Is there a tool that i can buy to get rid of this problem next time i build a plane ?
Like gp "Accupoint Laser Incidence Meter" or
"robart incidence meter" , wich one is the best or any other brand better ?

Hi Bidule,

Incidence meters are useful to align the wing to the tail and could, perhaps, be used to check if a wing is warped, but that's not the main purpose.

You can avoid warped wings when building if you ensure, at every step of the way, two things:-

1/ When the plans ask for the wing to be pinned to the flat building board, you continuously check to see if all ribs, etc are doing just that.

2/ Keep in mind what might happen to the structure if an off-centre load is being applied. By that I mean when starting to sheet the leading edge, or gluing one side of the two trailing edge sheets. I find placing weights on a long steel ruler over the main spar is useful when gluing either the leading or trailing edges in place.

Whatever caused it, the fact that it's possible to trim it out in flight with the ailerons means the warp is not particularly severe, and, has been said before, it can be taken out with the use of the heat gun on the covering and a pair of helping hands to carefully twist the structure. Do one side, then let it cool, then do the other side. For some reason, that seems to work best. Bear in mind though, that you are inevitably introducing a stress in the structure which might, over time, relieve itself (especially in hot weather) and the warp might tend to return. Just check occasionally at the flying field.

Hope all that helps; and good luck! It's a great plane to learn aerobatics with.

-David C.

Bidule:

Beore we get into wing warps, there is a more basic thing to check. Lateral balance. If one wing is heavier than the other, the plane will want to roll. And while it can be trimmed out, the trim will change with airspeed.

If the needed trim is the same slow or fast, it's probably wing warp. But DO check lateral balance first.

David is correct about care to avoid warping while building, and it's important. But you can induce a warp when covering , also.
They are both good, and work fine. I have the Robart, the GP came out after I got mine. Which is better? Don't know, haven't used the GP. Looks to me like the GP would be easier to read once set up, but the Robart looks easier to set up.

But do not expect an accurate reading with either if you mount on the leading edge of the wing, and the trailing edge of the aileron. Remember, the aileron moves up and down.

You say "I can eyeball the aileron to center." OK, eyeball the incidence too. Equal accuracy. Use the incidence meter only on the wing at a place where the trailing edge does not move.

This means, for most sport planes, an incidence meter is useless after the plane is finished. While building, or as a test during initial assembly checks, they are highly recommended.

So, since the plane is already flying, does this mean you're out in the cold? No,

But before we go on, let's define two terms. "Wash-out" is where the wing twist is toward a lower angle of attack at the tip. Put the airplane down, with the nose pointing to your left, and the wing tip toward you. Consider the wing, next to the fuselage being level. If the wing tip is then in a dive, twisted counter clockwise, that is "wash out." If the root is level, and either tip is "In a dive," that panel has "Wash out." Wash out is good.
"Wash in," is the opposite. Root level, tip in a climb. "Wash in" is bad.

Just a little more, and we'll get back to correction.

With wash out, the angle of attack at the tip is lower than at the root, so the root tends to stall before the tip That's good. "Wash in?" the tip tends to stall first. Stall/spin on final approach, broken plane, bad.

Put the wing on a flat table, with the trailing edge of the wing at the edge of the table. Get two blocks about 1/3 to 1/2 the thickness of the wing, be sure they are equal thickness. Put the first block under the wing at the inboard end of the aileron, be sure it's holding the wing and not the aileron. Now try to put the other block under the wing at the outboard end of the aileron, supporting the wing the same way you have the other block.

Now. If the second block is loose, the trailing edge is twisted up, you have wash out. If the wing lifts off the inboard block when you put the outboard one in, the trailing edge is twisted dowm, you have wash in. If both blocks fit nicely, the wing has no twist.

How's that for easy?

The problem is that it only works for a straight, untapered wing.

But if a tapered wing has enough twist to worry about your eyeball works well enough when using the "Flat board and block" method, you can see the twist.

