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Aircraft Performance

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Aircraft Systems

I was looking at the C172S CRUISE PERFORMANCE TABLE. @ 4000 ft PA and: 75% power 119 kts 11.7 nmpg 65% power 112 kts 12.4 nmpg 55% power 104 kts 13.2 nmpg I understand the relationship between the power and the IAS (power decreases, IAS decreases ), but what I don't understand is the relationship between the power and the fuel consumption. why does the fuel consumption decrease when power increases? Another thing I don't understand is why the nmpg increases as altitude increases using the same power setting. For example, at 75% power, nmpg is 11.7 at 4000 ft and 12.2 at 8000 ft?

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3 Answers

  1. John D Collins on Nov 10, 2013

    Fuel consumption increases with increasing power. I think you are reading the fuel values incorrectly as if they were miles per gallon, the nmpg value is nautical miles per gallon, in other words a higher number of nmpg is better than a lower number in terms of efficiency. 11.7 nmpg at 75% at 119 Kts would convert to GPH by dividing the 119 by 11.7 = 10.17 GPH. In the same way the other two power settings can be converted to GPH, so 65% is 112/12.4 = 9.03 GPH, and 55% is 104/13.2 = 7.88 GPH.

    The reason that nmpg increases with altitude at the same power setting is that the air is less dense at the higher altitude and provides less drag. Since the drag is lower, the true airspeed at the same power increases. Note that the fuel consumption will be the same for the same power (10.17 GPH), but the true airspeed increases. This makes the efficiency nmpg increase. If you check your POH data, it should show the TAS at 8000 to be 10.17 GPH times 12.2 nmpg = 124 Kts.

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  2. Lion on Nov 10, 2013

    Thank you John,

    You are right, I was reading the values incorrectly; I was thinking of the numbers as gallons not miles….Interesting how sometimes simple thing overlooked!!!

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  3. Jeffrey A. Baylor on Feb 19, 2014

    In addition… something called a “stoichiometric” ratio. At sea level on a standard day of 29.92 in Hg and 59F air weighs about 14.7psi and at 18,000 or FL180 it’s about half of that weight in density for a given volume. So…in order for an engine to run efficiently it must have a particular fuel to air mixture to support combustion. If I increase in altitude where I have less air but I fail to reduce the fuel volume accordingly, I burn increasingly richer mixtures until the engine “floods” itself out. But… if I reduce fuel input in conjunction with that increase in altitude, I reduce overall fuel consumption. The trade is less fuel and air to my engine = less power as we go higher. That now takes us to turbo-charging or super-charging or turbo-supercharging to keep “rated” power to altitude!

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