Why does the maximum allowable load factor vary with airspeed in some aircraft?

Question

In this ASE answer What is the maximum safe bank angle of a 747?, the author writes:

If we stay with stationary turns (without the "falling out of the sky" part), the maximum bank angle is given by the maximum load factor of the 747. At 400 KEAS, this is just 1.5g, so even with the 25° nose down attitude the maximum bank angle would be 53°. If you fly slightly slower, the load factor goes to 2g (equals 63° in a 25° descent) and tops out at 2.5g at 310 KEAS (68.7° in a 25° descent).

Why does the allowable load factor vary with KEAS in this aircraft?

Note that if the wings have any washout, the outboard portions of the wingspan would contribute less to any given total lift force at higher airspeed (lower angle-of-attack) than at lower airspeed (higher angle-of-attack), so for any given G-load, the higher-airspeed situation would seem to impose less upward bending moment on the wing structure.

Is it simply that imposing a drag load makes the wing less capable of absorbing the lifting load?

Answer 1

@JZYL is right. When I wrote the answer, I had buffeting limits in mind. Of course they can be exceeded at the cost of ride quality, but going further into buffeting will also reduce control effectivity. To be precise, the maximum safe load factor depends really on Mach and less on airspeed.

Your comment about washout is correct, and flying higher gs will increase washout from aeroelasticity. But rest assufred that those effects have already been applied when designing the structure for its limit load. Since the calculation was done for an airplane at 30.000 ft, the angle of attack for pulling higher gs is already close to what is needed at low speed and lower altitude.