Engineering YouTuber Weighs An Airbus A320 Plane Whilst It Is Still Flying

Material scientist Brian Haidet has weighed a plane while it was flying for his YouTube channel AlphaPhoenix. 

To weigh a plane, usually you would place scales underneath the landing gears, nose gears, and wing support points. But if you don't have access to a plane and plane scales – or you just want to do something cool for fun – it is possible to take a very rough measurement of its weight using some fairly simple devices.

In the new video posted to AlphaPhoenix, Haidet did just this, creating a set of sensitive scales before attempting to weigh a commercial plane. How?

"As any object moves through the air it has to push that air out of the way. And if that object is really strategically shaped, it can push most of this air downwards, which results in an equal and opposite reaction pushing up on the moving object, keeping it in the air. Typically, this process is referred to as flying," Haidet explains in the video.

"But that air that's been accelerated downwards below the wing, everybody always forgets about. Where does that air go? Well, it's moving downwards; it can't move down forever. Eventually, it hits the ground. And when that moving air reaches the ground, it can't go any farther. It stops. The air exerts a force on the ground, and the ground exerts a force on the air, decelerating the air. In this way, the total weight of any flying object is eventually transmitted to the ground."

Haidet first created a large scale using a flat piece of material and load cells, a type of transducer that deforms as a force is applied to it, altering its resistance. This difference in resistance can be measured in order to deduce weight (if the force were, for example, you standing on the load cells). 

He then conducted a number of tests on the homemade scales, including throwing a lot of paper airplanes over the device to check that it was registering downwards force created by the airplane. In cool demonstrations, the scales move as the plane exerts a downward force on the air to remain flying on a straight or upwards path.

While that was pretty much point proven, Haidet then headed to an airport to try the same thing with a full-scale airplane. For a real plane, he adjusted the scales a little. Instead, he measured pressure changes within a box and compared it to the outside, but again using load cells to do the job. 

The conditions were not ideal, with wind and a lack of a giant scale to measure the whole plane's weight, spread out as it is. But after a few attempts, Haidet was able to see the telltale spike of a plane passing overhead, as well as a "weird negative signal" that kept appearing as he measured other planes.

Taking the best read, with the smallest negative signal, he attempted to make some rough estimates of the plane's weight, using a number of assumptions about the plane's speed and distance. The weight he calculated using these assumptions was around 2 tons, far short of the Airbus A320's true weight. But it's pretty impressive that you can detect and (very roughly) measure the weight of a plane from the ground below.

"I was super excited to have detected any signal with this scale," Haidet added. "And I can say that, you know, although I probably had some systematic error in my measurement that I haven't yet accounted for, I was able to measure the weight of an airplane while that airplane was in the air. I think that's pretty cool."