Capstan Servo's

[Etienne]

Hi

Would it be possible to post a pic of a pre-release, subject to change, capstan servo? I'm a bit fuzzy on how this would actually connect to an existing cable set-up... How much movement will a capstan servo have at it's disposal? I'm guessing it would identical to the standard servos (+-60deg), just with a different fitting on top...?

Also, I didn't see it mentioned anywhere when the expected delivery date of the SV55 capstan servos was. My Citabria is all cable based, and has fairly heavy controls so I'm quite keen to get hold of a couple of these ;D

Thanks
Etienne

 

[dynonsupport]

Not sure we have any good pictures of a capstan right now, but I'll look around. It's not really subject to change, it's just subject to getting all the parts in from our vendors. The capstan servos are already in some beta planes and work fine.

The capstan can move much more than 60 degrees. It's a pulley on the output shaft of the servo. A cable wraps around the pulley 3 times in a machined groove. This cable is not your control surface cable, it's what is called a bridle cable. You wrap a second cable around the capstan, and you attach this second cable to your main control surface cable. This second cable pulls on the main control cable.

Because the cable goes around the capstan three times, the servo can move around about 360 degrees with no problems. The only issue with the +/- 60 degrees on the servos with arms is that an arm can go "over center" but a capstan can not. The 60 degree limit is not an internal servo limit, it's an install recommendation.

We don't currently have a specific date for the SV52's. You'd be surprised at how strong they are, and thus we don't really know of any airplanes that use them so far, besides really big or fast planes like a Lancair turbine or a Compair 10. Remember that heavy controls could just be because you have a short throw on the yoke/stick, not because the actual surfaces require a lot more force than other aircraft.

We'll happily sell you a pair when they're out though

 

[Etienne]

If these servos can go around a few times, then it sure reduces the amount of torque required!

The stick in my aircraft is close on 750mm long, and right at the end requires anything up to 30N to induce a 30 deg/sec roll-rate. Obviously, that's more authority than an autopilot should ever have, but it's still a fair whack.

40Nm *quickly does calculations* is nearly 30lb.ft...? Man, I'm sure I got a 55lb.ft last time I worked that one out. Ok, well that does answer that question then, and suitably reduces the requirement from my side! Whew :

Is there anything stopping me from ordering a standard servo and making my own capstan to replace the arm? Or do you expect to start shipping them by February next year?

Thanks again!
Etienne

 

[dynonsupport]

We'll be shipping them by Feb for sure. It is possible to field-change the arms/capstans, though we wouldn't necessarily encourage it, and there's a procedure that we've yet to write down to do it safely (certain torque values need to be adhered to, etc).

 

[Etienne]

I can wait until then... I need to wait for the ZAR/USD rate to recover!

Thanks for the reply