Several of us have been having trouble with throttle servos surviving on our 1/2A planes. I, and many of the folks I have checked with, have tried the HS55 and found them to frag quite rapidly, they usually start to slow down and then stop completely. The mounting ears also break off from vibration. There seem to be four possible explanations for these failures. 1) the servo is just not up to the application 2) vibration 3) exhaust/oil getting into the servo guts 4) the outlet of the Norvel muffler sometimes rotates around due to vibration, directing the exhaust right on the servo which makes 3 above worse, or flat out melts the case.
1 has been addressed by some folks using HS56 or 65 servos, but we've all groaned that it seems silly to spend more for the 1/2A throttle servo than for the elev/ail servo!! I did find a Tower Pro 9g servo for $8 which looks promising, it is the sams size and mounting pattern as the HS55 but has much more robust mounting ears and servo arms. Time will tell, I installed one on one of our planes but we're not doing much flying this time of year!
I tried a few modifications to our Hat Trick 1/2A Arrows to try to resolve 2,3, and 4. Sadly I cannot seem to get my camera's macro function to work, so will have to do without photos for now.
For 2, I took the foam pad that comes with Hitec Receivers (a wet-suit-like, dense but squishy, closed-cell foam with a woven backing) and cut it into four small pieces about the size of the servo mounting ear. Poked a hole thru it with a sharp wire. Made two rectangular washers of the size of the mounting ears, from leftover SPAD fuselage PVC gutter pipe (any similar material will do). Put the servo screw thru the washer, one foam pad, the servo mounting tab, and the other foam pad, on each side of the servo (the pads will hold the screws in place). Place the servo in the opening, and screw down the screws snug, enough to mildly compress the foam but not enough to squish it solid. Now you have a nice vibration-absorbing servo mount that fully supports the servo ear.
For 3 and 4, I took a pop (soda for you folks south of the Michigan/Ohio border) can and cut it open with scissors, giving me some nice flat stock. I cut a rectangle about 1 1/4 inch high by 1 1/2 inches long. With the assistance of a straightedge to act like a brake, bend a 3/16" wide flange along the long edge. The flange fits between the Norvel backplate and the front wall of the throttle servo case on the Arrow engine mount, other planes may vary. this gives you a standing wall that blocks off the exhaust sputum from spraying all over the throttle servo. The vertical portion of the wall starts at about the c/l of the servo output shaft, and extends to about 1/4" outboard of the muffler tailpipe. the tailpipe will pretty much blow exhaust onto the "wall" and the oil residue will run down the wall, puddle up, and blow off, but won't blow on the servo. At least that's the theory. If the tailpipe section rotates, the wall will protect/shield the servo. Drill a single vertical hole thru the flange, centered with the engine, into the engine mount material that is between the throttle servo and the engine cutout, and insert a servo screw to hold the deflector in place.
I think this will keep most of the oil and hot exhaust off the servo, and hopefully will save it. I've got one plane equipped with the Tower Pro, and two with HS55, all with the foam pads and deflector, but we won't know till spring if it helps. It's an easy mod, though, and might help!
Possible 1/2A Throttle Servo Saver
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