Props

     Just watched a little Youtube doco about early aircraft engines.   The presenter makes a serious mistake.  He pointed to the three bladed prop assembly on a mid-thirties airliner and described them as controllable pitch props.  Which is true, but he went on to define them as being able to control the pitch of the prop depending on the speed of the engine.  This is not only wrong but it's the exact opposite of what actually happens. 

   

     The pitch of the prop is based on the airspeed of the plane and not the engine speed.  The idea was to keep the speed of the engine, and its torque delivery, constant with the pitch changing depending on the flight regime.  Basically low pitch for takeoff and high pitch for cruise.  The controllable pitch propellor assembly is essentially the gearbox of the engine and is normally referred to as a constant speed prop.  The presenter in the doco seemed to be an aviation enthusiast and should have known better.