C172 Cruise Performance and temperature

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TopGun1234
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C172 Cruise Performance and temperature

#1 Post by TopGun1234 » Mon Feb 05, 2018 8:48 pm

Hello,

I've notice that for a Cessna 172, when you compare fuel burn rate vs temperature on the cruise performance charts the fuel burn will be higher at colder temperatures. E.g. for 2400rpm at Palt 2000ft & standard temp = 7.5GPH and for 2400rpm at Palt 2000ft & 20C below standard temp = 8.0GPH. Why is this the case. I thought aircraft performance would be better at colder temps.

Secondly, during very cold winters, how do you guys calculate your aircraft cruise performance (i.e. TAS & GPH) when temps are way below 20c below standard (or way below -5c). There are no columns beyond 20c below std.

Thanks!
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RocksAndProps
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Re: C172 Cruise Performance and temperature

#2 Post by RocksAndProps » Tue Feb 06, 2018 7:03 am

The engine performs better because the air is denser when it's cold. Same effect as if you go to a lower altitude. If you look at the standard temperature but change the PALT, you'll see that lower also means more fuel burn. Same concept. It's important to realize that the better performance doesn't mystically appear, it has to be sourced from more fuel.
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Chris M
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Re: C172 Cruise Performance and temperature

#3 Post by Chris M » Tue Feb 06, 2018 10:46 am

It's not just engine performance. The cold air is more dense, and while this gives you more engine power (and a higher fuel burn) it also gives you more drag (requiring yet again more fuel).

With regards to estimating performance at temperatures not in the book, you have to extrapolate based on the numbers given. Try to plot out the performance numbers in a chart and see what the relationship looks like. If it's a linear one you can pretty much just keep tracing that line along to wherever you need to be. When in doubt always err on the conservative side.
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photofly
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Re: C172 Cruise Performance and temperature

#4 Post by photofly » Tue Feb 06, 2018 11:57 am

TopGun1234 wrote:
Mon Feb 05, 2018 8:48 pm
Hello,

I've notice that for a Cessna 172, when you compare fuel burn rate vs temperature on the cruise performance charts the fuel burn will be higher at colder temperatures. E.g. for 2400rpm at Palt 2000ft & standard temp = 7.5GPH and for 2400rpm at Palt 2000ft & 20C below standard temp = 8.0GPH. Why is this the case. I thought aircraft performance would be better at colder temps.
Thanks!
"Better" performance: the word "better" is being used in two different ways, which is confusing you.

Engine performance is "better" at low temperatures and low altitudes because (at full throttle) you can pump more oxygen through the engine when the air is denser, and therefore maximum available power is higher: that means your takeoff run is shorter and your rate of climb is higher than when "hot and high". Of course you have to burn more fuel to get that extra power.

Aircraft performance is "better" in hotter air and at higher altitudes because the air is thinner there, so going from A to B involves less drag and therefore requires less fuel to make the same true airspeed; or, alternatively, for a given amount of power, you go faster. The airframe is more efficient under these circumstances.
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TopGun1234
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Re: C172 Cruise Performance and temperature

#5 Post by TopGun1234 » Wed Feb 07, 2018 8:27 pm

Chris M wrote:
Tue Feb 06, 2018 10:46 am
With regards to estimating performance at temperatures not in the book, you have to extrapolate based on the numbers given. Try to plot out the performance numbers in a chart and see what the relationship looks like. If it's a linear one you can pretty much just keep tracing that line along to wherever you need to be. When in doubt always err on the conservative side.
Thanks for the answers! I will make a chart.
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