120ccpm
Captain
Flight distance : 1245115 ft
United States
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Occams Razor Posted at 5-2 14:27
None of the literature talks about this force you keep mentioning. What is trying to rotate the drone back to level?
The force is gravity. In your diagram, you are basically rotating the drone around the center of mass. If all 4 propellers are producing roughly the same amount of thrust, gravity is going to want to bring the drone back to a state of equilibrium which is the horizontal position. Think of a seesaw on a fulcrum. If you tilt one end, it will not sustain because gravity is trying to achieve equilibrium and will push the seesaw level. Just like you need additional forces to keep a seesaw inclined, you need additional forces from one side to keep a drone in a tilted position. The extra thrust from rear propellers will put the drone in a new equilibrium position which is the forward pitch position. The only way to keep the drone in a forward pitch position with all 4 motors running at the same RPM's is to shift the center of gravity to the front of the drone but this would not be practical.
You picked the right example, but came to the wrong conclusion: an "ideal" seesaw where the pivot point is exactly at the CoG is in a state of equilibrium, regardless if it's tilted or horizontal. Thus, you need zero force to keep it tilted, it will stay in whatever position to put it, on its own.
The seesaw tries to rest horizontally only if the CoG is below the fulcrum.
As I said on the other thread, if you take the ideal quadricopter, perfectly symmetrical, 50/50 weight distribution, motors perfectly aligned to vertical axis, CoG right at center of the 4 corners, and if you ignore drag and other aerodynamic effects (which nobody is in a position to quantify, anyways), then you realize pretty easily that you don't need the rear motors to push more, to MAINTAIN a tilt. You need more power to cause the AC to tilt, but once it's there, all motors need to spin the same way.
The "byproduct" of the tilt, is a horizontal component of the combined thrust, that causes the AC to move forward.
Things in reality are much more complicated, well beyond my knowledge, as the CoG is not exactly between the motors, motors often are not perfectly aligned to vertical axis and as soon as the drone moves forward, you have drag and other forces that can have significant impact on the AC, causing imbalance. But again, in a simplified model, there is no reason for the rear motors to work harder. |
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