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Q & A: High-flying model planes

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Most recent answer: 10/22/2007
Q:
For my science class, we have to build a plane that can fly as high as possible. We have some set parts, but we can change the wheels, wings, and tail. What kind of wing will allow my plane to fly highest?
- Adam Chan
Hopkins Junior High, Fremont, CA, United Sates
A:
The height to which an airplane can fly depends a lot on the details of what it is made out of and how it is propelled. There isn't enough information in your question to answer it fully, but I will guess that the airplanes you are making are lightweight and are intended to be thrown by hand in a large, open area. And while I can only suggest experimenting with different wing shapes to get the final optimized design, there is a some guidance from the physics issues.

The forces on the plane are:

1) gravity -- pointing downards.

2) drag -- opposing the motion of the airplane, slowing it down.

3) thrust -- forces propelling the plane forwards. In this case, zero after the plane has left your hand. If the plane has an engine, then it can fly much much higher (until the air gets too thin for the engine to run well. For rocket engines, there is no real limit).

4) lift -- this force points upwards and is a result of the wing directing air downwards as it passes by.

Even if the thrust is zero and the drag pulls back on the plane, the plane will continue to go forwards because of the momentum from the initial push and Newton's first law, which says that objects in motion will continue moving at the same speed and direction unless external forces act. The drag will slow down the airplane slowly. You can get almost as much lift as you like from most reasonable wing shapes just by changing the "angle of attack" -- the angle the wing is tilted up relative to the plane's direction.

The gravitational potential energy of the airplane is proportional to its height. So if the goal is to maximize height, you want to maximize the gravitational potential energy. You can do this by -- throwing the plane harder (more total energy to start with, and a longer flying time), and by reducing the energy loss due to drag. The number one limitation of lightweight hand-thrown airplanes is the drag slowing them down. If there were no drag, then all of the initial energy can be used to make the plane go higher. In fact, if you are interested *only* in height (and not in soft landings or anything like that), taking the wings off and throwing the fuselage as hard as you can straight up in the air might work best (but if the fuselage is too light, it will flutter and not go as high -- drag will be a problem again.). The usual thing to work on is the total distance or perhaps even the time in the air -- then the wings make a lot of difference, and my suggestion to you would be to minimize drag. Designs that increase lift (and even increasing the angle of attack for increased lift) often result in increased drag too.

There is an effect of catching air currents -- if your plane is very very light, you might get lucky and it will get wafted up by an upward breeze of warm air (called a "thermal" by hang-glider and paraglider experts because it is often caused by warm air rising up from the ground). They don't always happen, and it takes some control to stay within the updraft -- you see birds taking advantage of these all the time, but they have to turn constantly to stay in the updraft.

Tom

(published on 10/22/2007)

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