Today I got my replacement parts for my Mustang PTS MkII that I crashed into a fence, including a new wing. Since I had to transfer the alieron servo from the old wing to the new one, I figured it would be a good time to upgrade to functioning flaps.

I used two old servos from an alpha trainer I totaled last summer. They work, but when the flaps are in the extended position, they seem to be constantly grinding, as if they are binding with something, but they are not (I have checked this many times). Grinding might be a bit of a strong word, its very quiet, but noticeable. The flaps are a little stiff, and take a little force to hold in the extended position, but it doesn't feel unreasonable. If I use the travel adjust on my dx6i radio to reduce the travel, it will eventually stop the grinding noise, but only if I set my flaps to only extend like 10 degrees, which makes them pretty useless.

Then I got to thinking about it, and decided that in the air, when the flaps are extended, there will probably a lot of aerodynamic force on them, with the airflow trying to push them back to the retracted position. And the servos will have to fight this force. So the servos will basically be binding, or working really hard, anytime the flaps are extended in the air anyway.

Is this a problem? How much will it reduce battery life?

Thanks guys

that "grinding" is the servo constaly applying force against the flap to hold it in the down position, any reistance at all will cause that. If it goes away at a lesser travel setting then put it at the lesser travel setting. There really is no predetermined amount of throw that has to be applied, even a 10 degree application of flap would have a very noticable effect on landing speeds.

If there is truly no binding in the flap or the linkage (I assume it's a straight line connect between the servo arm and the flap horn) then you might try to work the flap on both wings a while to sort of loosten them up a bit.  They may just be stiff.  You might be hearing a sort of buzzing noise as the servo pushes the flap against resistance.

You did say you are using old servos from an Alpha Trainer you totaled last year.  How certain are you that the servos are ok? 

With the cost of servos, heck, you can get a decent Airtronics 94012Z servo for something like $10.00, you might just consider picking up a pair and using them rather than the ones you may not be certain about their quality.  The 94102Z servos are just fine for flaps on the PTS.  I used them on flaps on my Goldberg Wildstick 40 which had full deflection in two stages.

CGr.

One more thing:

At the end of the deflection, where the air force against the little motor in the servo is maximum, the linkage should look somehow like the attached schematic.

Note that the pushrod should be perpendicular to the line formed by the hole of the horn and the hinge of the flap in order to have maximum leverage.

For the same reason, the servo arm should be as far from 90 degrees as possible.

Yeah, keep in mind that the flaps are one way (meaning down and up to center and not additionally center to up and back) so you can set it up for maximum efficiency in the center to down and back motion, and get the best resolution (not that you really need it for flaps).

CGr.

As already mentioned, the PTS linkages to the flaps should be configured for MAXIMUM MECHANICAL ADVANTAGE.

That way the servo uses the least amount of force to keep the flaps in position.

This means, the linkage should be as far IN as possible on the servo horn, and as far OUT as possible on the flap linkage at the rear of the plane.

On all of the PTS planes I've dealt with, I've also found it to be a good idea to also "break in" the flap movement by disconnecting the linkages, and toggling the flap possition back and forth a good number of times until things free up.

In most cases the wires bottom out against the cutouts, and you should really enlargen the cutout and restrict the flaps to smaller movement.