|
Modeling
took a hit during World War II because of the shortage of raw materials
and the need for rationing. Balsa was used extensively for air-dropped
supply pallets because of its light weight, strength, and shock
absorption properties. Rubber was in high demand for vehicle tires.
The author mentions in this 1946 Air Trails article that rubber
for free flight power was just starting to become readily available
again. We have it really good today with seemingly endless supplies
of everything! This article for the Miss Fortune III really
does a great job illustrating the thought and planning that goes
into creating a model that conforms precisely to contest requirements
for size, weight, configuration, etc., while incorporating
original methods and components to maximize effectiveness. A
Tale of Miss Fortune III
by Marvin L. Moss
 WITH
the return of rubber and more peaceful conditions we should soon
see the resumption of the annual international competition for the
Wakefield Cup. Along with the finest camaraderie, these contests
have always brought forth the best in models. We should do well
to review any progress that has been made since Dick Korda's outstanding
performance in 1939 left America in the position of defending the
cup. Let us first look at the problem which faces
us. The contest rules call for a ship of definite size (190-210
square inches) and a minimum weight (eight ounces). Other rules
limit the area. of the horizontal stabilizer to one third that of
the wing and require a minimum cross section area of length 2/100.
It should be noted that several defi-nitions differ from those used
in this country. Fuselage length is the total over-all length and
does not exclude the propeller. Wing area is defined as the actual
plan area; no allowance being made for projection. An unassisted-rise-off-ground-take-off
rule is strictly enforced in the three flights permitted for average
duration. Under these rules a more or less standardized
ship has evolved. In which directions, then, can we expect to make
progress? A look at the records reveals that since
1935 the winners, without exception, were aided by. thermals. With
the knowledge that the contest will be held during the fair weather
of summer we might as well design our plane with the expectation
that thermals will again be present. A ship best able to take advantage
of these currents is, therefore, the one for which we should strive
and not necessarily for one having maximum still-air time. This
means a model designed to climb to a great altitude, where the thermals
gain strength, and one having a low sinking speed to ride the weaker
currents. How can altitude be increased? First we
can increase the amount of stored energy available to do the work
of raising the plane. This means a greater amount of rubber, but
we do not wish to exceed the minimum weight of eight ounces so this
becomes a higher power-weight ratio. Second, we can reduce the amount
of work to be done. This would mean reducing the weight to be lifted
or the resistance to he overcome in doing it. but the weight permits
of no changes. We can, however, do much toward decreasing the drag
or resistance. Lastly, we can increase the efficiency with which
the available power is used. This translates itself principally
into the proper propeller and power combination to fit the ship
and the flying conditions. There are only two factors which
affect the sinking speed: weight and the overall lift-drag ratio
of our plane. The weight, we have seen, is to be held within definite
limits. There is no limit to the lengths to which we may go in improving
the lift-drag ratio. We see, then, that the ship we
need must be aerodynamically clean, highly powered, light, and efficient.
With this, we must combine stability, but we should also
like to have simplicity of construction, ruggedness, and ease of
adjustment. The war years saw no major strides in
this direction. We witnessed no such radical innovations as folding
props, new materials, tensioners, single-strut landing gear, or
monocoque fuselages which were introduced in past years. We have
seen a general refinement of detail and the emergence of such items
as retracting gear, two-bladed folding props, and spinners as almost
standard equipment. Several paradoxes may seem to
be implicit in the foregoing discussion. There are. Streamlining
is usually obtained at the cost of increased weight. A high proportion
of rubber weight leads to structural weakness. These and other problems
are encountered.
