Crosswind takeoffs



IT HAS BEEN a few months since I wrote about crosswind takeoffs and landings and the correct control inputs, so I guess it’s time for a reminder.

This time I’m going to talk particularly about crosswind takeoffs – that is, when the wind is blowing across the runway you’re intending to use.

The too-long, didn’t-read version is simply to apply full into-wind ailerons during the takeoff roll and bring them smoothly to neutral just before you raise the nose (rotate) at flying speed.

Let’s talk about why this is the appropriate thing to do, and the risk you run if you don’t.

Let’s look at the vertical stabilizer. If you look down from on top of the plane you’ll see the vertical stabilizer is an airfoil (wing shape), and the rudder looks a lot like an aileron – an adjustable surface that you can use to deflect the airflow over the rudder.

When there’s airflow diagonally across the vertical stablizer, say mostly from the front, but somewhat from the side too – this airfoil has an angle of attack and generates a “lift” force, which because the surface is sticking up in a vertical plane, acts to the left and right of the airplane.

It’s sort-of correct to say the rudder deflects the airflow over the stabilizer but it’s much more correct to think of it as changing the chord line and camber of the whole surface, which changes its angle of attack. When you move the rudder in flight you change the angle of attack of the vertical stabilizer from positive (in a leftward direction) through zero to negative (in a leftward direction) or equivalently, positive in a rightward direction. The control of the sideways lift force generated by the vertical stabilizer to either side lets you yaw the plane to left and right.

When you roll along the runway with a direct headwind, or with no wind at all, and with the rudder centred, there’s no angle of attack over the vertical stabilizer and it generates no sideways force.

But when there’s a crosswind, this sideways movement of the air adds (in a vector sense) to the forward motion of the airplane along the runway to generate an airflow diagonally over the horizontal stabilizer. This gives it an angle of attack, and generates a sideways force. That sideways force works hard to yaw the aircraft to face directly into the relative wind, which would be diagonally across the runway. And If you let it do it’s thing unchecked the airplane will indeed happily steer itself off the runway to the upwind side.

What can the prepared pilot do to keep straight along the centre line? Well, in the first instance, she can use nosewheel steering to keep the airplane straight. For as long as the nosewheel is in contact with the tarmac the pilot can steer with it. Right? Well, no, not really. On most airplanes the nosewheel is on a sprung castering mount. The pilot can influence the angle of the nosewheel to the left or right but doesn’t have fixed control of it: the wheel is also allowed to align itself with the direction the plane is moving, like the front wheels of a shopping cart. Above a certain speed – well below rotation speed – the nosewheel breaks away from the “centred” position that matches the pedals and free-casters in which ever way the plane is moving or wants to move.

The second influencer a pilot has is the rudder itself. By deflecting the rudder the right amount the angle of attack of the relative airflow over the vertical stabilizer can be brought to zero and the sideways force from the rudder removed. A little thought will tell you that as the aircraft speeds up the relative airflow comes more from the front and less from the side, so the rudder deflection needed to null out the sideways force decreases as the plane speeds up.

This leaves us with an awkwardness: we’re ok when the plane is moving very slowly because there’s little sideways force (the airspeed is low), and anyway the nosewheel steering is still effective. And when the plane is moving quickly the airflow angle over the vertical stabilizer is small and the sideways force is more easily handled by the rudder. But there’s an in-between zone, maybe half way or more through the takeoff roll, where you’re going to need a lot of – up to full – rudder input away from the wind; and even then in a stiff crosswind that might not be enough to keep the airplane straight.

So what else can the pilot do? Salvation lies in correct use of the ailerons. Every student pilot learns early on about adverse yaw and that the nose swings out to the wrong (adverse) side when you roll into and out of a turn unless you coordinate with the rudder. When you lower an aileron on one wing the lift on that wing is increased and generates an increase in drag too. When you raise an aileron the drag is decreased. More drag on one wing and less on the other causes a yawing moment – that’s what you have to counteract with the rudder.

In truth we get a bit complacent about aileron yaw. If you never use the rudder in turns, well, it won’t be comfortable, and it’s definitely bad technique, but honestly most of the time nothing terrible will happen to you. So it’s easy to put aileron drag out of your mind as something front-and-centre important.

When we’re in flight aileron drag is a transient effect: it lasts only as long as the aircraft hasn’t reached a steady roll velocity. As soon as the wing with the lowered aileron starts to go up, that is as soon as the airplane starts to roll, that wing has reduced its angle of attack (relative to the air it’s moving both forward but also now upward) and the extra drag goes away. Similarly with the other, down-going wing – the decrease in drag is only there for a moment or two.

But if we’re still on the ground, the airplane can’t roll. When you lower an aileron the increased drag on that wing is a force that remains for as long as the ailerons are deflected. And because it’s happening right out at the wing-tip where it has maximum leverage we can successfully use it to help keep the airplane straight in a crosswind.

Which way to turn the ailerons to achieve this? Turn them into the wind – fully, and hold them there, as you roll down the runway. That gives you the maximum benefit to keep the steering straight during the in-between speed when the nosewheel can’t help you but the angle of attack across the vertical stabilizer is still too big for the rudder to remove.

Lots of people learning to do this are scared that with full aileron input the aircraft will somehow roll upside-down on the runway. Well – don’t worry – it can’t. Until you reach flying speed and rotate, the wing doesn’t have enough lift to raise the airplane up, and unless it goes up first it cannot roll, because the wheels won’t allow it to – they’re in the way. If you do manage to rotate with some aileron input, all that happens is that one wheel picks up first, and it’s pretty easy and natural to feel that and bring the ailerons to neutral at that time. But ideally, you should bring the ailerons to neutral just before you raise the nose to leave the runway, and if you do that, you won’t be surprised by any rolling motion when you don’t want it.

So – for maximum steering benefit in a crosswind – begin your takeoff roll with full ailerons deflected into the wind, and keep them there until just before you’re ready to rotate. That gives you the best tools to keep the airplane rolling straight down the runway which is where you want it.

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