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Reprinted from - Flight Magazine and the Aircraft Engineer, December 1935, London, England

THE BURNELLI AEROFOIL BODY
By Dr. M. Watter

For very many years Mr. Burnelli has carried out research and experiment in developing his unorthodox type of aircraft in the United States. Briefly explained the Burnelli system consists in using a fuselage of aerofoil section, which gives remarkable passenger accommodation, and attaching to its sides wings of orthodox design. The Uppercu-Burnelli Corporation of Keyport, New Jersey, has granted to W. S. Shackleton, Ltd., the representative rights for Europe.--ED.

Theoretical Considerations

The problem of distorted aerofoil body-wing combination would seem to present rather complex aerodynamic phenomena, and from a purely theoretic viewpoint its accurate solution is laborious. The writer felt, however, that for all practical purposes in analyzing this problem one was justified to resort to the simplifying assumption of algebraically superimposed effects of aerofoil body and wings.

The following features are indicative of small aerodynamic interference supporting the above hypothesis. An examination of the drawing of the latest Burnelli design shows that in view of the continuity of the upper surface of the wing over the body portion it is logical to assume that the outer panels have an effective aspect ratio as if the wings were continuous throughout. As regards the body which has an aerofoil shape but a very small apparent aspect ratio it may be remarked that the presence of outer panels would preclude partial loss of circulation, and besides that the booms would offer a certain amount of end plating so that the effective aspect ration must be larger than the mere shape would suggest. To evaluate the effective aspect ratio of Burnelli type body the writer assumes an equivalent rectangular wing having the total area of the wing and body and possessing the body chord. It is interesting to observe that this simple assumption was very well verified in two instances by wind tunnel tests despite the use of two entirely different aerofoil body shapes. The satisfactory verification is considered to be definitely supported because of a very close check of the zero lift point, slope of the lift curve and maximum lift as well as the maximum L/D of the models.

Let Sb be the area of the aerofoil shaped body having chord Cb and span bb Sw the area of the wings having an average chord Cw and span b. We can write:--

Weight = C1QS hence for equal landing speed and weight C1S = const. Or aS (dC1/da = const. It can be shown (Fig. 1) that dC1da = A/(1+2/AR) where A is an experimental constant.

Aspect ratio of wings = b2/Sw

Aspect ratio of body = (Sw+Sb)/Cb2

In analyzing the relative advantages of the Burnelli monoplane design we may make the assumption that half of the body width is indispensable and would be present in any standard design without the benefit of additional lift effect. Let us take a numerical example based on one of the recent wind tunnel tests and representing a 14,000 lb., 14 passenger transport.

Sw = 587 sq. ft. through area S = 163 + 587 = 750 sq. ft. Sb = 240 sq. ft.; b = 70 ft., and Cb = 20 ft. The effective aspect ratio can be found from the following equation: --

S/(1+2/AR) = Sw/(1+2S/b2) + Sb/(1+2Cb2)/(Sw+Sb)

Substituting we find 2/AR = 750/(450+122) -- = .31: AR = 6.55.

In the case of standard design the semispan of each wing would have been (70-6)/ = 32 and the moment proportional to 32W, while in Burnelli design the moment is proportional to (70--12) x 450/572 = 22.7W or a saving of 27 per cent. with an additional saving in wing area of 158 sq. ft.

In this case the amount contributed by the fuselage is 26.5 per cent., which of course is conditional upon the fact that the zero lift points of both the wing and body aerofoil lift curves must coincide. A detailed investigation has shown that this condition represents the best comparison from both the aerodynamic and structural viewpoints. It results in practically the maximum obtainable L-max./D--min., gives high L-max. and contributing lift; it results in somewhat higher-minimum drag and lower L/D max. than could be obtained by delaying the lifting action of the body to higher angles of the wing. One of the other advantages of this setting is decided simplicity of stress analysis, since the contributing lift effect of the body is constant for all flying conditions. (Fig. 4).

continued ...

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