Franks LSA

[Frank]

I decided to cut and flange the rear spar lightening holes before drilling for cap strips. Cutting the .032 holes with the fly cutter is actually a little smoother than cutting thinner material. But flanging the holes is more difficult in that the distortion is greater than with thinner material.

The result is a longitudinal bow and a transverse (up/down) bow. I'm not too concerned with the long bow, it's about 1/4 inch and requires very little pressure to flatten, so I think it can be handled in the assembly. The transfers bow was easily corrected with pliers. I used heavy tape on the jaws to protect the aluminum. My bob sticks were pretty worn out and the .032 just caused the wood to split.

My method of flanging the holes is pretty lightweight. I re-used a MDF die from the ribs and made a form block sandwich. Dressing the business edge of the die with sandpaper and cold beeswax between each hole helped a lot. The method works okay but in addition to the bowing distortion, pressing the flange also tends to cause the spar flanges to widen a bit at each lightening hole.

So, while the rear spar bends remained straight the edges of the spar flanges needed some correction to get back to 12 and 90 degrees. This was easily done with clamping and light hits from a dead-blow hammer.

With the main spars I am taking a different approach. Since those lightening holes will not be flanged, I don't expect any distortion. So I will drill cap strips first and cut lightening holes after. I am laying out the man spar with all cap strips on one side (rear) using strings, clamps and gauges to establish straightness and center-to-center distances. And I made a drill guide to use with a six inch bit which tested great and will make straight and perpendicular holes.

[davzLSA]

Frank you are a true craftsman and do beautiful work. Your posts are very inspiring and is one of the things that keep me going.

[Frank]

My aviation adventure of the week: getting the left wing spars together.
I found that the 2X riveter is not strong enough to do AD5 rivets.
So I need to check the cfm output of my small compressor against the needs of a 3X or 4X rivet gun.

[davzLSA]

Looking good Frank.

[Frank]

Main and rear spars for the left wing are now substantially done. An EAA Technical Counselor visited and he went over every rivet with a flashlight and magnifying glass. He said my work was good but still found about twenty rivets to drill and replace. Having done that, I feel more confident about both the riveting and replacing rivets. Most of my problems were with the factory head. I learned that short rivets require less pressure than long rivets of the same diameter. And one long burst is better than three short bursts.

When bending the spar flanges on the brake, it’s important to pay attention to maximum bend radius as well as minimum. I exceeded the maximum and thus impacted internal parts dimensions. But Bob approved my fix and so I pressed on.

Basic sequence after the spar flanges were bent as follows:

Layout and mark front and rear spars together. Stack and clamp the cap strips together in position. Triple and quadruple check all marks and measurements.

Drill cap strips undersized using drill guides for plum holes. Cut and stack spacer bars at rib locations, lock into place and drill undersize holes with drill guide.

Spar attach and splice plates were difficult to cut using MDF template and flush router technique. Mostly cut these pieces with table-mount jig saw, hand saw and files.

Remake pieces as necessary to get good fit.

Cut lightening holes in main spar on drill press with fly-cutter.

Assemble the spars vertically mounted to bench, now with all pieces in their correct locations, front and back.

Cut rib attach angle blanks. Drill the rib attach holes only. Then bend the attach angles.

Using tape, square and clamps, drill the attach angles to match the spacer bars and cap strips.

Now that all pieces are in place (excluding wing strut attach fitting and its rib attach angles) , go through and drill to final size holes.

Disassemble, deburr and prep all pieces. Treat with Stewarts Eko Etch and paint with Stewarts Eko Prime.

Reassemble and rivet.

Flush rivet spacer to accommodate wing strut attach fittings.

Mark wing strut attach fittings and cut slot in spar flange.

Drill pilot holes for wing strut attach fittings. Hold in place with temporary screws.

Notch rib attach angles to fit over wing strut attach fittings.

This is where I will stop with the left main spar. When the time comes I will drill and ream the wing strut attach fitting holes for the AN3 bolts.

