So a banking turn (essentially a normally configured turn) could under the right circumstances put the aircraft into a stall. There would be an altitude where this would occur with a "normal" banked turn and therefore to turn (banked) you would need to go to a lower altitude?. Or would doing this manoeuvre lose some altitude which you would need to recover to get back to assigned FL?
I understand that increasing speed will eliminate stall but there is a limit at high altitudes
Any time that you pull some 'g', be it in a turn, pulling up, or even pulling down (if you happen to be inverted), you increase the stall speed. The stall speed will increase as the square root of the g loading. So, looking at an A4, which had a clean stall around 110 knots, it would stall at 6 g at around 270 KIAS. In manoeuvring that sort of aircraft, the term "pulling to the judder" was used...it meant pulling until the aircraft was mildly shaking in the early stages of the stall. You'd hold it there by feel, sitting right on the edge of the stall, as the speed varied through the manoeuvres.
If you're flying fast enough, you'll hit the g limit before the stall IAS. So, if I was faster than 295 KIAS (or so, this all varies with weight), then I'd hit the 7.2 g limit before the judder.
Applying this to an airliner....At altitude, your maximum speed is limited by mach number. The IAS for a given mach number reduces as you climb. At sea level, maximum mach is well above the maximum IAS allowed (by about 200 knots). At somewhere around FL300, max IAS will be greater than max mach, and from that point upwards, mach is used. The effect of this is that as you climb, your speed is being limited by mach to a reducing IAS.
But, the wings work on airflow, and IAS is a measure of this. The stall IAS remains more or less the same as you climb. So, your minimum speed, as measured by IAS, remains the same. (True airspeed, TAS, is increasing, but is not relevant to this.) If we climb high enough, the mach limit will continue to reduce until it meets the stall IAS. That's the real coffin corner, but it's also way higher than any airliner can fly. If you happen to have a U2 in your hangar, you'll be familiar with it.
The IAS for the stall is the most obvious limit, but a speed somewhat above that is more relevant. Vmin drag is generally only a few knots slower than the actual cruise speed. If you slow below that speed, then the aircraft may not have sufficient power to accelerate again, so it will continue to slow. The only solution is to trade height. The power margin may be such that as little as 5 knots below Vmin drag would be the tipping point. And, when you pull that 'g', not only does the stall speed increase, but so too Vmin drag.
Airlines and manufacturers counter this by placing some level of margin into the aircraft performance data. In the case of maximum altitude, the FMCs calculate it with a defined stall margin, specified in G. The Boeing standard is 1.2, but some airlines (QF) increase this to 1.3. That means that nominally, you could roll to a 47º angle of bank turn before running out of margin. You wouldn't have sufficient power to offset the drag, but that's a somewhat different issue. The is no need for 47º, so it's a nice margin. By using the 1.2 margin, an airline will be able to climb higher/earlier than an airline using 1.3. But, they also run the risk of issues if they reach any turning point that is more than the usual slight kink. I've seen this once in the Middle East, where an aircraft was unable to maintain altitude at a turning point (I'm told they have nice champagne).
This has all been about increasing the 'g', but what happens to the stall speed if we reduce it below 1...or even go to zero?
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