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Chapter 7 - “King Cobra” Speed Racer

Increasing speed is the modern trend with cars, boats, and planes. The model-airplane builders of the world have followed this vogue and have attained truly astonishing speeds with their small models. Specially designed "winged powerplants" have exceeded one hundred and fifty miles per hour, while the scale model with its more pleasing appearance is approaching one hundred and twenty miles per hour! These speeds can be achieved by employing brute power or by careful selection of model type through consideration of the various characteristics which contribute to successful speed flying.

It should be obvious that the primary factor responsible for high speeds is wing area. The smaller the wing the higher the speed, up to a given point. In other words, model efficiency can be impaired by using a wing that is too small to lift the model and, in these cases, the craft must be flown at a high angle of attack. This is the angle at which the plane flies in relation to the airstream; if this angle is too great, it can produce more resistance than a few added inches of wing area. This problem generally does not occur with scale replicas because of their very normal appearance.

Art Streith—General Petroleum  Photo

Speeds in excess of one hundred and fifteen miles per hour should easily be attained by our model when powered with a Fox .59 engine or similar powerplant. Much landing gear detail can be found in this photo. Engine on our model is mounted on its side in order to preserve the scale fuselage profile as well as to concentrate the cylinder weight on the outboard side of the model to keep the craft from banking towards the flyer.

A wing loading of from 60 to 75 ounces per square foot is generally accepted for efficient speed flying.

The airfoil or wing section is another important item which contributes to speed. Many enthusiasts rely on razor-thin wings, while others attempt to develop efficient airfoils suitable for high speeds and zero angle of attack during flight. In general, the airfoil should have a sharp leading edge, and the thickness can be between eight and twelve per cent of the wing chord. The chord is the straightline distance between the leading and the trailing edges of the wing.

In view of the fact that high speeds tend to cause sensitive controls, the tail moment arm on speed models should be long in order to prevent overcontrolling. When the speed of a plane is doubled, the control sensitivity is almost quadrupled! Small elevator surfaces with little movement aid in safe speed flying. A long tail moment arm will also eliminate erratic flight caused by the wing turbulence disturbing the tail surfaces. In other words, the air disturbed by the wing does not have time to smooth out before the tail of the model enters the area. The further apart they are, the better.

Flying Models Photo

Brute power for the size of this model was the reason for the eighty-five mile an hour speed of the author's replica of the experimental "Curtiss XF15C-1". Designed primarily for .099 engines as a maximum, we decided to install a .19 engine for experimental purposes. This is not, however, the correct approach when selecting models for speed flying.

Streamlining, of course, is synonymous with speed. The craft intended for fast flight should be polished to a glass-like finish. As many as 15 coats of "Sanding Sealer" should be applied. Care must be taken to insure good fairing of the engine cylinder. This should be partially or completely enclosed where possible. Spinners are universally used to fair the propeller hub into the fuselage nose. Although these are generally turned from aluminum, many plastic and nylon spinners are available.

It is advisable to select an engine designed for racing if the higher speeds are desired. These power plants have high compression ratios and attain up to 30,000 revolutions per minute. Any sport engine can be fitted, however, and a speed loss of only a few miles per hour will result.

Our candidate for scale-speed model flying is one of the most attractive racing planes ever to round a pylon.   Major Charles "Chuck" Tucker flew this beauty in several Thompson Trophy races but never was able to demonstrate his plane's real ability because of unusual bad luck.

Air Trails  Photo

When powered with an engine of .29 cubic inch displacement, this replica of Rudy Kling's Folkerts Racer attained speeds in excess of one hundred miles per hour. Plans of many planes like this and others shown in this volume are prepared by the author for American Modeler, Model Airplane News and Flying Models magazines. These are available at your local newsstand.

This racer was converted, as were most of the post-war racers, from a military fighting plane. However, instead of merely increasing the engine horsepower and removing all excess weight, Major Tucker and the General Petroleum Corporation went a step further and cut off eight feet from each wing, leaving an eleven foot stub on each side! This enabled the craft to qualify at 392 miles per hour for racing. However, the actual top speed was considered by many designers to be close to the 500 miles per hour mark.

This craft was originally a standard Bell P-63 "King Cobra" series, which in turn was a development of the earlier "Cobra" P-39 fighters which were designed to carry a 37 millimeter cannon firing through the propeller hub. In order to mount this cannon, the designers located the engine behind the pilot; the propeller shaft passes under the seat and between the pilot's legs. This engine location also allowed a much sharper streamlined nose to be fitted and also improved the maneuverability of the plane by locating this mass of weight closer to the center of gravity or balance point. An Allison twelve cylinder, "V" type, liquid-cooled engine of over 1,200 horsepower propels this craft; the radiators to cool this powerplant are cleverly concealed within the wing root to provide excellent streamlining.

