Trimming final rib length

[jaredyates]

stinger, didn't you have some angst about this topic when you built your wings?

[sjt]

I have pencil marks on my table. They are square. I used carpenter squares to extend up lines to attach hole centers.
11.jpg main spar attach
22.jpgrear spar
33.jpg 44.jpg I held everything in every possible way.

But then you come to the last thing that has to be satisfied -vertical rear spar distance.

cc.jpg ms.jpg
The last degree of freedom is the height of the rear spar (relative to main spar) off of the table.
The top edge has to continue curve along rear spar flange.
It gave me a good feeling so see that this also made the ends of the center rib nice and square to the to the vertical rear spar. I never trim - I only notch
I made the mdf template 32-032 inches parallel as perfectly as possible. The ribs are routed to this edge and should be the correct length and also parallel.

There does not seem to be any other choice.

Long ago I checked my master template by adding marks where the attach holes would be. Relative to the line marked as the cord line, the lines are not parallel.
co.jpg The bottom level follows the line marked as the cord line. It seems to be perpendicular to the lines marked as the center of the main and rear spars.

To the best of my ability I did not find the line drawn from the main spar attach and the rear spar attach to be parallel with the cord line. This is either true or not true and is the issue with the plumb bob.
There are no markings on the drawing 4 mylar. I used other drawings to allow me to plot where the hole would be on the mylar.


The picture above is the best I can do to illustrate the levels are not parallel.
You can still use a plumb bob with a vertical jig, you just would not expect the plumb bob to be at right angles to the spar. The spars can still be parallel just not level to the ground.
If you never put a level on the spar you could use your nice and squared up center ribs and that will force the two spars to be parallel - one of my reasons for not trimming ribs .


So what if they are not collinear, But it does suggest that the attach holes should be made undersized at this point and latter match drilled to the fuselage attach holes.
Or fix anything the way you want when you make the fuselage attach fitting.
The wing has to pivot a little when setting the dihedral. Even if the attach holes are not perfectly collinear at this point they will be when match drilled to fuselage.

[sjt]

Just as an aside, I think this is one of the new improvements made by Mark on his kits. He has the wings and fuselage fittings drilled at the factory.

The older kits required a tool to help make the attach holes. The tool is not that complicated and looks like what you would expect to make a collinear hole thru
fuselage and 2024 attach fittings.

Screenshot from 2024-01-11 13-08-12.png Screenshot from 2024-01-11 13-07-39.png


Also, as an aside; My issue is not a vertical jig or table method. You will have to have a vertical jig for skinning anyway. I am advocating for do not trim ribs - just notch

[Frank]

I remade the composite drawing as a check and is more accurate.
The front and rear attach holes are not in line, but separated by a little more than one inch.
This is the case whether laying flat or in a vertical jig.
My undersize pre-drilled holes are perpendicular to the spar webs.
So, I will keep studying and pondering.
Next step, study Stingers posts again and use the forum search function.
Because there seems to be an issue if these holes are to be co-linear hinge points for setting the dihedral, for instance. Screen Shot 2024-01-11 at 12.42.47 PM.png

[sjt]

I assumed the collinearity would be the result of using a drill jig when attached to the fuselage.

[Frank]

As I look at it, the only way for those holes to line up is to drill them at an angle.

[sjt]

I have a patrol so I am not sure how similar the drawings are to LSA

This is why I always expected it to be off by 1/2"

Starting at the main spar:
The spec is 1.5 inches from center of web
a1.jpg
a2.jpg It so happens that if I use Bob's line that I believe is at right angles to the spar I get 3.13 inches

b1.jpg I transfer that 3.13 inch mark to the rear spar again using Bob's line which I have always thought of as the cord. marked in red
b3.jpg The rear spar attach is specked at 1 1/8 above center line. I marked it with a dimple and circled in pink.


This picture shows that the projected (plumb bob) main spar attach point is 1/2 inches from the rear spar attach point.
I did assume that line is perpendicular to spars and as far as I can tell it is
cc.jpg

And from an earlier entry that means when attached to fuselage that is 1 3/4 inches up and wing is 1/2 inch down
so that is 1 1/4 inches over 32 inches giving 2.2 degrees up which is seems in agreement with Bob's spec of 2 degrees up.

