C/L stunt by its very nature is one of the
most demanding of leisure lime activities, it requires a thorough
understanding and integration of literally thousands of bits and pieces of
aerodynamic wisdom, engine and craftsman-ship skills, neuro-muscular
co-ordination of an extremely high degree, and hours of practice, honing and
blending these skills to the level of competitiveness desired by the
individual.
Because of the huge demand for dedication on the part of the individual,
stunt flying never has and never will appeal to the masses. There are just
too many simpler (and more remunerative for the manufacturer) amusements
available from TV, addiction to computer hacking. I'll make no comment on
the merits of other activities except to state that the easier an avocation
is to master, the greater the participation by both practitioners and
suppliers. Recognizing that, I applaud you all for your dedication to stunt.
I think it takes special people.
One unfortunate result of the small level of participation is a similarly
small amount of available user-friendly software. Unless you have a
catalogued collection of the last 40 years of modelling magazines – or
better yet, live next door to Werwage or Gieseke - access to informed help
in designing, building and flying stunt airplanes is a rare commodity. Most
of us learn through repeated trial and error...or we give up.
This month marks the beginning of a very ambitious undertaking for me. For
several years I have, in piecemeal fashion, tried to share some of the
design and trim skills I've learned in the course of almost 30 years of
flying stunt. In addition, some gifted builders and flyers have over the
years appended construction articles with good basic trim information. To
the best of my knowledge however, no one has yet undertaken a comprehensive
discussion of stunt model design development and trim from the design phase
through to competition-ready. This is what I hope to do in this and several
succeeding columns. I've no idea how long this will go on, but bear with me
as I feel we will all learn from it, "especially me" since verbalizing
perceptions clarifies the concepts in my own mind - in addition to
catalysing responses from some of my less supportive readers. Believe me;
I've learned plenty from them!
The format for a complex discussion such as this was difficult to develop.
What I've decided to do is the following. There are chronological phases of
trim input and evaluation:
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The Design Phase wherein we attempt to assemble the aerodynamic components into a vehicle which we hope will fly in the manner desired.
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The Pre-flight Phase - what Large James Greenaway termed "bench trimming" in his love letter to me published in stunt news - wherein the assembled airplane is configured to the design parameters, i.e. balance etc.
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The In Flight Phase wherein we empirically evaluate the success of the earlier two phases and make appropriate adjustments if necessary.
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The Post Flight Phase wherein we evaluate subjective and objective evidence of the planes performance.
Throughout the four trim Phases, we will attempt to optimize six major trim Objectives.
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Stability: Does it go where it's pointed without constant attention? Does it exit corners hard and flat?
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Response Rate: Does it turn readily and controllably?
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Line Tension: What causes good/bad line tension and what do you do about it?
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Speed Control: Too fast or too slow? How do we adjust it?
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Tracking: Does it fly consecutive manoeuvres concentrically, or does it wander? How do you fix it?
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Turn Uniformity: Does it turn equally both inside and outside?
These four
Phases of trim and six trim Objectives are plotted in the accompanying
table. Beside each Objective is a variety of design parameters and in-flight
factors which have an effect on the particular objective. They are listed in
approximately decreasing order of influence. The phase of trim where that
parameter is concerned is then marked by an X. For example, in the Stability
(A) objective, lead-out orientation is an element in the Design, Pre-Flight
and In-Flight trim phases, whereas sealing the flap hingeline is strictly an
In-Flight trim consideration.
Note in addition, that the table is broken down into a happy airplane side
and a happy pilot (the handle) side. It is very important to realize, as I
stated in the earlier columns on handles, that the handle is the interface
between the airplane and the pilot, it is the means whereby you make a
properly trimmed aircraft compatible with different pilots. While it is
often difficult to make the distinction, it should be kept constantly in
mind that the handle shouldn't be adjusted to camouflage what is an aircraft
deficiency except as a last resort. Our discussion will concentrate on the
happy airplane side with comments on the happy pilot only when absolutely
necessary.
Next month we will start with the "Design Trim Phase" and address each of
the parameters for the six Trim objectives. Subsequent columns will do the
same for each succeeding trim phase.
A few final comments as to what assumptions have been made plus a few
definitions of terms.
First, no attempt will be made to quantify any parameter. We will be
discussing concepts and principles in general, not specific terms. For
example, "the lift to drag ratio improves with higher aspect ratios"; not
the "L/D of a 7 to 1 A.R. is 137 - 267 of a 5 to 1 A.R." etc. I will however
provide some personal rules of thumb in hard numbers from time to time.
