Dremel US40 Instruction Manual download pdf (Page 45)

FLIGHT TRIMMING

... A model is not a static object. Unlike a car,

which can only hunt left or right on the road (technically,

acardoes yaw in comers, and pitches when the brakes are

applied), a plane moves through that fluid we call air in

all directions simultaneously. The plane may look like

it's going forward, but it could also be yawing slightly,

slipping a little and simultaneously climbing or diving a

bit! The controls interact. Yaw can be a rudder problem,

a lateral balance problem or an aileron rigging problem.

We must make many flights, with minor changes be-

tween each, to isolate and finally correct the problem.

The chart accompanying this article is intended

serve as a handy field reference when trimming your

model. Laminate it in plastic and keep it in your flight

box. You just might have need to consult it at the next

contest! The chart is somewhat self-explanatory, but we

will briefly run through the salient points.

First, we arc assuming that the model has been

C.G.

balanced according to the manufacturer's direc-

tions. There's nothing sacred about that spot - frankly,

it only reflects the balance point where a prototype model

handled the way the guy who designed it thought it

should. If your model's wing has a degree more or less

of incidence, then the whole balance formula is incorrect

for

you. But, it's a good ballpark place to start.

The second assumption is that the model has

been balanced laterally. Wrap a strong string or monofil-

ament around the prop shaft behind the spinner, then tie

the other end to the tail wheel or to a screw driven into the

bottom of the aft fuse. Make the string into a bridle

harness and suspend the entire model inverted (yes, with

the wing on!). If the right wing always drops, sink some

screws or lead into the left wing tip, etc. You may be

surprised to find out how much lead is needed.

At this point the model is statically trimmed. It's

only a starting point, so don't be surprised if you wind up

changing it all. One other critical feature is that the

ailerons must have their hinge gap sealed. If shoving

some Scotch tape or Monokote into the hinge gap to

prevent the air from slipping from the top of the wing to

the bottom, and vice-versa, bothers you, then don't do it.

To achieve the maximum lateral trim on the

model, the hinge gap on the ailerons should be sealed.

The easiest way to do this is to disconnect the aileron

linkages, and fold the ailerons as far over the top of the

wing as possible (assuming they are top or center

hinged). Apply a strip of clear tape along the joint line.

When the aileron is returned to neutral, the tape will be

invisible, and the gap will be effectively sealed. Depend-

ing on how big the ailerons are, and how large a gaping

gap you normally leave when you install hinges, you

could experience a 20 percent increase in aileron control

response just by this simple measure.

. . . Your first flights should be to ascertain

control centering and control feel. Does the elevator

always come back to neutral after a 180-degree turn or

Split-S? Do the ailerons tend to hunt a little after a rolling

maneuver? Put the plane through its paces. Control

centering is either a mechanical thing (binding servos,

stiff linkages, etc.), an electronic thing (bad servo reso-

lution or dead-band in the radio system), or C.G. (aft

Center of Gravity will make the plane wander a bit). The

last possibility will be obvious, but don't continue the

testing until you have isolated the problem and corrected

it.

... let's get down to the task of trimming the

model. Use the tachometer every time you start the

engine, to insure consistent results. These trim flights

must be done in calm weather. Any wind will only make

the model weathervane. Each "maneuver" on the list

assumes that you will enter it dead straight-and-level.

The wings must be perfectly flat, or else the maneuver

will not be correct and you'll get a wrong interpretation.

That's where your observer comes in. Instruct him to be

especially watchful of the wings as you enter the maneu-

vers.

Do all maneuvers at full throttle. The only

deviation from this is if the plane will be routinely flown

through maneuvers at a different power setting...

Let's commence with the' 'engine thrust angle''

on the chart. Note that the observations you make can

also be caused by the C.G., so be prepared to change both

to see which gives the desired result. Set up a straight-

and-level pass. The model should be almost hands-off.

Without touching any other control on the transmitter,

suddenly chop the throttle. Did the nose drop? When

you add power again, did the nose pitch up a bit? If so,

you need some downthrust, or nose weight. When the

thrust is correct, the model should continue along the

same flight path for at least a dozen plane lengths before

gravity starts to naturally bring it down.

Do each maneuver several times, to make sure

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Dremel US40 Instruction Manual Page 45

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