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" ASW 24" |
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This is what comes in "RDS" the kit. The kit fits, Airtronics,
Futaba, Hi-tec The
splined coupler top fits into the coupler bottom, that holds the drive |
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This is a drawing that comes in the kit. The installation is real
basic.
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These are servo cans that fit Airtronic's, Jr, and Futaba. |
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Here are the Aileron boxes showing the drive shaft with the aluminum Here the
rods are made from 3/32nd piano wire, bent to 45 degrees. |
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Here I have used a Futaba servo S3002 MG @ 46 oz torque and the The foam
is removed from the area where the drive shaft and coupler |
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I have used Robart hinges with allen sockets, I wanted to be able to
take |
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The aileron pockets are the only tough part to build, the trick is a
good fit to Here I
will defer to Mike Garton to show you the
best way to make Once you
make one the rest are easy. Now cut and fit the pocket in the |
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Here you can see the alignment of the aileron and wing, with all the |
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Not only does the RDS system give you the scale look we all want, |
The ROTARY DRIVER SYSTEM (RDS)
By Harley Michaelis, LSF
023, AMA 3234
e-mail harleym@bmi.net
This all-internal
method of deflecting hinged surfaces on R/C aircraft involves no traditional
hardware such as output arms, horns, clevises, threaded rods, etc. Properly
fabricated and installed, the system contributes no slop or bind. Surfaces can
be totally clean and free of unsightly hardware to catch on things and cause
drag and noise. Full expected deflections are provided for positive control.
The system adapts to most any size and type of airframe to move flaps,
ailerons, elevons, flaperons, elevator, rudder, etc. In any application,
structure must be thick enough to house a servo.
Installation of
this system is simplified using the Kimbrough Products servo accessory, item
#500. It weighs 1.2 grams and is 3/8” in diameter. This item can be ordered
from several sources as listed on the "Sources" page.
As pictured below
on the black background, there are three molded parts; (1) a coupler bottom,
(2) a smaller coupler top and (3) a tree of 5 splined adapters. The coupler top
has a .047” pilot hole to open to receive drive shafts of various sizes. The
#500 package contains a pair of the molded parts, needed setscrews and Allen
wrench.
With the shaft
oriented at 45 degrees, deflection possible either way is similar to the degree
of bend in the shaft. 45 degrees allows 45, 90 allows 90, etc. unless
restricted by hinging, beveling, servo rotation or other mechanics. For full
down flap, the shaft must be oriented 45 degrees to the hinge line. For ailerons
the shaft can also be run straight back.
ADAPTERS
Which one is known to fit which servo is illustrated below. Some fit very tightly. To attach the coupler, first press adapter into the coupler bottom, then over the gear. The servo screw seats both to the gear. Setscrews join the top to the bottom and seat against the shaft to secure it. Flats on a shaft prevent slippage.
CUSTOM
FITTING
A standard
adapter can be fitted to a smaller output gear as follows: Plug the screw hole
with wax. Wax the output gear and case. Put clear tape over the small adapter
hole. Almost fill it with epoxy. Press it centered over the output gear.
DRIVE
SHAFTS and POCKETS
These can be made from commonly available materials as
detailed further on. However, where only the very best hardware is acceptable
or where the modeler prefers to save time, hardened steel shafts and
precision-fitted pockets are commercially available, as later detailed.
KINEMATICS
During deflection, three motions take
place. (1) the bent section of the shaft works in a fan pattern within the
pocket. (2) the pocket moves slightly fore and aft on the fixed shaft. (3) the
shaft “floats” slightly up and down at the hinge line. If you are technically
oriented, click here to
access material prepared by guidance system engineer Steve Fujikawa.
These
motions let the RDS work. They avoid bind or pulling on hinges due to
differences between hinge line and shaft axis. The “float” is allowed by
placing wood blocks either side of the shaft as illustrated below.
To see and
feel what’s happening during deflection and to dispel misconceptions, it’s
helpful to make the simple mockup “demo” shown below.
