Q: Will this add weight to my airplane.
A: Absolutely not. A single mounting box weighs about 1/2 ounce. You get to throw away your
linkages and horns, which normally add up to more than that 1/2 ounce...in fact, often more than 2 ounces! In addition,
the material you remove to mount the box usually compensates for the weight of the mount itself. Direct Drive is lighter
any way you look at it!
Q: How is reliability?
A: Direct Drive is the most reliable system available. You are eliminating parts instead of adding new ones...so
there is nothing to break. You can say good bye to any chance of a clevis or ball link popping off, rod breaking, or
pull cable stretching/breaking.
Q: What are the airplane and radio requirements for Direct Drive?
A: You will need a computer radio. A substitute for this is either programmable servos or a JR Matchbox style
signal compensator. We also recommend a soft motor mount, although it is not essential. Virtually any aircraft can be
modified for Direct Drive.
Q: Can I retrofit a plane that is already built?
A: Yes. Naturally, the larger the plane, the easier the installation. But, virtually any aircraft can be modified.
Q: Do I need special servos?
A: In general, any standard servo will work. Practically, however, you will want a servo that can deliver
the precision this system is capable of achieving. Direct Drive especially optimizes the resolution provided by digital servos.
Q: What’s wrong with my linkages?
A: All linkages convert rotational motion into linear motion. Linear motion places push/pull forces on the
servo and the control surface. This accelerates the servo shaft bearing wear, gear wear, and control hinge wear. The resulting
slop destroys precision and causes flutter.
Linkage angularity is also a problem. Watch the link on a typical 3D capable aircraft as the control
is moved through it’s full range. The servo end of the link moves in one plane while the control horn end moves in a
whole different plane. The control motion varies depending on the direction the surface moves from center. The angles are
the worst at full control deflection, when you need the most power from your servo. For 3D capable aircraft, the linkage is
Q: Is the installation difficult?
A: Not at all. The hardest part is leaving your old ideas of servo mounting behind. A typical conversion on
a 35% plane takes roughly 4 hours…about what it takes to install a linkage system.
Q: For pattern flying I reduce my control travel to increase precision. Aren't I loosing resolution with
Direct Drive since I cannot mechanically reduce control travel?
A: Absolutely not! Here is why...
- Let's assume you are using the most precise combination on the market right now - a Futaba 2048 resolution PCM with 1
millisecond deadband digital servos. The servo gives you 120 degrees of travel, divided by 2048 PCM steps. That
means your best resolution is about 1/20th degree.
- Now, the least loss of resolution we have seen due to linkages is 2 degrees...and some set-ups
have more than 5 degrees of lost motion! This is the combined affect of grommet deflection, linkage play or
cable stretch, hinge play, and airframe deflection. None of these are a factor with direct drive.
- Practically, this means with a 2048 PCM system and Direct Drive, you will get the same resolution with 60 degrees of travel
that you are getting if you mechanically reduce control travel to 15 degrees through a linkage...And that
is worst case! If you have a loose linkage set-up and/or a 512 PCM system, you will not get the resolution of Direct
Drive at ANY amount of reduced control travel.
Q: Since I do most of my flying using just a couple degrees of control travel, aren't I wearing out just one or
two teeth in my servo using direct drive?
A: The answer is yes, but much less so than with a linkage set-up. Here is the deal:
- Any aircraft that is set-up for 3D control throws will spend most of its time working the same tiny range of servo travel,
so this is not unique to direct drive. Luckily, there is little air load on the servo near the center point of travel,
so you are not putting much load on the gears to cause wear. The steering gear on your car is exactly the same, and
look how long it lasts.
- Only the output shaft is working the single tooth, and this is a rather large tooth. The second servo gear
is working more than half it's teeth, and every other gear is working all their teeth.
- Because of air loads and engine vibration, a control surface will "shake" against the servo gears. This
is what causes almost all of your servo gear wear. The extra play in a linkage allows the control to
impact the servo gears much harder, since it has more room to build up speed. Direct Drive has no play...so the
magnitude of control shake is less, and the gears last longer.
- Here's more food for thought...If you have a set-up that uses more of the output shaft gear teeth, then you are actually
spinning the intermediate gears like crazy to move that big gear. You are then wearing the intermediate gears MORE in
trying to save the big gear! The intermediate gears are much more frail than the output shaft gear. You will break
the servo case every time before you strip a metal output shaft gear...not so with the intermediates.
Q: Am I damaging my servos by mounting them without rubber grommets?
A: It is best to use a "soft" engine mount with Direct Drive, or any other drive. Our experience with full
scale aviation has taught us a lot. Full size planes do not use rubber dampening in the control system, as it reduces
precision and flutter resistence, and it is not necessary. The piston engine is the component that produces damaging
vibration - and it is damaging to EVERYTHING on the airframe, not just the servos. Full scale planes put the rubber
into the engine mount, and so do we. That protects the airplane, radio, batteries, and servos. Merle Hyde (phone
702-269-7829) makes a full line of "soft" engine mounts that pay for themselves in equipment savings. His mounts are
much more effective in killing harmful vibration than any tiny servo grommet. Our planes are almost half the size of
the real things, so why re-invent the wheel? Soft mount the motor, not the servos.
If you insist on skipping the soft mount, then don't sweat it. We have tried that too, and the only down side
is a reduction in servo life...to about the same as a standard linkage set-up using grommet mounting.
And, if after reading all this you still want your servo soft mounted, read about a slick little trick to do it -
Servo Soft Mounting With Direct Drive
Q: Do I need to modify my servos to use direct drive.
Q: How do I save money by using direct drive?
A: Let's compare Direct Drive to a standard ball link setup. When using Direct Drive you will
A control horn - $5.99
Two ball links - $5.89
Control link - $4.50
That comes to $16.38 savings per servo. The net savings for a single servo mount is $4.38. For a dual mount
it's $16.76, and a triple saves $29.14! If you are using a complex pull-pull setup, the savings will be even greater.
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