Sideslip

[CJ1234]

Why don't big jet aeroplanes slip in the turn like the Cessna 152 does? I understand the principle of why the 152 does this, but why doesn't it apply to big aircraft? I have a feeling it has something to do with the dihedral angle of the wings bringing more lateral stability, but I don't nderstand how that happens. If anyone could explain that would be great.

[Capot]

I suspect that the answer is that you can sideslip an A380, or indeed any aircraft with normal aerodynamics (if the computer would let you) and that the main reason that big aeroplanes don't do much of that is that the pilots don't especially want to for lots of good reasons, passenger terror being one of them, and lovely, steep, curved side-slip approaches from 1500ft downwind are generally frowned on in public transport operations.

Someone from ETPS will now prove me totally and utterly wrong!

[MarkerInbound]

I'm not sure what your question is. Why don't transport jets slip or why don't pilots slip transport jets?

If it's the first, two words - yaw damper.

Capot has given an answer if it is the second. Don't think about dihedral, think about the sweep of the wing. If you yaw the plane one wing will grow in effectiveness as it is brought forward and the other wing will shrink. All sorts of interesting aerodynamics occur when this happens. Braniff lost a brand new 707 on a delivery flight when they were doing training enroute. The Dutch Roll got so bad they shucked an engine pod. I've been in a 727 sim that Dutch Rolled so bad I couldn't get my hand on the kill switch. Which leads us back to why they put a yaw damper on swept wing jets.

[Ka6crpe]

I believe that the 767 side slips quite nicely. As demonstrated at Gimli by Air Canada 143 The Gimli Glider / Air Canada (http://www.wadenelson.com/gimli.html)

[CJ1234]

If you bank to the left in the 152, you also have to apply left rudder because the aircraft slips in the direction of turn (pretty sure this is caused by the change in direction of lift which is now tilted - weight is still acting straight down but some of the lift is acting sideways, causing the a/c to slip into the direction of turn). My question is why don't big jet aircraft slip in the turn like the 152 when you roll them. I thought it was to do with wing shape, but I never thought about yaw damper so that is probs the answer.

[Dash8driver1312]

Are you talking about issues relating to adverse yaw?

[Tarq57]

Sounds like you need to study the forces acting on an aircraft (any aircraft) in the various stages of flight a bit more.

[halfofrho]

Large aircraft still need to be coordinated in a turn just like light aircraft, however it is controlled in a different fashion to your average cessna.

Larger aircraft use roll control spoilers to assist in the turn. Have a look at this video showing them in action. 738-800 Roll Spoilers.AVI - YouTube

In laymans terms, in a roll to the left, the spoilers will deploy on the left hand wing effectively dumping the lift on that wing causing it to drop (or roll) to the left. The other effect of the spoilers deployment is that it produces 'form' drag creating a yawing motion to the left as well. This yaw reduces/removes the requirement to use rudder to coordinate a turn.

Most modern jets will have some sort of a yaw damper as well. Depending on the aircraft these assist in turn coordination, but their primary use is to damp out any oscillations that present themselves such as dutch roll.

Hope this clears it up for you.

[john_tullamarine]

I'm a little confused.

Having flown Cessnas and a couple of airline jets, they both work the same way.

If you input a roll and nothing else, you get some slip and then yaw and so forth.

If you roll as part of intentionally turning, you may/will need some rudder input depending on the particular aeroplane's characteristics.

As observed above, adverse yaw and yaw dampers make for some specifics in the inputs required but the basics remain pretty similar.

There may be an element in your posts which suggests that you are mixing a straight roll input and a turn ?

[bravobravo74]

If you bank to the left in the 152, you also have to apply left rudder because the aircraft slips in the direction of turn

The aircraft initially slips into the turn but the keel surfaces cause it to yaw into the turn and actually change heading. If you don't apply rudder the aircraft will still turn.

The need for rudder is determined by how susceptible the aircraft is to adverse yaw as the aircraft rolls.

[5LY]

Transport jets are side slipped every day hundreds of times around the world in cross wind landings. If you cross the controls to counteract a cross wind, you're side slipping to touch down.

[Capot]

IMHO the X-wing landing technique is not so much a sideslip as flying into the wind to counter the sideways drift, then straightening out at the last minute to touch down straight; in other words the controls are not crossed in that technique, as they would be in a true sideslip or if the cross-wind is being countered with keeping the windward wing down instead of pointing into the wind.

