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Is the vertical tail the primary reason for causing landings to be skewed in a crosswind? Please explain the effect of the fuselage, the tail, or other components.

If the vertical stabilizer is the primary reason for crosswind landings, do flying wings with no vertical tails (say, the B-2) experience this issue?

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Short answer: The reason for crabbing (skewed flight, as you call it) during an approach in crosswind is to keep the wings level while maintaining sideslip-free flight. Flying wings are not fundamentally different, because they, too, have stability characteristics similar to those of conventional aircraft.

Airplanes want to be flown without sideslip. Flying in a sideslip (where a component of the airspeed comes from one side) incurs lots of complications:

  • Some of the lift is also pointing sideways, so the airplane drifts sideways.
  • Weathervane stability will make the aircraft yaw into the wind. This comes mainly from the vertical tail, but is only one of many components to make the aircraft controllable.
  • The dihedral effect will cause a rolling moment, lifting the windward wing.
  • Wing sweep will increase this rolling moment and add more yawing moment.
  • Since the vertical tail sits upward of the line of symmetry in most airplanes, this also adds to the rolling moment.

Most of these effects are desired, because they make the aircraft easier to fly, but during approach they make it harder to line it up with the runway. If the fuselage is held parallel to the runway, the windward wing needs to be lowered, so some of the lift points sideways into the wind, compensating the side force caused by sideslipping. This is achieved by keeping the ailerons deflected to work against the aforementioned effects, and the rudder needs to keep the airplane from turning into the wind. This is no problem as long as the airplane is high enough. On touchdown, however, such an attitude cannot be maintained, or the aircraft will hit the ground with a part of the wing or an engine nacelle first.

Only by pointing the fuselage into the wind can the course of the airplane be aligned with the runway in a sideslip-free way. This is called crabbing, because when seen from the ground the aircraft's speed has a lateral component, a little like a crab wich is also known for its sideways movement. Now the wings are level, so the touchdown will happen with the wheels first. Unfortunately, now the wheels' rotation direction is not aligned with the runway, and to reduce wear, pilots kick the rudder just before touching the ground to re-orient the aircraft parallel to the runway.

The Lockheed C-5 and the B-52 can rotate their gear struts, which enables both aircraft to touch down in crabbed flight. All other types produce more tire smoke and jerk the aircraft sideways if they touch down this way.

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    $\begingroup$ Small aircraft must be landed in cross-wind either in side-slip or decrabbed before touch-down, otherwise they would tip or ground-loop. $\endgroup$ – Jan Hudec Nov 2 '14 at 22:31
  • $\begingroup$ My one concern about this otherwise very good answer is that it makes it sound like flying in a slip is somewhat abnormal. It is not considered abnormal flight, and pilots are expected to maintain proficiency. $\endgroup$ – rbp Nov 5 '14 at 18:11
  • $\begingroup$ @rbp: That depends. If sideslipping means you touch the ground first with a wing, sideslipping IS abnormal. My point is that much effort goes into a design to make it fly straight, and the pilot needs to force it into sideslipping. By itself, this is just as normal as flying upside down, but should be avoided in situations like the one discussed here: During landing. $\endgroup$ – Peter Kämpf Nov 5 '14 at 20:20
  • $\begingroup$ Good answer! But it still does not clear my doubt about the effect of this on tailless aircraft. Except for maybe the point of sweep-induces-rolling-moment,say, a B-2 shall never face the issue of crabbed landing, right? +1 for the B-52 gear strut rotation fact! :) $\endgroup$ – Raj Nov 6 '14 at 6:25
  • $\begingroup$ Does a B-2 have a dihedral? $\endgroup$ – Raj Nov 6 '14 at 6:26
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The vertical tail is NOT the reason for "skewing" (I assume you mean crabbing) in crosswind landings. The answer is directional stability. But first, let's take a look at why crosswind landings are a problem.

Aircrafts fly relative to the air

Aircrafts fly in the air. They fly in a way such that the air pressure is balanced around the aircraft. Thus, if there is a 100 knot perfectly steady wind, any aircraft would fly just fine. A ground observer would see a 100 knot difference in the aircraft's velocity vector, but the relative movement between the aircraft and its surrounding air is exactly the same as a no-wind condition.

Try moving around a train. The train is travelling at 50 mph, yet you can move around the cabin just like on the ground.

Now if some pilot is going to land, he must transition from travelling through air to travelling on the ground. That's when the problem arises. His ground track must be along the centerline of the runway, and he must be pointing in the same direction as his ground track as well.

Back to the train analogy, try jumping from a 50 mph train to the ground. It's difficult because you have to match the 2 movements: one relative to the train and another relative to the ground.

