[quote=ChristiaanJ]
VLA (lowest admissible speed)
One would expect a curve for constant alpha max against IAS and altitude, not the staircase in the diagram.
Was this for simplicity of use of the diagram?[quote]
I don't have a complete explanation for all the regions - it was a long time ago and I'll need to dig, but:
Below 16000 ft Vla obviously needs to go as low as Vref to cover landing at elevated airfield altitudes. At present I don't have a satisfactory explanation for 250 kts between 16000 ft and about 45000 ft (250kts/Mach 1.0) A constant value in IAS is what you would get for a constant CLmax (the alphamax is not really the driver). Vla should give a margin above stall, and a quick sum suggests that 250 kts would be consistent with a 1.3Vs condition and a CLmax of about 0.8 up to Mach 1.0, which is not unreasonable, but I am not saying that is the correct interpretation.
From 45000 ft to 60,000 ft I think Vla may be set by manoeuvre requirements. Certainly the forward CG envelope boundary between 1.0M and 1.5M discussed in earlier posts is very close to the requirement to be able to pull 1.2g with half hinge moment available at Vla and heavy weights. Again not certain, but best guess at the moment.
Max altitude (60,000ft)
This is the 'simplest' one: it was the highest 'safe' altitude from which an emergency descent could be made, in the case of a window blowing out, without having the blood of the pax boil....
Test flights (without pax, and with the crew pressure-breathing oxygen) did go as high as 69,000ft.
Yes
[quote
]Mmo (max operating Mach number)
Mach 2.04 is usually quoted as having been chosen to assure an adequate life of the airframe.
But what effect does a higher Mach number
as such have?
Or are Mmo and Tmo (127°C) directly related?[quote]
I have always been puzzled by this statement as one does not normally associate Mach Number with a life limit. Going through my collection of lectures I found another, more plausible explanation:
quote" The scheduled cruise mach Number was 2.0. associated with a structural total temperature of 400 degK. Above ISA +5 Mc was cut back to maintain 400 degK.
To cope with variations of flight mach Number about Mc associated with often rapid and significant changes in wind and temperature which occur particularly in the vicinity of the tropopause (which can of course be as high as 60,000 ft in the tropics) a maximum operating Mach Number (Mmo) of 2.04 is selected" unquote [Leynaert, Collard and Brown, AGARD Flight Mechanics Symposium October 1983]
This is much more in line with my memory on this subject.
Vmo (max operating speed) = 530kts until 43,000ft
I suppose this is related to structural limits (qmax)?
Yes in principle, but it is a bit chicken and egg, since 530 kts also represents a very good choice for best performance, and I am sure that the stucture would have been built to cope if a higher speed was needed for performance reasons. To the best of my knowledge there is no structural design case that would be critical in this flight regime (other than flutter of course)
Vmo reducing to 380/400kts at about 33,000ft
What is the limiting factor here (other than qmax)?
Same as earlier - transonic manoeuvre requirements with failed hydraulics, matched to aircraft weight and CG envelope possibilities.
Vmo constant at 380/400kts down to 5,000ft
What is the limiting factor here? The answer will no doubt also explain why this is slightly weight-dependent.
Same again.
Vmo reducing to 300kts between 5,000ft and 0 ft
Why the sudden change below 5,000ft?
I'm not entirely sure, but:
a) there is absolutely no advantage is having a high Vmo at low altitudes as it could not be exploited even if one wanted to because of ATC limitations to 250 kts below 10,000 ft (in the USA at least)
b) there are a lot of things that get rapidly worse if you encounter them at high speed and which are anyway more likely at low altitude - hail, birds etc.
So why store up trouble for yourself!
CliveL