Props!

Hi guys,

 

I have been struggling to understand how an in flight adjustable prop works and how different settings relate to RPM, Manifold pressure and airspeed? Also when do you normally change the pitch (i am guessing the pitch on takeoff would be different to cruise)? Can anyone help ? Also how a constant speed prop works and affects the above variables? As I have only flown fixed pitch and never been taught variable I would like to know the workings.

 

Thanks, Andrew.

 

 

If you have a bit of time to read, I found the Trevor Thom "Aeroplane General Knowledge and Aerodynamics" has a good explanation. Also, maybe try AIRCRAFT PROPELLER CONTROL AND OPERATION or aircraft propellers and how they work or just google it. They explain it much better and in more depth than I could.

 

Bob

 

 

In-Flight adjustable Prop.

 

The above is like a gearbox on a car with a variable ratio without steps.. If you increase the prop angle (Pitch) it is like pulling a higher gear and the engine revs LESS but PULLs harder. There could be a slight manifold pressure increase when this happens.

 

The manifold pressure responds to throttle opening and altitude. More throttle gives you more and the increased altitude gives you less because the air pressure is less.

 

Fining off the pitch will give you more revs because it is like selecting LOW gear. If you are going fast the engine may over-rev even though the throttle is not fully open. The prop can act as a brake in this situation as can a gearbox in a car. You can use this to increase your rate of descent. (Not a blanket statement for the technique as some engines don't like it (Radials), and some aircraft require special training as the reverse thrust is very effective (Turboprops)..

 

When you are flying SLOW, ( T/O and approach), you will select finer pitch settings to make more of the engines power available, and when you are cruising or descending at high speed, you will have the pitch in a coarser setting to save the engine from over-revving and save fuel as well. Nev

 

 

The constant speed prop does what it says. Set a speed and the prop will keep there as long as the engine has the power to do so. You use full fine for takeoff, to get the max power at low speeds through the air, then coarsen up the pitch to give less RPM at cruise and descent. Less revs equates to less fuel burn and is also easier on the wear and tear, unless you try to rev very slowly at high throttle openings. They are good for saving fuel and also giving the best performance in any situation, but they are expensive to buy and to maintain.

 

 

Thanks Heaps guys for your help, understand the concept now and it is simpler than I thought!

 

Thanks, Andrew.

 

 

Got one more question though (or two). What type of prop does the majority of commercial turboprop planes use (dash8/kingair)? Also Yenn, you metioned set a speed, do you mean airspeed or rpm, and how (do csu's have different controls to an in flight adjustable)?

 

Andrew.

 

 

Constant speed props. General use.

 

CS props are the general thing out there. You dial/set a RPM figure. and the governor keeps the rpm you set regardless of the speed you do,(except at low throttle positions where the prop is on the "low pitch" stop and the prop behaves as if it is fixed pitch at that stage). There is a lot to cover regarding low pitch stops and prop overspeeding.. This is a design thing but something that you should understand. IF your prop mechanism fails then it normally reverts to the lowest pitch setting. This may require you to extend flap to be able to use the engine to get you where you want to go. If the low pitch stop is set so that you can continue to fly the aircraft under power then this is in essence a "fail safe" situation, and desireable to have. The propeller is pitch changed by hydraulic means most commonly using engine oil. Some props are actuated electrically. The pitch of the blades is influenced by the control mechanism, but reacts/ opposes with centifugal forces which tend to fine off the pitch, and aerodynamic forces which tend to coarsen the pitch.. The centiifugal forces are dominant ( of a higher order). Nev

 

 

Excellent, thanks for your help!

 

Andrew

 

 

Got one more question though (or two). What type of prop does the majority of commercial turboprop planes use (dash8/kingair)?

Couldn't tell you the fancy name for them, but they are usually constant speed, feathering and reversing propellers on turbine engines.

 

 

Just for your interest, here's a photo of a PT6 turbine prop setup. I forgot take a photo of the fly weights etc... before I put the spinner back on, so you just get this one.

 

Photo from my phone.