Wobbly prop - what to read?

[Gertrude the Wombat]

I have had a scan through several of the complex conversion threads but didn't see an answer to this ...

Before flying something with a wobbly prop I'd like to know something about it, why one should want one, what it does, how it works, how to drive it etc (at the principles and equations level rather than just the recipe "set these numbers and it will work and don't worry your pretty little head about the internal gubbins" level). In at least as much detail as I learnt all the other stuff for the basic PPL, but of course none of my PPL text books cover this as it isn't needed for basic PPL.

I've read a couple of articles on the web but can't get my head round it. So, what text book to buy, or what else to read please?

[deice]

I haven't checked any other threads and don't know what you already know but hope this helps some.

Why wobbly? Well, usually they would be fitted to an airplane and the performance of that particular airplane is what you want, not necessarily the prop.

Why they stick wobblies on some aircraft is a question of providing the optimum prop for each flight regime. In climb you want fine pitch. In cruise you want coarse pitch. Fixed pitch props will be either one or a less optimum choice in between. The Constant Speed prop on the other hand will transition from fine to coarse depending on the load placed upon it and/or the pilots input, i.e setting the prop rpm and adjusting manifold pressure (throttle setting).
So, basically it is a question of getting the most speed and climb performance from the same installation.

The prop operates via a governor which works to maintain a constant rpm (hence the name). The governer operates on the principle of centrifugal force. Rotating counter weights will act outwards, when the forces trying to throw the weights out are balanced against the force of driving the propeller, the speed is maintained.

The control of the prop is usually a hydraulic and mechanical spring set-up, on smaller types. Using hydraulic pressure the prop is forced towards fine pitch. The spring will act to force the prop to coarse pitch often together with weights acting due to centrifugal force. Larger aircraft (turbo props) typically have hydraulic drive for fine and coarse pitch settings. They also provide the possibility to feather the prop (twisting the blade so that the lowest profile is shown to the direction of travel)

If you set 2500 rpm and then reduce the throttle setting, the prop will need to adjust towards a lower pitch in order to maintain the rpm. This is because the load on the prop will act to reduce the rpm (try sticking your hand out the window of a car and tilt it at various angles - same force).

For takeoff you typically set the prop control full forward (fine pitch). In some aircraft you need to reduce rpm during the climb, typically to 2500 rpm, or you may do so to keep the neighbors happy. For cruise you select a power setting which is a combination of rpm and manifold pressure, usually from 2100-2500 rpm.

If you're looking for exact explanations of what happens to the prop, try the Oxford ATPL books (book 4), they should provide the insight you need.

[Circuit Basher]

GtW - not sure of your state of knowledge, but when I teach Propulsion to Air Cadets, we have a training manual which includes the basics of Variable Pitch Props / CSUs. The link to this manual (in PDF format) is Propulsion Trg Notes (http://www.aircadets.org/pdf/acp33vol3.pdf) and the bit on variable pitch props starts on Page 40.

For others who want a bit more info on aero engines (for Aircraft Technical), there may be some other info that is of use. The Web site also has other info (such as Air Nav, Principles of Flight) that may be of use to any PPLs who maybe don't find that Thoms explains it in a way they can understand. A word for the wise: The manuals are a bit dated now and contain some inaccuracies, so use that as supplements to other information sources.

[dirkdj]

You may want to google for 'Those marvelous props' by John Deakin.

Please also look at Manifold and Mixture. Mixture is by far the least understood of the trio and probably the most important for engine life.

[Gertrude the Wombat]

Thanks for the references.

I've already read the John Deakin articles - I understand why you want variable pitch (I'll revise my L/D curves and stuff to make sure I understand it), and can understand how a pilot would use a control which manually changed the pitch.

I can believe that manually adjusting pitch could be tedious and even dangerous (if fiddling with it during the take-off run were necessary) and therefore understand why an automatic mechanism is a good idea.

I understand how a governor maintains constant speed. What I don't get (and I admit to being slightly lazy here - I might be able to work out the maths for myself, I haven't tried) is why it is that "constant speed" is the right answer to the requirement for automatic adjustment, rather than some other automatic adjustment algorithm, and it's the maths that demonstrates this that I really want to see. Deakin doesn't do this; even Darrol Stinton doesn't (The Design of the Aeroplane, which is where I usually look if I actually want some nasty equations).

So, I'll read the references given ... thanks.

[IO540]

The prop governor is a standard negative feedback control system, where the setpoint (the value you want to achieve) is the rpm lever, and the "process variable" (the value you are actually getting) is the actual RPM. The control loop parameters (PID - proportional, integral, derivative) are embodied in the design of the mechanics of the prop governor. In fact I think it is just proportional only, possibly derivative, and almost certainly there is no integral element.

If setpoint > PV then it makes the pitch more fine (to raise the RPM)

If setpoint < PV then it makes the pitch more coarse (to drop the RPM)

The power to do it all comes from the engine oil pressure but equally could be electric.

I hope this helps :O

The other "control loop" on an aircraft is the pitch control system, where the trim wheel sets the airspeed (the setpoint).

[skydriller]

I have so far only flown fixed pitch types so far and so I have followed this thread and links with interest.

I had the oportunity to fly in a DA40TDi demonstrator last summer
and one thing in particular springs to mind from that experience..........It had only ONE power lever just like in a fixed pitch prop aeroplane.....However I know it had a variable pitch three blade prop!!

Following on from the info posted above by others about governers etc. why the extra pitch lever then, why do you need it?

I must be missing something blatently obvious I know,

Regards, SD..

[High Wing Drifter]

Skydriller,

Yes, the DA40 has a FADEC system (Fully Automated Digital Electronic Control). This box of tricks knows the best, most appropriate RPM/MP setting (and misture!) for your current calculated TAS. This technology was just not really available in GA until fairly recently.

However, thinking about it, didn't the Messerschmitt 109 have a single "go!" lever? If so, it is a good question as to why we have the RPM lever.

[Chuck Ellsworth]

Lets see how many different types of propellors I can remember flying behind or ahead of.

(1) Fixed pitch.

(2) Ground adjustable pitch.

(3) Variable pitch.

(4) Constant speed.

(5) Aeromatic. ( such as was found on the Globe Swift..)