Wednesday, September 23, 2015

Shake, rattle and roll

Here's an interesting video of an Airbus A380 airliner (the largest in the world) landing in a crosswind at Düsseldorf airport in Germany.  Note how the empennage, or tail assembly, continues vibrating and twisting from side to side as the aircraft decelerates after landing.  That's not just wind moving the tailplane;  it's the fuselage flexing around its longitudinal axis.  The aircraft is clearly designed to allow its parts to move in relation to each other - strength through flexibility, in other words.

Contrast that to the comments of a USAF C-5 Galaxy pilot that we read about here recently:

"The C-5 was designed in the 1960s and there are still some flying from the original production line. It was designed in the days when it was thought the best course of action for wings was to make them as stiff as possible."

There's more at the link.

I've noticed that difference myself, in other, smaller military transports of that vintage.  Stiffness to prevent such flexing of the fuselage is - or, rather was - regarded as a handicap in an aircraft that might have to land on rough fields and unprepared surfaces.  The flexing might become so great that parts would break - and getting such an aircraft out of a location like that with major damage might be a non-starter.  Nowadays, almost all aircraft are built to 'flex' more than earlier models.  Compare, for example, the current-production C-130J Super Hercules with the original version (I flew in the South African Air Force's C-130B's).  You can watch the different behavior of the wings in video clips.



Judy said...

As an engineer I worked with put it about a 737 that was 1.25 million parts flying in close formation.

Anonymous said...

I'm wondering how much of the twisting of the empennage is due to the exhaust stream from the engines affecting the horizontal stabilizers. I see a lot of movement in the horizontal stabs but not very much in the vertical stab. Doing engine runs on a Kingair C90 the entire airplane shakes due to prop wash over the low mounted horizontal stabs, while a power run in a Kingair 200 with its T-tail above the prop wash is much calmer.

Regarding Hercs, Peter, you're talking first of fuselages and then of wings. The fuselage has to be very rigid on a Herc or any other aircraft with a cargo ramp. If it were too flexible it would distort when loading/unloading and it would be impossible to get a good seal around the ramp in order to pressurize the aircraft. Imagine you have a pair of Bradleys strapped to the floor of a Herc and you encounter turbulence. When the plane drops (-g) there are suddenly many tons of force pulling the floor upwards, when the plane climbs (+g) all that force reverses and pushes the floor down.


Old NFO said...

Airbus has a tendency to 'move' the tail all the time. It's built into the computer software to try to give a smooth ride, AND it's not a stiff fuselage compared to a Boeing to start with.

Will said...

The stabilizers start bouncing around as soon as the wing spoilers deploy, which is done before they even get the aircraft pointing straight. I imagine that the thrust reversers are engaged at the same time. The combination of that suddenly disturbed airflow hitting the tail would seem to be the major influence on their movement.

This aircraft would seem to be a good candidate for steering main gear, like the B-52, as the side load on the tires and gear must be immense, judging by how quickly it is forced to track straight on the runway by those multi-bogy mains. I suspect that the tires aren't rated for many of those types of landings.

You can also see the large degree of wing droop when those spoilers deploy after it hits the runway.

Keith Glass said...

From my days flying B-52s, on the ground, with full takeoff fuel load, the wingtips are MAYBE 5 feet off the ground. That's with three feet of landing gear and two levels up to the wing root, which starts above and behind the crew compartment, call it 18 feet, so 13 foot flex downwards.

In flight, at altitude, wings flex upwards at least 10 feet above the wingroot. So, we're talking 23-25 feet of flex. . .