Originally Posted by
Scott Todd
Your model of 40 years ago that fluttered had very weak servos compared to today's standard. You probably also used 2-56 rods. I have seen lots of models flutter, including foamy park flyers. Every time it was apparent the linkages were weak/sloppy and the servos were not strong enough.
Full size aircraft textbooks almost always consider stick-free scenarios where the system needs to have a sort of positive stability (mass balance) to prevent flutter. Models are different because the 'stick' is fixed, based on the strength of the servo and linkages.
Hi Scott,
way back then, servos were a major expense and it was normal to drive both ailerons from one servo using torque rods (10 swg wire). I mass balanced and the problem vanished. Or rather, I rebuilt the model with mass balanced ailerons but same torque rods and weak servo. Now that servos are cheap, one per aileron makes sense and helps avoid flutter but it can still happen. Attempting to use excess servo torque to contain flutter is not the answer. Stiff controls and stiff airframe are the way to go.
It is always wrong to generalise, but all full size aircraft since about the 1930s have had mass balanced controls. They move the hinge line back, into the control surface, and add weights well forward.
Until, powered controls come along. Each surface has hydraulic rams (usually 3) fixed directly to the surface, which is made of sheet metal (or composite now) so there is very little flex in the control surface and zero flex in the drive. So the need for mass balance has gone away. I think the old B737 rudders still had mass balance as they had cable reversion in case of hydraulic failure. (I was an airline pilot before I retired, never on 737 but I flew L1011, 747, 757, 767, HS Trident and a couple of turbo props).
Most fully composite jet models are stiff enough to avoid flutter, but on my own designs I generally put a carbon sleeve over wire pushrods, and because they are driven at the inboard end I usually mass balance at the outboard end, just in case.
BTW, that 'stick fixed vs stick free' thing is a reference to longitudinal static stability. There is a shift of the Neutral Point stick free (I forget which way) but models are always regarded as stick fixed as you say.