Altitude Capability

[JIMBO01]

Hi Everyone,

Question. Today on the way out to the Atlantic entry point, establishing what our maximum altitude, optimum altitude and buffet margins will be at a given weight, temperature and altitude, I noticed that the figures for Mach 0.80 and Mach 0.81 were obviously different. But:

For an increased Mach number, the optimum altitude, maximum altitude and buffet margins were higher which surprised me as surely the higher the speed, the closer to coffins corner and buffet margins surely would be reduced.

Why is the increased speed giving higher numbers?

Thanks

[Mad (Flt) Scientist]

"Coffin corner" is where the high and low speed constraints come close together, ultimately leading to an altitude where at any except the perfect speed, you are either too slow or too fast. It's as much a problem with being slow as being fast.

So at some slightly lower altitude, the "point" speed becomes a range - with a lower and upper bound, below or above which you are again either too slow or too fast. And somewhere in the middle of that range of usable speeds is where you'd get the best in terms of margins.

So if, for instance, 0.79 was the lower bound at the altitude you were looking at, and 0.82 were the upper bound, it's entirely possible that 0.81 is a little better than 0.80. What's happening is that at 0.80, the low speed limits are predominating, and it's better to go a little faster and open up the lower speed constraint, as the upper speed one hasn't started to "bite" yet.

[FE Hoppy]

the power curve/drag curve is a.......curve.
Your cruise speed was obviously a tad behind the sweet spot for aerodynamic performance but probably chosen for cost/engine performance hence the aircraft could perform a bit better at a slightly higher cruise speed but at the cost of either fuel/engine life/overall economy (cost index).