Prop & windmill quiz

[Leif Ohlsson]

Reading about "Poppasmurf's" Bristol F2B build in the Marek Marathon, I came upon a collection of beautiful photos of the Shuttleworth F2B, among them this one, which has continued to intrigue me:



The title of the original photo says it is a generator, which I don't quite believe. It looks rather more like an hydraulic pump, possibly for the interruptor gear. But that's not the interesting point. Clearly it is a windmill, designed to be driven by the prop & airspeed airstream.

Have a look at the windmill - how could you tell that this is a windmill, not a propeller (if you didn't know already)? I never noticed the feature I'm asking you to identify until now; and it is really interesting when you come to think of it.

Here's another one: What could you tell about the rotational speed the windmill pump was designed for, in comparison with the propeller of the aircraft? (I think I know the answer to this one, but I'm not 100 percent sure, so fire away.)

You would of course have to know which direction is forward and backward, so here is an overview of the aircraft with both the propeller and windmill pump:



For a comparison with the propeller, this beautiful photo of the 11-layered laminated big prop should help:



And if you feel a need to brush up a little bit on propeller theory before attempting an answer, try this post and some related, like this one in the laminated props thread.

You may also want to think about this: Why were pumps & generators placed as separate units, unconnected to the engine (as distinct from, for example, cars) for such a long time? I remember noticing that an aircraft such as the DH 89 Dragon Rapide had an external generator at the wing root as late as the 1930s.

I'm not sure I have the correct answer to this question either, and it has always bothered me. But I have a pretty good guess, and it would be interesting to see if it coincides with the opinion of others.

So there are three questions/tasks in this quiz:

1. Indicate a surefire sign that this is a windmill, not a small propeller.

2. Is the windmill designed to rotate faster or slower than the big prop? Discuss why.

3. Discuss the most probable reason for mounting pumps & generators as separate, external, units?

You may now pick up your pencils and start thinking.

Leif

[Don Boose]

Wild guesses, hastily inscribed as I head out the door:

1. The very shallow pitch, or angle of attack (incidence), of the windmill blades as opposed to the sharper pitch of the propellor.

2. The shallow pitch indicates to me that the windmill turns slower than the prop, presumably because the pump would jam up if it were forced to cycle too quickly.

3. I don't know.

Don

[dansls1]

The answer to number 3 has a number of answers. a) You can. In an automobile, you don't really have a way to put a separate generator not connected to the engine by some belt or something, unless you run it off an axle - which leaves all sorts of nightmarish engineering quandaries, or run a separate wheel to the ground. (or have a windmill somewhere on the car which is an odd concept, but probably feasible if inefficient due to speeds) In an airplane it's much easier to take advantage of the flowing air. b) Maximizing engine power. In the same way your A/C will rob power from your car engine, so does any pump or generator. In the early days of aircraft design I'm sure they were trying to get as much power as possible out of the engine for best performance especially at slow speeds. c) If it truly is for the interrupter gear (actually synchronization gear), then the plane was not designed with it and it was an afterthought, so to speak - so it was easiest to retrofit as a separate unit.

[CharlieC]

Looks like a fuel pump to me - there's even a drip of fuel underneath the brass nut at the bottom. I'd guess the Bristol was started by manually priming the engine - once it was running the prop wash would keep the pump turning.

regards,

Charlie

[Kugelfang]

I'd agree that the angle of pitch is the telling feature that this is a windmill. It is designed to be pushed by air rather than to push the air.

My guess is that it is an airspeed indicator.

--jeff

[Darwin]

With respect to question 1....the direction of the bernouli force vector with respect to the rotational axis provides the answer. In the case of the propellor, the bernouli force is aligned with the axis, so would act to move whatever the propellor is attached to. In the case of the smaller device, the forces developed are tangential to the axis (torque), and would cause the airfoils to rotate around the axis, not produce linear motion of the device.

With respect to question 2, the propellor is designed to provide minimal drag in the direction of rotation, the windmill provides maximum drag in the direction of rotation, implying the propellor will operate at high speed, the windmill at low speed.

With respect to question 3, there are many possible answers, but one I've heard is that once the aircraft is in motion, the auxiliary units will continue to operate when the engine quits and the airplane commences to fall from the sky, thus allowing a semi-controlled fall rather than a simple ballistic path to impact.

[Leif Ohlsson]

I learned many things from this. First of all not to be so sure that you already are in possession of the one and only true or good or most well-reasoned answer...

From Don, and Jeff, and Darwin (with a very elegant reasoning, much to my liking), I take the shallow angle as a sign of a windmill. I agree, insofar as the shallow angle will make the windmill turn even at very low airspeed, provided the load isn't too high.

