109 prop pitch (rpm) and the supercharger

[bongodriver]

surely the only RPM that can be refered to is prop RPM, I don't think there is an engine RPM gauge in most of these aircraft, engines can run happily at a range of RPM's but a prop must run within a certain range for efficiency, typically between 2,600 to 3,000, therefore it is more important to know what the prop is doing, certainly this is the case with modern aircraft...engine RPM is practically irrelevent and only fixed pitch non reduction geared aircraft you can assume engine and prop RPM are the same.

jtd is correct that prop and engine RPM are fixed in a ratio but a prop turning at 1500 RPM is not producing much thrust, so it would be more like prop RPM at 2400 and engine is at 3720 with that 1:1.55 ratio, I know were talking 1930/40 tech here but I'm pretty sure the typical aero engine of those days could cope with RPM's that high.

[JtD]

Originally Posted by bongodriver

surely the only RPM that can be refered to is prop RPM, I don't think there is an engine RPM gauge in most of these aircraft....

Partial quote, sorry. But what you're saying is not correct. The rpm gauges you see in the aircraft are engine rpm, I'm not aware of any aircraft showing prop rpm (unless the gear is missing and the ratio is 1:1). Also the rpm limits as listed in manuals give engine rpm limits. In case of the DB 601, the 2400 rpm is the 5 minute engine limit - and the prop will be at 1550.

[bongodriver]

I'm not so convinced, the gauges normally just say RPM and dont apparently distinguish whether it's engine or prop, like I said a prop turning at 1500 rpm is not going to be very useful, I can believe the actual RPM gauge is being driven by the engine itself but I would think the actual calibration on the gauge is for propellor rpm, easy enough to do considering the ratio is fixed so no need for special gears, the manuals may state the engine limits but it may just be that the figures stated are on the basis of prop rpm taking into account the known ratio.

[JtD]

You can skip all the plane manuals and go directly to engine data, if you find plane manuals confusing. Engine limits are engine limits, they don't change.

[bongodriver]

Originally Posted by JtD

You can skip all the plane manuals and go directly to engine data, if you find plane manuals confusing. Engine limits are engine limits, they don't change.

Are you Crumpp in alias?....you seem to have much the same charm, no need to get patronising.

I don't find aircraft manuals confusing thank you, I just find the idea of a prop turning at 1500 rpm being efficient less than believeable.

[JtD]

Originally Posted by bongodriver

Are you Crumpp in alias?

I find that very insulting. I'm not calling you names just because you don't want to "believe" a simple technical fact.

Originally Posted by bongodriver

I don't find aircraft manuals confusing thank you, I just find the idea of a prop turning at 1500 rpm being efficient less than believeable.

For your consideration: 1500 rpm with a 3.1m diameter prop is 240m/s prop tip speed. Combined with 430km/h cruise you're at 270m/s. If you now consider going supersonic (300ish m/s at altitude) is a bad thing, you might find the figure much more believable. Hope that helps, even if my charm is still lacking.

[bongodriver]

Originally Posted by JtD

I find that very insulting. I'm not calling you names just because you don't want to "believe" a simple technical fact.For your consideration: 1500 rpm with a 3.1m diameter prop is 240m/s prop tip speed. Combined with 430km/h cruise you're at 270m/s. If you now consider going supersonic (300ish m/s at altitude) is a bad thing, you might find the figure much more believable. Hope that helps, even if my charm is still lacking.

I was eqally insulted by your sarcasm, I don't recall putting my query in any form that called your intelligence in to question.

[Crumpp]

Can we all specify exactly when we are referring to prop rpm, when we a re referring to engine rpm, and when we are referring to both?

What is referred to as propeller rpm is actually measuring engine rpm but you use the propeller governor to control it in a constant speed propeller. In a direct drive, meaning the propeller is bolted right to the crankshaft, fixed pitch propeller the engine control is just the throttle which sets the rpm.

In a variable pitch propeller, the throttle controls manifold pressure and the propeller controls are used to set the engine rpm. What is confusing people is at the limits of the propeller blade movement, it will change rpm. So if you pull the power to idle from cruise, your rpm will change too. You are not controlling rpm with manifold pressure though, it is just an effect because they are dependant upon each other.

The rpm and manifold pressure settings found in the Operating Notes correspond to a specific point of maximum efficiency. This point is based on the speed of the aircraft.

First of all, what is a propeller?

It is just a wing that moves in a circle. Like any wing, it has limited range of angle of attack from zero lift to maximum lift coefficient. It is also subject to critical mach effects like any wing.

All propellers reach peak efficiency for a give pitch angle at a specific velocity. Just like the wings on the airplane have a point of highest lift to drag ratio, a propeller is no different.

That is why fixed pitch propellers come in "cruise" and "climb" propellers.

A variable pitch gives the pilot the advantage of being able to use a wide range of peak efficiencies by adjusting the pitch angle of the blades and the speed at which the propeller rotates. It allows us to adjust the angle of attack and dynamic pressure our rotating wing requires to keep it at the point it can transfer the most thrust force to the air.

The "popping the clutch" analogy was very good for what the German pilots were doing. The engine has a finite amount of power and thrust it can generate. The point of maximum thrust production is defined by a velocity and the most efficient engine setting. We cannot produce extra power from nothing and just changing blade angle is NOT going to change that point of maximum efficiency or necessarily get us there quicker.

Popping the clutch in your car does not increase the cars power or speed it can attain. Done correctly, you gain an increase in acceleration to the maximum point of efficiency. Done poorly, you loose traction and it takes longer to reach the point of maximum efficiency.

[Crumpp]

Are you Crumpp in alias?....you seem to have much the same charm, no need to get patronising.

Bongodriver, he is just right. Can you accept that?

[bongodriver]

Originally Posted by Crumpp

Bongodriver, he is just right. Can you accept that?

as soon as I'm shown data that says a prop turning at 1500 rpm is going to be efficient.....yes I would happily accept it, it's all I'm asking for and not a tirade of sarcasm.....so you can stick your little blue face up your rectum.