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Back in the air again. We've
finally got the wing flying level, both upright and inverted. Now let's do a
few loops, both round and square, inside and outside, as we start to
investigate several objectives more or less simultaneously:
Stability - (A),
Response Rate - (B),
Tracking (E), and
Uniformity of Turn (F).
In addition, we're going to start thinking about our handle.
When you fly the loops, does the airplane turn a consistent radius, smoothly
ending each loop at the same altitude it began? Or does it jump into the
manoeuvre and consistently finish higher? Conversely do you have to force it
out of level flight and fight to keep it out of the ground at each bottom?
Does it react the same inside and outside, or does it turn easily one way
and hard the other? Keep these responses in mind as we wait for the gas to
run out. Don't fall asleep, however, as one of the most important trim dues
manifests itself when the engine quits.
Watch the reaction of the airplane closely as the engine dies. Its response
to power off is going to tell us a lot about whether the symptoms of mis-trim
we witnessed in flight are related to the airplane trim or if the handle may
in fact need to be adjusted. If your airplane took off and climbed smoothly,
tracked well in level flight and responded uniformly and positively to your
control inputs, chances are that when the engine quits the nose will drop
just slightly, tension will remain positive (slightly reduced of course with
power off) and you will have fairly precise control of the rate of descent
and touchdown. If this is your happy state of affairs, congratulations, the
hard part is almost over.
If, on the other hand, your ship leaped into the air, flew level flight
everywhere except where you wanted it, manoeuvred like a drunken cat chasing
a mouse etc. you have a bit more work to do. When the engine quit, did the
airplane pitch nose up? Did line tension nearly disappear? Did the landing
spot just appear rather than being selected? If this is the case, you have a
tail heavy airplane and I don’t care if the C.G. is exactly where Fancher
told you to put it. Add nose weight until it "don’t do that no more."
If the airplane flew much the same as the previous example, but when the
engine quit the nose did not pitch up and the line tension did not decrease
but the control of the glide was jerky and unsmooth, your control system is
most likely too sensitive. Do not add weight to the nose! Reduce the line
spacing on your handle depending on the severity of the problem. As little
as 1/16” is noticeable, but for this early-course adjustment, I would go for
no less than 1/8" at a time. At this point, move both lines an equal amount.
We may yet have to bias the line spacing, but we're nowhere near ready to
quit trimming the airplane yet.
Finally, if the ship had to be pulled into the air, grooved smoothly in
level flight, but responded slowly to control inputs and showed almost no
interest in cornered manoeuvres, your problem is a bit more complex. If upon
engine flameout the airplane noticeably drops the nose and you must react
fairly quickly to pull it up for a smooth glide; if line tension remains
good and if you can whip readily to extend the glide, the airplane is almost
certainly nose heavy and the appropriate adjustments should be made. Lighter
muffler! – lighter spinner, etc. or as a last resort add weight to the tail
until the response rate is correct.
If the preceding fits except the glide is smooth and controllable with no
abrupt tendency to drop the nose when the engine quits, you most likely have
too insensitive a control setup. Increase line spacing 1/8" per flight until
response improves.
However, if the ship took off smoothly, grooved in level flight, handled
nicely after engine shut down (or, heaven forbid, got light on the lines and
pitched up), yet turns sluggishly and unpredictably your problem is more
complex. For this situation we must determine what is keeping the airplane
from pitching at the rate we desire. We have determined by other means that
the C.G. is roughly correct (smooth take off, good groove in level flight,
and proper response to engine shut down) so simply adding tail weight isn’t
the best solution.
Checking the table for RESPONSE RATE (B) we see that most factors for
controlling pitch are built in; tail volume, tail effectiveness, aspect
ratio, etc. We do have a couple of aces in the hole, however. First, try
reducing the ratio of Flap to Elevator travel (B-4). This accomplishes two
things. It reduces the amount of flap travel thus reducing negative pitching
moment, and at the same time increases the amount of Elevator Deflection
(B-11) for a given amount of flap travel thereby increasing the Tail
effectiveness (B-3).
Another very effective way of increasing tail effectiveness is by Sealing
the Hingeline (B-7) between the stab and elevator. Lifting surfaces produce
lift by the difference in pressure between the top and bottom of the
surfaces. When the pressure is higher on one side, it will seek any means
available to get to the lower pressure area. If the hingeline is greater
than about 1/2 of 1% of the tail chord, there will be significant leakage
and therefore loss of lift. Sealing this gap will increase the lift and
thereby the tail effectiveness and should help increase your rate of
response.
