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Do you have an engineering drawing of Elevator Forward Spar?

D

drewhottub

Active Member
Team,

In order to be completely convinced that my RV-10 Elevator will be structurally sound (after the modification I made to correct a blunder), I need to be able to give my engineering group of friends, an actual dimensioned drawing of the E-1002, Elevator Main Spar.

Would anyone have one that they'd be willing to electronically send me?

By the way, this latest question, relates to a previous post of mine, shown here --> http://www.vansairforce.com/community/showthread.php?t=69985


Best Regards,

Andrew
 
I don't believe Van's releases this information.

What did Van's say about the extra hole?

-Rob
 
VAN's says that they don't like the extra hole!

I don't think they've done any formal analysis though, because they seem to be busy with other things.

Quite frankly, I believe that this a non issue.

Yes, I acknowledge that it wasn't a part of the original design, but I believe that drilling out all those rivets, to replace the main forward spar will weaken the elevator more than just leaving my extra "lightening hole", that is smaller than the larger lightening holes that the factory integrated into the main spar.


Also, I have an Aero Engineer friend who believes this is completely non issue, as well.

We (he and I are going to do some real formal analysis though), and in fact I even have some Aero Engineer buddies from the University of Wisconsin at Madison that are going to take a look at my problem too.

So, before I go with VAN's official, "don't hold me liable" answer, I'm going to have some formal analysis done, since I'm lucky enough to have people willing to do it for free.

This analysis may end up proving VAN's correct, but I'd like to do it with facts and data, rather than "feeling".

I have expended so much energy making these elevators "almost perfect", to this point, that I'm really reluctant to undo all of my work, unless the data proves that my friends and family, and I will somehow be at increased risk, as we're flying over the next 40 years!

I'll know more soon, and will reply with the results!

Regards,

Andrew
 
Not an engineer, so I'll ask a naive question to the engineering crowd. For Andrew's situation, instead of replacing the spar, could a doubler simply be riveted over the extra hole to replace the lost strength?
 
Todd,

Thanks for asking that question on my behalf!

This would also be an alternative that I could live with, if the VAF community agreed.

Good thought.

I do indeed wish to not take undue risks with my family on board, over the life of the plane. I just hope this does not require a full front spar replacement on both elevators.

I look forward to hearing the answers, that I'm sure will be forthcoming, here in the near future.

By the way here's the link to my problem --> https://picasaweb.google.com/drewho...?authkey=Gv1sRgCOiIhYfqpOaiVw&feat=directlink

-Andrew
 
Add a doubler

Well I had a long winded reply, but hit the wrong key and lost it. Here's a shortened version.

1. An engineering analysis of the elevator and spar is going to involve a lot of assumptions of loads and stress levels, even if you have an accurate drawing of the spar. These assumptions can only be verified by the manufacturer and I don't think they will do that.
2. What you want to do is make sure you have eliminated the stress risers as much as possible and restore the spar to equivalent strength.
3. Make sure you thoroughly debur the existing 5/8" holes (both elevators), in the pictures the holes look ragged with sharp edges on both front and rear surfaces. Make sure you don't introduce chatter marks with the deburring blade and polish the holes so all tool marks are eliminated. I have used Cratex polishing wheels with good success.
4. Devise a repair using AC43.13 Section 4 Metal Repair Procedures. Add a doubler on the front (using packers or spacers around the elevator horn attach bracket) or rear side of the spar whichever is easier. Consider Cherry Max blind rivets if unable to access both sides.
5. Be especially careful balancing this elevator.

Hope this helps.
 
It's fine IMO

Here's my engineering take on it just using reasoning (no calculations):

Bending stresses:
  • The horizontal stabilizer and elevator spars are basically straight beams.
  • Straight beams in bending have the same stress on the top and bottom flanges, although one flange is in compression and one in tension.
  • Normal loading for the horizontal stabilizer and elevator is downward, which means the bottom flange will be in compression and the top in tension.
  • Van's original design puts the large hole near the flange experiencing compression, which means they're obviously not worried about a compressive buckling failure due to the hole.
  • The "extra" hole at the top is near the flange in tension, so, again, it's even less critical than the hole near the bottom flange.
  • Thus, from a bending stress standpoint, there is no issue.

Shear stresses:
  • Shear stress in the web of a beam in bending run parallel to the length of the beam.
  • Shear stress is maximum along the centerline of the beam.
  • Holes in the web of a beam cause shear stress to travel around the hole resulting in increased stress on the remaining material on either side.
  • Since the "extra" hole causes there to be less material on either side of the "correct" hole, the shear stress in the adjacent material will be somewhat higher.
  • Because shear stress in the web of a beam is (never?) the critical factor for beams, it is common to cut large lightening holes in the web of a beam.
  • I have never seen a beam design fail due to shear stress being too high in the web. All beam failures I'm aware of are due to either buckling of the compressive flange or torsional deflection allowing a beam to twist towards it's "weaker" cross sectional moment of inertia.
  • Thus, from a shear stress standpoint, I believe the web is fine as-is.

Torsion stresses:
  • The torsional strength of the spar is essentially 0, so we must consider the assembly.
  • The elevator spar and skins form a triangular tube-like shape.
  • Tube-like shapes are good at reacting torsional stresses.
  • The closer a tube-like shape is to being round, the stronger it will be for a given weight. (i.e. a round tube is the ideal shape for reacting torsional stress).
  • Any cutout through the wall of a tube will reduce it's torsional strength.
  • The fact that there is a large cutout for a cover plate indicates to me that Van's doesn't believe torsional strength is the critical factor for this assembly. (note: installing the cover plate will bring the strength back up somewhat, but not to the strength of the original skin).
  • Thus, I don't believe the "extra" hole will significantly reduce the torsional strength of the assembly.

I won't tell you what you should do, but if it were my airplane, I'd just do a good job of deburring and polishing the edges of the holes to avoid stress risers, and move on.

Since the distance between these two holes is less than the recommended distance in AC43-13.1b (3D), I'd probably inspect for cracks in this area on each annual condition inspection. However, even if a crack formed between the two holes, I probably would not do anything other than watch it since the crack would be effectively stop drilled by the two holes.

My $.02
 
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