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Elevator Counterbalance skin issue

Antknee

I'm New Here
I am on step 9-6 and noticed Vans has us put the elevator skin on top of the counterbalance (CB) skin. Why would we have the elevator skin sticking up into the oncoming air stream? I called Vans tech support and they could not figure out why it is like this and confirmed their demonstrator is built this way. They could not state if there is an issue reversing this to be more aerodynamic either.

Has anyone reversed this? I don't see why it would be an issue but also am aware of not knowing if it would cause an issue down the road. I am suggesting putting the CB skin over the elevator skin to make the skins overlap in a more streamlined fashion, just like the fuselage is on 32-4 Figure 3.

Any advice on this would be appreciated.
 

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It's pretty minimal in terms of potential drag when you consider how many high-performance certified aircraft (Bonanza for example) have mostly universal rivets on wings and fuselage.

I've found that Van's designs parts with recommended orientation/positioning in mind - skin holes / rivets probably won't line up right with "reverse" alignment. I think it would probably also be harder to build the elevator if the skin needs to be tucked under while riveting the ribs and skins.
 
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This is a 14. Not mine, but pretty sure almost everyone on this board has seen this plane:)
 

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I’m at this step as well. Have we all decided that it is better to have the E-913 to extend over the skin as opposed to the way that the instructions say to do it?
 
I still don't understand what the issue is with the standard way. I've built my elevators and don't see any issue with it as is.
 
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It's been a while since I did this part, but I reviewed the plans to be sure my memory was correct. The E-903 ribs are shaped so that the elevator skin over the counterbalance skin lies flat. Reversing this joint will result in a slight bow in the elevator skins at the tip. Your choice as to which bothers you more, but the drag, especially on a movable surface that will often be offset to the relative wind, should be minimal.

As for why, I can only surmise but it seems to be related to the sequence of build. Since you build the counterbalance off the rudder, by the time you attach it, there is very little play to slide the elevator skins underneath. It is easier to insert the counterbalance, skins and all, inside the elevator skins. In the end, I built per the plans and am very happy with the fit.
 
On my -8, I'm pretty sure the tip rib had a joggle to allow the counterbalance skin to nest into, then the elevator skin over the top and fits nice. It looks cleaner the factory way in my opinion.
 
I did a check of a few 14s at Airventure 2 or 3 years ago and some had them over and some had them under. I think the 14 and 14A were different too. One of them was the East coast demo ride.
 
I reviewed all of the 10s at Oshkosh one year to see how the elevator/stabilzer tips were finished. It was about 50/50 blended with filler vs. joint left exposed.

I blended the tip faring with a little filler and shazam....overlap gone!
 
I read somewhere that forward-facing lap joints on a fuselage were less draggy. Counterintuitive and more likely to leak rainwater, but less drag. Anyone else remember seeing this?
 
I read somewhere that forward-facing lap joints on a fuselage were less draggy. Counterintuitive and more likely to leak rainwater, but less drag. Anyone else remember seeing this?

I recall this as well. Maybe we can attract Steve Smith to comment on this?
 
Fastener Analysis 101...

I am on step 9-6 and noticed Vans has us put the elevator skin on top of the counterbalance (CB) skin. Why would we have the elevator skin sticking up into the oncoming air stream? I called Vans tech support and they could not figure out why it is like this and confirmed their demonstrator is built this way. They could not state if there is an issue reversing this to be more aerodynamic either.

Has anyone reversed this? I don't see why it would be an issue but also am aware of not knowing if it would cause an issue down the road. I am suggesting putting the CB skin over the elevator skin to make the skins overlap in a more streamlined fashion, just like the fuselage is on 32-4 Figure 3.

Any advice on this would be appreciated.

It might be due to a strength requirement. Or it could be the designer just decided to make it that way. If it was due to a strength requirement, hopefully the discussion below will make sense on why it would be stronger.

