Bearhawk Five Plans #5053

[Redneckmech]

I just made the realization last night that all of my rivet calculations were not including primer. Does anyone have a measurement the thickness of an epoxy primer or a rib coated both sides with it?

Edit: From what I have been able to find, the thickness of the primer should end up at roughly 3-5 thousandths per side depending on how it is sprayed. Since I already put in my order for rivets I will likely have some that are too short. On the bright side, it will give my helpers (my three boys) some to practice driving and I had ordered 1/4lb of the next size up of every rivet so it will not put me line-down.

[geraldmorrissey]

I measured some of my parts, before and after primer application. Typically I added .0013-.0018 total thickness. And that's too much. Should not need longer rivets but they are nice to have around. Seems like what you compute and what you actually need can be somewhat different.
Gerry
Patrol #30

[Bcone1381]

RE: rivet length. You are talking about a single thousandth of an inch. The big picture on rivet shop head formation is.....If you use a rivet that is a bit too long, the shop head tips over when its being formed and its unsatisfactory. If you have to choose between a short or a longish rivet (for example, you need either a 3 or 4 length and its right in between) choose the shorter. You will have enough material volume on the rivet tail to form a shop head that meets The Mil-SPec standards. Those Mil SPec standards are more specific and generous....meaning there easier to "pass" than the general pictorials we all see.

Use table three on this document. (about p.9)

https://www.vansaircraft.com/wp-content/uploads/2019/02/MIL-R-47196A_MI.pdf

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[geraldmorrissey]

MIL-R-47196A, it is all there.

[Redneckmech]

Thank you both for your response. Glad to hear that primer appears to be thinner than I read elsewhere or I misread/misunderstood thicknesses I found off of the VAF forum. I have MIL-R-47196A pulled up on my computer, will need to print it so I have it handy in the shop. I tend to overthink things more than I should so it will likely save me a lot of drilling.

[geraldmorrissey]

I found that on my Patrol spars I had to trim about 90% of the rivets to get to the center of the driven tolerances. Make sure you get some rivet guages to check the buck tail dimentions. Make lots of test coupons and record what rivet length you need for a particular part stack. That way you will build a database and you will get very good at predicting the required cut length. Good work for the kids. Here's a sample of my notes, Kind of a mess but you'll get the picture. I squeezed my rivets so I had to deal with rivet length and squeezer die length. Good luck.
Gerry
Patrol #30

[Bcone1381]

On trimming Rivets, and selecting rivet lengths for purchase:

Avoid cutting Rivets to length. You will be greatful later...trust me.

Within reason, buy all the rivet lengths and especially seek out 1/2 length rivets, like the AN470AD3-3.5 (the dash 3.5 length is 1/2 way between the dash 3 length and the dash 4 length rivets) Just order small amount, they are cheap. These are available at both Aircraft Spruce and Vans Aircraft. But they might only have 3.5 and 4.5 lengths....maybe not 5.5 if I recall correctly.

I avoid a cut rivet. Maybe its my Cutter, or maybe it is my skill level. It shears the rivet to the length I choose, but in doing so it seems to work harden the material. So a large percentage of my cut are not driven properly....they are not smashing down properly and failing to meet the standard. So I no longer cut a structural rivet. I will grind them to length before I cut them.

[geraldmorrissey]

I actually use a mill file on my spar rivets and I can find no cracked buck tails. It was quite easy but time consuming.

[Redneckmech]

Thank you Brooks and Gerald for your feedback, I greatly appreciate your input.

Progress is still being made, I am very close to priming but wanted to complete all ribs first to minimize the number of times I have to set up a paint booth.

Aileron pocket ribs and flap nose ribs are now complete and it is almost time to start peeling all protective film off in preparation for cleaning and priming. I will be cleaning with prekote and priming with Stits EP-420.
My first rendition of my flap nose ribs did not have enough relief to prevent deformation at each corner so I notched my cutting formblock deeper and re-trimmed and polished them again. The second attempt appears to have worked well.
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I finally purchased a tungsten bucking bar at the recommendation of everyone here and my A&P friends. I also built Bob's bucking bar out of the Bearhawk book using some 1" barstock I had in the scrap-pile so I should be pretty well equipped once I begin riveting.