Air Trails Photo

One hundred and sixteen miles per hour was attained with this model of the "Caudron" racer. The author's model was powered by a Fox .59 engine. Wing span was 22 inches and construction was similar to the "King Cobra" presented in this chapter. Note the small wing area and long tail moment arm.  Engine was fully enclosed.

For the convenience of those who desire to construct a replica of the fighting version of this plane for sport use, we have illustrated the complete wing and tail. The fighting version would be colored olive drab on upper surfaces and sky blue on all lower surfaces. We shall, however, describe construction of the clipped-wing racer only.

The craft as presented herewith should be fitted with engines of at least .29 cubic inch displacement. Some of the powerplants that can be fitted are: K & B .29", Forster .29", McCoy .29", Veco .29", Ohlsson & Rice .29", Fox .29", and any other engine in this size ranging up to .49 cubic inch displacement. Our prototype model used a Dooling .29", and the highest speed attained with this engine was just over one hundred miles per hour. There is ample room in the fuselage nose for a Fox .59" engine which should attain even greater speeds. This installation is illustrated. For sport flying, any engine over .19 cubic inch displacement can be used. However, very soft wood must be used to keep the total weight of the model to a minimum.

Major Tucker's racer was developed from the Bell F-63 which was, in turn, a development of the "F-39 Cobra" pictured here. Our plans illustrate the fighting version of the "King-Cobra" as well as the racer. This photo will help those readers who care to construct the fighter F-63. Color is olive drab top and light blue bottom with insignia as shown. Note cannon protruding from spinner.

It is advisable to begin construction with the fuselage. The exterior is carved from a solid balsa block and then cut or sawed apart along the horizontal centerline. The two halves are then hollowed, as the plans illustrate. This procedure is identical with that described in Chapter Six. Care must be taken to cut the nose interior correctly to support the engine mounts, which should be firmly cemented in place at this time. These are then filleted with "Plastic Wood", not "Plastic Balsa", for added strength.

The fuel tank should be installed into the lower fuselage shell and should be of the approximate size shown on the plans.   These racing engines consume considerable quantities of fuel and a large tank is therefore in order. Feed, vent, and filling extensions should be fitted to the tank. Do not join the fuselage halves until the wing has been attached. Wing construction is accomplished by the method used in Chapter Five, except that the plywood spar-joiner should be securely cemented to the balsa main spars before the wing is assembled and covered. Construct one panel at a time. Once the ribs have been cemented in place, the landing-gear struts should be bent and then wrapped and cemented to the plywood. Fillet generously with "Plastic Wood". The wing can then be covered. Make certain that holes are provided in the ribs as the plans illustrate. These openings allow the passage of the control lines which will be installed later. While the wing panels are drying, a vertical slot can be sawed into the lower fuselage to fit the plywood spar in the manner described in Chapter Six. Slide the spar into the notch, cement well, and fill in the slot.

Art Streib—General Petroleum Photo

License and racing numbers, which appear only on the underside of the wing, are clearly visible in this head on view of the clipped wing speedster. Notice also the exhibition four bladed propeller with which our model can be fitted while at rest or during sport flights. This is black with yellow tips.  Wing radiator scoops are made from scrap balsa, openings are black.

Art Streib—General Petroleum Photo

Of special interest in this photo is the method of fairing the rudder into the wider fuselage. Note that the lower portion of the rudder is wider than the remainder. This is made by adding ⅛ sheet balsa to the model's rudder and then carving to shape and sanding smooth. All photos should be- carefully studied and referred to during model construction.

The wing joint should be generously filleted with several applications of "Plastic Balsa".

Securely fasten the lead-out control lines to the bell-crank by twisting and then soldering the spirals to insure a firm bond. The bellcrank should then be bolted to the hardwood support. This assembly is well cemented into the lower fuselage shell, first slipping the lead-out lines through the holes in the ribs until the ends protrude beyond the tip. When the bellcrank assembly is in place, the control rod can be bent and fitted. Solder a washer to the end of the rod to prevent disengaging. The after end of the control rod should pierce the fuselage side as shown.

Bend the wire nose-wheel-landing-gear strut to shape. This is then attached to the lower engine mount by fitting into a hole in the mount and cementing and generously filleting with "Plastic Wood".   The two fuselage halves can now be joined by applying plenty of cement. Add the fin and rudder, being certain to offset the rudder as required. Sand the entire structure after you fill in with "Plastic Balsa" any cracks, crevices, or other imperfections in the surface.

