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THE MULVIHILL TROPHY WINNER
How you can build the high-time stick model winner of the 1941 Nationals -- a consistent duration model in still air or in thermals
by RAY SMITH
Early in the spring of 1941 the third in a series of 200 square inch stick models developed by Bob Johnson won the Mulvihill Trophy. Its principal features are its 7 to 1 aspect ratio wing strong construction and extreme simplicity. This last factor is very important in any contest model, because it not only facilitates building but also permits quick repair on the "field of battle".
The first model in this series was built in 1940 and was lost two days after completion. It was a very simple job, with a freewheeling propeller and could fly three minutes without thermal currents assistance! The second model was somewhat more streamlined, with folding propeller and other refinements. This job took first place at our annual club invitational meet in 1940 with flights averaging 2:18 on a windy, overcast day. However it proved that the model could do 3:15 in still air. A short time later, with only one flight, it took fourth place in another contest.
The third in this series, the Mulvihill Trophy winner, has a very low aspect ratio wing. The efficiency of this wing may be questioned, but it has several advantages, such as lightweight, stronger, easier construction and less susceptibility to warping. It glides slowly at a ratio of 7 to 1, giving the ship a low sinking speed.
On its maiden voyage the model was lost after four minutes: several days later it was found about 9 miles from its starting point. The following Sunday two flights were made, one for 10:04 and the other for 6:33, after which it disappeared from sight.
Another ship was completed in time for the Chicago Spring Record Trials However after a test flight of 16:04, it was damaged on landing. It was repaired and entered in the annual meet where it flew out of sight after 7:22, receiving fourth place. Ray Smith built another in time to enter it in the Nationals, where it won the Mulvihill and C.W. Rogers Trophies. This model averaged a total time of ???4.5 seconds and a long single flight of 15:26. Bob Johnson received eighth place in the Junior Division with the same design. In all, four models have been made from this design: six out of sight flights were made and all the ships were lost except one.
CONSTRUCTION --- Before starting construction the plans must be scaled to full size. Note that the drawings are scaled 1/4" = 1 inch. The plan may be enlarged in one of three ways: photostating, laying it off with dividers or using a rule. Spread a sheet of wax paper over the drawing to prevent cement from ruining the plans. If you use the grades of wood suggested in this article, your model will probably weigh less than 6 ozs. Make up the additional weight required by adding more rubber.
FUSELAGE --- Pin the 1/8" sq. hard balsa longerons to the fuselage outline of the full scale layout, being careful not to split the wood. Cut the cross braces and cement them into place. After allowing sufficient time for drying, build the other side directly on top of the first. This insures perfect alignment. Remove the completed sides from the plan and join them together, as shown in the top view of the plan. It is to your best advantage to work slowly and carefully. Hard balsa fill-ins are added to nose and tail, as illustrated, and the brass plate is inserted for the rear hook.
WING CONSTRUCTION --- Cut 23 ribs from 1/l16" light quarter-grain stock. Pin the ribs together and sandpaper them as one unit to insure uniformity. Then cut all notches with great care. Cut wing tips from 1/8" sheet balsa according to full size patterns.
The wing is now ready for assembly. Pin the main spar and trailing edge onto the drawing in their proper positions. The trailing edge should be cut from very light quarter-grain 1/8" sheet. However, at the center section it should be very hard quarter-grain balsa to withstand wing rubber tension. Now cement the ribs in place and add the leading edge which should be hard 1/8" sq. balsa. The spar should be cut from medium straight grain balsa. From the third rib, taper the spar to 1/8" sq. at its tip. Cement wing tips in place and allow sufficient time for drying. Crack the wing at the dihedral points where shown and cement the 1/16" sheet gussets in place. When all joints have dried, sandpaper the entire wing to remove all excess glue.
RUDDER AND STABILIZER --- The stabilizer is built in the same manner as the wing except that the dihedral is omitted and two 1/8" sq. medium balsa strips are inserted along the top of the ribs. Rudder is made by laying out the soft 1/8" sheet balsa outline with the soft 1/8" sq. rudder post. When dry, remove from the plan and add the 1/16" x 1/8" ribs.
PROPELLER --- The propeller is the most important part of the ship; both blades must be equal in area and weight and should run with no vibration whatsoever. The curved line is about 5/32" from the straight diagonals at its thickest point. The propeller is carved from medium balsa with about 3/16" cup. To have a really beautiful propeller, finish with three coats of thinned out glue with a rub-down with 10-0 wet and dry sandpaper after each coat. After the folding unit is attached so that the blades fold snugly against the fuselage sides, wax the propeller and polish it with soft flannel cloth. The nose block is made from medium soft balsa with a 1/8" or 3/16" plywood backplate. The nose plug should fit snugly but should not necessitate forcing.
COVERING --- The entire ship should Be gone over with a 2-0 sandpaper wrapped block to remove bumps that spoil a good covering job. The model should then be covered with the tissue grain running spanwise on wing and tail. The fuselage bottom should be double tissue with grains at right angles. The rest of the body should be covered with single tissue, the grain running lo lengthwise. After water doping, the wing and tail are given two coats of light dope while the fuselage is given two coats of thicker dope. A good color scheme makes it easy for the builder to see his model while it is in flight. The Mulvihill Trophy winner had a red body with yellow wing and tail assembly.
FLYING --- The model is very simple to adjust. A 1/8" by 1/4" piece of balsa is glued under the wing spar to give incidence. Glide the ship until the approximate wing position is determined. Put a few turns into the rubber motor and adjust for a fairly tight right circle. Add necessary down and side-thrust to give it a tight t right spiral climb. Twenty-two strands of 3/16" brown rubber forty inches long will provide ample power.
Under full power the model will climb straight up for 15 seconds. Then without stalling, it will go into a tight right spiral climb. After about 40 seconds more, the model will level out, propeller will fold, and the ship will go "thermal Hunting" above 500 feet. In still air the model may be made to average between 4 and 4-1/2 minutes.
Scanned From April 1942
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