JAA Performance - conflicting feedback
Join Date: Jun 2002
Location: Malaysia
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Flying the Maxi Range
Sorry, but the answer is not complete
First
Specific Air Range = (TAS/Drag) x (1/SFC)
Secondly
At low altitude the SAR will be found at 1.32 Vimd
Tertio
Over the optimum altitude for the Specific Air range when the compressibility effects appear (Vimd = Mach crit) the aircraft is flown at constant Mach Number. Below the tropopause the TAS will decrease as the LSS will decrease. Over the tropopause the TAS will be constant. For flying the Maxi range, everything will depend on the aircraft characteristics. A fast aircraft for example B747-400 is already flown at constant Mach number at relatively low altitude. Others aircraft will be flown at constant CAS up to the optimum altitude for the SAR then at constant Mach Number when the compressibility effects appear.
Suggests to read SWATTON (Aircraft performance Theory page 117 and followings)
I was forgetting one point: the Long range or the Maxi Range are quite useful after the lost of one engine (or 2 on a big A/C) in such a case even my 747-400 is flown at 1.32 Vimd and not a constant Mach (this Vimd will reduce with the weight in order to maintain the same angle of attack but will be constant for a given weight throughout the whole range of altitude) example B747-400 on 3 engines for 270t .==> 303 kt from FL50 to FL 250 the IAS reduction begin at FL 260 for a Mach Number of .746 where we can suppose the increase of drag becomes significant - typical range of work for the Mach trim) . the Long Range is also useful after depressurisation (Low altitude and low speed...)
First
Specific Air Range = (TAS/Drag) x (1/SFC)
Secondly
At low altitude the SAR will be found at 1.32 Vimd
Tertio
Over the optimum altitude for the Specific Air range when the compressibility effects appear (Vimd = Mach crit) the aircraft is flown at constant Mach Number. Below the tropopause the TAS will decrease as the LSS will decrease. Over the tropopause the TAS will be constant. For flying the Maxi range, everything will depend on the aircraft characteristics. A fast aircraft for example B747-400 is already flown at constant Mach number at relatively low altitude. Others aircraft will be flown at constant CAS up to the optimum altitude for the SAR then at constant Mach Number when the compressibility effects appear.
Suggests to read SWATTON (Aircraft performance Theory page 117 and followings)
I was forgetting one point: the Long range or the Maxi Range are quite useful after the lost of one engine (or 2 on a big A/C) in such a case even my 747-400 is flown at 1.32 Vimd and not a constant Mach (this Vimd will reduce with the weight in order to maintain the same angle of attack but will be constant for a given weight throughout the whole range of altitude) example B747-400 on 3 engines for 270t .==> 303 kt from FL50 to FL 250 the IAS reduction begin at FL 260 for a Mach Number of .746 where we can suppose the increase of drag becomes significant - typical range of work for the Mach trim) . the Long Range is also useful after depressurisation (Low altitude and low speed...)
Last edited by malaysian eaglet; 4th Sep 2003 at 00:31.
Thread Starter
Thanks very much to all those who contributed to this thread. I managed to pass the exam despite being somewhat bemused by certain areas of the subject.
Very best of luck to all those taking the exam in the future but beware the dodgy feedback!
Very best of luck to all those taking the exam in the future but beware the dodgy feedback!
