Cessna 172 Cruise Speed

Cruising at 10,000ft with my Cessna 172. I set Auto Pilot Speed to 110 but it never gets past 97.
What am i missing?

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I think you might be a bit high for a Cessna 172

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Did you lean the mixture?

I think it’s OK :)

Oh, i usually never fly it that high…

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Me either, I just googled it :)

"As a general rule, a naturally aspirated combustion engine (not turbocharged) will lose 3% of its power for every 1,000 ft of elevation gain. If you have 100 horsepower at sea level by the time you get to 5,000 feet of elevation your engine is making 85 horsepower.

At 10,000 feet of elevation your engine will make 70 horsepower.
At 15,000 feet of elevation your engine will make 55 horsepower.
At 15,000 feet of elevation your engine will lose 45% of its power due to lower air density."

Turbocharging At Elevation - Garrett Motion

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Bruh really is that the AI overview? You can tell right from that it’s not sure how cruise altitude and max altitude relate. It’s usually wrong about most things related to aviation. 10,000 ft is definitely on the high side for a 172.

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Yes. Google Gemini

With increase in altitude, IAS decreases for a given TAS. While your indicated may show lower, your TAS should still be semi-accurate with performace for that altitude

But the

is IAS.

exactly. autopilots sets IAS. but since IAS decreases with altitude for a given TAS, you get the situation where you can’t reach the same IAS at 10,000 as you can at sea level (aircraft performance is based on TAS)

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What is the achievable TAS of a C172 at 10,000k feet vs sea level given the effect of a 30% or so reduction of engine power’s effect in dropping IAS at that altitude?

Does the loss in engine power make up for the gain in TAS relative to IAS?

This is not from the AI source, but rather an aviation website

i’ll use the C172S performance charts for this

performance using lowest chart (2000) shows, at 2550 rpm (77% of MCP with recommended lean mixture) at ISA, a TAS of 118kt can be expected. at Standard pressure and ISA, this puts our IAS at 116kt

at 10000 at ISA, at 2700 rpm (72% of MCP at recommended lean). a TAS of 123kt can be expected. Using ISA and standard pressure, this puts our IAS around 102kt

there are obviously going to be some variables here, such as temp and accurate modeling of performance in IF. but at 10k, only a difference of 5 knots (102IAS based on chart and temp/pressure alt correction vs 97IAS in game)

IAS formula i used, not entirely accurate i believe but close?
TAS = (IAS / (1 + (altitude in feet x 0.00002))) + ((Current-ISA)/5)

sidenote: if you redline it at sea level ,you will get faster speeds than the 118TAS the chart shows. as far as the 30% reduction due to the rule of thumb of 3%/1000ft, is close, you have 72% of available power at 10k (shown as % of MCP)

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Thanks. That look pretty good. Good reference info too!

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That’s the website the AI overview got the max altitude from if you look closely @SJSharkie’s AI overview result says confuses the max altitude with the cruise altitude, which the aviation website excerpt does not.

It’s honestly probably a little closer imo. He was probably not leaned, bc who does that in IF, and at 10,000 feet that effect isn’t small. Now he was also probably juicing the engine for all its got and overspending the prop in the process since full power in level flight isn’t really possible in the C172, so that might add a little extra speed the manual doesn’t account for, but the leaning will be a much bigger factor.

For what it’s worth, going to be a real pain in IF to get “recommended lean mixture” because this means 50 degrees rich of peak. In the real airplanes with a G1000 we can super easily see this through lean assist, but in infinite flight it isn’t so easy. You can see EGT though, so it is possible. Getting as close to that as possible will definitely help match that chart if people are trying to do this.

Sort of overly pedantic, but over 75% MCP “recommended lean mixture” is full mixture. If OP was firewalling it though, he also may have been above 75% MCP which would invalidate my prior point, but as indicated on the chart 72% MCP is just about the prop limit at that speed so anything above that and I can’t help you from the manual lol

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Also, for those saying 10,000 is too high for a C172, while you might be correct in practical terms, here is the manual from Cessna saying it is good up to 14,000!

I can’t help but wonder if it’s able to get higher as well, but they cut it off because it has no oxygen system and 14,000 is where it is required at all times for crew. Given that it can’t be certified to fly any higher. 14,000 is probably really pushing it though.

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So in terms of the big picture for the op’s question?: engine power decreases with altitude (as air “breathing” density drops)

…which reduces IAS (as the op observed),

…but you get more TAS (speed over the ground, ignoring wind), for a given value of IAS

…as the smaller drag from reduced IAS lowers how much power is needed to sustain the faster TAS (growing in relation to IAS as you go higher)

And you’re saying optimizing mixture would further optimize available power at a given altitude (to the extent it’s modelled),

…and that there is a limit to the fixed propeller’s ability to deliver full power with TAS maxed out (as occurs in level flight as opposed to in a climb; prop angle is fixed meaning it would have to spin too fast to “bite” sufficiently into the fast-moving air)

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