Sixteen sheets that are meant to be checked rather than trusted. This page says where every method comes from and where each one stops working.
What this is
Sixteen calculation sheets for people designing model aircraft and small unmanned aircraft. Put in the dimensions you are considering and get back the numbers that decide whether the aircraft will be pleasant, marginal or dangerous.
The gap it exists to fill is a narrow one. Plenty of free calculators will hand you a wing loading. Almost none will tell you that the airfoil you picked stops working at your Reynolds number, or that your balance point is a centimetre from unflyable. Every figure here comes with what it means for the aircraft on your bench.
Where the numbers come from
Standard methods, named rather than hidden, so you can go and read the original:
- Atmosphere
- The International Standard Atmosphere barometric relation, with water vapour from the Magnus form and viscosity from Sutherland's law. Every sheet that needs air uses the same block of code, so no two sheets can disagree about the day you are flying on.
- Aerodynamics
- The parabolic drag polar for the power curves; component build-up with Blasius and Prandtl–Schlichting skin friction and Raymer's form factors for the drag sheet; lift curve slope from finite-wing theory for the balance sheet.
- Stability and control
- Tail volume and lifting-line theory for the longitudinal axis; fin volume against the standard fuselage volume relation for weathercock stability; dihedral, wing position and sweep for the dihedral effect. Plain-flap hinge moments for the servos, and strip theory for roll rate.
- Reynolds number and skin friction
- Density times speed times length over dynamic viscosity, with viscosity from Sutherland's law in the same shared block every page carries. Skin friction from Blasius for laminar flow and Prandtl–Schlichting for turbulent, and flat-plate boundary layer thicknesses for each. The regime boundaries at fifty, two hundred and five hundred thousand are conventional rather than physical constants.
- Mission energy
- Power required from the same drag polar the rest of the set uses, divided by the propeller and drivetrain efficiencies, held for the duration of each segment. Pack energy is nameplate reduced by a compiled usable fraction and then by whatever reserve you choose. Loiter is flown at the minimum-power speed, which is the same condition as minimum sink and clamped to the stall in the same way.
- Ground geometry
- The standard undercarriage constructions used for full-size aircraft, applied to model dimensions: static wheel loads from moments about the contacts, tip-back and nose-over as arctangents of the main-wheel offset over the centre of gravity height, and the turnover angle from the perpendicular between the centre of gravity and the line joining a main wheel to the nose or tail wheel. Recommended wheel diameters are compiled from model practice, because grass bends and sand sinks.
- Gliding flight
- The classical results of the parabolic polar: best glide where induced drag equals parasite drag, minimum sink where it is three times as large, and the speed to fly into wind as the tangent drawn from upwind of the origin. Computed from your figures rather than quoted, and clamped to the stall when the ideal speed is below it.
- Field performance
- The ground roll integrated step by step from thrust, drag and rolling friction rather than taken from a closed form, with a linear thrust lapse to pitch speed and rolling coefficients compiled for model wheels on five surfaces. Distances over an obstacle use the climb and approach angles the aeroplane can actually hold.
- Structures
- The V-n envelope with sharp-edged gust loads, an elliptical spanwise lift distribution for the root bending moment, and a cap-and-web spar carrying it as a tension and compression couple.
- Propulsion
- Actuator-disc theory with a blade-quality factor for the propeller, a figure of merit for static thrust, and a battery modelled as an ideal source behind a fixed internal resistance.
- Airfoil data
- Twenty-seven sections checked against published tunnel and computational sources at model Reynolds numbers. Each row states whether its pitching moment was measured or computed; twelve of the twenty-seven rest on computation, and the sheet says so rather than presenting all twenty-seven as equally solid.
The convention that matters most
Where a number was compiled from practice rather than measured, the sheet says so, in those words, next to the number.
Areal densities, drag coefficients for wheels and legs, energy densities, blade quality, the mission bands on the balance sheet — all compiled. It is a device that only works if it is applied everywhere, because one soft number left unlabelled weakens every labelled one. Seventeen of the twenty-seven airfoils carry a warning of their own, by name, where the trap is known.
How the sheets check each other
The set is designed to close a loop, and the loop is run every time it changes. Ask Sheet 06 for an aircraft that stalls no faster than 31 km/h and it sizes the wing at 42.7 dm². Carry that to Sheet 05 and the weight budget comes back at 2,378 g. Carry that to Sheet 01 and the stall speed it gives is 30.6 km/h — inside the requirement the loop started from, on the first pass.
Smaller agreements are checked the same way: the pack mass Sheet 09 works out matches Sheet 05 to the gram, and the energy density behind both is one figure, not two.
What it is not
Not an airworthiness determination, not professional engineering advice, and not a substitute for flight testing. Several methods here are being used outside the range they were validated in, and each sheet says which and in which direction it errs. The drag build-up, for one, reliably reads low.
Corrections
A correction with a source behind it is the most useful thing you can send. Send it and it goes into the next revision, credited. Real measurements beat a compiled table every time, and the airfoil rows in particular improve only when someone who has tested a section says so.
Every sheet carries a revision letter so you can see what has changed.
What it costs
Nothing. No paywall, no donations, no advertising, no tracking, no sponsored airfoils, no data to sell — see the privacy page, which is short because there is little to say.
How it is built
One HTML file per sheet. No framework, no build step, no dependencies, nothing loaded from another domain. The typefaces are served from this site rather than from a font service. A service worker caches the pages after the first visit, so the sheets work at a flying field with no signal, which is where they are most useful.
It is a deliberately boring construction, chosen so the site still works in ten years and so anyone can read the source and check the arithmetic.