What causes a plane to snap when a lot elevator is abruptly applied at hight speed?

What is more prone to cause a high speed snap, being nose or tail heavy?

the reason i ask is because i maidened a NM extra 260, and the only bad tendency i found in it was that it snapped when i pulled back on high rates.... is there anything i can do to get rid it(HS snap) other than not doing it at all?

e.r.

I don't think that there is much you can do except not to honk on the sticks so quickly.. When a wing exceeds the critical angle of attack, it will stall.. If you are moving fast enough in the horizontal direction and have a small yaw moment at the time of the stall, you get a snap roll. You could try less deflection of your elevator, slower thumbs, or maybe moving your CG back a little bit.. If you have a forward cg, your aircraft will trim out in level flight with a higher angle of attack. When you are level at a higher AOA, you are closer to that critical angle of attack where the stall will occur.. With an aft loaded aircraft you will need a little nose down AOA to maintain level flight, keeping you further from your Critical angle. Of course moving your cg back too far will cause you more problems than you may be in the market for...

If your airplane has a split elevator are you 100 percent certain that they are moving in harmony with each other? If there is a difference in travel, this will give you enough of a roll/yaw at the stall to send you into the "snap"...

thankyou for the great explanation.... i have strikes against me, one im pretty nose heavy(i plan of working on that), and..2 , i have 1 servo for the elev's. i mgonna split the elevator into halves and use a servo to each

thanks

e.r.

I had a seagull yak that had this same problem in a bad way. I did a whole lot of research about it and this is what i came up with. Check the easy things first. Do both elevator halves match and move together. Could one half of the elevator be flexing under load? Check lateral balance. Seal all hinge gaps. Reduce elevator throw to only as much as you need. If you need 40 degrees for 3d then so be it. Lastly drop any un-needed weight. A heavy airplane will snap out because of the higher stall speed. The lighter the batter. My plane was snapping dangerously and I fiixed it by perfectly matching the elevator servos. I switched to CF landing gear and wing tube. Smaller battery. Lighter muffler. All in all I shaved 3/4 pound off the airframe. Also make sure you seal all hinge gaps. This is a major part of it. After I did all of these things the problem was gone. i did them incrementally and the problem got better and better. I believe it was a combination of all the things mentioned. One last thing, check the horiz. stab and wings for any warping. Get them as close to matching as you possibly can.

Bbagle1:

Getting your elevator halfs workint in union is frequently difficult due to the manufacturers design locations of the servos ending in different geometry thrust lines, or you simply locate each control horn a little different from the other. I enjoy walls and elevators, things have to be correct to do these with good stability. Bought a extra once because my fliing buddies flew so well. Finished with same engine, servos, battery and CG, mine flew like a dog. Spent weeks getting the thing sorted out. First check, elevator halfs were not even close at full deflection. Have a nice radio, JR 10X. No amount of ATV, servo speed or trying to match servos did much good. Finally cut off the hinges, removed the horns from each side, built a jig to hold each elevator halfs in the exact position and drilled a new control horn machine bolt through both at the same time. Now she flew fair compaired to my buds, but not good enough--screem @#@$. Now i'm mad. So, got out my crutch to hold the airframe in constant location, got a little lazer pointer, set tail direction toward a blank wall, made a X on a post-it note and started to play the the radio adjustments. When your lazer pointer spot you are trying to match is 10-15 feet from the modle, the end result is very fine. The end points were not a real problem to set, bring one side to match the other. Now, getting identical throws at 25-50% of throw becomes a little tough. After several complete charges of TX and RX, was finally able to get the halfs working the same all the way through the throw.

Ok, got it beat, sadly no. The wall was much better, but elevator was hard to control all the way to landing. Ballance was spot on, what could it be? Well, both wings were set correctly within .05 on 1 degree on my smart level, not there. By leveling the entire airplane in my hutch in all directions, things should be the same. Finally, found the right aileron drooped around 3-4% at the tip. Did the heat gun twist and shrink thing to where everything was 0-0, and ran to the field. Sweet, sweetness, for about 45 minutes, then back to goofy. Yep, when checked back home, the aileron had moved back to its original off position. The manufacturer was kind and sold me a new aileron, cut off old, installed new and low were at 0-0 and life was good.

Reality is when we fly 3D, we'r pushing design to its limits. A good stable 3D airframe is possible and there are some very good birds out there. Eyeball just is not good enough anymore. Gotta set ourselves up with good measuring devices. The standard mechanical guages are usually not adequate simply because the end reading is dependant on the angle you are looking at the meeter by, and the low friection quality of the device. ENJOY