Recognize that these are my opinions and may not be shared by all. I do
promise that they won't get you into trouble if you use them.
We will also assume that all systems, i.e. engine, tank, weights, lead-outs,
etc. are adjustable. Some are just harder to adjust than others. It will be
assumed that the control system has sufficient mechanical advantage to
overcome any reasonably encountered air-loads on the controls. Again, the
control system requires extensive coverage on its own merits and will not be
covered in detail in this series.
However, here are some "RULES OF THUMB FROM TED".
Modern full size stunters (.35 to .60) powered should use a minimum of a
three inch bellcrank (3.5 to 4 inches is strongly recommended). Control
horns should have the flap to elevator pushrod located at one inch on the
flap horn and be variable from .5 to one inch at the elevator. The bellcrank
to flap rod should be at least .75" from the pivot at the horn and its
location on the bellcrank should be selected based on your preference in
control sensitivity - .6" for slow response to perhaps 1-1/4" for very fast
response. I personally use .9".
Finally, since C/L stunters are flown tethered by one wing tip, we can
afford to be fairly casual in our treatment and understanding of both the
roll and yaw axes. Their importance will be largely confined to our
discussion of line tension. The pitch axis on the other hand is of paramount
importance inasmuch as all of our manoeuvres are accomplished through
various rates and directions of pitch change. We will, therefore, consider
pitch in detail.
So that we are all talking the same language, let's define some terms which
will recur throughout the series.
Firstly, the MAC or Mean Aerodynamic Chord. If the root chord is 13" and the
tip chord 9", the MAC will be that station on the wing span where the chord
is 11" in length. While the MAC is technically slightly different from this
average chord, we can for our purposes assume them to be equal and
co-located.
The MAC is important since many of the parameters we discuss will be
referenced to their position on the MAC. Also remember that the tail is
merely a smaller lifting surface and it therefore exhibits the same
aerodynamic characteristics as the wing, including an MAC.
Secondly, the C.G. or Centre of Gravity. It is important that we recognize
that the C.G. is the point about which any outside force will act, i.e.
drag, lift etc. Since that is true, any force moments (tail moments,
pitching moments, etc.) will be defined by their distance from the C.G. This
last differs from the historical stunt practice of measuring tail moments
hingeline to hingeline. This is a useless exercise unless when you are
comparing otherwise identical wing and tail plan forms.
Fore and aft C.G. will be spoken of in terms of its location on the MAC -
i.e. a C.G. located 1.5 inches from the leading edge of a 10 inch MAC is at
15% MAC. While it is recognized that the vertical and span-wise location of
the C.G. probably won't be located as defined, we must recognise that these
divergences are minor and not significant in affecting pitch.
Third, the C/L or Centre of Lift is that point on the MAC where the wing's
lift is concentrated. For symmetrical airfoils, C/L is located at 25% MAC
and moves somewhat aft when the airfoil is cambered, as with deflected
flaps. Lift generated by the wing and the tail act at their respective C/L's
and develop a moment about the C.G. The system works just like a "park-yard
teeter- totter." A force (lift) is applied a given distance from the fulcrum
(the C.G.) and a turning movement (pitching) results.
Fourth, Aspect Ratio is the measure of the relationship of a wing's span to
its area (or average chord for simple tapered wings). The shape of a wing's
plan form (top view) is of much more than cosmetic interest. As aspect
ratios get larger (longer spans for a given area) the wing becomes more
efficient in terms of lift produced compared to drag developed.
Fifth, L<co> or Coefficient of Lift is a non-dimensional number (no real
value like inches, pounds, sex-appeal, etc.) which reflects the relative
amount of lift which an airfoil section generates under a given set of
conditions. Some general statements concerning L for any given airfoil are:
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It will increase with an increase in air speed or an increase in angle of attack up to the point of stall, when the airflow separates from the wing's surface and lift rapidly drops to zero - a rare occurrence for a competitive stunt ship; as angle of attack increases, L<co> will increase at a faster rate for high aspect ratios than for low.
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Maximum L<co> will be roughly the same for high and low aspect ratios, however, low aspect ratios require higher angles of attack for the same L<co> and generate greater drag per unit of lift.
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Lastly, high aspect ratios stall at lesser angles of attack than low.
Whew! I think that sets the stage. Next month we'll dive right into the design phase and see what sort of options we have in developing a stunter that will perform to our expectations.
