The dynamic thrust (when the aircraft is in flight) will reduce as aircraft airspeed increases and is a
function of all propeller driven aircraft. However, it is worth reminding that the thrust on an unrestricted
turbo-prop falls off more gradually than an I/C engine due to the fact that as the propeller load reduces
due to forward speed (“unloading”) the propeller rpm rises 10-20% to balance the torque supplied. This
feature enables turbo-prop powered aircraft to achieve a higher forward speed than the static rpm
suggests, or the same speed achieved for a reduced throttle setting, saving fuel.
Activating the RPM limiter will reduce the whine up and down of a propeller on a plane doing aerobatics
in swoops and climbs so is worth setting. Note that as indicated earlier, in the air the prop will unload
quickly and can approach any preset rpm limit more easily so best is to set it slightly on the high side.
For example, a prop on the ground might give 5000rpm, you could set the limiter to 10% above this -
5,500 as the prop is sure to gain 10% in the air, it will be more like 20%. Experience will be your guide
here.
Maximum forward speed is mainly a function of forward thrust against airframe drag. A sleek and slippery
airframe will result in far greater airspeed on even modest power levels, whereas a large draggy airframe
may fly slowly on even exceptional power. Different brands of prop also add their own differences, some
appear more draggy than others or appear to make less thrust on the ground, but work well in the air.
Ground Clearance
If your plane has limited ground clearance then you can always go to 3 or 4 blade propellers. More pitch
may not be ideal with a slower flying model however.
Full-size “Pilatus PC7” – an ideal modelling subject for your new turbo-prop – who will be the first?!
Page 17
