The challenges of aircraft hydraulic design

The challenges of aircraft hydraulic design

A staff report

Designing a hydraulic system for an aircraft involves some challenges that differ from industrial or mobile applications.
 

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Aircraft hydraulic systems can be a challenge to design engineers, due to many constraints that are not encountered when designing a system for in-plant or mobile applications. Hydraulic technology first gained a foothold in aircraft flight control after World War II, when fluid power was introduced for some secondary systems' control. However, as aircraft flight performance and capabilities increased, hydraulics began to play a larger role in the critical operation and safety of airliners, helicopters, and high-performance military aircraft.

In modern aircraft, some of the places that hydraulics come into play include primary flight controls, flap/slat drives, landing gear, nose wheel steering, thrust reversers, spoilers, rudders, cargo doors, and emergency hydraulic-driven electrical generators. Military aircraft also use hydraulics on gun drives, weapons-bay doors, and hydraulic-motor-driven-fan heat exchangers. Factors that must be addressed on an aircraft include pressure conditions (both internal and ambient), temperature extremes, weight, speed, materials, reliability, fluid compatibility, leaks, cost, noise, and redundancy.

Pressure and temperature

Pressures on aircraft hydraulic systems run higher than on many industrial applications, which generally remain in the 1500 to 2000 psi range. Most commercial airliners run at 3000 psi, with military planes using 4000 psi systems (although some new military aircraft have made the move to 5000 psi). The impetus for higher pressures comes from space considerations and the need for light weight, because actuators can generate higher torque forces and power from a smaller envelope.

Not only must the systems deal with the expected variables in ambient temperature that are experienced across the whole of the planet's surface, but temperatures fall well below zero at an aircraft's cruising altitude. Most commercial systems are designed to tolerate conditions from &endash;65° to 160° F, while military aircraft can handle a temperature range of &endash;65° to 275° F.

Fluids for high and low temperatures

Fluids used on aircraft have a relatively flat (compared to industrial fluids) viscosity vs. temperature curve - in other words, they are somewhat thin. Additionally, these fluids must be fire resistant, a critical concern when the nearest fire department may be only five miles away - but five miles straight down. One characteristic of aircraft hydraulic fluids that makes them unique is that they remain fluid at &endash;65° F, a temperature at which water- and vegetable-based oils will freeze.

The current fluids of choice in the aerospace industry are:

* MIL-H-5606 - first introduced over fifty years ago and still used on many aircraft. Used on business jets and many U.S. Air Force aircraft, it is highly flammable and considered responsible for the loss of at least one military aircraft, due to the fire created.
* MIL-H-83282 - first used by the Air Force in 1982 and the U.S. Navy in 1997, it is less flammable than 5606, but much more viscous at low temperature. The lower temperature limit of MIL-H-83282 is considered &endash;40° F, and it is used in virtually all Navy aircraft.
* MIL-H-87257 - this newest fluid is used in C135, E3, and U2 aircraft; it is less flammable than 5606 (similar to 83282) but its viscosity at low temperatures allows use down to &endash;65° F. Considered the fluid of choice for newer aircraft being developed, and
* Skydrol and Hyjet - these alkyl phosphate ester based fluids are used on commercial aircraft, and are less flammable than the military fluids described above. Maximum temperature limit is 160° F. These fluids have been around at least since the 1960's.

Leakage is a concern whether the plane is on the ground or in the sky. A symposium has been held at the SAE A-6 meeting to discuss zero-leak technology, and future meetings involving aircraft manufacturers, component suppliers, and aircraft users are expected. (See page 30 for further details.)

Component and system reliability

There is a great emphasis on maintenance schedules, filtration, and general maintenance of fluid conditions on aircraft. Explains Ed Bush of Vickers Industrial & Mobile Group, Rochester Hills, Mich., "In industrial applications, the PLC is operating the system, so you design it accordingly. On mobile equipment, who knows who operates it? You hope people who know what they're doing. On aircraft, the reliability is so critical, you have to throw everything in - redundant systems, very specific maintenance schedules, and stringent guidelines on the components themselves."

Parts manufactured for use on aircraft do face much tougher guidelines on design, construction, and quality control than those used in most industrial applications. A typical hydraulic pump used on an aircraft may cost between $4000 and $25,000, compared with a comparable industrial pump's cost of $400 or less.

