A reader in Germany recently contacted me to see if I could provide a copy of an article from 1959 on servovalves. I told him I could, and that I'd be happy to fulfill his request on one condition: Why on earth did he want a nearly 60 year old article on servovalves? He explained that he was working on his masters thesis on servovalves and needed a comprehensive example of what was available in the 1950s to show how far the technology has advanced. And it's a great article: 12 pages.
Granted, this has nothing to to with a pneumatic airplane, but when I was looking for the article, I stumbled across the one about the airplane. And, no, it wasn't experimental. It was in use all over the world, and some of the aircraft are still flying today.
Continue on to read this interesting article.
By JOHN D. HULL, JR., and A. E. SCHMIDLIN
Walter Kidd & Co., Belleville, N. J.
Many factors favored the use of pneumatic systems on the F-27, most significant of which was an approximate 100-lb weight saving over electrical and hydraulic systems.
• Lighter weight
• Faster operation
• No flammable liquids
• Cleaner and easier to maintain
Fairchild’s F-27 is the first American-made plane to use a complete pneumatic system to operate landing gear, wheel brakes, nose wheel steering, propeller brakes, and passenger door operation. It also has an emergency air supply that will lower the landing gears and operate the brakes.
Compressed air for the system is supplied by two 2-cfm, 3300-psi compressors located in each of the engine nacelles. The compressors are driven through a gearbox by Rolls Royce Dart engines, which power the aircraft. They carry their own individual dehydration and filtration equipment to supply clean, dry air at a pressure of 3300 psi. These two compressor packages are isolated from each other by a system of check valves and can deliver air independently to replenish air to storage if an engine fails. Ground charging the system can be done through the dehydration and filtration circuit.
Three independent reservoirs isolated by a series of check valves store the system’s air volume. The main storage is a 750-cu. in. steel cylinder, and the emergency storage is a 100-cu. in. steel cylinder. It’s fed by the main storage and then isolated during flight so that the brake system will have sufficient pressure for a satisfactory operation. The main storage can be depleted to far below the braking pressure of 1000 psi because lowering of the landing gears only require 100 psi, and steering, 220 psi.
Locating most of the control valves on a panel simplifies service and maintenance. It also ensures uniform environment for most of the control elements. The F-27 system is divided into at least five functional sub-systems.
Landing Gear — This circuit has a solenoid-operated control valve, a manual selector valve for emergency, a pressure reducer to control lowering, and a blowdown valve in the gear extension line. At the selection of the landing gear switch, the solenoid valve supplies air at 1000 psi to retract the gears, simultaneously venting the 1000-psi extension air. The opposite is true for extension. Electrical failure will not affect the solenoid valve because a mechanical lock will hold its selected position.
Manual selection for emergency gear lowering simultaneously vents the 1000 psi on the retraction side of the actuator, and supplies 100 psi from emergency storage to unlock and lower the landing gear. Grease dampers built into the landing gear actuators snub the motion of the gear. The uplock actuators are single-acting, spring-return cylinders that operate with either normal or emergency air, as do all the actuators for the extension stroke.
Nose Wheel Steering — This circuit includes a solenoid valve, a pressure reducer, a servo valve, and an air motor-ball screw linear actuator. The solenoid valve controls the passage of air to the steering system and is operated automatically by switches arranged on the nose wheel which shut off the steering system when angles of greater than 60° port or starboard are reached or when the nose wheel is off the ground. The steering actuator has a self-centering feature that centers the nose wheel when the air is shut off to the air motor, ensuring that the nose wheel is centered before it is retracted.
The ball screw actuator is designed to give free castoring for the high loadings that occur during towing operations. The resulting use of a ball screw and many of the basic design features of the steering system were indicated by a nose gear design that had preceded the development of this nose wheel steering system. There is sufficient gain in steering performance in the F-27 to make it satisfactory.
The servovalve is operated through a linkage connected to a steering wheel in the cockpit to provide for steering under load at any condition between inching and maximum rates.
Wheel Brakes — The system uses two foot-operated brake valves, connected in series with rapid exhaust valves, which allow rapid depressurization of the wheel brakes. These brake valves are of the variable pressure delivery type and are operated through a torsion bar linkage from the brake pedals in the cockpit. An over-riding emergency system is controlled by a hand-operated dual brake valve. This valve allows individual control of each wheel, so that differential breaking for steering, as well as braking, can be accomplished during an emergency landing.
Propeller Brakes – This circuit uses two solenoid valves that meter the air to each of the propeller’s brakes, preventing wind-milling of the props with the engine off.
Passenger Entrance Door – This circuit is operated by a pneumatic actuator through the action of a solenoid valve. The system will damp properly during emergency door openings and yet will not leak air in the passenger compartment during flight, since no stored air is required.
Why pneumatics for aircraft actuation?
- Weight – Pneumatic systems are considerably lighter in weight than their hydraulic or electrical equivalents. This difference becomes more marked with the wider use of fire-resistant fluids.
- Viscosity – Air is stable within the wide temperature ranges encountered by present aircraft.
- Fluid Availability – Turbine or ram air is readily available for either storage or makeup.
- Rapid Actuation – Response rates in pneumatic systems are faster, increasing margin of safety.
- Cleanliness – Lack of fluid-trapped dirt built up on operating components and fittings reduce maintenance time.
- Versatility – Separate two or three pressure systems for the same air-craft are economically feasible.
This article was originally published in the May 1959 issue of Applied Hydraulics & Pneumatics magazine, pp 126 to 128..