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Power requirement of propeller

in 912 / 914 Technical Questions 13 years ago

  • Re: Power requirement of propeller

    by » 13 years ago

    Remy
    This is my opinion others might disagree.
    For the 912 (without a turbo) you cannot overtorque the engine.

    If you set the pitch too high (large angle) then the engine will run slower and you will not have all the power available except at high airspeeds when you don't really need it. But this will not overtorque the engine because the engine cannot produce more torque than it does at WOT and you cannot be at more than WOT. The overtorque problem on engines is usually related to those with a turbo and a manual control of the turbo. This means that the pilot can force the turbo to supply high pressure (boost) at low rpm which results in high torque that could be greater than the engine should be allowed to produce. The 914 has a Turbo Control Unit that ensures that this doesn't happen.
    Having said that it is not a good idea to run a 912 continuously at low revs, Roger Lee can give you the rpm limits that you should aim to run at when you are cruising which is presumably what you will be doing most of the time.

    If you set the pitch too low (small angle) as shown in graph 1 attached to my earlier post you would accelerate from a standing start very quickly because you have all of the power available at the very beginning of your take off run. But as soon as the aircraft gains speed the engine rpm would increase above the 5800 rpm max allowable and you would have to throttle back. You would also be cruising at higher revs and probably using more fuel.
    Therefore the pitch that you set depends on what you want to do.

    I take off from a very small field and enjoy flying slowly in my gyro so I tend to set the pitch low to give me the best take off performance and because I don't fly fast I still only need low rpm to cruise around. If I had a high speed aeroplane with a good long runway I'd tend to set the pitch much higher because I have the runway length available to accept a slow acceleration and once I get to high airspeeds my revs would be lower.

    So the pitch you choose depends on a multitude of parameters and there's no simple answer, it depends on what you are trying to do.

    Your second question is interesting because the theoretical answer is that for a normally aspirated (no turbo) engine it doesn't matter what pitch you set (within reason) the fuel consumption for a given aeroplane at a given airspeed will be more or less the same. Now many will disagree with me so I'll try to explain.
    The attached graph shows the 912 propeller power curve and fuel consumption for that propeller with whatever pitch it had when it was tested. The third line is what we call the specific fuel consumption which is the fuel consumption it needs for each kW produced. This is not something we consider in everyday use but is very important for engine builders and tuners and if you want to compare two engines.

    What you can see is that at part throttle (the power and fuel curves are NOT at WOT) above about 4000 rpm each kW of power requires about 0.4 litres per hour. Now your aeroplane needs a certain amount of power to make it fly at a given airspeed. Let's say your aeroplane needs 45 kW to fly at 120 k/h. From the power curve you can see that the engine would run at about 5300 rpm to produce 45 kW and the fuel consumption would be 0.38 L/kW.h x 45 = 17 L/h.

    If you increased the pitch of the prop you can see from my earlier post that the propeller power curve would move up and to the left and you would have run slower to get the 45 kW you need to fly at 120 k/h. If you run slower, lets say at 5000 rpm, the specific fuel consumption (L/kW.h) would still be about 0.38 (it would also vary due to slight changes in efficiency due to the reduced throttle setting but it wouldn't vary much) and so your consumption would still be about 17 l/h.
    I'm sure we will get lots replies telling us that this isn't true and strictly speaking it isn't because it's an over simplification and there are changes in engine efficiency at different throttle settings and propeller efficiency at different rpms that would give variations of fuel consumption but they are not as great as many think so set the pitch for the parameter that gives you the safest flying (mine is a short take off run because of my short runway) and don't worry about fuel consumption.
    Mike G
      
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  • Re: Power requirement of propeller

    by » 13 years ago

    [attachment]C:\fakepath\912 fuel consumption..pdf[/attachment]
    The file wouldn't attach first time.
    Mike
      
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  • Re: Power requirement of propeller

    by » 13 years ago

    Hi Mike,

    You can over stress the engine by having too much pitch in the prop. I think this is why many refer to it as over torquing. This even caused a few cracked crankcases. Too much pitch (i.e. 5100 WOT flat and level) causes too much stress on parts and eventually something has to give. Think of it like an old 3 speed on the column car and trying to drive 10-15 mph in third gear. It bucks and strains. When you give it gas it lugs down even worse and some times in the old days pre-ignition was added to the problem. Too much pitch causes loss in all performance characteristics.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Power requirement of propeller

    by » 13 years ago

    Roger
    I agree with you, that's why I mentioned the fact that there were other constraints and asked you to step in with the Rotax limitations. In my defence, my error was perhaps trying to be too simple and too specific in my choice of words.
    For me over torque is when a part (say a shaft) absorbs more torque and twists more than it is designed for and the part is over stressed and yields (bends) or fractures (breaks). So I would prefer to call it overload or over stress.
    To clarify my point the attached is the calculated WOT torque curve for a 912.

    You can see that the torque between 4500 and 5800 rpm is pretty well constant so running at lower rpms at WOT doesn't overstress the drive train simply because of torque.
    What Rotax are probably saying is that at this high torque (high because Rotax never designed the engine to run continuously at WOT, nobody ever would) the stress due to the torque plus the cyclic stresses could cause a fatigue failure. Now the theory says that failure due to cyclic stresses is usually within the first 10 million cycles and at 5000 rpm you arrive at 10 million cycles after 33 hours. This is part of the calculation they made when they limit the life of certain components. Also torque works both ways, the rotating parts must absorb the torque and so must the stationary parts, such as the crankcase in your example.
    So I agree with you that you can overload (high temperatures) and overstress (due to cyclic stresses) the engine if you run with too great a pitch but I wouldn't call it overtorque although I accept that most people do.
    I must say that your example was a bit extreme, if you pitch a prop so that it cruised at 5100 rpm at WOT, you'd need a very long runway to take off because you rpm at the beginning (brakes off) would be low and the power delivered by the engine to the prop would also be very low. However it certainly illustrated the point.
    I hope I've clarified my previous post and not given anyone a wrong analysis.
    Mike G
    912WOTtorqueurve.pdf (You do not have access to download this file.)
      
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  • Re: Power requirement of propeller

    by » 13 years ago

    Mike, I suspect Roger is considering the case where there's a VP prop.
    So take off is easy, then if coarsened a lot in S & L flight it could even require WOT to sustain, say, 5100 rpm.

    Also regardless of theoretical 10 to the power 'n' cycles, his business has demonstrated in practical terms what can break engines.

    I recall the VW engine as originally used on single seat a/c in GB: it had a struggling 1200 cc. To make it work several firms increased the capacity and redesigned the case, bearings etc. They are now a heavy lump of kit, if cheaper than the Rotax 912.

    Now the 912 series engine is reliable and gives - in my case - 80 h.p. from a measly 1200 cc.

    It's a bl**dy marvel !
    mike hallam
      
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