March 31, 2004

Krista Laursen
Director, HIAPER Project Office

Update on the GV Wing Pod Study and Selection Process

Work is continuing on the GV wing pod study and selection effort, and based on recent events, the HPO felt this to be a good time to send out another status message to all of you to outline where the pod effort presently stands.

Several days ago, Gulfstream (GAC) completed the aerodynamic studies of the new proposed NCAR pod. You will remember from previous messages sent out by the HPO about pods that after several weeks of studies and deliberations with ATD and GAC personnel, the HPO made the decision to stop considering the P-3 pod as the initial GV wing store choice and to instead focus efforts on the study of a new NCAR pod design proposed by Mark Lord (ATD/RAF aeronautical engineer). It is the aerodynamics studies of this latter pod that GAC recently completed.

This past Monday, 29 March, a number of HPO and ATD personnel participated in a teleconference with representatives of the Gulfstream HIAPER engineering and Flight Sciences team to go over the results of the NCAR pod aero. studies. A wealth of data has been generated by GAC, and I will only attempt to summarize the main points here. As was presented by GAC on Monday, the proposed NCAR pod aerodynamics are superior to those of the P-3 and the Sargent-Fletcher pods. There are, however, some issues pertaining to the aerodynamics of the proposed NCAR pod mount fairing and pylon that remain to be addressed. Specifically, the GAC studies have indicated that the current pod pylon and fairing designs introduce shock separations on the pod itself at GV flight speeds of between Mach 0.80 and 0.85. In essence, supersonic regions are induced on the pylon at higher flight speeds. However, it is the opinion of GAC Flight Sciences personnel that these shock issues can be addressed by the pylon and fairing structures being re-designed.

GAC has also determined that while the proposed NCAR pod displays drag characteristics that are improved over those of the P-3 pod, there is still a significant drag issue that remains to be addressed. GAC engineers feel that work to re-design some aspects of the pod fairing could yield desirable reductions in drag.

One significant area of concern still remains, however, with the NCAR pod. The airflow studies conducted by GAC yielded the result that the NCAR pod configuration may negatively impact the lift characteristics of the GV wing. This is, as you might imagine, of importance because should it be determined by modeling that the wing lift characteristics will be altered significantly by the mounting of the NCAR pod under the wings, substantial testing (wind tunnel, flight, etc.) could be required in order for the pod effects to be fully characterized and verified before use of pods on the aircraft. Further, re-programming of the GV air data computer could become a necessity for FAA certification and safety of flight in order to make sure that the stall performance of the aircraft is not impacted in a negative manner. GAC and NCAR personnel discussed the possible lift impact of the pod at length this past Monday, and this issue will require further consideration and analysis by all parties in the coming weeks. However, there is a possibility that by re-designing the NCAR pod pylon and fairing, some of the unfavorable lift impacts can be mitigated.

So, what will be happening next. It was agreed this past Monday that the GAC Flight Sciences Loads Group will continue with their study of the impact of the NCAR pod on the GV wing structure. (In fact, the pod aerodynamics data were passed on to the Loads group roughly two weeks ago, and they have already begun their work.) This loads study is an absolute necessity, as it will be used to determine if there any significant "show stoppers" that will require NCAR and GAC to cease considering the NCAR pod for use on the GV. After the loads study is completed (which is intended to occur in late June) -- and assuming favorable results are obtained -- NCAR and GAC personnel will then begin considerations of the specific design refinements that need to be made to the pod pylon and fairing.

The HPO will continue to update all of you as work on the GV pod study and development effort proceeds in the coming weeks. In the mean time, should you have any questions about the above information, please do not hesitate to contact either Dick Friesen (rbf@ucar.edu or 303-497-2004) or me here in the HPO.