14

Elements Of

Airplane Design Rauol J. Hoffmann

Designing an airplane or any other engineering unit is based on empirical or scientific data derived from tests of full-sized airplanes or reduced scale models in co-ordination with some workable theories. To simplify the method of preliminary designing and to have a fair degree of accuracy, the design must

follow along conventional lines with only minor aerodynamic and structural improvements. The estimate should ascertain the general dimensions and performance

expected of any airplane carrying passengers with an engine of known output as well as the approximate gross weight.

The gross weight, a decisive factor, may be estimated if 550 lb. are taken for the pilot, 500 lb. for each passenger, 4.75 lb. for each horsepower of the engine, and 1 Vz

times the weight of cargo or the weight of special equipment. Another factor dominating the airplane design is the stalling speed (often called the landing speed) which is the lowest speed possible with a certain wing loading

and a certain wing section. For estimating purposes a medium thick wing (12 to 13 per cent) is used, which will give the stalling speeds indicated on Scale No. 1.

For wings with flaps the lower graduation is used. Usually the stalling speed is assumed and the wing loading found; then the total weight is divided by the wing load-

ing and thus the wing area obtained. The area of a rectangular wing is equal to the span times the chord, which makes it possible to have a mul-

titude of plan forms. The ratio of span to chord is called the aspect ratio (A.R.) usually from 5 to 8, with an average of 6. Dividing the area by the aspect ratio the

chord factor is obtained. Then the chord may be found by using seal No. 2. Multiplying the chord by the aspect ration the span is determined.

Having found the span and the chord, a rectangular wing may be laid out. In case a

tapered wing is wanted of the same aspect ratio and area, the

plane may be made a function

to be considered are the diamter of the cowling, the overall

are multiplied by it. Use half of the outline for the vertical tail surfaces.

ed for small areas and a plan

of the span; a 70 percent ratio would make a good average which is used on scale No. 3 to determine the overall length. The next item in design is the fuselage. Using the minimum dimensions required for the pilot's compartment and for the spacing

form of parabolic curves should

of the passenger seat, a general

per cent of the wing area. Scale

balance or the center of gravity (C.G.). Ninety per cent of all

greatly improve the performance.

outline may be made with a maximum thickness at 33 per cent

No. 4 gives the span in inches

failures may be traced to the

of the tail surfaces (13 per cent) if all dimensions of the sketch

wrong position of the C.G. in experimental planes.

leading or trailing edge may be

rotated about a point located at quarter span. A taper of 2 to 1 will be as good as any curved

plan form. The often-mentioned elliptical plan form is too point-

The overall length of an air- of its length. Other dimensions

length of the engine (including the engine mount), the fastening

For aileron dimensions 25 per

of the rudder post, the space for

cent of the mean chord and 40

the tail skid and the ample distance between the fastening bolts for the tail surfaces.

per cent of the semi-span should be used.

The area of the horizontal tail

done in estimating the center of

surfaces should be at least 61/2

A very exacting job should be