“Bench Trimming A Stunt Ship”
"Bench Trimming" - this refers to setting up the initial trim of the airplane in the shop prior to flight. Since people have been flying stunt in its current form for more that 40 years, the range of acceptable trim settings has pretty well been figured out. Some variation is required to accommodate different designs and construction quality, but the basics are remarkably constant. What follows is a cursory example of how to set up your airplane prior to flight to get at least get close to begin with.
Fore and Aft balance:
For reliable control response, the proper fore and aft balance is absolutely crucial. If the balance point is too far forward, the ability to turn tight corners is inhibited. If the balance point is too far aft, the control response will be unpredictable (one time it will turn the amount you want, the next time it will over- or under-shoot). Way too far aft will make the airplane completely uncontrollable. The balance point is usually expressed in terms of a percentage of the "mean geometric chord" (MGC) of the wing. For constant chord or straight tapered wings, the MGC is equal to the average chord. To figure this out, add the root and tip chord (including flaps if any) of the wing together and divide by 2. This will give you the length of the mean geometric chord. The CG is specified as a percentage of the length of the MGC measured from the leading edge. The percentage range should depend on the model design. This table will give a guide:
|Design||Relative tail volume||Recommended CG location|
|Non-Flapped old-time/nostalgia (Ringmaster, All-American etc.)||small||14%-18%|
|Flapped Old-time/nostalgia (Ares, '52 Nobler)||small||14%-16%|
|Later flapped nostalgia (Kit Nobler, Shark 45, Olympic, Twister)||moderate||15%-18%|
|Early modern (Genesis, Stiletto, Cardinal, Imitation, Magnum)||large||15%-20%|
|Modern (Impact, Trivial Pursuit)||very large||18%-24%|
It is worth noting that the sensitivity to CG position depends to a large
extent to the relative tail volume. More modern designs are much more tolerant
of CG shifts than older designs. Also note that the CG is usually measured
with the fuel tank empty. The CG shifts during flight quite a long way, causing
the sensitivity to increase towards the end of the flight. This is very noticeable
on nostalgia type planes.
Refer to the drawing below as a guide as to how to measure all this stuff. Note that these are only starting guides. Find the MGC then multiply by the percentage from the above table. Measure this distance from the leading edge at a point halfway out the span on both sides. Add/remove weight to/from the nose/tail to balance the airplane in the range specified in the table. NOTE THAT THE BALANCE MUST BE RIGHT EVEN IF IT MAKES THE AIRPLANE HEAVIER. If it takes 3 oz. in the nose, so be it.
Locating the desired CG range
The CG can be adjusted for up or down location as well, by adding or subtracting
weight to the landing gear. The value of adjusting this is very dubious and
it is recommended that it be ignored.
If you don't have adjustable leadouts, skip ahead. Note that all leadouts are adjustable, some are just harder to adjust than others. Moving the leadout position can have a dramatic effect on the line tension. If the leadout position is too far aft, you will get tons of line tension in level flight but dramatically less overhead. If the leadouts are too far forward the line tension will suddenly go away or get dramatically less in square corners (usually outsides). The goal is to get the airplane to fly exactly tangent to the circle, not yawed in or out to any noticeable degree. The degree of rearward bend in the lines (due to drag) at the conditions of flight determines the leadout angle. Fast and heavy airplanes need less sweep than slow and light airplanes as a result. Changing line sizes (say from .018's to .015's) makes a big difference as well.
The leadout position is specified as an angle relative to a line through the CG, parallel to the span of the wing (see drawing below). Since angles are hard to measure accurately, it is usually easier to figure out the distance behind the CG at the wing-tips. This is done by multiplying the half-span of the wing by the tangent of the desired angle. Note that the desired angle has nothing to do with the position of the bell-crank pivot, the high point of the wing, or the spar location. The fact that you need to measure the angle relative to the CG means that it is necessary to properly locate the CG before you can determine the leadout position. In theory, every time you move the CG from then on, the desired leadout position should change. In practice almost everyone leaves the leadout position alone when changing the CG and fine-tunes the leadouts by feel. A good starting position is in the range of 1.5 to 2.5 degrees behind the CG. The tangent of 1.5 degrees is .02618 and for 2.5 degrees is .04366 degrees, so multiply these numbers by the half span to get the range of starting leadout position.
