Motor and propeller physics basics in Drone Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Calculate thrust from motor RPM and propeller pitch

Given the formula thrust = k * rpm * pitch, where k is a constant 0.1, rpm is motor speed, and pitch is propeller pitch in inches, what is the output of the code below?

Drone Programming

k = 0.1
rpm = 5000
pitch = 4
thrust = k * rpm * pitch
print(round(thrust, 1))

A 2000.0

B 1000.0

C 20.0

D 200.0

💡 Hint

Multiply the values step by step: 0.1 * 5000 * 4.

The thrust is calculated as 0.1 * 5000 = 500, then 500 * 4 = 2000.0. The code prints round(thrust, 1), which outputs 2000.0 (option A).

❓ Predict Output

intermediate

Calculate power consumption from voltage and current

What is the output of the following code that calculates power consumption of a motor?

Drone Programming

voltage = 11.1
current = 15.5
power = voltage * current
print(round(power, 2))

A 26.6

B 1.43

C 172.05

D 0.71

💡 Hint

Power is voltage multiplied by current.

Power = 11.1 * 15.5 = 172.05 watts.

❓ Predict Output

advanced

Calculate torque from power and angular velocity

Given power in watts and angular velocity in radians per second, torque is calculated as torque = power / angular_velocity. What is the output of the code below?

Drone Programming

power = 150
angular_velocity = 50
try:
    torque = power / angular_velocity
    print(round(torque, 2))
except ZeroDivisionError:
    print('Error')

A Error

B 0.33

C 75.0

D 3.0

💡 Hint

Divide power by angular velocity.

Torque = 150 / 50 = 3.0 Nm.

❓ Predict Output

advanced

Calculate thrust-to-weight ratio

What is the output of the code that calculates thrust-to-weight ratio given thrust and weight?

Drone Programming

thrust = 120
weight = 100
ratio = thrust / weight
print(round(ratio, 2))

A 1.2

B 0.83

C 220

D 20

💡 Hint

Divide thrust by weight.

Thrust-to-weight ratio = 120 / 100 = 1.2.

❓ Predict Output

expert

Calculate propeller efficiency from thrust, velocity, and power

Propeller efficiency is calculated as efficiency = (thrust * velocity) / power. Given thrust in newtons, velocity in meters per second, and power in watts, what is the output of the code below?

Drone Programming

thrust = 15
velocity = 10
power = 300
efficiency = (thrust * velocity) / power
print(round(efficiency, 3))

A 0.05

B 0.5

C 5.0

D 50.0

💡 Hint

Multiply thrust and velocity, then divide by power.

Efficiency = (15 * 10) / 300 = 150 / 300 = 0.5.

Practice

1. Which factor directly increases the thrust generated by a drone's propeller?

easy

A. Lowering the drone's weight

B. Increasing the motor speed (rpm)

C. Reducing the battery voltage

D. Decreasing the propeller size

Solution

1. Step 1: Understand thrust formula basics

   Thrust depends on the square of the motor speed (rpm) and the propeller area.

2. Step 2: Identify which factor increases thrust

   Increasing rpm increases thrust because thrust ∝ rpm² x propeller area.

3. Final Answer:

   Increasing the motor speed (rpm) -> Option B

4. Quick Check:

   Thrust ∝ rpm² [OK]

Hint: Thrust grows with rpm squared, so speed up motor [OK]

Common Mistakes:

• Thinking smaller propellers increase thrust
• Confusing power with thrust
• Ignoring rpm's squared effect

2. Which of the following is the correct Python syntax to calculate power given rpm and propeller_area variables?

easy

A. power = rpm ** 2 + propeller_area

B. power = rpm * 3 * propeller_area

C. power = rpm ^ 3 * propeller_area

D. power = rpm ** 3 * propeller_area

Solution

1. Step 1: Recall Python exponent syntax

   Python uses ** for exponentiation, so rpm cubed is rpm ** 3.

2. Step 2: Match formula for power

   Power depends on rpm cubed times propeller area, so multiply rpm ** 3 by propeller_area.

3. Final Answer:

   power = rpm ** 3 * propeller_area -> Option D

4. Quick Check:

   Use ** for power, not ^ or *3 [OK]

Hint: Use ** for powers in Python, not ^ or * number [OK]

Common Mistakes:

• Using ^ instead of ** for exponent
• Multiplying rpm by 3 instead of cubing
• Adding instead of multiplying

3. What is the output of this Python code snippet?

rpm = 1000
propeller_area = 0.05
thrust = rpm ** 2 * propeller_area
print(round(thrust, 1))

medium

A. 50000.0

B. 50000

C. 50000.00

D. 50.0

Solution

1. Step 1: Calculate rpm squared

   rpm ** 2 = 1000 * 1000 = 1,000,000.

2. Step 2: Multiply by propeller_area

   1,000,000 * 0.05 = 50,000.0.

3. Step 3: Check print rounding

   round(50000.0, 1) = 50000.0 and print outputs 50000.0.

4. Final Answer:

   50000.0 -> Option A

5. Quick Check:

   Thrust = 1000² x 0.05 = 50000.0 [OK]

Hint: Square rpm, multiply by area, then round [OK]

Common Mistakes:

• Misreading rounding precision
• Calculating rpm squared incorrectly
• Confusing thrust with power

4. Identify the error in this Python code that calculates power:

rpm = 1200
propeller_area = 0.04
power = rpm ^ 3 * propeller_area
print(power)

medium

A. Multiplying instead of adding propeller_area

B. Missing parentheses around rpm and 3

C. Using ^ instead of ** for exponentiation

D. Incorrect variable names

Solution

1. Step 1: Check exponentiation operator

   In Python, ^ is bitwise XOR, not exponentiation. Use ** for powers.

2. Step 2: Understand impact on calculation

   Using ^ causes wrong calculation or unexpected result, not power of rpm.

3. Final Answer:

   Using ^ instead of ** for exponentiation -> Option C

4. Quick Check:

   Use ** for powers, not ^ [OK]

Hint: Remember ** means power, ^ is bitwise XOR [OK]

Common Mistakes:

• Confusing ^ with **
• Assuming parentheses fix ^ operator
• Ignoring operator precedence

5. You want to program a drone to maintain constant thrust while reducing motor speed to save battery. If you reduce rpm by 20%, how should you adjust the propeller area to keep thrust constant? (Thrust ∝ rpm² x propeller_area)

hard

A. Increase propeller area by 56.25%

B. Increase propeller area by 25%

C. Decrease propeller area by 20%

D. Keep propeller area the same

Solution

1. Step 1: Calculate new rpm ratio squared

   Reducing rpm by 20% means new rpm = 0.8 x original rpm. Squared: 0.8² = 0.64.

2. Step 2: Find required propeller area increase

   To keep thrust constant, propeller_area x 0.64 = original propeller_area. So, new propeller_area = 1 / 0.64 ≈ 1.5625 times original, or 56.25% increase.

3. Final Answer:

   Increase propeller area by 56.25% -> Option A

4. Quick Check:

   Thrust constant -> area ∝ 1 / (rpm²) = 1/0.64 = 1.5625 [OK]

Hint: Square rpm ratio, invert it for area change [OK]

Common Mistakes:

• Forgetting to square rpm ratio
• Adding instead of multiplying percentages
• Assuming area stays same