Plugging Along . . .

The winds and temperature keep me from flying so I resort to building.  I have been working on a Stik now for three or four months.  I have to think about it a bit, then do the work.  If one follows the instruction manual to a "T" sometimes things do not come out correctly.

I have mounted the engine, a Saito .65 four stroke engine in the nose.  It is a little on the heavy side so that means I will have to mount the tail surface servos in the aft fuselage to shift weight aft to keep the center of gravity (CG) within bounds without adding additional weight.  Additional weight is usually a no no for us modelers as it steals some of the performance of the model in flight.  So shifting the servos requires cutting new holes in the aft fuselage and bracing the holes to take the servo loads.  Not bracing will lead to premature stress failures of the aft fuselage.  Breaking the tail off in flight is usually a terminal event, thereafter all one has is a bag of sticks.  We often call that "rekitting" the aircraft. 

I have already moved the landing gear forward to make the airplane a "tail dragger."  That simply means instead of a tricycle landing gear arrangement, it has tail wheel.  The tail wheel is attached to the rudder for steering.  In the tricycle landing gear  a nose wheel is used and it is up front (and thus adds to the nose weight of the airplane, not desired!).  And the tail wheel adds to the aft CG weight so it is desired.  It really does not make much difference from a flight characteristic, both configurations work just fine except for the weight distribution effects.  Models typically have lots of power so long take off rolls are an exception not a normal event.  Take off usually is not more than 20 or 30 feet depending on the power to weight ratio and the angle of attack on take off roll.

One balances an airplane over its center of gravity (CG) which is about 25% chord of the wing.  In other words about one quarter of the way from the leading edge of the wing's total width.  One usually finds the spar located in that area.  The spar is the internal structure that carries the weight of the aircraft in flight.  The CG is usually a little aft of the spar.  So when one finishes with the building one must check the CG.  Sometimes one can move the battery around to fix things.  But sometimes one has to add weight to the aft end of the airplane to get the CG correct.   Beter to think it through and compensate for the weight and CG effects early in the build.

It is near impossible to land an airplane with a bad CG arrangement.  And the airplane flies better when properly balanced too.    Some aft CG is desired by those that do extreme acrobatics.  Those that like a more gentle flying machine will have the CG near where it belongs.  I like that arrangement.

So building is a thoughtful process not to be rushed into, especially if you have a heavier than normal engine to deal with such as I do.  The Saito .65 is a relatively powerful engine in its class but is a little on the weighty side.  It has been discontinued by Saito for several years now.  But one can still get repair parts for the engine.  Thus it will be around for along time. The Sait .65 swings a big prop, a 13 X 6 normally.  That is a 13 inch propeller with a standard pitch.  Most engines this size swing a 12 X 6 prop, so the Saito is really quite powerful for its size.  .65 means it has  a .65 cubic inch displacement.

I found a bad servo and took it apart.  As usual, the internal gears were stripped.  It has a simple stack of five gears to drive the servo arm that moves the control surface.  The gears in this case are made of nylon and do not take shocks very well.  Thus a crash can strip the gears, knocks a few gear teeth off.  They are not hard to replace and gear sets are available.  Thus I will replace the gears on this servo and put it away for future use. 

It is a Hitec 425BB, meaning it is a relatively powerful little machine.  BB means double ball bearing on the motor (yes, it has a small electric motor inside the case).  It is a little more powerful than the standard servo but is the same size as the standard servo.   The case also has the electronic circuitry, a follow up potentiometer and the gear train.  The motor turns pretty fast, so the gears reduce the speed to acceptable travel rates for the control arm and provide sufficient torque (muscle) to more the control surface.  Amazing devices that move in proportion to the control sticks on the transmitter. 
That is another story.

Hitec is Korean made equipment.  The US market drives the consumption of Radio Control equipment, so the design is probably US in origin.  But the orientals make servos, transmitters and miniature receivers at much better labor rates than in the US.  And the equipment is usually adaptable to foreign markets,  Their transmission frequencies are often different than US/Canada standards.  Items like servos are standardized by connection and general performance now days.    So servos are generic in nature, but transmitter/receivers are not, it is peculiar to the manufacturer.  I happen to use Hitec transmitter and receivers and have grown to really like them.  Since I have been using them I have not had a failure.

Now off to do a bit of building.