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This week-end, I took my first lesson in a High Performance aircraft (a Cessna 182), and had my first "hands-on" experience with a prop pitch control and a Constant Speed prop.

Before the lesson, I'd read a good deal about these in reference books like the Airplane Flying Handbook. Still, the idea of generating more thrust with a finer pitch setting (prop taking a smaller "bite") and higher RPM seems counter-intuitive.

A prop is an airfoil -- and as an airfoil, the Lift Coefficient should increase as the angle of attack increases (until you reach the Critical Angle of Attack).
Increasing the angle of attack also increases your induced drag -- however, induced drag decreases with forward speed (and the prop is an airfoil that moves very quickly.) So it seems that a prop spinning at high RPM would generate more parasite drag than induced drag (and the other way around for low RPM settings.)

I realize that the engine must work harder (at courser pitch settings) to move a greater quantity of air, still -- it seems counter-intuitive.

I know it works -- we flew around and various throttle/prop pitch settings, and I saw it all happen for myself, exactly as described in the books.

But I'd like to get a clearer understanding of the physics and aerodynamics of what's going on.

Thanks!!

-- Charlie

Think of the pitch as a gearbox.

The power you get from the engine goes up as you burn more fuel, which increases both with increasing manifold pressure (more air/fuel coming in) and RPM (more air/fuel coming in).

What the gearbox (sorry - prop) does is transfer all that power to throwing the air back (which yes, the airfoil of the prop is doing for us). If you want more horsepower from a higher RPM, we need to allow the prop RPM to increase, which intuitively means flatter pitch.

The pitch also needs to change depending on our forward velocity. For a given engine power output, a higher forward velocity will need a steeper pitch to maintain the same prop airfoil angle of attack (and avoid an RPM increase and consequent change in power). Conversely, a slower airspeed requires a flatter prop angle of attack to maintain RPM & horsepower (as in starting a takeoff roll).

To directly look at your thought that a flatter pitch would give less, not more thrust. consider: If you want more horsepower from the same RPM (higher MP, same airspeed), the prop pitch intuitively needs to increase. We can do that, but only by increasing the MP to get the power to maintain RPM at the increased pitch, and if we go too far in that direction we get detonation (VERY bad - melt piston in 60 seconds). So we allow the RPM to increase too.
Taken separately, how do we get the RPM to increase? Flatter pitch, intuitively.

We're just gearing the engine power to moving the air. All the power always goes into thrust (well, except for the part that makes all that noise )

And remember - that blue pitch control knob doesn't really control the pitch. It sets a target RPM, and the governor adjusts the pitch as needed to allow the engine to reach that RPM. The pitch changes every time our airspeed (or MP) changes. Thus lower pitch to allow the engine RPM to increase for greater power output could (maybe) be considered intuitive.

Does that mean that you can never overspeed the prop on a plane with a constant-speed prop?

no you could, theoretically. the "full forward" prop setting means the prop pitch is adjusted to maintain redline, maximum. but if you keep pushing it with more power and more airspeed, the prop will eventually get to the coarsest pitch possible and still be at redline. from there the governor can do nothing to stop it from going fastest. however, this would take a lot of power/speed and is not very realistic.

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There's another way to overspeed a constant-speed prop.

Have a bunch of power in at a lowish RPM (most constant speed prop/engine combos permit a somewhat "over-square" operation - like 23", 2200RPM).
Then push the prop in all of a sudden. The pitch takes time to adjust. It will go max flat at first, and it will run past the setpoint RPM (redline in this case) before the governor catches up and increases the pitch again to bring the RPM back to the setpoint.
Alternately, have the prop full forward with the engine at idle, then push the throttle in all of a sudden - as in ham-handling the start of a takeoff roll. Again, you'll go past the setpoint before the governor gets the situation handled.

So in the interest of being nice to your engine (and hoping it follows the golden rule ), it's a good idea to take 2 or 3 seconds to smoothly push these controls to their limit, even if it's a sudden change in plan like going-around.

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Joe --

This was probably the clearest explanation I've heard. Thanks!

As a follow-up, I've noticed that during a flight, the postion of Throttle control can vary from full-forward to all-the-way-back.

I've never seen this done with the Prop Pitch control. In my (limited) experience, I've seen it either full-forward (fine), or "tuned/twisted" a little bit back.

Are there situations where you'd want the Prop Pitch Control "all the way back" (full-coarse) ?

in the event of an engine failure with unsuccessful restart, pull the prop to low RPM to reduce the drag of the windmilling propeller.

_________________
Everyone must believe in something, I believe that I will go flying.