The way the plans have the layout, the aluminum strip from the counter balance weight would be in double shear. When a joint is in double shear (i.e., a sheet on top and bottom), you essentially get double the shear strength of the rivet since both sides of the rivet (top and bottom) provide load transfer. There is a knockdown factor associated with relatively large rivet diameter to thickness ratios, such as in this case. I haven't checked the actual thicknesses of these parts but assuming they are 0.032" thick, the knockdown factor for a 3/32" rivet is 0.814 per side (see Table 8.1.2.1(b) of MIL-HDBK-5J). Since the single shear allowable for a -3 AD rivet is 217 lbf (Table 8.1.2(b)), the total shear strength of the rivet would be 353 lbf (2*0.814*217) or 62% higher than a single shear rivet (see Table 8.1.2.2(c) for the Ultimate shear strength of a -3 AD rivet = 217 lbf).

You also have to check the bearing strength of the sheet. Alclad 2024-T3 sheet has an A-Basis allowable of Fbru = 121 ksi (e/D = 2, Table 3.2.3.0(e1)). A-Basis is a statistical knockdown that is normally used for primary structure (this would be considered primary structure since it's flight critical. See Appendix A, section A.3 for more info). Since the hole size is 0.098", and the sheet is 0.032" thick, the bearing strength of the sheet is 379 lbf (0.098*.032*121,000). That means that the rivet will shear (353 lbf) before the sheet will "bear out" (379 lbf). You normally prefer to have a "fastener joint" fail via bearing vs. failing by shear. If a rivet let's go in shear, all of the load that the rivet was carrying now has to be "picked up" by the adjoining rivets. If a rivet "bears out" (i.e., it "yields"), it will continue to transfer the load it was seeing when it yields. However, any additional load in the joint will be transferred to the adjoining rivets. This results in a joint that has some "give" to it. It shouldn't completely fail until all of the fasteners in the joint reach their ultimate strength. With a "shear first" type joint typically the joint fails when the first fastener fails. However, I would expect that the overall strength of the joint is much higher than what it actually sees. Even though the failure mode is not ideal, the joint is probably much stronger than what it needs to be so it really doesn't matter what the failure mode is.

If you place the sheet "on top", then the fastener is in "single shear" (i.e., a sheet on only one side). That would make the rivet shear at 217 lbf. So by placing the sheet in double shear (like the plans suggest), the strength of a particular rivet is 62% higher.

I hope all of this makes sense. I know it's a lot to swallow so to speak. There's more to this to figure out the actual load on a specific fastener, but this is what a structural engineer looks at to find the strength of a single fastener in a "mechanically fastened joint".

Oh, and BTW, it doesn't matter if the joint has primer in it or not... :)

Jeff
 
Dimpling vs. Countersinking

Dimpling usually increases the effective shear strength of the rivet due to the interlocking sheets. Take a look at Table 8.1.2.1(b). You can see that for a single shear 3/32" rivet in a "flat" sheet (think a protruding head rivet), you need to be 0.032" thick to get 100% the shear strength of the rivet. However, Table 8.1.2.2(c) shows that you only need to be 0.025" thick to get 100% the shear strength of the rivet in a dimpled sheet.

Countersinking will almost always give you less effective shear strength. Look at Table 8.1.2.2(f). You can see that for the same -3 AD rivet, you need to be 0.063" thick to get the same (well, almost the same...) shear strength.

What's the rivet shear strength of a dimpled sheet going into a machine countersunk sheet? Well, you'll probably have to run some tests for that. I haven't seen too much data for that.

Also, solid rivets are typically used in "shear" type applications (i.e., the primary load is in shear). Solid rivets can take tension loads, just not a whole lot of tension load. There are some company specific tests that have these values. There are also "interaction" equations so you can see how the combination of tension and shear affect the strength. For more substantial tension loads, that's where bolts and Hi-Loks come into play. They can take both tension and shear load fairly well.

You even have to worry about shimming too. If the shims get too thick, then bending comes into play. Again, there are some equations you can run to get the reduced effective shear strength of the fastener. You can play around with how you fasten the shim and make it a "structural shim" (i.e., it picks up load). That will get you a bit more strength in the joint. The list goes on and on for mechanically fastened joints. And if you really want a "fun" task, look into bonded joints. Especially, composite bonded joints (not that composite bolted joints are a breeze...).

