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Inc Perf above Cross Over ???

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Inc Perf above Cross Over ???

Old 30th Jan 2009, 11:11
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Inc Perf above Cross Over ???

Can anyone tell me why above cross over altitude, the climb performance improves?

I have looked in several books, but they only make tell me that the perf increases.

Thanks

Last edited by PAPI-74; 30th Jan 2009 at 11:40.
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Old 30th Jan 2009, 11:45
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Perhaps that's because above the crossover point you climb at constant Mach, which means decreasing IAS (closer to bast R.O.C. speed)? Just my guess...
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Old 30th Jan 2009, 12:10
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Thanks, but there is a definate pitch up at x over and it is an LPC question as to why.

Polar curve
Mach meter is at ISA

Dunno???
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Old 30th Jan 2009, 12:27
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Thanks, but there is a definate pitch up at x over
That makes sense - the a/c pitches up to maintain Mach number (ie. decrease IAS)
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Old 30th Jan 2009, 13:05
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Constant IAS = increasing TAS in the climb. Once on constant Mach No, TAS also pretty constant, IAS reduces and ac pitches up. Happens about FL300 ish in the 75
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Old 30th Jan 2009, 14:21
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During the climb at a constant CAS, the TAS is increasing, thus, a portion of your excess thrust is used to ACCELERATE the aircraft.

At Cross-over, Mach is constant and CAS and TAS are decreasing, thus (1) the excess kinetic energy of the aircraft is translated into climb performance as the aircraft DECELERATES, and (2) the portion of the excess thrust previously used to accelerate the aircraft is now available for improved climb performance.

Regards,

Old Smokey
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Old 30th Jan 2009, 16:30
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Airbus doc

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Old 30th Jan 2009, 21:31
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Perfect - thanks guys.
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Old 31st Jan 2009, 01:39
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At Cross-over, Mach is constant and CAS and TAS are decreasing, thus (1) the excess kinetic energy of the aircraft is translated into climb performance as the aircraft DECELERATES, and (2) the portion of the excess thrust previously used to accelerate the aircraft is now available for improved climb performance.
A superb explanation, and true from the B707...onwards.
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Old 31st Jan 2009, 14:48
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So how did it work before the B707?
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Old 31st Jan 2009, 15:21
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So how did it work before the B707?
Comet excepted, there were no jet transports prior to the 707, so the reference to mach number crossover was not a consideration in the climb.

However, of one looks closely at the limitations for some 4-engine piston transports, there will be a reference to maximum mach number, which could be a consideration during descent.
This was investigated during original flight test (with a mach meter fitted) however the aircraft, when delivered, had the mach meter removed.
Instead, the Vne was listed as a sliding scale, dependant on altitude...IE: the higher the altitude (beyond a certain point), the lower the Vne.

Mach number effects were a definite consideration on two specific 4-engine piston types, the DC-7 and the L1649 Constellation, as these two aircraft were designed for long range high(er) altitude, high(er) speed flight.
Specifically for descent, both of these types were fitted with 'speed brakes'...although not the type you might think....the main landing gear could be lowered (independantly of the nose gear, which remained retracted) for speed reduction.
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Old 31st Jan 2009, 20:50
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Did you have a "barber pole'' on the ASI (Air Speed Indicator) to denote the changing Vne values, (what really was Mmo, I guess) or did you just have a look-up table in the Constellation and the DC-7?

I remember asking someone what value that barber pole was using in a Cheyenne turbo-prop when I got just a shrug. I went home and worked out the different values it gave and found it meant something like Mach 0.33! Coming from the Twin Otter I had never seen anything such as a barber pole before and transonic flow and a Mach limit were just abstract concepts.
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Old 1st Feb 2009, 01:10
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Did you have a "barber pole'' on the ASI (Air Speed Indicator) to denote the changing Vne values, (what really was Mmo, I guess) or did you just have a look-up table in the Constellation and the DC-7?
No barber pole on the 1649 or DC-7 (nor Stratocruiser, which was in the same category of high altitude flyer)...you had to make corrections manually (table), if necessary.
It was rarely a problem however, as normal descent speeds were operationally lower.
The Lockheed Electra had one however, as that airplane, as originally certified, could outrun a 707 at lower altitudes (Vmo limits).
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Old 1st Feb 2009, 09:38
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WOW! My hat off to you.
With everything else to deal with in that generation of aircraft, I bet you guys had your hands full - but rewarding at the same time.
I would love to fly an old 727, 707 and ditch the glass EFIS for a bit. EFIS does make life very simple at times.
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