Flush vs Protruding Rivets

[PlaneDan]

I think I would go with protruding. Not because of the rivets or the dimpling, but because of the countersinking. I have tried everything I can think of and am not convinced that I can get repeatable counter sink with each execution. I have tried using ice water to keep the micro stop at the same temperature, and that works, but it is messy. The last attempt on the upper longerons of the aft fuselage, I decided to try using the air blower gun and that gave the best results so far. It was cool in the shop and the air kept the debris out of the way while keeping not only the micro stop cool, but helped keep the part cool.

I could still go either way because I am sure there will be proud stems with the protruded.

I was wondering on Tony's post much earlier in this thread, about pushing the stem back in on a fuel tank. How can you do that without creating a leak?

[len@i5t.co.za]

Hi

When you guys who have built and are building the Panther will need to do it again...Who will do it with flush rivets and who will do it with Protruding Rivets?

What is the general consensus now?

Regards
Len

[GlenNJ]

Hi Paul Salter,
I am curious about your statement concerning solid rivets,

"I mentioned the above, because some people think that solid rivets are always better than pulled rivets. And in some cases that is true, but not necessarily in all cases. The Panther was designed around the strength of the rivet chosen, do not use a replacement fastener that has less strength than the ones chosen. Do some research or ask us if in doubt."

Doesn't the Panther brochure state that you can use pulled or solid rivets. If so, is there a list of recommended solid rivets that can be used in place of the pulled rivets? I plan to use pulled rivets on most of the skins, except for the trailing edges of the wings, tail sections and the leading edge attachments of the hinges. I don't like the looks, or sharp, rough finish of the exposed shop heads of the pulled rivets. I'd like to set the solid rivets with a rivet squeezer, to lesson the chances of dents on the skin. I'm aware that pulled rivets and solid rivets have different head angles.
Glen

[Waiex116YX]

I built a Waiex with protruding heads. If I build the Panther or Cougar, it will be with flush pulled rivets. I don't think I want to go to driven rivets but the thought is intriguing. Perhaps doing driven rivets on the top and pulled on the bottom.

[sactu1]

Hi Glen.

Regarding solid rivets in the tail group. A builder here just finished a Onex with solid rivets - he managed solids in the horizontal and vertical stabilizers as well. I asked him how he did it, and you have hit the nail on the head below (mixed metaphor, but it's New Year's eve and I'm tired) . He had to keep the rear spar off until he'd gotten the bucking bar inside to buck the rivets. Top skin wasn't a problem, but the bottom skin required that he worked from the leading edge back, and for the last couple of rows, getting in through the rear spar opening - if that makes sense. It's relatively easy going until about half way to the trailing edge. as you simply peel the skin back and get your arm in to buck the rivets. But, as I said, the last few rows are done through the rear spar opening.

From the photos I've seen of the Panther, it appears to be the same construction technique as the Sonex family in the tail group area (please correct me if I'm wrong Dan or Rachel), which means solid rives can be applied to the tail group. Control surfaces look like they're accessible for solid rivets too (on the flat surfaces, not around the hinges), with the stiffeners the way they are - but I've only had a cursory glance at the way they're put together. For what it's worth, I plan to use flush pulled rivets for my aeroplane (will be ordering a kit mid-late 2014), having previously built a Sonex with protruding head pulled rivets.

Hope this helps.

Cheers,
Jim

[Rpellicciotti]

Just thought I would chime in here with some first hand experience. I helped a company in Eastern Europe certify and import a S-LSA airplane. The airplane was built entirely of protruding head pulled rivets. This was done to keep the price down through reduced labor costs. People in the LSA market didn't like the looks of the protruding rivets and didn't believe us when we said they had minimal effect on performance at the speeds these airplanes fly. So, the factory built two airplanes with flush pulled rivets. These airplanes did look better to the eye but there was no measurable difference in performance. That said, these were 100 knot airplanes. The faster you go, the more noticeable the drag would be.

I like Dan's approach of using flush rivets where they are visible and protruding in other places. When I build mine, I am sure it will have pulled flush rivets just for looks.

Rick Pellicciotti

[psalter]

One thing that I will point out about solid rivets, in new commercial and military aircraft, solid rivets are becoming almost as archaic as exploding rivets. For those that don't know, exploding rivets were a soft solid rivet with a small amount of gunpowder in the end. You would stick the rivet in the hole, heat it with a soldering iron and the charge would explode, forming the tail of the rivet. Pulled rivets replaced exploding rivets in the late 1940s early 1950s. Ercoupes had some of them to in areas they could not use a solid rivet. Some years ago, I actually found a bag of exploding rivets at Sun-n-fun in one of the junk booths. I almost bought them just for fun, but thought 60-70 year old explosives might not be very wise to keep around the hangar.

