What's the advantage of the F4U Corsair's gull wing design?

Question

What is the advantage of the F4U Corsair's gull wing design?

Is there also a disadvantage to this? The wing design just seems odd, and as far as I know it is one of the only planes to use this design.

Is this because the technology advanced so fast, or because the wing design was flawed?

Answer 1

The wing shape in the F4U Corsair is called the inverted gull wing. The main reasons for use of this is the large propeller used in the aircraft.

The Corsair design was in response to the US Navy RFP (Request for Proposal) in 1938, which mandated the following things:

• It should have maximum attainable speed
• The stall speed should be 70 mph
• Minimum range of 1000 miles
• Bombs should be carried under the wing (this was supposed to be dropped on aircraft formations from above, which didn't work out)

In order to achieve this, the Vought engineers used the largest available engine, the Prat and Whitney R2800 Double Wasp radial engine (this was done as the Navy, in a change of heart, vowed to consider designs with liquid-cooled engines, in contradiction with a policy settled in 1927 that required air-cooled engines for shipboard aircraft). Also, the wing area of F4U was the largest naval aircraft of its time.

During design, the oil cooler and supercharger inlets were moved to wing leading edges. In order to extract power from the engine, a large (~4m dia) propeller was chosen. This large propeller caused problems with ground clearance, which necessitated an increase in the landing gear length.

The Vought team overcame this by choosing an inverted gull wing, which starts out with a strong anhedral, and then curved upwards to strong dihedral, with landing gear at the lowest point in the bend.

^{"F4U-Corsair OE-EAS OTT 2013 04 main landing gear" by Julian Herzog. Licensed under CC BY 4.0 via Commons.}

The main advantages of this configuration are,

• Shortened landing gear
• Elimination of the need for wing fillets, and reducing drag as the wing and fuselage were perpendicular.
• Simplified wing folding (the wings were folded at the lowest point), with the folded wings nearly at the height of the propellers. This enabled simple automatic mechanism for folding/unfolding, unlike the F6F Sto-wing which was manual, as the hydraulic mechanism added too much weight.

^{Source: www.airport-data.com}

The main disadvantages were:

• The design added weight compared to straight wing.
• (To some people) It was ugly

This design has been used in a number of other aircraft too, mostly to reduce the length of the landing gear and carry large external stores, the most famous being the Junkers Ju-87 Stuka.

^{"Bundesarchiv Bild 101I-646-5184-26, Russland, Flugzeug Junkers Ju 87 edit 1". Licensed under CC BY-SA 3.0 de via Commons.}

The inverted gull wings were also used in Fairey Gannet, an ASW aircraft of Royal Navy.

^{Source: www.maritimequest.com}

This was also used in some twin engined aircraft like the Caproni Ca.331.

^{Source: flickr.com}

Note that in some books, this design is called the gull wing instead of inverted gull wing.

Answer 2

It is mainly related to the folding nature of the wing and the need to accommodate the landing gear,

...it was difficult to make the landing gear struts long enough to provide ground clearance for the large propeller. Their solution was an inverted gull wing, which considerably shortened the required length of the main gear legs.[20]

note how the landing gear has to be pre fold. The wing design uses the wing to add height to the gear which in turn allows for shorter struts that are easier to fit in the wing.

Answer 3

This "inverted gull wing" design is not flawed, it just shows that the designers arrived at a different solution in their desire to reduce drag and weight, and to make the result easy enough to build.

This wing shape is not so unusual - the Ju-87 "Stuka" (below, right) used it, and so did the Klemm 35 (below, left), a litte sports plane from the 1930s. In both cases the idea was to have a shorter gear with lower drag.

(picture sources: Left and Right)

Apart from the obvious benefit of a shorter landing gear, the angle at which the wing meets the fuselage in a low-wing configuration is increased (closer to 90°), so that interference drag should be smaller. The added complexity in manufacture, however, outweighed the benefit of lower drag in most cases. Both of my examples used fixed gears, and using an inverted gull wing on them was a trick to reduce the drag of the gear leg.

In case of carrier-borne aircraft like the F4U, the lower height of the folded wingtips allows to fit the aircraft into lower hangar decks; or the airplane can afford to have a higher wing span for a given hangar deck height.

The Heinkel He-70 did have a retractable gear and an inverted gull wing, and here the motivation for the inverted gull wing was reduced interference drag, but its superb aerodynamic quality was mostly due to it being the first aircraft to employ countersunk rivets.

Heinkel He-70 (picture source)

To sum it up:

Advantages:

• shorter, lighter landing gear with less drag.
• Less interference drag between wing root and fuselage and between gear strut and wing
• In case of carrier aircraft like the F4U lower height of the folded wingtips.

Disadvantages:

• More wing area needed for the same lift, since both inner and outer wing have a sizable sideways component of lift …
• … which also causes more friction and inducded drag
• Wing construction becomes more difficult and expensive