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I also drilled the bucking bar to accept dimple and squeezer dies for when the need arises and turned a set for the rivet gun to accept the other half of the die, as long as I keep it square to the material it works very well and should allow easy dimpling of areas that are hard to reach with the benchtop C-frame I have in process.

If you have not studied the Bearhawk book with the same detail as your plans, I highly recommend it. There are all sorts of hidden and not-so-hidden gems of information in it including answers to a question that comes up frequently here on the forum :-)

[geraldmorrissey]

Nice job building that bar. Seems like bucking bar designers never stopped to look at a human hand. My guess is your hands will be hurting when you start setting -5's in the main spars with those rectangular bars. Looks like you have some machinery, why not try a new design. My best bar that recently grew legs, started life as a 14" long 3" diameter steel cylinder that has been cut, machined and ground on for 40 years. Ugly but its cylindrical shape fits my hand. Flat sided bars with small corner radaii hurt my hands after just a few rivets. A pair of riveting gloves will help but you loose some "feel". Got to get that bar back. Good luck.

Gerry
Patrol #30

[Redneckmech]

Thanks for the advice Gerry! I have a stockpile (my wife calls it a horde) of roundstock under my lathe. I think I have some 2-1/2 or 3" so may have to make use of it for a bucking bar.

[SpruceForest]

Just had a chance to look through this thread - lots of good stuff here from the OP and others. Belated congrats on the check ride... that first is always the toughest, and keep flying... surprisingly perishable skill until you get things wired in. Will have to go back and do some editing on the rib forming stuff to better emphasize super-thin CA and wood hardener to stabilize MDF. Might have been SJT, but cheaper MDF is the devil, and some of what I have seen in Home Depot is def not high enough density to qualify as MDF... more like the low-density stuff.

A Five is on my list, assuming angle-valve 540 jugs drop below $5K per. Looked at bigger engines and it just gets worse. May end up making a radial attractive, assuming the Eastern European stuff comes back online at some point in the next 7-8 years.

[Redneckmech]

Thank you! I guess I didn't mention it here, but I managed to finish up the overhaul on my Cherokee's O-320 in July and have about 32 hours on since. I am sure hoping parts costs come down for when I get to the BH engine but that is going to be quite a few years out at my current build rate.

Thank you for everything you have posted, I used your guide extensively for making my ribs.

[Redneckmech]

OK, time to fess up..

A couple weeks ago I was getting ready to begin making my rib stiffeners and as I was preparing to, there was an article in Kitplanes magazine about scalloping non-critical parts for weight savings
(link below)
After reading it, I built a test piece and figured that I could save a bit over a pound of weight by doing that for all of the .025 rib stiffeners without affecting the strength. With the goal being to build as light as practical it seemed like a great idea. Over a three day weekend I built all of the .025 stiffeners with scallops.

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My initial thinking was that it should not affect the strength of the parts since the scalloped leg was riveted to the rib. Well, not so fast... The more I thought about the mechanics of it, the more I began to doubt the advice in the article and my initial thinking. Since the stiffener angles are an L shape and we are riveting along one leg of the L offset from the bend it is going to impart a rotational moment along the rivet line as the rib flexes with the wing. This in turn is going to force the length of the stiffener to change to match which will impart a stress concentration in the valley of the scallop.... And thus, increase the likelihood of a crack forming....

So based on that, I started digging into it further and asking myself the question, why, if the only obstacle to scalloping is the amount of labor, and Airbus and Boeing have extreme CNC capabilities, do they not do it....

From my research the reasons are threefold....

First, as I had considered, a scalloped edge cannot carry significant lengthwise tension or compression without significant increases in peak-to-valley stress concentrations - compression buckles concentrating at wave valleys.

Second, Transverse bearing-tear-out occurs on a 40 degree angles from hole center. Scalloped edges reduce tear-out strength by reducing the hole-edge-to-free-edge distance. "Analysis and Design of Flight Vehicle Structures" by E. F. Bruhn

Third, scalloped edges are significantly longer than a straight edge and cracks tend to form at edges; so the longer the edge, the greater the potential for cracks.

