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Aviation History
1939
1939 - 0551.PDF
FEBRUARY 23, 1939. Supplement to jfl& Eft? 98a c AIRCRAFT ^ ENGINEER No. 157. (V0Xe3XVU) I«hYear February 23, 1939 INERTIA STARTERS Advantages of Torque Damper Over Slipping Clutch By G. BROULHIET, Ing.A.M. THE inertia starter can be described as one in which energy is first given to a mass represented by a rotating flywheel. This mass is then coupled to the engine to be started, the energy thus being used to start the engine. Owing to the large amount of energy that can be stored in a flywheel of quite small mass, rotating at a high speed, this principle is obviously one which commends itself for starting of aero engines, but, unfortunately, up to now the proportion of the stored energy which can be utilised has been quite small, owing principally to the difference between the speed of rotation of the engine to be started and the starting dog. Generally, in order to overcome this difficulty, the means used is the provision of a form of friction clutch, which is allowed to slip until such time as the whole assembly of flywheel, starting dog and engine are all rotating at the same relative speeds. A large amount of the energy given to the flywheel is, therefore, dissipated in the form of heat by the slipping clutch. A further considerable proportion of it remains unutilised in the flywheel itself, which rotates with a fairly high velocity at the end of the operation of starting. The disadvantage of this design is that the torque needed to slip the clutch has to be quite high to avoid excessive loss of energy ; and this torque is applied instantaneously to the engine, giving considerable stress on certain parts thereof. Another method of overcoming the difficulty of the relative speed is that designed by the writer, in which tin elastic mass is introduced between the flywheel and the starting dog in such a way that this mass can store the energy of the flywheel which is, in the case described above, lost in the form of heat. Moreover, it allows the flywheel to come to a stop, thus giving up all its energy into the resident mass which, in turn, passes it on to the starting uog and the engine. In addition to the higher efficiency obtained by this design, which will be discussed later, an important advantage of the system is that there is no torque on the starting dog when the latter is engaged with the engine, and that the torque builds up in a progressive manner, thus avoiding any abrupt shock to the parts of the engine. Mathematical analysis of the ultimate distribution of the energy given to the flywheel in both cases can be set out as follows : Inertia Starter Fitted* with Slipping Clutch For the sake of clearness, we can consider the engine simply as a second flywheel; that is to say, we can take the resistance of the engine as being inertia resistance only. Let m be the moment of inertia of the flywheel, V its" angular velocity, and M = m the moment of inertia of the engine to be started. Let v be the speed of rotation which flywheel and engine reach together as a single unit, when the clutch is fully engaged and is no longer slipping. By the conservation of momentum, we have : mV = (m + M) v that is to say: v m mi V m + M m (1 + a) 1 +. a The energy in the flywheel before engagement was : - m V2 = a 2 The energy transmitted to the engine after engagement is: - Mv2 = - m V2 = b 2 2 (1 + a)2 The energy remaining in the flywheel after engage ment is : mv 2 = - m V2 • -2 (1 + a)2 ENERGY ABSORBED BY FRICTION CLUTCH ENERGY REMAINING IN FLYWHEEL ENERGY TRANSFERRED TO ENGINE ANGULAR VELOCITY
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