The aerospace industry has some of the most difficult operating conditions imaginable. Sensors of all kinds used in this industry must stand up to the environment and be able to perform under harsh conditions, fit in extremely tight spaces, and have electronics that can compensate for variable conditions in order to provide precise, repeatable results.
In particular, flight instruments measuring fluid flow in aircraft fuel and hydraulic systems are required to operate over extreme temperature ranges, while enduring heavy vibration and shock, and withstand electromagnetic interference and voltage transients. And, as aerospace test environments become more severe (higher temperature, greater vibration) and test application requirements for measurement devices grow more stringent (smaller, lighter, more stable, higher accuracy), obtaining meaningful data can be challenging.
Typical Flow Applications
From monitoring the fuel consumption of rotary and fixed wing aircraft, missiles and drones, to evaluating the performance of hydraulic fluid and lubricants, aerospace applications place high demands on flow sensing technology. Accurate and dependable flow meters are required for numerous fluid measurements on board aircraft and in component test stands, such as:
Fuel Management: Aerospace companies rely on flow transducers to provide accurate, instantaneous fuel flow rate and total fuel consumption data. Paired with cockpit displays, flow meters must offer easy installation and high reliability. Fuel Ballast: As fuel is consumed, there is a ballast issue that can be resolved by fuel manipulation. This is often accomplished by pumping the fuel to different tanks, which requires measurement of the fuel transferred to balance the aircraft.
Hydraulic System Verification: Measuring hydraulic fluid leakage or displacement during a flight test requires a flow meter that is highly repeatable, capable of differential flow, and extremely compact and rugged. Instrumentation suppliers are called upon to answer the size, safety and durability requirements of flight test engineers in a wide range of hydraulic system applications.
Product R&D: By pairing a flow meter with a flow computer or smart transmitter, aerospace equipment manufacturers can perform precise metering of fuel flow in engine test cells, fuel and hydraulic fluid flow in component test stands, and hydraulic fluid flow in hydraulic mules.
Qualification and Acceptance Testing: During the development process for new flight-qualified components or systems, turbine meters can be used to validate compliance with performance requirements over a wide range of operating conditions. In the process of acceptance testing, the excellent accuracy achieved by turbine meters prevents manufacturers from narrowing the window of acceptance due to the uncertainty of the flow measurement. For example, a ±100 pound per hour (PPH) flow requirement can be narrowed to ±90 PPH by a flow measurement with ±10 PPH uncertainty, thereby increasing the risk that a good part will be rejected.
Performance Evaluation: Unmanned aerial vehicles (UAVs), drones and missiles are frequently called upon to stay in flight for extended periods of time, or even loiter over an area. Aerospace designers developing these and other advanced platforms seek instantaneous fuel flow rate data that can be used as feedback to control engine performance.
Maintenance, Repair, and Overhaul (MRO): When flow specifications are involved, turbine meters offer a precise tool to verify that fuel and hydraulic components meet the original manufacturer’s specifications following maintenance, repair, or overhaul. As with qualification test procedure (QTP) and acceptance test procedure (ATP), the accuracy of these meters reduces the chance that a conforming component will fall out of the acceptance window.
Why Turbine Meters?
One of the preferred types of flow meters among aerospace engineers is the turbine meter (Figure1). It is widely used for obtaining precise flow measurements in clean, known liquids with relatively low viscosity. Available in compact, lightweight packages with rugged materials of construction, turbine meters are used to measure diverse fluids ranging from aircraft fuel to hydraulic fluid, lubricant, cryogenic fluid and coolant. They also offer a versatile metering solution that can be designed and manufactured based on custom specifications for packaging, end-fittings, mounting, electronics, etc.
Turbine flow meters employ a proven measurement technology, which is digital in nature and provides exceptionally reliable outputs. The meter functions by sensing the linear velocity of the fluid passing through the known cross sectional area of its housing to determine the volumetric flow rate. The fluid, as it passes through the meter, imparts an angular velocity to the rotor, which is proportional to the linear velocity of the flowing fluid. Since the linear velocity of the flowing fluid through a given area is directly proportional to the volumetric flow rate, it follows that the speed of rotation of the rotor is directly proportional to the volumetric rate.
The meter’s turbine rotation produces a train of electrical pulses sensed by an external pickoff mounted in close proximity to the rotor blades. The frequency of the pulses can be converted to scaled pulse or analog output, and displayed as gallons per minute, pounds per hour, cubic feet per minute, or in other engineering units.
Since the turbine flow meter is digital in nature, the task of signal processing is greatly simplified. The meter electronics are not subject to drift and eliminate the need for analog-to-digital conversion. This is useful for monitoring dynamic step-changes in flow for aircraft hydraulic systems, flight measurements for telemetering flow rates, or fast delivery fuel dispensing systems. It also allows linearizing and temperature conditioning circuits to be adapted to military levels of temperature and electromagnetic interference (EMI) noise immunity, with all of the signal conditioning electronics packaged in a compact, rugged housing that is quiet to EMI susceptibility and emissions.
In addition, turbine flow meters can withstand the extreme G forces encountered during the flight of high-performance or military aircraft. Their electronics can operate in temperatures from -55°C to 125°C. The meters themselves (when used with remote electronics) are used from cryogenic to 450°F.
