The Sabre Dance.
The F-100 Super Sabre was an American single engine fighter bomber and the first of the “Century Series” of fighters produced from 1950s onward. These jets played a vital role during the Vietnam war by mostly providing close air support and air superiority. The Super Sabre was an advanced derivative of the proven F-86 Sabre, however there was one peculiar trait that only the Super Sabre seemed to possess : The Sabre Dance.
What is the Sabre Dance?
Contrary to general associations with dancing, it was an uncontrollable deep stall condition arising due the very nature of swept back wings. At low speeds, the stall began at the tip and caused the nose of the aircraft to pitch up rapidly further intensifying stall, but this time in the form of trailing edge stall. These series of behaviors was called by aviators as the “Sabre Dance”. The Sabre Dance is associated with a significant number of total F-100 fatalities, as a result the F-100s were also called as “Widow Makers”.
Why did it happen?
A wing is swept to reduce wave drag arising at transonic and supersonic speeds, this is achieved by splitting the incoming air into span wise component in addition to existent chord wise. Although this is very beneficial at high speeds, but as the aircraft approaches low speeds(such as during landing), the tip loses lift due to lesser effective free stream velocity compared to rest of the wing. Since the wing is swept the wing tip is aft of the aircraft CG, loss of lift at tip along with lift acting at root and mid span generate a strong pitch up moment leading to a super stall and loss of directional control which can be almost impossible to escape from.
Can this be corrected?
Wing fences or potential fences are aerodynamic devices that not only help counter tip stall, they also improve roll stability and act as vortex generators thanks to the span wise component of velocity in swept wings. The term potential is with reference to the effect these devices have on the potential flow(Zero Vorticity).
Wing fences usually cover 1/3rd of the chord and even extend ahead of the leading edge(region of maximum lift). These plates redirect the span wise component of air into chord wise flow, thereby giving good handling qualities at low speeds by delaying tip stall.
Where should they be located?
Typically wing fences are located between 40% to 60% of half span and directly in front of inner edge of aileron or elevon, this is to take advantage of the single core and opposed dual core vortexes that form at low and high angle of attacks respectively that delay flow separation over the control surface, thereby making them more effective. When it comes to high aspect ratio steep sweep wings, it is beneficial to use 2 wing fences per wing located at 1/3 and 2/3 distance of the semi span.
The wing fences work by splitting the wing into 2 parts of lower aspect ratio lifting surfaces having greater stall angle than the previous single wing.
The invention of the versatile wing fence is attributed to a German aerodynamicist, Wolfgang Liebe in the year 1937. Apart from the eventual application into the Super Sabre, and a majority of other non delta commercial and transonic(and supersonic) warplanes, they can also seen in a variety of modern war planes, especially from Russia such as the MiG-21, MiG 31, Tu-122, Tu-160(in swept mode), etc.
