Friday, June 10, 2005

Johan Beyers on wing servos

In my Shongololo and now my Pike Superior I have experimented a bit with different wing servo's to try to find ones that last longer than 6 months.

I have used JR 368 digital servo's, JR digital slim wing servo's, Hitec 125 analog wing servo's and various other types of plastic geared servos.

These planes suffer a huge aerodynamic load, and they have quite large flaps and ailerons. On landing you need crow braking and every so often I forget to tuck the flaps back in place before landing. This causes substantial force on the flaps and thus the servo gear trains.

As a result, my plastic geared servo's just did not last. In my view, the JR331 servo's are simply not up to the job. Considering your investment in the plane I think these servo's represent false economy.

I have thus resorted to metal geared servos.

I have found that the slim wing servo's - the Hitec 125 and the JR digital wing servos - suffer stripped gear trains on a regular basis. I have also experienced servo motor failures on the JR slim servo's. The JR368 servos lasted long, but it is a pricey servo, and there is a big current draw with digital servo's.
My suggestion would be to go for a 13mm thick metal geared servo like the Hitec 85MG. These servos are economical (about US$ 35 each from, compared to US$45 for the Hitec 125) and I think the added thickness (approx 13mm) gives a much stronger gear train than the slim wing servo's.

One must also bear in mind that, every time you want to replace the gears, the whole servo has to come out. If, like me, you epoxy the servo's in place, this is a mission, even if you tape the servo's before epoxying them in.

In the Peninsula Soaring competition on Sunday, I suffered a stripped gear train on a Hitec 125MG flap servo, and I was out of contention. If I had installed proper servo mounts in my wing, I could have replaced the stripped servo in 5 minutes and continued flying.

So I have now resolved to buy proper servo mounts from (about R40 each) which are secure and which add no thickness to the servo installation. This also allows one to replace the stripped gears without damaging the servo - my JR wing servo's were glued in so securely that I had to router them out of the wing (even though they were masking taped before epoxying them in) thus destroying the servo. Had I spent a little extra to start off with, I would have been able to replace the gear train only with no damage to the servo.

Servo mounts also allow you to fine-tune the position of the servo output arm after installation - this is impossible once you have glued the servo in place.
As regards your Shongololo - mine suffered a catastrophic wing failure on launch due to a manufacturing defect related to the installation of the wing joiner tube. Unlike international manufacturers of planes like the Tragi or the Pike, you will find the manufacturer of the Shongololo singularly unsympathetic to your plight if this happens to you, so I suggest that you check that the wing skins are securely glassed to the joiner tube before you fly the plane. I have been told that several Shongololo's experienced this problem - maybe the manufacturer has attended to this defect since, I don't know.

Hope this helps.

Kind regards

Johan Beyers

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