Conversely, like Don and Darwin conclude, the low incidence will set an upper limit to the rpm of the windmill. Even if the airstream is strong enough to overcome a high load, at a certain rpm the relative airstream (a combination of the airspeed and rotational speed of the windmill) will cause the incidence to be too low for producing enough rotational force.

The equilibrium will be at a much lower rpm than the prop, would be my guess, like Don & Darwin. But my reasoning is based on viewing both the propeller & windmill as basically wings, and looking at optimizing angle of attack rather than minimizing drag (Darwin).

From Charlie I take the suggestion that it might be a fuel pump. An airspeed indicator (Jeff) is less likely, in my view, given the fact that there seem to be pitot tubes on the plane.

The answers to the third question is what shook me most. My original thinking was along Dan's last suggestion - namely a retrofit, and difficulties adapting existing engines to include the pump (or generator, or whatever extra equipment was needed). But this wouldn't explain why the method of having an external windmill-driven generator would continue for so long. Here I bow to Darwin - if batteries weren't on board, an engine failure would mean loss of current, or hydraulic pressure or whatever.

When effective batteries came along, together with effective generators integrated with engines, the external generator became obsolete.

I still have a little difficulty in accepting Dan's reasoning about loss of efficiency if the extra equipment were to be mechanically coupled to engine. Wouldn't the loss incurred by drag and lack of efficiency in the windmill far be far worse? At least, that's what I always thought - but I don't know, so I'll listen to the car expert.

Finally, my only contribution to the discussion will be the answer to the first question - what striking feature (beyond the low incidence already discussed) reveals this as a windmill? My suggestion is this:



I was really intrigued by realizing that I had never spotted this before, nor thought about the requirements of a windmill along those lines!

I will look at windpower generators with a completely new eye from now on - always had a little difficulty accepting their direction of rotation, given the shape of the blades.

It now turns out that I have been thinking about them as propellers, with the engine shut down, windmilling in the slipstream. Not so, I now realize - windmilling propellers in fact are very inefficient windmills, with their profiles all backward!

Thank you for all you taught me; hope it was a little bit of fun as well.

Leif

[shrike]

A windmill has an 'inverted' airfoil versus that of a driven propellor. That is that the flat 'underside' of the airfoil faces into the wind

<sigh> once again I rush to answer a question that has been answered already.



It is desirable to design your windmill to stall out at certain wind speeds, which acts as a speed regulator. Somewhere I have a site bookmarked with a set of sections for a windmill that does just that (it gets windy here and I like the idea of building a small wind generator)

The windmill in the photos is an auxiliary fuel pump, pumping petrol from a fuselage mounted tank up to the tank in the wing where its fed by gravity to the engine.

As to why there are seperate wind generators on some aircraft, you have to look to the needs of that airplane.
Aircraft engines almost invariably have magneto ignition (not all, but 99.99%) which is completely self-contained and doesn't require a battery of any kind. Taking the Bristol as an example, it has no starter, no lights, no wireless, no need for any electricity aside from that provided by the magneto for ignition, so it has no battery or generator.

If it needed electricity for any of those purposes, it is much more convenient to add a wind driven generator and an electrical system completely separate from the engine, plus it is redundant. If the generator fails, the engine still runs. And one kit can be fitted to any aircraft no matter what engine it has.

Engine driven accessories require additional engineering, a potential loss of parts commonality for maintenance, additional weight and complexity of driving gears and the potential that if your accessory packs it in (generator seizing etc) it can destroy the engine.

<double sigh> this is like some of our arguments at work - airplane engines/parts and their accessories is what I do

[Golden Bear]

Windmill blades will be designed differently for different situations. With low angle of attack this is clearly designed for highish air speed with low revolutions (in relation to how fast the driver propellor is turning). In terms of parasitic energy loss you must be correct when taking the entire airframe as a free body. However, given the enormous losse engendered by the non-smoothly-airodynamic airframe it might be felt that a loss to at an otherwise fairly neutral site had little effect. Also, mechanisms coupled directly to the powerplant, not particularly effiecient compared to what we accept as the norm nowadays, could have the effect of impairing the performance in momentum maintenance and immediate effectiveness. Still the free body argument is compelling.

It seems clear to me that imposing it as a separate mechanism is at least much easier and detached from the concept of altering the existing drive mechanism.

BTW, I've built a model of the Bristol "Fighter" which is posted most likely at the inheritor of our predecessor site. That is, at Zealot, home of giant robots and imagined spacecraft.

Carl

[whulsey]

The Voisin Laboratoire Grand Prix car's water pump was driven by a small windmill to help cut down parasitic loss on the engine. Since he came from early AC design that's probably where the idea came from. Some race cars do drive various accessories and pumps off the drive shaft, differential or transaxle primarily to cut down the length of fluid lines with the added benefit of removing some of the clutter from around the engine.