Originally Posted by
Captain Beaker
The graph shows 3 things:
1. Engine/Gearbox is not the cause of the drag, e.g. compare the scenario at say 30 degree pitch.
2. Flat pitch and wind milling creates very significant drag, the drag coefficient increases massively as the blade moves towards 0 degrees. This is entirely consistent with the need to feather in an aircraft cable of fine pitch. It is also consistent with safety devices needed to ensure blade angle remains outside the 'beta range' while in flight.
3. It's consistent with my own observations and the flight manual, for an aircraft fitted with a fixed pitch prop of approx 18degrees found in a C-172. i.e. wind-milling or stationary does not seem to have a very noticeable effect. It's consistent with the need to set low rpm on a constant speed single.
All of this is entirely consistent with every flight manual I have come across.
The original poster asked an interesting academic question, so maybe I provide some thoughtful input on that academic discussion. I haven't proposed anything contradictory to what you expect to find in the flight manual.
Couple of thoughts, just for fun
How come the propeller creates so much drag in the positive beta range, even when driven by the substantial torque of a turbine in flight idle?
While a helicopter is in auto-rotation, the blades free wheel because it is fitted with a sprag clutch. If we consider the blades to be a large windmilling prop rotated 90 degrees wouldn't the resistance from the engine be advantageous?
Probably best I get my coat.
1 - It is important to note the context of the reference material - the graph was used to illustrate the substantial drag of a windmilling prop on a failed engine. The fact that it shows the drag curves cross over in the higher pitch angles is interesting, but without knowing the other variables - RPM, airspeed, prop dimensions and propshaft torque - it is meaningless, and very misleading - a change in any of these variables will totally change the graph.
To illustrate - note the stopped prop line represents infinte torque/friction from a stopped engine. Now lets reduce the friction so that the prop barely rotates - say 1 RPM. What would the incurred drag now be? I can tell you it will not be the windmilling drag line on the graph.
This is the interaction I was talking about earlier - the engine/gearbox friction dosen't CAUSE the drag - but is one of the inputs that will most definately affect the drag by changing the windmilling RPM at varying airspeeds. If you have an engineering background we can continue this discussion on how these higher order effects stack up.
I dont think anyone has issues with point 2 - you are pretty much reiterating the consensus.
3 - Please elaborate on your testing methods and results.
And on your 'fun' thoughts
Idle beta 'creates so much drag', but guess what happens when you go deeper into beta?
You missed the whole point of autorotation - its not done to increase drag to reduce the ROD, but to build up rotar momentum so there will be enough energy built up by the time you get to the ground to flare out for a survivable landing.
No need to get your coat - but try to consider your audience before speaking and modify tone accordingly.