Investigation of the Lift Distribution Over the Separate Wings of a Biplane Page: 4 of 41
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N.A.C.A. Technical Memorandum No. 889
l~dCa , if: lpe3
1 d cai , liftslope
I. REVIEW OF PREVIOUS WORK DONE ON THE BIPLANE PROBLEM
The solution of the monoplane wihg problem having been
obtained, that of the biplane lifting system consisting of
the combination of two. airfoils presents new problems for
the airfoil theory to solve, because of the fact that each
wing is located in the disturbed flow-produced by the other.
Each wing of the cellule therefore exerts an essential in-
fluence on the position and magnitude of the aerodynamic
forces on the other. From the practical viewpoint, an an-
swer is required to the following two questions:
1. How large is the total lift of each of the wings
and of the biplane combination?
2. How is the lift distributed over the span of each
wing?
In connection with the second question, a third one
arises, namely,
3. What lift distribution gives the minimum induced
drag of the lifting system?
The first question has already received the attention
of a large number of investigators, whereas an attempt to
answer the second will be the object of the present paper.
To the third questions a general solution had already been
found very early in the development of the theory.
I should like first to give a brief review of the
work that has so far been done on the biplane problem.
This work is almost entirely concerned with the answer to
the first question. It is convenient to distinguish two
groups of theoretical investigations. A first group con-
siders wingsof finite span (three-dimensional problem).
In those investigations in which only the total lift is to
be determined, assumptions are made on the lift distribu-
tion so that each wing is replaced either by a horseshoe
vortex or by a vortex system with elliptic lift distribu-
tion. It is then possible to consider theoretically the
mutual interference of such vortex systems. The finite
chord of the airfoil is only approximately taken into con-
sideration. In contrast to this group there is a second_ _~ ___
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Küchemann, D. Investigation of the Lift Distribution Over the Separate Wings of a Biplane, report, March 1939; (https://digital.library.unt.edu/ark:/67531/metadc63276/m1/4/: accessed March 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.