RPM on a fixed pitched propeller

[arflyingman]

G'day guys,
Came across this question today and not sure if I have the correct answer.
Anyone shed some light?

Why does an aircraft's RPM, with a fixed pitched propeller, decrease when slowing down and conversely increase when speeding up?

My reasoning is that when the A/C is fast the AoA on the prop is less thus a decreased total reaction on the blade. This leads to a decrease torque on the blade and the engine effectively exerts more force rotating the prop therefore a higher RPM.
(the above might be a complete load of crap)

Care to share insights anyone?

[6080ft]

I am not even sure I understand your question!?

[sprthompson]

I think you're correct, When the aircraft and thus the prop is moving faster (forward) through the air the effective AOA is less so there is less drag and the prop can spin faster, I think?

[VH-XXX]

You are half way there.

If the Power is set at a specific point on the throttle position. Eg. During a run-up. The engine will be at a certain RPM during this time.

If the aircraft is in a dive at the exact same "throttle position" the engine will be operating at a higher RPM as there is less load on the engine due to the air passing over the propellor.

It is as simple as that. Don't get confused about setting RPM at static and in flight because this is irrelevant, you are referring to the position of the throttle.


To add to that an observation in my aircraft is that even though the documented best climb speed is around 80 knots, I get better performance when climbing at 100 knots because the engine revs faster at the same power setting because there is less load upon it. Not a problem with one of those Rotax engines though as they have a gearbox, hence more torque.

[Mark1234]

If there was no engine, or you were considering a wind generator/windmill type thing, it would be intuitive that it will windmill faster, the faster the airflow, would it not? All the engine does is add (somewhat) fixed amount of power.

Alternatively, consider that the pitch of the prop gives some set amount of forward motion per turn. In order to go faster, it must turn faster.

If you slow the aircraft down (e.g. by pointing it straight up), the angle of attack of the blades will increase, creating as you say, more drag - that's drag in the sense of resistance of the prop to turning. Drag in the plane of the prop disc; hence the engine slows.

[djpil]

The best explanation that I've seen is in Noel Kruse's Book #1, Aerodynamics and Other Stuff - see page 85 onwards. Free download. (http://www.flybetter.com.au/)

Although while I was digging up the reference you seem to be all sorted out. Anyway, the whole book is a good read.

[tea & bikkies]

Sort of like sticking one of those little props on a wooden stick out the window of a car, when the faster you go, the faster it spins:}

[ksa5223]

Why does an aircraft's RPM, with a fixed pitched propeller, decrease when slowing down and conversely increase when speeding up?

Imagine a boat, you add power and the propeller spins faster, decrease power, prop slows down :}

[Jabawocky]

Geez you guys

XXX has described it best...... and he is a Geek not an engineer!

Its all about load, and the torque available at a given throttle setting.

Just a thought..... take your manual car...... a small throttle setting = high RPM, same setting but increase the load.....RPM drops.

Now think about gravity and climb and descend...............

how hard is that!:hmm:

[arflyingman]

alright guys, thanks for the replies.

so in a nutshell

when an A/C accelerates with a fixed throttle position, the RPM increases because there is less "drag (torque)" acting against the engine (due to the decreased AoA of the propeller).
and conversely when decelerating with a fixed throttle position.

Thanks!

[VH-XXX]

Personally I'm still wondering how the aoa can change on a fixed pitch prop blade purely because of it's speed through the air? Does the aoa of a wing aerofoil change as the speed of your aircraft increases???

[QJB]

Because the AoA of a propeller blade is a function of its forwards velocity (TAS) and its rotational velocity (RPM). That's my understanding. Hence root of propeller blade is at a different angle to the tip, since it rotates more slowly TAS has a greater effect on its AoA.

[Mark1234]

What QJB said.. Thought experiment - stop the propeller with the aircraft gliding - what is the AOA of the leading edge of the propeller? Now spin it really fast?

[Runaway Gun]

XXX - in a simple word, Yes.

I mean, in straight and level flight, the wing may be at say 12 AOA at 50kts
Flying faster at 140kts, still in S&L, the wing will be at less, maybe 2 AOA.

In both instances, the life required to be produced in exactly the same, as the aircraft weight remains the same too.

So the old equation: LIFT = CL 1/2 Rho V squared S must remain contstant.

V increases, and for the total LIFT to remain, something else decreases. That is the AOA in this case

[arflyingman]

http://www.stefanv.com/quiet/2002-03/figure2.gif

XXX, my understanding is that when the propeller travels faster through the air the vector of "air flow due to incoming air" is greater, as when combined with "airflow due to blade motion" you get a relative airflow closer to the chord, ie. a smaller AoA.

Basically what QJB has said.

[VH-XXX]

I mean, in straight and level flight, the wing may be at say 12 AOA at 50kts
Flying faster at 140kts, still in S&L, the wing will be at less, maybe 2 AOA.

Would be interesting to see a graph of that.

I would imagine that there would be a big difference from the stall speed to cruise speed, but a very small change in AOA from cruise to VNE.

[training wheels]

I'm sure this was covered in CPL aerodynamics theory .. did you skip that class XXX? ;)

[VH-XXX]

I was probably up flying on a pri-mercial flight and missed it ;)

[Runaway Gun]

Regardless, you have your answer. The AOA changes throughout.

If you want to score an A+, check out the negative AOA required on an an asymetrical wing, for a zero-lift part of flight.

[MakeItHappenCaptain]

To add to that an observation in my aircraft is that even though the documented best climb speed is around 80 knots, I get better performance when climbing at 100 knots because the engine revs faster at the same power setting because there is less load upon it.

The speed for performance is a function of the thrust required vs the thrust available (for angle of climb) or powers (for rate). If you haven't made any changes (to the engine output), there is no reason why the certification data will be any different.
You stack and the coroner finds out you made up your own figures, well.........

The AOA theory due to airspeed is best used to explain static RPM (which many more people should be looking at as a part of their take-off checks - see your flight manual).
A "laymans" way of looking at thrust available decreasing (also increasing rpm)as speed increases (for fixed pitch props of course) is the prop is able to accelerate the mass of air to say 100 units of velocity. At a standstill, there will be max acceleration of the air mass available and max load (torque) on the prop. If you are already flying at say 50 units of speed, there can only be a further 50 units of acceleration. The prop doesn't work as hard, therefore spins faster (less torque present and also less thrust available).:ok:

[arflyingman]

If you want to score an A+, check out the negative AOA required on an an asymetrical wing, for a zero-lift part of flight.

Approx. -4 degrees for a light aircraft? Again, depending on the design of the wing. The reason why at 0 degrees AoA the wing is still producing lift is because the upper surface of the wing is still accelerating air, therefore decrease in static pressure and producing lift ?