The other method is just as simple, but gives mixed results..

If the plane wants to roll right, taking left aileron trim, twist the left tip to add wash out. If the roll is to the left, twist the right tip down. Problem is, you don't know where you're starting. You could end up with six or eight degrees of wash out on both tips. It would fly, but it could fly better.

Sorry if I've been too wordy, I wanted this to be such that even a total beginner could understand.

Hope it helps.

Bill.

Dont over tech this wing relies on the covering for strength . even if it was built straight it is easily twisted when covering. and if you straighten it with the heat gun it will stay. This is a very light wing with little structure. This is a very good plane with light wing loading it is the next step from a trainer. The orange one I pictured is 2 years old with more than 200 flights the only weird thing it does is its slow to recover from a flat spin.

Thanks a lot William for your really good explanations.
You'll never be too wordy for me, what i like is detailed explanations just like you did because i am considering myself still a beginner. When i ask a questionn, i read everybody's reply twice or sometimes even more just to make sure i really understand eveything. I have never reallly understand what wash-out was before you explained it to me. I know there is a wash-out on my wing (Mid-Star 40) because it says in the manual that it has a "built in wash-out" and that's all it says. I don't how how many degrees. Since there is no difference in roll rate when i change the aircraft speed, like you all said, i guess my wing is warped. I know now that my aircraft is rolling to the left because i just checked my ailerons on the wing. So i'll give it a try and do like you just suggest me William and "twist the right tip down" a little bit.

Again thanks a lot...

One other thing, is your wing centered properly?

If your wing is longer on one side, it'll generate more lift and roll the plane.

Also, if the vertical stab straight and centered properl. A stab cocked just a bit to one side or the other would also roll the plane.

Before you do anything check your lateral balance. I just finished building a Midstar and it the lateral balance was off quite a bit. You will love this plane once the problem is fixed.

Robot's name was "Gort", I believe.

1/4" is a lot of aileron, so warp sounds the most likely.

Having the wing mounted crooked, so one side is further forward, will also give a slight roll; and a dutch roll in turns.

Have you checked the aeroplane by holding at the spinner and rear fuselage tip to check if it is in balance around the long axis? One side of the wing could be heavy.

Just some additional things to check.

A heavy wing panel is more pronounced in a high-g maneuver compared to level flight. 1/4 inch seems like a lot.

Try this. Put your plane on a table and stand about 10 feet out in front, looking at the nose. Gradually kneel down until you are looking at the leading edge and just across the top of the wing. you can easily tell if one trailing edge is higher than the other.

Measure both sides from the fuselage out to make sure they are the same. If one side is only slightly longer, you can boost it out with a soft balsa "rib." You should weight the other tip to balance.

I've had a couple of 4* 60's and one thing about all of Sig's Stars I do not like is the type of wing construction. They use a multi-spar type construction rather than a sheeted leading edge. This type construction is easier to warp than the D-tube, sheeted type.

I have stiffened these type wings with a couple of extra stringers. I used a 1/8 square spruce spar, mounted from the main spar at the center sheeting diagonally back to the trailing edge on top and the opposite diagonal on the bottom (center sheeting at trailing edge to the main spar at the tip). Looking through the wing they make an "X." This stiffens the wing in twist.

Hello...
i just checked the lateral balance of the wing only (not mounted on the plane) and one side is a little heavier than the other side.
i have to put about 2 quarters on one side to make it balance. But that doesn't seem a lot to me , considering that the plane has a really fast roll to the left. Maybe i have a combination of many factor affecting the flight. Should i check lateral balance of the wing only, or better to do lateral balance of aircraft with wing bolted on it ? or maybe i should do both ? wing and a/c ?
My os 46 fx in mounted straith up, so the muffler in on the right side, and is probably affecting lateral balance.

Bidule:

Check lateral balance with the plane assembled. If you find the right side heavier, that is working against your left roll. But it still should be corrected.

Bill.