Hi everyone. Help needed!.
I also posted this question in the pattern forum as it is a pattern plane, however it is aerodynamics thats probaly my problem.
When I pull in up elevator my plane snaps to the right. Any ideas on where to fix problem anyone?. When Im inverted and push down elevator it seems to track okay.
It just uses one servo for both elevator halves, with a "y" joint in the pushrod.
I have adjusted one elevator up, and down in relation to the other one, and that seems to make no difference, it always snaps to the right when loading up the elevator. Recently I just relocated the elevator servo right back in the tail of the plane, so its now a direct connection from the elevators to the servo, via a 4/40 rod. This hasnt helped at all!.

I've also tried putting some weight in the left wing tip, (about 20grams) that did not help either.

Now I will re-hinge one aileron, as one of the hinges has broken, could this be the problem?
The aileron still works fine, just the hinge closest to the centre section is broken, I have some tape on it and have been flying like this for months. I will re hinge and test, but I cant see this being the problem?.

Also the wing seems very weak, as in esily flexed, could this be my trouble? maybe one wing flexes more than the other when I pull in up elevator?. Both sides appear to be just as flexy.

What about CG, could this cause this problem? I have played around strapping weights on here and there but to no avail so far!.


The harder I pull in up elevator the worse it is, eg: slow speed pulling in up will just break, roll to the right, medium speed will cause full snap(maybe more barrell) roll, its scary stuff.

Its a Flair Swallow with an OS91FX.

Basically I'm just looking for suggestions, cause I have run out of idea's!.
Thanks Guys.

Thats what happens with to much elevator! You basically stall the wing, one wing stalls first, and it "snaps" in one direction. Most of the time this snap will be in the same direction everytime.
How much ele throw do you have? Sounds like it could be to much.

Thankyou for your input.

The elevators have around 25-30degrees travel.
The plane feels real touchy(sensative) around neutral elevator when flying, so for this reason I preffer to fly with as little elevator travel as possible, however when I dial less elevator in, I seem to strugle with inverted stalls, and negative spins, so for this reason I have around 25-30degrees travel on elevator to make it possible to snap, spin, and stall. Its so sensative I even have to have 100%epo on to soften it out. I tried adding heaps of weight to the nose,around 100grams, and this did not help to soften the elevator out, and it made it harder to stall,spin and snap, so I took the weight off.

So in a momment I will head out to the shed and re-hinge my dodgy aileron, I will let you know how it flys after testing.

Thanks again.

A guy at my flying field has this exact model but with an OS 91 four stroke.

He describes it as having good flying characteristics at speed, but a vicious tip stall. Also he said the structural integrity of the body was suspect - could it be that the wing flexibility you mention is also an indicator of suspect design/build?

Like a lot of pattern planes the swallow has a straight trailing edge and swept leading edge.

This will have a number of effects aerodynamically and strcuturally. E.g. for the same wing area/AR, a tapered wing will have a larger chord at the the root - this would normally mean a deeper section - which is structurally stronger than a non-tapered wing with the same AR and area.

Aerodynamically, the lower reynolds number of the tip section on a tapered wing may induce a stall which progresses from the tip rather than the root. If you check out Low RE lift/alpha polars you can see that some sections don't perform well at the Re of a typical model stall - i.e. full-size planes may get away with more taper before a nasty tip stall than the models we fly.

The wing of the swallow is effectively a tapered wing with moderate sweep. Sweep has a stabilising influence equivalent to dihedral - except of course it will work upside down too!
So this will help these pattern planes track smoothly during manouvers - earning them more points in competitions.

Also aerobatic ships should be capable of snapping on elevator - this is built into CAP, Edge, Extra etc. designs - because without a swift spin entry some manouvers are hard to induce on demand.

Swept wings even with no taper have a stall which progresses from the tips rather than root - so the snappiness you are experiencing could be behaviour by design.


However it seems that your Swallow snaps when you don't want it too. My guess is that this could be to do with the finish on the leading edge on the wing.... Is it really sharp? or really round? Sharp will make it snap into a stall more easily, round would be better for avoiding a premature stall.

wing flex could also be a problem however I would expect the wing to flex with induced washout on a swept wing whereas a swept forward wing would flex towards wash-in. and wash-in is tip stall inducing - so maybe your flexible wing is not a culprit.

tricky to know what to suggest. All i can say is keep your elevator travel down or put some serious exponential on the elevator stick on your TX.

I have this same problem. I fly a CAP 232 from yellow aircraft and the only solution I have come up with is that there is none. It is merely a design flaw/attribute i guess. Theres realynothing you can do about it from structural standpoint. I have split elevators with a "y" in the pushrod, pretty much exact same setup as you, and i have the same issues. I just use the plane as a speed demon and use elevator throw gently. I only havea 25-30 degrees of throw like you and mine still snaps. It rolls like a dream though!

As an experiment, you might try raising both ailerons about an 1/8 of an inch. acts like washout. Might tame it down a bit.

