RADS in Molded Airframes
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Home > Articles & Tips Index > Construction > RADS in Molded Airframes

[Courtesy of Harley Michaelis harleym "at" bmi.net, January 2000]

Jim. . .although I am the inventor of the RDS, I have never done an installation in a hollow-molded wing, but perhaps can make some observations and suggestions, considering what I know to be the requirements of a successful installation.

Viewing the underside of the wing, I'd first determine just where the servos will be placed, maintaining the 45 degree angle on the drive shaft. Knowing the servos must not move around, the well should provide room for a base with rails, between which the servo can be bracket-mounted with a so-called "hat bracket" made of sheet aluminum. This is detailed in the thumbnail sketch in the main Instructions under http://www.hollyday.com/rads/rads.htm. Just click on the thumbnail. I'd then apply masking tape in the area to mark on. I'd first outline the perimeter of the well, allowing clearance for the base, rails, servo and the attached coupler. The base is not attached until the bearing tube, discussed below, is in place. The base with rails and the waxed servo mounted on it is handled as a unit. That is, you do not mount the base and then mount the servo. Tilt may be required and tilting the base is easy. See below.

After marking the well outline I'd then mark another outline 1/8"-3/16" beyond the perimeter and plan to cut the skin there, using a razor saw. Use the removed piece for a cover. To support the cover, I'd plan to carefully epoxy some strips of thin ply, say 1/32", inside the bottom skin to form a lip or shelf 1/8" wide on which the cover can lay and be attached with tape.

The pocket will have to be next installed in the movable surface. I'd plan to mount the pocket inside the bottom skin, with its front edge 1/16" behind the hingeline. Be very sure you put the pocket in the correct spot, laterally, so the bent end of the drive shaft is not going to get jammed against the spacers. (Not much of a concern for the aileron drive shaft, but a major one for the flap shaft with its 90 degree bend) As you epoxy the pocket in place, be sure to keep the slot free of epoxy. You can either wax the slot or cover the opening with balsa. I'd have pre-added balsa fill over the top of the pocket so it fits snugly between the skins.

Your next step is to put in the bearing tube which stabilizes the rear end of the drive shaft and also aligns the shaft between the pocket and the coupler on the servo. If the main wing is truly hollow in the area, you might want to first build a channel from balsa in which the bearing tube can lay. I'd consider make a channel from say a piece of 1/2" x 3/8" balsa. I'd bevel that to match the opening between the skins along the drive shaft path and see that it could be inserted from the well. 

The idea would be to then angle the channel so the tube could be aligned as mentioned above. I'd try to get that angle perfect and then epoxy the tube in it before epoxying the assembly in place. However, as the website details, you can use a waxed piece of the drive shaft material, with tube on it, to run between the coupler and the pocket to get the tube perfectly angled and then apply epoxy through a hole in the skin. I can't pass up a challenge, so if I were doing it, I'd fuss with it until I got that tube properly epoxied into the channel and then epoxy the balsa/tube assembly in place.

As the website notes, the servo may be tilted, relative to the skin. That is, the drive shaft may not be in plane with the underside of the wing. It depends on wing thickness, servo thickness, etc. No problem. A glob of epoxy putty on the underside of the servo base will set the proper tilt. Be sure you have the servo cases well-waxed as some of that putty may get on the cases as the base is pressed in position.

The website has just been updated with new material on Drive Shaft length, Coupler Positioning on the output gear, making flats and seating setscrews. All that should be reviewed so you know what you are doing. 

Questions? . .e-mail me. Maybe I have answers.

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