Aeroplane Construction and Operation

by John B. Rathbun

Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.

Location of Wheels. The exact location of the wheels, in a fore and aft direction, is of the greatest importance. If they are too far ahead of the center of gravity, too much weight will be placed on the tail skid and excessive running will be required to get the tail off the ground. If the wheels are too far back, the machine will be likely to nose over when landing or running over the ground. In any case, the wheels must be well ahead of the center of gravity so that the weight will resist a forward overturning moment. In the majority of orthogonal biplanes, in which the leading edges of the upper and lower wings are on the same vertical line, the center of the wheel is from 3 to 6 inches back of the leading edges. In staggered biplanes the wheel center is from 6 inches to one foot in front of the lower leading edge. This difference is caused by the fact that the center of gravity is nearer the leading edge of a staggered wing than with the Orthogonal type, and hence the wheels must be further forward.

Fig 15 (upper diagram) shows the conditions when the machine is running over the ground with the body horizontal. The vertical line a-a passing through the center of gravity C G is a distance N from the center of the wheel. The weight acting down has a tendency to pull the tail down, this moment being equal to the weight of the machine multiplied by the distance N, or W x N. The elevator flap M exerts a lifting force Ky which acts through the lever arm L, and opposes the moment due to the weight. The force K must be equal to K = WN/L. The distance I is the distance of the wheel center line from the entering edge of the wing. The weight on the tail skid S when the machine is resting on the ground will be equal to S = WN/M, and this may range anywhere from 40 to 200 pounds, according to the size of the aeroplane.

Fig. 16 illustrates a principle of wheel location advanced by Capt. Byron Q. Jones, and published in "Aviation and Aeronautical Engineers," Nov. 16, 1916. The body is shown in a horizontal position with the propeller axis X-X horizontal. The center of gravity is at G on X-X, the weight acting down as at P with the line prolonged meeting the ground line at B. A line E-E is a line drawn tangent to the wheels and the tail skid at D, the angle of this line with the ground determining the maximum angle of incidence. E-E is the ground line when the machine is at rest. For the best conditions, Capt. Jones finds that the line connecting the point of tangency C, and the center of gravity at G, should make an angle of 13 degrees and 10 minutes with the vertical GB dropped through the center of gravity. With the line GA drawn perpendicular to the resting line E-E, the angle BGA should be 10 degrees as nearly as possible. This is for a two-wheel Vee chassis, but with a third front wheel as with the training of type the angle CGB can be made less. Capt. Jones has found that with the wheels in the above location there will be no tendency to nose over even with very poor landings, and this method has been applied to the training machines at the San Diego Signal Corps aviation school. If the angle BGA is greater than 10 degrees it is difficult to "taxi" the machine on the ground, this tending to make the machine spin or turn into the wind. Capt. Jones claims that a two-wheel chassis arranged according to these rules is superior to the three-wheel type for training purposes since the tendency toward spinning is less.

The location of the tail skid S should be such that the elevator and rudder surfaces are well off the ground with the skid fully deflected, and yet the skids must be low enough to permit of the maximum angle of incidence or an angle of EXX = 10 degrees. To a certain extent, the maximum angle of incidence determines the chassis height. If the angle EXX is made greater than the greatest angle of incidence, the wings can be used as air brakes in bringing the machine to a quick stop after landing.

The track, or the distance between the centers of the wheels measured along the axle, must be about 1/7 or 0.15 of the span of the lower wing. This makes the track vary from 5 to 7 feet on the usual types, and as high as 15 feet on the large bombing planes. The track must be great enough to prevent overturning when making a landing on soft ground or with a cross wind. If the track is excessive, there will be a heavy spinning moment in cases where one wheel strikes a depression or soft spot in the ground.

Shock Absorbers. The axle movement allowed by the elastic shock absorbers and guiding appliances averages from 5 to 6 inches. The greater the movement, the less will be the stresses induced by a given drop, but in practice the movement is generally limited by considerations of chassis height and propeller clearance. It can be proved that a movement of 5 inches will produce a maximum stress equal to 8.6 times the weight of the machine under conditions of a one-foot drop, while with an absorber movement of 6 inches the stress is reduced to 7.5 times the weight. This calculation takes the tire deflection into consideration. With the absorber movement limited to one inch, the stress may be as high as 35 times the weight of the machine.

F=W (2 + 2.77/x) where W = weight of machine in pounds, F = the stress produced by the fall, and x = the absorber movement in inches.

Landing Gear Wheels. The wheels are generally of the tangent laced wire spoke type, and are enclosed with discs to reduce the resistance. They must have very wide hubs to resist the heavy end stresses caused by landing sidewise. The length of the hub should be at least twice the diameter of the tire and a greater width, say three times the tire diameter, is preferable. The narrow hubs used on motorcycle wheels are not safe against side blows, although they may be capable of withstanding the vertical load. The wheels are rated according to the outside diameter over the tire, and by the diameter of the tire casing. A 26" x 4" wheel signifies that the outside diameter is 26 inches with a casing diameter of 4 inches.