RC Electronics & Airframe Calculator

AUW is the weight of the plane ready to fly: airframe, motor, ESC, servos, receiver, battery, prop, glue, tape and wires. Every other number depends on it, so weigh the finished plane on a kitchen scale if you can.

The weight budget mode lets you plan before you build. Small things like wiring, connectors and hot glue easily add 5–10% of the total.

Many countries treat 250 g as a regulatory threshold (registration, Remote ID, pilot tests). The badge shows which side you are on. Always check your local rules.

Wing loading is weight divided by wing area, usually in g/dm² (or oz/ft²). Lower wing loading means slower, gentler flight. Rough guide: under 20 g/dm² is floaty, 20–35 suits trainers and sport planes, and 45+ is fast and demanding.

Stall speed is the slowest speed at which the wing can still carry the weight: V = √(2W / (ρ·S·CLmax)), where W is weight in newtons, ρ is air density, S is area in m², and CLmax is the wing's best lift coefficient (about 1.0 for a simple model airfoil). Cruise is typically 1.3–1.5× stall.

Static thrust is the push the prop makes while the plane is held still. Thrust-to-weight (T/W) compares that to the plane's weight. A trainer flies well at 0.5–0.8, a sport plane around 1.0, and an aerobatic plane wants 1.5+ to hover and climb vertically.

The estimate uses momentum theory: T = (FM × P × √(2ρA))^(2/3), where A is the prop disc area and FM (figure of merit, about 0.5) covers real-world losses. Bigger discs make more thrust from the same power, which is why slow planes like large props.

The simplest way to size the power system is watts per kilogram of AUW:

• Powered glider: 80–120 W/kg (default 100)
• Gentle trainer: 100–150 W/kg (default 150)
• Sport: 150–250 W/kg (default 200)
• Aerobatic / 3D: 250+ W/kg (default 300)

This is electrical input power at full throttle. It sets battery current, ESC size and motor size.

A LiPo cell is 3.7 V nominal and 4.2 V full. Never discharge it below about 3.3 V at rest. Under load the voltage sags to about 3.5 V per cell, so the calculator uses that for current. Packs are named by cell count in series: 2S = 7.4 V, 3S = 11.1 V.

mAh (milliamp-hours) is capacity: 1000 mAh can deliver 1 A for one hour. Bigger packs fly longer but weigh more, and a small plane quickly runs out of room.

The C rating says how fast a pack can be discharged: maximum current = C × capacity in Ah. A 450 mAh 20C pack is rated for 0.45 × 20 = 9 A continuous. The burst figure (often written 20C/40C) is only good for a few seconds. Size by the continuous number.

Required C = current ÷ (mAh ÷ 1000). Buy at least 1.5× that, because labels are optimistic and packs run cooler, last longer and sag less with margin.

Flight time (minutes) = capacity in Ah × usable fraction ÷ average current × 60. Use only about 80% of the pack so the LiPo stays healthy.

Full throttle gives the worst case. Most flights average 40–60% throttle. The calculator scales current linearly with throttle, which is conservative because real current falls faster than throttle.

KV is RPM per volt with no load. A 1400 KV motor on 7.4 V would spin about 10,360 RPM with nothing attached. With a prop it slows to roughly 80% of that (the RPM load factor).

Low KV turns big props slowly, which suits gliders and trainers. High KV turns small props fast for speed. More cells raise RPM and power with the same motor, so always check the prop is still within the motor's limits.

A prop's name gives diameter × pitch in inches. Pitch is how far it would screw forward in one turn. Pitch speed = RPM × pitch × 0.0254 ÷ 60 m/s is roughly the top speed. Aim for 1.5–2.0× cruise.

Diameter matters most. The load on the motor grows roughly with the 4th power of diameter (at fixed pitch ratio and RPM; the full power formula P = Cp·ρ·n³·D⁵ grows even faster). Going from 8″ to 9″ adds about (9/8)⁴ ≈ 60% load, enough to overheat a small motor. Bigger, slower props give more static thrust per watt. Smaller, faster props give more top speed.

Brushless outrunners make roughly 3.3 W per gram of motor weight. Pick a motor whose maximum power rating is at least 1.5× your target power so it runs cool. Suggested motor weight ≈ 1.5 × P ÷ 3.3.

