one for the aerodynamics forum

[tprop]

Can you safely sideslip a light/medium/heavy jet?
How much? Why not? Have you tried it? Will the DFDR rat you out if you do?

[Mad (Flt) Scientist]

For a FAR.25 certified aircraft, paragraphs 25.147 (Directional and lateral control) and 25.177 (Static lateral-directional stability) together effectively require that the aircraft be safe and controllable (as defined in the details of the paras) up to full rudder angle or high pedal forces, whichever is encountered first.

Therefore, aerodynamically, any FAR25 certified aircraft can be sideslipped to the maximum rudder capability throughout the flight envelope. (JAR 25 rules are similar, and the rules haven't changed a huge amount, so that statement is pretty much generally applicable)

There are, howvere, other limitations of the designs which may limit the capability of the aircraft: fuel systems, for example, may not be designed for sustained lateral acceleration and lateral fuel migration and engine fuel starvation may result from a sustained sideslip.

Other assumptions used during the design may also be violated if the aircraft is routinely sideslipped - loading cases for e.g. gusts usually assume that the initial aircraft condition is trimmed wings level; a design-limiting gust acting on a sideslipping aircraft could conceivably exceed the design loads for the aircraft. And, of course, this assumes a slow and steady control application - stamping on the rudder at high speeds is never a good idea.

These requirements are met through demonstration in certification flight test.

And the DFDR will probably reveal what was going on if someone bothers to check - lateral acceleration and rudder angle are usually recorded, and will be a fairly conclusive indication of what was going on.

[xyz_pilot]

Why would you want to do a max sideslip in a pax a/c?

[Milt]

Sideslipping Heavies

Presume the questioner wants to know whether sideslippng a heavy is a useful manoeuvre.

Effective flaps and high lift devices which also substantially increase drag have long since negated any usefulness from sideslipping to increase an angle of descent.

So there is a lot of consideration in design to keep sideslipping to a minimum. It increases drag and particularly with swept wings introduces strong rolling forces.

Maximum sideslipping for a multi engined aircraft usually occurs at Vmca when the pilot will have maximum rudder input that he can provide to offset asymmetric thrust.

There may still be the odd aircraft around which suffers from rudder overbalance. This is nasty and in the worst case will cause the rudder to lock hard over. TPs approach that area very carefully.

[Volume]

Two obviously very good pilots already demonstrated, that it can be done, and saved an aircraft that way.
The 767 that ran out of fuel in 1983 was landed using a sideslip, as the spoilers were not available.

Six miles out Pearson began his final approach onto what was formerly RCAFB Gimli. Pearson says his attention was totally concentrated on the airspeed indicator from this point on. Approaching runway 32L he realized he was too high and too fast, and slowed to 180 knots. Lacking divebrakes, he did what any sailplane pilot would do: He crossed the controls and threw the 767 into a vicious sideslip. Slips are normally avoided on commercial flights because of the the tremendous buffeting it creates, unnerving passengers. As he put the plane into a slip some of Flight 143's passengers ended up looking at nothing but blue sky, the others straight down at a golf course. Says Quintal, "It was an odd feeling. The left wing was down, so I was up compared to Bob. I sort of looked down at him, not sideways anymore."

The complete story can be read
here (http://www.casa.gov.au/avreg/fsa/03jul/22-27.pdf)

[Mad (Flt) Scientist]

Maximum sideslipping for a multi engined aircraft usually occurs at Vmca when the pilot will have maximum rudder input that he can provide to offset asymmetric thrust.

Sideslip angles for Vmca/l and for a max crosswind are probably going to be similar. Say up to about 10 degrees. Operationally you'll rarely if ever need sustained sideslips, but the aircraft will be theoretically capable of some greater level of sideslip.

[Intruder]

Sideslip angle on landing in a crosswind will also be limited by the angle at which the wingtip or engine pod/prop will hit the ground. In the 744, that's about 8 degrees. For very heavy crosswind, crab is used in conjunction with a slip of 5 deg max, and the airplane can be landed in the crab. Inertia straightens out the airplane on the runway very nicely, as long as you are tracking straight at touchdown.