Flutter
Many years ago I asked my friend Pete to elaborate on some information he had put out on Modelnet (back through the mists of time). Pete is an aeronautical engineer with BAE and knows things.
Hopefully what follows will be of interest even if you conclude maybe Pete knows too much 🙂
<Peter, I am interested in your comments on the subject of aileron flutter on models.>
Gosh, this must go back quite a way – I can’t remember making any such comments for ages. How one’s past comes back to haunt…<G>
<We can all agree that slop free control linkages are mandatory.>
Well, yes and no! In general I would suggest that, while they will certainly exacerbate an existing flutter problem, sloppy linkages wouldn’t actually *cause* flutter. My reasoning is given at the end.
<A popular first suggestion is to seal the hinge gaps of the ailerons. I suspect that the effect of this is less aerodynamic magic than changing the stiffness of the hinge system. Damping would be increased and natural frequency of vibration raised, given the method of adhering monokote to both wing and aileron.>
I would agree that sealing the hinge gaps is probably just adding damping (but not “stiffness” which is mathematically a different thing). Hinge-gap leaks don’t in themselves cause flutter – if they did there could be major problems with slotted ailerons and flaps!! (and the elements of slotted flaps are rarely mass-balenced).
<Also the notion of static balance of the ailerons about the hinge line, I believe the greatest effect here is lowering the natural frequency of vibration of the ailerons.>
Static balance *will* reduce flutter tendencies where the flutter (like 90% of flutter cases) is actually a wing-aileron system flutter rather than just the aileron itself. It works by reducing the gain produced by the phase-shifted negative feedback (see below).
OK, here are my thoughts on the mechanics of flutter. As I said, I can’t remember what I said last time so please forgive me if I’m merely repeating myself, and all the more so if it includes any lessons in egg-sucking<G>:
Flutter occurs when a structural resonant frequency coincides with an aerodynamic vibration. The structural vibration must produce aerodynamic positive feedback – the usual wing-aileron system flutter is caused by the phase-lag between the wing and the aileron, which is why mass-balencing works – in some cases mass *over*-balencing has been successfully used to combat a flutter mode by providing phase-lagged positive feedback (which makes negative feedback). This technique isn’t used that often because it will usually cause more problems than it solves!
The frequency of most structural resonances will generally remain constant with airspeed, whereas most aerodynamic vibrations increase in frequency with increasing airspeed. We can therefore remove a flutter problem by raising the mechanical resonant frequency such that the aerodynamic vibration won’t achieve this frequency within the defined flight-envelope. This is why aircraft certification requires a set of “flutter clearance” dives to a particular speed; Vne+10% or limiting Mach number+1% (whichever occurs first) for UK military aircraft.
As for the parameters which affect flutter, they are basicly the same as for any other oscillating dynamic system: stiffness (in the mathematical sense) determines the frequency while gain and damping determine the magnitude of the problem. A system with a stiffness which would produce a flutter mode within the normal flight envelope *can* be overcome by providing a damping term which reduces the feedback to a value where the oscillation isn’t self-sustaining. This is mostly what slop-free linkages do for us, but to rely on this technique is dangerous. There will still be a small vibration and this will cause wear in the linkages, which means that a previously cured (or even unsuspected) flutter mode could appear at a later date – usually when you are demonstrating a well practiced high-speed pass with “old faithful” to your local safety officials (yes, I hold to the theory that Murphy was an optimist!).
A better cure for a known problem is to mass-balance. Even though this *reduces* the resonant frequency it reduces the gain to a point where there is insufficient feedback to amplify the resonance. The only other really practical solution is to modify the structure to raise the resonant frequency and offset the flutter mode to a region outside the flight envelope – an increase in torsional stiffness would be the classical solution, but this is much harder to do as a retro-fit!
A final thought: As I’ve said, sloppy linkages will only exacerbate an existing problem and will not actually *cause* flutter. Probably the best examples that illustrate this are those 1930’s and 1940’s aircraft which had unpowered “servo-tab” control systems. In these aircraft the ailerons (and/or elevators, and/or rudder) were completely unconnected to the control system and were totally free to move, while the pilot’s controls were connected (in reversed sense) to the tabs on the back of the aileron to “gear down” the forces required to operate them. These could be considered as “100% sloppy” controls(!). Such systems remained in use for a remarkably long time – the Boeing 707 had unpowered servo-tab controls and most of the twin turbo-prop feeder-liners still use them even now because of the mechanical simplicity (and thus lower weight and reduced maintenance costs) they offer compared to powered hydraulics.
Hope this was of interest…
Peter
PS – In the interests of accuracy (ie before someone else picks me up on it!!), I’ve just remembered that the 707 did have a powered hydraulic rudder-boost system, but this was only used to reduce the pedal-forces in the event of a double engine failure where both engines were on the same side (in normal flight it was not activated). The numerous cases where 707’s have successfully landed after shedding engines in flight have shown that the servo-tab system is probably actually *safer* than a powered hydraulic control system in the event of “severe structural trauma”, although my personal opinion is that it would be better not to have shed the engines in the first place…<G>
3 and 4 blade propellers in place of 2 blade
Peter also answered a question of mine regarding how to determine what size 3 or 4 blade prop should be chosen to replace a certain 2 blader, Here is his answer to that.
<Given a certain size of two blade prop…How do you determine the size of equivalent three or four blade prop for the same motor?>
Not a simple question, but I’ll break the habit of a lifetime and try to give a simple answer<G>
If you want to load the motor for the same RPM then the pitch obviously remains the same. The required diameter is complicated by the fact that the outer portions of the blade cover a greater distance than the inner portions. We can approximate an answer using the “swept area” concept – reduce the diameter until the total swept area is the same. I’ll skip the derivation and just give the answer: reduce the diameter by the square root of the ratio of the number of blades.
For the 12” 2-blade prop, the equivalent three-blader will be:
D = 12*SQRT(2/3) = 12*0.82 = 9.8”
And for the four-blader:
D = 12*SQRT(2/4) = 12*0.71 = 8.5”
Don’t take this result TOO literally though, there are many other factors that come into play (like the fact that for a given RPM the smaller diameter reduces the Re of the blade), but it’s a good starting point. You can always check by checking the static RPM before flying. Bear in mind that the multi-blader will usually be less efficient than the two-blader for a number of reasons:
1. A greater proportion of the propeller disk area is “blade” and this effectively reduces the aspect ratio of the prop.
- There are more tips, so the “tip losses” will be higher.
- The smaller diameter will increase the disruptive effect of the fuselage on the propeller efflux.
Anyway, at least it gives the rather more analytical answer you were looking for!
Regards,
Peter
