The F-35: What Will Happen While Exploring ‘High Angle-of-Attack’, Part 2

 “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.

 The Profound Truth:

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:

Modest asymmetries increase the departure and spin susceptibility and come with undesirable fight manual limitations on the maneuverability. Large asymmetries impose severe limitations, which must be rigidly observed, thereby reducing the airplane’s safety and operational flexibility (Heller).



Well 'Connected' Vortex Flow
at Moderate AoA

The other really ‘big’ thing discovered in the YF-17/F-18A development effort was that the leading edge extension (LEX) has all sorts of advantages (up until it doesn’t). It generates vortices over the top of the wing and fuselage that increases lift at higher AoAs until it reaches a point where the AoA is so steep the vortices break down and turbulent flow takes over. The LEX was modified, ‘fences’ were added to help, but at Max AoA the sudden onset of turbulent flow beats up the vertical tails and knocks controllability out the window. Later, it was discovered that the tails took such a beating that the attachment points were reinforced with additional structure. (Aerospaceweb has an excellent short summary here.)    

As an aside, I must add that a detail design engineer on the F-18 program once noted in a lecture I attended that the early F/A-18 (Correction: YF-17) wind tunnel models shed verticals like crazy at high AoA. The model makers assumed it was the models’ fault, so they just built the models stronger. In retrospect, the wind-tunnel models were telling them something.

'Broken' Vortex & Turbulent Flow at
High AoA (NASA HARV Program

NASA got involved with fixing some of the Heritage F-18 ‘controllability problems’:   

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.

 

Many of the design objectives behind the F-18E/F were focused on eliminating the extant problems and limitations of the Heritage Hornets.

Keep in mind the Heritage F-18’s discoveries when the rabid army of F-35 haters start sounding off.

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REFERENCES:

“The Impact of the F/A-18 Aircraft Digital Flight Control System and Displays on Flight Testing and Safety”; Kneeland, B. T. , McNamara, W. G. , White, C. L.; NAVAL AIR TEST CENTER PATUXENT RIVER MD; 1983.

“Partners in Freedom: Contributions of the Langley Research Center to the U.S. Military Aircraft of the 1990's”; Chambers, J.R.; NASA SP-2000-4519; 2000.

“F/A-18E/F Super Hornet High-Angle-of-Attack Control Law Development and Testing”; Heller,M., Niewoehner, R.J., Lawson, K.P.; JOURNAL OF AIRCRAFT, Vol. 38, No. 5; 2001.