Newtonian Mechanics – Do Pilots Need to Consider Earth’s Angular Spin Velocity?

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First imagine a pilot takes off from a North South runway situated on a specific northern latitude, the circumference of which is 888 miles, where the angular spin velocity of the earth (and the plane) would naturally be calculated at approximately 37 miles per hour in an Easterly direction. Then imagine that he attempts to land on a North South runway at the equator where the angular Easterly spin velocity is 1037 mph.

Explain how this 1000 mph difference of angular spin velocity is gained with enough exactitude that his plane will not flip over upon touchdown?

For extra points explain how such a plane under these conditions could lose the greater angular spin velocity if the trip were reversed and started out at the equator and landed at the northern runway?

Best Answer

If the pilot were flying in a vacuum, then it would take an enormous amount of energy to catch up to a landing site near the equator. One way of thinking about this is that the vehicle has to overcome the Coriolis force that comes from moving across a sphere in a direction parallel to the sphere's axis of rotation (i.e., North-South). Artillery and battleships guns have to account for this deflection in the trajectory of their shells in order to hit targets to the North or South.

This extra velocity near the equator is why rockets that put spacecraft into orbit are located as close to the equator as possible. In this list of American rocket launch sites, 10 out of 12 are in the southern half of the continental Unites States. The most famous launch sites, Cape Canaveral and the Kennedy Space Center, are in Florida since it is the state that is closest to the equator. This location takes advantage of the rotation speed of the Earth to give rockets an extra boost to get to orbit. This is also why nearly all satellites orbit west-to-east, and nearly none orbit east-to-west.

Luckily, the problem for airplanes is much reduced because they travel in Earth's atmosphere. Due to air being a fluid with non-zero viscosity, it gets dragged by the Earth's surface and ends up rotating with the Earth in the same direction as its rotation. The air's circulation speed does not exactly match Earth's rotational speed and lags behind it, as evidenced by the existence of trade winds, which blow east-to-west near the equator. Since aircraft are subject to the movement of air around them, their movement over the ground is a combination of their movement through air and the air's movement over the ground. For example, if an airplane flies with an airspeed of 100 miles per hour flies towards the East while the local wind is 100 miles per hour towards the west, the airplane with have zero ground velocity--essentially hovering (note the very steep landing angles starting at 0:42). If the wind is constant, airplanes do not feel it as a crosswind that pushes them. It only affects their velocity over the ground. Aside from the force from the engine, an airplane will match the local air movement due to its drag. This is similar to how a swimmer in a river does not feel the river pushing them downstream, they only notice that they have a downstream velocity with respect to land.

So, as the airplane flies South, the air at each latitude that is rotating with the Earth drags the plane with steadily increasing velocity to the East so that, by the time it reaches the equator, it already has the 1000-mph sideways velocity needed to land. This sideways speed is measured from an inertial reference frame in which the Earth rotates, not with respect to the ground. The airplane will have to expend fuel to counter the trade winds, but these winds are nowhere near 1000 mph. At the destination runway, the airplane, the surrounding air, and the ground are all moving with the same speed in the same direction, so the airplane can easily land with no threat of flipping over due to sideways velocity. In the reverse direction, the slower-moving air to the North puts a drag on the initial sideways velocity of the airplane so that it matches the northern runway's slower rotational speed by the time it lands.