Direct measurement of aircraft-engine thrust by use of strain gauges offers several advantages over traditional model-based methods of calculating thrust, provided that care is taken during the installation and calibration of strain gauges, and provided further that secondary load paths are understood. Advantages of the strain-gauge-based thrust-measurement method depend upon the specific engine/airframe interface, but can include the following:
- Simplification of the sensor installation, relative to the sensor suite that would be needed to support model-based calculations;
- Immunity (unlike in model-based methods) to drift and to the associated loss of accuracy as the engine deteriorates over time; and
- Excellent dynamic response.
Flight tests were performed to assess the suitability of the strain-gauge-based thrust-measurement method for application to the full flight envelope and power range of the F-15 airplane powered by F100-PW-229 engines. Other objectives of the flight tests were to determine whether unmeasurable secondary load paths affect the accuracy of this method significantly and to compare direct measurements against proven model-based thrust calculations.
Preliminary results from the flight tests showed that by properly accounting for secondary forces, and through the use of output data from a digital electronic engine-control system, excellent gross thrust-data accuracy was obtained during a subsonic-to-supersonic acceleration maneuver (see figure). Preliminary assessment of measurements throughout the remainder of the flight envelope show similar encouraging results.
At the time of reporting the information for this article, an in-depth analysis of the results from full-flight-envelope tests was planned for the near future, and the results were presented at the 1998 Joint Propulsion Conference. A description of the direct thrust sensor system, along with installation and calibration issues, were to be included in the presentation.
An F-15 Airplane at an altitude of 30,000 ft (9.1 km) was made to accelerate from mach 0.9 to mach 2.0, then to decelerate from mach 2.0 to mach 0.9 in a flight test of the strain-gauge-based thrust-measurement method.
This work was done by Tim Conners and Robert Sims of Dryden Flight Research Center. DRC-98-81