This video explores the principle of how flying objects transmit their weight to the ground through the air, essentially demonstrating the conservation of momentum. The presenter constructs a large, sensitive custom-built scale utilizing load cells and foam core stiffened with rocket-part-inspired internal structures to measure minute changes in pressure. Initial experiments successfully demonstrate that a paper airplane flying above the scale, or a hovering quadcopter, registers its full weight, with the force transmitted through displaced air.
Further investigation into the nature of this force reveals that the weight is transferred by a broad, low-magnitude pressure wave traveling at or near the speed of sound, rather than localized wingtip vortices. The video details the challenges encountered during the experiments, such as signal-to-noise ratio issues, the need to contain the air column, and the subtle complexities of air pressure dynamics. Finally, the presenter attempts a full-scale experiment at an airport using a modified, sealed pressure sensor to detect the weight of a commercial airliner flying overhead. While quantitative measurements were challenging due to instrument limitations, a detectable pressure blip confirmed the principle, demonstrating the continuous transfer of a flying object's weight to the ground.
Summarizing "How an airplane's weight is transmitted to the ground (and how I measured it)"
0:00 The "Troll Physics" Concept: The video introduces a comic of a bear bouncing a ball to fly, highlighting the underlying principle of momentum exchange. It then posits that this is fundamentally how airplanes "bounce" off the air to fly.
1:41 The Large Scale Setup: The presenter builds a very large, sensitive scale using four load cells, an Arduino, and a laptop to measure small forces across a wide area.
3:40 Core Demonstration - Paper Airplane: The scale successfully measures the weight of a paper airplane as it flies above the platform, not touching it.
3:58 The Physics Explained: Any object flying pushes air downwards, which eventually impacts the ground, thereby transmitting the object's weight due to conservation of momentum.
5:05 Quadcopter Test: A hovering quadcopter's weight is also accurately registered by the scale, whether it's flying above or resting on the platform.
6:33 Scale Construction Details: The custom scale's platform is made from lightweight yet stiff foam core, inspired by rocket structures with internal "veins" to enhance rigidity without adding significant mass.
6:41 Load Cell Mechanics: The video provides a detailed explanation of how load cells work: tiny wires inside them change electrical resistance when stretched or compressed by mechanical stress.
13:17 Initial Challenges and Solutions: Early paper airplane tests were inaccurate because air displaced by the plane would spread under the scale, causing negative readings. This was solved by adding paper ramps and walls to contain the air and direct it onto the scale.
14:41 Nature of the Pressure Wave: The scale's response time revealed that the weight is transmitted by a wide, low-magnitude pressure wave moving at the speed of sound, not the slower-descending wingtip vortices.
20:15 Clarifying Lift and Weight: The presenter explains that even a descending paper airplane still generates lift (and registers its weight) because it's not accelerating downwards; it's supporting its weight by pushing air down.
21:28 G-Force Demonstration: By modifying a paper airplane to pitch upwards, the scale briefly registers a weight three times higher than the plane's actual weight, simulating 3Gs.
22:45 Playing Card Experiment: Attempts to measure the weight of a flying playing card demonstrate the extreme sensitivity required and the challenges posed by low mass and complex flight dynamics.
26:12 Full-Scale Airplane Experiment: The presenter builds a sealed pressure sensor from a wooden box with a load cell to measure atmospheric pressure changes caused by real airplanes.
28:35 Successful Detection (Second Attempt): After initial failures due to weather interference, a second attempt yields successful detection of small pressure "blips" corresponding to commercial airliners flying overhead at lower altitudes.
30:22 Quantitative Limitations: Despite successful detection, attempts to calculate the plane's total weight from the single sensor's data (extrapolating with Gaussian curves) yield significantly underestimated results (2 tons vs. actual plane's much higher weight).
32:28 Source of Inaccuracy: The main culprit for inaccuracy was determined to be the flexible tape used to seal the pressure sensor box, which allowed minute volume changes that distorted the pressure readings. Stiffer, more precise materials are needed for accurate measurements.
34:11 Conclusion and Takeaway: Despite the quantitative challenges, the experiment qualitatively confirmed that an airplane's weight is continuously transmitted to the ground through the air, reinforcing the principle of conservation of momentum.