3 Day Open Conference
3 Day Open Conference
Aerodynamic Design of the Lockheed Martin Cooperative Avionics Testbed
Tuesday 18th November 2008
15.00 - 15.30
Auditorium 3
The Lockheed Martin Cooperative Avionics Testbed (CATBird) is a heavilymodified B737-300 airplane. The CATB will be operated in the ExperimentalCategory as a testbed for JSF sensor operations and data fusion in anoperational environment. External modifications to the basic airplane includereplacement of the nose radome, addition of sensor wings on the forwardfuselage and strake antennas on the aft fuselage, and the addition of extensivespine and canoe antenna fairings over the airplane’s upper and lower fuselage.
The airframe has also been heavily modified internally to provide asimulated JSF cockpit and engineering and observer stations. The subject of thispaper is the aerodynamic design of the external aerodynamic modifications.The most critical external modification was the sensor wing/radome combination.The JSF radome completely replaced the airplane’s nose radome with a longer,chined structure. The aerodynamic fairing between the JSF radome and B737forward cockpit bulkhead was designed to minimize downstream disturbance,particularly at the existing pitot-static probe locations which were to bemaintained for CATBird.
Portions of the JSF wing leading edges (called ‘sensor wing’) were mounted onthe CATBird. The radome and sensor wing had to be installed on CATBird so asto maintain their geometric relationship on JSF. Consequently the sensorwing was required to be mounted on the forward fuselage ahead of the wing andengines, with possible negative effects on airplane stability and engine inletperformance.
Attaching the JSF LE section to the B737 required the development of chordwiseand spanwise fairings to complete the sensor wing loft. These sectionswere design for conflicting requirements. Reduction of the destabilizing effect ofsensor wing required minimum area. This became a requirement to minimizechord length, as the span was fixed by the JSF components. However, avoidingvibration and buffet in the airplane’s transonic operating environment placedupper limits on the airfoil thickness-to-chord ratio. Because thickness was fixedby the JSF part these two requirements produced conflicting design constraintson chord length.
The sensor wing loft was designed to use the JSF leading edge part, minimizethe stability impact, and avoid transonic flow separation. The airfoil design codeMSES and 3D analysis codes VLAERO+, VSAERO, and MGAERO were used. There is a reduction instability due to the sensor wing, which is accounted for in CATBird operation by achange to the aft cg limit (compared to the basic airplane). The predictedstability increment of this planform was later verified by wind tunnel and flighttesting. In addition, the sensor wing is free from vibration and buffeting over theoperational speed envelope of the CATBird.
The aft band antenna structure was designed to mount a portion of the JSFhorizontal tail and a separate sensor. Similar to sensor wing theremaining wing sections were designed using VSAERO and MGAERO tominimize the possibility of high speed buffet. The final loft was a compromisebetween the compound curvature aerodynamic loft and a simpler loft which wasmore easily manufactured.
The fuselage spine and canoe fairings were designed for minimum suction peaksto reduce aerodynamic loads and high speed buffet. The cylindrical constantcross-section was designed with the maximum curvature consistent with therequired footprint. The forward nose was designed to minimize suction peaksand the aft section designed to minimize the severity of the recovery pressuregradient.
Each external OML was developed through analysis and wind tunnel testing.Candidate lofts were developed and evaluated by aerodynamicists and structuralengineers. The resulting lofts were the simplest buildable shapes that producedthe minimum impact on the existing airplane.Flight testing of the modified airplane demonstrated that the impact on airplanestability was as predicted. The modifications did not create high speed buffet orvibration within the operating envelope of the modified testbed.
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