ETLB Squawk Forums — Archive

Looking at DWG #17 of the EROS plans (wing spar construction) I notice the upper struct attachment reinforcement is RS9 but the lower reinforcement is RS8 for both front and rear spars. I assume this is a design decision based on loads (Gs?, landing?) limitations/expectations. (e.g. 1100 plans have only upper reinforcements on rear spar) One ramification of this is that the RS9 diagonals require notching at the end for gussets to fit appropriately. My question is, hypothetically, other than very small weight difference and need to shorten verticals in that section, as well as make the RS23 filler in rear 1 1/8 width instead of 1 1/4), what would be ramification of just using RS9 for both upper and lower reinforcements? Gussets would lay flat without notching. Am I missing something else that would be impacted?

I realize notching the diagonals is not really a big deal. Just curious if there are other reasons beside my assumptions about loads on upper vs lower attachment points that design decision was to use different stock for upper vs. lower reinforcements.

Again, not a biggie. Just trying to learn more.  Thanks!

Tony

If you're talking about spar cap doublers, they increase the local bending strength.  The lower cap needs less because the negative g load requirement is less.  By going to RS9 you would be increasing the negative g loading allowed.  The doubler has nothing to do with the strength of the strut attachment.  Its proximity to the strut attachment is only because wing bending stress is at a maximum there.

To explain the rear spar doubler at the rear spar of the 1100 mentioned in the above post Bob once said it was to strengthen the area in a steep dive at high speed with full aileron deflection.I hope I remember correctly.

Quoted from Bob Daly If you're talking about spar cap doublers, they increase the local bending strength.  The lower cap needs less because the negative g load requirement is less.  By going to RS9 you would be increasing the negative g loading allowed.  The doubler has nothing to do with the strength of the strut attachment.  Its proximity to the strut attachment is only because wing bending stress is at a maximum there.

Thank you Bob. I was hoping you would reply. That makes good sense. My first guess was g load, second guess was strut attachment... But, in both cases I was guessing. Thanks again.

Quoted from mullacharjak

To explain the rear spar doubler at the rear spar of the 1100 mentioned in the above post Bob once said it was to strengthen the area in a steep dive at high speed with full aileron deflection.I hope I remember correctly.

The design case for the rear spar is VD and 2/3rds of the positive load factor or 2.93 g with 1/3 aileron deflection.  Aileron deflection increases the lift and moment coefficients of the rising wing so along with the high dynamic pressure at VD wing torsion is highest and bending of the rear spar is highest.  Obviously this is also the design case for the rear strut.