Weightlessness During Parabolic Flight:
How to Get Zero Gs in an Ordinary Airplane


To experience weightlessness for relatively long periods of time, but without the danger and expense of going into outer space, scientists fly an airplane in a parabolic flight path. The airplane follows the same path as a freely falling cannon ball that has been fired into the air. Passengers and equipment in the airplane experience weightlessness just like in outer space.

Why must the airplane fly in a parabolic path to cause weightlessness? And why aren't passengers weightless when the airplane is flying normally?

NASA weightless airplane
NASA C-9B Reduced Gravity Aircraft during parabolic ascent. Photo courtesy of NASA.


Weightlessness in a Flying Automobile


To understand how this works, first imagine that we'll use an automobile to achieve the same effect. In the following figure, the driver speeds along on a flat, level road, then hits a ramp and flies up into the air at a 45-degree angle. The occupants are weightless while the car flies freely through the air. The car lands on another ramp and changes back to moving on level ground again.

Weightless
                    automobile
Closed course, professional driver. Do not attempt.

There are five phases, labeled A through E in the diagram.
Let's examine in detail what is happening in each phase.

During phase A, the car travels horizontally at a constant speed. The force of the road pushing up against the car's tires holds the car up. The car is held steady (not falling freely), so the occupants feel normal weight.

During phase B, the car ascends the ramp. The car is forced upward against gravity, changing its direction of motion from horizontal to diagonal-upward, causing the occupants to feel greater-than-normal gravity, or "Gs" of force. The amount of G-force depends on the curvature of the ramp. The tighter the curve, the greater the G-force.

During phase C, the car flies freely, without any external forces acting against it, except for wind (aerodynamic drag), which we will consider negligible in this discussion. Accordingly, the occupants are weightless during this part of the ride.

During phase D, the car descends the ramp. The car is forced upward against its downward path, changing its direction from diagonal-downward to horizontal, causing the occupants to feel greater-than-normal gravity. As with the first ramp, the amount of G-force depends on the curvature of the ramp.

During phase E, the car travels horizontally at a constant speed, just like phase A. The force of the road pushing up against the car's tires holds the car up and the occupants feel normal weight.

If the car leaves the first ramp at a speed of 65 miles per hour (105 km/hr) at an angle of 45 degrees, it will achieve a height of about 70 feet (22 meters) above the ramp, providing about 4 seconds of weightlessness between the two ramps.


If there is no second ramp at region D, but a cliff instead, the car would continue to fall along the parabolic path shown in the figure, perhaps adding another second or two of weightlessness, depending on the height of the cliff.


Weightlessness in a Flying Airplane

Now let's consider the path of the zero-G airplane. It works just like the car, as shown in the following figure.

Parabolic flight for
                  weightlessness

Again, there are five phases, labeled A through E in the diagram.

During phase A, the airplane travels horizontally at a constant speed. The force of lift on the wings holds up the airplane. The airplane is held steady (not falling freely), so the passengers feel normal weight.

During phase B, the pilot pulls back on the control stick, forcing the airplane upward against gravity. The strong aerodynamic force against the wings pushes up the airplane, just like the ramp in the previous example. This changes the direction of motion from horizontal to diagonal-upward, causing the passengers to feel greater-than-normal gravity, or "Gs" of force. The amount of G-force depends on the curvature of the airplane's path. The tighter the curve, the greater the G-force.

During phase C, the airplane flies freely, without any external forces acting against it, except for wind (aerodynamic drag). The pilot can exactly cancel the effects of drag by keeping the engines running at just the right level. The passengers are weightless during this part of the ride.

During phase D, the pilot pulls back on the control stick, forcing the airplane upward against its downward path.
The strong aerodynamic force against the wings pushes up the airplane from its downward path, just like the ramp in the previous example. This changes the direction of motion from diagonal-downward to horizontal, causing the passengers to feel greater-than-normal gravity. As with the first upward change in path, the amount of G-force depends on the curvature of the airplane's path.

During phase E, the airplane travels horizontally at a constant speed, just like phase A. The force of lift on the wings holds up the airplane and the passengers feel normal weight.

If the airplane starts its ascent at a speed of 550 miles per hour (885 km/hr) at an angle of 45 degrees, it will achieve a height of about 5,000 feet (1500 meters) above the "ramp," providing about half a minute of weightlessness in the parabolic flight path.
If you need some additional weightlessness time, you can allow the plane to continue falling along the parabolic path shown below the letters D and E.


Wait a Minute ...

You might notice that the airplane is flying horizontally during
both phase A and at the exact middle of phase C. So why do the passengers feel normal weight in the former case and weightlessness in the latter case, even though the direction of travel is exactly the same?

The answer lies in what happens just before and after the point in question. During phase A, the motion of the airplane is steady and unchanging. In the middle of phase C, the direction of travel is changing -- before the airplane it traveling upward, then level, then downward; it is following the path of a freely falling cannon ball. Freely falling objects don't travel in a straight horizontal line.


How to Experience Weightlessness

If you would like to experience weightlessness in an airplane, book a reservation for a flight on the Zero-G Corporation Boeing 727. The fare is $4,950. If you are a student and you want to do a zero-G experiment of your own design, submit a proposal to the NASA Microgravity University. You might earn a slot on a NASA zero-G flight at no cost!

For an even simpler zero-cost alternative, see the
next web page.

For more information about parabolic flight, see the Wikipedia article "Reduced gravity aircraft."

For a highly technical description of parabolic flight, see "The dynamics of parabolic flight: flight characteristics and passenger percepts," by Faisal Karmali, Ph.D. and Mark Shelhamer, Sc.D., published by the National Institutes of Health (NIH).

Next: Weightlessness in Outer Space




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2009 Gray Chang