Wingtip Attachment
Some builders have been attaching wing tips with hinges similar to the process with the cowling. This is a great way to save weight, improve appearance and increase strength. I've heard some discussion expressing worry about the strength of this applications compared to nutplates and screws.
The aircraft, L IVP, weighs about 3500# maximum and it's effective weight is doubled to 7000# at two G's. The wingtips or winglets need to support less than 10% of the total effective weight. Ten percent of 7000# is 700#. Taking half of this number since this load is shared by two wingtips, you get 350# max load per wingtip or winglet. The Carbinge material on the bottom of the wingtip will be in shear load during positive G's while the aircraft is airborne. Carbinge is rated at 3000# per lineal foot in shear and it takes about 20" of Carbinge on top and 20" on the bottom for the wingtip attachments. 20" of Carbinge is rated for 5000# of shear stress. It would take over 14.25 G's to reach the rated limit of the Carbinge attachment on the wingtips or winglets. It would take twice that or 28.5 G's to reach the limit load of the Carbinge. By contrast the old method spreads these shear stresses to multiple points along the attachment line concentrating and increasing the loads at smaller areas of the wingtip. This produces a weaker attachment.
Comparing this approach to the use of nutplates and screws you find that the common method of attachment spreads the loads to 8 or 10 points along the top and bottom of the wing. Carbinge spreads the load equally and continuously along the entire length of the attachment because it is bonded to place. The old method requires a row of screws in the skin while the Carbinge method combined with Carbinge Keepers is virtually invisible. The old method causes small waves in the surface of the wingtip or winglet, because it holds the surface down at multiple points. Carbinge leaves no irregularities. Maintenance of the old system requires unscrewing and rescrewing multiple screws into locknuts, taking a good bit of time and leading to stripped out screw heads. Removing and replacing the hinge pins is straightforward, rapid and repeatable countless times without breakdown of the parts.
Want to see how it's done? Art Bertolina has been kind enough to provide photos of his wingtip installation using Carbinge attachments. See them below.
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Carbine trimmed, fitted and clamped to place, then drilled for clecoes. |
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Carbinge clecoed and bonded to wing. |
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Second half of Carbinge is assembled then wingtip is positioned, marked and drilled for clecoes. |
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Wingtip assembled with pins partway in and Carbinge Keepers fitted. |