SCAMPER (Jul, 1956)
Using an air propeller, this model zips along at 40 mph as a car and does 20 as a ski-equipped boat.
By Paul Del Gatto
BUILT as a car, this model is a supercharged bundle of energy. Free-running, it surges forward as if shot from a cannon and tops 40 mph. Most people won’t have the space to let it go and will have to use a tether. Even at that, it will do better than 35.
Personally, our favorite version is the one featuring the hydro-ski arrangement. Though not as fast as the car, 20 mph is still very high for a boat of this size. Yet it isn’t the speed that impresses us so much as the sight of this unusual water bug rising up on the skis. The air prop lends to the fascination by creating the illusion of some weird form of aircraft skimming across the water. Of course you may experience a somewhat different type of reaction, but one thing is certain: no matter which version you try, you will enjoy it every bit as much as we did.
The chief material used in the construction of the model is balsa wood. However, if you have hardwood handy don’t hesitate to use it. It may mean a few more hours work on the construction, but the time lost there will be made up when it comes to applying the finish. Hardwood, of course, also makes a more durable model.
Beginning with the main hull, cut out two blank shapes from 3/4-in. thick balsa or hardwood. Then cement them together lightly to obtain the desired thickness of the hull. Roughly shape the hull to a symmetrical outline with a whittling knife, using the cross section templates as a guide. When complete, sand the hull to the desired final shape.
At this point the wheel or hydro-ski axles can be assembled and installed. Before doing this, it is first necessary that the shaped hull be pried apart for their installation. Both the wheel and hydro-ski axles are bent from 1/16-in diameter wire. The bottom half of the hull is then recessed to take the axles and then the two halves of the hull are cemented together with the axles in place.
The skis for the water version are cut from approximately .020-in. thick brass and soldered to axles. Try to obtain a similar angular setup to what we have shown, as we have tried it this way and know it works fine.
The engine nacelle mount can be cut out from 1/8-in. plywood or laminated from two 1/8-in. thick hard balsa sheets. Shape it to a symmetrical piece as was done for the hull; then recess the top of the hull V8 in. and cement it firmly in place.
The engine nacelle is blanked out from two 1×2-in. balsa blocks cemented lightly together just as the hull was. Here again the shaping procedure is similar to that of the hull. When this phase has been completed, pry the two halves apart and hollow them out approximately as indicated on the plan. Then mark the position of the firewall and cut the two halves across at that point. Since only about ^ in. or less will be lost in cutting, just remove the difference from the cowl portion to account for the Va in. thickness of the plywood firewall.
Before cementing the two nacelle halves together all necessary cutouts for air, fuel, needle valve and cylinder head will have to be made. This is best done by first cementing the firewall in place to the lower shell and cowl, then mounting the engine permanently in place. Next check the top shell and cowl portion against the engine installation for the exact location of the required cutouts. When the assembly has been completed, recess the nacelle to fit on the mount and cement it in place. All that remains is to add such fixings as the canopy, tail piece, exhausts, bridle hooks for tether and the 3/16-in. hard sheet balsa or hardwood side fairings.
Before attempting to apply a finish, the next phase is to fillet all joints with Plastic Wood to obtain greater strength and a better appearing model. After this is done, give the model a final smooth sanding.
If a glow plug engine is used, remember the finish will have to be fuel proof. Begin by applying three to four coats of sealer, sanding smooth between each coat. Then apply three to four coats of clear fuel-proof dope. The colored dopes should be of medium consistency when applied.
Brush on at least five coats and, when dry, rub down the finish with a fine grit rubbing compound such as Duco No. 7 to obtain a high gloss.
In testing the models, particularly as extensively ‘as we did, we undoubtedly experienced all that you will in operating your own car or boat. If you are free-running the car, be certain that the engine is offset about 2° to the right as we indicated on the plan. The reason for this is there is a natural tendency for the car to veer to the left due to the propeller’s rotation. This force is known as torque. Now it may be necessary to use a little more or less offset, but this will best be determined while the car is being tested. In all probability your air-car will have to be tethered for lack of space. Braided wire of .015 in. diameter and 50 feet in length will do. It, in turn, is fastened to an .040 in. diameter wire bridle strung between two hooks. For a center post, drive a big nail or spike into the ground.
If you are testing the air-boat, the first thing you should do is add a little right turn to the rudder, even if you have offset the engine to the right. Make all your test runs from moderate to high speeds so that the model will quickly get up on the skis and not dig in and porpoise back and forth. In launching the model, keep the bow out of the water and follow through with a gentle sweep of the hand. About the most troublesome thing that might occur is a dunking if the turn is too sharp. Remedy: decrease the turn, dry out engine, run it and let her go again. Your efforts will be well rewarded.
BILL OF MATERIALS
Quantity Dimension Use
1 piece 3/4 x 4-1/2 x 26-in balsa or hardwood Hull 1 piece 1 x 2 x 18-in. balsa Engine nacelle.
1 piece 3/16 x 2 x 18-in. balsa or hardwood Side fairings.
1 piece 1/8 x 3 x 18 in. balsa Nacelle mount, tail piece Miscellaneous: 1/8-in. plywood firewall; 1/16-in. dia. wire for axles; 1/4 in. O.D. tubing; celluloid canopy; 1-3/4-in. dia. wheels for car or ,020-in. brass for skis; 1-1/4 in. dia. spinner; .020 to .049 engine; fuel-proof cement; fuel-proof clear and colored dopes; Plastic Wood; wood sealer.