RE: Flutter
 

It is a combination of aerodynamic and mechanical instability. The quickest fix for our models is to make sure your linkages are tight, servos adequate, and surfaces strong and properly hinged.

Action and Reaction:

Flutter happens when some force (usually an aerodynamic force) impacts part of the control surface system - it could be a change in attitude, implementation of aileron/elevator/rudder, a wind gust, or possibly even something like vortex shedding (spiraling air that breaks off opposing edges of a surface creating an oscillating instability). Whatever force is present is the action.

Every action has a reaction. When the above action is beyond the limits of the surface/servo/linkage, something will bend, break, or give. If it doesn't break, it will snap back like a rubber band (servo regains control when force reduces, push rod flexes and reflexes back, etc). It snaps back with a force that is about equal to the initial force, however the aero force is probably gone or moved. Now you have a large reactive force with nothing to balance it out, and the process continues until it finds some kind of balance. Hopefully it oscillates out, but often something breaks.

Flutter has a lot to do with the bending and resonant properties of the materials of the control surface and linkage, and even the wing and airplane frame. So the cause of flutter is usually inadequate structure (including loose parts) for whatever situation the airplane is in. The aerodynamic force sets off the flutter, but is not really the cause. Airplanes should be able to fly within their operating envelope and not experience flutter.

RE: Flutter
 

I just took the LiFe battery out, it was puffed. It took charge, 30 mins at 1A, up to 7.1 volts.<div>I'm not trusting it. Just slided a new freshly charged one. I will test fly her tomorrow and let you guys know if flutter is back!,!!</div>

RE: Flutter
 

Binding in the control system .... or slop.

RE: Flutter
 

You were lucky surviving the first case of flutter, I would not suggest flying the airplane again until you are sure it is fixed. I am 35 years into R/C, 15 of those flying IMAC. I have NEVER seen a battery cause flutter. I have seen poor linkage setup cause flutter more times then I can count. Your previous posts are leading me to beleive that you are running ball links on plastic servo arms. On any size gasser that is a bad idea. IMO at this point you should take your aileron servos apart and inspect the gears very carefully. If all is well with the gears then replace the ball link at the servo with a metal clevis and have it no more then 3/4" out on the arm. This would be the minimum I would suggest, best case senario you would replace the servos with 150+ oz metal geared servos and metal arms.

RE: Flutter
 

Unless that weigth was some ultra dense material I'd doubt the CG of the moving system was anywhere near the hingeline or just in front of it.

Mass balancing of a surface does two things (among many):
(1) changes the frequency at which it flutters and as a result the airspeed at which flutter occurs
Some motorcycle use lead weights in the handle bars if vibrations build up at cruising speeds to shift the natural frequency a bit so that they build up at speeds the bike will be expected to pass through quickly.


(2) moves the CG of the moving system to the hingeline or very slighty ahead so that aeroelastic movements in the wing/stab/fin can't excite flutter in the moveable surface.
Grab a bit of timber at one end and move it up and down and you'll need a strong wrist to holt it level as it tries to tilt up and down, grab it at the GC and move it up and down and it will stay close to level with much less effort.

RE: Flutter
 

Generally when people talk about 'balanced control surfaces' it is not about mass balance, it is about aerodynamic force balance. A large, balanced stabilizer has some portion of the surface in front of the hinge line, such that when elevator is applied (say down elevator), the smaller portion that goes (up) in front of the hinge line is exposed to the flow path and takes some of the load off the actuator/servo.

This is about rudders in marine vessels, but the point is the same:
http://en.wikipedia.org/wiki/Balanced_rudder

RE: Flutter
 

Years ago, early 70's..many RC'rs flew a popular design called the UGLY STICK...The early STICKS had ailerons all the way out to both tips of their wing...and a few modelers that had a penchant to go Mach 2 with their hair on fire, they spent bigger bucks and installed one the new Schnerle ported fire breathing engines on their STICKS and shortly thereafter began experiencing FLUTTER..... Some of the RC club "wise guys" cautioned everyone to keep their aileron/elevator hinge lines tight to reduce FLUTTER.. That fix didn't entirely solve the problem and the Mach 2 guys still suffered from gross aileron/elevator vibrations on their STICKS.... Since I was a member of the local EAA chapter and had a modicum of aerodynamic learning & experience (engineering degrees sticking out of my ears and other orifices) I told them the FLUTTER problem was caused by TIP VORTICES or TIP RECIRCULATION and to cure the problem.. INSET or shorten the length of their ailerons to inside of the wingtip(s)...Several followed my advice and inset their ailerons and wouldn't ya know it.... VOILA'... The FLUTTER disappeared... But I still cautioned them, to assist in eliminating aedrodynamic FLUTTER...one must strive for a zero surface airleron/elevator gap to help cure the problem...then reduce the DOWNWARD movement of each aileron and increase the upward movement...referred to as AILERON THROW DIFFERENTIAL, Almost all RC modelers now own a computer radio which have the added ability to 'dial in' differential aileron deflection (ATV)..just takes a few moments to accomplish... :)

RE: Flutter
 

Well, good thing it is windy and raining today sine I was ready to fly her.<div>I'll certainly look at replacing those servoes.</div><div>Thank you guys, there is a 1000 yrs of wisdom here</div>

RE: Flutter
 

<span style="background-color: rgba(255, 255, 255, 0);"></span><div><span style="background-color: rgba(255, 255, 255, 0);">
</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">I never knew they would do that. Have always heard they were tolerant of abuse. </span></div><div><span style="background-color: rgba(255, 255, 255, 0);">
</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">Charlie, John B., Rodney et al, great info. The point behind Charlie's 'unaerodynamic' mass balance pic is to get as much weight ahead of the hinge line. We've also gotten rid of flutter on ailerons by tapering the last 10-20% of the aileron trailing edge. </span></div>

RE: Flutter
 

LiFe's are very sensitive to being overcharged or fully discharged. They are more physically tough than Lipos, but that's it. That said, I have a couple of puffed ones in planes I fly now. They both cycle fine and maintain voltage fine, so I've kept using them. I saw something a while back that said early Lipo puffing was caused by water vapor that got trapped inside the cell casing and didn't affect the battery's performance. I have to wonder if that's the same situation with LiFe's as well.

RE: Flutter
 

In the magazine article where the author subjected the LiPo gasses to chemical analysis, they found the puffyness to be caused by expelled Nitrogen gas, that cannot work it's way out of the sealed covering.

I've taken many puffed packs, re-shrunk the covering, and subjected them to extensive testing...

All were good to their rated capacity...

That said, while puffed up, I do not believe they were overdrawn to an extreme.