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Power requirement of propeller

in 912 / 914 Technical Questions 13 years ago
Hello to everybody and happy new year 2012.

I have looked at the performance graph in ISA conditions of the 912 ULS many times. While it is easy to understand the meaning of the torque and power curves, I always wonder what is the exact meaning of the "Power requirement of propeller" one. Can anybody give a clear explanation of this curve and what is it used for?

Thank you very much in advance,
Alb

  • Re: Power requirement of propeller

    by » 13 years ago

    Alberto
    I'll try to answer this for you.
    The power curve shows the power that the engine can supply at different rpms when the throttle is wide open.
    The propeller curve shows the power that a propeller absorbs at different rpms. The curve given in the manual assumes that the propeller is a fixed pitch and that pitch has been set such that at wide open throttle (WOT) the propeller requires 63.5 kW at 5800 rpm which is just the amount of power the engine can supply so the engine will not be able to go higher than 5800 rpm.

    I drew the attached curves

    with excel and the first shows as an example that at about 4700 rpm the propeller requires 40 kW and the engine is capable of supplying 63 kW at WOT. Therefore the engine will accelerate because there is 23 kW of power available. As the engine speed increases the power available gets smaller until at 5800 rpm there is no power left to accelerate any more.

    The second graph shows the same curve but I've added what happens if you increase or decrease the prop pitch. You can see that as you increase the prop pitch it absorbs more power for a given rpm hence the extra power available is less, so that if you increase the pitch enough you can get the engine to run no faster than 4700 rpm.

    If you decrease the pitch you have the reverse effect and you could easily over rev the engine.
    I hope this helps.
    Mike G
    912powercurves.pdf (You do not have access to download this file.)
      
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  • Re: Power requirement of propeller

    by » 13 years ago

    Interestingly for the 80 h.p Rotax data shown above, Full Throttle [WOT] gives 5,200 rpm in my climb out - 75 mph & Rans S6-116 - after take off. It means all ~69 h.p available are being absorbed by the propeller.

    In horizontal cruise flight at ~100 mph the same rpm require much less power & thus throttling back. And in level flight WOT for my WD ground adjustable prop is ~5550 = 72 h.p.

    So just to complicate things apart from the usual static rpm holding rev's down, the a/c speed through the air effectively changes the pitch as seen by the air entering the prop disc.


    mike hallam.
      
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  • Re: Power requirement of propeller

    by » 13 years ago

    Mike
    You are of course right when you say that as the aircraft flies faster the effective pitch of the propeller reduces and you have more power available. The example given would overspeed if you went to WOT once in the air.
    I was tempted to add the part throttle curves as well but thought that it would be too complicated.
    Mike G
    Thank you said by: Alberto Aragones
      
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  • Re: Power requirement of propeller

    by » 13 years ago

    Excellent explanation Mike, thank you very much!
      
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  • Re: Power requirement of propeller

    by » 13 years ago

    Hello everybodies
    Hello Mike

    Interesting topic, and answers!
    About all of this

    First question:
    Is there an ideal setting (pitch propeller) to not endanger the engine and propeller?
    I think a risk of over torque or under torque depending on the pitch of the propeller at full power when take off. (and what are the settings to not trespassing the limits)

    Second question:
    The pitch of the propeller does it influence the consumption of the motor according to the rpm displayed in flight? (according to the graph given by rotax about the consumption -Can we make the similar comparison you made with the pitch?)
    If so how to calculate it without giving instrumental?
    Best regards
    Thank you very much for all the responses giving along the different topics...and sorry for my poor english writing !!
    Remy
    Flying on BRM okavango high wing (copy of savannah) rotax 100cv
      
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