Sizing a fixed-wing power system isn’t the same job as sizing a multirotor. A multirotor holds itself up with thrust, so thrust is the whole design variable. A wing holds the aircraft up, and the motor only has to beat drag. Get that backwards and you’ll end up with a plane that cruises at half throttle while carrying a motor twice the size it needs.
Quick answer: A wing holds the aircraft up, so the fixed-wing motor only has to beat drag. Start from wing loading: a 5kg fixed-wing UAV with 0.6 square metres of wing sits at roughly 8.3kg per square metre, a comfortable, forgiving figure for hand or catapult launch. Cruise drag is commonly 0.1 to 0.2 times weight, so a 5kg aircraft may need under 1kg of thrust in level flight. Size for launch thrust first, then confirm the pitch speed matches the cruise you want. Fixed-wing ESCs run sustained part throttle inside a fuselage, so mount them in the airstream and size with real thermal margin. See our propulsion overview and the motor selection guide.
| Step | What it decides | Reference figure |
|---|---|---|
| Wing loading | Launch method and wing area | 5 kg on 0.6 m2 is about 8.3 kg/m2 |
| Cruise drag | Level-flight thrust | 0.1-0.2 times weight |
| Launch thrust | Motor and propeller peak | Set by hand or catapult launch |
| Pitch speed | KV and propeller pitch | Must match the cruise speed |
| ESC cooling | Continuous part-throttle heat | Mount in the airstream, keep margin |
| Propeller | Drag and damage on landing | Folding prop on belly or runway landings |
Here’s the order that works, wing loading all the way through to the propeller.
Start from wing loading, not from thrust
Wing loading is all-up weight divided by wing area. It sets the stall speed and the cruise speed, and those two numbers set just about everything else. A lightly loaded wing flies slowly and wants a big, low-pitch prop. A heavily loaded wing needs speed, and speed means pitch.
As a working reference, a 5kg fixed-wing UAV with 0.6 square metres of wing sits at roughly 8.3kg per square metre, which is a comfortable, forgiving figure for a hand- or catapult-launched airframe. Double that and you’re into a faster, less forgiving aircraft, one that needs a longer launch and a more pitched propeller.
Thrust requirement is much lower than a multirotor’s
This is where fixed-wing sizing surprises people. A multirotor needs thrust equal to its weight just to stay in the air. A fixed-wing needs thrust to beat drag, and drag at cruise is usually a small fraction of weight, commonly 0.1 to 0.2 times weight for an efficient airframe.
So a 5kg fixed-wing UAV may need well under 1kg of static thrust in cruise. What it does need is take-off thrust. For a catapult or a hand launch you want enough to push the aircraft past stall speed in a short distance, which is why the launch method changes the motor requirement more than cruise does.
The practical rule: size for launch, then confirm that cruise throttle sits around 40% to 60%. If cruise demands more than three quarters throttle, the motor or the propeller is too small. If it sits at a quarter throttle, the aircraft is carrying a motor it doesn’t need.
Pitch speed tells you the cruise envelope
Pitch speed is rpm multiplied by propeller pitch, converted into a speed figure. It’s the speed the prop would advance through the air with no slip. In practice an aircraft cruises at roughly 60% to 70% of its pitch speed, so a target cruise of 90km/h implies a pitch speed around 130 to 150km/h.
That gives you the two variables to balance: a large, low-pitch propeller with a low-KV motor for a slow, efficient airframe, or a smaller, high-pitch propeller with a higher-KV motor for a fast one. Pitch is what sets speed, and diameter is what sets the static thrust you have at launch.
Motors by airframe class
Small electric fixed-wing and glide-test airframes
The AX435-A and AX435-B cover the light end, with the larger AX525 and AX530 pairs for airframes carrying more weight or camera payload.
Larger electric fixed-wing and VTOL pusher
The AT4120 comes in KV250, KV500 and KV560 with a long shaft, which suits both a tractor configuration on a fixed-wing airframe and a pusher position on a VTOL. Choosing between those KVs is a pitch-speed calculation, not a preference.
Petrol-class and heavy fixed-wing
Where the airframe is big enough that electric power stops being practical, displacement sizes the motor: the AT8020 55CC, the AT8025 70CC and the AT8030 85CC. These are matched to airframes of 15kg and above, where endurance and fuel mass dominate the design.
The whole electric range is grouped under fixed-wing UAV motors and the AT line specifically under AT series fixed-wing motors.
The ESC on a fixed-wing build
Fixed-wing ESCs live in a fuselage with poor airflow, and they get asked to hold a steady part-throttle for a long time. Two consequences: thermal margin matters more than peak current, and the cooling path matters more than the number on the label.
Size for the take-off current with the usual 1.5 to 2 factor, then mount the ESC where air actually moves past it and add a heat sink if the fuselage is sealed. The AT 195A 6-14S ESC is built for this configuration, with a VTOL variant in the AT 195A VTOL version.
Propellers: folding, and why it matters here
A fixed-wing that lands on its belly or a runway needs a propeller that folds, or it needs a brake to stop the prop horizontally. Folding is the cheaper answer and the one most builds use. Polymer folding propellers such as the TF15*8 and TF16*8 cover the light and mid-size airframes; the AUZ21x16 and AUZ20x13 carbon propellers cover heavier and faster airframes.
Spinners and adapters matter as well. A folding propeller needs a hub of the right diameter to fold cleanly, and an unbalanced spinner shakes more than an unbalanced propeller on a multirotor, because it turns at higher rpm against a longer moment arm.
Worked example
A 5kg electric fixed-wing with 0.6 square metres of wing, a target cruise of 90km/h and a catapult launch. Wing loading lands at 8.3kg per square metre, so stall speed is modest and the airframe stays forgiving. Target pitch speed is around 140km/h. A 12-inch propeller at 7-inch pitch reaches that at roughly 10,500rpm, which on 6S points at a motor in the 500KV to 600KV band, the sort of figure the AT4120’s KV560 option delivers. Launch thrust target is around 3kg static, comfortably inside what that combination produces.
The check afterwards: cruise should sit at about half throttle, the ESC should stay warm rather than hot, and a full throttle pass should be a brief manoeuvre, not the normal flight state.
Frequently asked questions
Do I size a fixed-wing motor by thrust or by speed?
Both, at different phases. Size for launch thrust first, then confirm the pitch speed matches the cruise you are after, which is the order in our propulsion overview.
What thrust-to-weight does a fixed-wing UAV need?
Far less than a multirotor. Drag at cruise is usually 0.1 to 0.2 times weight, so a 5kg aircraft may need under 1kg of thrust in level flight.
How do I choose between KV250, KV500 and KV560?
Pitch speed decides it. Higher KV spins the propeller faster for a given voltage, which suits smaller, higher-pitch props and faster airframes, as the motor selection guide sets out.
Should the propeller fold?
Yes, on anything that belly-lands or lands on a runway without a brake. A fixed propeller is likely to get damaged on touchdown.
Why does my fixed-wing ESC run hot in cruise?
Restricted airflow in the fuselage, plus sustained part-throttle running. Mount the ESC in the airstream and size it with real thermal margin.
Send us the all-up weight, the wing area, the launch method and the cruise speed you need, and we will size the motor, ESC and propeller around it. The range sits on our motor and propulsion page, with the method in the propulsion overview.
