HI guys,

I'm trying to figure out why BVM did not recommend digital servos on the flaps for the Kingcat. BVM recommends JR 2721 non digital servos instead of JR 8611a, I was thinking it was because of air load/battery drain issues. Any thoughts?

Thanks.

Hi,

I am using DS8411 on my KC flaps with no problems at all.

Cheers,

Darryl

A regular control surface does not have a stop near the normal operating range. A flap has a stop right at the end of the up range. If you have a very small setup error, the digital servo may try to drive the flap against the stop at full power, which would burn up the servo and possibly drain your battery. I have seen this once with standard servos on a flap that where way out of rigging. One servo had melted it self down and was locked, the other was in-operable. had it been a small error there would not have been a problem, digitals would have just made it worst. Better to stay with the recomended servos, they work great on my KC.

Steven

Hi Steven,

Just reading through your reply and could you clarify what you mean by a "stop" at the uppermost travel limit for the KC flaps. Do you mean a physical stop made of balsa, ply, fiberglass or carbon fiber. I don't own a KC - just curious.

Thanks, Gene

Yes Gene, that is exactly what he means.

Gene

That is what Steven means, the physical stop.

Sorry John, we were answering at the same time, have a good day.

But wouldn't a digital or NON digital do the same thing.. TRY to get the surface to the commanded position until it is told otherwise. I can see that the digitals are more power consuming. I always thought the main reason was due to the air load on the surface, especially KC "barn door" flaps...Hopefully Doug C. will chiime in.

Thanks fellas

S. A. Leo

My experiance suggests that an analog servo will still pull higher amps, but will "give" so to speak and not eat itself up.

This is somewhat the same situation why I suggest using an analog 4131 on nose wheel steering. If you hit a rut in the runway high speed, the analog servo won't eat itself up trying to hold against the rut. While an 8411 will pop that nylon gear trying to hold.

My JR manual says not to use digitals on flaps......

This all comes down to how the servos are controlled. A conventional servo applies power to the motor that is propotional to the error in its position. For example; you commmand the servo to move 12 degrees, the air load is trying to stop this, in the end the servo moves less than 12 degrees. It might take a degree or two of error for the servo to generate enough power to hold against the air load. A digital servo on the other hand will command it self to near full power to get to the commanded position. This is why digitals are better, they go where you tell them to go, analogs only get close, they must by design travel short of the command when under load.

This is easy to see. Power up an analog servo and try to move the arm, try hard. The output arm will move a ways before it really fights back. Then try it with a digital, you can not move it. It is the nature of the two beasts.

Steven

FlyBoy 2000:

No disagreement with all the comments so far on this thread.....they all sound legit to me......... but when I was building my KC 1 year ago, I had several N.I.B. 8411's and wanted the 2721's for another project.....I now have over 100 flights on my KC......zero flap problems....I use all digis on my KC, except for N/G steering(2721) most flights off grass and still works fine..... and UP-6 brake valve(341)

I set up all of my servos with an in-line ammeter to be sure there is no high-current drag or stalling occuring at any point in the travel......I have found 8411's pull just a little more current than a 2721.....now the 8611's are a different armadillo.....they can pull big mama amps when you drag or stall them.....

Another saving grace is the fact that with the KC linkage set up the way it is, and with the long live hinge line (no where near as tight as on my SuperBandit)....... there is inherently enough slop to prevent any stalling at either end of full travel, unless you really screw things up......you don't really have to bang the flaps up tight with the servo when you set them up, as airloads do that for you, for the last 1/16" or so of travel.......

While the 2721/4721 remains a great servo in selected applications, it really is getting long in the tooth......the output bearing and flimsy case top design really belong in the 1990's, as does the analog technology.....the Giant Scale gurus know all about this....when they want to hold that monster rudder rock steady on their 50% Edges, in both full push and full pull, or hold that elevator rock still with 60 degress +/- of travel, they gang 8411's or 8611's not 2721/4721........

I use an SR 2200 RX pack with no regulator......my UltraDuo Plus records about 400-450 maH consumption for an 8 minute in-the-air flight, so I think current draw is well within safe range.....

Just my dumb***** opinion......

Tom

A digital servo develops its max power just 2 degrees off center, so if you have the slightest setup issue and overdrive your flap such that it hits the stop while being told to move, the servo can do one of two things;

1) Draw stalled current putting you at risk of running your batteries very quickly.

2) Burning out the servo motor and ultimately giving you an asymetric flap extension..

Both of these would be very bad.

Because an analog servo doesn't develop max torque until around 17 degrees off center, this becomes much less of an issue. Also, I don't like running digitals on anything that will experience a continuous high power demand like a flap. A regular control surface needs a lot of power but only for momentary bursts.. whereas a flap needs it the entire time it's deflected and ultimately can put you into the same situation as above ESPECIALLY if you put the flaps down at a higher speed than recommended.

Yep,

Thanks Doug...thats what I thought! It is 'better' to use analog on a flap....also BVM recommends it..nuff' said.


Anton

I use digitals on flaps never had a problem, BUT I set up the flap up position with the wing held vertically , ie, le up, so that the servo can be set at its exact up position within a hairs breadth of the up stop without the servo having to carry the weight of the flap. In flight the flap will be sucked up hard aginst the up stop and the servo movement is virtually nil, eliminating any high current. Works for me.
Regards, David Gladwin.