I have a plane which seems to roll a little and pulls to one side when I pop the elevator - nose up from level flight. Im wondering if the elevator halves- on 2 servos - are aligned properly throughout the travel from start to finish. Any tips on how best to measure this? Could you more experienced guys tell me if I am on the right track in terms of figuring this out?

First, what Transmitter are you using. The 9C has an issue when mixing elevtor halfs, one noticably lagging the other. If you are flying a 9C, that may be what you are seeing. There was a thread recently about it, and the solution is to use a match box of sorts and Y the elevators off one channel. The match box allows you to reverse one half and then tune the center point so both halfs are syncronized. I've got on plane with the two servo elevators on a 9C, and I havent noticed it. Of course, I not that good of a pilot that I would notcie something like that.

Don

I am using the Futaba 7Cap with the Futaba version of the matchbox (Futaba MSA-10 Servo Synchronizer).

I would tape 2 pushrods on the elevators and have them pointed back and almost touching behind the rudder. Then go through the movements and see if the are working the same

Here is a bit more info on the method described by AirWizard,

Scroll down this page until you get An Idiot Proof Elevator Differential Gauge on this page [link=http://www.rcpattern.nxs.net/tipsB.htm]http://www.rcpattern.nxs.net/tipsB.htm[/link].

Cheers
Pupmeister

Wow.. there has to be a better solution than mixing in opposite aileron to counteract the differential.

Perhaps matching servos, but this could be a pain in the butt. Or remove the two servos and adding one with two pushrods.

My Venus II has one servo and a dual pushrod setup to control the elevator movement. I used Sullivan Carbon rods and a method to join them at the servo end. The servo was in the regular servo tray. The pushrod guides were secured at hard points along the formers to prevent any flexing. At least they both move the same amount. But, I do have a Super Star 120 that has a pair of elevator servos and this was one of my concerns. I am anxious to see the replies to this.

CGr

I don't like the idea of mixing aileron. It's really just covering up an existing problem. That solution also adds unnecessary drag.

Face the front of your airplane so that the elevators are just hidden behind the wings. Now slowly pull back on the elevator. Does one half appear before the other? If so, that's the high half. The halves may also look straight when neutral, but misalign after being moved because of asymmetric travel. This is usually caused by one of the servo horns not quite being set up square at neutral (perpendicular).

You could also have two servos that move the same but one has gotten a bit weaker over time. Put 'em under an equal load and see if they still move the same.

May be that
Probably isn't, but still worth the look.

Also, if the airplane always rolls left on inside loops, and always rolls right on outside turns............

It just might be P-effect.

Depends on the power of the engine, size and pitch of the prop, and a few other factors for how significant P-effect can effect what it effects. But it happens.

The prop sees a favorable change in AOA on one side of the nose while the other side sees a less favorable change. Has to do with the upgoing blade versus the downgoing blade. It's real and causes the fan to pull in a yawing direction. And when you yaw an airplane, the faster wing gets a favorable change in AOA while the other gets a degraded AOA. Sound familiar? We can usually see roll pretty easy. Seeing yaw ain't so easy.

It just might be caused by this.
This effect happens more than we usually identify. How often do you see a model that's taking off turn left. Nose is pitching up on what amounts to an inside loop (sometimes more loop than takeoff) and the p-effect gives some yaw. This has a probability of effecting what you're seeing about on a par with how often you see your model want to turn left on takeoff. That said, if your model has tricycle gear, discount the amount. They don't pitch up as much on takeoffs. Anyway.... it's worth considering.

It's also a fact that if something else is the main contributor to what you're seeing, p-effect will keep helping yaw left on insides and yaw right on outsides. So don't look for a cure that tests out with complete removal of all rolling.

I put together a quick drawing of how I did it on my Venus II. I used an Airtronics 94731Z heavy duty coreless servo (high torque). Of course, any similar servo will work, I just happen to prefer this particular servo.

Anyway, I used the Sullivan Composite rod, http://www3.towerhobbies.com/cgi-bin...?&I=LXFV44&P=7 and a home made piece of plywood. See the below drawing. This setup, if secured properly (sand the sleeve at the hard points at formers, cut a slot the size of the sleeve or drill a hole in the former to accomodate the sleeve, then epoxy it in place at each former) is very strong and works well even up to 2 meter pattern.. so it will work on just about anything. It takes some time to do right, but it works and allows adjustment for both length and differential.

CGr

Since this is the beginners forum, I'm going to make an assumption that the OP is unfamiliar with the concept of the snap roll. What plane are you flying? "Popping" the elevator as you describe it, depending on the plane, can change the angle of attack of the wing abruptly, puting the wing tip into a stall. This is the normal way to initiate a snap roll, and that might be exactly what you're seeing. In which case the answer is to use less elevator to bring the nose to vertical. Your plane may be doing exactly what you're telling it to do.

Brad

Brad: This is off of the subject matter, however, perhaps a quick answer (maybe a PM to avoid hijacking the thread). How do you intentionally snap roll? Just curious. I've tried it but can't seem to get it right... perhaps my knowledge of the subject needs some honing.

CGr.

Snap rolls are initiated by first feeding in full up or down elevator, followed almost instantaneously with full left or right rudder.

The idea is to stall the wing with elevator and as soon as it's stalled, initate the snap with rudder. Some aircraft also want some aileron added, but usually this slops the snap up a bit.

Some aircraft will do an ugly snap roll with full aileron and elevator simultaneously applied.

Hold the settings for the snap (at altitude!) (full rudder and elevator) and most planes will then enter a spin. Neutralize controls, apply opposite rudder to stop rotation if required, elevator out of the resulting dive when sufficient airspeed is attained. What fun!

J

It is the most common cause.

I have some time with out using a Futaba radio but I can say that those problems can be solved with subtrim and travel adjust on my JR radio. (after first checking the servo horns)

Thanks, Jburry.

I will give that a try next time out.

CGR

Flying a Hanger 9 Showtime. This is my 6th plane thats flown...but am just getting into precise trimming and adjustment as opposed to masking by using trim tabs on radio.

If you're flying a showtime and pulling full elevator quickly to "pop" the elevator, I'd bet a large sum of $ you're inducing the snap. That plane is designed for 3D, which means large control surfaces.

Get your hands on a trim chart and follow that step by step. That will help you identify any trim problems. Others have told you how to see any misalignment between the elevator halves, and its worth looking at, but the snap roll is a tendency of any aerobatic plane, and learning how to manage it can be fun.

As far as initiating the snap roll, I normally do it at about 1/2 throttle, then simultaneously input full elevator (up for positive, down for negative) and kick the rudder (left or right depending on the planes preference, and usually left rudder for positive, right rudder for negative). I release the controls about 3/4 of the way through, but again this is plane dependent. If you start with sufficient airspeed, the plane will be able to continue flying on almost the same line it entered the snap, but the higher your airspeed, the more violent the snap. And none of my planes need ailerons to snap.