Why these extra costs? Consider the following differences on a pump:

* great attention must be paid to shoe bearing plates and plate faces, due to poor fluid qualities
* the pump must be compatible with these poor-quality fluids
* aluminum is often used, due to weight considerations
* it must be smaller and have a higher operating speed, and
* it must be built to withstand severe vibration, shock and g-forces (helicopter applications see more extreme vibration and shock forces than a shop floor does.)

Even the packaging of many components is more exotic than with the average industrial component. Phil Galloway, engineering manager, military programs, for Vickers Aerospace Marine Defense Fluid Power Division in Jackson, Miss., explains, "Since minimum weight is the single most important objective, it is unlikely that the exact valve is available. Thus, it will most likely need to be modified. Each added valve is usually custom designed to suit its function. Since aircraft quantities are relatively small, modular, low-cost, mass-produced valves are not generally used."

Bootstrap (pressurized) reservoirs are used on military aircraft to keep the charge inlet of pumps pressurized to prevent cavitation. One could easily imagine a military jet performing a roll, and the ensuing problems that would result from using a traditional industrial reservoir. Commercial airliners often use reservoirs with an air charge. (Note that due to weight limitations, reservoir size will be reduced dramatically from non-aircraft systems.) Reservoir size is optimized for aircraft so that only the amount of fluid needed for proper function is carried. Sizing considerations often include:

* differential volume requirements due to actuator differential areas
* volume required to fill accumulators when totally discharged
* volume required to make up fluid when total thermal contraction and expansion of fluid is experienced, and
* amount needed to minimize the frequency of filling

There is currently a focus on building quality systems on a system level. As part of this movement, aircraft companies are increasingly looking to component manufacturers to provide complete aircraft hydraulic systems, instead of a myriad of unconnected parts.

The majority of aircraft have three or four redundant hydraulic systems, which are geographically separate in many cases (especially on fighting aircraft, which must be able to survive being hit by enemy fire).

Particularly interesting is the RAT, or Ram Air Turbine. This last-gasp system comes online in an extreme emergency. The RAT is a spring-loaded device usually located near the nose of the plane, which is deployed in case of a total loss of power. The RAT basically consists of a propeller that turns a hydraulic pump - providing enough power to allow the pilot to make rudimentary landing gear and aileron adjustments upon landing. Its only drawback is noise - commonly on the scale of 120 dB.

Other factors

When people talk about noise control, they usually aren't referring to airline hydraulic systems. After all, why bother tweaking a system to achieve a noise level several decibels lower, when the plane's engines themselves are drowning out any noise that the hydraulics could possibly make? However, aircraft engines have been engineered to run quieter in recent years both in takeoff and landing situations - to appease communities in close proximity to airports - and in midflight - in an effort to increase passenger comfort. Thus, noise is starting to become an issue that can no longer be overlooked.

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F-22 fighter hydraulics efficiently operate weapon bay doors

In the year 2004, the F-22 Raptor is scheduled to take over the air dominance role with the Air Combat Command from the F-15. Unveiled last year, the fighter was developed by a team of individuals from Lockheed Martin, Boeing, Pratt & Whitney, and the U.S. Air Force. The F-22's primary objective is to establish complete control of the skies - and it does this via the latest technology in low observables, avionics, materials, engine performance, aerodynamic design, sensor capability, and of course, weaponry. The F-22 is capable of carrying existing and planned air-to-air weapons, including medium-range missiles - such as the AIM-120A - and short-range missiles such as the AIM-9 Sidewinder.

Hydraulics has a hand in many functions of the Raptor - the 4000-psi system plays a role in rudder control, landing gear, nose wheel steering, flight control surfaces, and in the weaponry.

A hydraulic gun drive, the M61A2 20mm cannon, is comprised of two discrete component subassemblies integrated into one assembly, and provides rotary power to the gun when commanded by a computer signal or manual input. The system is integrally mounted in the aircraft on its starboard side between the top side of the wing and the fuselage. A gun door, located in the wing root area, is hydraulically controlled to open before the gun can be fired, which allows the rounds and blast pressure to clear the muzzle. A 480-round ammunition feed and storage subsystem is housed under the right wing for easy ammo upload and download of empty casings.

