A Tale of Miss Fortune III Article & Plans
November 1946 Air Trails
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 IIIby 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 definitions 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.
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Posted December 31, 2011