What is in this article?:
- Flow meters pass the test
- Data accuracy
Engineers require valid flow data to understand the operation of individual components across a flight envelope.
Ground testing of hydraulic and pneumatic components installed on aircraft, missiles and unmanned aerial vehicles is critical to ensure flight worthiness. Whether it be a hydraulic pump or valve, accurate flow measurements help determine system performance.
Today’s demanding aerospace market demands implementation of meticulous quality control systems in the manufacturing process so that the final product meets all the internationally set standards. For example, manufacturers of hydraulic and pneumatic components used onboard aircraft or in other aerospace or defense equipment are faced with not only government and military standards, but also industry cooperative standards such as the National Aerospace Defense Contractors Accreditation Program (NADCAP). The goal of standards such as ISO 9000 and NADCAP is to develop basic quality programs that provide for continuous improvement emphasizing defect prevention and reduction of waste and variability.
Demands on manufacturers
The risk to human life inherent to aircraft failure must be a serious consideration for all suppliers of aerospace equipment. The risk factor means the reliability of all systems used to control the aircraft must be given a high priority.
Manufacturers of aircraft components use hydraulic and pneumatic test stands to verify 100% certification of performance and reliability over the part’s full range of inputs. Each component must pass rigorous test procedures before it is shipped to the ultimate customer. Inspection through sample testing is not an option for such a high liability product.
Designers of aerospace equipment are also developing more efficient systems to meet the demands of increased industry competition. Improving designs often includes a review of the hydraulic system, as engineers seek more efficient means of transmitting power. Increasingly, the focus is on reducing the size of hydraulic components or concentrating power into smaller package sizes. This can be accomplished by using higher operating pressures.
During the prototyping of a new helicopter platform, for example, component manufacturers may need to test the performance of an orifice or pump. Flow measurements not only have to be extremely accurate, but data must also be provided on a real-time basis. Once the flow data are collected, the manufacturer makes the necessary design changes, keeping the aircraft within specified performance tolerances.
Common testing requirements
Hydraulic components intended for commercial and military aircraft are typically designed for maximum power density and reliability. Because low maintenance is critical to aircraft of all types, hydraulic component makers continually seek to optimize existing products and systems, as well as manufacturing processes.
Although every component of a hydraulic system undergoes stringent testing, valves, pumps, and actuators are scrutinized especially closely. Liquid flow measurement is commonly used as a fine-tuning process to monitor the machining and assembly of the hydraulic component to achieve the desired flow characteristics.
Most hydraulic test stands are dedicated to test one or a limited set of hydraulic devices. These systems, which typically run in either semiautomatic or fully automatic mode, contain a cabinet with flow regulators, pressure regulators, manifolds, and connections to the unit under test. A computerized data-acquisition and control system, along with a full suite of sensors, enables the test stand to perform a variety of tests.
The most widely used technique for precise leakage measurement of aircraft parts is to connect one end of a hydraulic cylinder to the component under test, and the other end to a pressure source. If any fluid leaks from the unit under test, fluid enters from the cylinder to replace it, and the piston moves. Therefore, movement of the piston over a known time period is a measure of the leakage rate.
Latest flow meter technology
For test stand applications requiring the best flow measurement accuracy available with the benefits of high resolution and dynamic response, proven reliability and compact size, a precision turbine flow meter is the flow sensor of choice. The high resolution of the turbine meter makes it ideal for detection of leaks in aircraft fluid systems. With resolutions to 48,000 ppg (pulses per gal) for small turbines, minute fluid flow can be detected.
A turbine flow meter paired with a flow computer can be used to measure the hydraulic fluid flow through a component under changing conditions. Tests can involve multiple types of hydraulic oils, each having unique characteristics related to temperature, pressure, and viscosity.
Turbine flow meters rely on a proven, highly accurate measurement technology, which provides exceptionally reliable digital outputs. These flow meters incorporate a freely suspended turbine rotated by the flow of the fluid (liquid or gas) through the meter body.
Because the flow passage is fixed, the rotor’s rotational speed is a true representation of the volumetric flow rate. The rotation produces a train of electrical pulses, which are sensed by an external pick-up mounted directly above the rotor. The frequency of the pulses can be converted to an analog current or voltage or can be displayed as gpm, lb/hr, cfm, or in other engineering units.
The 2- to 3-msec response times typical of turbine flow meters provide rapid updating of flow rate. Furthermore, calculations are made in conjunction with real-time temperature, pressure, viscosity, and density values — which lends itself to output as mass flow rate.
Turbine flow meters also have a relatively high turndown ratio, with a linear range of 10:1 and a repeatable range of up to 100:1. This capability often enables a single turbine meter to replace multiple meters with a lesser turndown capability and can significantly reduce cost in applications requiring accurate rate and totalization measurement over a wide flow range.
Unlike other flow sensing technologies, turbine flow meters are compatible with remote electronics. Also, when paired with a heat-tolerant electronic pickup and amplifier, turbine flow meters can be located in areas exposed to extreme temperatures. In this setup, data acquisition electronics are safely mounted elsewhere.