Missiles are stored inside main and side weapon bays. Hydraulics controls the operation of these doors, as they are required to swing open and allow the missile to be deployed in a fraction of a second. The F-22's primary weapon is the AIM-120 Advanced Medium-Range Air-to-Air Missile (AMRAAM). The AIM-120 was developed to provide an all-weather, all-launch environment capability not only for the F-22, but for the Air Force's in-service F-15 Eagle and F-16 Fighting Falcon and the Navy's F-14 Tomcat and F/A-18 Hornet as well. The missile has multiple-target engagement capability, increased maximum launch range, a reduced-smoke rocket motor, and improvements in maintenance and handling. The AIM-120 (which has no official nickname, but is called "Slammer" by pilots) is carried internally in the F-22's main weapons bay, which is located on the underside of the fighter tucked under the inlets. The main bay is covered by two thermoset composite bifold doors that open outward.

Each missile is carried on an EDO Corp.-built LAU-142/A hydropneumatic launcher, called an AMRAAM Vertical Eject Launcher (AVEL). The AVELs are substantial - nearly 113 pounds each - in order to minimize missile movement in the weapons bay. They are made mostly of aluminum, have a 9-in. stroke, and eject the missile out of the bay at more than 25 ft/sec with a force of 40g at peak acceleration. Unlike conventional missile launchers on other aircraft, the AVELs require no pyrotechnics, and need less logistics support than other launchers.

The entire launch sequence (door opening, AVEL ejecting the missile, missile ignition and flyout, door closing) takes just seconds. The combination of the F-22's stealth characteristics, its integrated avionics, the help of hydraulics, and the AIM-120 missile gives this fighter a first-look, first-shot, first-kill capability.

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Pneumatic muffler minimizes exhaust air noise for Northwest Airlines

There's nothing like a little peace and quiet. This phrase is particularly meaningful for those who work at manufacturing facilities where pumps, motors, valves, and other air-operated equipment keep things abuzz. Now, thanks to a new pneumatic muffler developed by the 3M Bonding Systems Division, St. Paul, Minn., worker comfort levels, as well as noise limits established by the Occupational Safety and Health Administration's (OSHA) Hearing Conservation Program, may be within reach.

Just ask Northwest Airline's pneumatic shop manager Jacques Mezin and Fluid and mechanical accessories manager Robert Steele. Working in a shop-repair and overhaul facility for pneumatic components in the Northwest Airline's fleet, the two encounter a great deal of noise every day. "Testing air cycle machines, starters and flow pressure regulating valves make our facility noisy," explained Mezin. "We had one valve in particular that was very loud. So we fabricated an adapter to the valve port to try the muffler. We saw a reduction in noise of more than 30 dB, without sacrificing production."

Under OSHA's program, employees are required to wear ear protection in working conditions where noise levels exceed 90 dB. The pneumatic muffler can help reduce equipment noise considerably and in some instances completely eliminate the need to wear earplugs.

The muffler is comprised of a rugged polymer housing with a replaceable acoustic composite insert, designed to effectively quiet the exhaust on air-operated equipment. It is porous to air and fluids, helping to minimize back pressure.

Most of the real noise is generated around the test cells and test benches. On average, there are 30 people testing various types of equipment periodically throughout the day, with the number dropping to 15 at night - thus high noise levels are present almost continuously. Northwest installed 5 mufflers on one test bench and saw a noticeable difference - enough that workers are no longer required to wear ear plugs when conducting some tests and other workers are no longer aware that tests are being done.

Joe Nelson, plant safety engineer for a 3M manufacturing facility in Aberdeen, S.D., agrees that the new mufflers can speak volumes to employees. "We've been using the mufflers at our facility for about nine months. When our machine operators noticed the difference the pneumatic mufflers were making, they wanted them on all the equipment. Now more than 80 of our 300 employees, who were required to wear earplugs or muffs to protect their hearing, no longer have to. In fact, we can actually hear mechanical cylinders actuating. Before, all we ever heard was a lot of air."

