More than a feeler!

Traditional gap measurement methods, such as feeler gauges using typical precision metal shims cannot meet today’s tighter quality specifications for accuracy and repeatability due to the metal gauge’s variable frictional resistance ‘feel’ in a gap. In addition, manually recording and transferring error-free data can result in headaches with certain added time to retain the high-quality manufacturing standards.

Lightweight, but rigid, carbon fibre composite content is mandatory in new aircraft platforms, making assembly techniques much more challenging. Inboard hidden gaps between CFRP and metals, where the dimensions expand to larger values, make it impossible for feeler gauges to quantifiably measure gap values at all.

Released in 2011, the Gapman Gen3 is a portable battery operated, non-contact gap measurement system. It includes two capacitive sensors – one looking up and the other looking down installed at the end of a flexible Kapton wand. The GPD Family of gap sensors is connected to the Gapman Gen3 electronics and use a separate ground lead being attached to the conductive target surfaces or temporary fasteners. The sensor pair acts as one plate of a capacitor ‘looking’ up and one looking down. The grounded targets act as the other plates of the capacitive return path. The gap between each sensor face to the grounded target face is linearly calibrated to engineering units. The results can be recorded into the internal Gapman Gen3 memory and entire files sent by USB or wirelessly sent individual readings or data files sent via Bluetooth to a laptop computer.

The Gapman Gen3 and its historic predecessors of the Capacitec Gapman Gen2 portable non-contact electronic gap measurement system has been used for aircraft shimming applications since 1996. It has replaced feeler gauges and other mechanical contact methods at all major commercial aircraft manufacturers with a non-contact ‘electronic feeler gauge’.

Mind the gap

Over the past several years, Capacitec has met the challenge of providing higher volumes of GapmanGen3 to match the increased production volumes of passenger aircraft. The Gapman’s value is a large realised saving of significant labour costs compared with feeler gauge methods and helps speed aircraft delivery schedules. The Gapman has a payback factor of at least 3:1 to its cost for this fast digital gap gauge.

Capacitec consistently develops new Gapman Gen3 wand models to solve unique aircraft assemblers’ pain, such as measuring gaps in hard to access gap locations, non-conductive targets (one or both) and painted target surfaces.

One of the examples is the Boeing 777X. The wing configurations and materials are different from the older 777 versions. The new wings use composite skins attached to metal ribs versus the older version of metal to metal. This larger structure which can require shims to fill the voids. Engineers were required to profile large gap areas so they contacted Capacitec for a solution. The new Gapman Gen3 remote reach probes now enables aircraft builders to profile large gap areas reaching into gaps along 500mm while mapping every 25mm.

Attaching skins on the large number of rib pads was also a challenge. Such large wing surfaces require thousands of gap measurement to see if they are needed to insert shims (liquid and solid) to fill the voids. In some cases, measured inside the fuel tanks requiring a custom angled probe due to the limited access or operator convenience.

Gapman Gen4 developments

Larger shims in applications, such as fuselage joins, wing assemblies and elevator and engine pylon joins are driving aircraft builders to look for new gap measurement systems with the ability to measure higher volumes of gaps with one electronic systems. To meet these challenges, Capacitec is in the development of the new Capacitec Gapman. This will be a multi-channel measurement system (4 to 16 gaps) to map out the dimensions/contours of larger and very large shims required in critical join locations.

www.capacitec.com