Welcome to Brainstuff from house Stuff Works dot com where smart happens. Hi Marshall Brain with today's question, how does the spedometer in an airplane work? Air speed is a measurement of the plane speed relative to the air around it, not to the ground, but to the body of air that the plane is flying through. The pet tube is an ingenious device used by airplanes and votes for that matter,
for measuring forward speed through a medium. The device is really a differential pressure gauge and was invented by Henry Peto in seventy two. An example of an air pressure gauge is a tire pressure gauge. It measures the pressure inside the tire. A Peto tube is doing that same kind of thing, measuring the air that's flowing path us the airplane. The open end of the pet tube, usually mounted on a wing or with a little angle bracket on the fuselage, faces towards the flow of air or water.
The air speed indicator actually measures the difference between a static sensor not in the air stream and a sensor the p DO tube in the air stream when the airplane is standing still. The pressure in both these places is equal. In the air speed equal zero. The rush of air in flight causes a pressure difference between the static tube and the p DO tube that's in the air stream. That pressure differential makes the pointer on the air speed indicator move. An increase in forward speed raises
the pressure at the end of the pet tube. In turn, the air pressure pushes against a flexible diagraphragm that moves a connected mechanical pointer on the face of the indicator. The indicator is calibrated to compensate for winds in the air or the speed of the opposing current in the water. In airplanes, electronics also compensate for altitude and air temperature
to make the air speed measurement accurate. It's possible to take this basic pressure difference idea and make it really complicated with digital readouts and digital sensing and all kinds of computer stuff. But it's also possible to take it in the other direction and make it incredibly simple. A little mechanical device that you might hang off a hang glider or stick outside a car, or use on a boat. It could be just a little slider inside a tube.
Such a device is known as a direct reading airspeed indicator. The instrument is for paragliding or ultralight aircraft, or any kind of simple plane like that that needs a lightweight, inexpensive airspeed indicator. The difference between the static pressure and the pressure of the air rushing into the opening of the bottom of the tube causes a red indicator disc to rise or fall, since the upward pressure in the
tube is greater than the weight of that disc. If you look at one of these, you'll notice that there's a slight taper in the design of the vertical tube. The red disc has a hole in the center of it for the slighter rod to keep the disc level. The airspeed indicator fastens to a strut of the hang glider. Peedo tube devices and minometers are also used in commercial heating and air conditioning systems for duct air flow measurements.
You just stick the device in the duct and you can read off the speed of the air flowing through that duct. Be sure to check out our new video podcast, Stuff from the Future. Join House to Park staff as we explore the most promising and perplexing possibilities of tomorrow. The House of First I Find app has arrived down at it today on iTunes