39% Krill Katana Elevator Flutter
11-28-2015, 08:57 AM
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39% Krill Katana Elevator Flutter
I recently bought this plane used and finished putting it together. I've flown it 3 times now and have started noticing some severe flutter of the entire horizontal stabilizer. After the first flight I took the elevator halves apart. There are 2 tubes that run through the fuselage and into the elevator halves. I epoxied nuts into the tubes so I actually had thread to bolt the elevator halves in to and then threadlock was used to keep the bolts secure. The tubes into the elevators fit very snug, but flying it yesterday produced the same flutter. Anyone else ever have a problem like this on a Krill plane? My whole setup seems pretty rigid and I can't seem to figure out where this problem would be coming from. Thanks!
11-28-2015, 09:25 PM
#2
Post pictures of your elevator linkage along with specs on the servos used. Also include battery power and chemistry. The more information you can provide concerning the mechanical setup and electrical setup with your elevator servos the better. The tubes should be going into your stab halves not elevators.
11-30-2015, 11:21 AM
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I am using JR8711 servos throughout the plane. Also using Fromeco LiIon batts going through a Smart Fly Turbo regulator. Tubes are going into the stab halves not elevators. See below attached pics. Just to clarify, the entire stabilizers are fluttering, not just the elevators. There is a small amount of gear slop in one of the servos but it's pretty darn small. I can't imagine this would cause both stabilizers to shake violently.
Something that does concern me is that the stab tubes do not protrude that far into the stabs. You can see in the pics the 2 screws on the bottom holding them in. The tubes extend just past those screws. On the edges of the stabs there were cutouts to mount extra servos in the stabs. I bought the plane used but it does not look like servos were ever mounted there. I did cover the exposed holes there but just with Monokote. Could having that cutout make the edges weak enough to flutter? I can try to upload a picture of that later.
Something that does concern me is that the stab tubes do not protrude that far into the stabs. You can see in the pics the 2 screws on the bottom holding them in. The tubes extend just past those screws. On the edges of the stabs there were cutouts to mount extra servos in the stabs. I bought the plane used but it does not look like servos were ever mounted there. I did cover the exposed holes there but just with Monokote. Could having that cutout make the edges weak enough to flutter? I can try to upload a picture of that later.
Last edited by Andy Stemple; 11-30-2015 at 11:34 AM.
12-01-2015, 09:06 AM
#4
Picture # 3 shows the problem very clearly. The mechanical advantage in your linkage setup is the culprit. You have the linkage all the way out on an SWB 1.5" arm and the linkage all the way in on your control horn. Move the linkage out a couple holes on the control horn and the issue should go away. It is the elevators fluttering and that certainly has enough force to make the entire tail vibrate. You are lucky to still have an airplane.
12-09-2015, 06:24 PM
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This....
Picture # 3 shows the problem very clearly. The mechanical advantage in your linkage setup is the culprit. You have the linkage all the way out on an SWB 1.5" arm and the linkage all the way in on your control horn. Move the linkage out a couple holes on the control horn and the issue should go away. It is the elevators fluttering and that certainly has enough force to make the entire tail vibrate. You are lucky to still have an airplane.
12-16-2015, 09:33 AM
#7
There really is no best arrangement. It really depends on how the airplane is going to be flown. If being set up for extreme 3D then the set up shown is close to correct but one really has to make sure it is not overdone to the point of flutter. IMO one should never go past a ratio of 1:1. What this means is that if running a 1.5" servo arm then the control horn from center of hinge to linkage should not be shorter then 1.5". This gives you the true torque of what the servo is rated getting delivered to the surface. Say we are dealing with a servo with 500 in oz of torque. Then we set it up to 1.5" servo arm and .75" control horn. The reality is that you would only be delivering 250 oz of torque to the surface. Now if the airplane were being set up for IMAC type aerobatics then you want as much mechanical advantage as possible. A servo arm of 1.25" and a control horn of 2". This increases power to the surface while improving centering and resolution. I suspect the OP has way more throw then needed and has dialed it down with either ATV or dual rate and the loss of resolution has made the airplane jumpy on elevator so he has used a ton of expo to try to compensate. Moving the linkage out on the horn should still give him enough throw for 3D if he maxes out his ATV while increasing servo power and resolution. The airplane overall will be more manageable and the flutter issue fixed.
