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Old Sep 24, 2009, 07:13 AM
SoarScale2 is offline
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United States, WI, Wind Lake
Joined Nov 2004
814 Posts
I personally would not use a fast acting fuse as the potential for falsly blowing the fuse is great. There are two main failures for the retract servo. 1), if it is an analog servo, you'll likely strip the gears and 2). if it's a digital, you'll likely burn out the power amp inside the servo.

The problem with most retracts is the combination of the friction-based actuation mechanism and the chosen gear door solution.

Most "Hock/Fema" style retracts use a pin that slides in a slot within the actuating mechanism - something like a pin/slot based lever. This mechanism has a fair amount of friction that needs to be overcome at the full in and full out extremes as well as when the retract is being pulled in IF the counter-balancing spring is not well selected. It is better if this linkage is lubricated in order to reduce the friction. The downside of lubrication is the collection of dirt within the lubricant that further adds to the friction if not monitored and cleaned. The act of landing and taking off causes debris to be pulled into the fuse through the retract doors when the retract is down. This has a cumulative effect for increased debris over time.

Combine this with the standard method of gear door attachment, opening and closing (the retract pushes open the doors and rubber bands pull them closed) and you present large and sometimes variable forces that the servo needs to overcome to consistently operate the retract.

If this setup is not done with due diligence to reducing the friction forces involved as well as ensuring that absolutely nothing can catch on some other part of the setup, it is quite feasible to cause everything to lock up and jam.

The key is that the retract mechanism is different than almost ALL other servo driven items in a sailplane because of the friction involved. It therefore represents the greatest risk of stalling a servo. If it is a digital, you could be talking a couple amps being drawn from the pack during the stall. If left to it's own devices, the result can be catestrophic if you are using the receiver power source.

My earlier comment about a fuse stands in that it is quite feasible to see varying loads on the retract servo over time - up to the full stall current of the servo if the retract mechanism is not regularly maintained. Placing a fast acting fuse in the cct could cause premature failure of the retract.

On the other hand however, it might very well be a useful approach as an early warning indicator of when maintenance is required on the retract mechanism (i.e., the forces on the servo have increased). I personally choose the additional pack with common ground.

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