Why do 20-series Learjets have such an extreme susceptibility to mach tuck?

Question

Mach tuck is a change (usually nosedown) in an aircraft’s pitching tendency during transonic flight relative to its pitching tendency in below-transonic flight, caused by the appearance of areas of supersonic airflow (with attendant formation of shockwaves, sometimes accompanied by regions of flow separation) around the aircraft when it exceeds its lower critical mach number. It first showed up immediately prior to and during World War II, as a completely unexpected, and quite deadly, type of loss of control at high speeds (especially on aircraft using the very thick airfoils then in fashion); however, by the mid-1950s, advances in wing and airframe design had reduced mach tuck to little more than a nuisance, allowing even first-generation jetliners to safely fly at transonic speeds without risking a loss of control.¹

In contrast, 20-series Learjets, although dating from the 1960s (by which point we already knew very well how to make aircraft highly mach-indifferent), tuck like an aircraft from the 1940s, pitching down suddenly and violently at approximately mach 0.81. This extreme susceptibility to mach-induced loss of control at high speeds caused a rash of crashes in the 1970s and 1980s, and is one of the many factors that make first-generation Learjets some of the most unforgiving aircraft ever sold to the public (at least, if the NTSB’s AAR archive is anything to go by).²

It is quite hard to see what about these aircraft would cause them to have such evil overmach characteristics; admittedly, they do have straight, rather than swept, wings, but this should impact more where the lower boundary of the transonic flight regime is - not so much how the aircraft behaves once it’s already in transonic flight - and, besides, the thinness of the wings should compensate for their lack of sweep.³

What is the cause of the extreme susceptibility of first-generation Learjets to mach tuck?

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¹: First-generation jetliners are, nevertheless, generally kept below their lower critical mach numbers in normal line operations, due to the greatly-increased drag associated with transonic flight; however, further design advances, first seen on the Convair 990, greatly mitigated even the transonic drag penalty, with the result that second-generation and later jetliners routinely cruise at transonic speeds.

²: Other factors included a balky autopilot, spoilers that make the aircraft go faster when extended (by causing a large pitch-down tendency of their own), and the fact that their original certificated service ceilings were well into the coffin corner.

³: The reason swept wings help with transonic flight is that they make the wing appear, to the air flowing over it, to be thinner than it actually is, thereby helping to delay and mitigate shockwave formation and flow separation. These same benefits can also be gained by actually making the wing thinner.

Answer 1

The early series 20 Lears have always had boundary layer separation issues, so much so that any time the wings are removed and reinstalled the aircraft must be test flown. Mach tuck is an issue with all aircraft capable of trans-sonic flight, but on the early Lears this is apparently exacerbated by the susceptibility to boundary layer separation from the laminar flow airfoil.

To address this and other shortcomings in the 20-series wings, a number of modifications have been developed. Vortex generators were installed in front of the ailerons, and then a boundary layer energizer "fence" was developed. Dee Howard and Gates Learjet jointly developed a "cuff" for the Lear 25 wing that resulted in the 25G variant, with better performance at cruise and an increase in range of some 360 nautical miles.

There has been some talk that the problems of the Lear wings come from the FFA P16 design. Bill Lear's son has clarified that his father did not use the P16 as the basis of the Lear 23. The two aircraft use different, though similar airfoils.