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THE MULVIHILL is one of the most handsome of the National trophies. It is a bronzed figure of the mythical Greek hero, Icarus, testing his wings. Icarus met disaster when he flew too high and the sun melted the wax, which was holding his wings, and he fell into the sea and was drowned. The Mulvihill Trophy was a donation of B. H. Mulvihill, vice president of the N. A. A., in 1923. A yearly contest is held and the winner holds the trophy for one year. It's impossible to get permanent possession of the trophy, regardless of how often the same builder wins it. And since the age limit is 21, few modelers have the privilege of winning it more than once.
But to have your name inscribed on the trophy entitles you to a place on the honor roll of model building. Bruce Luckett of Tulsa, Oklahoma, was the 1936 addition. And this month we're privileged in being able to include the plans for his winning Mulvihill entry, along with some of his ideas and viewpoints on modeling.
Bruce can best describe this model and what features he tried to incorporate in its design. Here is a portion of his letter:
My primary purpose in building this ship was to produce a suitable plane to enter in the outdoor duration contest. The factors that I wished to put into this design were:
1. Good climb and a good flat glide to enable the ship to take thermals more easily.
2. To distribute the weight in such a manner as to make the ship both inherently stable and able to stand the punishment than outdoor contests are sure to inflict.
A plane with these features was produced only after ten months of experimentation on my part and the part of various other members of the Model Aeronautical Engineers of Tulsa. The following features were outstanding on the first ship: extremely high dihedral, low aspect ratio wing, fairly short moment arm, and a wing mounted rather high above the center of gravity. In the ships that followed several basic changes were made, as the above-mentioned features were far from ideal.
After many test flights with the following models I found it necessary to lower the dihedral, increase the aspect ratio, of the wing, lengthen the moment arm, and also to lower the wing itself. The finished product far exceeded my expectations. It had an unusually flat glide and was very sensitive to thermals.
As a contest ship it proved to be a consistent performer and I was more than pleased with its performance. Three weeks prior to the 1936 Nationals held in Detroit, the plane was entered in a contest sponsored by the members of the Tulsa Airmen's Club. The plane placed third, with a flight of better than 15 minutes, and its second-best official flight way 10:50. Three weeks later this same model won the Mulvihill Trophy at the Nationals, with a flight of 41:41, making a 9:06 flight on its first official try.
And this is Lockett's very modest account of how a converted scale modeler set about to win an outdoor contest.
The model has the advantage of combining excellent contest performance, and yet its construction is well within the range of the beginner. This proves again that the most successful contest model is usually the simplest. Simplicity and efficiency go hand in hand in modeling, as in most other things. And, too, the model should be attractive to the builders who are working on a limited budget. The cost of the material is small, compared to the usual expense of duplicating a contest winner.
MOTOR STICK AND TAIL BOOM
Both stick and boom are triangular in cross section, built up from three pieces of flat sheet balsa. The construction procedure is to build a trough out of two pieces by cementing the edges together and then the third piece is cemented across the top, as cap. Hard-grade balsa is used in both stick and boom.
The motor stick is 18" long and uniform in cross section. 3/32 " sheet balsa is used. The edges of two pieces of 5/8" balsa are angled so they fit tightly together and then joined with liberal coats of cement. The width of the 'trough or "V" formed by the two pieces should be 3/4". The edges of the "V" should be bevelled so they fit snugly against the cap strip which is 3/32 x 3/4".
The tail boom is built the same way. It is 8" long and tapering. The cap strip tapers from 1/2" to 1/4" and the sides of the "V" taper from 7/16" to 3/16". 1/16 sheet balsa is used and the thickness of the walls does not vary.
The stick is used with the cap strip on the bottom, whereas the boom is turned so the cap strip is on top. This provides a flat surface for mounting the tail surfaces.
The front of the motor stick is cut off at an angle, and a small piece of balsa is cemented to it to provide a solid base for attaching the thrust bearing. The bearing itself is bent from #20 wire. It extends up the front of the stick and over the top. To prevent the bearing from bending, it is reinforced with a of piece of plywood. It can be either balsa or hardwood. However, it should be three-ply and at least 1/8" thickness. The front of the stick is equipped with a thrust holder. It is a wire hook from #20 wire and cemented to the plywood. The hook should be the same level as the hole in the thrust bearing or just about 9/16" below the bottom of the motor stick. This thrust holder can be bent to give the propeller any desired negative angle.
The rear hook is bent from #20 wire and attached to the rear of the stick. It is reinforced with plywood in much the same way as the thrust bearing. The center of the rear hook should be 1/4" below the bottom of the stick.
The tail boom and the motor stick are butt joined. That is, the edges are cemented thoroughly and then pressed together, keeping in position until the cement has dried. Remember that the flat part of the boom is on top and the flat part of the stick is on the bottom. Naturally, the edges of the boom will protrude where the joint is made with the motor stick. Don't cut these off. Fare any rough edges by filleting with pieces of balsa. These balsa pieces supply additional strength, in addition to maintaining the symmetrical shape of the stick. The boom is cemented at a slight upward angle. As is visible from the three - view drawing, the extreme rear top of the boom should be just about level with the top of the motor stick.
