Mach # gear limitation

[palgia]

Hello all.

As I was looking at a poster of the 744 cockpit I noticed that the gear operation is limited at max .82M (I then saw that many other aircraft have this sort of limitation)

What is the reason for a Mach number as a limitation for the landing gear extended/extention speed?

Is there concern for shock waves forming on the gear? (I guess there is otherwise they wouldn't have put it there:))

So my next question is why would the gear develop enough shock waves to be of concern, at a lower Mach than the wing does?

Not sure what the exact Mmo is on the 744, but I'm sure its more than .82M


Thank you in advance for your answers.

palgia

[Mad (Flt) Scientist]

That's pretty fast. I'm surprised they bothered to be honest - I'd expect a combination of a max IAS and a max altitude to override any need for a Mach limit. (i.e. any applicable Mach number ould be aoutside the IAS/Alt envelope).

It must be related to the gear mucking up the flow on the wing; maybe they had asym rolls due to that? You'd expect the gear - which is aerodynamically an ugly bit of junk sticking out of the wing - to be a great deal worse than the wing alone, which is why you might have a limit lower than Mmo say.

I'm still surprised a Mach limit was necessary, though.

[Notso Fantastic]

This is for the emergency descent case as in depressurisation. In the old days on 707/VC10 type equipment, it was a discussion point as to whether it was better to commence an immediate descent with speedbrakes out or slow first to gear lowering speed with speedbrakes out, then lower the gear and let rip. The problem with the 747 is the transition with gear extending/retracting when extra landing gear doors are open and it's not a good idea to blast them with 550 mph airflow. Nowadays, we don't bother with the gear- with full speedbrake and accelerating to near IAS speedlimit, you get down as rapidly as you can handle with all the other things going on at the same time.

[Old Smokey]

palgia,

It's not at all uncommon to see a Mach Number limit for Gear AND for Slats / Flaps. In fact, for 6 of the 7 heavy jet aircraft which I've operated, predominantly Boeing, all had a Mach limit for the gear (the odd one out was A300 and I cannot recall, it was too long ago).

In the case of the gear, there are 2 main concerns -

(1) The very unaerodynamic gear assembly has a considerably lower Mcrit than the rest of the aircraft. Drag increase beyond it's Mcrit would be rapid and severe, causing excessive pitch-down moments, and

(2) Shock wave formation on the gear door assemblies causes high torsional stresses (as opposed to dynamic pressure from the subsonic consideration). These torsional stresses oscillate due to the fairly low rigidity of the doors, causing flutter and inevitable separation from the aircraft.

Some manufacturers (like Douglas used to) published a Max Operating Mach No. for Slats / Flaps, others simplify the situation by imposing a cut-off Flight Level (Typically F/L 200) for Flap operation at which Pressure Height the Flap Limit CAS is below the limiting Mach No.

(Away for 6 days - look forward to further responses)