Zenith Aircraft Builders and Flyers
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CH 801 Pitch Stability
The stick forces in my 801 are very light, particularly so with the dual stick conversion. So maintaining altitude except in calm air (the light wing loading doesn't help either). Every time you hit a bump that kicks the tail up any the elevator gives strong feedback to the stick and it moves. no other aircraft I have ever flown had such strong feedback. A moderate bump and if you dont have a tight grip, the stick can fy out of your hand. Forget trimming it to fly hands off for longer than it take to scratch your nose. I suspect that it is because the elevator is not mass balanced, nor is there an elevator bob weight in the tail. and the overhung mass of the elevator exceeds the aerodynamic forces trying to hold it in position.
It's a work out in bumby air, and trying to fly IFR in anything but super smooth air is an invitation to get busted for altitude excursions.
Is anyone elses 801 behave so twitchy in pitch? If so, what have you done to correct it?
I am considering either some means of mass balancing the elevator or just adding some controlled friction to the elevator controls. Anyone have any suggestions?
Thanks.
Sam McNair
Replies to This Discussion
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Permalink Reply by Rob Lankford on
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I have an 801 kit but its a ways away from first flight. My first thought is a hydraulic dampener. Popular with high speed desert racing motorcycles, hydraulic steering dampers greatly reduce terrain induced steering twitch that can oscillate into a catastrophic lock-to-lock handlebar rotation which can be fatal at freeway speeds across baja. They dont noticeably restrict normal steering inputs and the adjustment valves allow for fine tuning.
I think something along these lines might help, maybe with a quick disconnect feature as a failsafe, but wait for someone with more experience flying an 801 to chime in.
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Permalink Reply by Samuel McNair
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Thanks, do you have any suppliers that you would recommend?
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Scotts steering dampers makes an adjustable rotary unit. My 2001 Ducati 996 had a linear damper (looks like a small hydraulic cylinder) but it wasnt adjustable.
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Permalink Reply by David Green
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Hi Sam, have you done a full weight and balance to check your c of g. Very aft c of g can make it twitchy....not stable in pitch
Also check if you have the bigger elevator/tail which was brought out as a modification later....around 2002 I think sure this would help stability.
Can also try putting in a fairing at tail to avoid air being dumped there....if you search for 801 upgrades by Mark Patey he explains how this can help elevator authority.
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W&B has been done and well within limits
It has the large elevator/tail and it has vortex generators.
I have the fairings at the intersection of the stabiizer to the empenage.
Every time you hit even a small bump in the air that kicks the tail up or down, you feel a definite feedback "kick" in the stick in the direction which exacerbates the deflection of the tail. Reading up on it, that exactly describes the characteristics of a control surface that needs to be mass balanced. Damping sufficient to overcome the "kicks" would probably just make the controls too stiff in both axis instead of just one axis.
So I am going to go in the direction of mass balancing the elevators. I just need to figure out a way to get the mass where it is needed. The design of the outboard ends of the elevators would mak adding balance "horns" like you see on a Cessna elevator a bit difficult.
Thanks for your interest and suggestions
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Permalink Reply by Matt Pope
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I was wondering if that might be it. Maybe adding something like the 750 Cruzer's flaperon weights would do the trick.
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I dont have any info on the 750 cruzer. Do you have any pictures or references for the balance weights?
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It's like a little steel bomb slung forward of the control surface's CG: https://www.youtube.com/watch?v=QG94SOuCtg0
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Permalink Reply by Jim and Amelia Isaacs
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What did Sebastian recommend?
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Matt, thanks for the suggestion. Jim & Amelia, I haven't consulted Sebastian about it. I have gone over it in detail with a friend who is a retired structural and aerodymics engineer from Boeing and reserched the topic on line. The overhung weight Matt suggested wll be difficult to implement since the stabilizer is in the way. My Boeing friend cautioned me to stay away from adding overhung weight like the Cessna "balance horns" or the flaperon weights on the outboard of the elevator because of the torsional stresses and chance of introducing a torsional resonance. He also cautioned about the stress concentraion where the 750 weight design would attach and possibility of interference with bounday flow if too close to the rudder.
So, if my measurements and calculations work out and the necessary mass is within reason, I am going to try to cast rod shaped lead wieghts curved to fit the inside of the elevator leading edge. They could be slippled in thorugh the inboard end of the elevator and fastened to the leading edge. That way there is no concentrated load at one point, they add some stiffness decreasing the chance of flutter, and do not mess with the airflow.
I will have to take things apart to get all the necessary data to do the calculations. So this little project may have to wait until I get moved.
I will share the results of my findings.
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