Departure with a crosswind needs less runway than a departure with no wind?

Peter wrote:

But it is 16 degrees off axis. The Q is: how much does this matter? How much is the wing lift sensitive to the deviation from perpendicular of the airflow across it?

It is the same as slipping with 16 degree yaw. Pretty sensitive!

AnthonyQ wrote:

But the lift at Vr is calculated from the 70kt component at zero degrees…which is the same as the nil wind case

Yes I believe you are correct. The total relative wind doesn’t matter as for a lift equation, the wind perpendicular to the wing is used. On a runway this is 0 at 0kts and 70 at 70kts.

Peter wrote:

After liftoff (Vr) the picture changes. Over a few seconds, the plane turns into the relative wind (i.e. a heading change from 360 to 016 in this case). … a departure into wind gives you a briefly higher rate of climb.

Yes I believe you are correct, so many times as I have taken off with a XW, I have used this. At lift-off kick the nose into wind (you don’t need to do much for this really due to the weathercock effect), and get an instant lift. Now you experience the fact that all this time you were moving into 20kt XW, but you could not use it… until now!

That’s why I think you need less runway, as you can lift-off as early as ground effect kicks in, turn nose into wind, and fly away.

Unless there is trees or hangars lining the runway… then you will be held down due turbulence and rotors…

@huv: we are discussing the takeoff run, not the landing. I have flown quite some hours in the Cirrus of which some in the SR20 and most in the SR22T. I can’t remember having to use differential braking on the takeoff run, but lots of left rudder. I do agree that with heavy crosswinds, you need more distance in takeoff and landing to accommodate any gusts. However, this is all becoming a very theoretical discussion.

Has no pilot of Cirrus, Diamond, AA-5, or Columbia/Corvalis contributed?
All of them lacks a steerable nose wheel. So when taking off in max demonstrated crosswind, some differential braking is necessary to maintain direction in all of those airplanes, until the slipstream gives the rudder enough authority. Braking during take-off does only one thing for the distance required.
The above effect is limited as it only applies in the beginning of the take-off run. But during landing in a strong crosswind, the need for differential braking is much more significant since there is much less slipstream on the rudder, so differential braking is required during most of the landing run, obviously increasing the distance required compared to max braking on both main wheels.

Generally I find that the operationally most significant point is that if a strong crosswing is coming 90 degrees on the runway, even a small change in wind direction will change the headwind/tailwind component a lot.

The above two points are on my mind in strong crosswind operations, both calling for bigger margins.

(I did read, or at least skim, all previous posts, and did not really find any convincing argument that a crosswind could benefit take-off or landing performance.)

Peter wrote:

I think a useful discussion would involve carefully dismantling the points I made in my post #29.

Debating airspeed is a circular argument because that is just an indication on the ASI. It doesn’t directly represent the action of the airflow across the wing.

An alternative view might be to consider the effect of increased lift in isolation.

As a thought experiment, consider three tb-20s, two of which were built on a Friday… of which one has shorter wings than standard and the other longer. They are otherwise identical (same weight). The handbook gives lift off at 71kts and 78kts at 50ft, the stall speed in this condition is 65knots, Vx is not given but a reasonable guestimate might be 75kts, i.e. a little less than take off speed. The Friday specials have stall speeds of 63 and 67kts respectively. This difference is chosen to represent the 3kt difference in actual take off speeds, referred to in post #29.

Lets assume the take off roll is not effected and the pilots all operate as per handbook, so how are the take off distances thus effected? I would assert that there would be small differences, as climb angle is dependant on excess thrust versus drag. Indeed the smaller wing variant would be subject to the most drag, the standard one less so, and so on. However these differences are much smaller than if the speeds are recalculated and the correct safety margins used.

I think this makes a number of points.

JasonC wrote:

That is true but takeoff is not the opposite of landing and you will need to accelerate to 60 knots to rotate assuming that is the airspeed needed for rotation.

I would say it depends on how theoretical you want to look at this and what kind of aircraft you fly. What you are saying is merely that we cannot take advantage of it, because we chose not to (for good reasons I guess, but nonetheless). It’s not physically impossible to rise the nose wheel at 56 kts, yaw into the wind and have 60 kts instantaneous, then lift off the main wheel and continue straight ahead in a climbing crab. Accelerating to 60 kts take longer and requires more runway.

LeSving wrote:

Therefore accelerating to 56 kts take less distance than accelerating to 60 kts.

That is true but takeoff is not the opposite of landing and you will need to accelerate to 60 knots to rotate assuming that is the airspeed needed for rotation.

Peter wrote:

It says This site is not available in your region

Myth busters…

Another way to look at this. Landing is the opposite of take off. Landing in a cross wind, you can crab or sideslip. Let’s say you crab. IAS is 60 kts, cross wind component is 20. This means the SOG is 56.6 kts. The moment you touch down the SOG is only 56.6 kts compared to 60 kts for no cross wind. Clearly braking from 56 kts requires less distance than braking from 60 kts. Therefore accelerating to 56 kts take less distance than accelerating to 60 kts.