Most new commercial and military aircraft designed in the last 10-20 years, are using versions of pulled rivets, some of which are about the same strength of an AN bolt of the same size much stronger than any solid aluminum rivet. Solid rivets were from days when labor costs were cheap and hardware was expensive. Now a days in industry the reverse is true, fasteners are put in by robototic drills and riveters, pulled blind fasteners are much better for that style of manufacturing than solid rivets.

There are hundreds of versions of pulled fasteners, some stronger than the ones used in the Panther and some not as strong. Dan in keeping with the philosophy of an affordable airframe that is easy to build, but strong enough for aerobatics chose a fastener, that wasn't much more expensive than a solid rivet, but easier and faster for most builders to install.

I mentioned the above, because some people think that solid rivets are always better than pulled rivets. And in some cases that is true, but not necessarily in all cases. The Panther was designed around the strength of the rivet chosen, do not use a replacement fastener that has less strength than the ones chosen. Do some research or ask us if in doubt.

There is one thing that solids rivets are better at than pulled fasteners, and that is the hole quality tolerance. A solid rivet can expand a little more and fill a slightly larger mis-shapen hole than pulled fasteners. But good technique drilling and sharp bits minimizes bad holes. And if a hole gets to bad, you can usually enlarge the hole one rivet size up, and still have good edge distance.

But, the bad side of solid rivets is poor technique can damage the structure, work harden and crack the rivet. Go look at homebuilt aircraft with solid rivets and look to see how many "smiley faces" you see around protruding head solid rivets. You will probably see quite a few, unless the builder was good. I know I have created a few smiley faces, and had to remake parts.

[csjohnson65]

Chris,
If you go the solid rivet route with your build, please let us know how you rivet the skins on difficult to reach areas like the tail feathers. Because they have a frame, I don't know how the tail feathers can be solid riveted...

Glen

Hi Glen - Will do, though I'm afraid that most of you will be well ahead of me. I plan to build the Sport model and am waiting for the prototype to be built, flight tested and if there are any resulting design updates before I get started. Then I plan to purchase the full kit all at once to take advantage of the discounted pricing.

But if there are places where it's difficult to set a solid rivet with a gun or squeezer, and I find myself spending too much time contemplating ways to get it done, I'll likely look here to see if someone else developed an ingenious method, otherwise I'll go ahead and pull those rivets. But if I do stumble across something unique and valuable, I'll be sure to share!

Chris

[Tony Spicer]

Once all the little details get figured out, proud stems aren't really a problem. First set of tanks I built used flush closed end aluminum rivets. Lots of proud stems. Not happy. Used Paul's method to knock the stems down. Decided no more flush tank rivets for me. Second set of tanks used protruding head SS closed end rivets. Third set built with protruding head alum closed end rivets. Decided to give the flush rivets another shot on the 4th set of tanks, but to do some testing first to figure out best practices. Worked just fine, as I finished tank #7 today. 218 rivets and one stem that was proud about 1/64".

And there's a really easy way to handle proud stems. Take a 1/16" punch and press on the protruding stem. If it doesn't move, tap the punch with a small hammer. It will move down. The stem on a non-structural rivet is like a bee's stinger. Once its job is done, it serves no useful purpose.

What works:

Use a rivet one size longer than the stack height would dictate, as there's no way to measure the thickness of the tank sealant. I used #42's except for the filler neck where I switched to #43's.

Use the smallest diameter nose piece on the gun that will not jam stems.

Use only enough pressure to break the stem. Start at 40 psi and work up.

When the rivet is seated, push hard on the gun, make sure it is square to the work, and slowly squeeze the trigger.

Tony

[psalter]

A couple of quick notes, Chris is correct that sometimes the pulled rivet will break off proud of the surface, while not a very common occurence on most of the flush pulled rivets, it is more common on the closed end pulled rivets used in the fuel tank. Using a dremel tool with a sanding drum, or a die grinder with a sanding wheel, you can sand the stem off to be flush. Just be careful not to hit the wing skin, or take off the head of the rivet. I had to do this on maybe 20 rivets in the prototype Panther fuel tanks, and occasionlly elsewhere.

It doesn't take long to do, maybe a minute or so per rivet.

For solid rivets, there are a multitude of ways to do them in a wing. Most likely, you would lay the skin out just like normal, then roll the trailing edge skin up and work from leading edge back. You could certainly use solids on the top, and some pulled rivets on the bottom in the hard to get to areas. And with most solid rivets, at some point, you have to make your own bucking bars to get to rivets. The builder definately needs to be more creative to use solid rivets, and have small hands to get through lightening holes in the ribs.