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Well, I expect that since
reading last months wind up on the design phase of stunt model trim, you
have all spent the last 30 days designing, building, and painting a brand
new stunter. You are at this very moment standing at your mailbox with the
new ship in one hand and the car keys in the other waiting in anticipation
of this month's column on PRE-FLIGHT TRIM and finally flying the thing.
Well, here it is, so jump in the car and let's go.
Our first step will be a careful examination of the ship to ensure that it
not only conforms to our design criteria but also that all the bits and
pieces are ready for flight. The first step is to double check that all
screws and keepers are secure. Frequently forgotten items are: spinner and
prop nuts. engine mount bolts, the fuel line, tank hold downs, lead-outs and
line connectors, wheel keepers, hinge glue joints secure, all controls and
hinges free and lubricated, tail wheel secure, and (if accessible) all
pushrod keepers secure.
Next locate the MAC on both wing panels and let's check to see if the C.G.
is located per our specs (A-1) under the STABILITY objective. Locate the
design C.G. on the bottom of each wing panel; place an index finger at each
place, cross your other fingers and lift. If you are extremely lucky the
model will balance. Most likely it won’t and weight will have to be added.
Do so until the design C.G. is achieved.
Now pick up the airplane by the wingtips so that it balances fore and aft
(you might need a friend if the span is very large). Locate the position of
the C.G. on the inboard tip and measure aft to the design location of the
midpoint of the up and down lead-outs. Remember this spot should be about
three degrees aft of the C.G. from the centre of the aircraft - roughly 1 to
1-1/4 inches aft of the C.G. at the wing tip for most conventional stunters.
Adjust the lead-out to spec.
Moving right on to the RESPONSE RATE (B) objective we find C.G. once
again. Since we've already resolved that, let's go on to Flap/Elevator Ratio
(B-4). If your controls are accessible and adjustable, I suggest that you
begin with a one to one flap/elevator ratio and stay with it until you find
some reason to try something else. One to one ratios are the closest thing
to a universal setup I can think of. There are some exceptions, but really
only enough to prove the rule.
Ensure that both flap and elevator are neutral at the same time as you check
for Control deflection (B-11). You should have 40 to 45 degrees of travel
available whether you think you'll need it or not. No law says you have to
use it all, but Murphy says that "if you don’t have it, you will need it!"
For the first time let's mention your handle. The line spacing should be
adjusted so that comfortably full wrist motion results in full elevator
deflection, and the handle should be vertical at neutral. Oh yes, as you're
at the handle, double check to ensure that up is up and down is down. No
joke. Check it!
A thorough pre-flight check is especially important relative to line
tension. First flights are always a nervous time and you don’t want to have
to deal with an airplane that wants to fly closer to you than the length of
line you want to use!
The easiest way to give away LINE TENSION (C) is to have the airplane
roll toward the pilot. This unsettling phenomenon can be caused by Warps
(C-15, Misaligned Flaps (C-16), or a combination of both. In addition,
inadequate Tip Weight (C-3) will allow the plane to roll toward the pilot in
both upright and inverted flight. To check for warps and misaligned flaps,
very carefully view the aircraft from the rear so that your vision has the
trailing edge of the flap exactly centred on the wing. If the flaps are
aligned, both should be bisecting their respective wings. If not, twist the
flaps to align them. To check the wing for warps, again view the trailing
edge with the flaps centred and slowly move the airplane’s rear end away
from you while watching the trailing edge of the flaps relative to the top
of the wing. As the trailing edge moves away from you, the flaps will start
to obscure your view of the top of the wing. They should both do so the same
amount. If one flap starts to cover its wing faster than the other, one of
the wings is warped. Investigate closely to determine which one and remove
the warp.
A rough initial tip weight can be arrived at in the age old fashion of
adding it until the outboard wing drops easily but not rapidly with the
airplane supported by the crankshaft and the tail. If you are using equal
span wings, add enough more to cause the wing to drop smartly, probably
another 1/2 to 3/4 ounce. This should get you in the ballpark.
If your ship is a full fuselage type, you probably have built in the desired
Engine Offset (C-6). If it's a profile, ensure that you have washers under
the front engine lugs to provide offset. A RULE OF THUMB FROM TED: I use
about 1-1/2 degrees of offset on my .40 • .46 sized ships and up to 3
degrees on .19 - .35 size ships. A number of good ships have been built with
no offset. Theoretically, this should work fine. Practically speaking, I'm
more comfortable with a little bit for good measure.
