An ability to trim a model aircraft
for proper flight with no supplementary control surface input has, since the advent
of precise, reliable radio control (R/C), been the domain mostly of the relatively
small number of free flight (F/F) and competition fliers of control line (C/L) and
R/C. Most models can be made to fly very well when a human or electronic pilot is
able to make corrective deflections of control surfaces. Warped and twisted wings,
misaligned tail surfaces, and even a dangerously mislocated center of gravity can
have their otherwise detrimental - even dangerous - effects mitigated by a skillful
flyer. Authors have written that a properly trimmed model of any sort will fly more
precisely and successfully. This article from a 1951 issue of Air Trails
magazine offers very useful instruction on how to configure a model airplane for
both a proper power-off glide and a power-on climb.
I have been guilty of applying the wrong technique when attempting to countering
the severely nose-high attitude that sailplanes with a flat-bottomed airfoil wing
exhibit when a motor and propeller are mounted in the nose. A large amount of down
thrust is required to counter the tendency to climb at a very high angle. With electronic
mixing, down elevator can be progressively coupled to an increase in throttle. Moving
the center of gravity rearward (but not too far) does reduce - but not eliminate
- the effect. I learned something useful here. BTW, I have also noticed that mounting
a power pod above the wing, near the center of gravity and the center of lift, does
not require any down thrust to achieve a good climb attitude under power. You can
see the many configurations I have tried with my Great Planes
2-Meter Spirit glider.
Wikipedia states: "The phugoid has a nearly constant angle of attack but varying pitch,
caused by a repeated exchange of airspeed and altitude. It can be excited by an
elevator singlet (a short, sharp deflection followed by a return to the centered
position) resulting in a pitch increase with no change in trim from the cruise condition.
As speed decays, the nose drops below the horizon. Speed increases, and the nose
climbs above the horizon. Microlight aircraft typically show a phugoid period of
Free Flight Model Trimming
You may have read many articles on adjustment,
but we say this is the most important ever to be presented ... peruse and profit!
By William F. McCombs
Many well-built and well-proportioned models are unable to give the best performance
of which they are capable simply because they have never been correctly adjusted
or trimmed for flight. In fact, as is well known, a very large number of models
are destroyed or severely crippled in the initial test flying stages.
It is the purpose of this. article to give the reader, whether his experience
be extensive or limited, an understanding of how a completed model can best be adjusted
for flight. It is also intended to point out how to observe from the flights what
features of the design should be corrected or changed.
Before making test flights on any free-flight model, rubber or gas, the thrust
line or axis of propeller rotation should be tilted down two or three degrees, as
in Figure 1. This is called adding downthrust.
Longitudinal trimming is done to correct any stalling or diving tendencies which
may be present and to secure the best duration of flight. It is important to understand
that in all cases the model must be trimmed in such a way as to take care of two
flight conditions: (a) flight without power, that is, in the glide, and (b) flight
with power applied, that is, with the propeller turning and driving the model.
In all free-flight designs, rubber or gas, the leading edge of the wing should
be tilted up so that there's an angle of four or five degrees between the wing chord
line and the centerline of the fuselage as shown in Figure 1. This angle is called
the angle of incidence from the fuselage, and the amount mentioned will result in
the fuselage having the smallest drag or resistance to the airstream in flight.
This is because the angle of attack of most airfoils used in models is around four
or five degrees for best duration (minimum sinking speed).
Consider the case of a typical rubber contest model of the tractor type (propeller
in the front). First, the wing is strapped onto the fuselage at what is believed
to be the correct position. As a first guess, this may be at a point where the model
will balance midway between the leading and trailing edges of the wing. Enough incidence
is then added to give the four or five degree angle mentioned above.
The model is then glided gently into tall grass and the flight path is noted.
If the tendency is to dive in, the trailing edge of the stabilizer is blocked up
slightly. This is called putting negative incidence into the stabilizer. If the
tendency is to stall, the leading edge of the stabilizer is blocked up slightly.
This is called putting positive incidence into the stabilizer.
Note that the position of the wing is not changed to correct for diving or stalling
tendencies as is so often recommended. Several more hand glides are then made and
the stabilizer setting corrected in the above manner until a smooth steady glide
is obtained. This only represents a first guess for correct gliding trim since the
model must glide in smoothly from 30 or 40 feet of altitude before we can be fairly
certain the trim is correct.
The next step is to put 40 or 50 hand winds into the motor, launch the model
from the hand, and observe the flight path. Suppose that it climbs up a little,
then begins to stall under power or hang on the prop, but finally manages to recover
and glide in smoothly when the power gives out. This means that the gliding trim
is pretty nearly correct but that the "power on" trim is not. The most commonly
used method of correcting such a condition is to add excessive downthrust, eight
or ten degrees or more, until no stalling under power shows up as in Figure 2.
This is not at all the most efficient manner of adjusting. When a model is trimmed
for good glide but noses up under power with two or three degrees of downthrust,
the real trouble is that the model is too stable. It is an established fact that
both large-scale aircraft and models which are quite stable tend to nose up considerably
under power when they are trimmed for a good glide. While it is true that the usual
manner of adding excessive downthrust will relieve this effect, it will not produce
best duration results.
The nose-up tendency is best corrected by making the model less stable. This
is done by moving the center of gravity or balancing point to the rear, or by making
the model balance at a point that is further to the rear with respect to the wing.
Of course, this adjustment will destroy the glide trim and the model must again
be trimmed in the glide for the new location of the center of gravity by changing
the stabilizer incidence as previously described.
Continuing with the case of our rubber model which at present balances at a point
halfway between the leading and trailing edges (at the. 50% chord point), the next
step is to destabilize the model slightly and thus reduce the nose-up under power
tendency. This is done by shifting the wing forward about one quarter of an inch.
The model now balances at a point one quarter of an inch further to the rear. The
glide is checked and a slight stall is noted, which is trimmed out by stabilizer
Another test under power is tried and it is seen that there is still a slight
stall under power, but that it is not nearly so severe. Another shifting forward
of the wing is made and, after trimming for glide, the model is seen to climb steadily
and glide smoothly in.
Successive flights under increasingly larger number of winds are made and in
each case a smooth flight is obtained. After two or three hundred hand winds have
been tried the winder can be used until the maximum number of turns of which the
rubber motor is capable have been used. The correct position of the wing on the
fuselage is, then marked for future assembly. After this all trim adjustments should
be made by changing the stabilizer incidence for glide and by offsetting the thrustline
to the right or left for changing the amount of turn under power.
The important thing is to determine, as described, where the wing should be located
on the fuselage and to hold this for future flying.
It might have happened in the case of the rubber model just described, that,
upon being flown under power for the first time, the model would dive to the ground.
This would indicate that the model was not stable enough, and the trouble should
be corrected in a manner just opposite to that previously given. That is, the wing
should be moved rearwards a little, the stabilizer incidence changed to trim the
glide and another low power flight made. This process is repeated until smooth power
flights and glides are obtained, the number of winds being increased with additional
flights until maximum winds are put in.
It should be noted that adding weight to the tail to move the center of gravity
to the rear has the same effect as moving the wing forward: both operations decrease
the stability of the. airplane. And, by the same token, it is true that adding weight
to the nose, which moves the center of gravity forward, has the same effect as moving
the wing to the rear: both actions increase stability.
Further discussion of the subject will be found in a forthcoming issue.