Propellers, propellers...

Hi everyone,

As previously said, I wanted to do an article specially about propeller operation on the PC-6, and generally speaking, constant speed propeller used in almost every high performance ACFTS, both piston or turbine powered. But of course, in our case it will most likely be about a PT6 powered airplane. So, it will also apply to Twin Otters, King Airs, Turbine Otters, etc etc

The basic principle of a propeller is commonly known by anyone who is interested in the technical side of aviation. The explanation of the blade being similar to a wing is the easier one, and most complete. The higher the AoA ( Angle of Attack ), the greater the lift ( here thrust ), for a fixed RPM and until a certain limit, the stall angle. On those aspects, both systems ( blade and wing ) are very similar.

If you want to go further in the technical side, a fixed pitch propeller, like the early wooden one or even today's metallic ones used in small GA planes such Cessnas or tourism planes are the simplest propellers, but can't offer great performance on every flight regime. They are optimized in the optic of a compromise of great performance on cruise ( high blade angle, high airspeed ), and slow speed regimes ( take off, landing ). The great idea that come to engineers just prior to WWII was to create something new, using the same principle but adjustable, so the pilots could set the blade angle on ground, according to the performances, the kind of flight, RWY type etc they would achieve during the flight. That wasn't very reliable as ground crew always had to set this priot to flight. Then it was adjustable manually but in flight, like the Supermarine Spitfire Mk. 1a... It rapidly appeared that it required an important work load, because it was very easy to " rev up " the engine by applying full throttle in a low pitch ( low prop load ) setting.

The next idea was the great, the final one, always used on today's latest props. You would set the target speed by a lever in the cockpit ( simply called Prop Lever ) which directly controls the governor's speeder spring. Then that Governor ( the whole system is called the CSU, Constant Speed Unit ) precisely manage the amount of oil dumped into the hub to actuate the blade pitch.

Because yes, the blades in the case of the PC-6 prop are actuated by : hydraulic pressure ( oil from engine ), natural twisting moments tending to fine blade pitch, counterweights on blade roots that tend to coarse pitch and finally the spring inside the piston that also tends to feather ( coarse ) blades.

The advantage of CS ( Constant Speed ) system are multiple : you don't have to worry anymore about overspeed, managing manually the blade according to what you want to achieve ( best climb, best cruise speed, low noise etc... ). You just have to select the prop RPM you want then the CSU keep it automatically... So, why the prop don't spin at same RPM from engine start to shutdown ? Because at idle, on ground, when there is no airspeed, the engine can't spin the prop at the normal operating speed. In flight, as there is a consequent amount of relative wind passing through the prop and spinning it like wind does on wind turbine, the prop basically spin at same speed, whatever the throttle setting is ( as long as prop lever is not touched ). This allow for quick thrust availability ( a fixed pitch prop takes more time to spool up as it's fixed pitch is generally situated in a relatively coarse position, so the prop load is consequent ) and best performance for take-off, climb, cruise, descent and landing ! That is a constant speed prop

What would be better ?

Bêta range is better !

As the propeller technology allowed the blade angle to be modulated in flight, why not bring the system to all his capabilities ? Until then, the ability for a plane to slow down after touch down was always the task of wheel brakes or speedbrakes. Now, even the propeller can help for slowing down ( also going backward ! ). Some devices has to be created to avoid such conditions in flight, as a reversed prop would destroy all incoming slipstream and so, makes control surfaces like elevator and rudder inefficient in the case of a nose mounted engine, or a dramatic loss in lift in planes such the Twin Otter. Some of these devices, like WOW ( Weight on Wheels ) allow the prop to enter reverse range only when weight is sensed on landing gear, meaning the plane is on the ground.

Before going further, we have to define some ranges :

Alpha range : The range of forward thrust. To be simple, a fixed pitch propeller, as long as it spins in the correct direction, can only be in Alpha range. On a variable pitch constant speed prop, Alpha range is the range of blade angles where the prop produce positive thrust.

Bêta range : The range of zero OR negative thrust. The reverse is the extreme end of Bêta range.

On PT6 powered aircrafts, the Bêta range is controlled by the PCL ( Power Control Lever ) which also directly control blade angle. Yeah, the governor is overriden and by moving the PCL past the idle gate ( you usually have to lift the PCL against that gate ) you directly schedule blade angle in correlation with Ng ( Gaz generator ) speed. Note that the overspeed governor ( another device, that prevent the Np - prop speed in % of max RPM -to exceed 106% ) is always active and has full authority over Bêta control.

On the PC-6 airplane, the propeller ( here the 4 bladed Hartzell HC-D4N-3P ) is put into Bêta range( until max flat pitch of -0°30, not in reverse ) to create a fabulous amount of drag in order to slow down the plane, or at least avoid it to go too fast while it makes itsfamous dive descent ( -30° pitch attitude, -7000 Ft/m without exceeding 115-120 KIAS )

Here is some schemes I've made to illustrate all I've said, as well as show you the different parts of that particular prop. As the blade angles are measured at 30in. position of blade lenght, I cutted it there to place the chord line.

If you have any questions/remarks/comment, please ask me via comments or directly contact me !

Hope you liked this reading, ( I wish to excuse me for my english, it's not my native language at all, I'm french )