Go Back  PPRuNe Forums > Wannabes Forums > Professional Pilot Training (includes ground studies)
Reload this Page >

Altitude of Commercials Aircraft : economic reason

Professional Pilot Training (includes ground studies) A forum for those on the steep path to that coveted professional licence. Whether studying for the written exams, training for the flight tests or building experience here's where you can hang out.

Altitude of Commercials Aircraft : economic reason

Old 31st Mar 2020, 07:26
  #1 (permalink)  
Thread Starter
 
Join Date: Jan 2016
Location: Norway
Posts: 69
Altitude of Commercials Aircraft : economic reason

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.
ced0802 is offline  
Old 1st Apr 2020, 06:58
  #2 (permalink)  

 
Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 69
Posts: 3,927
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.
paco is offline  
Old 1st Apr 2020, 08:26
  #3 (permalink)  
Thread Starter
 
Join Date: Jan 2016
Location: Norway
Posts: 69
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?
ced0802 is offline  
Old 1st Apr 2020, 08:33
  #4 (permalink)  
Gender Faculty Specialist
 
Join Date: Mar 2002
Location: Stop being so stupid, it's my turn
Posts: 1,730
Reduce your ROC.
Chesty Morgan is offline  
Old 1st Apr 2020, 10:26
  #5 (permalink)  
 
Join Date: Nov 2016
Location: FLSomething
Posts: 166
Originally Posted by ced0802 View Post
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 doesnít know or care what TAS youíre doing, hence the speedo doesnít shot TAS. TAS is calculated and is then displayed, it canít be measured directly. Itís your IAS that gives you your lift and what the wings think theyíre 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 itís 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.





Don't get too hung up on it, thereís some lovely CDMVT charts that youíll 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 🤷‍♂️
VariablePitchP is offline  
Old 2nd Apr 2020, 07:42
  #6 (permalink)  
Thread Starter
 
Join Date: Jan 2016
Location: Norway
Posts: 69
Thanks a lot guys !
ced0802 is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.