Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/393

Rh '''§ 242. Author's Experiments. Method' (continued).—In addition to the free flight experiments enumerated above, an attempt has been made to effect the direct measurement of $$\xi$$ by means of a new instrument, which may be appropriately termed an aerodynamic balance''.

The magnitude of $$\xi$$ as determined by the method of free flight suggested that, in spite of the failure of previous experimenters, it should be quite possible to effect a direct measurement of this quantity by the aid of a suitably designed appliance.

The aerodynamic balance, Fig. 153, consists of a horizontal arm or beam $$A ,$$ pivoted about a vertical axis $$B ,$$ the amplitude of motion permitted being regulated by the screws $$C\ C ,$$ which also form electrical contacts.

For the determination of $$\xi ,$$ a normal plane $$D$$ is attached to one end of the beam and a friction plane $$E$$ to the other, the areas of the two being adjusted until they exactly balance. The instrument is used either by being exposed to the wind and held stationary, or fitted in front of an automobile vehicle in still air.

In either case the planes require to be carefully balanced about the vertical axis so that gravitation and inertia forces are inoperative. When the instrument is held stationary this precaution is unnecessary so long as the axis is exactly vertical, but it is more convenient to have the instrument properly balanced in any case. In spite of every precaution, when the instrument is mounted on a motor car the beam is found to be in a continual state of oscillation between its stops, probably due to slight rotational movements of the car body produced by the unevenness of the road. This difficulty was actually experienced to so great an extent that the employment of the instrument in its present form on a motor vehicle was abandoned.

The uses of the aerodynamic balance obviously are not Rh