The aerodynamics of the Stratos 714 is being developed using modern computations fluid dynamics software along with empirical processes.
The Stratos 714 will cruise at Mach 0.7, yet have approach speeds comparable to piston singles. Developing an aircraft capable of flying at 400 knots but with excellent handling qualities at 80 knots was a tough challenge. The key to achieving these competing demands is the Stratos 714’s unique airfoil and wing design.
Stratos 714 Wing Design
The Stratos 714 wing has been designed to sustain extensive laminar flow in the cruise. The drag benefits from laminar flow are well known to pilots of gliders and the new generation of composite piston singles. However, unlike these aircraft, the Stratos 714 has to maintain this laminar flow at transonic speeds. In addition, the Stratos wing must have a generous buffet margin to allow for gusts and maneuvers at high speed. At the other end of the speed range, the wing must generate high lift with benign stall characteristics. This combination of requirements is unique to the 714 and required the development of a correspondingly unique wing.
The highly efficient wing is the result of the application of a novel design approach. Many companies now use computational fluid dynamics (CFD) software to simulate the airflow around airfoils, the wing or the whole aircraft. However, Stratos have taken the application of CFD to the next stage by writing software to carry-out analyses automatically as part of the wing design process. An optimization method implemented in the software modifies the wing profile according to the CFD analysis results. After many hundreds of such modifications, the software converged on the wing profile which best matched the aerodynamic and structural requirements.
In addition to its use in the wing optimization, CFD has been applied throughout the design process. As illustrations of its ability to visualize the flow, providing rapid feedback on design changes, the following figures show example CFD analyses from the design of the Stratos 714.
Figure 1 is from a CFD analysis of one of the Stratos airfoils at conditions close to clean stall. The colors in the plot represent the velocity of the local flow. Separation of the flow over the aft upper surface is readily apparent. The upper and lower boundary layers are also discernable.
Figure 1 – Velocity of flow over wing section at high incidence.
Figure 2 shows the pressures on the aircraft at conditions equivalent to a pull-out maneuver at high Mach number, one of the more extreme design cases. Figure 3 shows surface streamlines for the same case. The green surfaces in this figure mark the periphery of regions of supersonic flow. Flow leaving this region does so via a ‘shock wave’. This shock has the potential to separate the flow over the wing causing severe buffet. It is therefore important the wing be designed such that these shocks are weak.
Figure 2 – pressure plot during high speed pull-out maneuver.
Figure 3 – Streamlines and iso-Mach surface during high-speed pull-out.
CFD has also been applied to the integration of the power plant. Figure 4 shows the flow over a plane cut vertically through the inlet face. Figure 5 shows an analysis of the entire flow path of the air through the Pratt & Whitney Canada JT15D-5 engine. The mass flows, temperatures and pressures in these analysis were set to the values output by Pratt & Whitney Canada JT15D-5’ engine performance program.
Figure 4 – Streamlines on section through inlet face.
Figure 5 – Analysis of flow through inlets and exhaust.
Stratos 714 Wind Tunnel Program
In August of 2011 a 1:5 wind tunnel model was tested at the University of Washington wind tunnel. The objectives of the wind tunnel program was to verify much of the theoretical CFD analysis.
Figure 6 – 714 Wind Tunnel Program.
Over 90 wind tunnel runs were completed. The results verified and showed excellent correlation to the CFD analysis. Some areas of concern proved to be non-issues. However, an issue with airflow separation was discovered that is being addressed.
Figure 7 – The patterns shows the results from one of many tests to verify local airflow.