Saqr ConsultingDesign sheets
Saqr Consulting

Wing Sizing Sheet

Fixed-wing models & small UAVs — free tool
A file you keep, not an account. Nothing is stored on this site.
Dimensions
g
Everything on board: airframe, battery, receiver, payload.
mm
mm
mm
Same as root for a straight rectangular wing.
deg
Backwards is positive. It moves the mean aerodynamic chord aft, which is where the balance point is measured from; nothing else on this sheet depends on it.
Conditions
Two-dimensional section values measured at model Reynolds numbers. A finite wing reaches less than its section can, so 90% of this is carried through to the stall speed below.
Lowers the landing stall, not the clean one: cruise and the Reynolds number are worked out with the flaps up. The gain is Raymer’s ΔCLmax = 0.9 ΔCl · flapped area ÷ wing area, with the flaps over 60% of the span; slats add 0.9 × 0.44 over 80% of it.
m
Thinner air raises stall speed.
°C
Leave blank for the standard atmosphere. Worth filling in: a 45 °C afternoon at sea level carries about 9% less density than the standard 15 °C, which is a larger effect than most fields are high enough to produce.
%
Humid air is lighter than dry air, not heavier — water vapour weighs less than the air it displaces. Small, but a humid coastal summer adds a percent or two on top of the temperature.
Scale top view of the wing with dimensions The wing drawn to scale from above, with the span, the root chord and the tip chord dimensioned, and the mean aerodynamic chord marked at its distance from the centreline. Every figure on it is repeated as a number in the readout or the notes below.

How these are worked out — the equations behind this sheet, their sources, and what each one assumes.

Where people go wrong

The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.

Using a section's two-dimensional lift figure straight. A finite wing never reaches the maximum its section manages in a wind tunnel: the tips leak and the flow runs spanwise, and about a tenth of it goes. The picker's figures are two-dimensional and the sheet applies the 0.90 for you. Type a number in from a polar without taking that tenth off and the stall speed reads about five per cent low, which is the wrong direction to be wrong in.

Leaving the temperature blank on a hot day. Blank means the standard atmosphere, which is 15 °C at sea level. At 45 °C and dry the air has lost 9.4% of its density: the stall speed goes up 5.1% and the Reynolds number down 16.1%. The sheet will not guess the weather for you.

Comparing wing loading between models of different size. Grams per square decimetre is not scale-independent — double every dimension at the same wing loading and you have a different aeroplane. Wing cube loading is the figure that compares, and it is beside it for that reason.

Reading the Reynolds number as the one the wing always sees. It is worked out at 1.6 times the stall speed, which is a cruise. On finals at 1.3 times the stall it is a fifth lower, so a section sitting near the bottom of its useful band in the cruise is below it when it matters most.

Taking the mean chord for the mean aerodynamic chord. On a tapered wing they are not the same number, and MAC is the one the balance sheet's tail volume and centre of gravity both hang off. The sheet computes it properly and publishes it to Sheet 03.