Of course you sometimes need to combine a sideslip (used to lose height quickly and/or improve forward visibility in some single-engine aircraft) with countering sideways drift in a cross-wind by pointing into it, but that's not quite the same thing.

Is it?

[BOAC]

If you cross the controls to counteract a cross wind, you're side slipping to touch down. - you certainly are, but as Capot says, you are doing it wrong:)

[john_tullamarine]

then straightening out at the last minute to touch down straight

Worked beaut in lighties but I could never get it right in the jets. I was always much more comfortable

(a) tracking with drift until a suitable height (100-200 feet depending on crosswind strength)

(b) flying to the flare in an intentional wing down sideslip. This gave a really good feel for what the wind was doing.

(c) flaring and touch down wing down to suit

Maybe I got away with it so often because I had my eyes closed ?

[galaxy flyer]

5LY,

If you are slipping to do a crosswind landing, you are indeed not doing according to any of the OEMs' recommended "decrab" procedure for crosswind landings. Striking various airframe parts are highly likely depending on component, design geometry, and inputs.

Of course, the esteemed John Tullamarine's "eyes closed" method may be appropriate in difficult and trying times.

[5LY]

GF
Don't disagree with you that full on sideslip is not always the correct method to use, but in a strong crosswind I'll go half way with it to the last 150 ft. and then kick the rest off in the flare. The B777 does a variation of that on an autoland. It starts the correction at 500 ft. and then is in a full sideslip by the flare (and always does a beautiful landing).

[5LY]

Further to the above, the point of the thread was can we sideslip and the answer is that we can and do.

[Tinstaafl]

I think the OP is confusing adverse yaw as a further effect of rolling (in an aircraft using ailerons) and slideslip as a further effect of angle of bank.

Adverse yaw: When the aircraft is rolled, the wing's ailerons generate more lift on the upgoing wing, and less lift on the downgoing wing. More lift = more drag and less lift = less drag. Look down on the aircraft and picture how having more drag on the upgoing wing compared to the downgoing wing will yaw the aircraft. It will yaw the aircraft away from the turn that will eventually happen as a result of rolling to an angle of bank. This requires some amount of rudder input to overcome. Manufacturers try to reduce the adverse yaw effect using various aileron designs but it's not really 100% effective.

An aircraft that uses spoilers for roll control tends not to have this effect. A spoiler reduces lift & adds drag on that side so it avoids the whole problem. The downgoing wing (from the reduced lift from using the spoiler) also gets extra drag. Many (or most) jets use only spoilers for roll control at high speeds due to Mach considerations.

Many aircraft also have yaw dampers which act to prevent yaw, including the adverse yaw I mentioned. Some work better than others.

Once the aircraft is in an angle of bank then other effects become apparent. Lift is inclined which provides a force pulling to one side. As the aircraft moves in that direction its directional stability causes it to yaw into the direction of turn ie towards the low wing. It also no longer has all the lift opposing weight so will descend.

A further effect of yaw is roll so the banked aircraft that starts yawing will roll. Roll has yaw as a further effect so the aircraft will yaw even more. The end result of this sideslip induced yaw --> roll --> yaw --> roll pattern is a spiral dive.

This pattern is complicated by the lateral stability characteristices of the airframe. Straight wing aircraft are designed to be slightly laterally stable so a *small* angle of bank will be corrected (or at least not develop further) but a large angle of bank will develop into a spiral dive (directional stability trumps lateral stability). Swept wing aircraft tend to have additional lateral stablity effects from the swept wing. How the lateral & directional stability interact can lead to Dutch roll or snaking - and a requirement for a yaw damper for dispatch.

[CJ1234]

If I'm the "OP" (don't know what that means) then I am not confusing anything with adverse yaw. My question has nothing to do with adverse yaw. Neither is it about sideslipping in crosswind landings. I know big jets and small SEP aircraft can sideslip for crosswind landings - again that has nothing to do with the question.

halfofhro's answer was perfect. I knew there had to be something counteracting that slip motion and clearly on big jets that is the roll spoilers and yaw damper. Thank you. :ok:

[Tarq57]

OP = original poster.

And since halfofhro's answer is perfect, I'd suggest it probably has rather a lot to do with adverse yaw.