There are whole books that go over this subject. It's a bit tough to go into all of the details here. Needless to say, it's not as simple as it looks...

I hope this helps.

Jeff
 
“ If you place the sheet "on top", then the fastener is in "single shear" (i.e., a sheet on only one side). That would make the rivet shear at 217 lbf. So by placing the sheet in double shear (like the plans suggest), the strength of a particular rivet is 62% higher.”

That is as it relates to the skin of the elevator correct? If the counterbalance skin were in the middle of the sandwich, it would have the extra strength, right? I’ll admit I’m having a hard time keeping up with your math. It seems like the rivet would have the same strength regardless. It doesn’t know which sheet is on top, only the holding power of the skin that is in the middle would be increased. If so, the question is which skin you want to have more holding power.....
 
single shear vs. double shear

It's a little tough to get these points across on a forum such as this. As long as there is enough sheet thickness for a rivet, the rivet will shear at its maximum amount. If the sheet is too thin, then the thin sheet has a tendency to slice thru the rivet like a knife. That's where Table 8.1.2.1(b) comes into place. If the sheet is too thin, then it wants to cut thru the rivet, reducing it's effective shear strength.

The thing about double shear is you have two shear planes instead of just one for single shear. This pic below does a better job of explaining this than what I can do...

9-10.gif


What it looks like in the plans is that you're taking the load from the counterbalance weight and trying to get it into the elevator via the outboard rib and the elevator skin. With the counterbalance skin in between the elevator skin and the rib, half of the counterbalance load goes into the elevator skin and the other half goes into the outboard rib. With the counterbalance skin in double shear, you get up to 62% more load thru the rivets than if you put the counterbalance skin in single shear (assuming all the skins are 0.032" thick).

I hope this makes sense. I know it's a rather complicated subject. Don't feel too bad if this is as "clear as mud". I've had to explain this to many a young engineer in my career. It's a bit tricky to grasp.

I hope this helps.

Jeff
 
Thanks for all the feedback. I do appreciate it. I have built RC planes for over 30 years and this has been a goal since I was a kid. I seem to fly through parts and then hit something I don't understand or isn't clear and get stuck for a bit, then work through it and am clecoing and drilling again.

After as much images of RV elevator counter balances I could handle viewing and talking to Vans Tech Support, I decided to do the counterbalance skins over the elevator skin. I realize it is a trivial 1mm gap. I was going to college for aerospace engineering and life led me to computer engineering. From an aerodynamics perspective, anything sticking out into the air stream takes HP to make it go through it. The less of this the better. E.g. It takes ~60 HP to make the side mirrors alone go through the air at 200MPH on most cars which can do this. If you omit, them, that's 60 more HP you have for speed. Most racing events that allow street cars require the mirrors, but you get the point. I'd like to see the math behind a raised surface being more aerodynamic.

There isn't a notch in the rib for it, I did cleco them both ways and they sit flush either way.

I know, it's a ~1mm skin, but it's the principle to me. I'm looking at making the exterior as clean as possible. This is what I love about the EAA community.

Vans tech support said it could be either way and provided some pictures of their demonstrator elevator. Their's is under the elevator skin. For all we know, it could be for aesthetics, the lines line up better since that's where the LE roll is and the elevator skins go all the way to the edge as a more rectangular surface.

PS... As for the rivet thread you've got here, I get what you're saying but shouldn't we be multiplying by the number of rivets between the two pieces that are bonding them to determine the lbf in which that piece would fail. Shear strength is determined by everything holding the two surfaces together, including friction. It's my understanding/belief that the number of rivets in the Vans aircrafts are key to their strength.
 

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I asked the same question on this fulcrum when I was at this stage since to me too it was counter intuitive.....I ended up doing it as per plans. I didn’t want to find out later “oh that’s why they did it that way”. That being said I can’t imaging reversing it would be much of an issue.
 
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