From 1/8”
balsa sheet, make a wing and an aileron. 12” span by 3” or 4” chord is fine.
Bevel edges as shown on the left to allow down deflection. With aileron spaced
slightly at the hinge line, hinge on top with a good clear tape that sticks
well, such as book tape. Flex the tape. From 3/32” softer wire (coat hanger,
etc.) make a low radius 45 degree bend. Bend the front end to point up with the
rear bend horizontal. From balsa, ply, a piece of compact disc material, etc., make
a pocket to nicely fit the shaft, neither too tight or sloppy. Cut away some of
the hinge to glue the pocket to the top of the aileron. Place it 1/16” behind
the hinge line.
It is not
uncommon for a servo to require tilting to direct the bent end of the shaft
into the pocket. To simulate that, raise the shaft 1/8” or so by gluing a piece
of ply in the “shim” spot. Glue the whole assembly to a block ¾” or so thick
placed under the wing. Drive some small nails through the shim either side of
the shaft, to allow only rotary movement there. Glue “bearing blocks” (shown in
black) to the wing either side of the shaft to prevent sidewise motion. Find
the position for the elbow that allows smooth deflection. Glue a “Butt block”
to the wing to put the elbow at that position.
Operate
the mockup. Note what makes the RDS work well. The servo must be mounted,
possibly with a tilt, to direct the shaft into the pocket and to keep the elbow
in the right place. The pocket needs to be located a little behind the hinge
line. It needs to fit the shaft without bind or play. Blocks by the hinge line
prevent lateral play. Vertical play is controlled when the shaft is in the
pocket and the surface hinged. There is enough flex in the coupler and the
shaft to allow slight vertical motion during deflection.
The most
common applications are to operate flaps and ailerons. If you have something
else in mind, e-mail harleym@bmi.net. Some material on
it may be available.
“HAT
BRACKET” SERVO MOUNTING
This works
well for flaps and ailerons. Servos are easy to attach/detach. The base has a
small footprint and can be very thin where overhead space is at a
premium.
Make a base (dark gray area)
of ply as long as the servo, plus the width of the rails (light gray ). Cap
rails with ply to prevent splitting out. Glue rails to the base so the servo
cannot shift side to side.
The bracket should snugly
fit the case and tightly cinch it down as the screws are run into the rails.
Bending over the case results in a bracket that is short, side to side. Make up
a "Width Block" about 1/32" wider for that. Make a "Bend Form"
to get the bracket the right height. Start with 1/8" ply and add shims of
thin ply on the top as needed to size the bracket for cinching down.
Tempered aluminum
siding is great bracket material. Sig sheet aluminum is fine. Tin can is okay.
Soda can is okay if you place a cleat of 1/16" ply etc. under the screw to
keep it from buckling. Epoxy the base to the skin. If using the RDS, which may
call for tilting the servo, epoxy putty can be globbed under the base to set
the tilt.
Servo
Placement
The sketches below illustrate
typical installations. With plug-in panels, servos can go most anywhere along
the hinge line. Where a foam core is involved, tunnels are made between the
hinge line and the servo wells.
<For a typical flap and aileron installation,
click thumbnail to
enlarge.
In a 3-piece wing, the
flap servos are usually clustered at the center. The aileron servos are placed
a bit inside the ends, as illustrated below.
The drive
shaft slips into one half of a regular pocket to become an open-ended pocket in
the aileron. The end cap structure acts as a bearing for the rear end of the
shaft, which typically exits the center end cap 5/16” ahead of the hinge
line.
CAVEATS APPLICABLE TO ALL INSTALLATIONS
1. Use servos and
drive shafts suited to the application. Favor larger shafts if there is space
for the pockets. Pockets may be beveled to get in the space available.
2. Vertically
position the pockets as close as practical to the plane of the hinges. For
example, if the flaps are bottom hinged, place the pocket inside the bottom
skin. This is a concern only in thicker
surfaces where the shaft could come out of the pocket in extreme deflection.
(Example: Aileron full up, hinge high, pocket low.)
