Okay, I have a 40 size Tequilla Sunrise ARF.
Im using an O.S. 61 FX on it.
My question is this. If I yank back on the elevator, it seems to snap roll. If I do it softly, it dosn't do it. I converted the elevator to use dual servos with the servos in the middle of the fues to balance it a little better(Still a little nose heavy). Is this occuring because I am using a 61? Is it because it is too nose heavy? Or is it because I have too much elevator input? I was thinking about just putting my 46 FX on it cause I crashed my H9 Big Stick [:@] Anyway, please give me some input. Thanks guys!!

Too much elevator.

Yep, too much elevator.

You can try shifting the CG forwards, that usually allows you to deflect the elevator more before stall (by making the elevator less effective, so it's really counter productive here).

Check your lateral balance. If you have one wing heavier than the other, the plane will snap sooner than if the plane is balanced. The difference isn't usually huge though.

Yep, ANY airplane will stall the wing with enough elevator. The question is do you think it's doing the stall and snap prematurely or not.

Well, I checked the Lateral Balance, and its okay. I really dont want to move the CG forward because its pretty nose heavy as it is. One of the reasons why I put dual elevator servos on was to put more weight in the back so it wasnt so bad. The book says the cg is 5 1/2 inches from the leading edge and I have it at about 5. Im wondering if I should just put the 46 on and see how she does... What else could it be?

Okay, Im an idiot, I dont know why I said I didnt want to move the CG forward because its already nose heavy, since moving it forward would make it more tail heavy. Anyway, I think im going to try that and see what happens... Ill let you know. Thanks!

If the plans call for the CG at 5.5" from the LE and you have it at 5, moving it farther forward isn't going to help. In fact, you should consider moving it back to the recommend point to start with.

Actually, this made me think a bit more. I suspect you'll snap less if you do, in fact, change engines down to the .46. I think the extra weight of the larger engine is driving up your wingloading, and that will always increase snapping tendancy. So loose some weight, and the snapping will reduce.

There are many factors that can cause an airplane to snap. Basically one wing is stalling before the other. CG does have to do with how violent and what airspeed the snap, but it is not causing the snap. A forward CG will cause and airplane to stall at a higher airspeed. An aft CG likewise will cause a stall at lower airspeed. Your problem is that one wing is reaching the critical angle of attack (stall) before the other. It may be cause by incorrect incidence or maybe a twist in the wing. If you have an incidence meter check bother wings and make sure they are the same. Also check your elevator(s) to make sure that they have the exact same travel in up, down and neutral positions. You want both wings to stall at the same time. Having a forward CG will just make the stall occur sooner when you apply backpressure.

Hope that helps you a bit.

If you have a computer radio try mixing in some flaperons.Be careful try a little at a time.The effect is pretty noticable,even with a little bit.I use this only on my 3D planes!

Oh,yea.TOOOOO much elevator try dual rates.If you have a computer radio try to change the rate % till you get it right!!

Hold the phone, CG has nothing to do with the high speed stall that you are describing.

It is caused when the airfoil passes its critical angle of attack. In other words, when the wing is headed into the relitive wind at too great of an angle, it will stall, no matter what.

Moving the CG will effect your elevators ablility to force the wing past its critical angle.

I had a Tecate biplane. It was prone to doing the same as your plane is. My guess at the time was that the extreme wing taper was causing the tips to stall early, hence the relitivly low speed of the "high speed" stall.

In the end, there is not much you can do but change the airfoil, fly the manuver faster, or use less elevator. Darn physics is so legalistic

I'm no engineer, but my first impression was that you're a little overpowered on a .40 size model using a .61 size engine. This being the case, when you yank back hard, your airspeed slows and engine torque may take over, causing the snap roll. Does your ship snap opposite to the rotation of the engine, if so this may be possible. Just a thought, but at least (although for different reasons) I agree with the thread about reducing your engine size.

Engine size would probably only effect the speed at which the airplane counter-rotates to the prop-during a stall, not normal flight. Your airplane(specifically your choice of engine size) is not broke, so there is no reason to fix it. If your airframe can take the engine, I'd say keep it! The fix is not in the engine size but rather in usage of left rudder or less deflection. Like its been said, lots of elevator will put the wing at an angle of attack where it stops flying, and stalls-it has nothing to do with lateral balance, but physics. The air over the top of the wing is disturbed, and no longer laminar-destroying all lift. Ailerons would be useless at that point, rudder would stop the rotation which is caused by the torque of the engine.
I would keep the high rates on a dual rate radio and fly the airplane on low rates to get to know it. Chose the low rates based on your airplane, not necessarily the recommended deflection in the book. That's just a good place to start. It'll be a trial and error process untill the airplane stops snapping. My expierience is that every plane is different.
You may need more deflection for flaring when you land then your low rates will allow{(when your rates are adjusted right to fly at higher speeds without stalling the wing(high speed stall)}, so you may need to land with high rates, but be carefull so it doesn't bite ya', because it will still snap at lower speeds also.

As a side note, lateral balance may not sound like it should have an effect, but it does.

I spend a LOT of time eeking out every bit of turning radius I can get from my planes. It's not uncommon to be able to add 3-5% to my elevator deflection after balancing laterally, sometimes more. It shows up even more dramatically in the negitive direction. An out of balance plane really suffers when it comes to tight outside loops and turns as well.