The airfoil used is the Eiffel 400. 20 ribs of 1/16" medium-hard balsa are used in the wing. The ribs are notched to take a 1/8 x 5/16" main spar. Considerable checking will have to be done throughout all stages of wing construction, to make sure you build the correct amount of dihedral and sweepback into the wing.
Building the wing over a full size outline will insure correct rib spacing and sweepback. Sketch a full-size pattern on a piece of scrap wrapping paper. It need be only the simplest outline of a drawing. Build the wing flat on the workbench ignoring the dihedral until the wing has been entirely assembled.
Dihedral is put into the wing at three points. The center of the wing will have to be broken and then cemented to raise the ends of the wings. Halfway out each side of the wing, the dihedral should be 3/4". Break the wing at this point and raise the tips until they are 5" above the center. Liberal coatings of cement will restore the wing's strength where it was necessary to break it.
ATTACHING THE WING
The wing is mounted to the motor stick with a balsa saddle that rests atop the stick and is secured with rubber bands which pass underneath the stick. Two wing clips are bent from #20 wire. The front clip is 1/4" higher than the rear one. The size of the clips gives the wing the correct angle of incidence. These clips are cemented and bound to the leading and trailing edges of the wing. The wing saddle consists of two pieces of 1/16 x 5/8 x 4-1/2" balsa which are cemented edge to edge to form a "V" which fits snugly on top the motor stick. The wing clips are cemented rigidly to this saddle. The rubber bands are illustrated in the drawing.
ELEVATOR AND RUDDER
The elevator is free from dihedral and sweepback, but there is the taper to consider. Elevator ribs C and D are shown exact size. The other three sizes of ribs are cut so they taper in length and thickness. Good practice is to assemble the elevator with only the center and two end ribs in position. Then fit in the other ribs, cutting them to the correct depth. By resting a straight edge across the top of the center and end ribs you can readily determine the depth of the intermediate ribs.
The same process can be followed with the rudder where ribs A and B are shown exact size and the other ribs are fitted in. Part of the rudder is made from 1/8" sheet balsa. Using the full-size pattern, cut this part and sand it to a streamline shape.
No special treatment is used in this operation. Banana oil is used to attach the tissue. The bottom sheet-balsa section of the rudder is covered with tissue. The stick is protected by one or two coats of banana oil, followed by a rubdown with sandpaper. The model is treated with light dope - using as many coats as necessary to give the protection and strength for all-weather flying.
ATTACHING TAIL SURFACES
The elevator is mounted flat on the boom. It is set at a two degree negative angle. If you've attached the boon to the stick at the indicated angle, the elevator will have the correct setting. Cement it to the boom after it has been covered. If you want to check this setting, the leading edge of the elevator should be about 3/16" below the trailing edge. This setting is taken with the top of the motor stick as the reference line. The rudder is cemented atop the elevator. The part of the rudder directly above the elevator is left uncovered. The sheet-balsa part of the rudder is cemented to the bottom of the tail boom.
Luckett used a hard balsa propeller. If this is your first large propeller, you better substitute a medium-hard balsa block. Being easier to carve, it will give you experience before you tackle the more difficult hard-balsa propeller. The block is shaped as shown in the drawing. And then the blades are cut. About 1/8" camber is put into the rear face of each blade. The maximum blade thickness is about 1/8". The hub of the finished propeller is about 1/4" thick. The height at the hub is about 3/8". The excess wood at the hub is cut away after the blades have been cut to the desired thickness. The type of freewheeling is a spring-and-catch type. However, your favorite variety can be easily substituted. Three or four 3/16" washers are used between the propeller and the thrust bearing. Washers are cemented to the hub of the propeller to protect it against wear. The propeller is given 2 degrees negative and 2 degrees right thrust. Merely adjust the thrust hanger to get the required angles.
The model is powered with 16 strands of 1/8" flat, brown rubber. About 5 inches of slack are used. The rubber will take about 720 turns. The model climbs in tight right circles and after the power cuts out, eases into a wide right circle. An estimate of the model's climb is 225 feet under power, in calm, current-free air. The propeller run is about 55 to 65 seconds.
Bruce lost his original model out of sight at 41m and 41s. But he made a duplicate for the purpose of illustrating this article. He took it out for a few test flights, to make sure he hadn't lost his touch. And he hadn't -- it flew out of sight on one of its test flights. But he had neglected to take photos, so he built another ship. Wisely, he took photos before making any test flights. But when I asked him for a detailed list of weights for the article, he wrote despondently that a short time after he took the photos, he took the ship out for a few flights and -- you know the rest of his story. He lost it high in the clouds.
It was Luckett's skill in building and flying which won the trophy. Yet he graciously acknowledges that he drew from the common storehouse of knowledge which accumulated from the experiments conducted by his fellow club members. We'd like some of the material in this storehouse of model knowledge. With it, success in contests should be easy. Last year the Tulsa boys proved they had developed a formula for winning contests. The complete assortment of trophies and prizes they took home from every meet is the evidence.
Scanned From April, 1937
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