Captain Beaker

How might that make any difference?

The great thing about providing data, is that it can be scrutinized. I suggested it be taken with a pinch of salt. Others factors relevant to a particular installation and flight condition may well influence the relationships posited. No consideration has been made of how propeller effects the lift characteristics of the wing.

Following is an abstract from another dangerous fool.

Abstract:
This investigation sought to determine which has more drag, a windmilling propeller or one that is held stationary. A measurement of the drag on stationary and windmilling propellers was conducted considering the length, pitch, rotational frequency of the propeller and the wind velocity. It was found that there is a point at which the drag of a windmilling propeller and a stationary propeller are equal. This crossover point was found to be dependent on the pitch and length of the propeller and independent of wind velocity.

You can find the report here. Keep in mind, unlike the NACA report this one does not consider or provide the necessary data to account for the negative torque on an attached engine. (i.e. friction)

flyingchicken

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.

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.)
You should highlight this a lot more - it might take away a lot of the hostility shown towards you.
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.

UAL T38 Phlyer

^^^Post of the year.

Captain Beaker

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.

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.

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

Captain Beaker

FlyingChicken,

I reread your post, I wanted to edit my post and wanted to include...

I think you do appreciate some of what I have posted, and took some time to read the data. I apologise if I suggested how to suck eggs. Regarding specific speed/rpm and propeller advance, both reports covered variations in airspeed. Certainly through the gliding range of a 172. On putting the cart before the horse, how else would you suggest analysing this other than actually looking at the quantitative data?

This is primarily an engineering question, and explaining the basis for certain pilot actions and aircraft performance. It is not about changing any pilot actions.

Freight Dawg

????? So what does the MP read when you are on the ground with the engine stopped? Have you ever started a chainsaw or lawnmower? Hint, it's easier to pull the motor through with the throttle open. With all due respect, it's time to stop arguing. You're not making any sense.

Captain Beaker

Atmosheric, The scenario of starting the engine is not a steady state. You are attempting to create vacuum on the intake stroke, but you do not have increased pressure in the other cylinders on a power stroke. (The 'spring effect' that flying chicken referred to)

But to be fair, I have not provided any data to support this, so perhaps lets put that aside...

Freight Dawg

Why put it aside if it's germane to the conversation? So if you are windmilling the prop with the throttle at idle the MP would be what? Where does the extra energy come from to create a vacuum? Try holding a flat plate out the window at 90 mph and compare that to the force required to rotate an engine at 1300 rpm. Remember that energy has to come from somewhere.

(I tried not to chime in but couldn't help myself....next.....)

Captain Beaker

We don't have to, we can just park it and come back to it and address it. I am fairly certain what I have presented has not been understood. Perhaps when we are on same page. Memory is a bit sketchy on the later, but 11 inches is in the ballpark. I felt old when you asked that one Is my memory correct?

'All that extra energy' ultimately comes from the forward movement of aircraft. Almost all of the energy of a moving aircraft is either stored potential energy, or being used to displace the air (at high speed it warms it a little as well). If you use a little energy spinning an engine. Then the question becomes Is the energy used driving the engine less than the reduction in the energy used displacing air? The data presented suggests that the correct answer is it depends...

Your more than welcome, I had never though about the starting scenario. So you learn something everyday Your even more welcome if you want to contribute a critique to the data that's been presented.

I will make the widely unsupported assertion that some would like to prove this Muppet wrong. Well you can count me in on that group, but you'll need to provide some new data or demonstrate the research reports I submitted are incorrect.

dontcare4U

Captain Beaker, listen to JohnBurke and flyingchicken, please!!