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Okay group. Let's design
ourselves a stunt ship. Remember - we're going to look at each of the
parameters listed on the trim table as they relate to design. We'll look at
each and discuss briefly its effect on aircraft performance and, if
appropriate, how that performance is altered by change in that parameter.
Again, let me remind you that I'm dealing in concepts and that my
conclusions are for you to consider in developing your own ship. Some of the
things I'll be saying will be generally accepted and a few will, I'm sure,
be quite controversial (controversial, from Fancher! Say it isn't so!) In
those cases I'll clearly label my opinion as such and you may want to seek
further enlightenment from another source. Of course, I'm sure I'm right
however.
STABILITY AND BALANCE (A). Our first objective is to design for
stability. There is a subtle difference between stability and balance. An
airplane is 'balanced' when all the forces acting on it are in equilibrium;
lift equals weight and thrust equals drag. An airplane in balance will
continue on course and on speed until this equilibrium is disturbed. Once
disturbed, its tendency to return to its balanced condition is the measure
of its stability. If it tends to return to the balanced state it is said to
be positively stable. If it maintains the new attitude it is neutrally
stable and if it increases its divergence from the balanced state it is
termed negatively stable.
Stability is inextricably bound to the relationship between the C.G. and a
point known as the Aerodynamic Neutral Point (1-1). All we need to know
about the N spot is that its location is relative to the overall lift
capability of the total aircraft, wing plus tail. It will always fall
between the 25% positions of the wing and tail's respective MAC's. The
larger the tail in respect to the wing, the further aft the "N" point will
be. As long as the C.G. is located forward of the "N spot, the ship will be
positively stable and therefore flyable. If the C.G. were moved aft of this
point, the aircraft would be uncontrollable. Since for stunt ships with very
large tails the N spot is far aft of any normally usable range of C.G., it
is safe to say that reasonable stability is inherent in any remotely
conventional stunt design. It should also be stated that the further forward
the C.G. is, the more stable will the airplane be – also the less
manoeuvrable. In practical terms, a C.G. aft of the C/L, although
technically stable, rapidly becomes too sensitive for reasonable control and
should be avoided for our purposes.
Tail effectiveness (A-2) is a parameter I've developed to compare the
capability of the tail to both stabilize the wing, and also to cause it to
change direction, as in manoeuvres. Tail effectiveness is the product of the
distance from the C.G. to the C/L of the tail, and the tail area. The larger
the tail, or the longer the distance from the C.G., the greater the
effectiveness of the tail to do either of its jobs.
Tail Volume (A-3). For our purposes tail volume is the area of a lifting
surface multiplied by its average chord. A tail of given area can be
designed in a variety of shapes, either long and skinny (high aspect ratio),
or short and stubby (low A.R.). In terms of stability, tail aspect ratio is
important because the lift coefficient (L<co>) for a given angle of attack
increases with higher aspect ratios. Therefore, minor control deflections on
a high A.R. tail will produce more lift than on a low A.R. tail. If tail A.R.
becomes too great, smooth level flight will become more difficult since
minor control inputs will result in large L<co> and aircraft pitch changes
will result.
Okay, let's have some practical applications. What we as pilots are looking
for is a ship which flies straight and level without constant attention (in
balance) and when it is disturbed by either outside influences (winds, wake
turbulence etc.) or by the pilot (for manoeuvres), returns to the balanced
state readily (stability). The basics for good stability were designed into
our ship with the C.G. and "N" spot. Some less important items to consider
follow.
Lead-out Position (A-4) affects stability inasmuch as the airplane should
fly with the C.G. in line with mid-point of the up and down lead-outs. If
the lead-outs are too far forward (rare) line tension and controllability
will suffer. If too far aft, line tension will be good in level flight but
the airplane will start to hunt as a result of sustained vertical P-factor
component. There is only a narrow range of correct lead-out location for a
given aircraft weight, speed, line length and line diameter.
The Leading Edge Radius (A-6) or relative bluntness of the wing has an
effect on stability. Blunter wings tend to be more stable and exit corners a
bit flatter. I suspect this is because minor up and down movement of the
forward-most point at which the airfoil strikes the air (the stagnation
point) alters camber (and thus lift of the wing) less than it would with
sharper leading edges. On the debit side, blunter airfoils have poorer
penetration in the wind and they are just plain ugly to look at.
P.S. this one is my opinion, only.
Nose Moment (A-7), the distance from the C.G. to the prop is thought by some
to be significant in stability. While I don't entirely agree, some merit
exists in having a longer nose so that the Gyroscopic Inertia of the prop
will act as a stabilizer on a longer arm. My personal opinion is that the
gyro effects are minor and that the best length for the nose is just long
enough that the airplane balances correctly without adding weight.