Art Streib—General  Petroleum  Photo

This is Major Charles Tucker's long distance racer for the Bendix Transcontinental Race. Also a reworked F-63, this plane does not have abbreviated wings like the closed course racer. The wing tanks are surplus "Shooting Star" tip tanks. Plane is silver with black trim and red horse.   Plans illustrate this plane for those who care to construct it.

Apply ten coats of wood filler ("Sanding Sealer"), sanding well between each coat. Now very carefully cut away the nose hatch as the plans illustrate. Fit the engine in place, cutting away the nose interior here and there as required. The hatch must have a hole cut in it to fit the engine cylinder. No portion of the cylinder should be closer than 3/32" to any wood surface. Bolt the engine in place after the nose interior has received two heavy coats of colored dope. The hatch is held in place with droplets of "Fuel-proof Cement". A gentle slice with a razor blade can remove the hatch at any time desired.

The cockpit canopy can either be cut from a Berkeley plastic bubble canopy available at most hobby counters or it can be carved from a block of soft balsa in the same manner as a fuselage is cut to shape. This item can be cemented in place at this time.

Several additional coats of "Sanding Sealer" can now be brushed on until a very smooth surface is achieved. The entire airplane is colored white. Apply three coats and rub the finish briskly.   Paint the black anti-glare panel, using masking tape. All license numerals and racing number are cut from black "Wondur-cal". The red horse "Pegasus" is carefully cut from red "Wondur-cal". It is advisable to study the plans and photographs during this decorating operation.

Art Streib—General Petroleum Photo

Much detail is visible in this close up of the "King Cobra". Note the radiator scoop in the wing, exhaust stacks on the fuselage and lettering. The area forward of the cockpit is colored dull black to prevent the fuselage nose from reflecting into the pilot's eyes. Nose wheel doors on our model are cut from tin can metal while the horse is red "Wondur-cal" detailed with white ink.

Spot solder the tin-can metal wheel covers to the struts after the wheels have been added. Paint the covers white. The entire craft should now receive two coats of clear fuel proofer.

An aluminum spinner is recommended. A "Froom" spinner is one of the most popular; this can be fitted at this time and painted white. The correct propeller must be chosen if we are to attain maximum speeds. Although each engine requires a slightly different propeller, as a general rule these propellers should possess a very narrow blade and have a very high pitch of from 9 to 13 inches. Use a two-bladed propeller for speed flying and for realism; a four-bladed propeller can be fitted for appearance. If speed is not desired, the model can be flown with the four-bladed propeller. This is made by half-lapping two standard two-bladed propellers together.

Smashing speed records was a habit of Ernest Babcock's specially designed model racer "Jughaid". We have compared this famous 130 m.p.h. model to Howard Hughes’s Record Racer" and it is very evident that these two craft are quite similar in basic arrangement. A model of "Hughes' Racer" attained over 90 m.p.h. with a .49" engine while a .60" engine powered "Jughaid".  Suppose the "Hughes' Racer" were .fitted with a .60"!

Model Craftsman Photo

One of the author's earliest attempts at scale speed models was this "Heinkel He 112" fighter. Top speed was only 87 miles per hour because the wing area was too large and thus presented too much drag for higher speeds. Interesting feature of this model was the extension shaft fitted to the standard .60 engine in order to enclose the engine cylinder in the scale belly "radiator"

Once the craft is balanced, it can be test flown. Safety is important, especially with speed models. Do not use lines less than .012" in diameter of braided stainless steel. A smooth, paved surface is almost a necessity because of the high speeds during landing and take off. It is suggested that the first few flights be conducted on forty-foot lines using a propeller of relatively low 4 to 6 inch pitch in order to hold the speed down to a safe figure.

It will be found that this type of model does not leave the ground as quickly as the models with lower wing loadings.   It may require three-quarters to a full lap in order to become airborne because the take off speed is much higher. It may even be necessary to apply very slight up elevator to lift the craft off the ground.

All control actions must be gentle and not with the wrist alone; rather, the entire arm should be raised and lowered in order to control the craft safely. As usual, the performance should be carefully observed during the entire flight. The plane should fly level with the controls at neutral. If "up" or "down" elevator is required for level flight, weight should be added in the nose or tail until the neutral control-handle position produces level flight. This will promote higher speeds.

When the engine stops, these craft must generally be "whipped" or "led" into a landing because of the high landing speed required. Normally, these models will drop sharply if not "led". Speed flights should be made with fifty- or sixty-foot lines. Make certain you guard the spectator's safety.

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