The RotorLab newsletter

A lithium pack is not a constant-voltage device. Pull hard on it and the voltage dips below its resting value; the harder the pull and the colder the cells, the deeper the dip. That dip is sag, and it is the difference between the endurance you calculated at the desk and the low-voltage warning that interrupts a winter survey.

Cold matters because internal resistance rises as the chemistry slows down. The same pack that holds voltage comfortably in July can sag through the floor on a frosty morning at the same throttle — the energy is still in the cells, but you cannot get it out fast enough without the voltage collapsing.

Two habits pay for themselves: warm packs before winter flights, and plan margins against sagged voltage rather than resting voltage. RotorLab's flight advisor computes the sag margin for a build explicitly, so a plan that only works on a warm pack gets flagged before anyone is standing in a cold field finding out.

Every fleet has a story that starts with a spec sheet. The motor page said 1.2 kg of thrust, four motors, airframe weighs 2 kg, so the math said fine — and the aircraft sat down in the far corner of the field anyway.

Spec-sheet numbers are bench numbers: a fresh pack at full charge, no payload, sea level, and a throttle the pilot never actually flies at. Real endurance and real thrust live on curves, not in single numbers — the battery's voltage falls as it discharges, the motor's thrust falls with the voltage, and the aircraft needs more of that shrinking thrust the heavier and windier the day gets.

This newsletter exists to walk those curves one at a time: sag, thrust-to-weight, prop choice, hover efficiency, radio range, and the operational side that keeps an aircraft flyable on paper as well as in the air. RotorLab computes all of it from real physics rather than optimism, and each issue will show the math so you can check it yourself — the same math the free Log Analyzer and the Builder run on every plan.