Tech Lab · Electric Powerplants
300 SHP is 300 SHP — Assuming All ‘I’s Are Equal
A practical look at drop-in electric replacements for common Lycoming/Rotax class engines in the 200–300 HP range — where they make sense, why 300 keeps showing up, and what’s still hard.
Thesis
For the airframe, shaft horsepower is agnostic. If cooling, mass, and installation are comparable, a 300 SHP electric behaves like a 300 SHP piston at the prop shaft.
What the “I”s are
Assuming all I’s equal → Installation, Inertia, Integration: mounts/CG/prop; rotating mass/torque delivery; cooling/controls/cert.
Why 300 SHP keeps popping up
In GA, ~300 HP is a sweet spot: real utility without turbo-prop complexity, enough margin for seaplanes, short fields, mountain ops, and generous training envelopes. ~200 HP remains the trainer/learner workhorse.
Airframe reference points
| Type | Typical HP | Why it matters |
|---|
| Cessna 172 / DA40 | 160–180 | Trainer baseline; ~200 eHP hits parity. |
| Cessna 182 / SR22 | 230–310 | Utility/IFR family; 300 eSHP ≈ like-for-like. |
| Bonanza / Mooney | 250–310 | Speed + load; 300 eSHP attractive. |
| Light amphibs | 180–300 | Water drag: torque helps; 300 eSHP shines. |
Electric advantages
- Torque peak torque at 0 RPM → strong takeoff/step-up.
- Altitude less power loss vs NA pistons (no mixture drama).
- Simplicity fewer moving parts; fewer maintenance items.
- Modularity motor + inverter + battery packs as swappable modules.
The drop-in thought experiment
Install sketch
- Motor: 220–300 kW (295–402 SHP) continuous-rated unit, direct drive or geared.
- Inverter: mounted firewall side; liquid-cooled manifold.
- Battery: modular packs in cowl + wing-roots for CG; quick-release hard-points.
- Thermals: chin radiator with controlled shutters; coolant loop shared across drive.
- Prop: retain certificated prop hub; map e-torque to stay within prop limits.
What must match the piston baseline
- Installed mass & CG window
- Cooling drag (don’t give back efficiency on the cowl)
- Electrical safety & fault isolation
- Propeller envelope limits (torque spikes, RPM)
If these match, the airframe mostly “sees” torque at the shaft and cooling drag at the cowl — not fuel type.
What’s still hard
Energy density
Avgas ~12,000 Wh/kg (usable); modern packs ~220–300 Wh/kg. Mission length and reserves define feasibility. Hybrids (genset or fuel cell) bridge some missions.
Cooling & performance
Motors are efficient, but continuous 200–300 kW needs robust liquid cooling and careful cowl design to avoid drag penalties.
Certification & ops
Power-class components exist; the path is standards, test hours, and maintenance practices. Retrofit STCs vs new type certificates is a strategy question.
Who’s pushing (at a glance)
China
Rapid prototyping on 200–500 kW class motors & packs; aggressive airframe retrofit experiments; strong supply chain control.
MagniX / Rolls-Royce
Demonstrated flight on Caravans, Beavers, and demonstrators; maturing inverter + motor stacks toward cert pathways.
EU programs
Hybrid-electric projects (Clean Sky, etc.) exploring distributed propulsion and regional-lift categories.
This page is a living note — not endorsements; just markers on the map.
Rules of thumb (back-of-the-hangar)
Power mapping
- 1 kW ≈ 1.341 HP; 300 HP ≈ 224 kW.
- Design for continuous power, not 60-sec burst.
- Target cruise: 45–65% of max continuous for thermal margin.
Battery budgeting
- Energy needed (kWh) ≈ cruise kW × hours + reserves.
- Mass estimate (kg) ≈ kWh ÷ pack Wh/kg × integration factor (1.15–1.3).
- Urban circuit / seaplane hops are early winners.
Closing thought
Maybe the question isn’t if electrics can fly, but when we treat them as engines, not experiments. If installation, inertia, and integration line up, then at the prop: 300 SHP is 300 SHP. The sky doesn’t care what fuel you burned to get there.