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what if you only used pop rivets on a -7?

K

kritsher

Well Known Member
OK, this is a crazy "what if..." but seeing as how the -12 uses only pop rivets, what would be the implications of assembling a -7 kit the same way?

There would be more drag. The plane would be slower. It would be uglier.

Structurally, how significant is the difference? would it be equivalent? or almost equivalent? would it be safe? If a -7 is strong enough for aerobatics with solid rivets, would it be strong enough for more "normal" use if assembled in this manner?

What else would change?

I understand that this is a question that only the engineers at Van's can answer authoritatively, but I'm curious to know what the collective wisdom on the topic is here.
 
Not sure it's even possible. Most of the rivets on a -7 are 426-3, while I think the pulled stuff is typically -4. Bigger holes, but fewer of em. One thing is for sure, you'll spend way more money on rivets if you use the pulled type.
 
cost, weight, and pride

There are many substitute rivets that could be used. Some don't weight too much more Like Cherry 332 and they are a direct replacement for the solid -3 rivets. They do have a hole in the head that will be visible unless filled with something. Cherry also make structural rivets in -4 size that have pins that break off flush with the heads. These can cost $0.50 each for some sizes and you have to use a guage to make sure you are putting the correct length in. They are also heavier. My personal policy was to use the solid rivets wherever practical and then use the Cherry rivets where bucking was going to be a real pain. I figured that a properly set pop rivet was better than a badly set solid rivet anyways. On to pride, you will take a lot of ribbing from the guys who persevered and used the solid rivets everywhere.
 
OK but ...

Structurally, the pop rivets aren't as strong as solid of course, and I haven't done any analysis on the structure (I'm an AE), but my guess is that using pulled rivets would not be a problem structurally - at least in the short term (for the skins only - internal rivets should be per plans - always). The 7 structure is pretty conservatively designed.
The most significant issue I see is that over time, you'd find out which areas of the structure are the most highly stressed because those rivets would tend to flex more and loosen. If the stresses are low enough, it may take a very long time for this to happen - that's the unknown.
And you're right - it would be ugly :eek:
 
I thought that -4 cherry max were a suitable replacement in all respects to a -3 driven rivet.

Am I missing something?
 
Jconard said:
I thought that -4 cherry max were a suitable replacement in all respects to a -3 driven rivet.

Am I missing something?
Click to expand...


Yes, in some cases the structure your riveting isn't large enough to give you the edge distance you need for the larger 1/8" hole the -4 rivet requires. For instance, all of our ribs have an ~5/8" flange. That gives you just a touch of leeway with a -3 rivet, but the edge distance is marginal at best when using a -4 rivet. That doesn't mean that the occasional -4 rivet will cause the plane to self destruct, but you are eating into your engineering margin.
 
Reasonable compromise

I recently purchased a 450 hour 1987 -4 that has pull rivets on the bottom of the wings. Did not affecty my buying decision materially, but if these were everywhere I probably would have kept looking. Cosmetics do matter.
 
Not that much difference

I doubt that pop rivets would save that much time. It takes me as much time to set a pop rivet as it does to set a solid rivet. Maybe if I had to do the whole plane with pop rivets, my pop rivet tools would be better. However, I really don't spend that much time riveting, anyway.

Where pop rivets are really useful are for those places that are hard to do on you own. That could save some time, but not all that much. Also, they might help in those instances where you quit early because you are a good neighbor and you do not want to pound rivets late at night. But if you are just a weekend warrior, then that's rarely a concern, either.

I haven't seen that many planes with pop rivets, but those that I have seen were not flush rivets. So, in that case, what you are saving is not the time to set pop rivets, but rather the time spent dimpling.

Of course, I am building a airplane with mostly solid rivets, so what do I know?

Cheers,
Tracy.
 
SMO said:
I recently purchased a 450 hour 1987 -4 that has pull rivets on the bottom of the wings. Did not affecty my buying decision materially, but if these were everywhere I probably would have kept looking. Cosmetics do matter.
Click to expand...

IIRC, using blind rivets on the bottom of the wing was specifically allowed in the plans on the older designs. I would imagine they took it out of the newer plans because they couldn't get any of us fanatic RV guys to actually use the blind rivets :rolleyes:

Rusty
 
Pop Some Buck Some

I understand that Supermarine tested the Spitfire by glueing split peas on the flush rivets to find out which ones would effect speed. They wanted to make the plane easier to build by using 470's in any area where 426's were not beneficial...
So where could you use pop rivets on the airframe exterior that would not slow down the airplane?:confused:
As for the title, get you mind out of the ditch so mine can float by....
 
DGlaeser said:
Structurally, the pop rivets aren't as strong as solid of course ... QUOTE]

Most folks assume a pulled rivet isn't as strong as a bucked one, but they're wrong. At least in the case of the all stainless Cherry N rivet as used on the Sonex.

