Why does an aircraft not spin out of control right after touch-down?

Question

When a plane is landing, how does it not immediately start spinning out of control?

Given the massive amount of forward momentum and the fact that all the various braking forces (wheel brakes, spoilers, thrust reversers) are applied separately on each side of the plane, if there was even a small imbalance between the two sides wouldn't the plane want to start spinning (yawing) in that direction?

Is this something a pilot has to account for while landing or is there some physical effect that makes it Just Work?

Answer 1

how does it not immediately start spinning?

Physics, and engineering. But I guess that you would like a bit more detail, so let's try dive in.

wheel brakes

They tend to have a stabilizing effect by shifting some of the weight on the front wheel and allowing the pilots to control the aircraft with more effectiveness.

spoilers

These are automatically controlled by the Fly-by-Wire system, so if required they would be retracted.

thrust reversers

If they are functioning properly, the resulting force will go through the vertical plane containing the C.o.G. (but a bit below it), so they do not pose much of a problem.

Another thing you have not mentioned, but I am going to add, is crabbed landings (such as this one), since in these situations the aircraft DO tend to spin a little. In these cases, in the timeframe between touchdown and braking, 2 things help the aircraft remain on the runway: the vertical tail (with the rudder), and the position of the main landing gear w.r.t. the center of gravity of the aircraft. As it can be seen in this image taken from the lecture notes of the course I followed (the title translates to "Braking run in crabbed landing"), normal tricycle gear aircraft tend to be stabilized by their own landing gear in this situation, while the same cannot be said for old school taildraggers.

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So, what really could cause a problem? The answer is wind gusts (especially if the wind is perpendicular to the runway), uneven conditions of the runway (say, a puddle or a thin layer of ice on one side of the runway will cause one set of main gear to have less grip than its counterpart on the other side, causing a yawing towards the side with more grip), asymmetrical failures in the thrust reversers, excessive braking while also turning the aircraft to realign to the centerline (particularly a problem if the distance between the two main gears is not sufficient).

How does the aircraft remain on the runway despite these possible problems? At high speeds, thanks to the vertical tail; and at low speeds thanks to the front nose gear. At in-between speeds, the situation is slightly more complicated: the tail is not much useful anymore, while the front gear would risk breaking if used to correct too much. So, for example, a FbW system can help through the asymmetrical use of spoilers. An example of what could happen, but seen during a take-off, is illustrated in this video (video kindly provided by MichaelK in chat).

Answer 2

At landing the vertical stabilizer and rudder have a lot of aerodynamic force to counter any yawing motion. The pilot does have to account for any imbalances while landing.

There are many different sources of imbalances in yawing force at landing. As you mentioned, the brakes can be uneven. Engine thrust can be uneven. Crosswind is another large yawing force. The aircraft must be designed so that the vertical stabilizer and rudder are able to overcome these forces. Airliners are designed to be able to land safely with one engine out, with one-sided brake failure and crosswind up to a certain limit.

As the aircraft slows and the rudder loses effectiveness directional control passes to the wheels. Pilots use asymmetric braking and wheel steering to keep the plane on the center of the runway. It's at this point, later in the rollout, that problems are more likely to occur. This is when things such as skidding become more of a problem and runway excursions do sometimes occur. Anti-lock braking was designed to help prevent this.

Some aircraft, usually small tail-draggers, have no means of directional control other than the rudder. So that shows how important the aerodynamic forces from the rudder are to controlling the direction of aircraft both in the air and on the ground.

Answer 3

Small planes do spin out all the time. It's called a ground loop.

When a plane lands the pilot has to quickly do two things to keep the plane going straight: get ALL the wheels tight on the ground, and use the rudder and to steer the plane straight. If there is a strong crosswind, it can be quite tricky to do this in a small plane. The larger the plane, the more immune it is to crosswinds. One of the big dangers is overcompensating. Often a ground loop happens when the pilot oversteers.

