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Old 26th Feb 2019, 17:34
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oggers
 
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Originally Posted by double_barrel
Looking at the figures for endurance/range in the C172M handbook, I see that it gives figures for various engine rpm values and altitudes, but assumes max gross weight for all. This led me to wonder how much influence weight has on endurance. I guess that there are a bunch of other factors too, obviously mixture setting is one, and I suppose that trim will have a significant influence on drag and thus range? How important are these factors? Is there any way to quantify them?
Theoretical best endurance will occur at the minimum power speed in level flight. If the weight is reduced minimum power will also be reduced and it will occur at a lower speed. If the performance data in the POH doesn't give your endurance speed you would find it by slowing down until you start to have to add power back to maintain level flight, then speed up just enough to get yourself back on the right side of the power curve.
  • As mentioned trim and cg are factors. A rear cg although less stable is more efficient aerodynamically as the tailplane is providing less downforce.
  • Altitude: unlike a turbine, the piston is not optimised for cruise efficiency at 36000'. Lower is better. The aerodynamic reason is that for any given IAS your TAS is lower at lower altitude and therefore so is your power because power is TAS x drag.
  • Mixture would be leaned to 25LOP if permitted, or recommended best economy for your aircraft. 25LOP is theoretical best specific fuel consumption for the Lyconental engines ie it is lowest fuel flow per horsepower produced, but it might not be recommended.
  • RPM with a VP would be low eg 1800. Volumetric efficiency is higher at low RPM. For a fixed pitch you don't have a choice, the RPM wil be whatever it needs to be to maintain speed.
All the above apply to flying for best endurance as per the thread title. Theoretical Best Range is slightly different, you fly at minimum drag speed (L/D max, also best glide speed) instead of minimum power, and you operate at the altitude where you can just get the requisite power with a wide open throttle (or somewhat below to give you some reserve power for maneuvering). In a headwind your best range speed would occur somewhat above minimum drag speed and vice versa. A rule of thumb is to add or subtract quarter of the headwind component.

Last edited by oggers; 27th Feb 2019 at 07:23.
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