Turn and slip

[chillie]

Please could someone answer a simple question for me.

If you are flying along straight and wings level but cross controlled i.e. Crabbing along, where will the ball be?

Thanks

[Amadis of Gaul]

If you're wings level, then you're not cross-controlled by definition. Cross-controlled means rudder is opposite to ailerons, as in a crosswind landing. If you're crabbing along, wings level, rudder neutral, the ball in centered.

[Microburst2002]

You can maintain a constant heading with crossed controls, but you have a slip angle because you have a lateral speed with respect to the air and the ball is, of course, down. The reason is gravity: since there is no turn and no curved motion, there is no centrifugal force and gravity alone acts on the ball.

This is what you do when you approach to land to a runway with a cross wind using the slip technique. You make the nose to be aligned with the runway using the rudder and apply enough bank with ailerons to stay on the extended centerline, thus making the lateral speed within the air equal and opposite to the crosswind component. but you can spill the coffee...

[Microburst2002]

In case you were using rudder and opposite aileron to maintain wings level, then that is another story:

You would be skidding, now, and turning (heading would change because of the rudder creating the slip angle) therefore the ball will be pushed out of the turn by centrifugal force. The larger the skid, the more the ball, and your body, will be pushed. Centrifugal force alone acts on the ball. Well, gravity does, too, but has no effect in the ball indication.

Coffe will spill, too...

[FCeng84]

If you are cross controlling and thus generating sideslip there will be an aerodynamic side force on the airplane. The classic forward slip involves dipping the up-wind wing to keep the flight path from turning.

If in a slip the wings are held level, the side force will cause a flat turn. The ball will be off center.

[Capt Pit Bull]

Straight (constant heading), Cross controls (out of balance), wings level.

3 items.

You can have any 2 of them you like, but not all 3.

I.e. This is an impossible question.

[Feather44]

I guess the ball will be deflected to same side than your dead feet.

[chillie]

2002

Thanks. Your first reply is what I expected. The Wikipedia article on turn and slip indicator says the slip ball does the same as a yaw string on a helicopter.

So, if you are flying straight and level with some yaw, the ball will be in the middle but the yaw string will be offset?

[AerocatS2A]

Wiki doesn't say that. It says they have the same purpose but It doesn't suggest they work the same way. If you have some yaw then you are not flying straight so the second part of your post is meaningless.

[Capt Pit Bull]

chillie,

Speaking as an instruments and POF instructor; You need to get your POF sorted out before trying to understand the turn and slip.

Your question is an impossible situation, so trying to establish what the turn and slip will be doing is equally impossible.

Until you appreciate that your question is impossible you are just wasting your time.

[chillie]

Thanks all for the replys.

CPB probably spotted that I am not a pilot. It was more of a theoretical question trying to figure out what forces the slip instrument measures. ( gravity and centripetal?)

Maybe this would have been a possible question to answer? If a twin has one engine out but is flying in a straight line with wings level a yaw string would show it was cocked off to one side, while the ball would be centred?

Thanks for your help.

[Microburst2002]

I don't fly choppers, so I can't say.

If the string is just a wool string like those of sailplanes, the string will be offset, too, because there is a slip angle, the air doesn't come from dead ahead. It should be on the opposite side of the ball, I think. yawing right with wings level, the ball would move to the left by centrifugal force and the string would be floating to the right because relative wind comes from the left.