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Steve Snyder of Washington State has done an amazing amount of research on these instruments
and has been kind enough to share his knowledge with us.
History of Clinometers
by Steve Snyder
During the evolution of flight from the earliest of manned flight contraptions, indicators of flight conditions evolved at a slow pace from simple manometers to gauge forward airspeed and crude compasses from marine applications, frequently mounted on the floor of the aircraft to avoid interference from the control panel electrics and metal parts.
After the advent of major armed conflict in the Great War, the development of aircraft instruments accelerated primarily because of the requirement to fly in bad weather conditions to accomplish the mission of observation, ground or air attack. The British tended to fly in worse conditions than the Germans and their allies. A good reference for the daily life of a Sopwith Camel pilot is "Winged Victory" by Yeates. As a result, British flight instruments were by necessity more complete that the Germans; resembling modern flight instruments in many respects. These instruments were two category aircraft performance and flight information. Initially there was very little reliable information about the flight information other than airspeed, altitude and compass heading. When in the clouds, involving instrument meteorological conditions (IMC), there was no real reference to keeping the aircraft level or in a controlled/coordinated state such as in trim.
The clinometer, also taken from a ship, was one of the first attempts to provide instrumentation for this reference. In straight and level flight it would give a reference as to a gravity level position of a bubble in a circular tube. One of the drawbacks of this system is that the bubble has very little mass and will not indicate if the aircraft is in a trimmed condition (rudder and ailerons coordinated) so that a turning skid may be happening in level flight. That is why a ball was added later as well as gyro stabilization and so on.