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RAISBECK ENHANCED PERFORMANCE LEADING EDGES (EPLE) MODIFICATIONS — AN ANALYSIS BY TOM CLEMENTS


Note: The following analysis was written by Tom Clements and submitted to Stevens Aerospace. We’re grateful to Tom for a lifetime of experience and insight in the aviation industry.


I have been recently asked (February, 2020) to provide my thoughts on the Raisbeck Enhanced Performance Leading Edges (EPLEs) and I am happy to do so.


First, a little history. Back in the 1960s, as some old-timers may remember, the Robertson STOL Company was developing improved short field performance modifications for many airplanes. Robertson’s drooped leading edges became a very popular modification and starting in the ’70s the Cessna factory made their own wings with drooped leading edges. You might say a drooped leading edge “fad” existed at this time and Beech designers added the drooped leading edge to the center section and the first few inches outboard of the nacelle beginning with the Beechcraft 99 and then on the King Air 100-series. Later, the F90-series would also use this wing design.


Design of the Super King Air 200 was taking place at this time – late 1960s, early 1970s – and although it had a 50-inch wider center section than previous King Airs, the drooped leading edges were included in the design. But, perhaps due to the higher cruise speeds of the 200 along with the increased span of the center section, it was determined that the droop was actually having a detrimental effect. Namely, the droop – at cruise angles of attack – was causing the wing center section to provide “negative lift.” By that I mean that the inboard wing sections were producing less lift than the outboard sections, becoming more pronounced as speed was increased. Only when the angle of attack was strongly positive, as in slow flight, did the droop work as desired.


The center section providing this lower lift component in cruise meant that the wing outboard of the nacelle had to provide lift not only to balance the weight of the airplane but also to overcome the loss of lift from the center section. The engineers raised concern about the long-term affect this might have on the wing attachment mechanisms. By now, however, the design was finalized and the redesigning and building of a new center section leading edge would have delayed the program too long. The “fix” was to change the specification of the aileron rigging, to make both ailerons, in neutral, have their trailing edges a small degree higher. This caused the effective angle-of-attack of the outboard wing to be reduced and forced more lift to be generated inboard.


This slight aileron adjustment caused a minor problem: The test pilots reported a degradation of the outstanding aileron “feel” that was exhibited prior to the adjustment. The fix to this complaint was the installation of a very small triangular strip affixed to the bottom wing skin just in front of the aileron’s inboard end. This strip “tripped” the boundary layer flow such that it better attached to the slightly raised aileron’s bottom surface. Here’s a picture:

The triangular strip affixed to the Super King Air 200's bottom wing skin just in front of the aileron’s inboard end

It’s likely you had not even noticed these before. Right?