FUSELAGE AND OTHER EFFECTS

The effect of the fuselage is destabilising. Any long, slender body moving through the air tends to turn broadside to the airflow. This can easily be confirmed by experiments with throwing sticks and arrows. Even the addition of a weight at one end does not change this much. As any archer will confirm, arrows which lose their fletching will yaw wildly sideways in flight Rockets and bombs require fins for the same reason. The fuselage therefore must be regarded as moving the neutral point forward of the location determined by wing and stabiliser alone. In full-sized work, attempts are made to estimate this numerically, but the upshot is invariably that the static margin must be large enough to cope with die fuselage effect That is, the centre of gravity must be far enough forward to achieve the desired stability, whatever may interfere. The propeller also has a destabilising effect which is considerably greater with power on than when the propeller is stationary or feathered. There is usually a difference in stability power on and power off, which is caused by this, among other factors.

Every other component of the aircraft which has air flowing over or through it will exert some influence on the neutral point position. For instance, a large horizontal undercarriage axle, faired, ahead of the centre of gravity, will tend to destabilise, but a broad strut, bracing a tailplane, will act in some respects like a second tailplane. Ultimately, the static margin has to be adjusted to cater for all these factors to produce a satisfactory result