What if an electric plane carried more batteries?

Some small planes run on electricity from batteries.

More batteries could hold more energy. But batteries are heavy! What if the plane carried more of them?

Most planes burn fuel, but engineers are building and testing planes that run on electricity from batteries. Batteries hold the energy for the trip, so carrying more of them sounds like a way to fly farther.

There's a catch: batteries are heavy, and every kilogram a plane carries has to be held up by its wings. In this lab you can load a small plane with extra batteries, then see whether its wings can still make enough lift, and what you could change to get it flying again.

An electric aircraft trades energy for mass: more battery capacity means more stored energy but also more weight, which the wings must support. Weight includes the aircraft, its energy source and its payload (NASA).

This lab isolates the lift side of that trade-off. It uses the airfoil model to ask how added mass changes the lift-to-weight ratio, the speed needed to fly at a given wing tilt and the stall speed. Range, battery energy and drag are separate questions the model does not address.

A few small electric planes already fly. One is used to teach new pilots.

Big airliners still burn fuel.

In June 2020, the European Union Aviation Safety Agency certified the Pipistrel Velis Electro, a two-seater meant mainly for training pilots. It was the first fully electric plane in the world to get this kind of approval, called a type certificate.

NASA also built an experimental electric plane, the X-57 Maxwell. The project ended its aircraft work in 2023 without a first flight, but NASA says it produced hundreds of lessons on batteries and electric motors that other designers can use.

On June 10, 2020, EASA issued the first type certification worldwide of a fully electric aircraft, the Pipistrel Velis Electro, a two-seater intended primarily for pilot training, powered by the first certified electric engine.

NASA's X-57 Maxwell project concluded aircraft operations in September 2023 without a first flight, after mechanical issues and shortages of critical components; NASA reports it produced hundreds of lessons learned, including on battery technology and cruise motor control, shared with regulators and industry.

In the lab, the plane gets heavier: from 1,100 kg to 1,500 kg.

The wings make the same lift as before. But now it isn't enough! The plane needs more speed.

Start with a small plane of 1,100 kg flying at 55 meters per second with its wings tilted 5 degrees. Its wings make about 13.0 kilonewtons of lift, 120 percent of its weight, so it flies.

Now add batteries until it weighs 1,500 kg. Same speed, same tilt, same wings: the lift is still 13.0 kilonewtons, but that's only 88 percent of the new weight. The plane can't stay up. It now needs about 59 meters per second instead of 50. Try going faster, tilting the wing more, or giving it bigger wings.

Model, S = 16.2 m², AR 7.5, α = 5°, ρ = 1.225 kg/m³, v = 55 m/s: lift 13.0 kN. At 1,100 kg (weight 10.8 kN) the ratio is 120 percent; at 1,500 kg (14.7 kN) it is 88 percent. Speed needed rises from about 50 to about 59 m/s and stall speed from about 29 to about 34 m/s.

Three fixes restore flight: 60 m/s gives 105 percent; tilting to 6° gives 106 percent; wings of 22.2 m² bring the speed needed back to 50 m/s. Each has a cost the model doesn't show: more power and drag, closer to stall, or a heavier, draggier wing.

Wings push air down, and the air pushes the wings up. That push is lift.

A heavier plane needs a bigger push. Going faster gives a bigger push.

NASA explains that lift depends on the air's density, the wing area, the wing's shape and tilt, and the square of the speed. To fly level, lift has to at least match the plane's weight.

Extra batteries don't change how much lift the wings make; they only make the plane heavier. So the pilot has to make more lift: fly faster, tilt the wing more, or use bigger wings. Because lift grows with the square of speed, a plane about 36 percent heavier needs only about 17 percent more speed.

NASA's lift equation, L = C_L·½ρV²·A, collects shape and inclination effects into the lift coefficient. Setting L = mg gives v = √(2mg/(ρSC_L)), so the speed needed scales with √m: √(1500/1100) ≈ 1.17, about 17 percent more.

Model limits: a symmetric wing that stalls at 15°, no flaps or slats (which real planes use for slow takeoffs), no drag, thrust or motors, steady flight in still air. It compares lift with weight only; it says nothing about how far the batteries would take the plane.

Batteries hold much less energy than fuel of the same weight.

That's why most electric planes today are small and fly short trips.

Researchers at Carnegie Mellon University wrote in 2018 that the best batteries store about 40 times less energy per kilogram than jet fuel. Electric motors use their energy more efficiently, but even so, jet fuel held about 14 times more usable energy for the same weight.

There's another difference. NASA notes that a fuel-burning plane gets lighter during a flight as it uses up fuel. A battery doesn't get used up that way, so an electric plane carries all of its battery weight from takeoff to landing.

Viswanathan, Sripad and Fredericks (Carnegie Mellon, 2018) estimated that the best batteries store about 40 times less energy per unit weight than jet fuel, and that after accounting for electric drivetrains' higher efficiency, jet fuel holds about 14 times more usable energy per kilogram than a state-of-the-art lithium-ion battery.

NASA notes that an airplane's weight constantly changes as it consumes fuel. A battery's mass does not fall as it discharges, so an electric aircraft lands as heavy as it took off. Adding batteries therefore raises energy and weight together, which is why battery-electric designs today tend to be small, short-range aircraft.