1/8 Cherry N 450 lbs shear
3/32 bucked 179 lbs shear
1/8 bucked 319 lbs shear

I beat my Sonex pretty hard for 450 hours and almost six years and never saw one smoking/loose rivet.

And with a $30 pneumatic puller from Harbor Freight I could leave the volume on the radio turned down and still hear Neal Boortz just fine.

Tony
Click to expand...
 
SHIPCHIEF said:
I understand that Supermarine tested the Spitfire by glueing split peas on the flush rivets to find out which ones would effect speed. They wanted to make the plane easier to build by using 470's in any area where 426's were not beneficial...
So where could you use pop rivets on the airframe exterior that would not slow down the airplane?:confused:
As for the title, get you mind out of the ditch so mine can float by....
Click to expand...
Driven vs Pop has nothing to do with Round head vs Flush.
Both AD and pulled rivets have both.
 
Tony Spicer said:
DGlaeser said:
Structurally, the pop rivets aren't as strong as solid of course ... QUOTE]

Most folks assume a pulled rivet isn't as strong as a bucked one, but they're wrong. At least in the case of the all stainless Cherry N rivet as used on the Sonex.
1/8 Cherry N 450 lbs shear
3/32 bucked 179 lbs shear
1/8 bucked 319 lbs shear
I beat my Sonex pretty hard for 450 hours and almost six years and never saw one smoking/loose rivet.
And with a $30 pneumatic puller from Harbor Freight I could leave the volume on the radio turned down and still hear Neal Boortz just fine.

Tony
Click to expand...
You're right about SS Cherry rivets, but I think he was asking about aluminum pop rivets like those to be used on the RV12. I should've been more specific. Steel rivets would also add a few pounds to a 7!
Click to expand...
 
Pull Rivets and Dimpled Skin

Someone on another thread was saying that, flush pulled rivets should not be used where both parts to be riveted together were dimpled. His theory was that these pull rivets require a square surface, where the shop head is formed, to form a good bond and that the structural integrity of the riveted joint would be weakened. Is that true?? If so, that sure limits the use of flush pull rivets!!

Tom
 
Not sure where that came from...

todehnal said:
Someone on another thread was saying that, flush pulled rivets should not be used where both parts to be riveted together were dimpled. His theory was that these pull rivets require a square surface, where the shop head is formed, to form a good bond and that the structural integrity of the riveted joint would be weakened. Is that true?? If so, that sure limits the use of flush pull rivets!!

Tom
Click to expand...
Flush pulled rivets work just fine on dimpled surfaces.
 
In one of the RVator issues, Van talks about the decision of using "pulled" rivets for the RV-12. The cruise speed of the -12 was low enough that using round head (I think he said round head) pulled rivets did not detract from the performance of this aircraft.

The airspeed of the "other" RV's was high enough that round head rivets did make a noticeable difference, so flush rivets are used.

I believe that the manufacturer of the "cherry" brand of pulled rivets is TEXTRON. I remember downloading the data sheets (books really) from their website some years ago. It contained all of the data, for all of the sizes and styles, of their rivets. And yes, the Cherry Max brand did exceed the shear strength of "normal" bucked AN rivets.

So, from a strength issue, pulled rivets can take the place of a "normal" rivet. The choice of flush head vs round head certainly will affect the speed of the RV's. In addition, there will be a measureable weight increase with the pulled rivet solution. This list has at least one poster that has tallied up nearly 14,000 rivets in his A/C. It would be an interesting exercise to tally up the weight of those 14K rivets and compare that with a therotical 14K of pulled rivets that will include the small portion of the steel mandral that gets left in the pulled rivet.

The Spitfire testing with split peas to simulate the effects of round head rivets instead of the labor intensive use of flush rivets resulted in the use of both flush and round head rivets...flush on the wing leading edges back to some non-critical flow seperation point, and then the use of round heads from there on back.

Hope this helps...
 
The pros: Less critical riveting--pop rivets pretty much all come out the same.

The cons: Resale value, questionable safety & liability (you built it, sell it a few years later, something comes off the plane in flight, you get sued even if it didn't fail because of a pop rivet; after all, you didn't follow the plans), ugliness, weight, cost. Drilling solid rivets is easier--too often once you get so far, the drill bit just spins pop rivet in the hole, which says to me it didn't fill the hole like a driven rivet does.

The uncertainties: Time saved? Safety--after all, this is an aerobatic airplane.

Don't be afraid to use solid rivets. After less than a hundred rivets or so, you'll have it figured out. And if you attend a workshop, it won't take a hundred rivets. You'll be happier with the final job and won't regret having used solid rivets.
 
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