Question for Aerodynamics Experts

[Jr. Airman]

Can someone explain why the 777-200 and 777-300 have
different Mmo Limits (-200 is .87 while the -300 is .89)

They have the same Vmo limits, and apart from being longer and heavier the -300 uses the same engines and wings.

[Uptrim Disable]

Sounds strange! Are you sure it isn't the other way around? The heavier the a/c the lower the Mmo and Mcrit?

[FlyingForFun]

Warning... uneducated reply coming!

As far as I know, Vmo and Mmo are not related at all. Vmo is a speed above which there's a danger of the aircraft breaking up due to the dynamic pressure forces and associated stresses. Mmo is a speed above which there's a danger of the aircraft breaking up due to the shockwaves on the wings. So, where you say that "they have the same Vmo limits", I'm not sure that's relevant.

Also, I don't see how the weight of the aircraft would affect Mmo. It may well affect Vmo, since the wings will have to provide more lift, and will therefore be subject to more force in all stages of flight. But the mass of the aircraft wouldn't affect the shockwave formation.

I'd have thought shockwave formation would depend mainly on the shape of the wing. So if the two aircraft have the same wing, then the only reason I can think of for the different Mmo would be the shape of the aircraft as a whole affecting the airflow over the wing, presumably giving a lower local airspeed over those parts of the wing where shockwaves are most critical.

But, like I said, this is largely guess-work. Will be interesting to hear from someone who actually knows!

FFF
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[Jr. Airman]

Very sure about the Mmo numbers. They behave identically below round about 30,000' but after that the -300 is a whole different sort of bird.

FlyingForFun, I merely made the Vmo and weights comment more as an observation rather than anything else.

Thanks for the answers though. Would love to hear other views. If anyone could post an answer with a link to more information would be all that much better.

[LOMCEVAK]

I know very little about the Flight Control System of the B777 so the following is pure hypothesis. I suspect that the difference is related to the ability of the FCS to provide adequate flying qualities at high Mach number. Is the FCS software the same for both versions? Perhaps you should consider the difference in Mmo from the view that it is reduced to 0.87M in the -200, possibly due to the functioning of the FCS between 0.87M and 0.89M. If the -200 is shorter and lighter than the -300 then its pitch inertia will be less and perhaps the tail moment arm is less (shorter distance between the c.g. and horizontal stabiliser/elevator). In this speed regime of transonic airflow there may be some aspects of stability and control that are very sensitive to these issues. This may be the reason for the reduction in Mmo.

FFF, for the record Mmo is far more likely to be related to stability and control/flying qualities aspects than potential structural failure.