[Rob]

I don’t want to come across the wrong way. I am not trying to argue. Or say one way is right over the other. I’m just trying to educate myself and learn. I am not new to building wings. Have built many. All on the table flat. Stearman, Waco, Taylorcraft, Pietenpol, Piper. All fabric covered. I have never hung one vertical to skin. I never gave it much thought. Assumed you would line up attach holes. Awhile back I saw something about this topic. Then while talking to Bob awhile back about something else, I then asked him about this topic. He said to hang and aline holes with plumb bob. I thought…ok that is what I was thinking. Now this comes up again. I’m thinking how can it be. Bob, the designer himself told me to do it this way. Also have seen videos as I linked of others doing the same. Lining up center holes. Unless Bob and I wasn’t understanding each other in this conversation? I’ve read though old post in search. Maybe I haven’t seen them all. But I’m more confused as ever. I would think this would be a simple answer. Apparently not. Does anyone know how the factory does it? I don’t have a problem doing it either way. Just want to do it the right way the first time. Too much time and wasted money on materials to get it finished and find out it’s wrong. My last thought is….I might have to go back and re-read, but Stan, I believe you said once match drilled they would line up? So why can’t you start out by lining up? Also how far off can you be by lining them up? If your scratch building couldn’t you adjust your rear attach fitting on the fuselage to get the incidence you want? A lot of Bearhawks have been scratch built. Have there been 2 different ways of building wings all these years? I’m assuming the Bearhawk, Patrol, LSA are all built the same? Are they not? Any multiple builders (Collin, maybe?) care to share what they did? I appreciate any and all responses to get this right. Or if somebody could point me to a informed thread I might have missed? Are there any old Beartracks articles on this? Thanks Rob

[sjt]

Whenever I thought I could be clever and do it differently then Bob I paid for it in time and money. I think everyone has to eventually match drill at an angle with a jig.

[Frank]

Stinger said:

https://bearhawkforums.com/forum/bearhawk-lsa-plans-build/25019-bearhawk-lsa-wing-plans-build-caution

[sjt]

Whenever I weld something it moves. I always assumed that there would be the need to match drill at some point.

There are a couple of possibilities. The plumb bob in a wing stand does guarantee that the main and rear spar are aligned long ways (a necessary condition before skinning.)


If I put my table built wing in a vertical wing stand I could still twist it 0.9 degrees so that the plumb bob points at center (still not coaxial though) .
My center ribs have perfect parallel not trimmed ends. My center ribs are what forces the main and rear spar to be parallel.


The end result is the the spars are parallel to each other but not to the floor. That would not effect the skinning operation. The string would not be at right angles to spar - how would you know? The string is not at right angles to the floor. My floor is not level in any direction. I use some wedges under a 2x4 to help support the rear spar. No precision -just however many wedges to make the 2x4 touch rear spar.

You would then be in the same boat as building on a table. If you made your center ribs with perfectly parallel ends, you will still use the same technique I used by stretching a scrap piece of aluminum from the spar flange smoothly to the rib profile.

I also have to keep going back to Bob's fuselage drawing I have not built it yet but I think it is angled up 1 3/4 over 32. That is 3.1 degrees. On the first page of the drawing set Bob says 2 degrees. Something makes that wing 2 degrees when match drilled.

[Frank]

I'm glad this topic came up and gave me an excuse to stay inside and study. It's too damn cold out in the shop!

[sjt]

From Frank's reference post :

"I am building the wing in the vertical mount. I have now learned the spar mounting holes should not align with a plumb bob and stated on page #17. When the spars are in there correct position the mylar sheet line is the plumb bob reference. I will start over again . Stinger"


I have only looked at patrol.
But I find Stinger's comment to be consistent with the table approach.
my.jpg
Spars are right angles to table.
Mylar line (that I had always assumed represented cord ) is parallel to table.

plumb.jpg The plumb bob is only useful for setting plumb in the long direction. It is expected to be off by 1/2 inch(patrol) in short direction.
Like Stinger said in his post - there is a line on the mylar drawing that is at right angles to the spar and that would be a reference using a plumb bob in all directions.

Unfortunately, you can not hang a string from that location.

That line would be plumb, you just do not have the ability to find it. There is no piece of metal on main or rear spar that still has any metal where that line was on Bob's
mylar drawing. There is an attach hole in the main spar that you could drop a plumb thru. But such a plumb bob only has value in setting the long direction.

I used a shim against 4x4 post to make sure it is plumb the long way. If the spars are level, if the spars are parallel, then you are just making sure the rear spar is aligned the long way relative to the main spar. If you built on a table that structure could never be moved in any direction other then the long way.
The rear spar could not ever be moved in the short direction if the ribs are already riveted; nor would you want to.


The above picture shows alignment in the long direction only.

It is what it looks like when projecting the main spar attach with a plumb bob.
The black washer has a 1/2 inch radius. I was just showing off my nice plumb bob with replaceable sharp tips.

[marcusofcotton]

Too many years ago I looked at this exact issue on my four place a model project. The math simply doesn't work out properly, the holes through the spars have to end up drilled at an angle, not perpendicular to the spars. Frustrating issue.

Russ Erbs CD addressed the issue as well, iirc.

