| |
Last month we were discussing
RESPONSE RATE (B) and up to the point we left off everything we had spoken
of, namely pitching moment, was in opposition to our desired pitch change.
Let's discuss how to make it turn - the right way.
The prime motivators are the elevators, to make a pun. More correctly, it is
the lift produced by the tail (stab and elevator as a unit) and its distance
from the Centre of Gravity (B-3). The more efficient the tail is at
producing lift. and the further that lift is produced from the C.G., the
more leverage it will apply about the C.G., first overcoming the opposing
pitching moment of the wing's lift and then rotating the aircraft about the
C.G. until the controls are again neutralised.
Once more we should consider Tail Volume (B1-2), (area times average chord).
Higher aspect ratios will produce more lift at given angles of elevator
deflection and will, there-fore, be more responsive. Again, if the A.R. is
made too high, smooth control inputs will be difficult to maintain and, for
instance, round manoeuvres will tend to show little flat spots as minor
control corrections create comparatively major pitch changes. Notice the
dichotomy between the implied preference for fairly high aspect ratios for
response rate and low aspect ratios for stability! Compromise – compromise!
I opt for lower aspect ratios and accept the need for more deflection to
obtain the lift necessary for a given rate of pitch change.
Speaking of Control Deflection B-4). It has been stated with some authority
that only 20 or 30 or some such degrees of control travel is necessary to
perform a hard stunt corner. This statement is true for only a particular
set of conditions for a given airplane, not as a general rule for all
designs. The amount of control deflection for a given rate of turn is
variable based on airspeed, aspect ratios, and most especially on the C.G.
relationship to C/L and the amount of pitching moment. It will take more
control deflection for a given turn rate early in a flight with a full tank
than at the end of the flight when the C.G. has moved aft a considerable
distance.
Inasmuch as the flaps and elevator move dependent on one another, i.e. a
fixed relationship, and inasmuch as lift is mindlessly increased by the
flaps as a result of control deflection with no consideration of the amount
of lift actually necessary to support the "G" loads of the corner, it
becomes obvious that we are "probably" always generating more lift than
required. If this were not the case, we would be fairly consistently
stalling our stunters in corners. In consideration of response rate, this
additional lift means additional pitching moment, which means reduced rate
in pitch.
When we first began the discussion of response rate last month, I spoke of
pitch rate which is predictable and controllable in addition to being rapid.
This is where it becomes important to keep the lift required/lift produced
equation in balance. LIFT IN EXCESS OF THAT REQUIRED BY THE "G" FORCES
PRODUCED, WILL ACCELERATE THE AIRCRAFT IN THE DIRECTION OF CHANGE ONCE
PITCHING MOMENT IS OVERCOME.
While on the surface this sounds like a good deal, what actually occurs is
that the lift acts at right angles to the wing chord line and the airplane
leaps in the direction of the lift ... which is not the same as rotating to
a new body angle which is what we are seeking. The result is an airplane
which jumps in hard corners, making their shape difficult to control and
their rate and final position difficult to predict.
This phenomenon also argues for moderation in our pursuit of high aspect
ratio's many virtues. Because higher aspect ratios increase L<co> at a
greater rate with angle of attack increases, it can be seen that excessive
high A.R.'s will induce this same leaping-type of acceleration in sharp
corners. Only an educated guess, but I bet that's part of the reason Denny
Adamisin tries to fly his very high A.R. stunters at very slow speeds; to
mitigate the very rapid build-up of lift that would be the combination of
high speed and high L<co> at a given angle of attack. One man's opinion.
LINE TENSION (C) Ever onward, let's see what we can do in the design
stages to optimize line tension. Not a whole lot it would seem from a
perusal of the Trim Table. That's basically the truth. We can decide whether
and how much of the following to use: Tip Weight (C-3); Wing Asymmetry
(C-4); Engine offset (C-6); Lead-out position (C-7); and Rudder
Configuration (C-8). For the most part these items all act in concert with
one another more to ensure we can take advantage of the real source of line
tension, the engine's horsepower, than to actually produce it themselves. It
is worth stating that there is little we can do to produce line tension but
a great deal we can do to lose it.
From the design standpoint we should consider wing asymmetry, i.e. building
the inboard wing larger to equalize lift since it flies slower than the
outboard. There have been National and World Championship stunters with and
without wing asymmetry and I, therefore, conclude it to be not significantly
important. (That'll really ring some bells, ifs my opinion again). Since
equal panel wings require more tip weight to balance lift requirements for
both wings, I would opt for a certain amount of asymmetry to reduce the need
for significant amounts of weight a long ways from the C.G. While wing
asymmetry seems to be a matter of taste, the outboard flap should probably
always be slightly larger than the inboard. Here's why. The wing tip weight
is used to balance both the lift differential of the two wings and to
overcome the weight of the lines and control system all located in or
attached to the inboard wing. The amount of weight that is proper in level
flight is increased in mass by the "G" loads in a hard comer and will
therefore cause the ship to roll in that direction ... popularly known as
hinging. By making the outboard flap a little larger, when the corner is
being flown the outboard wing will have a slightly greater increase in lift
thus offsetting the increased "G" load of the tip weight.
