Question about DB RPM and power

[Dirty Rotten Flieger]

Hi

I am wondering about Daimler benz engine and max power versus RPM

 

Take for example the 2400rpm limit in a bf109E3. Does this mean that if you rev the engine to 2600rpm for 30sec that you are getting more power or is the power plateaued or is the power dropping off after a peak at 2400rpm?

 

I am asking because I have been thinking that under circumstances of low IAS it is useful to rev the engine to 2600rpm for short burst of acceleration. But if the engine does not make more power at 2600rpm than at 2400rpm then maybe I am mistaken.

 

It also occurs to me that the prop pitch is made very fine while reaching 2600rpm and at low IAS this may be giving improved acceleration even if the engine is not producing more power.

 

Any Daimler benz engine experts who would be able to offer some insight into this ?

[Kurfürst]

The engine power should increase in itself and should be especially useful above rated altitude of the engine, since then the supercharger would be also driven faster.

 

However the downside is that since there is a fixed reduction gear ratio to the propeller, its likely that the propeller blade tips will start to exceed the speed of sound (easy to calculate - RPM x reduction gear gives you the rotation speed of the prop, then consider the length of the prop blades - that is 3 meters on the 109 - and their speed of them at the tip ) above the permissible max rpm and the propeller efficiency will rapidly decrease. The end result is that while you pump a lot of power to turn the prop faster, it cannot turn that motion into useful thrust.

 

Some of the early 109Es and Fs were officially allowed to briefly increase the engine rpm beyond max of 2400, to 2600 at high altitudes above the rated alt, which did yield some extra speed, however on the 109K this max speed was already 2800 and the only "increase of rpm" trick or allowance that can be found is for using the full rpm for lower boost settings.

 

In any case, prolonged exceeding of allowed rpm will stress and break the engine components sooner or later, at which point power drops to zero. :)

[Dirty Rotten Flieger]

Thanks for the reply Kurfurst. I hadn't even thought about the Mach number of the prop.

 

Specifically I was thinking of a situation where the Bf109 was maneuvering in combat and had just leveled off after a steep climb and was just above stall speed. Very low IAS say 250km/h or 300km/h. With an enemy plane climbing up who is also near stall and unable to lift his nose and fire on me... I have been over revving the engine to 2600rpm hoping this will mean better acceleration. But then I realise that the engine may not even be making any more power doing this below critical altitude. I think it still may give a benefit because of the finer prop pitch for low speed acceleration.

 

Have you ever seen a graph of a DB601 RPM versus engine power? Does the power curve peek at 2400rpm? Or does it continue to produce more power at higher RPM. Maybe it does produce more power but the heat would be too much for the radiators to dissipate at 2600rpm?

 

The more I fly with these planes I realise there is always more I can learn.

[Yo-Yo]

Generally, there are two main cases: below the critical altitude and above it. The first case usually operates with constant boost, so increasing the rpm will increase air mass flow, thus indicator power. But there are several factors that counteract: air filling can decrease due to valve timing optimised to certain rpm and mechanical losses decrease too.

As you take a look at a car aspiration engine you will see that power curve flattens up to max rpm.

 

THe second case, already described - increasing rpm increases boost and the power grows significantly.

 

But the engines are very sensitive to overreving...

 

By the way, there are two main ways to add special power: by boost or by boost and rpm together. Depends on how the engine bears increased rpm.

[Zee Pet]

Ok, I'm lost now.

Am I correct in saying horse power is fixed at constant altitude, with the exception of using MW-50? But if the propeller was rotating at 2000 rpm at fine pitch, and 2000 rpm at coarse pitch, then coarse pitch is biting more air, therefore more thrust therefore airspeed at the expense of fuel consumption?? But then there is some rpm at which a further increase in rpm does not result in more thrust. So then because the power is linked to the amount of air being taken in and a supercharger is in the mix, higher rpm spins supercharger faster which gives more air and hence more power? But to get this power from the supercharger means adjustment of the prop pitch to finer, which means its pushing less air. So wouldn't that be generating more power but power that is unusable because the prop isn't changing speed? Is there a simple way someone can explain this to me or a resource thatIi can learn about from the ground up?