Safety and reliability considerations dictate the use of turbine flow meters in some aerospace applications. The meter’s in-line rotor configuration ensures the presence of fluid flow (including fuel, hydraulics and coolant) even if its rotor is not rotating (locked). Other flow sensing devices, such as positive displacement meters, would not permit the passage of fluid. The result would be engine overheating or fuel blockage.
Latest Technology Innovations
Even though liquid turbine flow meters have existed for decades, there have been many design enhancements offering impressive flow performance. These improvements enable turbine meters to satisfy application requirements in ways once considered infeasible.
Dual-Rotor Design: Turbine flow meters have traditionally offered a repeatable flow range of approximately 100:1 and a maximum linear flow range of 30:1. This performance is adequate for general-purpose applications, but may not be suitable for aerospace users. The dual-rotor turbine design extends the lower flow range far below what has been possible before. These meters have a wider turndown than single-rotor configurations, and offer a repeatable flow range up to 500:1 and a universal viscosity curve (UVC) turndown range of 60:1 (Figure 2).
Hydraulically-Coupled Rotors: Although several versions of dual-rotor technology are patented, only a few have been successfully marketed. One version uses two rotors rotating in the same direction. However, inlet swirl will affect both rotors, reducing or increasing the revolutions per minute (RPM) depending on the direction of the swirl, and shifting the accuracy of the meter output similar to that of a single-rotor device. An alternative approach uses two closely coupled rotors, turning in opposite directions. The flow exiting from the first rotor greatly affects the inlet incidence angle on the rear rotor. The two rotors become hydraulically coupled and their sum (or average) provides a cancelling effect on fluid swirl. Therefore, flow straighteners are not required in most applications, making it possible to install the flow meter in tight spaces where an added length of straight piping cannot be tolerated.
Another benefit of hydraulically-coupled rotors is the RPM ratio can be monitored to determine bearing integrity due to wear or particulates. Bearing diagnostics have many practical applications in an aerospace environment (e.g., noting anomalies when securing test data over long periods of time).
Helical Rotors: Additional turbine meter advantages are found in the use of helical rotors, which optimize energy transfer over the entire surface of the blade — enhancing speed-of-response to step changes in flow rate. Helical rotors also produce less pressure drop across the meter, as compared with the traditional flat-bladed rotor design.
Advanced Electronics: Turbine flow meters equipped with new, sophisticated electronics provide total compensation to enhance flow measurement accuracy, while extending linearity over their repeatable range. The electronics receive a signal from the rotor, temperature sensor and, occasionally, a pressure sensor to correct for viscosity effects on the flow meter output.
Embedded Processors: Electronic processors can be packaged with the turbine meter for remote, direct, or embedded mounting to meet any installation requirement. Modern meter designs generally employ embedded electronic processor boards. This approach allows for a small envelope, eliminates the problems associated with EMI/RFI and mismatching flow computers to flow meters, and provides an amplified signal for transmitting long distances. It also allows for meter interchangeability without the need for rescaling, and delivers multiple process outputs for temperature, flow, and pressure (Figure 3).
Ceramic Bearings: In recent years, there have been major advancements in turbine meter bearing technology. Ceramic bearings have proven themselves to be superior in wear and less susceptible to particulates than stainless steel bearings. Furthermore, ceramic bearings can be used in water applications — providing a low-friction bearing system with longer service life than traditional journal bearings (Figure 4).
Secured Internals: The repeatability of the turbine meter not only requires high-quality bearing systems, but the rotor and supports (referred to as the internals) need to be rigidly mounted into the housing. In bi-directional applications, it is important the method of securing the internals prevents any movement changing the flow profile. This is best done with a rod-through clamping system that secures the internals on a machined step in the housing. This type of clamping system not only enhances bi-directional flow, but also provides a solution for high-shock applications.
Finding the Right Solution
Aerospace applications are among the most rigorous for any type of measurement instrument; strict industry regulations, extreme environments, and the ability to customize devices are just a few of the demands placed on sensors in these applications.
Aerospace projects frequently call for a flow meter supplier with the resources to provide a custom solution based on specific needs such as:
- Ambient requirements
- High/low temperature operation
- Lightweight materials (aluminum, titanium, etc.)
- Bi-directional flow
- High-pressure capability
- Extreme shock loads
- Custom physical dimensions
- Special end connections
- Material traceability
- Specialized testing requirements
Experience has shown project management for development and testing is essential to a successful outcome when devising a specialized flow measurement solution. The right approach establishes a joint venture between the flow meter supplier and customer, which is based on clearly defined specifications, expectations and costs. The project should start by identifying the following key parameters:
- New or existing technology
- Environmental conditions
- Performance criteria (e.g., accuracy, pressure drop, flow range)
- Certification requirements
- Standards compliance
- Delivery schedule
- Product cost
Long development cycles and high qualification costs require aerospace companies to identify stable, reliable, cost-effective partners. Such firms value speed in prototyping and development of flow measurement devices in specialized packages. The precision output of these products helps reduce risk and cost in key applications while also minimizing the need for unscheduled maintenance.
This article was written by Mike McCoy, Senior Applications Engineer, Badger Meter (Milwaukee, WI). For more information, Click Here .