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Brake Horsepower Question

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  • Kev A
    Hello and hope you can assist Power required on my aircraft is a movable feast, it needs around two thirds throttle and 5,800rpm to cruise at 58-60mph burning
    Message 1 of 9 , Oct 1 2:32 PM
      Hello and hope you can assist

      Power required on my aircraft is a movable feast, it needs around two thirds throttle and 5,800rpm to cruise at 58-60mph burning 11 litres per hour on 'fast' trim.

      By adjusting trim to 'slow' it can run at 5000rpm at just below 50mph, on around one third throttle.

      This is currently using a Rotax 447, which I want to change for a more frugal alternative but keeping as much simolicity as possible.

      Before buying a mildly tuned Vee twin industrial unit and redrive, need to be certain the engine can be run in the cruise at or below the continuous rating; if I buy a 28bhp unit (Subaru or Briggs) and bin the governor, make alloy shims for the valve springs and keep the fan cooling, it should be possible to run it over the revs to say, 35bhp for short periods to climb to height.

      So rather than chase the same 40 bhp the 447 is giving me now, it appears to make sense to slow the aircraft down 5mph and enable it to work with 35bhp max instead.

      Question is, what power is a 447 giving me burning 11 litres per hour at 5,800? whatever that number is, forms the minimum continuous rating in bhp I should be seeking.

      Anyone help?

      Cheers

      Kev
      UK
    • Peter Walker
      Hello You can get a close approximation by timing a measured quart (or litre) at WOT tied down If it runs at 5800RPM look up the HP on the Rotax chart @5800
      Message 2 of 9 , Oct 1 5:15 PM
        Hello
        You can get a close approximation by timing a measured quart (or litre) at WOT tied down If it runs at 5800RPM look up the HP on the Rotax chart @5800 to get the HP you use as the rated HP A burn of 1 quart(litre) in 4 minutes is a burn 15 Quarts (litres) an hour 60(minutes) /4( burn time) = 15 
        A fuel burn at cruise of 10 quart(litre) is ~65%of the HP off the list  
        Not perfect but lots closer than a guess
        BTW I have a flywheel dyno program Simply replace the prop with a flywheel hook up a speed sensor (revs) and do a run from idle to peak It calculates the acceleration and gives a readout To get cruise do a run up at cruise throttle setting
        Peter

        [Non-text portions of this message have been removed]
      • Johan
        Hi Kev, If my memory is correct, a 2-stroke burns 0,4 liter/HP/hour @ full throttle. If you close the throttle, let s assume to 20% of full throttle, it can
        Message 3 of 9 , Oct 2 11:58 AM
          Hi Kev,

          If my memory is correct, a 2-stroke burns 0,4 liter/HP/hour @ full throttle.
          If you close the throttle, let's assume to 20% of full throttle, it can burns up to 0,8 liter/HP/hour.
          So if you consume 11 liters, it could be from 13,75 hp up to 27,5 hp.
          If you know the intake manifold pressure you can estimate a bit better the power-output.
          Knowing the position of the throttle is not that accurate as intake manifold pressure also varies in the rpm range with unchanged throttle.

          This is how I estimate power-output if it's not that important. (20% accuracy)
        • George Bearden
          ... I assume this is at peak hp rpm? Some additional discussion: Since some 2-strokes are abnormal in that they either operate slower (Chotia 460) or faster
          Message 4 of 9 , Oct 2 5:49 PM
            > a 2-stroke burns 0,4 liter/HP/hour @ full throttle.

            I assume this is at peak hp rpm?

            Some additional discussion:

            Since some 2-strokes are 'abnormal' in that they either operate slower
            (Chotia 460) or faster than the typical 2-stroke,and since internal engine
            power losses [1] are highly subject to rpm, if you have such an odd-ball
            engine this formula may need tweeking before use.

            Typically best BSFC (brake Specific Fuel Consumption) is found at torque
            peak. Since engine's peak hp is higher than torque peak you may get better
            mpg at lower than peak rpm, and higher than the 20%.



