UAS Leading Edge Heat Transfer for Icing Investigations

Aircraft icing is a significant flight hazard that affects, with increasing frequency, uncrewed aircraft. Heat transfer analysis is required in order to predict, model, or mitigate ice accretion. Convection heat transfer is one of the primary phenomena for determining aircraft ice classification and is needed to determine the weight and volume of ice accretion. The flight regime of unmanned aircraft, however, has substantial differences from manned aircraft. Therefore benefit little from their studies on convective heat transfer. A robust study on convective heat transfer for uncrewed aerial systems(UAS) would provide a necessary foothold in addressing the icing problem in UAS. The work is for the design and development of a leading-edge convection heat transfer instrument The intended instrument is a system of heated wires embedded in a custom built composite wing. The benefits of conducting a flight campaign were proven by Newton, by addressing the need for accurate levels of free-stream turbulence (Newton). The chamber and flight campaigns are needed to understand icing on low altitude low velocity operations for small UAS and AAM.

Initial data sets from a cylindrical sensor were able to estimate heat transfer correlations at the lower velocity range expected for UAS. This data was then able to function well in an ice accretion model. The cylinder data however, is limited in accuracy to the region close to the stagnation line. The presented work uses a system of leading edge heat transfer sensors embedded into a UAS representative wing section. The sensored wing section is tested across the envelope of standard UAS conditions. A set of heated wires will be integrated along the wing’s airfoil with variable power input to allow for heat flux (convection heat transfer) calculations. The work consists of detailed installation with a custom composite wing, internally powered electronics, and highly observed wind tunnel tests. Since heat transfer is highly dependent on Reynolds number, the system is tested across the entire reasonable stretch of its flight envelope (22-65 knots). This then equates to an approximate Reynolds number range of 211,000-572,000 for the airfoil. The experimental tests will use angle variation of the sensored airfoil to consider rotational changes of convective heat transfer. Additionally, free-stream turbulence has a significant impact on heat transfer. Measured values are, as a rule, higher in wind-tunnels than in flight. However, reported values of free-stream turbulence for UAS in flight regimes are non-existent. In order to measure this quantity, the sensors for observation will include a multi-hole probe. It has been shown that except for cases with high levels of axial turbulence, the multi-hole probe is able to match the accuracy of a hot-wire anemometer up to 1%.

Airfoil heat transfer data for UAS will provide unique and important information to the UAS and heat transfer communities. The sensor data resultant for this campaign will directly feed into the existing model on UAS icing. The sensor design and initial data will also provide foundational work for the future UAS flight test. The knowledge of icing classification and accretion rates is paramount to winter or cloud operation of small UAS and AAM. Efforts to provide icing mitigation systems on UAS and AAM will greatly benefit from heat transfer flight data. Size, weight, and power are costly attributes to any onboard system, and any icing mitigation installation will need to be designed for maximum consideration.