Can your pit man tell if the wings are a level extension of the lines during upright and inverted flat laps? If so, the tip weight is probably okay, and the wing doesn't have any twist. All I can think of is maybe the stab is not parallel with the wing.
Rusty

Maybe reversing the prop rotation helps?

Hi All,

I recently built a simple .40 sized flapless electric powered stunter, and I am totally puzzled as to why it has the outer wing tip going down on UP elevator and going up on DOWN elevator. It hinges on the lines as if I have too much tip weight in it. I have even removed the tip weight and moved the lead-outs back and forwards and it makes no difference to this hinging. I have included a 3-view with the wing dimensions on it. The wing halves have equal span and are made from foam cores that are covered in medium tissue and white glue. The weight is 1140 grams (40.2 oz.) all-up with battery on board, and the wing area is around 576 sq. inches. The wing loading is not excessive in my view, but maybe the wing itself is too heavy?? Just btw, I am flying at 5800 ft. ASL, so this also means thin air.

The CG is at 18 to 20% of the wing chord from the leading edge and it flies pretty well for a trainer. Maybe the wing span is too short, or the aspect ratio at 4:1 too low?? Any idea's?? Thanks.

  You probably need a rudder adjustment. It's very unlikely to be a design issue. Constant-chord wings on Profiles are particularly sensitive to roll/yaw coupling.

    Does this one have a reverse-rotation prop?  That can also cause substantial "funnies", too.

     Brett

Some or other world champ in Slovakia kindly pointed that out to me this morning.

I am sure I know one wch who will recommend trivial solution - less tip weight ... and since I know whole storry, I will also point out, that if you cannot remove tip weight from outer wing, you can add little bit instead ... but to the INNER wing tip    :- ))))

Thanks Igor....I will try the inner wing tip weight first. Looks like this week-end is going be blown out in our area with rather strong wind, but I'll report back as soon as I've done these tests.

I really like the .40 size models, especially now when the 2820 size electric motors are popular and inexpensive (not cheap!). The APC 11 x 5.5 E prop is also readily available, and a 2200 mAh 4S Lipo works well at our altitude as well. When I had the old Big Eze with the Tower .40 on it, I asked the kids why it was that they all preferred to fly the diesel version. Eventually it came down to the fact that they felt confident with it because they knew that it would not quit on them suddenly. The glow versions often spluttered and died near to the end of the fuel, just before the overhead eight or clover. Even if the diesel spluttered a bit it simply would never quit, and the other thing was that it ran at a very predictable constant rpm. Today, electric power is very similar, only easier now and "usually"" smells better as well. The pilots know that the motor will run the same all the time.

Keith R

What is the 2mm misalignment direction?
Is the inner wing forward of the outer wing, relative to the stab (looking down on a "plan" view)?
If so, this combined with a forward CG would cause a right roll on up elevator (and even in level flight I think if it was really cg forward) if it was an RC plane.
So that coupled with a couple other things might be the culprit.

I hope you find out what it is, because now I'm curious!

Good luck,
Chris

Weird indeed.
Hope you find the cause.
Goodluck and regards.

Yes Theo is indeed a fast learner, and his flying and building skills have come on so nicely. His enthusiasm is also contagious so I enjoy being around him. He is as you know, an avid 1/2A guy.

I'll get back to trying to sort out my rolling plane in a few days.

Keith R

'
   Oh, I missed that part. Certainly, remove tip weight or add it to the inboard until the hinging goes away, then see what you have.  Brett

I'm very pleased to see this. I have never "trimmed" a stunt model so far, but it did occur to me that E-Z-Duzzit, (although not flapless) with its unusually long inboard wing, might benefit with a little inboard tip weight.

I'm very pleased to see this. I have never "trimmed" a stunt model so far, but it did occur to me that E-Z-Duzzit, (although not flapless) with its unusually long inboard wing, might benefit with a little inboard tip weight.

    There aren't any "tip weight police", so if you start taking it out of the outboard and it still hinges when it's all gone, there's nothing wrong with putting it on the inboard. It needs what it needs.

     A lot of people tend to create this situation by selecting the wood and putting all the heavier wood on the outboard wing - which saves a bit of weight overall, usually,  but also makes the flex properties of the wings different, which is potentially a MUCH bigger problem than fractions of an ounce of overall weight.