Reference classes (check local availability):

• E-flite Park 180: 8.5 g, 2200 KV, up to 30 W, props 3×3 to 6×3 on 2S
• E-flite Park 300: 24 g, 1380 KV, up to 85 W, 7 A continuous, props up to 10×3.8 on 2S
• E-flite Park 370: 45 g, 1360 KV, up to 150 W, 12 A continuous, props 8×3.8 to 10×4.7

The ESC (electronic speed controller) should be rated for 1.25–1.5× the full-throttle current, then round up to a common size (10, 12, 15, 20, 30, 40 A…). Mount it where it gets airflow.

The BEC (battery eliminator circuit) inside most ESCs supplies 5 V to the receiver and servos. Small linear BECs are fine for 2–4 micro servos on 2–3S. On 4S and up, or with many or large servos, use a switching BEC or a separate UBEC.

Each independently controlled function needs a channel: throttle, ailerons, elevator, rudder, flaps. Two aileron servos on a Y-lead share one channel. Elevons on a flying wing need 2 channels mixed in the transmitter. A 6-channel system such as the FlySky FS-i6 (AFHDS 2A protocol) covers most beginner planes and leaves spare channels for flight modes.

Failsafe decides what happens when the signal is lost. Always set it so the throttle goes to idle/off, and test it on the bench by switching the transmitter off with the prop removed.

CG is the balance point. A conventional plane is usually balanced at 25–33% of the mean aerodynamic chord (MAC) back from its leading edge. Nose-heavy is sluggish but flyable. Tail-heavy is often unflyable. MAC is the "average" chord of a tapered wing, and the diagram draws it for you.

Dihedral (wings tilted up toward the tips) makes the plane roll level on its own. Trainers use 3–6°, rudder-only planes more, and aerobatic planes about 0°.

Tail volume measures how powerful the tail is: Vh = Sh·lh / (S·MAC) (typical 0.35–0.6) and Vv = Sv·lv / (S·b) (typical 0.02–0.05). A longer tail arm or bigger tail gives more stability.

Depron / EPP sheets (3–6 mm) are light and easy to cut, and they suit park flyers. Paper-faced foam board (as used in many scratch-build plans) is cheap and stiff but a little heavier. Peel the paper off one side to bend curves. Reinforce wings with a carbon or bamboo spar.

Converting a chuck glider: add a 1S micro motor as a pusher behind the wing, a tiny ESC/receiver (or a brick), and 2 micro servos for rudder and elevator. Keep the total under about 70 g, and re-check the CG because the battery usually goes in the nose.

• Charge LiPos with a balance charger, on a non-flammable surface or in a LiPo bag, and never unattended. Store them at about 3.8 V per cell. Retire puffy or damaged packs.
• Treat a powered motor as live. Plug in the battery last, with the throttle at zero and your hands clear of the prop disc.
• Do bench tests with the plane restrained, with nobody in line with the prop, wearing eye protection. Use a watt meter to confirm current stays below the ESC, motor and battery ratings.
• Balance props, tighten prop nuts, and replace any chipped or cracked prop.

When you open the throttle, the motor does more than push the plane forward:

• Thrust line height. If the thrust line is below the balance point (common on high-wing trainers), power pitches the nose up; above it (many pushers and low wings), down.
• Prop wash on the tail. The fast air behind the prop changes the tailplane's load, so the pitch trim changes with power.
• Spiral slipstream. The wash also rotates with the prop. On a normal (clockwise from behind) prop it hits the left side of the fin and yaws the nose left.
• Torque. The airframe tries to spin the opposite way to the prop: a clockwise prop rolls the plane left. Thrust angles can’t fix this; a touch of aileron trim does.
• P-factor. At high angles of attack the descending blade bites harder and pulls the nose to one side (small on fast-spinning electric props).

Tilting the motor cancels most of it: down-thrust points the thrust a few degrees below the fuselage line; right-thrust points it towards the right wing. Typical tractor values are 1–4° down and 1–3° right. On a pusher the same correction is made by moving the prop end of the motor the other way (for example, “up-thrust” with the prop end lower).

The Designer, this calculator and the Build page work out angles for your exact layout with the simulator’s physics (they make the plane’s reaction to full power match an ideal airframe), and the Build page gives the washer thickness to shim a standard motor mount. In the simulator you can switch these motor effects on or off and compare.