[Frank]

My conclusions at this point:
1. The spar-end wing attach holes are perpendicular to the spars. They are not "co-linear" and there is no reason for them to be. I don't know where the idea of passing a rod through them as a check comes from.
2. The plumb bob between attach holes is a red herring. The nose, center, back ribs and spars are puzzle pieces: they can only be assembled in one correct way. And that is to match mylar drawing no. 4 and the tip rib.
3. If assembling in a vertical jig, the most sensible vertical reference line is the center/chord line of the airfoil. Then, the spars will be level widthwise as well as lengthwise.

[sjt]

Agreed it can only be assembled in one way with smooth transitions to spar flanges.

I planned to build on a table so this would not have been something that would have been useful to me but:

If you also glued mylar to the 032 sheet like me - you could have drilled two tooling holes coincident with that straight line on the mylar sheet (i always thought it was the cord line). One hole toward main spar one hole toward the rear spar.

Then you would use those two holes to drill thru the mdf form blocks, which then could be transferred to all rib blanks.


Then you could somehow use those two holes as a plumb bob reference. There would be absolute certainty in a vertical stand. It would not take much effort to have these marks permanently on the ribs if they are of use someday.

I think it would be ok to do the above but it is not likely to be of great use - since the ribs that have never been trimmed (only notch them) contain all of the information forcing them to be assembled in one way.

The plumb bob sets the direction the long way to keep it from be askew for the wing skinning stage.


In a post above Bob's reference line can be seen when building on a table. A level can be used to confirm the same thing.
a.jpg

[Rob]

Well I found another thread to read I had not seen. And re-read others. I understand the situation now. If I have got to the assembly stage and seen issues while trying to assemble I might of figured it out like others have. And if I was reading these post while having trouble actually assembling, they would have made sense. I re-read the plans booklet again last night. It clearly says center holes with plumb bob in fixture. After also asking Bob, and he said to plumb holes centered, and watching B-spot video, when you hear it being said to not make them centered the first reaction is it can’t be. Especially after my last project they were centered and a rod could go through both attach fittings. But apparently that is not the correct way.

I understand it is a scratch built airplane and it is what you sign up for building one. But sure would be nice if there was better information on this in the booklet for somebody to start out on the right foot. Just another couple sentences could help a lot. Too many of these types of issues on one project knocks the fuzz off a project. Maybe why so many projects never get finished…

I appreciate the efforts of others, and the time they spend posting there experiences, fixes, answers. They save others a lot of aggravation sharing there pitfalls so others don’t make the same mistakes.

I will continue to follow for anymore updated information.
Thanks Rob

[Frank]

If you drill the mounting holes at an angle through the spar ends then it seems that the steel mounting brackets will also be drilled obliquely. That would seem to mean the head of the mounting bolt washer and nut will not sit squarely.

There may be some other reason for a plumb bob between the mounting holes, but I don't yet see what it is.

Carry on!

[sjt]

Frank, there is a picture above that is green and purple. Only the 2024(purple) attach plate will need to have a oblique angle drilled/reamed.
The fuselage attach holes are parallel and collinear, The main and rear spar are just parallel. In the picture the green circle is seen straight on.
If you looked straight on you will not see the green circle inside. The purple circle -before reaming - will have an edge that you could perceive if looked straight on.
In principle, the fuselage will not be cut during the drilling/reaming. Only the purple will have material removed.

[sjt]

I entered the thread as an advocate for not cutting ribs early. I made the case above.

I have always been intrigued with how few tools are required to make an airplane. You can do a lot of work with the $7 orange plastic hammer that harbor freight sells. And it dates me a little when I say I used coupons at harbor freight to get their tiny sander I use to make notches for $35.

One tool that I would not try building an airplane without is a table saw with an aluminum 10" blade.
I cobbled together a board to make a track system for that saw:

All mdf forms start with at least one good edge.

I start with a mdf board that has at least one straight edge. Then I make the first cut at right angles with sled. I slide board 32"-0.032" over and make another cut with the sled.
Now I have a mdf board with 3 good edges. I no longer care about the first edge.

I line my two new edges up against the mylar 032 template using those two edges:
I then pin and use a trim bit on a router table - using the edge of the 032 template to make the airfoil profiles out of mdf.
Using the first mdf profile I can make additional mdf blanks for each purpose I need but they all have two good edges.

Using this mdf template I can cut blanks and using orange hammer make ribs that are guaranteed to have parallel edges 32-032 apart. Ribs made this way
are helpful in the assembly stage.

I liked building on a table. You have to climb around like a monkey to drill and rivet every thing but if the table is level you end up with a wing structure that is
solid in every way except the rear spar can be moved in the direction of the spar. Until the skin is applied it can slew in that direction.

I made a vertical stand for wing skinning. It has the following features that may or may not someday be useful:






rrr.jpg

t.jpg