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?
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?