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I don't know if I'm overthinking this, but it has been causing a bit of confusion in my head.

Knowing that the headwind component affects the angle of climb/descent, I was wondering how it affects landing when transitioning from a crab to slip technique?

During a crab the nose is facing more into the wind, but when kicking the nose out to align with the runway, I am trying to imagine if that causes a slight loss of lift or a sudden decrease in descent angle? As there would be more of a crosswind component in relation to the direction the wings are facing?

I might be missing something here, but it seems important to know the physics of this when you're only 100 or 200ft above the runway!

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  • $\begingroup$ I think you are over-thinking this. If you have a left crosswind on final, just prior to touch down add right rudder to align the nose with the runway centerline and add as much left wing down as necessary to keep from drifting. Touch down will be on the left main first. Works in a Cessna 150 and a B767. I think also that 100 to 200 feet is too high to begin the correction from the crab to runway alignment. In a light aircraft, I think transition from the crab to a slip (aligning with the rwy) should start at about 30-40 feet. (that's my opinion) $\endgroup$ – 757toga Oct 15 '18 at 3:30
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    $\begingroup$ What he said... you shouldn't really start the side slip until you are starting to flare. The application of rudder, dropping the wing, and flare should be one coordinated motion. The side slip increases the sink rate, but at this point you are in ground effect and pulling pitch so the effect isn't noticeable. $\endgroup$ – John K Oct 15 '18 at 3:39
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    $\begingroup$ Overthinking is in the eye of the beholder-- if you are curious about the underlying dynamics, more power to you. $\endgroup$ – quiet flyer Oct 15 '18 at 4:36
  • $\begingroup$ @quietflyer - I agree with your comment. However, the OP implied in his question that he was interested in feedback regarding whether or not he is "over-thinking" the aerodynamics associated with a crosswind landing. $\endgroup$ – 757toga Oct 15 '18 at 14:24
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It seems the question has changed significantly since I last answered. Is the headwind component defined relative to the direction of the flight path over the ground, or relative to the aircraft heading? It would seem to be the latter. If the former, the headwind component stays constant (at least momentarily-- not long term) after you "kick out the crab". If the latter, the headwind component decreases, and the cross-wind component increases, when you "kick out the crab". At the first instant this is true regardless of whether or not you lower the upwind wind to stabilize the situation, while in the long run the decreased headwind and increased crosswind can only be sustained if you lower the upwind wing to stabilize the situation. Either way, at the very least you will see an increased sink rate, and in most GA aircraft, a tendency to roll in the "downwind" direction if the ailerons are neutralized, immediately after you "kick out the crab". This is due to the fact that after you "kick out the crab", the relative wind is coming more from the side and less from the front, in the aircraft's reference frame. This creates drag and also creates a roll torque due to aerodynamic coupling between slip and roll.

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  • $\begingroup$ Once again I'll say my personal preference is to eliminate the crab angle and establish the wing-down angle with plenty of altitude-- I'm guessing typically 200 to 400 feet up-- and hold it all the way to rhe ground, adjusting as needed for differences in windspeed and wind direction with altitude. This is in the context of GA light aircraft. $\endgroup$ – quiet flyer Oct 16 '18 at 3:04
  • $\begingroup$ You know what, the second you talked about ground track it clicked! The heading isn't all that's at play here, such a simple concept I don't know how I missed it! Thanks.. $\endgroup$ – Jesse Oct 16 '18 at 6:57
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Let's start by looking at a pure "kick out the crab" technique, where we don't lower the upwind wingtip at all.

Even though we don't usually use the word "sideslip" when we talk about "kicking out the crab", we do in fact cause a sideslip when we press that downwind rudder pedal, even if we don't lower the upwind wing.

When you kick out the crab you force the aircraft to slip (or we could say skid) sideways through the air. A yaw string on the front of the windscreen would blow sideways, in the downwind direction (if there were no propwash). This is not an efficient way to fly. Initially, the slip angle is fully as large as the crab angle you just kicked out. As the flight path starts to curve (because you didn't lower one wing to stabilize the situation)-- i.e. as the aircraft starts to "accumulate drift" (which in reality is actually a turn -- a curve in the flight path--a rudder-commanded "flat turn" or skidding turn)-- things change, but that's the initial situation. So the more crab you kick out, the more the aircraft is forced to fly sideways.

For any given approach airspeed and any given crosswind component, as headwind increases, so will the required crab angle, so when you "kick out the crab", you'll have more sideways flow over the aircraft, and the sink rate will increase. You may also notice an increased tendency to roll toward the downwind wingtip, due to slip-roll coupling.

It may help clarify things to understand that "kick" is a little bit of a misnomer. After the "kick", if touchdown is delayed, to keep the plane aligned with the runway, you have to keep HOLDING some rudder pressure. If you released it, the plane would tend to swing back into line with the original flight path-- the nose would swing back toward the original heading. After a few seconds though, the plane has "accumulated enough drift"-- i.e. the flight path has TURNED or curved enough-- that the aircraft heading will be aligned with the runway even with no rudder pressure. You do not want to be in this situation, because it means you have TURNED your flight path through the airmass so much that it is fully aligned with the runway heading, and you have TURNED your ground track toward the downwind edge of the runway. You'll soon be in the weeds. That's the trouble with the pure "kick out the crab" technique-- if touchdown is delayed, you have a problem.