  Beware the other problem, though.  If the fuselage is way off-center, the engine thrust line is also way off-center and adding tipweight on the inboard side can make the takeoffs even more hairy. Same issue as the All-American. The solution is to use a lot of engine offset to keep the thrust line through the CG.

  Brett
   

I built a bunch of airplanes with this wing shape.  They have significant "dihedral" effect.  Thus if they go through the air left wing first (as they will downwind, especially), they will roll out in maneuvers as if they had too much tip weight.  Balancing that by moving the CG left can cause other perversions, e.g. the right tip stalling before the left.  I suggest that you ascertain that the rudder or leadout position isn't causing a lot of sidslip.  Take out the warp first, though.

Edit: I looked above and saw that Brett already said this:

 You probably need a rudder adjustment. It's very unlikely to be a design issue. Constant-chord wings on Profiles are particularly sensitive to roll/yaw coupling.

    Does this one have a reverse-rotation prop?  That can also cause substantial "funnies", too.

     Brett

You might wait until you change prop direction to do the dewarping.

Again, I have yet to see any particularly interesting effects of changing the spin vector direction that cannot be readily explained by looking at the CG and thrust vector.

I'm not sure what you mean.

I am curious about the forces and moments on propellers with the axis at an angle to the free stream.  This, I think, is amenable to measure using a Miata, a broomstick, and some baggage-weighing-scale strain gauges.  I'm also curious about forces and moments as functions of angular rate.  So far I've been too lazy even to do a literature search.  If anybody wants to conspire on an experiment, please let me know.

I'm not sure what you mean.

  What it means is that if you aim the engine somewhere, the effects seem to be consistent with assuming that the thrust vector moves accordingly. In other words, it *does* seem to work about like a rocket engine, in motion, or not.

    Brett

O.K. yesterday we had really good weather, and I even finished painting the beast......except for the inner wingtip because I needed to stick some weights on it. I could not "bend the wing straight" so I opted for a trim tab on the outer wing. It did not take much tab to level the wings. I replaced the pusher prop with the standard APC 11 x 5.5 E and the same problem was still there. It actually flies fairly well for a stunt trainer, and if you don't whack the controls really hard, it can perform a pretty good pattern.

I then added 10 gram of weight to the inner wingtip and tried it like that. I could feel the difference but's it's hard to explain, and the outer wing still drops in a hard inside loop, and the same with the outside loops. I added another 10 grams and it still does exactly the same but no longer flies as well. I played with the lead-out guide and could get the outer wingtip hinging forward when I went back too far. The best position I found was with the front line close to the CG. I did consider removing the outer wing tip weight but then did not bother because the extra 20 grams in the inner wingtip did not seem to effect the problem at hand.

I checked the balance between the wings by removing the tip weight and the inner wing drops due to the extra weight of the lead-outs. I added 10 grams to the tip weight box and the model balances level. I fly it with 20 grams and like that the outer wing drops, so I would guess that the wing halves weigh close to the same amount. Right now after reading what Brett and Howard say about this sort of wing, I think that maybe I got the aspect ratio wrong, and it could be too wide. I will most likely build another one with more wingspan and a narrower chord to see what it does. Like I said, it flies quite well even with this problem, but that is most likely due to the good "engine" run! Here is a pic of how it looks now.

Keith R

I then added 10 gram of weight to the inner wingtip and  ..... the outer wing still drops in a hard inside loop but ... no longer flies as well.

so I would say you are hiding some trimm issue by excessive tip weight, I would firts set proper tip weight and only then try to solve "that other problem" :- )))  ... may be PW trimming chard will help ... I should have a link somewhere in my incomming mails, if you need it, I can find it somewhere, someone sent it to me recently :- )))))))))))))))))))))

Hmm....Igor, I wonder who sent you that link to PW's new trimming articles?? 
Keith R

     Good one Keith!

I was a bit sad to sell my Sport-E before I tried the idea of adding more outer wing tip, so this week-end I had a chance to try this on my .15 size Elec-trick which exhibits similar problems. The outer wing is 12 mm shorter than the inner wing. The first thing I tried was to remove all of the tip weight. I set the timer for some 1 minute flights and flew it. The outer wing still dropped badly in inside loops and outside loops. I then added a foam wing tip that makes the outer wing much longer as you can see from the first photo below.

