FAA Aerodynamics of sUAS 2 — Questions and Answers
Question 1: What is 'pitch' as it relates to sUAS propeller blades?
- The rotational speed of the motor in RPM
- The theoretical distance a propeller would advance through the air in one complete revolution (Correct answer)
- The total diameter of the propeller arc
- The weight distribution along the length of the blade
Correct answer: The theoretical distance a propeller would advance through the air in one complete revolution
Pitch is the theoretical forward distance a propeller travels per revolution based on blade angle — a higher pitch moves more air per revolution but demands more torque from the motor.
Question 2: Why do most multirotor sUAS use counter-rotating propellers on opposite arms?
- Counter-rotating propellers generate more total lift per unit of power
- They cancel out the reactive torque effect, preventing the aircraft body from spinning (Correct answer)
- They reduce electrical interference between adjacent motors
- Counter-rotating propellers are significantly quieter than same-direction propellers
Correct answer: They cancel out the reactive torque effect, preventing the aircraft body from spinning
Each spinning rotor creates reactive torque that would rotate the aircraft body opposite to the rotor's direction; counter-rotating pairs cancel these torques to maintain yaw stability.
Question 3: What aerodynamic principle allows a multirotor sUAS to accelerate into forward flight from a hover?
- Tilting the aircraft forward converts a portion of rotor thrust from vertical lift into horizontal propulsion (Correct answer)
- Differential thrust between front and rear rotors pushes air backward
- Increased motor speed creates a low-pressure zone that pulls the aircraft forward
- The landing gear extends to create a rearward aerodynamic drag surface
Correct answer: Tilting the aircraft forward converts a portion of rotor thrust from vertical lift into horizontal propulsion
By pitching nose-down, the total thrust vector tilts forward — dividing into a vertical component (lift) and a horizontal component (forward thrust) to produce acceleration.
Question 4: How does increased payload weight primarily affect the hover efficiency of a multirotor sUAS?
- It improves hover efficiency by stabilizing the aircraft
- It reduces hover efficiency by requiring higher RPMs, increasing power consumption and reducing flight time (Correct answer)
- It has no effect on hover efficiency below maximum takeoff weight
- It reduces power consumption because added weight pushes air down more effectively
Correct answer: It reduces hover efficiency by requiring higher RPMs, increasing power consumption and reducing flight time
Heavier weight requires more thrust, demanding higher motor RPMs that consume more battery power, directly reducing hover endurance and overall flight time.
Question 5: What is 'translational lift' and when does it occur in sUAS operations?
- Lift generated by horizontal tail surfaces during forward flight
- Improved rotor efficiency that occurs as the sUAS transitions from hover into forward flight, entering undisturbed air (Correct answer)
- A sudden loss of lift when crossing between two different air masses
- Lift generated by the aircraft fuselage shape during high-speed forward flight
Correct answer: Improved rotor efficiency that occurs as the sUAS transitions from hover into forward flight, entering undisturbed air
As the aircraft moves forward, rotors continuously encounter undisturbed air rather than recirculating their own downwash, making each revolution more efficient and producing more lift for the same power.
Question 6: What is the aerodynamic effect of propeller blade damage such as chips, cracks, or nicks on sUAS flight?
- Minor blade damage has no measurable effect on flight performance
- Blade damage causes vibration, reduces thrust efficiency, and can lead to catastrophic failure mid-flight (Correct answer)
- Damaged blades only affect operations above 200 feet AGL
- Blade damage causes increased lift due to additional disturbed airflow patterns
Correct answer: Blade damage causes vibration, reduces thrust efficiency, and can lead to catastrophic failure mid-flight
Even minor blade damage creates aerodynamic imbalance that produces vibrations stressing motor bearings, reducing sensor accuracy, and risking catastrophic structural failure during flight.
Question 7: What role does the flight controller's IMU (Inertial Measurement Unit) play in maintaining aerodynamic stability?
- It adjusts rotor blade pitch to compensate for wind gusts
- It measures acceleration and angular rates, enabling the flight controller to adjust individual rotor speeds to correct attitude deviations (Correct answer)
- It measures air pressure to maintain a constant barometric altitude
- It controls the camera gimbal to compensate for aircraft movement
Correct answer: It measures acceleration and angular rates, enabling the flight controller to adjust individual rotor speeds to correct attitude deviations
The IMU detects tiny changes in pitch, roll, and yaw many times per second, enabling the flight controller to make rapid rotor speed adjustments that keep the aircraft aerodynamically stable.
FAA Aerodynamics of sUAS 2
What is 'pitch' as it relates to sUAS propeller blades?