 How,
then, are we: to solve them? We don't. That is, we don't solve them
completely. We cannot carry anyone quality to its limit .without
sacrificing some other. The only other course left open to us is
to compromise wisely. Miss Fortune III is presented as an optimum
series of such compromises. An important feature of the
design is the attention paid to "cleanlining." For instance, the
wing has been mounted on the fuselage at what may seem to be an
excessively high angle. This was done expressly to have the fuselage
in line with the path of the relative wind .at the angle at which
the wing will work. It will be noticed that the wing-to-fuselage
junction has been handled to create little interference through
the use of a small reflex fillet. The rubber wing anchorage has
been fully enclosed without loss of the ability to spring free in
a collision. An eight-sided fuselage, roughly approximating a circular
cross section, blends into a fair-sized spinner. Some
other items deserve mention. A wing of medium aspect ratio is used
with a high-lift airfoil section selected for its good performance
at high angles. At the center section, the undercamber becomes flat,
further to reduce interference at that point, and approaches a symmetrical
section at the tip to delay the stall and decrease drag there. A
long dorsal fin is employed to counteract the peculiar center of
pressure travel of streamlined bodies at low angles of yaw. Good
handling strength is insured by the strategic location of soft blocks
and the use of double· covering on the fuselage. The
fuselage structure is a type which was first introduced by Roger
Hammer some years ago and has since gained popularity, especially
in the New York area. It is built around a square basic frame. Stringers
are cut from quarter grained sheet and are finished to shape after
mounting. A small block plane will be found most helpful in this
operation. It was expected that some trouble might result
from the low position of the stabilizer. No adverse effects due
to downwash over the stabilizer or blanketing of the rudder have
been noted. The whole assembly has proved entirely satisfactory.
Wing construction is a bit different in that the main spar is
completely assembled first, insuring accurate dihedral angles. Placing
the spar at the top of the ribs provides a good base for cementing
the leading edge sheeting. This sheet is sanded to a slight taper
before assembly to minimize the amount of sanding to be done afterward,
thereby preventing bumps from rising over the ribs. Cap stripping
and the leading edge sheet maintain a smooth unsagging covering.
Adequate provision is made for handling the power of eighteen
strands of one-quarter-inch brown rubber. Use of bobbins and the
two-piece prop shaft permit quick removal of the motor. Placing
the winding hook outside the spinner eliminates much of the fussing
under stress of full winds. An accurately aligned wheel
is imperative for smooth take-offs with a single-wheel gear. It
is for this reason that the special wheel construction is used.
Many types of propeller hinges have been developed during
the years of their use. The old brass strap type shown is still
among the best when durability, rigidity. case of construction and
freedom of motion are considered. To secure flush folding the angles
must be made as shown. The performance of Miss Fortune,
like that of any other model, depends finally upon the quality of
its adjustment. These notes may prove to be valuable. All directions
are from the cabin. During doping the right wing washed in about
one eighth of an eight of an inch from the center to the tip dihedral
break. The left stabilizer tip is also given a slight wash-in.
Because the wing position is fixed the center of gravity
must be located as shown before any adjustments are made. Thereafter,
the procedure is normal. Glide adjustments are made by altering
incidence of the stabilizer and warping the rudder trailing edge.
The power flight is controlled by offsetting the thrust line.
When trimming is complete, a flight of this character should
result. Under the first burst of power the ship takes cleanly and
climbs straight ahead, almost approaching a stall before beginning
its wide turn to the right. The washed-in right wing prevents the
circle from becoming tight in the climb and holds the wings level
during the small circle of the glide. This type of flight seems
to get the greatest altitude and have the best thermal-hunting ability.
Because of the unusually high angle of wing incidence the fuselage
will be inclined at a slight angle to the ground giving the impression
that the model is gliding downward. That's exactly what is doing.
At present, Miss Fortune is powered with prewar brown
rubber that is more than six years old. It is still possible pack
950 turns into its forty-two-inch length. With its weight of 3.63
ounces, it is a good approach to our goal of a power-weight ratio
of 50%. Whatever you are forced to use for power,
Miss Fortune will offer an indication of what cleanlining and weight
control can accomplish, as well as a hint to what the postwar Wakefields
my be like.
<click for larger version
Notice:
The AMA Plans Service offers a
full-size version of many of the plans show here at a very reasonable cost. They
will scale the plans any size for you. It is always best to buy printed plans because
my scanner versions often have distortions that can cause parts to fit poorly. Purchasing
plans also help to support the operation of the
Academy of Model Aeronautics - the #1
advocate for model aviation throughout the world. If the AMA no longer has this
plan on file, I will be glad to send you my higher resolution version.
Try my Scale Calculator for
Model Airplane Plans.
Posted December 31, 2011
|
