“Heritage F-18: Surprise!”Part 2 in a series of posts where we document 'The Profound Truth' of High Angle-of-Attack (AoA) flight testing of high performance aircraft.
Discovery and rectification of undesirable aircraft behaviors during High Angle-of-Attack testing of High Performance Aircraft is not only the ‘Norm’, but those behaviors needing rectification/mitigation are usually complex, sometimes bizarre, and often ‘spectacular’.For this post, we will avoid mentioning all other problems the F-18 program dealt with that did not have to do with the High AoA performance, behaviors and testing. They would be helpful highlighting in yet another way, how the F-35 program isn’t as ‘concurrent’ as some would lead us to believe: but I’ll resist the temptation to beat that dead horse (this time).
The ‘Heritage’ F-18A/B/C/D provides an excellent exhibit of 'The Profound Truth'The Heritage Hornet (F/A-18A thru D) was one of the ‘first-generation fly-by-wire (FBW)’ aircraft developed in the 1970s. While other notable 1stGen FBW aircraft of the era (such as the F-16 and the Mirage 2000) employed AoA limiters “within their control laws to avoid out-of-control-flight (OOCF) losses due to departure, spin, or deep stall”(Heller, Et al, 2001), it was found the Heritage F-18 did not need one….but only in a ‘clean’ (and therefore nearly useless militarily) configuration. High AoA testing revealed a design that would let the Blue Angels boggle John Q. Public’s mind with precise aerial displays, if you hung a weapons load with almost any real asymmetry the Max AoA allowable for the Heritage F-18 is reduced and other bad things happen:
|Well 'Connected' Vortex Flow |
at Moderate AoA
|'Broken' Vortex & Turbulent Flow at |
High AoA (NASA HARV Program
In 1979, an F/A-18 test aircraft at Patuxent River suddenly and unexpectedly departed controlled flight during a wind-up turn maneuver at high subsonic speeds. None of the baseline wind-tunnel data predicted this characteristic, and the F/A-18 Program was shocked by the event. The fact that the free-flight model had also exhibited such a trend did not go unnoticed, and a joint NASA, Navy, and McDonnell Douglas team was formed to seek solutions with the free-flight model at Langley. Following exhaustive wind-tunnel tests in the Full-Scale Tunnel, the team recommended that the wing leading-edge flap deflection be increased from 25 deg to 34 deg at high angles of attack. Following the implementation of this recommendation on the test aircraft (via the flight control computers), no more departures were experienced, and the flap deflection schedule was adopted for production F/A-18’s. (Chambers, 2000)Between late 1979 and end of Full Scale Development (aka FSD --closest corollary is today’s SDD) there were FIVE different series of F-18A/B’s control law changes. These major changes “…were incorporated in each of the major PROM series. Control law changes have been incorporated to improve handling qualities at all flight conditions (including high AOA and out-of-control), improve roll performance, reduce structural loads, improve departure resistance characteristics, incorporate and refine pilot relief modes, and provide an active oscillation controller to suppress undesirable in-flight oscillations.” (Kneeland et al)
Fortunately, these changes mitigated or eliminated most of the Heritage F-18’s early untoward behaviors, but one in particular remains to this day: the ‘Falling Leaf’ departure mode (aka ‘alpha hang-up’). The mode remains “suppressed”, but as the video below illustrates, still remains a threat to all but the most wary Heritage F-18 pilots.
Keep in mind the Heritage F-18’s discoveries when the rabid army of F-35 haters start sounding off.