3. Keep shafts short
and thus less subject to axial flex under extreme loads by using thinner servos
that can be mounted closer to the hinge lines. Flex is not a concern with
typical R/C craft, but is in high speed, heavy duty, high load applications.
Flex is actually an advantage in such craft as EPP combat slope machines.
4. To open the pilot
hole, insert top in the bottom and align setscrew holes. Run screws in enough
to prevent rotation. Grip assembly with sandpaper and progressively increase
hole size. Use #51 bit for a 1/16” shaft and #41 bit for a 3/32” shaft. Mark
the coupler so you always reassemble the top and bottom the same way so the
threads made align to each other.
5. Until shafts have been sized to precise length and ready to install, there is no need to seat setscrews on them. Don’t overdo seating setscrews. When you’ve hit bottom, too much torque will strip the couplers. Torque from the short end of the Allen wrench between thumb and forefinger is about it. Position the coupler on the output gear so that, in neutral, the setscrews stick out sideways, like ears. (Radio on, neutral ailerons, trim tabs neutral.) Each spline on the coupler changes the position about 15 degrees, identical to an output arm. Getting a precise neutral is a function partly of positioning and partly of programming.
6. Position the coupler on the output gear so the setscrews stick out sideways, like ears, in neutral. (Radio on, neutral ailerons, flaps not deflected.)
7. After shaft length is set, a flat can be made with a grinder. Figure out a jig and practice on scrap to do a nice job. Always file/grind flats on a shaft in a spot where a setscrew can be accessed. See sketch below. If the flat is put on the “outside” of the aileron shaft, the screw there can be easily accessed in ”up aileron”. When that screw is seated, if necessary, use the shaft as a handle to rotate the servo to access the opposing setscrew. Similarly, the flat goes on the “inside” of a flap shaft so the setscrew there becomes accessible when flaps are dropped.
8. Where possible, install the shafts and program the radio. Then attach flaps and ailerons. For skin-hinged surfaces, a wider slot is needed in the pocket to get the shaft in. Manually deflect the surface. Make clearance in the wing at the hinge line to run the shaft into the wing. This may require the servo to be raised in the well. Neutralize the surface and back the shaft into the pocket. Mount the servo with coupler. Run the shaft into the coupler.
9. For heavy duty, demanding applications, or where time is more important than money, check out the fine RDS accessory line provided by modeler/machinist Walt Dimick at http://www.irfmachineworks.com/rds. The tempered aluminum coupler tops, used in place of the plastic ones, allow firm seating of setscrews on the shafts without any danger of stripping.
10. Pockets go a little behind the hinge line. For ailerons, the elbow of the bend goes a little ahead of the hinge line. For flaps, it is where you want the flap, in full down, relative to the wing.
11. A lesser bend equates to using a closer in hole on a servo output arm. For example, if you can get by with 30 degrees of aileron deflection, you need only a 32 degree bend. This gives a two-fold advantage in power and resolution. If you are technically oriented, click here to access material prepared by guidance system engineer Steve Fujikawa.
MAKING SHAFTS AND POCKETS
SHAFTS
Music wire in other than small diameters will not take a sharp bend. It’s also subject to rust. Great for combat foamies, though, in smaller diameters. A good material is stainless steel rod found at welding supply outlets. It takes a 90 degree bend without cracking. Start with a 5” to 6” piece. Clamp ¾” in a vise. Pound near the vise with a hammer. Determine exactly where the servo goes and then incrementally shorten the shaft to put the elbow in the right place when it butts the servo screw.
Aluminum welding rods make good “dummy” shafts to find exact lengths and then make the real shafts made to match. Pieces can be used to check tunnel alignment and align servos so shafts go straight into pockets.
POCKETS
Tops and bottoms of smooth Formica, CD, etc. spaced to make a “slightly snug” fit with the shafts, works very well. The sketch below illustrates some typical sizes for 1/16”, 3/32” and 1/8” shafts. Slots need to be large enough sidewise to allow motion of the bent end within them and so the shafts can be backed out of the couplers to service the servos. Typically. the slot will be at least half the overall length.