Up till now this answer addressed a pure "kick out the crab" technique, but you did in fact mention sideslip in your question. If at the same time you give the downwind rudder input, you also lower the upwind wing, you'll have the same initial slip angle as described above, and a headwind will affect the required slip angle-- and the resulting sink rate-- in the same way as described above. But if you lower the upwind wing to stop the rudder-commanded turn, then the situation will be stabilized and you'll see that same slip angle (and that same sink rate) all the way to the ground -- assuming of course that the crosswind component and the headwind component are both constant all the way to the ground, which may very well not be true in reality.

(We're ignoring here the effect that banking has on sink rate, which is real but but probably quite a bit smaller than the effect of flying sideways through the air (slipping/skidding) due to the rudder deflection.)

At any rate, whichever technique you use, for a given crosswind component and a given approach speed, more headwind = a larger crosswind component = a larger slip angle = a higher sink rate.

If you are crabbing along on final approach with NO wing-down correction, and you haven't yet "kicked out" any of the crab angle, then the plane doesn't "know" anything about the wind at all, for a given power setting and angle-of-attack. It's only after you "kick out the crab", or after you initiate a wing-down cross-controlled correction, that the strength of the headwind matters, because of the way you must adjust your control inputs to match the required slip angle, which is affected by the strength of the headwind.

In actual practice, with the cross-controlled wing-down technique, it seems to work well to imagine that the lowered upwind wing is somehow "opposing the wind" and "stopping the downwind drift", while the rudder is controlling the heading. The real truth though is that what the lowered upwind wing is really doing, is stopping the rudder-commanded flat (skidding) turn that would otherwise take place if we "kicked out the crab" and didn't touch down immediately after.

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  • $\begingroup$ PS my preference is to establish a wing-down cross-controlled slip early-- at least a hundred feet up, maybe double that or more-- and hold it all the way to touchdown. I like the practice. This is in the context of GA. $\endgroup$ – quiet flyer Oct 15 '18 at 4:28
  • $\begingroup$ Thanks for that, love the detailed answer that did help me visualise what's going on better. I was just curious if there was more headwind component in a Crab, compared to a Slip (With lowered wing). I suppose, trying to imagine the relative airflow over the wings and how that is affected between the two techniques, but I think the action of lowering the wing must counteract that, because I haven't noticed any major change in practice.. $\endgroup$ – Jesse Oct 15 '18 at 6:51
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    $\begingroup$ If you want to compare headwind component in a crab (with no "kicking out" yet initiated) versus in wing-down slip that is being conducted purely for the purpose of aligning the nose with the runway-- well-- the aircraft are pointing in two different directions but have the same ground track. Do we define headwind component relative to ground track or a/c heading? Whichever way we answer, defines which situation has the greater headwind component. But I think my official answer above was actually a better fit to your actual original question! $\endgroup$ – quiet flyer Oct 15 '18 at 7:19
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I think you are over-thinking this. If you have a left crosswind on final, just prior to touch down add right rudder to align the nose with the runway centerline and add as much left wing down as necessary to keep from drifting. Touch down will be on the left main first. Works in a Cessna 150 and a B767.

Since leaving the crab for a slip should (in my opinion) occur as the aircraft is entering ground-effect and starting the flare any noticeable aerodynamic consequences from the slip are minimized to irrelevancy.

I think also that 100 to 200 feet is too high to begin the correction from the crab to runway alignment. In a light aircraft, I think transition from the crab to a slip (aligning with the rwy) should start at about 30-40 feet or even a bit lower depending on the aircraft type.

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Coming into land in a cross wind seemed, in training, to have 2 schools of thought, drop your wing and apply opposite rudder about half way through final approach, or after rounding out (before flare). Never liked to even think about doing it while rounding out, as not hitting the ground with my nose wheel had top priority.

One thing to remember is that rudder one way and ailerons the other is cross controlled. Luckily for me, the Cessna 172 is incredibly tolerant of this. I practiced cross controlled stalls at altitude with nothing significantly unusual about them. But I liked to hold the Crab all the way down past round out as it was more coordinated. It just made more sense (in a small light plane) to crab it all the way in.

Once rounded out and floating over the runway (in lower winds than aloft) rudder straight/wing down, flare, and land. Wing down is important as it "holds" the plane from being pushed sideways by the cross wind component.

Now, as far as how much airspeed you lose in the "kick", only a fraction from the cross wind. For example 20 knot cross wind from 25 degrees off nose 65 knots total airspeed. If it were a 20 knot headwind and was suddenly taken away (highly unlikely), your airspeed would still be 45 knots. However deflecting your nose 25 degrees, your headwind component is cosine 25 degrees x 20 knots = 18 knots! Your airspeed is now 63 knots, your vertical lift slightly less. So if it is a little windy, carry 5 extra knots in and you're in good shape. The remaining headwind component brings ground speed down considerably.

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  • $\begingroup$ Possible improvement to this answer: clarify does "rudder straight" mean "straighten the rudder" or "use the rudder to align the nose of the plane (a/c longitudinal axis) straight with the runway centerline". A slight change in language may make it much more clear. . $\endgroup$ – quiet flyer Oct 17 '18 at 3:13
  • $\begingroup$ It would mean using rudder to align nose with runway center line (while applying aileron into wind). I forgot to mention it should be held while plane slows to a stop to help control "weathervaning". $\endgroup$ – Robert DiGiovanni Oct 17 '18 at 4:32

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