The muffler is engineered for long service life. It can last for weeks, even years, depending on the in-use environment. Factors such as intermittent or continuous use, the cleanliness of supplied air, amount of oil or dirt in the environment, and the malfunction of pneumatic equipment can all affect the insert's service life. At the end of its service life, the insert need only be replaced - not the entire muffler.

A threaded cap screws on to retain the insert, making replacement quick and easy. The disposable core eliminates the need for cleaning with solvents. It can be disposed of with standard methods, eliminating the need for special handling.

Six NPT sizes range from 1Ú8- to 1-in. The muffler's housing is non-corrosive and its simple aesthetic lines complement modern designs.

"These mufflers have definitely made a world of difference to us," Steel concluded. "We continue to come up with innovative ways to use them."

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Zero Leakage Hydraulics initiative launched by SAE Committee

The Society of Automotive Engineers (SAE) Committee A-6, Aerospace Fluid Power, Actuation and Control Technologies, launched its Zero Leakage Hydraulics initiative with a kick-off one day symposium in Lake Tahoe, Nev. late last year. Significantly, this attracted over 200 delegates, demonstrating just how seriously the aerospace hydraulics community is treating the goal of zero leakage.

Symposium Chairman and A-6 member, Peter Amos, vice-president, sales and engineering, The Advanced Products Company, North Haven, Conn., explained why the symposium was so important. "Hydraulic flight controls have achieved outstanding levels of reliability, and have been proven over millions of flight-hours on every major commercial and military aircraft. The technology has continued to evolve, from mechanical linkages direct to the pilot's stick, to digital fly-by-wire, and now optical databuses offering immunity to electromagnetic interference."

"System pressures have been increased to save weight, and variable pressure systems offer reduced energy losses and enhanced component fatigue life. New fluids have extremely good properties over a wide temperature range and offer excellent fire-resistance, and since the fluid is constantly circulating in a filtered, closed, airless system, thermal and contamination management is straightforward and effective."

"SAE A-6 has been at the forefront of all these advances over the last 56 years, but one pesky problem still exists: leakage. So A-6 decided it was time to deal with this head-on and the idea for the zero leakage initiative was born. It was clear that tolerating drips was creating customer dissatisfaction and an unnecessary negative image, that was bad for the entire hydraulics community."

The symposium was structured into different parts: firstly to answer the question, "How much do aircraft leak?" and secondly to seek advice from the experts in the industry, "What can be done to eliminate leakage?".

Major contributions to part one were provided by Boeing, several major airlines (data collated and analyzed by Dr Ron Zielinski), the U.S. Navy, and Boeing McDonnell Douglas Aircraft and Missiles. These presenters showed an improving trend, but still problems in the area of tubing, fittings, and seals.

Part two invited input from several seal manufacturers, U.S. Air Force Wright Laboratories (fluid formulations and elastomers), Moog Inc. (flight controls), Parker Abex GmbH (hydraulic pumps) and two fitting suppliers. Kelly Fling provided an insightful and down to earth summary on eliminating leaks and all the presenters gave examples of leakage success stories, where attention to detail, supported and encouraged by the customer, had virtually eliminated leakage in specific cases.

Another goal of the symposium was to promote a vision of zero leakage, as an achievable and realistic goal. In his keynote address, Jim Bloomquist, of Vickers, Inc. encouraged aerospace engineers to look at advances made in zero leakage industrial hydraulics and refrigeration, as an example of the progress that can be achieved. Vickers also closed the Symposium with an uncompromising call to overcome the leakage paradigm.

Following this successful meeting, a special task-force within A-6 continues to gather and sift data from commercial and military aircraft operators, feeding this back to the appropriate sub-committees and technical panels for action. A-6 is therefore offering leadership on this issue, by focusing attention on the problem, challenging the industry to do better and then through detailed committee work, seeking to apply much tighter leakage requirements through improved aerospace standards.

The first tangible results of SAE action are expected to be a revised standard for qualifying separable and permanent fittings and tubing (AS 4401); inclusion of tighter tolerance options in the AS 568 elastomeric O-ring standard; and a comprehensive upgrade to AS 4716. This is the aerospace gland design standard which replaces the widely used, but now discontinued MIL-G-5514.

For more information on SAE A-6 technical standards activities and membership in the organization, call Aleita Wilson at (724) 772-7160 or e-mail: aleita@sae.org.