Ground testing proves what can be proved before leaving the ground: weigh the aircraft on its wheels to find its weight and centre of gravity, proof-load the wing, check every control direction with a second person, run the engine tied down and measure thrust with a spring balance, then taxi at walking pace and build speed slowly. High-speed taxi near flying speed is not a safe step, because an untested aircraft can lift off by accident at a height and speed nobody chose.
Part of Aerodynamics and Flight
The weighing, load testing, control checks, engine runs and taxi runs that precede any flight.
1. Weight And Balance
Put each wheel on a scale (platform scales, or the beam methods of Improvised Weighing), with the aircraft level, fuel and pilot loaded as in flight. Centre of gravity is the sum of each weight times its arm, divided by the total weight, measured from a datum.
Worked example (illustrative weights, datum at the main axle, arms positive forward): main wheels 340 kg at 0 m, tail wheel 60 kg at −4.2 m. Total 400 kg. Moment = 340 × 0 + 60 × (−4.2) = −252 kg·m. Centre of gravity = −252 ÷ 400 = −0.63 m, which is 0.63 m behind the axle. Convert that to a fraction of the wing chord and compare it with the limits for your design (Tail Assembly). The carrier’s 15-30% range is not to be treated as your limit. Repeat after any change of engine, tank, seat or ballast.
2. Proof Load The Wing
Load the wing, upside down on supports at its fittings, with sandbags spread to follow the lift pattern, in small steps up to the multiple of flight load that your design standard prescribes. Measure the tip deflection at each step and unload to check it returns. Stay clear of the wing: a collapse at test is the cheapest place for a wing to fail. This test supports, and does not replace, the calculation (Structural Spars).
3. Control Checks
Move each control through its full travel while a second person watches each surface and the cable run. Direction, travel, stops, locks, free play and binding (Control Surfaces). Check the pitot, static and instruments (Flight Instruments).
4. Engine Runs And Static Thrust
Tie the tail to a strong anchor through the spring balance, with the aircraft chocked and nobody in the plane of the propeller. Run the engine in steps up to the power you will use. Watch temperatures (Cooling Systems), vibration (Propeller Balance), fuel flow (Fuel Systems) and oil pressure. The balance reads static thrust. Use an anchor and rope whose breaking strength exceeds the thrust by a wide margin (Rope Strength Testing): if it fails, the aircraft is loose with the propeller running.
5. Taxi, Slowly
Taxi at walking pace to test brakes, steering and throttle; then a little faster, with the tail down. The carrier proposes accelerating to near lift-off speed and then reducing power, repeated 5-10 times. That is not recommended here: at speeds near flying speed an aircraft can lift off, or its control surfaces bite, and the pilot may be in the air before the field is chosen. Raise taxi speed in small steps, and stop at the speed at which the tail begins to feel light.
Common Mistakes
- Weighing without the pilot and fuel.
- A rope or anchor that is weaker than the thrust.
- Skipping the second person on the controls.
- Taxi speeds that creep up.
Safety
Propeller, anchor and accidental flight
The propeller cuts, a failed tie-down releases a running aircraft, and a fast taxi can become an unplanned first flight. Keep everyone clear of the propeller plane and of the line of the tie-down rope, and stop whenever anything is unexpected.
What’s Next
- Learn next: Glider Testing, the first steps off the ground
- Combine with: Flight Testing, the framework