I’ll also remake at least one of the rib attach angles over the wing strut attach fittings to get better edge distance for the ribs at that location.

Here's a short video:

https://youtu.be/ibsnJjDfwvQ

[Frank]

Three of the four wing spars ready to go. The fourth (right main) is ready for etch and prime, with warm temps forecast for Tuesday this week. Perfect timing!

[Frank]

All four spars are substantially done except for some touch up painting. But there are a few rivets I'm only let's say 85% confident of. I have drilled and replaced quite a few already. Sometimes the 2nd attempt is not much better than the first. Although I think these structures are very strong, I don't want someone to say "you really ought to replace those rivets" at some later stage.

So while I wait for a visit from at least one persnickety EAA technical counselor, I'm marking everything which could conceivably be a problem and then get expert advice. I am improving but the main problem is deformation of the factory head. You really have to keep your eye on the ball.

The main goal is to get a thumbs up on airworthiness of the spars. Only then will I tear down my elevated spar bench.

In the meantime, I can do some more rivet replacing, touch up painting, organizing and clean up. The plan for reconfiguring the shop for initial wing assembly is starting to take shape.

[Frank]

When it comes to preventing deformation of the factory head, a very helpful suggestion from Facebook Bearhawk group: use masking tape to get a better grip on the factory head. The other thing I am doing is cleaning the rivet set, hitting the contact cup with emery cloth to keep a slightly roughened surface, also to maximize the grip.

Another big help is I started to give support to the bucking bar, raising it as necessary so as to take gravity out of the situation. It's so much easier to keep the bucking bar in place when I don't have to hold it up!

Of course, before I started in with replacing rivets, I had to clean and reorganize the shop, throw out and recycle a lot of scrap. The anxiety around these spars was not helping!

And then set up a practice piece to rivet, drill out and rivet again.

[Bcone1381]

Two common errors I recently read from a professional, mis-alignment of the gun and rivet and a bounce between the rivet set and the factory head after trigger release. The fixes are to 1) point at the rivet with your index finger on the gun hand to help keep the gun in alignment, press the set into the rivet and squeeze lightly the trigger with your middle finger. Then keep pressure on the gun for a two count after releasing the trigger to avoid a bounce of the set off the factory rivet head which creates a smile on it.

[Frank]

Wing spars are done for now and hung up out of the way.


The spar rivets are generally shop-head facing forward and factory-head facing rear, unlike most Bearhawk examples I’ve seen. My research leads me to believe rivet direction was irrelevant with regard to the spars. This was a consequence of how the spars were set up on the bench and ergonomics.

In some cases the direction was reversed, the reason being I had better luck this way with some rivets of the rib attach angles. The shop head af the AD5 rivets had a tendency to deform the ends of these attach angles. I think the shop head forms from the center outward and the developing force tends to pull the metal toward the center. Unlike the factory head which has a larger bearing surface and the force is uniform. Replacing those few problem rivets and reversing the rivet direction mostly took care of the problem.

Drilling and replacing rivets turned out to be a fairly routine matter. Even so, there were maybe a dozen rivets between the four spars that I felt needed to be upsized after drilling them out.

I used the rivet cutter a lot. More than I would have expected. There are just too many in-between and impossible to calculate lengths to deal with.

I went over all of this with an EAA technical counselor and he had no issues.

The wing strut attach fittings are drilled undersized and held with hardware store bolts for now. I’m uncertain as to when I should drill, ream and permanently fasten those. The fuselage attach holes, also undersized, obviously will be drilled to final size when someday the wings are mounted.

[Frank]

After the spars were hung up and out of the way the next item was to fabricate the two center ribs at the wing roots from .032 material. Although the factory apparently flanges the lightening holes on these two pieces, the plans indicate otherwise. I checked with Bob who said it’s really unnecessary to flange these holes. This plus the thicker/stiffer material makes it a lot easier to get a flat and straight rib.

