What is the purpose of a wing Yehudi? [duplicate]

Apart from covering the landing gear leg, the Yehudi also increases the wing root cord which allows the build height for the root to increase for the same wing relative thickness. This is useful as the wing has its highest stresses in this area, it is where the wings bending moment is at its highest (leehamnews.com).

I don't understand the text in bold. Apart from landing gear 'packaging'. What are the advantages of the Yehudi? Aerodynamic or otherwise.

(commons.wikimedia.org) An early jetliner without the Yehudi.

I'm also interested in the etymology.

marked as duplicate by mins, Ralph J, Sanchises, David Richerby, Peter Kämpf aerodynamics StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Aug 13 '17 at 16:29

• The official history explains in a footnote that the name Yehudi in then-contemporary slang meant "the little man who wasn't there".[1] The slang may perhaps allude to the popular catchphrase and novelty song "Who's Yehudi", or "Who's Yehoodi". The catchphrase is said to have originated when violinist Yehudi Menuhin was a guest on the popular radio program of Bob Hope, where sidekick Jerry Colonna, apparently finding the name itself humorous, repeatedly asked "Who's Yehudi?" – TomMcW Aug 13 '17 at 0:18
• That's from the footnotes in this Wikipedia article but it doesn't ring true to me – TomMcW Aug 13 '17 at 0:20
• – Peter Kämpf Aug 13 '17 at 7:27
• Having said that, I would love for some answers on this question to be put there, one way or another. – Sanchises Aug 13 '17 at 13:10

Mitteneffekt

The pressure fields of the left and right part of a swept wing interfere at the center, causing a drop in lift. The Horten brothers called this "Mitteneffekt", and it was never properly translated into English, so the German term is used. The Hortens increased chord at the center trailing edge of the wing, reducing local sweep and increasing the area for lift creation. This helped to fill the "hole" at the center and at the same time allowed them to better enclose the pilot.

Lift distribution over unswept and swept wing (picture source)

The same is achieved with the reduced training edge sweep at the wing center of airliners. Since this part is occupied by slotted flaps, the rounded trailing edge contour of the Horten planforms is not possible there, so a straight line is the best solution.

Structural Efficiency

Large wings need heavy spars, and saving some spar weight goes a long way in reducing lift requirement and, consequently, drag. While smaller airplanes do not profit much from an increased wing root thickness, larger ones can save hundreds of kilograms by increasing their root thickness. Since the flight Mach number must not suffer from this thicker root, the simplest solution is to extend the wing chord at the center. (Sorry, the link leads only to a library, but the thesis gives extensive proof.)

A nice side effect is the increased space for housing the landing gear.

Separation Control

Interference between fuselage and wing will cause separation there to start earlier than on the clean wing. Also, the superposition of the flow fields of fuselage and wing will cause shocks at a lower flight Mach number. Therefore, aerodynamic efficiency both at low and high speed is increased when the local lift coefficient drops near the wing root, so the flow has more margin before shocks or separation start. By increasing root chord over what is required for an elliptic lift distribution, the local lift coefficient can be lowered while the circulation distribution stays elliptic.

Flap Effectiveness

With an unchanged leading edge sweep, Mach tolerance is not much affected, but the lower trailing edge sweep is very helpful for increasing the effectiveness of the trailing edge flaps.

Regarding the word "Yehudi": This is the first name of a famous violinist, but I do not connect it with a wing planform.

• I was also imagining a violinist sitting on the wing. :-) – David Richerby Aug 13 '17 at 13:37
• Great answer! Obert mentions 'the wing trailing edge shows a kink (sometimes called a Yehudi after the inventor at Boeing).' in Aerodynamic Design of Transport Aircraft p207. – ROIMaison Aug 13 '17 at 15:38
• @ROIMaison: What "inventor"? The Me-262 already had that straight center section trailing edge; Boeing studied the Horten IV when starting work on the B-47 and even hung a Me-262 drawing in the office when they started on the 737. There was nothing left to invent. – Peter Kämpf Aug 13 '17 at 16:24
• @PeterKämpf, as always you've got inside information :) I just recalled the mention, and thought it would be a good addition. Can't say anything about the trueness of it. – ROIMaison Aug 13 '17 at 20:20

The straight bit of trailing edge at the inboard section of swept wings is indeed to house the undercarriage, without having to interrupt the rear spar structure: behind the rear spar, outside of the fuel tanks. The top view in the OP is of a caravelle with engines mounted at the rear fuselage, with a relatively much shorter landing gear.

If this gear housing would just be a bit of extra skin cladding, the local wing profile would have:

• a lower sweepback angle, decreasing drag divergence Mach number.
• a lower wing thickness, increasing drag divergence Mach and (partly) compensating for decreased sweep angle.

That's aerodynamics. Structure wise, a low wing thickness at the root is not desirable, because that is where the highest bending moments are: the more distance we can get between upper and lower skin, the lighter we can construct. An early solution to this was seen in the B737, where the leading edge was kinked as well and the average thickness increased proportionally, maintaining more or less constant profile geometry.

Aerodynamic optimisation of the wing root is a very complex procedure, and half a century after the design of the 737-200, CFD enables much more extensive aerodynamics testing. Wing root optimisation is a bit of a dark art, with parameters including:

• Increasing local twist.
• Forward shift of the point of maximum thickness.
• Extra thickening of the root section: for low wing aircraft, part of the root section lacks the upper surface where the drag divergence shock are created.

On the etymology: I've never seen it referenced as a wing yehudi before, only as a trailing edge kink :)

There are some people on here that have much more technical knowledge about this, but just from the words of the text you are asking about it seems that they are saying that the wing can be built "fatter" (i.e. the distance vertically from top of wing surface to bottom of wing) when the chord (distance from leading edge to trailing edge) is increased.

The wing root chord, is simply the chord (distance from leading edge to trailing edge) at the root of the wing (where it connects to the fuselage).

It seems to make sense that being able to build a fatter wing where it has the highest stress would be beneficial for improving structural strength.