Elevator Stall
 

Does this exist, and if so, any supporting documentation would be greatly appreciated.

I found a few references to elevator TRIM stall on a Google search, but what I'm talking about is what occurs when aerobatic types snap out of loops, Immelmans, split-S's, etc.

My H9 Cap was doing it yesterday at the top of a Split-S. I commented it was doing the dreaded elevator stall, and the predictable flying-field argument ensued...<G>

Help me out here, guys; I need some ammo!

Steve

Elevator Stall
 

As the CG is moved forward, more and more down load on the horizontal tail is required to achieve trim. If, for example, the CG was within 5 or 10 percent of the MAC from the leading edge, the down load on the tail would be excessive and the tail might stall in an attempt to achieve trim. This never happens on a plane that is properly set up for aerobatics with neutral stability. The horizontal tails of such planes are low aspect ratio, of ample area and very lightly loaded with very ample stall margin until well after the main wing has stalled.

When a control surface is deflected more than about 10 or 15 degrees, the flow seperates at the hinge line and that might be thought of as a partially stalled condition. However, if the lift of the tail is not seriously reduced, then control isn't lost.

Most horizontal tails have low aspect ratios and the induced angle of attack goes up as the aspect ratio is reduced and the squared coefficient of lift increases. So, these low aspect ratio "wings" with variable camber airfoils can still function at very high (extreme) angles of attack.

Wide control surfaces at high deflections put huge loads on the servo. A stalled servo can give symtoms of lost control that could easily be mistaken for horizontal tail surface stall. See:
http://www.multiplexrc.com/calcservo.htm
to calculate the required servo torque for the application.

Stalls
 

If your plane stalls and snaps out of the top of a loop, or any other high-g maneuver, you are using too much elevator for the weight and wing area of the plane.

When you first start a loop, for example, your plane due to weight or amount of elevator, is close to a stall. As it slows down, it drops below the stall speed so it snaps. This usually occurs at the slowest point, the top of the loop. On a split-S, you probably pull up first, or you pull harder since you are going downward, again causing the plane to drop below stall speed.

Basically, you are trying to get more that your plane can give. You can reduce the amount of elevator and/or add more power without adding weight or maybe add flaperons.

Here's how I would reduce the elevator. Fly straight and level and do a full up loop. If the plane snaps out, land and reduce the elevator movement. Take off and try it again. Keep doing this until you can do a loop using full up without snapping. Use this amount of up elevator for your normal flying. You should do the same thing using down elevator.

Elevator Stall
 

Thanks for all the replies. Mine isn't snapping; it is doing sort of a half-corkscrew ONLY at the top of a split-S, and then not all the time; weird. Elevator halves are synched, and throws are well within specs. I could be feeding in some aileron without realizing it.

I've since found out that while this phenomena (elevator stall) does indeed exist, it is rare among full-scale and practically un-heard of with models. What I was thinking of as elevator stall is actually the wing stalling due to excessive elevator throw causing wing AOA to go past the point where it stops flying.

Looks like I was half-right... :-(

Ollie, this was happening at low speed (the split-S was executed at the top of a 45 degree climb, to do a sort of FAI turnaround), so I'm pretty sure the servos weren't stalled. I hope not; I've got a 4721 (120 oz/in) on the elevator.

Steve

test message from florida
 

reply if you get this note.

Elevator Stall
 

hmmm....I have a 1/4 scale Extra 300s (13 lbs) that sorta does this also, only from the strart of a loop. If I pull to hard and hold it, it will start to look like a stop sign. Goes up a little and rolls to the right a little, goes up more, rolls more. Even if I pull moderatly it wants to roll to the right. If I make a very big loop it is much better.

CG dry is where the mfg recommends (Lanier) but wet (22oz) it would be nose heavy. Power is ST 2300, elevator servos are regular JR 42oz servo's. Throw's are less that mfg recommends.

Could I have a bad servo and stalling out, to nose heavy, or something else?

10-4
 

Hey Goggles! Thanks for checkin back...just starting to get my sea legs on RCU.

Hi there.
 

Are you sure that your servos are really synced up? I had a large Extra doing this. Although the travel was the same on both halves of the elevator, they were traveling at different speeds. The result was a sort of departure when entering moderate to hard elevator inputs. Well, that and it was found to be very tail heavy. I guess they sell synchronizers now to insure that the elevators stay together . Just a thought, good luck, I know this one can be really annoying.