Tiny dents and cracks on drone propellers and high-frequency jelly vibrations caused by dynamic imbalance

1. Physical centrifugal force and mechanical amplification of dynamic balance imbalance

The brush/brushless motors of aerial photography drones (such as DJI Mavic 3, Inspire 3 or various FPV flying machines) usually rotate as high as 5,000 to 12,000 revolutions per minute (RPM) when hovering and flying at high speeds!

1.1 Geometric explosion of centrifugal force and centripetal acceleration

According to the centrifugal force formula of classical rotational mechanics:
$$F_{\text{centrifugal}} = m \cdot r \cdot \omega^2$$
Among them, $\omega$ is the angular velocity of the motor, which is proportional to the rotation speed; $r$ is the blade tip radius, and $m$ is the deviation mass of the center of mass.

When the blades are intact, the masses of the two symmetrical blades are strictly aligned with the geometric center of aerodynamic lift, and the resultant centrifugal force vector sum is absolute zero;
Lethal amplification of tiny damage:
Suppose a blade tip loses only 0.05 grams (equivalent to the weight of a tiny grain of sand) of plastic due to a collision:
Under high-speed rotation of 8,000 RPM, this insignificant 0.05-gram unbalanced mass will generate 130 ultra-high-frequency centrifugal alternating shear forces per second on the motor shaft at the level of several kilograms! **

1.2 Conduction of high-frequency vibration and Jello Effect

The motor violently tears the carbon fiber arm at a frequency of 100-200Hz;
Violent high-frequency micro-tremors penetrate the drone body frame and directly impact the gimbal damper ball (Gimbal Damper);
When the vibration frequency breaks through the physical damping attenuation limit of the gimbal’s silicone shock-absorbing pad, the microshocks are directly transmitted to the camera CMOS sensor;
Rolling Shutter and vibration resonance:
CMOS sensors expose and read pixels line by line. When the exposure row is scanned from top to bottom, the camera is vibrating violently up and down at ultra-high frequency, and the pixels in each row are slightly jagged and misaligned in time and space coordinates. The video image shows a disgusting and crawling disaster like wavy jelly.

2. Four hidden forms of physical damage to blades and fatal dangers

Damage pattern	Visual appearance characteristics	Physical hazard mechanism	Treatment decision
Tip Chipping	0.5mm of plastic missing from the tip edge	Destroying dynamic balance, causing severe high-frequency vibration and jelly effect	Scrap and discard immediately
Stress Whitening	The plastic at the bend of the propeller root shows white matte fine lines	The polymer chain of the material is broken and fatigued. When rotated at high speed and pulled by centrifugal force, the propeller will explode in the air and disintegrate in an instant!	Absolutely no flying! Cut and discard immediately**
Folding shaft limit wear (Hinge Play)	The shaking at the root of the rotating shaft of the folding blades becomes larger	The angle between the propellers is asymmetrical after unfolding, causing periodic shaking of the rotating lifting surface	Replace in pairs immediately
Aerodynamic Angle of Attack Deformation (Warpage)	Exposure or heavy pressure causes the blades to produce weak helical distortions	In order to maintain balance, the left and right motors are forced to output asymmetric violent compensatory speeds, and the ESC motors become rapidly hot and burnt	The entire set is scrapped and updated

3. SOP for blade inspection and dynamic balance testing for professional pilots

3.1 "Nail scraping tactile inspection" before each flight:

Seeing with your eyes is not enough! Use the nails of your thumb and forefinger to run along the edge of the paddle from the base to the tip.
The tactile nerves in human fingertips can detect micron-sized pits, micro-cracks and barbs. Once your fingernails slide over and you feel any jams or scratches, you must stop immediately.

3.2 Practical operation of dynamic balance physical inspection:

For film and television teams that pursue the ultimate in picture quality, use the Propeller Balancer to inspect newly unpacked propellers:

Mount the two paired blades on a frictionless strong magnet suspension frame through a precision metal mandrel;
If one end droops due to slight gravity, it means that the end is too heavy;
Use very fine 2000-grit water sandpaper to lightly sand the back of the heavier side of the blade twice until the blade can maintain an absolutely horizontal static balance in the air.

4. Avoidance of pits and defense line of life and property

[!CAUTION]

Never buy cheap third-party propellers:
Original blades (such as those made by DJI) have undergone high-precision laser dynamic balancing weight testing before leaving the factory, and are wrapped with soft shock-absorbing rubber at the tip of the blade. The low-quality counterfeit hard plastic propellers on the market that only cost a few dollars have rough pneumatic molds and huge dynamic balance errors. Not only do the motors make abnormal noise and vibrate violently, but the hard plastic will act like a sharp blade when it hits the human body, causing extremely terrible lacerations!

The iron law of "pair replacement":
If only one blade of the front right motor is damaged, it is absolutely forbidden to replace only one blade! Both blades on this motor must be replaced as a complete pair**. There is a tiny microgram tolerance in the factory weight of propellers from different batches, and mixing them will inevitably destroy the dynamic balance.