Same story with Rudder Configuration (C-8). A large diversity of opinion
exists concerning the value of rudder offset for tension. I leave you to
your own devices. The Nats has been won by airplanes with Rabe style wiggly
rudders and by airplanes with no rudders at all. No big deal I say.
Prop selection deserves some attention in terms of tension. To hedge our
bets, choose a prop with a little more pitch or a little less diameter than
you predict you will finally use. Either one will provide slightly better
tension for your first flights. I would opt for less diameter, especially
if, as is so often the case, your new stunter has a new engine as well. Less
diameter will reduce the load on the engine while allowing a little more
horsepower to provide the additional line tension.
Dealing with props brings us to SPEED CONTROL (D). Aside from prop
selection, the only pre-flight consideration is line length (D-6). When
discussing line length we are talking about apparent speed to the pilot. The
airplane speed is dependent on thrust and drag but the pilot sees only how
long it takes to cover a given segment of the circle. Pick lines of moderate
length for the airplane/engine combination so that the speed (lap times) may
be slightly faster than eventually desired, but line tension will be
improved.
There is almost nothing we can do in terms of TRACKING (E) at this
stage of the game. We've built in most of the aircraft parameters, we've
located the C.G. (V-4) to specs previously and we've already set our Flap
relationships (E-5). Further tracking trim will depend on our evaluation of
the airplane in flight.
Finally, UNIFORMITY OF TURN (F) will pretty much have to wait for
in-flight evaluation. Check once again to ensure that the flaps and
elevators are neutral at the same time and eyeball the wing and stabilizer
carefully to convince yourself that you really did build them with no
Decalage (F-1). i.e. both installed in the fuselage with zero incidence.
IN-FLIGHT TRIM PHASE
All right! Double check your lines for kinks, give the control system a
reasonable pull test, and let's fly! Be advised that in the In-Flight Trim
Phase that we will approach the trim objectives as we encounter them rather
than in Trim Table order as we have been doing. This is simply in order to
allow the discussion to have a flow that approximates the actual progression
of flight trim.
If at all possible, when making your first flights on a new ship, do so with
less than a full tank of gas. This has nothing to do with flight trim; it is
for the sake of the engine. Any number of things can make for a poor engine
run on a new airplane. Engine cooling can be inadequate, new fuel tanks can
release impurities into the fuel system, fuel tubing can kink, etc. Almost
anything that can go wrong will cause the engine to run lean and hot. If
you've a full tank of gas, you can be facing 8 to 15 minutes of torture for
a brand new engine. An experienced helper can hold the airplane in an
attitude ensuring fuel to the pickup tube until you launch. Done this way,
if an engine does overheat, chances are much better of it running out of
fuel before ft runs permanently out of gas. Besides, the first flight or two
will serve to check only a few of the trim parameters ... in addition to
allowing us to check for loosening components, tanks, engines, cowlings,
etc. on a timely basis.
Our very first indication of trim or mis-trim will be the takeoff itself. If
the airplane lifts off smoothly, climbs gradually and flies level without
excessive attention (even if it grooves a little high or low) we can be
reasonably assured that C.G., response rate and handle neutral are in the
ballpark. If on the other hand, it leaps into the air without your
invitation, tries to stay on the ground until you force it off, hunts in
level flight and/or requires constant control input to maintain level
flight, we probably have a problem. Further evaluation will be necessary.
As long as we're up and can’t come down until the gas runs out, let’s try to
fly straight and level and have our experienced helper check the wings for
any banking tendency. Check both upright and inverted flight unless in
upright flight the outboard wing is noticeably down. If the wing is down an
extreme amount, going inverted will most likely result in the ship banking
toward you and causing a loss of tension. If you've done a good job of
checking for Warps (C-15) and Flap Alignment (C-I6) this shouldn't be the
case. If the outboard wing is up upright and down inverted, or vice versa,
you are dealing with a warp or flap misalignment. Recheck and straighten. If
you are convinced the wing and flaps are straight, then and only then, tweak
the flaps to oppose the roll. A better solution is to add a small tab to the
outboard wing (not the flap) and bend it like a small aileron to level the
wings.
If the wing is up both directions, or down both directions, you have either
too little or too much tip weight respectively. Add or subtract weight to
trim level. If you're lucky, the weight will be good for both level flight
and hard manoeuvres.
We'll check the dice later!
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