If the rib stiffeners were parts that were easily inspected and easy to replace, I would likely use them. Also, if I was building a STOL competition aircraft the reward may outweigh the risks. For my purpose, I do not think the risk is worth the reward and I am nearly finished building a full set of .025 stiffeners per the plans.

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The moral of the story here, is to make sure to do the research before putting the effort into any plans deviations and if there is any question, stick to the damn plans!

If anyone is building a no-limits STOL competition BH, I have a set of rib stiffeners for you!

[Redneckmech]

Making more progress! It is nice to have primer on some of the parts!

Rather than the typical Etch - Alodyne - prime I am instead using PreKote since I have small children that tend to get into everything and PreKote isn't nearly as toxic. It seems to have advantages and disadvantages in that it does not leave a conversion coating like alodyne so priming must be done withing 24 hours after application. It also requires a lot of elbow grease with scotchbrite pads and I had grossly underestimated the amount of scrubbing that would be needed. I started about three in the afternoon and finished shortly before midnight on just the nose ribs and nose rib stiffeners. I then got up bright and early the next day and primed. So far the primer appears to be very well adhered but I will likely get a better idea of it once I start dimpling parts.

I am tempted to try an acid etch prior to using the PreKote so as to reduce the needed scrubbing, I don't foresee any ill effect but will have to research if there is any reason not to.

For primer, I am using Stitts EP420 in white. I honestly like the look of the green better but in thinking ahead for assembly and internal inspections to the wing I chose the white instead since there never seems to be enough light where you are working and I figured white will help with that.


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[marcusofcotton]

...For primer, I am using Stitts EP420 in white. I honestly like the look of the green better but in thinking ahead for assembly and internal inspections to the wing I chose the white instead since there never seems to be enough light where you are working and I figured white will help with that.

Not an expert here and didn't stay at Holiday Inn last night but generally speaking, primer doesn't provide lasting corrosion protection without topcoat.

[Redneckmech]

That is a point I had wondered about. In my previous experience with autobody work, there were primers and primer-sealers. Primer-sealers did provide "more" corrosion protection but still did not provide any UV protection so if external would need a topcoat. I do not know which category that EP420 falls into but I had not seen where any other builders were top-coating internal wing parts and only primer was called out in the plans. I may have to dig into it a bit more.

[rodsmith]

I never top coated interior aluminum parts, steel yes. EP420 is a good primer. The factory primes the interior wing parts, no topcoat.

[Redneckmech]

So I did some digging on primer just to ease my concerns, With EP-420 being an epoxy primer I am comfortable not top-coating it inside of the wings. For any steel parts I will likely use an etching primer in conjunction with the epoxy primer before top-coating.

Here is the spark-notes version of what I was able to find on the different varieties of primer:


Etching primer - used primarily as a tie coat to link bare un-sanded steel to topcoat or build primer. It is not a good rust inhibitor, but will prevent flash rust from humidity.

Vinyl wash primer - primer developed for military and marine applications to seal out moisture from steel. This type of primer is extremely durable and water proof. The drawbacks are it's poor adhesion to un-sanded steel, and it's lack of compatibility when used with an underlying tie coat.

Epoxy primer - The most commonly used primer over bare steel or Aluminum. This primer provides very good protection from rust and is very durable. You should use an etching primer tie coat over un-sanded bare steel.

2K Urethane build primer - Contains large amounts of solids to lay down thick coats, and is easy to sand. It is not intended to be placed directly on bare steel, it is a poor rust inhibitor and and it also absorbs water, due to its high solids content. This type of primer should be used over top of an epoxy primer, or a vinyl wash primer before applying finish coat to this primer you should use a sealer.

DTM primers - (Direct-To-Metal) primers are relatively new. They are corrosion resistant urethane primers that can be applied directly to bare sanded steel, and are high build, and do not require a sealer coat. They are a fast curing urethane and are ready to sand within 1 hour. These types of primers do not protect as well as others against corrosion.

Epoxy primer/sealer - this type of primer is used after build primer immediately before top-coating to seal moisture from the build primer. most topcoats are not very corrosion resistant because they are porous and allow oxygen to the metal. This extra step insures that moisture will not de-laminate body fillers, or create lifting of the build primer, or surface oxidation. Most of these primers are the same as regular epoxy primers but will have a different mixing ratio so it is thinner and easier to lay down.