Propellers (B-8) and Horsepower (B-9) go hand in hand. One of the forces
that retards response rate is drag (primarily induced drag). We must have
adequate thrust to overcome the drag induced in sharp manoeuvres. Where we
run our stunt engines in the classic manner, they are well below their peak
power RPM. We can increase airplane response rate by increasing the RPM of
the engine and thereby its thrust. More nitro and or larger intakes will
increase power without necessarily increasing airspeed. Both will also
increase fuel consumption, so this must be used sparingly. Smaller diameter
props are a very useful method of increasing power and will almost always
improve turn rate. Additional pitch will move more air over the tail and
thus increase its effectiveness. However, care must be taken not to overload
the engine. More pitch will also increase the Airspeed (B-10) (if the engine
can handle the load) which will also make the airplane more responsive,
assuming that your control system has sufficient mechanical advantage to
overcome the air-loads on the flaps and elevators.
A final, albeit distasteful, approach to increasing response rate is to
either increase elevator size or reduce flap size. Use of knives, etc. is
mandatory in either case. Yuucch!
Okay, we've got the thing gliding good and turning great but we can’t for
the life of us get it to fly straight and level at four to six feet. First,
remember that a controlliner, especially one with a light wing loading, will
naturally climb into the wind and dive a little down wind as the velocity of
the wind adds and subtracts from airspeed and therefore lift. We can't
really trim that out completely. However, remember that a comparatively nose
heavy airplane must fly at a slightly higher angle of attack in level flight
to compensate for the C/L pitching moment about the C.G. Therefore, a nose
heavy airplane will aggravate the climb/dive syndrome in the wind.
Controversial, but I believe it.
If we still have the problem of hunting in level flight, let’s consider the
handle neutral position. Any reasonably straight airplane should want to fly
straight and level if it is in balance and stable. If it wants to do so at
an elevation different from the rule book altitude, we have a handle trim
problem. If in upright level flight the airplane wants to climb and inverted
wants to dive, shorten the down line a bit each flight and the problem
should disappear. The same solution applies in reverse for the opposite
condition. Many pilots mistakenly fly with the handle in a relaxed,
"natural" position in neutral. I think this is a mistake for both this
problem and for Turn Uniformity. More on that later.
If the blamed thing doesn't respond to the handle adjustment, we have a
couple more zingers. If the Lead-out Position (A-4) is too far aft, a
vertical component of P-Factor (A-10) is produced due to the airplane flying
with the nose pointing outward of a line tangential to the circle. The lower
half of the prop arc (upright) will produce more thrust than the top and the
ship will want to climb. Control input must resist this. Again the reverse
is true inverted.
If this "lead-out too far aft" problem exists, it is probably manifesting
itself in another form of instability. In tight corners the airplane will
jerk on the lines in a fashion similar to hinging but will be more felt by
the pilot than seen. As the lead-outs are moved forward, the jerk should
disappear and hopefully the hunting in level flight will decrease. If not, I
have one last defence: sealing the flap hingeline.
I have personally fought sealing the flaps over recent years because I felt
that unless you are stalling your ship in corners, you have no need for
additional lift (the primary by-product of sealing any hingeline). While I
still feel that statement is true, I witnessed a near miracle at Reno - one
that I was able to repeat. Gary McClellan's three year old stunter had
steadfastly resisted all of our best efforts to eliminate an almost
uncontrollable hunting problem. We had given up and he just flew it that way
... to both our embarrassment. Yet on arrival at Reno, and ever since, that
airplane flew like it was on rails in level flight. I finally threw my ego
to the winds and asked Gary what on earth he had done. Turns out he Sealed
the Flap Hingeline (A- 5) to get a little more lift for the high density
altitude. Lo and behold! It totally cured the hunting.
The last two days of that Nats, my "Celebration" started to hunt badly.
Something it had never done before. Inspection (after the Nats of course)
discovered a broken hinge which was allowing the flap to flex and open the
hingeline gap. Repairing the hinge and concurrently sealing the hingeline
eliminated the hunt. Take it for what it's worth.
One last element while we're discussing stability. If your airplane doesn't
come out of corners hard and flat (over or under-turns, or hunts) you are
experiencing another form of stability problem. It is my controversial
opinion that the conventional solution of adding nose weight is totally
wrong unless you have other indications of the C.G. actually being too far
aft. Moving the C.G. forward increases pitching moment and, therefore, the
amount of control input required for a given corner increases. I feel a
better solution is to reduce the line spacing at the handle and to move the
C.G. aft a little. At some point the plane will exit the corner hard and
flat repeatedly.
As always, avoid extremes. A little at a time.
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