The outer wing immediately flew high in level flight. I managed one rather scary inside loop and one outside loop which was a bit less scary due to (I think??) the standard prop rotation. I added 10 grams of tip weight and this was a lot better but still lifted quite a lot in level flight and inside loops, so I cut the foam tip in half and flew again. Once again there was an improvement but still flew outer wing high upright and inverted. I then cut the foam tip so that the wing panels are equal, and what do you know?? The wings flew level with level wings through the loops!

I would therefore guess that the wing panels work the best when they are of equal length in these basic models. It still dropped the outer wing in a hard inside turn like a square so I moved the lead-outs forward and this was a lot better. I most likely have too much elevator throw. I also may find that a narrower chord will work better overall as I could be getting too much drag when the wide chord changes angle of attack.

Keith R

Hi Chris, I am a "suck it and see" engineer!

And why you did not try it with tip weight?

Do you mean tip weight on inside wing tip?

O.K. Thanks I'll try that and see what happens. Can I borrow some of the modelling clay that you use please??

Why not? you have to match center of lift with center or gravity somehow. Does not matter if you move one to another or another to that one   ... usually we move CG by tip weight, so you used tip weight, who says it must be only in that outer wing?

The only difference between those 2 methods is, where is your fuselage (thrust line) relative to CG or center of lift after first or second modification.

Why not? you have to match center of lift with center or gravity somehow. Does not matter if you move one to another or another to that one   ... usually we move CG by tip weight, so you used tip weight, who says it must be only in that outer wing?

The only difference between those 2 methods is, where is your fuselage (thrust line) relative to CG or center of lift after first or second modification.

   The issue here is the fact that all the heavy parts of the engine are on the onboard side of the fuselage, creating "built in" tip weight. Plus whatever mass asymmetry you might already have built into the structure.

  Note that the real differences are that the *tail* is also effectively off-center, AND, that the thrust line relationship to the CG changes.

    Whether it is better with an asymmetrical wing, or tip weight, or in what combination, is open to question. *I* would probably opt for about the right asymmetry (1/2"-ish) and the engine straight ahead or slightly canted outboard (right). With symmetrical wings you build in some effective engine offset, because once you get the right tip weight (inboard or outboard) your thrust line will pass inboard of the CG (since with equal-span wings, the lateral CP is shifted a bit outboard and that's where the CG will wind up, too.

  Brett

Keith, higher aspect ratio wings(anything or 6ish) require a fairly steep taper.  A rectangular wing with a 6+ aspect ratio will be quite sensitive to rough air.  I went at far at a 15in. root, 6in. tip(40% of root).  That seemed to work pretty well.

Hi Phil,
Thanks for that. I was thinking more about the wing stalling and dropping the outer wingtip. My wing on both of these models has an aspect ratio of 4 so I was thinking that maybe 4.5 to 5 would be a better idea to try next time?
Keith R

I think it's unlikely the tip is stalling from  getting overloaded.  If the leadouts are too far back the plane will be flying in a sideslip.  Any kind of maneuver tends to make the trailing wing tip drop.  If you get the leadouts are put too forward the plane will actually be turning in at you.  It gets rather hairy if you do a sharp corner upwind and the plane tends to point its nose in when the motor quits, requiring a lot of whipping to land it smoothly.  Get the LO in the right place(with everything else right) and the wings will stay level in maneuvers and it will transition better in the eight maneuvers.

The easiest way to figure out what is going on in get copies of Wild Bills articles in American Aircraft Modeler and the various Round and Round columns in Model Airplane News in the early '70's for some more aerodynamics for C/L.  For example, on a 48in. wing the MAC is offset outboard about .44in(~11mm).  On a full body stunter the thrustline is down the centerline.  On a profile plane the motor is typically mounted on the outboard side, leaving the thrustline only a bit over an eighth of an inch(3.3mm) from the MAC.  That makes profiles much for susceptible to other minor spanwise imbalances.  A good reason to put in 3-4 degrees of engine offset.