Free Flight Sensation: Bob Linn's B-70 Article & Plans
May 1961 American Modeler

May 1961 American Modeler Table of Contents These pages from vintage modeling magazines like Flying Aces, Air Trails, American Modeler, American Aircraft Modeler, Young Men, Flying Models, Model Airplane News, R/C Modeler, captured the era. All copyrights acknowledged.

Jet airplanes were still somewhat of a novelty with actual flying models when this article about a free flight North American XB-70 Valkyrie appeared in a 1961 issue of American Modeler magazine. This rather large - 21" wingspan and 45" long at 15 ounces - craft is built of 3/32" and 1/4" balsa sheet and is powered by a Cox .049 engine in a pusher configuration. That's not a lot of power for such a large ship, but it must have been. The plans are easily scaled up or down with pencil and paper thanks to a 1" square grid provided. Today, the B-70 could probably be built lighter and be powered by an electric propulsion setup.

Free Flight Sensation: Bob Linn's B-70

The first B-70 to take to the air made its maiden flight over the recreation field at the Los Angeles division of North American Aviation and reached a top speed of 30 mph.

Powered by a 1/10th hp glow plug engine the lightweight balsawood bomber climbed to 200 feet and circled the field in wide, smooth sweeps that testified to the success of its sleek, delta-wing design.

Owner-designer Bob Linn from the NAA Engineering Flight Test Dept., a modeler for 30 years, expressed a model-man's highest praise for the B-70 replica when he reported, "It flys well in every way, you couldn't ask for anything to perform better." Bob, who built the 8-ounce model in two evenings, said it's a sure attention-getter among other model enthusiasts ... "Everyone comes over to look when I fly it."

Linn had tried building canard models, prior to his B-70 effort but never had much luck with them. Bob has made 7 of these in various sizes. Competition Models (Sal Taibi) is bringing out a smaller version for .020 power. The "middle" size is that presented here. It is preferable for .049 size power plants. An R/C version is under development. Here is Bob Linn's construction suggestions. Bob says you can add a fixed landing gear if you desire, but that for open field flying it lands better with wheels "retracted" (in other words, without an l.g).

Building Procedure. First step is to glue fuselage pieces in shape shown on drawings. It is a good idea to pre-glue all edges before joining. While this is drying the wing pieces may be cut out; after cutting each piece, check each left-over end to see if it will make one of the other needed pieces. After all the wing pieces have been cut, assemble them on a large, flat surface with the recommended pre-glue method. One word of warning, on the wing construction try to select wood of similar grain and "bend" characteristics and be certain the edges make good joints before starting gluing operations. It is a good idea to allow the wing to dry over night.

Be the first executive on your block to have your own B-70 supersonic transport! Los Angeles' Linn of North American Aviation Engineering Flight Test Department has built a number of these delta bombers, in .020, .049 and R/C sizes, reports "a sweeter performing model you couldn't ask for." Version presented here is for .049-size power (weighs 15 ounces) ... plans are quarter-size enabling you to scale it up quickly. Landing gear shown above is optional, Bob's B-70s work equally well with or without l.g., designer prefers to leave gear off for "open field" flying.

After the fuselage pieces are dry, the fuselage profile is plotted from the "squared" plans. Layout the airfoil as shown - this is very important. Cut out the fuselage including the stabilizer slit and the cut that separates upper and lower fuselage. After making this cut, check to be sure it is a right angle so the pieces will set on the wing at 90 degree angle.

After the wing is dry, sand it on both sides to smooth the joints between sheets. Mark a center line down both sides. Glue the upper fuselage to the wing surface using plenty of pins to keep the wing tight to the fuselage to form the airfoil. After this has dried add the lower fuselage.

Cut the stabilizer and rudders from sheet wood and sand them to shape, add dihedral to the stabilizer as shown on the drawings.

Regardless of what engine is used weight must be added to the nose to bring the C. G. to the desired position. The best way is to insert the weight in the center shell before adding the side halves; however, if the builder wishes, he may add it afterwards externally. Glue the stabilizer in place; add the fuselage side pieces, then carve and sand the fuselage to shape.

After the wing to fuselage joint is dry the final work on the wing can be completed. Glue a strip along the leading edge and sand the wing to final shape. Cut the slots for the rudders and glue them in place.

All of my models have had the wing covered with Jap tissue: I feel this is a good idea ... not only does it make the wing easier to paint but it helps to keep the airfoil in shape, strengthens the surface, and prevents fuel soakage. Install the engine using either beam or radial method depending on whatever power plant is selected. I recommend engines that will run backwards since pusher props are sometimes hard to come by. If a separate commercial tank is used the outlet in most cases must be changed to allow the engine to feed fuel in a climbing attitude.

Flying Instruction. After completion of model, weight is added to nose to bring C. G. to point marked on drawings. You are now ready to begin flight tests. Make a number of glides from overhead via hand launch. This model must be heaved quite hard to give it flying speed. Don't be afraid to really throw it as good forward speed is very essential. Correct any nose-up or down glide by adding or subtracting weight from the nose. After glide tests are satisfactory prepare for power flights; for the first flight use zero thrust adjustments.

If you have a real hot engine try about 3/4 power for the first flight. The B-70 can be made to turn either direction by a very small movement of the edge of both rudders or a washer under the edge of the engine. A few words of warning ... remember this is a pusher installation, be sure the air from the prop is blowing the right way before launching.

One other word, if you are using a separate tank be sure it will feed in a climbing attitude. If landing gear is installed make certain it is long enough to give prop clearance when nose rises for takeoff.

Free Flight B-70 Bomber Side Plans Sheet

Free Flight B-70 Bomber Top Plans Sheet

Posted November 29, 2019