## How High Do Planes Fly

Article Updated: 24 Dec , 2015
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Have you ever asked how high do planes fly? The answer is easy to understand when you remember how flight for aircraft works. The first thing to know is that air is a fluid just like water. So it works under the same rules. Any object that moves in a fluid is under the influence of four forces, drag, lift, weight, and thrust. The net total has to be positive so that the influence of thrust and lift keeps a plane in the air. Thrust and lift depend on the density of the air. So it is easier to achieve the ideal lift and thrust at higher elevations than lower elevations. So how high a plane flies is not fixed except for the limit of the vacuum of space of where the atmosphere becomes too thin for aerodynamics to work.

Lift and thrust are the main forces that make flight possible. As long as they are greater than weight or drag, plane will fly. Thrust is the forward acceleration produced by a plane’s engine. The less dense the air the more thrust a plane must produce to create the needed lift. The full explanation is pretty complicated but the best way to put is that every plane has a maximum condition it achieves to fly. This maximum is the best possible combination of density, speed, and lift to fly the plane. That is why the height a plane can fly can vary so much. It depends on the needs of the plane.

A good example is commercial turbo jets. Turbo jets fly below the speed of sound. The also weigh a lot. In order to reach optimal flight conditions and fly at speeds convenient enough to make air travel profitable, most commercial planes fly at 30,000 feet. This is high enough that a plane has the least amount of drag and can reach the top speed its engines can produce safely. Supersonic craft like fighter jets and spy planes can fly much higher. This is because they design of the plane makes it easier for the plane to resist drag and produce greater thrust to compensate for the thinner air.

So we see that how high a plane can fly is determined by its use, the drag, the lift, thrust, and weight. We also know that a planes absolute limit will be where air becomes too thin to act like a fluid which is the uppermost level of the atmosphere. Right now scientist are looking to take advantage of this upper level of the atmosphere to help planes fly even faster. However there are still barriers such as friction and engine design.

We have written many articles about airplanes for Universe Today. Here’s an article about the largest airplane, and here’s an article about pictures of airplanes.

If you’d like more info on airplanes, check out these articles from How Stuff Works. Here’s an article about How Airplanes Fly.

We’ve also recorded an entire episode of Astronomy Cast all GPS Navigation. Listen here, Episode 212: GPS Navigation.

Sources:
NASA
How Stuff Works