Trailing edge construction -- riveting plastics and aluminum

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addaon

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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?
 

DeepStall

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Dec 26, 2010
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Colorado
Given the modulus mismatch (~70GPa for Aluminum, ~3GPa for Delrin), I wouldn't worry much about the CTE mismatch. I'd expect the delrin is just going to go along for the ride and not have a chance to develop anywhere close to yield stress.

Will creep / cold-flow be a problem for the delrin under the rivet load? I'd expect the glass-filled to help with that.
 

Himat

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May 5, 2011
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Combining aluminium and POM?
(Delrin is the trade name for what I know as POM, or Polyoxymethylene.)

The main issues have been brought forward, that is difference in coefficient of expansion by temperature and creep/ cold flow. Run the numbers for the length of your wing and a 70 degree temperature range. I suspect a “wavy” trailing edge at elevated temperatures and a bent one at low temperatures. If the insert is made in shorter sections this would be less troublesome.

Next, POM is a thermoplastic, there is a risk that at creep/ cold flow, especially if it do get hot. Fasteners then might lose their tension.

Last, POM is degraded by water containing chlorine. Now, the tap water contains enough chlorine to may make this a problem if you wash your plane frequently. (Someone that made tube fittings from POM did run into this. The fittings cracked resulted in water leaks.)
 
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