The reason is simple. If one wing is heavier, than the airplane has to do something to create more lift on that side. Usually this results in the ailerons being slightly deflected, basically putting one wing at a higher AOA than the other. So when you pull hard on the elevator, that wing stalls first. Equalize the weight, and both wings track at the same AOA, allowing both wings to get to the critical AOA (or just shy of it) more consistantly, which to the pilot feels like a later stall, and my experience bears this out. Lateral ballance IS physics

Engine size will only matter if, as I said, it results in too much weight and too high of a wingloading. Lighter wingloadings mean lower AOA for level flight and less lift requirements at high rate-of-turn, so the airfoil operates at a lower AOA while making the same radius turn as a heavier plane. As a result, the lighter plane has the ability to further increase AOA, allowing more elevator devlection and a tighter turn than a heavier plane. Mass is physics, too

I will second that. I correct the lateral balance on my models as well.

Schmleff

Try left rudder and see if I'm correct. If that doesn't correct it, then I'd be more concerned with lateral balance, or elevater halves.
My reply is based on what was taught to me by an aircraft designer for the US Air Force. His name is Mike Fayhe, and he is also a rocket scientist. He worked on the design of the A-10 Warthog and other missles. I think he knows what he is talking about.
[sm=punching.gif]
I never said lateral balance wasn't physics, sir.

It seems consistant that most replies to these similar question is about elevator deflection.
I agree, and I think it makes sence to all of us. It is setting the wing to a critical point where it destroys the laminar flow over the top of the wing, destroying lift. I am still convinced it is engine torque spinning your airplane. Please let me know so I can eat my deserved humble pie if rudder doesn't fix it.

Well, you would with an avatar like that.



-David C.

Laminar slaminar, since when did all airfoils become laminar

What is his prop size?

Ok, revised opinion:

Check the lateral balance, check the elevator for eveness (I am not in this camp however), try diving for a little extra speed and pull the power right about where the thing usually stalls to take the torque out of the picture, and last but not least, consider that it may be ca combination of all of the above.

I still vote for the lame airfoil, because if that plane shares the same wing taper that its bi-wing brother, it will tipstall more than any other model I have flown. The Tecate was the worst plane I have ever owned. Now that I think about it, I know I guy that had the Global Raven, and it tip stalled constantly. He hated the thing.

only my $.02

Schmleff

Airflow over top of wing must remain laminar-AIRFLOW-not airfoil.
Good Grief!
The only way you'll eliminate the engine torque problem would be to shut the engine off.

Textbook:

A certain amount of air turbulence occurs on the surface of most aircraft wings, regardless of the shape and sizeof the wing. As air moves across an airfoil, it is changed by the frictional force between it and the airfoil's surface from a laminar (smooth) flow at the forward area to a more turbulent flow toward the trailing edge. The 'perfect" wing would demonstrate laminar airflow across the entire surface of the wing, with no sign of turbulence. This turbulence affects flying performance by increasing aerodynamic drag and fuel consumption.

Just because the flow is not laminar does not mean that the wing stalls. That was my point.

MMmm, no. Laminar is not the same as unstalled. Laminar is a special condition of airflow having little or no turbulence. This is why some folks develop laminar-flow airfoils, to help decrease drag. However, most planes do not have very much laminar airflow over the wings at all. In some cases, laminar flow is a bad thing. Laminar flow generally leads to more abrupt stalls at lower angles of attack. Laminar flow also causes a drag rise on many rc sailplane designs. I have one myself, where a strip of tape is used to make the airflow turbulent on the wing, which increases performance.

Back to the original question. I have a variety of airplanes. More since modern radios stopped crashing them for me. I tend to set the high elevator deflection really high and low for nice flying. With the high setting almost every airplane that has sufficient power can be put into a loop and held there. As I gradually increase the elevator deflection there will come a time when the airplane will snap.

Conclusion is that there are a lot of things that don't cause the snap roll but might help it out a little. Among the helpers are prop torque, prop wash, one wing heavier, asymmetric aero forces from any asymmetric configuration effects. etc.

But the big and main factor is that the wing stalls. Things in nature and models being like it is, one wing will always separate before the other. It results in a snap.

The 60 just enables the airplane to carry energy into the maneuver better, keeps the speed up and allows the elevator to be more effective such that it will hold the high angles of attack needed to stall/snap.

This is related to his question, and not simply banter!

I guess my point is this; When a wing-any wing-is put at a steep enough angle, the airflow over the top of the wing(whether perfectly laminar or not) is disturbed to the point that it no longer works to create lift. This is why the military still uses washout in their wings still today.
As I recall, the gentleman said he already checked for lateral balance, and checked his elevator halves, as well as tinkered with CG, none of which helped him any. What else could it be?
The issue here is that this problem was so frustrating to me, when no one would even try to explain what in the world was going on. And I had access to many people who competed in major events(TOC for ex.) as well as design the airplanes when I lived in CO, and when I approached them, even though they knew how to fix the problem, refused to help! I hate to see it when other people go through that too. This is why I'm so passionate about this topic. It is SOO important to understand if you want success!

Adding a bit of washout would not be difficult. You would need to have an incedence meter and a heat source.

Oddly enough, I did go back and do this to my Tecate.

Twist the wing a bit at the tip, heat the top covering and measure to see what you have got. It is then important to duplicate the same amout on the other wing. 2-3 degrees works well.

Sorry about being anal about the laminar thing. We are talking aerodynamics so it helps the rest of us to understand someone elses views when we use the correct terminology.

Schmleff