The Basic Physics of Drones
Now, the effects of loading and weight on aircraft (sUAS) performance
- Flying an aircraft overweight is detrimental to its performance
- It is critical to adhere to the weight and balance limits specified by the manufacturer of your aircraft (you will see this document referred to as the ‘UAS Flight Manual’)
- If your manufacturer does not provide this information, you should use the best information available to you to derive your own limits)
An overloaded aircraft (excess weight) will exhibit the following:
- flight time is reduced
- climb rate is reduced
- stability is reduced
- available thrust is reduced/maneuverability reduced
- windy conditions, high altitudes, high humidity will magnify the above issues
An out-of-balance aircraft will result in the motors on the heavy end of the aircraft to work harder, resulting in possible:
- stall
- loss of altitude
- loss of control
- windy conditions, high altitudes, high humidity will magnify the above issues
Center of Gravity (CG)
- The point at which the aircraft will balance
- Center of the aircraft for multi-rotor
- Forward CG gives us:
- higher stall speed
- slower cruise speed
- more stable
- Aft CG gives us:
- slower stall speed
- faster cruise speed
- less stable
Load Factor – the load we are imposing on our aircraft
- Anytime an aircraft is at an attitude other than straight and level flight, load is imposed on it (very noticeable while turning)
- Performance decreases due to an increase in load factor when the aircraft is operated in maneuvers other than straight and level flight
- The PIC should be mindful of the increased load factor and its possible effects of the aircraft’s structural integrity and the results of an increased stall speed
- Critical Angle of Attack: The angle at which lift is no longer generated thus entering into an aerodynamically-induced stall; (another way of defining it: the angle of attack which products the maximum lift – above this angle, the aircraft stalls)
- Stall occurs when airflow is separated from the wing (airfoil)
Load Factor Chart
- 0º bank, the load factor is 1G
- 60º bank, load factor increases to 2G
Furthermore, if an airplane weighs 30 pounds, the wings will need to support a load of
- 60 pounds if banking at 60º (2 * 30)
- 42 pounds if banking at 45º (1.4 * 30)
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(Question from https://jrupprechtlaw.com/part-107-knowledge-test/)
If an unmanned airplane weighs 33 pounds, what approximate weight would the airplane structure be required to support during a 30° banked turn while maintaining altitude (refer to the load factor chart above)?
[Explanation: In a turn of 30 degrees of bank and while maintaining level flight (no altitude loss because you slightly pitched up), you will have a 1.154 load factor. This means that in this turn you will be feeling like you are pulling 1.154 G’s. 33 pounds x 1.154 = 38.082 pounds].
A.34 pounds.
B. 47 pounds.
C. 38 pounds.
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Density Altitude
- Flying in dense air is ideal
- Standard Conditions: true altitude (above sea level) = density altitude
- For example, in standard atmospheric conditions (ISA), the density altitude at the beach in San Diego (sea level) is 0 feet, while the density altitude in Denver is 5500 feet.
- High density altitude:
- Temperature increases, density altitude increases (air becomes less dense)
- Humidity increases, density altitude increases (air becomes less dense)
- Air pressure decreases (from standard), density altitude increases (air becomes less dense)
- Aircraft performance decreases
- Another example:
- On a hot summer day in Denver, with high humidity and a temperature over 100º F, the density altitude would be over 10,000 feet even though the true altitude of 5500 feet is unchanged. The aircraft “feels” as if it was at 10,000 feet and its performance will be decreased.