When the parts are ready to join, here’s how to do it: Cut spacers (gray) a bit oversized.
With CA glue, bond one to either end of the smooth side of a Formica piece. Trim away the excess spacers. Place pins around the ends, spacers facing up. Apply CA to the other Formica piece. Press it to the spacers. Practice to learn how little CA to use so it does not get in the slot. When finished, wrap around the spacer areas with a single, uniform layer of fine thread to prevent potential splitting apart.
LOCATING FLAP POCKETS IN THE CENTER SECTION OF A BAGGED WING
After bagging, cut the flaps off for tape hinges, or loose for skin hinging. Bevel as needed for reflex. Tape a piece of paper on the wing underside at the center. Make a triangle (gray) from the back of a legal pad, etc. Mark a perpendicular line to the base to center the triangle over the wing centerline. Position the pointed end ½” or so behind the spar. Mark lines A and B as the drive shaft paths. Position the servos forward along those lines to just leave space between the cases to mount them and to route the leads toward the LE.
On the paper, mark key outlines for the wells, servo mounts, servos and where to cut the covers. Make pinholes through the paper into the skin to outline the corners of the cover. Remove the paper, score the paint and cut the covers loose. A sharp, unbacked razor saw works well. If the servos are located outboard in a panel, just use ½ of the triangle to establish the drive shaft path and the pocket location. Route and connect leads as with any other installation.
The opening in the pocket is to be centered where the shaft paths bisect the hinge line. Make loose tunnels for the shafts from the hinge line to the servo well.
TAPE HINGING
The shafts can be sized and secured in the couplers. The little spruce blocks at the hinge line can be put in. The opening in the pocket needs to be only wide enough to slip the flaps over the protruding end of the shafts without jamming the edges of the opening.
SKIN HINGING
Know where the servo and its mount will be positioned. As closely as you can, size the shaft to length so the elbow of the bend will be at the right spot when the shaft butts the servo screw holding the coupler on the servo.
Drop the flaps. In the wing, either side of where the shaft exits the tunnel, make clearance in the core for the spruce blocks. Fit the blocks, but do not glue in. Deflect the flap to full down. Run the shaft into the tunnel and far enough into the well to get the bent end ahead of the hinge line. Then the flap can be neutralized and the shaft backed into it if the pocket is sized to allow it. Clearances are illustrated in the drawing below.
The gray areas represent the spacers between the top and bottom of the pocket. The heavy, solid black line represents the shaft with the elbow of the bend in operating position for the left flap as viewed from the underside of the wing. The opening between the spacers must be wide enough so the end of the shaft clears the spacer when the shaft is backed out of the wing and further back enough to get the front end of the shaft into the Kimbrough coupler on the servo.
Make the pocket, then deflect the flap and clear out the foam where it goes, leaving room either side for a wedge-shaped piece of balsa. Add balsa to the top of the pocket and bevel it so the pocket nicely fits between the skins 1/16” behind the hinge line. Epoxy the pocket to the skins first and let cure. Then fill in either side with the wedges and epoxy. Take care to keep epoxy out of the opening. Finally with the shaft in and the flap deflected, epoxy the spruce blocks in place either side of the shaft.
If the fit between the shaft and pocket is good (slightly snug) and there is play in a surface, one or more reasons may be found below.
1. There is slop in the servo gears.
2. The surface itself is flimsy or flexible.
3. The flat is convex.
4. Setscrews are not well seated on the shaft.
5. The servo or its base is loose.
6. The shaft used is too springy.
7. Hinging allows play between the fixed and moving surface.
8.The servo screw is not fully seated allowing the adapter to slip on the gear.
9. An adapter may seem to fit, but is the wrong one and allows slippage.
10. The pocket is wiggling around in the surface.
11. The pocket has split.
The above information essentially covers typical applications. If you have other questions, email me at harleym@bmi.net
Harley Michaelis