An inclinometer, the slip-skid ball, is a ball in a curved tube of fluid that rolls to whichever side the apparent gravity in the cockpit leans toward: centred means the aircraft is turning without slipping or skidding, and displaced means the rudder is wrong for the turn. The ball reacts to the sideways force on the pilot, so it shows the quality of a turn but says nothing about bank, heading or airspeed. Build one from a bent glass tube, a heavy fluid and a steel ball, then check it level on the ground.
Part of Aerodynamics and Flight
The curved tube and ball that shows slip and skid, and the same device used as an angle tool for rigging.
What The Ball Senses
The ball sits where gravity combined with any sideways acceleration points. In straight, level flight that is straight down. In a turn the aircraft accelerates sideways toward the centre of the turn, and the apparent gravity tilts; if the bank is exactly right for the turn rate the apparent gravity is perpendicular to the cockpit floor and the ball stays centred. Too little bank for the turn (a skid) throws the ball to the outside; too much (a slip) lets it fall to the inside. The ball’s offset angle θ obeys tan θ = sideways acceleration ÷ g, so a sideways acceleration of 0.1 g displaces it by arctan(0.1) = 5.7°. The 0.1 g is an illustration.
Why It Matters To Safety
A skidding turn at low speed loads one wing more and stalls it first, the classic route to a spin. Pilots tightening a turn near the ground with an uncoordinated rudder have died this way. The ball is a prompt to centre it with rudder.
Building One
The carrier describes a curved glass tube partly filled with liquid and a ball, built from a bent glass tube, kerosene and a steel ball bearing (Aerodynamics and Flight).
Step 1: Bend the tube to a gentle curve with its lowest point at the centre. The radius of curvature sets how far the ball moves; the corpus gives no figure.
Step 2: Fill with the damping liquid so the ball rolls slowly and does not bounce. Thicker liquid damps more, but its thickness changes with temperature.
Step 3: Seal both ends with no trapped bubble that might interfere.
Step 4: Mark the centre with two marks the ball fits between. Mount the instrument with the tube horizontal when the aircraft is level, checked on the ground.
Check It On The Ground
Level the aircraft sideways with a spirit level or water level (Levels and Plumb Bobs); the ball should sit between the marks. Tilt the aircraft by a measured amount and mark the ball’s position at several angles. This also shows how sensitive the instrument is.
A Second Use: An Angle Tool
A tube of fluid with a ball or bubble is a levelling device. Used with a straightedge on the ribs, it gives wing twist and incidence relative to level (Wing Twist). Its resolution is limited, and Angle Measurement (the workshop article, not the field-survey ones) shows how protractor and bevel methods compare.
Common Mistakes
- Mounting it off-centre or tilted.
- Reading it as bank angle.
- Using a liquid that evaporates or freezes.
- Staring at the ball.
Safety
A glass tube in a cockpit
A glass tube that breaks in a hard landing cuts the pilot and spills fluid; shield it, and keep any flammable liquid in it to a very small quantity. Prefer a non-flammable damping liquid if you have one.
What’s Next
- Learn next: Flight Testing, where instruments meet the air
- Combine with: Control Surfaces, especially the rudder