reduce servo load

[Quasimorte]

I have a fairly quick p-51 3d plane. it flies great but i have had to replace all three servos at one time or another because of burn out. now while i have some more powerful options i wanted o see if there was a way to redesign the ailerons, rudder and elevator to reduce the torque needed to move them under load. the plane is a flat wing foamy so i can easily re-cut and redesign all the moving pieces. I have seen mention of "boosters" which effectively move the pivot point of the ailerons from their leading edge to a few cm behind it. what are the problems with this type of system? the only thing i could think of would be added stress to the hinge. and how big can these be? could i literally cut the rudder and ailerons in half and move what i cut off into the body of the wing and stabilizer? Do these have a name because i could not find anything on the forums but a few small pics.

Thanks for any input
Quasi

[Sparky Paul]

How large is the plane?
How large are the surfaces?
What servos are you using now?
How far do you deflect them?

[Quasimorte]

81.5 cm fuse
85.5 wing
CG is 8cm behind LE
don't know the weight atm need to get a scale.
40.5x11.5 cm ailerons
15.5x9 cm elevator
15x10.5 cm rudder
blue arrow ba-ts-4.3 servos
and between 7.5-9.5 cm off center depending on the surface

Thanks
Quasi

[harveyweewax]

Quasi,

I can't help you with the "boosters" question but this link may help you determine the size of servos required.

http://www.coloradogliders.com/servotorquecalculator.htm

Hope this helps.

Sven

[Brandano]

You could look at the solutions used on full size planes. Boost tabs are small "sub-ailerons" on the aileron trailing edge, that are linked to the fixed portion of the wing so that they deflect in the opposite direction of the aileron itself. These are quite effective, but mechanically complex, you can see them in more detail here: http://www.geocities.com/roger_forgues/Boost-tabs.html (found with google). Some other methods are detailed here: http://history.nasa.gov/monograph12/ch6.htm , mostly they deal with the control surface weight, but both the last two systems can be easily retrofitted to a model and work to balance out both the aileron weight and aerodynamic forces. For the aileron inertia, I am afraid, there's nothing you can do other than to lighten them up.

[Rodney]

On your foamy, the easiest solution is to move the hinge point on the moveable surfaces aft. I find that you do not want more than 10% of the surface ahead of the hinge line or the servo will hunt (oscillate) when the controls are at neautral. The same goes for the booster tabs (the moveable trailing edge); keep them at 10% or less of the total moveable area. Either way does take a big load off the servos.

[Quasimorte]

Thanks for the calc Harvey it will help if i do upgrade the servos.

brandano those articles are great but that looks a little more complicated than i wanted to do. what i was looking for is info on what the first guy calls paddle balancers i think. leading edge of the aileron extended into the wing body. I tried googleing paddle balance and a few other descriptive terms but could not come up with anything more detailed.

thanks all for the help, keep it coming if you have any better luck with this info than i do.

Thanks
Quasi

[Quasimorte]

Rodney any idea of how much it takes off? I am trying to get an idea without building 50 models but that may be where i need to go. I saw the charts on one of the articles brandano posted so that might give me a clue but do you have a specific source?

Thanks
Quasi

[Sparky Paul]

More powerful servos is the easiest fix.
My QQ Yak 54 and BYP YAK use Eflite S-75 servos. The BYP YAK has 4 enormous ailerons with only a single servo, no aerodynamic or static balance.

[Rodney]

Quasimorte, no I have never measured the change between with/without changes. I have done a fair amount of just experimenting with different ratios of area in front of the hinge line. Anytime that I got more than 10% of the area ahead of the hinge line, the planes would tend to porpoise when in level flight although worked great during tight manuevers. I was able to use rather small servos on big planes with no problems, even on the rudder on some quarter scale bipes. I used the booster tabs on three different planes on elevator with great success but never exceeded 10% of the total surface area on the tabs relative to the moveable surface total area.

[Quasimorte]

Rodney

When modifying an existing plane did you add surface area to add the boosters or did you simply turn part of the elevator into the tab? I would think adding would be counter productive but i am not sure. Exactly how bad was the porpoise? I fly slow inside planes in the winter and can deal with that with throttle adjustments, if it isn't too bad. I am assuming it is cause by the servo oscillating. Finally how high of a percentage did you go? would the wing possible settle down if more of it was used forward and back of the pivot point? I am trying to get a picture of the physics behind how it will act and why the servo is oscillating. I am also wondering if a pull-pull setup on the servos would be more effective or not in settling the oscillation.

Thanks for all the input
Quasi

[BMatthews]

Boosters, assuming they'll work decently at "our" sizes, only work if you're zooming around at higher speeds. If this is a hovering, tail sliding, harrier'ing rapid rolling model that uses a lot of throw at lower and down to no speeds then boosters are going to do very little for you. And a large part of your load is just plain old prop blast which is in close to the fuselage.

Paddle balancers that are within the outlines of the wings are not the best in this case as they are blanked for the first few degrees by the hinge line and then suddenly open up. And figuring out how much they help would require some fancy measuring setups. Otherwise you're just guessing.

I'm afraid that you're really stuck with just moving to slightly larger and stronger servos. Are 6 gram types really that much more weight?

[Bobber Bob]

Hi Quasi...

I fly light (down to two oz, so far) scale-like TomKats indoors, and only control them with elevons (large elevons), and the area ratio is fully 20% to 80% (ahead to behind, of the pivot), and have NO hunt or flutter with them.

The 2oz swing wing TomKat, uses the Micro Jewel servos for the elevon (and the swing wing), and they are quite happy moving those huge elevons.

Heck, I have even moved the same sized "elevons" by just the differential thrust of the Aero Ace stuff out of one of their biplanes.

There are two "secrets" here, the 20% aero balance, and a VERY STIFF control surface. To get the required stiffness on an aileron, you would have to make it hollow (an upper and lower surface, seperated by some ribs).

Even on my elevons, I use fore to aft strips of 1/64th ply (at or near the root), and that helped a lot. Balanced surfaces HAVE to be STIFF !!!!!

Bob Reynolds
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