Originally Posted by
flyingchicken
Originally Posted by
captain beaker
1. Energy used to turn over the engine, or the resistance/friction of the engine is the not the cause of the drag. It does have an effect, depending on the blade angle it can mak ce or in the case of a very flat blade angle (> 7degrees) engine friction can actually reduce the drag. Energy used to turn the engine is a red herring.
drag is an aerodynamic effect. It is affected by the profile drag of the prop (this is obvious), and the rpm the prop spins at. The term windmilling itself is the best way to visualize what is happening. You are using the forward speed of the aircraft to drive a (very inefficient) windwill. The energy of this windmill goes to two things - overcoming engine friction, and spinning the blades. The faster the blades spin, the more energy it soaks up just from spinning the planks. Engine friction goes up relatively linearly with rpm, where as the prop drag goes up as the square of rpm, and hence the cube of forward speed. Think of how much energy it takes to drive those prop tips to near mach 1. So prop drag dominates massively at high rpm and you can see this clearly in the shape of the graphs.
So far so good. The issue here is you've been putting the cart in front of the horse in your analysis. We do not fly airplanes based on propeller drag curves - our characteristic speeds are dominated by wing performance. So the fact that these graphs are done as a function of nd/v (inverse of prop advance ratio) means that you are only looking at data at specific speed/rpm combinations that results in the prop blade flying at a specific angle of attack for the blade. Useful if you are specifying a propeller design for an airplane, but dosen't make much sense to us in an engine out situation. Here, forward speed is the primary input - and if you are flying in a fixed pitch prop (which seems to be the focus of your analysis), the resulting rpm is whatever you end up with - being a function of both forward speed and engine drag.
You have said
'The energy of this windmill goes to two things - overcoming engine friction, and spinning the blades.' Taken in isolation that is true. But it is misleading, yes energy is expended overcoming engine friction, and the mechanical friction of the blades. Yes those are two pathways for expending energy or 'doing work' but the other pathway is the aerodynamic 'work' done by the actual blades. An aerofoil travelling at high speed, operating at high angle of attack, will displace a lot of air, this expends a large amount of energy, and 'lots of work is done' i.e. HP
Actually the NACA diagram (figure 4) has a lot of interesting information if you take the time to understand it.
Originally Posted by
flyingchicken
Originally Posted by
captain beaker
3. For the blade angles 12,17,22 a reduction in negative torque results in reduction in drag, i.e. Less friction is better, this is consistent with flight manual check list for closing the throttle in a piston engine aircraft.
playing with the throttle is not going to do much - you don't really get to pick how much drag your engine makes. In a fixed pitch prop, the engine drag torque is mainly going to be a function of rpm, which is going to be a function of forward speed, which is your input. - you are analyzing this backwards.
(and throttle position for minimal pumping losses is also engine specific. For most normally aspariated engines like your little 172, opening the throttle fully actually results in the lowest pumping loss - a closed throttle inpedes flow while any additional air drawn into the cylinder merely acts as a spring regardless of throttle position. What you are saying generally only applies to turbocharged engines due to higher backpressures there.)
I may appear to analyzing this backwards, because I am attempting to provide an illustration.
Closing the throttle reduces the manifold pressure, so that mass of air that is compressed and heated and then finally expanded is reduced. While the air acts a spring that heat loss is not recovered. Open throttle may be most efficient when the engine is running for real, the reverse applies without ignition. It is a exceptionally minor point in any case.
Originally Posted by
flyingchicken
Originally Posted by
captain beaker
none of this contradicts any flight manual or training manual text, with regard to pilot actions
you should highlight this a lot more - it might take away a lot of the hostility shown towards you.
Originally Posted by
captain beaker
how might that make any difference
relevance. You dont come across as someone who knows a lot about either flying airplanes or the engineering behind them. But you present your 'findings' in a tone as though you do, captain beaker.
I foolishly
made a light hearted reply to the simple question posted the OP. I have since posted two research papers, with some annotations and comments.
As it happened I do have relevant qualifications, but effectively saying 'I have lots of experience, so what I say is right' does not count. I have supplied data to support my assertions, and would happily have anyone show how the data is wrong...
Happy New Year, and safe flying
