Retract Land Gear Setup

[FLOYD CARTER]

This retract gear is installed in a RSM P-51 with OS46LA power.  The retracts are self-contained each with its own motor and controller.  They come from "Hobby King" in Hong Kong, and are called "Change Sun".  They have several types.  This one is for "2.5KG", which is 5 1/2 lbs, good for a medium-size stunter.  The electronics access is in the bottom of the outboard wing.  The electronics uses my modification of the Jomar unit, with a small microphone that "listens" to the engine noise.  Battery pack is 4.8 volt NiMh of 300 mAh.  I didn't weigh the total but I estimate this will add about 4 oz. to your plane.

Flying with retracts is slightly different!  The plane definitely notices the absence of wheels hanging down, but only takes a moment to get used to.  Note that the strut covers seal up the "hole" in the wing, except for the lower half of the wheel.  The resulting "hole" doesn't seem to affect flying.
Floyd

[Tim Wescott]

And if you're flying 'lectric, you don't need the batteries -- as long as your ESC can supply the necessary current to the retracts, the necessary power will come from your flight battery.

(I don't know if this would rule out any of the popular ESCs -- you'd need to check the ESC capabilities against the retract's needs).

[FLOYD CARTER]

Yes.  The electric ESCs that I use will supply a lot of 4.8 volt current for all your R/C flight servos.  The retracts take about 200 mA but only when moving up or down.  The standby current is 10 mA, which is constant during the flight.
F,.C.

[FLOYD CARTER]

Now a larger retract installation.  The plane is a 61" Zero stunter with RO-Jett 76.  Retract units are from Hobby King and are called "EPF Hobby".  The set is DSR2-46B. The struts are nicely machined and feature shock springs inside the tubing.  I first tested the entire setup on the workbench and the system adds 6.4 oz to your plane.  They are rugged and hold up under the plane, which comes in at 75 oz!  Once again, the electronics is built into the outboard wing and is my own version of the Jomar unit, with some modifications.

Floyd

[FLOYD CARTER]

You can see a picture of my P-51 flying with gear up.  www.flyinglines.org and look for "Oregon Fun Fly #4" and "Oregon Fun Fly #2"

F.C.

[Tim Wescott]

You might be able to pull an ounces or so out of that installation by using a pair of 300mAh lipos and a linear regulator.  Just a cruddy old LM78M05 ought to work, as long as it doesn't heat up too much during the gear movement.  Going to surface mount would save enough space and weight on the board that you could use a switching regulator without pain, which should let you either use a slightly smaller pair of cells, or a single cell that's less weight than the two you'd need with a linear regulator.

(And BatchPCB will sell you prototype PC boards in onsie-twosie quantities for a $10 set-up fee plus $2.50/square inch of board: http://batchpcb.com/index.php/Faq.  $15 for a bare board of your own design is pretty good; $25 for three bare boards is even better).

[Mike Palko]

And if you're flying 'lectric, you don't need the batteries -- as long as your ESC can supply the necessary current to the retracts, the necessary power will come from your flight battery.

(I don't know if this would rule out any of the popular ESCs -- you'd need to check the ESC capabilities against the retract's needs).

An external BEC should allow any ESC to work with retracts. It might be the safer way to go anyway.

Mike

[FLOYD CARTER]

I use four NiMh cells at 300 mAh for the retracts.  LiPo would be lighter, but the main weight of the system is in the retracts themselves, and not so much the battery or the electronic circuit board.  You can see that there is plenty of room in the wing-box for everything including the 2" X2" circuit board.

My installations are for glo engines, so a BEC does not apply here.  For electric motors, the on-board BEC is a handy way to get 5 volts for the retracts.  Most electric motor controllers have this capability built in.

Floyd