Fastest True Airspeed

[coloradobluesky]

What altitude does fastest true airspeed occur? This is for a non-turbo, carbureted engine like an 0-360. Is it sea level? or sometimes something else? Never mind the gas mileage, fuel burn isn't important for this question. Just wondering what altitude the manufacturer figures the highest level TAS is?

 

[frcabot]

What altitude does fastest true airspeed occur? This is for a non-turbo, carbureted engine like an 0-360. Is it sea level? or sometimes something else? Never mind the gas mileage, fuel burn isn't important for this question. Just wondering what altitude the manufacturer figures the highest level TAS is?

Depends on all sorts of factors. Air deaf is decreased at higher altitudes but a non-turbo engine can't produce as much power. The sweet spot for me is around 6500 or so. Look at your POH performance tables.

 

[NoHeat]

The info you need is in the POH performance tables.

A tricky part of the comparison is asking what are you keeping constant, as you change altitude. RPM? Percent power? If it is % power that you are keeping constant, there will be a max altitude that you can attain that power, and that will sort of spoil the comparison.

 

[Pilawt]

Manufacturers usually quote "maximum" speed for normally-aspirated aircraft at sea level, because that's where maximum power is available. It's not efficient, and miles per gallon will be lower, but the brute horsepower overcomes the thicker air somewhat.

"Cruise" performance is generally quoted at 75% power. Manufacturers have settled on this arbitrary figure as a reasonable balance for speed, economy, engine wear and cabin noise; and some engines should not be run continuously above 75% rated power. The highest that a normally-aspirated engine can produce 75% power is in the neighborhood of 7,500-8,000 feet density altitude. So cruise performance is usually quoted there, where the air is thinnest (i.e., least drag) with 75% power still available.

Higher altitudes are more efficient (i.e., better range), but less engine power is available, so true airspeeds at full throttle are less, despite the thinner air.

Here's a screenshot from the POH for my C-172N.

 

[flyingron]

Generally, if you're going to run 75% (or some other less than full) power, the best true airspeed is obtained at the highest altitude you can maintain that power. As pointed out, range (notwithstanding winds) is always better at lower speeds/power settings for the same reason it is in a car (drag).