Whew!
Engine offset (C-6) is helpful, particularly in the instance of lost line
tension as it tends to return the ship to the end of the lines where it
belongs. The same can be said for a small amount of rudder offset. An
adjustable rudder is probably the best alternative although I have
personally gone to a fixed surface air-foiled on the inside per the Nobler
and been happier for the lack of yet another adjustment to make.
The final design parameter influencing line tension is Lead out Location
(C-7) relative to the C.G. This location is the same as that established in
the Stability objective, roughly 3 degrees aft of the fore and aft location
of the C.G. at the fuse centreline. An additional consideration which is
very important if you are building a stunter of unusual plan form, such as
Al Rabe's semi-scale ships with dihedral, is to consider the lead out's exit
point relative to the vertical location of the C.G. They should be in line.
If they aren’t, say the lead outs exit below the C.G., in flight the C.G.
will align itself with the exit point, resulting in the aircraft banking
toward you. The resulting bank is the single most effective way of throwing
away line tension (short of running out of gas).
Now let's look at SPEED CONTROL (D). Speed control is again only
influenced peripherally by design considerations. Our primary attention will
be given in the flying phase. Some things can, however, be said.
The speed of an airplane is governed by two things: thrust and drag. Here's
another irreverent statement from Fancher: reasonable amounts of form drag
(from the shape of an object moving through an air mass) and parasitic drag
(drag from skin friction, lines, wheels etc.) is a good thing for a stunt
ship. We are all flying engines which are operating significantly below
their horsepower potential, i.e. at much lower RPM figures than optimum, in
part to keep airspeed manageable. The sleeker we make a ship, the faster it
goes for a given amount of thrust. Ergo, the slower we must run the engine,
ergo the lesser the amount of horsepower we are getting from the engine.
By not only accepting, but even seeking sources of drag, we develop the
means whereby we can operate the engine closer to its peak horsepower
without losing control of the speed. So go ahead and put on the bombs and
antennas and big wheels etc ... however, make sure you've really got the
horsepower available first.
There is one other form of drag about which we can’t be too cavalier and
that's induced drag (drag induced by the production of lift). Here again
higher aspect ratios will reduce the amount of induced drag for a given
amount of lift. Just be sure to consider all the caveats mentioned earlier
about excessive aspect ratios.
TRACKING (E) has several design aspects which can be discussed
together and have been mentioned previously. An explanation of tracking may
be helpful. The manoeuvre descriptions require that consecutive manoeuvres
be flown concentrically, in the same spot, within two feet of the track of
the first manoeuvre. If a stunter locks on to a rate of turn in a round
manoeuvre and tends to follow the same radius smoothly with little control
input, it is said to track well. If it requires constant control trimming to
maintain the radius, and the input causes noticeable flat spots in the
rounds, it is poor tracking.
Let's consider Tail Volume (E-3). If, again, the aspect ratio of the tail is
too high, the change in lift with minor control inputs will be larger,
resulting in noticeable pitch changes and flat spots. Fairly
straight-forward.
Less simple are the effects of pitching moment C.G. to C/L (E-4) and Flap
effects (E-5) which we will consider together. This gets a little
controversial, so consider the source.
The lift required in a loop is not constant. Going up, lift must oppose
gravity and pitching moment and will therefore be large, requiring larger
control inputs to overcome it and to establish the desired radius of turn.
Over the top of the loop and coming down inverted, required lift is lessened
inasmuch as gravity and lift now work together. This reduction in lift
reduces the pitching moment and requires that control be backed off to
retain the radius of the loop.
As you can see, the greater the variation in pitching moment of the wing,
the greater the variation in control input required from the pilot to
maintain constant loop radius. Therefore, contrary to popular opinion,
adding nose weight is not the proper solution to most tracking problems
since it will increase the arm between C.G. and C/L. This is another sound
argument for keeping the lift produced in line with the lift required.
Still my own opinion here. The flaps (E-5) have a strong effect on tracking.
Particularly influential is the span-wise distribution. As flaps get
shorter, tracking suffers. Flaps should be long in span and narrow in chord.
A RULE OF THUMB FROM FANCHER: Flaps should be full span, or nearly so, and
their chord should range from 15 to 20% of the wing chord, depending on
aircraft weight. In addition, their hingeline should be straight. Since a
stunter flies in a constant left skid, a straight hingeline assures that the
airflow across the airfoil is as consistent as possible for the entire span.
The final design objective is UNIFORMITY OF TURN (F). The prime
design factor is Decalage (F-1). Decalage is a fancy term for alignment of
the wing and tail. To be blunt, there should be no decalage. The Centrelines
of the wing and tail, and for that matter the thrustline, must be parallel.
Any misalignment of any of these in either design or construction will
result in a plane which turns differently inside versus outside.
DON'T DO IT.
|
|