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Trailing edge construction -- riveting plastics and aluminum

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addaon

Well-Known Member
Supporting Member
Joined
Feb 24, 2008
Messages
3,976
Location
Kanab, UT
Folks, I'm trying to nail down the trailing edge construction for the elevons of my design. A folded trailing edge doesn't work for me, so it's going to be built up in some form. One option I was considering was RV-style, with a single rivet fastening the two skins and a wedge-shaped piece of material in between.

Unfortunately, my trailing edge angle is significantly greater than in the RV designs, so I can't use their AEX wedge even if I could get it in the necessary lengths. Also, Because of this higher angle, the weight of the trailing edge wedge becomes significant -- over 10% of the control surface weight, plus the impact on mass balance weights.

Since I'm not going to get a custom extrusion done, I'll be machining the wedge (if I take this approach) anyway. Fortunately, wedges tend to be easy! I was considering, though, the feasibility of using a Delrin (or similar) wedge rather than aluminum in this context. Reasons I like this idea:

1) Depending on grade, Delrin is at least 40% lighter than aluminum. This is a 4% decrease in control surface weight, plus impact on mass balance.
2) Delrin is great to work with; certainly no worse than aluminum, so no tradeoffs there.
3) Delrin is commonly riveted, no worries about local stress fractures from rivet expansion if using a common process.

One thing I'm uncertain about is effect of riveting materials with different coefficients of thermal expansion. I'm sure that in the extreme case there are issues; but what's extreme? Aluminum has a CLTE of around 13 microinch/°F; most stock Delrins are around 60 microinch/°F. But some of the glass-filled stuff (525GR is 25% glass filled and readily available) is down around 20 microinch/°F, which to my mind seems close enough to ask about...

Anything else that would pop up and bite me?
 
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