Take off rpm

Hi Roger,

What prop preformed best?

Thanks,

Jim

Actually they all did pretty close to the same and it was all rpm dependant. Different WOT rpms were tested with all the props. Anything under 5500 WOT in level flight was detrimental. The 5600 - 5650 WPT setting was the best all around and well balanced rpm for cIimb, cruise, fuel economy and engine temps. If you have a need for a better climb prop then up to 5800 rpm was better, but you lose a bit of cruise and fuel. I have been reading many post though that the newer E-Prop is performing better than many others, but it wasn't available when I did the study. Prop mfg's all think they have the best prop since sliced bread, but testing of 2 blades, 3 blades, long vs short, flexible vs stiff showed no big differences. Warp Drive suffered a lot in climb. The testing used 4 identical planes, one person setting prop pitch, one person telling each pilot what way to take off, planes taking off side by side, flying within 75' of each other at the same time of day and same direction. So this removed most variables that other single plane aircraft mfg's or owners use. 

I am confused about Mr. Wizard’s prior statement that the 5200 RPM minimum really just applies to the climb phase of flight. The word “climb” is not in SL912- 016R1 Para 3.1.2 but rather it states “takeoff speed” at WOT should not be below 5200 RPM, and this is the situation our author of this post believes he is in. What is the Rotax definition of “Takeoff Speed” and how does this differ from Static RPM at WOT or Climb RPM at WOT?  Engine RPM (with a fixed pitch prop) varies with forward speed, is some standard climb speed assumed?

Hi Jim,

Rotax recommend 5200 WOT at Static - ie the aircraft is not moving, the throttle is wide open (WOT) and the engine speed is at 5200 rpm

Further Rotax recommend that the take-off (TO)  & climb rpm should not drop below 5200 rpm. TO and particularly climb-out (CO) are the periods, in the engine's operation, of maximum stress/load. 

(Assuming you have set engine to static WOT) In practise this will mean, irrespective of TO technique (short field/rolling), the engine speed will, on application of full power (WOT), quickly rise to 5200 rpm.

As the aircraft gathers speed, in ground role & then standard CO, the rpm will rise accordingly,  likely to max permissible of 5800 rpm (for 5 minutes). UNLESS the pilot chooses a higher CO angle.  The pilot can choose to "govern" WOT engine speed using the aircraft climb out angle - steeper the climb, lower the rpm & visa versa. The pilot in doing this should keep the engine rpm above 5200 for the duration of the climb. The pilot is not limited to 5200 rpm, he/she may elect any rpm between 5200 and 5800.

The ability of the pilot to choose TO/CO engine speed between 5200-5800 rpm is, in my mind, a safety feature, in that this engine setting facilities short field, high density altitude and high aircraft load operations, while minimising engine stress.

FYI - Roger L advocates a diffrent approach (to static) to setting WOT

Hi Jim

The service letter is trying to prevent people from over/underloading the engine.  The major issue in fixed pitch is not getting the engine to that peak torque point of the engine in the phase of flight from lifting off to leveling off.  For sure static and at the end of the runway will give you different RPM readings at WOT. 

I don't care to argue the semantics of the wording as long as you understand what the intent was.  

Climb speed is not at all relevant, what we are talking about is the engine load.  (RPM)  If you spend a lot of time in the climb phase of flight below peak torque you have the highest cylinder pressures.  The engine is more susceptible to detonation and crankcase loading is at its highest.  With fixed pitch you always are dealing with a compromise, it is like a manual transmission car with one gear.  The prop settings dictate what RPM you will see for the most part.  

I don't know if that helps you or not.