Weight in the outboard wing-tip is almost always required. This weight compensates
for the weight of the lines and corrects any construction imbalances. If the
design contains wing asymmetry less tip-weight is required than if you have
equal-span wings. The theory is that the fuselage being off-center acts as tip
weight. Whether this is true or not is arguable but a large amount of asymmetry
does reduce the need for tip weight. Use this table as a guide:
Asymmetry relative amount suggested tip-weight
- 0-1/2 inch small 1 1/2-2 1/2 oz.
- 1/2-1 medium 3/4-1 1/2
- 1-2 large 1/2-1
- >2 very large 0-1/2
These are only suggestions for starting out. The values are probably more than most people will end up after flight trim but it is safer to have more than you need than to have less. Tip weight is the most sensitive of all of the adjustments so expect to have to make day-to-day alterations based on changing conditions.
Right (or outboard) rudder offset is very frequently shown on plans to induce an outboard yaw that supposedly enhances line tension. In level flight this is true to some extent, however maximum line tension is not really the goal. The goal is really to get the most constant line tension throughout the flight. The most constant tension will be achieved when the yaw angle varies the least. Experience shows that very little rudder offset is desirable. The rudder should be set to the smallest possible offset angle that will ensure you that it isn't left (inboard). Too much outboard rudder will make the airplane "doggy" or sub-optimal; any inboard and it may very well start chasing you around!
Thrust (Engine) offset:
Some outboard thrust-line offset is also usually specified on the plans, for
much the same reasoning as rudder offset. It is generally a more reliable adjustment
than rudder offset and much less critical. The amount of yaw torque given by
rudder offset varies strongly with airspeed. Yaw torque from thrust line offset
is more or less constant. Once again, very little thrust line offset is needed.
Start with just enough to ensure it isn't inboard. This can be adjusted only
slightly on full-fuselage aircraft by loosening the bolts and twisting it to
the edges of the tolerance. If necessary the engine lugs can be slightly enlarged
by drilling out to get more slop. This also messes up the spinner fit, so it
is better to build in approximately the right amount. On profiles the offset
can be varied by different angle wedges or in a pinch by adding or subtracting
washers under the front mounting bolts. Beware of the washers sinking in to
the wood and changing the angle! The bolts also tend to loosen themselves up
at the same time so beware. Make sure you use the same number and thickness
of washers on both sides of the engine to avoid stressing the crankcase out-of-plane.
Several sources of angled wedges are available and this generally works out
better in the long run.
Obviously the thrust line can be adjusted for up-and-down (pitch) torque as well. While there is some evidence that having an offset in pitch may affect (cure or cause) "hunting" in poorly understood ways, it still seems best to start out with the engine at 0 pitch angle relative to the wing. On full-fuselage airplanes this is best set during construction but can be adjusted if it looks noticeably off by very thin (.001"-.002") shims under the front or rear lugs as necessary. On profiles the "loosen up the bolts and twist" is about the only adjustment method possible after construction. Measuring the angle is tricky but possible with a big flat table (like when gluing the tail in place). Eyeballs seem to frequently be good enough as well. This adjustment is quite insensitive so a fairly large error seemingly has little effect on performance (except when attempting to find "hunting" problems).
Tweak the flaps and elevator so that both flaps are at the same deflection
angle. Eyeballing it usuallt works well enough. You might need to tweak the
flaps to correct wing level problems during flight trim, but it is always better
to start out with everything 0-0.
Move the flap to neutral and check if the elevator is neutral as well. Tweak the horns or adjust the pushrod longer or shorter to correct. Having the elevator up with the flap neutral makes the inside turns quicker than the outside turns. Most airplanes do best with the elevator neutral or slightly down at neutral flap, but the best setting can only be determined by flight testing. It should start neutral.
This should get you started and save a lot of time at the field.
mod1 bb 12/15/97