China Civil UAV Theory Questions — Questions 346–355

Study note: Ni-Cd has strong memory effect, is cheap, and must not be discharged; otherwise choose lithium batteries. Li-po lithium polymer batteries have the highest energy density and the largest capacity for the same weight.

The stem's “same multi-axis aircraft” means both prop types are driven by the same motor. The longer the prop diameter, the greater the pull at the same motor speed; the smaller the diameter, the smaller the pull. With the same diameter, pitch, and motor speed, more blades give more pull, but also greater rotational inertia and air resistance. For a three-blade prop to reach the same speed as a two-blade prop, the motor needs more current, and when current rises to ten or even tens of amps, the motor's and ESC's internal resistance consume some power as heat, reducing the kinetic energy the motor imparts to the prop. Reaching the same speed with a multi-blade prop therefore consumes more power. So normally three- and four-blade props are less efficient than two-blade props.

Study note: Ni-Cd has strong memory effect, is cheap, and must not be discharged; otherwise choose lithium batteries. Li-polymer batteries are the most expensive; Ni-MH batteries are more environmentally friendly than Ni-Cd and also cost more.

Multi-axis aircraft power systems mainly use electric motors, which are simple, light, and convenient. Drive motors are brushed or brushless; brushed motors are less efficient and have gradually fallen out of use in UAVs.

The more coil turns, the lower the kV value. The more turns in a motor's coils, the lower the speed at the same voltage: each coil carries magnetic flux, more turns mean greater flux, and for the same voltage the motor faces greater resistance, so its speed drops.

1 positive lead and 6 negative leads, so a 6S battery has 7 leads.

From the lift formula, fitting 4 much larger propellers increases wing area S and reduces disk loading, but the larger rotors increase the drive motor load and speed V drops, so greater lift is not guaranteed.

Study note: Ni-Cd has strong memory effect, is cheap, and must not be discharged; otherwise choose lithium batteries. Li-polymer batteries have no memory effect, Ni-Cd batteries have severe memory effect, and Ni-MH batteries have less memory effect than Ni-Cd.

An airfoil is understood as a two-dimensional wing: its front is rounded, its pointed end is called the trailing edge; the point on the airfoil farthest from the trailing edge is called the leading edge; the line connecting the leading and trailing edges is the chord, whose length is the chord length. Inside the airfoil, a series of inscribed circles tangent to the upper and lower surfaces is drawn; the line connecting their centers is the airfoil's mean camber line, the largest inscribed circle's diameter is the airfoil's thickness, and the maximum distance between the mean camber line and the chord is the camber; the leading edge's radius of curvature is the leading-edge radius. A supersonic airfoil's leading edge may also be sharp. Relative thickness and relative camber are defined as thickness and camber divided by chord length; an airfoil with zero camber is called a symmetric airfoil, whose mean camber line coincides with the chord.

This question tests battery capacity and discharge rate C. C is the battery's discharge capability, called “discharge rate”: discharge current divided by battery capacity. A 1C discharge means emptying the battery in 1 hour; 2C means emptying it in 30 minutes; 0.5C means emptying it in 2 hours. Option A: 2000mAh × 30C = 60000mA = 60A. Option B: 20000mAh × 5C = 100000mA = 100A. Option C: 8000mAh × 20C = 160000mA = 160A. Option C has the largest value and the largest discharge current.