Now we understand that aircrafts move through steady wind just as they do in still air. If a crosswind is 20 knots from the right, the B2 would be drifting to the left at 20 knots if its heading is exactly the same as the runway.

Directional stability

Let's assume what you said is true: on a B2 (which has no vertical tail), the aircraft is not affected by crosswind, and thus can land by pointing the nose straight at the runway.

Then it logically follows, that a crosswind of any magnitude would have an neglectable effect on the aircraft. Which also means, if the aircraft starts floating left or right, there is no way the pilot can control it! He can hold winds level and a constant heading (say 360), but the aircraft may drift sideways!

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    $\begingroup$ the B2 is affected by crosswind just not in the same way as a plane with a vertical stabilizer would be. $\endgroup$ – ratchet freak Nov 2 '14 at 15:08
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    $\begingroup$ @ratchetfreak - a plane has no idea whether it is flying through still air or not. Any steady crosswind appears transparent to any aircraft. If you're talking about a sudden change in crosswind, those are gust, and it's not what the OP is asking. $\endgroup$ – kevin Nov 2 '14 at 16:52
  • $\begingroup$ Last para : the crosswind will push the fuselage around! Also, if the aircraft begins to drift, the pilot has access to control surfaces to yaw or give directional forces. First line : Why is the vertical tail not a reason for crabbing in crosswinds? It will give the aircraft all of rolling, yawing and side force. $\endgroup$ – Raj Nov 6 '14 at 6:51
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    $\begingroup$ Last paragraph: the crosswind will not push the aircraft around because the aircraft has zero response along that axis. If it does, then the assumption (nose pointing straight at runway) will not stand. The reason is directional stability, not the vertical tail. The vertical tail is only one of many ways to achieve directional stability. $\endgroup$ – kevin Nov 6 '14 at 7:35
  • $\begingroup$ @Raj: the pilot has access to control surfaces to yaw or give directional forces is the answer to: Why is the vertical tail not a reason for crabbing in crosswinds. Basically, it is the reason - the vertical tail (specifically the rudder) is the control surface that yaws the aircraft correcting for the drift. So it is the cause but it is only the cause because the pilot made it do it. If the pilot doesn't deflect the rudder on the vertical tail then the plane would not crab and will only drift further from the runway. $\endgroup$ – slebetman May 15 '15 at 6:57
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For unaccelerated flight, an aircraft can fly in wings-level zero-sideslip (assuming symmetrical drag/thrust such as asymmetric stores or engine failure) or steady heading sideslip (SHSS). To induce SHSS, the pilot would apply rudder, which in-turn causes sidesplip (Beta). At this point the behaviour of the aircraft is determined by it's inherent aerodynamics as not all behave the same. Just looking at the B2, you will observe that it isn't as 'slab sided' as most commercial or military aircraft might be. Hence there would be less SIDEFORCE than normal aircraft configurations, especially given there is no vertical tail. The effect of this reduction in sideforce means that less angle of bank (lift vector used to counter the sideforce) is required to maintain the SHSS. Now modification of other aircraft configurations such as dihedral and sweep will determine the amount of control that is required to maintain a given SHSS, but doesn't affect the AoB(read sideforce) required due to sideslip.

So the objective in crosswind is to land within the landing gear side load limits. We need to align the aircraft longitudinal axis as close as possible to the runway direction, but we also must have minimal sideways(lateral) velocity.

On a typical crosswind approach, The aircraft will fly a crabbed profile in a wings-level zero heading sideslip for the majority of the profile as this is the most efficient in terms of drag reduction. How the pilot controls the aircraft close to the flare depends on the geometry, control power and handling qualities of the aircraft. This will also determine its crosswind limits. There are two options, land in a SHSS or kick-off drift (KoD), or a combination of both.

Generally, if geometry is limited (ie large wings, engine pods etc) then you're only option is to kick-off-drift (KoD) while trying to keep the wings and close to level as possible. This would be done at the very last moment as the resultant sideforce created by sideslip will make the aircraft drift laterally away from the runway axes, which if left unchecked, might overcome the structural limits of the gear.

If geometry isn't limited but handling qualities are an issue, then the option to use SHSS (wing down) in the flare may be valid. This manoeuvre can be easier to perform in some cases as it is less dynamic in the late stage of the flare whereupon you might encounter other delayed secondary effects seen in KoD such as roll due to sideslip - swept wing aircraft especially. Coming back to aircraft that have low sideforce due to sideslip, we might expect this manoeuvre to be even easier to perform as only small amounts of side slip are required.

Of course, The handling qualities of the aircraft will depend on how the flight controls are implemented and the stability characteristics of the aircraft. This will determine the best techniques to land the aircraft.

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