The following paper is a summary of a presentation, given on the 13th October 1991, at the 8th Nurflügelsymposium (tailless planes symposium) of the Oskar-Ursinus-Vereinigung (OUV, a german amateur aircraft builder association), in Scheidegg, Germany.
Besides lift and drag coefficients, the moment coefficient cm is of importance for the behaviour of an airplane - it has a big impact on the longitudinal stability. While a conventional airplane can compensate the moment of the wing with its horizontal tail, a tailless plane obviously can't.
It is possible to divide tailless airplanes in three groups, depending on how they achieve longitudinal stability. The requirements for the moment coefficient of the airfoil is a direct result of the stabilizing mechanism.
Classes of tailless airplanes and their typical moment coefficients.
To achieve static longitudinal stability, the center of gravity (c.g.) must be located in front of the neutral point, which makes the momentum derivative dCm/dAlfa (rel. c.g.) negative. Placing the c.g. in the neutral point results in dCm/dAlfa = 0, making the plane indifferent, i.e. it will not stabilize itself after a disturbance. These requirements lead to the moment coefficients presented above.
To achieve dynamic stability oscillations must be damped out, which can be difficult, when the c.g. is located too far ahead of the neutral point and the moments of inertia around a spanwise axis are small (as with an unswept flying wing).
Last update of this page: August, 13, 1996
Last modification of this page: 08.09.03
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