Windmilling?

[cookmg]

I am studying up on constant speed props and I have learned a great deal. But, so far in my training I have failed to learn the aerodynamics of the propeller's windmilling property. I have looked through all of my books and can't find a satisfying explanation. Can anyone help? What are the aerodynamics that cause a propeller to windmill?

Thanks!!

 

[A Squared]

Well, the simplest explanation is that it's like a child's pinwheel turning in the wind, or a real windmill. Does that answer the question?

 

[VNugget]

http://142.26.194.131/aerodynamics1/Multi/Page2.html

 

[tarp]

Propeller is an airfoil (or better thinking a set of tiny airfoils all set at different angles due to propeller twist).

Airfoils do their work through lift and drag. i.e. lifting force and drag force (either induced or form/surface drag).

As said above everything else is basic aerodynamics you have studied for the wing and works like a windmill.

If you are trying to figure out in constant speed props, why coarse pitch, fine pitch and feather - remember you have to think about the angle of attack into the relative wind - the only difference on the prop is that it is bolted to a crankshaft that is bolted to pistons that are reasonably hard to move unless the engine is running.

In fine pitch, high RPM, you have made the prop as efficient as you can in creating torque. i.e. in low pitch the prop is going to turn as fast as it can and has enough power to drive the pistons up and down and hence you have a good windmill.

As you make the pitch more coarse, i.e. turn more into the wind you start losing a little of that torque power (the prop airfoils try to lift up and down but not sideways). Think of the engine running, we slow down the engine and take bigger bites of air with each bite. This economizes the activity (low power, more bite) but you don't have as much power (we do this in cruise not in climb, because there isn't enough torque in each bite of air to create excess power to climb). So when you go the other way - engine stopped and wind power trying to overcome the internal friction of the engine - the airfoil is not as efficient in reverse and therefore the windmill doesn't work as well.

As we go to feather - the props are turned right into the airstream - the one prop is turned right into the wind and makes a beautiful little wing, but it can't develop enough torque to move the prop against the friction of the engine. However, since we aren't making all that torsional lift we are also not making torsional drag - the spinning disc stops and we get a little break from drag enhancing the performance of the hopefully other engine (on a twin).

Now that you know that, now you know why a single engine fails to low pitch, high RPM and a twin fails to high pitch, low RPM. In the twin, we want to get rid of that drag to help the other engine. In the single, we are hoping that by turning the prop against the wind and turning as fast as possible, it will aid in the restart of the engine - if we can get fuel, air and spark to the single engine, the windmilling prop works as a starter.

And with that, I'm going to go iron shirts for my next 4-day.