BTW the other answers on the page have a lot of incorrect statements in them because none of the other posters are pilots. The main error they make is thinking that brakes are used to keep the plane straight. This is absolutely untrue. A pilot never applies the brakes until he has the plane going straight first, and then both brakes are applied simultaneously. A pilot never tries to steer with the brakes on landing--that would be a good way to get a ground loop started.

On nearly all planes from small to large, the brakes are at the top of the pedal (they are called "toe brakes"). What the pilot does is use just the balls of his feet when he lands to steer. For a small plane this can require rapid and accurate movements. Once he has the plane going straight and does not need to steer anymore, he shifts both feet to the top of the pedal and presses the brakes together to slow down. Once the pilot starts doing this, the plane generally keeps going in whatever direction it was pointed in, so it better be pointed straight. Sometimes the plane will not be pointed perfectly straight and the pilot ends up to the right or left of the centerline.

Answer 4

The plane does not spin out of control because:

• It takes the path of least resistance: on a dry runway, rolling friction is about two orders of magnitude less than sliding friction, so if the brakes are not applied there is a natural tendency to autocorrect.Your car also does not spin out of control in a corner, as long as the wheels don't slip.
• If the plane lands with a sideslip velocity vector, once all three wheels make contact the nosewheel autocorrects: because of the way it is configured, the weight of the aeroplane pushes it straight. In small planes, the nose gear is inclined forward and is mounted in front of its axle, so that it lifts the aircraft up when turning. Same reason you can take your hands off of a bicycle handle bars.

In large planes, the long wheelbase provides stability. From the Wiki article for wheelbase:

Because of the effect the wheelbase has on the weight distribution of the vehicle, wheelbase dimensions are crucial to the balance and steering. For example, a car with a much greater weight load on the rear tends to understeer due to the lack of the load (force) on the front tires and therefore the grip (friction) from them.

And understeering is exactly what you want. From the Wiki for understeer&oversteer:

When an understeer vehicle is taken to the grip limit of the tires, where it is no longer possible to increase lateral acceleration, the vehicle will follow a path with a radius larger than intended. Although the vehicle cannot increase lateral acceleration, it is dynamically stable.

When an oversteer vehicle is taken to the grip limit of the tires, it becomes dynamically unstable with a tendency to spin out.

This self stabilising configuration is the main reason why an aircraft can be landed successfully with a crosswind and when not aligned perfectly with the runway. In these situations the aircraft needs to slam down, not touch down - transfer weight onto the wheels and the undercarriage configuration does the rest. The main wheels are placed apart wide enough to make sure that the aircraft cannot topple over, even with the most aft and highest centre of gravity.

Braking further pushes the nose wheel onto the runway, and increases the self-aligning force. All the braking system has to do is make sure that the wheels don't block: that removes the low rolling resistance. And then it needs to bring the aircraft to a standstill of course.

For correcting the lateral acceleration, the nose wheel steering is connected to the pedals after the main gear has detected ground contact, this enables the pilot to continue controlling the aircraft yaw when the velocity has become too low for the rudder to be effective. Pedal nose wheel steering has a much smaller range than nose wheel steering via the tiller.

The pilots have two brake pedals,one for each side, and can correct for a-symmetry via differential braking as well if required. The auto-stability of the touched down aeroplane is a must for successful landing, like aerodynamic stability is for successful flying. Airliners will never depend on active fly-by-wire systems for controlling landing stability - they may be helpful, but the aircraft must also be directionally stable when the systems are inactive, in any emergency for instance.

Answer 5

The control surfaces on the plane still work when the wheels are touching the ground. Also, planes have left and right toe brakes, unlike a single brake pedal in a car. The pilot can still steer with the control surfaces, tail rudder, using the pedals and also steer with the brakes by adjusting how much left and right side braking. When the plane is too slow for the control surfaces to be effective they use just the brakes to steer. A helicopter can use the pedals to steer even with little or no forward/backward movement.