Question about effects of RPMs on acceleration and decceleration

[Kweassa]

Hi guys. Thanks for the last time when you guys answered my questions on CSP systems.

 But I'm getting confused again, and I'll have to ask some more questions to fully understand the effect of the throttle, RPM, and propeller management of a plane.

 Now, the instance I want to ask, is from Forgotten Battles. Don't yet raise your eyebrows, because this isn't about cheerleading. I'm bringing it up because all engine systems in AH planes are same, but FB has two different types - and there is a lot of confusion revolving around the understanding of the difference between those two systems.

 ...

 The question is simple and straight forward:

Plane A, and plane B are is identical in every aspect. However, plane A uses a single-lever interlinked system which regulates both throttle and engine RPM. Plane B uses a two-lever system which the throttle and RPM control is separate.

 Question1: Both A and B are flying at the same speed. Both pull their throttle(throttle/RPM lever, in case of A) back as much as possible. Which plane, will deccelerate faster?

 So, which of the two will deccelerate faster?

 In my guess, I always thought A would deccelerate much faster than B, because, I thought the main contributor to the speed of the plane was the revolution of the propeller.

 Now I thought of it this way:

1. With the interlinked system, when the throttle is pulled back, the prop angles also increase to pull down the RPM rate.

2. Fuel is not dumped into the engine anymore. So, what was once powered by the fuel to create thrust, by meeting air and pushing it back, now loses it's power, and creates only drag.

3. But the props are still turning, and this means the momentum of the plane is turning the props by windmilling it.

4. Therefore, if the prop angle is very coarse(lower RPM), the prop meets a lot more air. It has to push a "thicker" wall of air which windmills the prop.

5. However, with the non-linked system, the constant speed prop reacts by itself. When the fuel is cut by chopping throttle, the windmill will slow the plane down, and the windmill effect will become weaker, and the props will begin to turn slower. This will kick the CSP into action, and it will automatically fine it's prop blade in order maintain the RPM level.

6. So, with a non-linked system that has a separate acting CSP, the props will fine itself out, and less and less blade area will meet the air. This means it pushes a "thinner" wall of air which windmills the prop.

7. Therefore, plane A will deccelerate faster than plane B.

 Would this be correct? That, is how it's portrayed in Forgotten Battles, also.

 In FB the German planes, when the throttle is pulled back, chops both throttle, and pulls down RPM.

 The VVS planes don't do that. They only cut throttle, and for a quite long time the RPM level stays put. (until the airspeed becomes really really low, so even the CSP can't contribute to maintain the same RPM rate)

 ...

 From what I think, I thought my explanation was pretty much simple to understand and logical.

 But what confuses me, is when the engine is totally shut down - as per the "glider" status.

 People clearly advise us to lower RPM(coarsen prop angle) to the max, and feather it even, if possible, to create less drag, and maintain speed, in the case when the engine is damaged and have to be shut down. This seems to contradict my understanding....!

 If lowering RPM(coarse prop angle) creates indeed lesser drag than windmilling the props at high RPM(fine prop angle), and that's what people advise us to do when the engine is shut down.. then howcome the throttle/RPM inter-linked system offers a faster decceleration than just pulling the throttle lever??? ??

 What am I missing here? Is what is portrayed in FB wrong? Should it be opposite, so plane A deccelerates slower, than plane B??

 Or is there some difference, between just pulling the throttle back while running the engine, and the engine being totally shut down???

 Need some explanations

[Ecliptik]

I think it has to do with the amount of energy it requires to keep a prop windmilling against the compression of the engine.  The energy used up making the prop turn translates effectively into increased drag - a lot of drag relative to the drag of a stopped prop.

Note that a truly windmilling prop (engine out) does not have the same effect as a prop turning at low or idle power.

[zmeg]

Feathering the prop will ALWAYS produce less drag weather engine is running or not.

[Ecliptik]

Yes, but all I meant is that running at idle will not produce a similar descent rate as cutting the engine and windmilling the prop.  Sometimes pilots try to "simulate" engine out descents with very low power settings.

[flakbait]

Engine-out simulations on prop aircraft are usually done by either feathering the prop or by pulling the engine off to idle. On old warbirds it's almost always done by pulling the throttle for that engine to idle, simply because some feathering systems can't be used all that often. I know of one emergency system that actually dumps the oil inside the prop dome overboard to feather the prop.


Kweassa, the correct answer is aircraft B. Because the prop governor is trying to maintain a given RPM level, the prop pitch stays parked at max RPM. This is the most unaerodynamic place to stick a prop with the throttle off because the blade pitch stays at a high angle, causing a ton of drag. Aircraft A's prop would get pulled off to almost a feather point, resulting in lower prop drag and thus a much more gradual reduction in speed. Max RPM is a rather low blade pitch (45-55º) turning at high speed, while low RPM is a rather high blade pitch (60-75º) turning at low speed. If you set a protractor on your desk with the flat edge facing you, you can see the angles. With the blade at a relatively low pitch, unpowered, an enormous amount of drag is being created. At a higher pitch angle the blade is closer to 90º (feathered) so there's less drag. You'll often see this quoted in prop books backwards, such as 25º instead of 65º, which only adds to the confusion.



Ecliptik, that's not quite true. Pulling an engine back to idle while at cruise speed and actually killing the engine at cruise will produce the exact same result. At idle no engine is producing all that much power, and when turned off the cylinders will continue to pump air because they're being driven by the prop. An engine equiped with a gear-driven supercharger will actually produce the same amount of power in flight whether at idle or turned off. It does this because unlike a normally aspirated (carb equipped) engine, a supercharger is forcing air into the engine under pressure. Plus, it's geared off the crankshaft. So the prop will not only drive the engine it'll also drive the supercharger and force air into the cylinders under pressure. Turbochargers are driven by exhaust gasses and not the crankshaft, so they'll act just like any carb-equipped engine when at idle or switched off. During a decent an engine at idle and an engine that's off will still pump air, still turn the prop, and still make drag. An idling engine, in flight, produces enough power to swing the prop. When switched off, air moving through the prop (along with inertia) is driving the engine. Although there is a difference in the power required, it's minimal. The primary reason pilots do engine-out procedures with the engine at idle is simple; less danger. If you're in a twin-engined AC and practicing engine-out procedures, you want at least one engine turning. Actually killing one engine, and then having the second fail on its own for some reason, would really cause problems. And since there's not much of a diff between an engine at idle and a dead one from a flight-characteristic stand point, there ya go.



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Flakbait [Delta6]
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