sidgates

My Feedback: (2)

[QUOTE=Len Todd;11795013]To move a control surface with a given amount of air pressure on it, it takes a certain amount of watts to move the surface. If the voltage is lower, due to whatever reason, more current is required. (Watts = Voltage x Current.) To hold that surface in place on a flap, it takes that amount of current until the surface is returned to neutral position.
With analog servos, they each operate a different times. With digital servos, they can all operate at the same time. When they all operate at the same time (potentially), all the individual currents add up. When you constrain all that current on a typical Rxer buss, the buss is essentially a resistance and voltage on the output side of the Rxer will go down (voltage sag) resulting in an increased current draw to get the power needed to move all the surfaces. That is why there are power distribution systems out there. When Resistance goes up the Voltage goes down (Power = Current x Resistance.) You may not ever see a drop on the bench. But when the plane is moving, it is there. The faster it moves, the more prevalent it is. At some point, the Battery's capability may even begin to factor in. Long servo leads and maybe a little resistance in the connectors and now what is the voltage at the servo going to be?

When you throw these conditions together with several digital servos all servos operating off that tiny Rxer buss, in a high speed plane, you have a situation looming (i.e. Many or all the servos operating at the same time with high loads on the surfaces.) Now, you throw in a flap servo, at maybe high speed, and there is now a constant high current load placed on that Rxer buss and the battery, plus all the other control surface loads. The flap's constant load can be up around 5 Amps+. As the flap servo begins to heat, the resistance goes up and even more current is needed resulting in an even higher current draw. That's why the Ultra Premium servos have obvious heat sinks. They can handle the higher currents and resulting heat.

Bottom line: Slow down before dropping the flap. Make sure you have a servo that can handle the load and resulting heat for a long period of time. Set the servo's failsafes. Keep the battery near fully charged (as the Voltage goes down on it, the above negative effects are amplified.) When using several digital servos, consider a power distribution system. Remember, the flap servo, when down, is a constant load versus just momentary like most of the other servos. It has to be a pretty healthy servo capable of handling the load and the resulting heat for a long period of time.

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Len,
I have a different understanding of some of the statements made above. The start time of a servo is not determined by whether it is analog or digital but by the receiver design and the software design for 2.4GIG radios. I know of one radio where all the servos start at the same time whether they are analog or digital. I don't like this because it puts a very high current drain for a few milliseconds when more than one servo starts at the same time.

I am surprised there are not more servo failures when I see servos being run above the rated voltage. I don't recall seeing any mention of the voltage used on the servos that are mentioned in the start of this thread. Servo electronics failure is caused by combination of too much voltage and current (total power) so if we only overvoltage but keep the current low we may get away with it. Flaps tend to keep the load(current) higher for longer periods of time so the possibility of failure goes up.

Sid