Dateline: 11/24/97 Composites are ideal for applications where high stiffness and low weight are required. A classic example of such a product is an airplane wing. In this week's feature, we'll take a detailed look at a typical composite wing design.
Terminology
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The sample wing section shown above was made while I was working at Aurora Flight Sciences of West Virginia. It was built for a display on lightweight aircraft in the Smithsonian Air and Space Museum, but I don't know if it actually made it to the museum.
In case you are unfamiliar with wing designs, there are a few terms you need to know. The curved edge on the left is called the leading edge; it is the forward edge of the wing. The pointed edge on the right is called the trailing edge.
The distance from the leading edge to the trailing edge is called the chord. This sample has a chord length of 18 inches. A full wing may have a constant or a varying chord.
The two vertical members are the spars. The one closest to the leading edge is the forward spar; the one closest to the trailing edge is the aft spar. The forward spar is actually an I-beam--the vertical piece is the spar web and the thick graphite plates at top and bottom are the spar caps. The spar carries most of the bending load in the wing.
Wing Design
The wing is made in several parts. The upper and lower skins are layed up separately in female tools (the outer sides are placed against the tool face). Nomex honeycomb is used to increase the bending stiffness of the skins.
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The spars are layed up on a flat table; the web and caps of the forward spar are layed up separately. Both webs have a Nomex honeycomb core, but a real wing might have local reinforcements of solid graphite or some other material for point loadings.
To assemble the wing, the spar caps are bonded to the upper and lower skins. The high loads carried through the spar could cause the honeycomb to fail, so the skin core must be tapered at the spars. This wing uses a 30 degree taper, which is typical for most structures. Larger angles would create a high stress concentration at the taper; smaller angles would be difficult to manufacture.
Next, the webs are then bonded to the lower skin or spar cap (it could just as easily be the upper). A relatively thick layer of adhesive goes between the web and skin, and then one or two plies of graphite are layed up over both the web and skin to provide better load transfer. These latter plies form an "L" shape, and are sometimes called angle plies.
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Once the spars are in place, the upper skin can be bonded in place. The spar bonds on the upper skin are similar to the lower skin (the actual process is rather tricky, because you don't have access to the inside on a full wing).
The leading edge bond is a simple lap joint. The honeycomb on both the upper and lower skins tapers at the leading edge--the curvature of the wing provides enough stiffness here. The upper skin curves around well onto the lower skin (the red line of edhesive shows the end of the upper skin). The lower skin curves roughly halfway up the sharp bend of the leading edge. Because the lower skin goes on the outside, the upper skin must have a slight step (equal to the thickness of the lower skin) to keep the leading edge smooth.
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The trailing edge bond looks simpler, but it is actually more difficult. Because it comes to a point, the skins can't wrap around each other. Instead, a thick adhesive paste layer is placed on the lower skin and the upper skin is pressed into this layer. Ideally, the honeycomb would be continued all the way to the edge, but it must be tapered earlier to fit the skins together.
The wing is finished by priming and painting. In this case, standard automotive paint is used. Some paint has been left off the trailing edge to show the original surface. The thin gray band is the primer coat.
This sample is typical of a composite wing, but it is not identical to any existing wing designs. In particular, the forward spar caps are much thicker than they need to be for a wing this size. Also, this wing section lacks any control surfaces such as ailerons.
Related Links
For more on the use of composites in aircraft, see John Wert's Materials for Future Aircraft and Spacecraft.
The wing section described in this article is similar to the wing of Aurora's Theseus aircraft. A set of Theseus pictures can be found in the Dryden Photo Gallery. A more complete description of Theseus is on NASA's Mission to Planet Earth Web site. That page has links to some other UAV pages, including the Perseus UAV, Aurora's predecessor to Theseus.
Theseus is manufactured by Aurora Flight Sciences of West Virginia (AWV). AWV's parent company, Aurora Flight Sciences, specializes in the design and manufacture of unmanned aircraft (UAVs).
Aurora's aircraft designs are similar to those of Scaled Composites and Burt Rutan. Air & Space Magazine interviewed Mr. Rutan earlier this year.
Finally, Chris Fouquet has a page which shows how to make vacuum bagged wings.