Powerplant and Control Link Knowledge
This domain tests the electric propulsion chain and the command-and-control link - the two systems most often responsible for a forced landing.
Electric propulsion
- LiPo cell voltages: 3.7 V nominal, 4.2 V fully charged, land by roughly 3.5 V per cell; store at about 3.8 V.
- C rating describes safe discharge current relative to capacity; internal resistance rises with age and heat, producing voltage sag under load.
- Motor KV multiplied by pack voltage gives approximate no-load RPM; propeller diameter and pitch convert that into thrust and current draw. Oversized props overheat ESCs.
- A multirotor typically wants thrust-to-weight near 2:1 for control margin.
Control link and video
2.4 GHz control and 5.8 GHz video are common; both need line of sight and a clear Fresnel zone. Buildings, terrain, wet foliage and the aircraft's own body shadow the signal. Antenna polarization must match. Interference from Wi-Fi and broadcast towers shows as RSSI fluctuation. Radio equipment must be NTC type-accepted and within EIRP limits.
Worked example
A 5000 mAh pack powers a 20 A hover. Usable energy at 80% depth of discharge is 4.0 Ah, so endurance is 4.0 ÷ 20 = 0.2 h, or 12 minutes; a 20% in-flight reserve brings the planning figure to about 10 minutes. Reasoning: capacity in Ah divided by current in A gives hours, and the usable fraction must exclude both the reserve and the aging margin.
Traps
Fitting a larger pack without recalculating weight and current; ignoring sag in cold or hot conditions; dismissing a low RSSI warning as normal.