It was a fun experience to pull out my old MDF templates, bender and backer boards, and make a couple of more ribs and the rib stiffeners. And what a pleasure it is to use the rivet squeezer on the AD3 rivets.

Now, finally, I have the complete set of ribs (plus a few extras), spars and ailerons and can start to reconfigure the shop for wing assembly.

I recommend having EAA technical counselors examine a project. I’ve had four visits so far. Though they won’t spend a lot of time with you, they will certainly address your concerns and point out any obvious defects. It’s a great feeling to get their thumbs up and be able to proceed with some confidence. And copies of their reports in the builder log will add to the seriousness of your efforts when it comes time for the FAA examination or in case you need to turn the project over to someone else.

Linked here is a short video:
https://youtu.be/TP7E3ewnmFY

[Frank]

I cut the worktable down to 2X16 and did my level best. Reduced wing contact points to four supports. Focused then on using spirit level(s) and strings. Also rigged up a water level.

Thanks to sjt and his analysis of how to assemble the wing. Also, videos from the factory confirmed the choice to assemble the center wing horizontal on the table. After preliminary assembly, the center wings will then be set up parallel in vertical jigs.

At first I wondered why Bob would specify the aileron support frame in decimals… 3.78 rather than fractional. But if you subtract front/rear webs and cap strips (for that location) from the nominal front/rear dimension on the mylar (31.0), then that is the exact result.

All the center ribs were initially cut to the same nominal front/rear dimension (31.0). For assembly, I subtracted cap strip thicknesses from either end of the center ribs as required by each rib station. Also subtracted 1/32 from each end for wiggle room.

Aviation snips removed the wider bits. Then bench grinder to the pencil line, hit it with flat file and scotch-brite wheel. The trimmed ribs don’t just drop into place. It requires some relaxing of clamps and loosening the structure to allow the ribs into place.

The center ribs don’t fit tight against the spars and cap strips. But the edge distance for riveting is more than adequate.

The spars and ribs of the left wing are now set up in a basic configuration held together with clamps.

At this point the aileron support frame is only mocked up. It is the correct dimension and square. But points to the fact that I need to get started on welding asap.

After triple checking for flatness and level I will go ahead and start drilling. I have a nice compact air drill to use but my little air compressor isn’t up to the task. So at the moment I’m shopping around for a suitable air compressor. My research indicates the basic requirement for an air drill is about 5 CFM.

[davzLSA]

Hey Frank, those wings are looking good. It looks like you will be skinning them soon? I looking forward to getting started on mine. Im almost done with the fuse, I still have a few things I want to get done before I start the wings. Im thinking about making the cap strip a one piece milled out of a solid billet of aluminum. My son is a machinist and we have a bridgeport mil. I think Mark had someone build a cap strip like that but I dont think it went anywhere. If I do it ill share it on the forum here.

[Bcone1381]

I believe factory wings are now all made from the one piece milled aluminum cap strip.

[Frank]

Wow Dave that's quite an idea! But if you have the means to do it then go for it. Tell Bob and see what he thinks about it.
Me skinning soon? Not likely. Currently left wing center skeleton is riveted and set aside. Now beginning the same for right wing.
Then both to be set up side by side in vertical jigs for nose and back ribs.
Then there will be a stretch of time while I make the steel parts.
I don't suppose you already have the steel parts for the wings, do you?

[davzLSA]

Hi Frank, yes I have the steel parts already, I just bought them from Mark. I actually have all the parts for my wings I just need to get started on them but I want to get the fuse done first. Brooks Cone was saying that the factory is using the 1 piece spar caps. Im going to make a full spar assembly per the plan so I will have a good model of the spar to guide the making of the 1 piece spar cap.

[Frank]

Keep the steel parts handy, Dave. One of these days I’ll be bugging you for some detailed photographs. ?

[davzLSA]

Sure no problem man. I make some pics and get them to you.