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.
Taking the ground roll as the field length. On the default trainer the three-metre obstacle adds 17 m to a 21 m take-off roll and 27 m to a 46 m landing roll, so the strip it needs is 73 m, not 46. The field you need is the greater of the take-off and the landing totals, because the strip has to serve both.
Picking the strip for the take-off only. Long grass lengthens the take-off by half and shortens the landing to a third of the tarmac figure. The worst strip to take off from is the best one to land on.
Ignoring the lift coefficient on the wheels. It is 0.35 by default and it matters more than people expect, because it is what unloads the wheels and takes the rolling friction out of the equation as the run develops.
Discounting the wind. A headwind is the cheapest runway there is, and the two effects compound: the aeroplane starts the roll with airspeed already on the wing, and the ground distance accrues at airspeed minus wind. Six metres a second takes the default aeroplane from 21 m to 4 m.
Reading the landing figure as a braking model. Brakes here are one coefficient added to rolling friction. Fade, wheel lock-up, wet surfaces and crosswind are all outside it, and ground effect is not credited during the roll either.
Reading a thrown aeroplane's ground figure as a take-off run. Nothing runs. When the launch is a hand, a bungee or a catapult the figure is the landing alone, and the sheet says ground needed rather than field needed for that reason. A strip long enough to land on is the whole requirement; the launch needs an arm, not a runway.
Treating a dip below the stall as a failed launch. It is what most hand launches of anything heavy actually are: the aeroplane leaves your hand slower than it can fly, falls while the propeller accelerates it, and flies from there. What matters is not whether it dips but whether the height it loses is less than the height you have. The sheet integrates the fall rather than declaring the launch impossible.
| Surface | Rolling resistance | Take-off roll | Over the obstacle | Landing roll | Field needed |
|---|
Rolling resistance figures are compiled for model-sized wheels, not measured, and vary enormously with wheel diameter and grass height.