I'm going to build a 1/6 scale Comanche and would like to keep it close to the full scale with retracts, flaps, and a flying tail. The rub is that my club has a grass strip and I have read that retracts on grass is a no-no. Is this true for both mechanical and pneumatics? If one had to have them, which would be better? (I am leaning toward the Spring Air type, since the failure mode allows the gear to extend.) I am aware that the gear operation would have to be checked before each flight for proper operation. Opinions are welcome.
Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:
Both mechanical and pneumatic retracts will work from grass.
Depending on the weight of the model AND the length of the strut, you want A MINIMUM of a 3/16" diameter strut. 5/32" struts bend too easily on anything less than a perfect landing (especially on grass or if the plane is heavy ).
I know Robart has retracts that use 3/16" struts and I am pretty sure other manufacturers have them also.
Depending on the length of the strut AND the weight of the plane, you may want to consider having the struts made up in a larger diameter and boring out the hole in the retract.
Mechanicals are less expensive (and usually a little lighter in weight ), but require more "fiddeling" on your part to get them set up and locking properly. Pneumatics require pumping the tank up every few flights and running the "plumbing", and if you lose air pressure your gear may not come down or lock (Spring Air will fail and lock in an open/down position in the event of loss of air ).
Retracts work fine on grass IF you have thick enough strut material and the gear blocks are anchored sufficiently well.
I preferred pneumatics in spite of the extra weight. Easier to install, they isolate the retracting action from the main Rx battery (stuck, non closing retract) so the battery is drained prematurely and, for me, they are more reliable. I know that doesn't seem likely, but they were for me. Granted, it depends upon whose retracts you are using.
Ed, I took a 'Y' cord and cut one leg of the positive line off and supplied a different pack for the mechanical retracts. The mere thought of a fly away makes my stomach churn since I have had 3 or 4 and was lucky enough that no one was injured. With the second pack, if the retract system sucks juice the RX and flight pack servos are still provided motive power.
Tim, somehow I suspect that for the size airplane you mentioned 1/4 inch might work better
The servo will be getting ALL of its power from the second battery, and will be isolated from the RX battery.
I won't call out wire colors, because they may be different for different servos.
The servo needs three things; power for the motor, signal (+) from the receiver, and ground (negative) for both of these functions.
If you cut the power wire between the RX and servo, the RX will continue to send the servo's signal and return the signal on ground. Now take a second battery, and splice the battery negative to the ground wire of the servo wire. That does not affect the signal to the servo, but it will provide a ground path from either battery.
Now, take the positive of the second battery and splice it to the power wire going into the servo. Cut off, or isolate the power wire that was cut coming from the receiver, so it does not short out on something and drain the receiver battery.
Now, the servo will get its instructions from the RX, and _that_ power will still come from the RX battery, but that is very little power, and it will not increase if the servo is stalled or at idle.
The motor will get its power directly from the second battery, but that is the only place the power from that second battery can go. If the servo is stalled and pulling a lot of current, it will drain that second battery, and the retracts will not work, but the rest of the plane will still be working, just fine.
I hope that clears things up. If I was too basic, sorry, but perhaps someone with little electron smarts (like me) will be helped by the basic explanation.
If the second battery runs down, the input signal to the servo will be at a higher voltage than the servo power voltage. Seems this could damage the servo input circuitry unless the input circuit schematic is known and accounted for.
How so? The servo has no idea what the voltage of the other battery is. There is NO interconnection of the control circuit to the motor, other than the transistor "switches."
It could care less what the power voltage is. If it isn't high enough, it won't run the motor.
And that's precisely the location of the potential problem described by "uncle_vito".
The receiver is sending a control signal to the servo. The transmitting gates in the rx are referenced to the main battery, the receiving gates in the servo are referenced to the second battery. If the second battery voltage is significantly lower than the main battery, there is the potential that the servo control signal high state will exceed the maximum input voltage allowed by the servo signal input.
That said, there are ways that the servo can be designed to minimize such potential damage. Clamp diodes immediately comes to mind. Without knowing what the control input circuitry designed into the little chip inside the servo looks like, all that remains is speculation.
That's looking in the wrong place for the problem...
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