GLIDING

Gliding, either with engine throttled back or with no engine at all, is best understood if the forces are resolved as shown in Figure 1.4b. The weight alone acts vertically downwards, but may be resolved into one force acting along the flight path and another at right angles to it The glider, or gliding power model, moves forward and slightly downwards under the action of the weight component along the flight direction. The total air reaction force is similarly resolved into lift at right angles to the flight and drag opposing the forward-acting force. The result is a diagram very similar to that for powered flight but it has been rotated through a small angle, known as the glide angle. A steeper glide would cause a larger weight component to pull the model along its flight path. It would accelerate until the drag component of the air reaction once again grew large enough to restore equilibrium.

1.4 DIVING

In a dive, the four force diagram has rotated further, as shown in Figure 1.4d, and in the limiting case the flight path is vertically downwards, weight and thrust (if any) both pull the model down, the only opposing force is drag. The speed at which drag becomes large enough to equal weight-plus-thrust is usually very high and probably before this ‘terminal velocity’ is reached, the model would hit the ground (Fig. 1.4.f).

1.5 CLIMBING

In a climb, the total support comes from a combination of wings and propeller. The weight may be resolved into two components, one opposing lift and the other directly opposing thrust, assisting the drag. Again, the result is a four force arrangement in balance, but rotated through the angle of climb (Fig. 1.4.c). The limiting case is the vertical climb, when the weight plus drag is opposed only by the propeller. Such flight is commonplace to the helicopter, but a model of orthodox type, if sufficiently powered, is capable of vertical climbing in this fashion also. As the diagram shows, in such a climb the wing lift force must be zero, and its angle of the attack to the air flowing over it must be such as to give no lift It is therefore obvious that to obtain a steep climb there must be sufficient thrust from the motor since this, rather than the wing, provides the necessary reaction to equal the weight and drag resistance.

1.6 HOVERING

For a helicopter to hover, the thrust from the rotor must equal the weight plus a relatively small addition to compensate for the drag of the rotor’s slipstream over the body of tire aircraft. In a helicopter ascent some additional rotor thrust is needed because the air drag of the hull in the rotor wake increases. Ordinary model aeroplanes are, given sufficient power, capable of climbing vertically but although they can be made to hover briefly, it is usually not possible to hold them in this position because the airflow over their control surfaces is too slow. Control is quickly lost and the model falls out of die vertical attitude.

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