            [1] pumping losses (loss due to moving air around in the crankcase, effort
            to expel exhaust gases, etc), and other rotational losses such as bearing
            and ring friction



            [Non-text portions of this message have been removed]
          • George Bearden
            ... We have to know the weight of the flywheel, right? Who publishes this, and the name please! Thanks GeoB [Non-text portions of this message have been
            Message 5 of 9 , Oct 2 6:23 PM
              > Simply replace the prop with a flywheel hook up

              We have to know the weight of the flywheel, right?

              Who publishes this, and the name please!

              Thanks

              GeoB



              [Non-text portions of this message have been removed]
            • Peter Walker
              Hello  The program has a hub calculator You can use a mic on the exhaust as a pulse http://www.sci-spot.com/Mechanical/dyno.htm Remember that the dyno only
              Message 6 of 9 , Oct 2 6:57 PM
                Hello 
                The program has a hub calculator You can use a mic on the exhaust as a pulse

                http://www.sci-spot.com/Mechanical/dyno.htm

                Remember that the dyno only takes the added flywheel into the calculation If you did a run on a Generac and removed the factory flywheel HP WILL go up If you start with a stock engine adjust the dyno flywheel mass to match the factory HP If you remove the factory flywheel adjust the dyno flywheel mass to to match the last run with the flywheel
                 Peter

                ________________________________
                From: George Bearden <gab16@...>
                To: Small4-strokeEngines@yahoogroups.com
                Sent: Monday, October 3, 2011 9:23 AM
                Subject: RE: [Small4-strokeEngines] Re: Brake Horsepower Question


                 
                > Simply replace the prop with a flywheel hook up

                We have to know the weight of the flywheel, right?

                Who publishes this, and the name please!

                Thanks

                GeoB

                [Non-text portions of this message have been removed]




                [Non-text portions of this message have been removed]
              • Peter Walker
                Hello  The program has a hub calculator You can use a mic on the exhaust as a pulse ________________________________ From: George Bearden
                Message 7 of 9 , Oct 2 6:57 PM
                  Hello 
                  The program has a hub calculator You can use a mic on the exhaust as a pulse



                  ________________________________
                  From: George Bearden <gab16@...>
                  To: Small4-strokeEngines@yahoogroups.com
                  Sent: Monday, October 3, 2011 9:23 AM
                  Subject: RE: [Small4-strokeEngines] Re: Brake Horsepower Question


                   
                  > Simply replace the prop with a flywheel hook up

                  We have to know the weight of the flywheel, right?

                  Who publishes this, and the name please!

                  Thanks

                  GeoB

                  [Non-text portions of this message have been removed]




                  [Non-text portions of this message have been removed]
                • Peter Walker
                  Hello  The program has a hub calculator You can use a mic on the exhaust as a pulse http://www.sci-spot.com/Mechanical/dyno.htm Remember that the dyno only
                  Message 8 of 9 , Oct 2 6:57 PM
                    Hello 
                    The program has a hub calculator You can use a mic on the exhaust as a pulse

                    http://www.sci-spot.com/Mechanical/dyno.htm

                    Remember that the dyno only takes the added flywheel into the calculation If you did a run on a Generac and removed the factory flywheel HP WILL go up If you start with a stock engine adjust the dyno flywheel mass to match the factory HP If you remove the factory flywheel adjust the dyno flywheel mass to to match the last run with the flywheel
                     Peter


                    [Non-text portions of this message have been removed]
                  • Johan
                    ... It s at peak torque rpm. In 2-strokes peak torque rpm is close to peak hp rpm. For the Rotax 477 is this 6000 and 6500 rpm.
                    Message 9 of 9 , Oct 3 1:38 PM
                      > I assume this is at peak hp rpm?

                      It's at peak torque rpm. In 2-strokes peak torque rpm is close to peak hp rpm. For the Rotax 477 is this 6000 and 6500 rpm.
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