"Very" High Speed Performace Question (Sorry)
Joined: Sep 2002
Posts: 113
Likes: 0
From: Hampshire
A few points...
Just a few points for consideration:
1. Once above about Mach 0.3 you must take compressibility into account when calculating TAS from IAS or CAS. The effect of it (scale altitude correction), i.e. the difference between CAS and EAS, becomes significant quite quickly.
2. Pressure errors (which affect the relationship between IAS and CAS) can do some crazy things in the transonic region (above about Mach 0.85) and can also be a bit odd when supersonic. For solving this problem a modern air data computer really helps.
3. Another consideration is the aircraft Specific Excess Power (SEP). This changes with height and speed and is a combination of engine performance and airframe drag factors. The best climb performance is obtained by flying speeds that optimise SEP and can involve climbing at a set speed, then descending to gain speed, then climbing again. An efficient climb is all about energy management.
4. The tropopause has a huge effect on climb and cruise performance because once through it you no longer benefit from reducing temperature and air density as you climb (temperature above the tropopause is constant).
5. You are probably wasting your time trying to back-calculate speeds from non-technical publications. When speeds are expressed in MPH there is no way of knowing if they converted from a Mach No of TAS, and whether the conversion used the sea-level Mach No or the Mach No at altitude.
6. All subsonic high performance jet aircraft I know of tend to use a constant IAS climb until a specific height, at which point a constant Mach No climb is conducted. Supersonic is a totally different ball game because getting through the transonic region requires an awful lot of thrust so this acceleration is normally done in level flight or a descent, before climbing again at constant Mach.
Hope this all helps!
WF
1. Once above about Mach 0.3 you must take compressibility into account when calculating TAS from IAS or CAS. The effect of it (scale altitude correction), i.e. the difference between CAS and EAS, becomes significant quite quickly.
2. Pressure errors (which affect the relationship between IAS and CAS) can do some crazy things in the transonic region (above about Mach 0.85) and can also be a bit odd when supersonic. For solving this problem a modern air data computer really helps.
3. Another consideration is the aircraft Specific Excess Power (SEP). This changes with height and speed and is a combination of engine performance and airframe drag factors. The best climb performance is obtained by flying speeds that optimise SEP and can involve climbing at a set speed, then descending to gain speed, then climbing again. An efficient climb is all about energy management.
4. The tropopause has a huge effect on climb and cruise performance because once through it you no longer benefit from reducing temperature and air density as you climb (temperature above the tropopause is constant).
5. You are probably wasting your time trying to back-calculate speeds from non-technical publications. When speeds are expressed in MPH there is no way of knowing if they converted from a Mach No of TAS, and whether the conversion used the sea-level Mach No or the Mach No at altitude.
6. All subsonic high performance jet aircraft I know of tend to use a constant IAS climb until a specific height, at which point a constant Mach No climb is conducted. Supersonic is a totally different ball game because getting through the transonic region requires an awful lot of thrust so this acceleration is normally done in level flight or a descent, before climbing again at constant Mach.
Hope this all helps!
WF
Joined: Nov 2009
Posts: 88
Likes: 0
From: Near Puget Sound
The reason civil transport limit themselves to 0.75 or thereabouts is to avoid the drag rise as the airflow goes supersonic over the top of the wing. Years ago, when fuel was cheap, we would cruise at 0.85 and you could see the shockwave on top of the wing.
Goldfish
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