The problem Smoothpass notes, can really be a bugger.
Typically -it is a simple high speed stall.
the fix for ONLY that problem is to reduce pitch to eliminate the stall.
But -on models there are a couple of other gotchas
If the model is constantly yawing - (hard to see)- any sudden pitch makes lift change from wing panel to wing panel -and -over she goes.
try various rudder trim
Also if the elevators actually move at different rates -- this will do the same thing - make the model yaw -then pitch.
The best "FIX"- is to make the airframe as stiff and accurate as possible
BUT also, add no weight
The higher wingloadings typically operate closer to stall angles -so the problem is worsened
just keep pecking away at it .
Looks like a lot of good answers posted here - just start with the easiest fixes first-

Is this snap a speed-sensitive behavior? Specifically, if you fly faster, can you perform the maneuver you want without snapping?

Just wondering if you're flying too close to stall speed.

Good luck,
Dave Olson

My guess would be to much elevator deflection. Do you have a computer radio? you could dial in some expo to smooth it out near neutral.

smoothpass,

I don't know if a flying buddy's experience with a Cap232 (ARF) will be of any help. He experienced problems quite similar to yours. We tried all that you have, as well as all of the great suggestions in the other replies provided here. None of this worked. Then, purely by luck, while we were checking wing/stab incidence angles for the umpteenth time, he thought he saw that the left half wing thickness was greater than the right. We immediately got out the measuring tools and found almost 3/8 inch thickness difference (at max thickness point) between left and right at the fuselage sides, and 1/4 inch difference near the tip.

He contacted the manufacturer of the model (I apologize that I don't remember which one) and they sent a replacement. With the new wing the plane flies beautifully!

My Yak does the same thing. Happened to me coming out of a spin...

25 - 30 degrees of elevator is lots of elevator on a pattern plane! If absolutely required for snap/spin maneuvers, you might think about dual rates for just those times when you need it.

But if the plane is always snapping to the right, that indicates some form of imbalance. For some reason, the right wing is always stalling first. Two things come to mind that can cause that. Lateral balance and wing twist.

In my experience, the usual culprit is wing twist. Have you checked the plane with an incidence meter? Block the plane up so that the wing center is at zero degrees. Check both wing roots, are they at the same angle? They should be! Then check both wing tips. Are they also both at zero? They better be! Any twist in the wing will be compensated for with aileron trim. Now you have two wing tips with different cambers that are flying at different angle of attacks and will stall at different speed/AOA. That is the recipe for a consistent tip stall, one wing tip always stalls before the rest of the wing.

The other possibility is lateral balance, usually not a player. Trim out for big, long straight and level passes. Then roll inverted and make a long pass with no aileron input. Does the plane roll? If it rolls right, the left wing is heavy. Add a chip of lead to the right wing and repeat. When you've got it balanced to trim out in roll both upright and inverted, the lateral balance is right. Even if this isn't the cause of your problem, you need to do it for your maneuvers to be true so do it anyway.

What is the wing construction? If it's balsa and shrink covering, you can adjust the wing twist with a heat gun. Twist the wing the opposite direction and reshrink the skin top and bottom. Retest with the meter. Get both wing tips to the same incidence. This has tamed many tip stallers in my past.

Dave

I would check the leading edge of both wing panels. A poorly shaped leading edge on one panel can cause it to stall quicker on one side than the other. Also you said that inverted it acts different.

It's a combination of nose heavy and too much elevator travel.The OP is running excessive elevator travel to compensate for a nose heavy condition. This would be evidenced by having to hold a fair amount of back stick at slower speeds and a fair amount of forward stick while inverted. I'm also betting that the airplane took a fair amount of up trim when maidened.

I agree.

Let me suggest that the guys whose planes are showing this behavior make all the basic checks such as CG, but don't stop there because the CG shown on the plan may be incorrect. Since the guys who are observing this issue are very observant and cognizant of model behavior, I suspect they would like a more thorough approach. Actually calculate it using one of the online calculators. I've used "RC Aircraft Proving Ground" recipe with good results and use 15% Static Margin for the calculation.

15% Static Margin produces a stable model with no tip stall or high speed stall tendencies. More static margin moves the CG foreward and less moves it back.

One more thing, if the CG is pretty close to where it should be and the elevator required is still 30 degrees, the model has insufficient elevator area. Add area and reduce the throw. None of my designs require more than 15 degrees for the most violent snap rolls. I usually fly with a max of 10 degrees of elevator (mid rate). Even at this deflection, the model snaps fine, just a little more slowly. No reason a pattern model should need any more than that

Make sure that you didn't leave any rolls of coins, tire weights, rounds of ammunition, lug nuts, or any tools inside the fuselage. Overweight planes never fail to behave like this.
Yes...a nose heavy plane will fly "heavier" than what it really is. It puts the wing at a disadvantage. How well does this plane perform after the prop stops spinning..?