Altitude of Commercials Aircraft : economic reason

[ced0802]

Hello everyone,

I have read that commercial aircrafts fly at a certain altitude for several reasons, with one of them being for "economic" reason.
I would like to understand why. I know it has to do with the relationship between IAS & TAS, but not sure to grap correctly why.

My understanding :
As we fly higher, less air molecules will fly over the wings as density is decreasing.
To maintain our lift, we therefore need to increase our speed (so our TAS increase) to generate more air to flow over the wings.
But more TAS, wouldn't mean more thrust and therefore more fuel? I am definitely missing something here :)

Thanks in advance.

[paco]

The point is that the drag value remains the same, so you get "less" drag for more speed. But there is a practical reason why there is a maximum altitude - the stalling speed increases as well, so you will get bunced around, but the speed of sound also reduces - and you will get bounced around! To keep the flight smooth and not bend the aircraft, you need to back off from both of those and let the computer fly.

[ced0802]

Thanks a lot Paco.
Where I am missing something I guess is, as we go higher, our IAS decreases due to Density decreasing. TAS must increase to maintain our lift.
But how can we generate more TAS without adding power/thrust?

[Chesty Morgan]

Reduce your ROC.

[VariablePitchP]

Thanks a lot Paco.
Where I am missing something I guess is, as we go higher, our IAS decreases due to Density decreasing. TAS must increase to maintain our lift.
But how can we generate more TAS without adding power/thrust?

The aircraft doesn’t know or care what TAS you’re doing, hence the speedo doesn’t shot TAS. TAS is calculated and is then displayed, it can’t be measured directly. It’s your IAS that gives you your lift and what the wings think they’re doing. It reduces for a given TAS due to compressibility effects, more obvious at altitude.

That this relates to a certain TAS is only relevant for you really when it’s converted to your Mach number, which their will be a limit of. Have a look at the picture below. Quite happily at 260kts, TAS is about 455, which puts the Mach number at 0.79.


https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1024x683/6e7518a1_53ab_4869_b0de_01c2aaaf25c5_bf9ef6442101560ceac6e4a 3e712c969d6c683cf.jpeg


Don't get too hung up on it, there’s some lovely CDMVT charts that you’ll be fed to learn it all, and the question bank for when you actually take your exams. And when actually flying, just avoid the red bits 🤷‍♂️

[ced0802]

Thanks a lot guys !