A "RULE OF THUMB FROM TED" - on designing for stability. Locate the Centre
of Gravity at approximately 16% of the MAC. If you plan to use the currently
popular ST .60 in a light airframe, say under 55 ounces, you might move it
aft to 18% - 20%. This will allow for the additional fuel burnout of the .60
and the resulting initial nose-heaviness compared to the same aircraft using
a .40 or .46 on a couple of ounces less gas.
Use low to moderate aspect ratios for your tails to reduce the sensitivity
to pitch inherent in high aspect ratios. I use 4.5 to 1. Use as long a tail
as you can balance with normal hardware in the nose. My jury is out relative
to Tail Thickness (1-8). I do feel you can get too thick (over about 12%),
at which point response gets sluggish and if built too thin, rigidity
becomes a problem. Use about 8 to 12%.
The design midpoint of the lead-outs position should be at approximately a
three degree sweep from the designed C.G. at the aircraft centre-line, with
allowance made for 1/2"' adjustment fore and aft. The bellcrank (a-9) should
be mounted at the C.G. so that the lead-outs will exit the wingtip in a
straight line and not be forced to bend (remember the C.G. will line up with
the lead-out exit mid-point).
Finally, use a moderate to blunt leading edge. Most important is that it not
be sharp.
So much for Stability. Now, how do we make the blamed thing turn?
RESPONSE RATE (B) Once again, the C.G. must take star billing, along
with its relationship to the C/L of the wing (B1-1); Tall volume and
effectiveness (B-2 & B-3); Flap/Elevator Ratio (B-4); Aircraft weight, G
loads and Lift required (B-5); and Aspect Ratio (B-6). Since these are all
bound together, let's discuss them as a group.
I might just as well start out with the controversial stuff so we can draw
the battlelines early. The rate at which a stunter turns (pitches) is not
primarily a function of lift and light weight! While reasonable weight and
adequate lift are desirable, light wing loading is not the primary requisite
for rapid pitch change - especially a predictable and controllable change.
Let's take a quick look at why.
When a stunt ship is in level flight, the lift exactly equals the weight. A
three pound airplane is developing three pounds of lift. If we now desire to
pitch that three pound ship abruptly, as for a square corner, we will cause
an acceleration or change of direction of the aircraft's mass which will
result in the apparent weight increasing. This resultant weight is termed a
"G" load. One "G" is normal weight, 5 "G’s” is five times normal weight etc.
Just for fun, here's a formula for figuring "G" forces:
where V is speed in mph, and R
is the radius of the corner in feet.
For example, if our three pound stunter turns a corner of 10 foot radius
while flying at 55 MPH, it would incur "G" forces of 20 times its own weight
or 60 pounds. This is the lift that the wing must produce to support the
stunter throughout the corner. Please note that the wing can produce more
lift than required. However, if it is unable to produce this amount of lift
we must then increase the radius of the corner or else the aircraft will
stall as the wing is forced past its critical or stalling angle of attack.
As we discussed in the previous instalment, when the wing develops this lift
it is concentrated at the Centre of Lift which is normally aft of the C.G.
Therefore, the lift causes a pitching moment in a direction opposite to the
desired pitch change. This is a force which must be over-come by the tail
before any change in body angle can be made. Since the lift is necessary to
support the "G" forces in the corner, we can only reduce the pitching moment
by lessening the distance between the C.G. and the C/L In the design phase
this can be done by increasing the aspect ratio of the wing. This allows us
to narrow the distance between C.G. and C/L while still retaining the
desired percentage location of the C.G., about 16% MAC.
Another significant factor re high aspect ratio wings is that for a given
amount of lift they will develop less induced drag (drag which results from
the production of lift, primarily wingtip vortices and downwash). This
reduces the thrust requirements in a corner which is good and will slow the
airplane less, also good. As with the tail, remember that high aspect ratios
increase the L<co> more for a given angle of attack and there is therefore,
a real world limit to the maximum aspect ratio that we can smoothly control
in abrupt manoeuvres.
While discussing lift, let's talk about wing thickness. It has often been
stated that we should use thick (18 to 20 plus %) wings to increase lift
potential. After reading the foregoing you probably realize that I no longer
share this opinion. While reasonable thickness is desirable (12% or more for
a ballpark figure) to prevent early flow separation and stall, very thick
sections are seldom needed for the sake of lift. Structural considerations
are probably a more important factor. The higher the aspect ratio, the
thicker the wing should be to prevent flexing. This is apt to be very
controversial, but since I don't feel extra thickness does any particular
harm (thickness in an airfoil shape causes minimum drag build up, and is
much less significant to lift than area) make the wing as thick as you like.
Remember that flaps are a high lift device that contributes a significant
percentage of the increased lift generated in a corner.
We'll get back to that next time.
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