When our sailplanes leave the ground, there are countless hours and precious dollars invested in that single event. Far too often simple steps that can provide an added degree of insurance and assurance are not taken. One of those is redundancy in our power sources while another is isolating the retract gear system power in the plane from the receiver and servo power source.
The retractable landing gear in any large scale sailplane takes a fair amount of abuse over its lifespan. This would include side loads and jolts and bangs from the occasional hard landing. These stresses can tweak the alignment of a retract and cause binding in the mechanism. Sticks, grass clumps or dirt have also been known to become lodged in moving surfaces. Additionally, if a control arm slips or pushrod bends, and goes unnoticed, the next time the unit is retracted, it may not fully engage the over lock mechanism on the retract. This in turn can put a strain on the retract servo, causing it to drain its power source. All of this happens while you as the pilot are happily enjoying a big thermal unaware of the problem facing the model’s electrical system. The result can be disastrous.
Even with a redundant battery system, if one flies long enough, there can be danger of draining the receiver batteries. However, having a separate 4.0 volt or 6 volt pack just for the model’s retract provides an additional level of redundancy without the necessity for voltage regulators. The power goes directly to the retract servo without first going through your receiver. Essentially, the retract servo sees the ground and hot from the battery and the ground and signal, but not the hot from the receiver.
Here’s how to do it:
Take your dedicated 4.0 or 6.0V pack of sufficient mAh capacity, run the red power lead directly to the retract servo, split the black neutral lead from the battery and Y it, one leg to the servo, and the other to the RX for continuity, the white signal goes only from receiver to the servo. Use a separate switch to turn on the isolated power source. Big retracts need the power, and while you can use six volts for everything including the RX and other servos, separating the retract gives you a failsafe in case of an unobserved jamming due to a hard landing or general gremlins in the mechanics.
With this separate battery for the retract in place, it’s then possible to use a redundant power receiver like the “PowerSafe” DSM2 series from Spektrum, or a battery back-up system like the SmartFly BatShare, Electronic Model Systems Ultimate Battery Backer or a similar product from a host of vendors. These are inexpensive and proven devices that allows the use of two flight pack batteries of any capacity, (they may be different MAH as long as they are both the same voltage, 4.8 or 6). Another example, recently HobbyKing came out with a dual pack BEC for under $20. In the event of a significant battery failure, these uints are worth many hundreds of times their weight in gold. And, yes, some folks do simply use two batteries, two switches and therefore two inputs to the receiver which is a positive step foward, but this does not address a hard short in one pack as the damaged pack remains still fully connected to the receiver.
SmartFly's Ultimate BatShare offers 8A continuous current per input!
In summary, batteries can and do occasionally hard short leading to catastrophic loss of the receiver pack. Using a dedicated battery for the plane’s retract, and or a simple battery backup device for receiver flight packs may save your plane. This very approach saved one of my 1/3 scale planes due to a similar failure.
This article contains content created in part by John Derstine. It was expanded and edited for rcaerotowing.com by Steve Pasierb
