Front spar question

[Peterross]

I have all ribs complete, and the front and rear spars glued up.
I now need to cut the front spar and have a few questions.
First, should the back side of the spar which butts up to the rib be 1.6mm shorter than the rib? This way the 0.8mm plywood would be level on both the top and bottom with the rib.
Second, what angle should I cut the rib at? I do not have the nose rib template yet and will fit it to the front of the spar.

Thanks

Pete

[scottiniowa]

I have all ribs complete, and the front and rear spars glued up.
I now need to cut the front spar and have a few questions.
First, should the back side of the spar which butts up to the rib be 1.6mm shorter than the rib? This way the 0.8mm plywood would be level on both the top and bottom with the rib.
Second, what angle should I cut the rib at? I do not have the nose rib template yet and will fit it to the front of the spar.

Thanks

Pete

Pete,  I will answer this a bit in reverse of your question.  If you were to have your nose rib template, that matches your rib jig less the spar spacing this would/could be done the way you stated-.just like you said, 1.6MM less height than your  rib that is glued into place on the aft side of spar, thus allowing your .8 mm leading edge be flush to the start of your rib- " over the spar and under the spar " 

That being said, this very template- will then determine the answer to your second question if you meant "spar" instead of Rib.  If you meant  rib as you wrote, then I am not sure what is meant because the rib would be a vertical  cut up and down the face of the leading edge of the rib.- Which I suspect you already have.  

Sounds like your coming along nicely.

Scott

[scottiniowa]

If this worked there should be a close up photo showing this all coming together.

[Peterross]

Hi Scott

Thanks for the response as well as the drawing. Let me see if I can further clarify what I was asking.
The front of my rib is 7". And the top of the rib needs to be cut so that there is a slope down toward the nose rib. I'm asking two things: first, what is the angle of the cut? Or what should the height difference be between the front and rear side of the front spar?
Second, should the side of the spar that butts up against the rib be 1.6mm lower than the rib? This would be to allow the plywood wrap to smoothly transition onto the rib.
The reason I do not have the nose rib template yet is because my drawing appears to be quite a bit smaller than what is needed. So I thought I would put the angled cut on the top of the spar and the. Create the template so it is flush with the spar.

[scottiniowa]

Hi Scott

Thanks for the response as well as the drawing. Let me see if I can further clarify what I was asking.
The front of my rib is 7". And the top of the rib needs to be cut so that there is a slope down toward the nose rib. I'm asking two things: first, what is the angle of the cut? Or what should the height difference be between the front and rear side of the front spar?
Second, should the side of the spar that butts up against the rib be 1.6mm lower than the rib? This would be to allow the plywood wrap to smoothly transition onto the rib.
The reason I do not have the nose rib template yet is because my drawing appears to be quite a bit smaller than what is needed. So I thought I would put the angled cut on the top of the spar and the. Create the template so it is flush with the spar.

NO, would be my first words, you say, "the top of the rib needs to be cut so there is a slope down towards the nose rib"  So NO, you would not be cutting ANY slope in your rib. SEE the photo.  
Now your spar could be 1.6 MM less in height than the total height of your main rib. Thus with the .8 MM nose skin (leading edge) you would have a flush line with your main ribs. Again, no cutting on your main ribs.
So again, NO angle cut, and yes to the height difference as explained and shown in the photo.

I guess that above answer would be in part answer to your question you call " second"

So now we are to your nose template again,  this template determines the final shape of your upper spar cap. While that doesn't have to be perfect for a aircraft of this speed, getting it as nice as possible is rewarding and this would not be a "flat cut"  Some have glued on the nose ribs and sanded spar down to the ribs to keep things nice and smooth for the transition.  Doesn't take long. And yes you can saw an angle on spar top to get darn close.

[Peterross]

I'll try one more time, as I noticed I said rib when I meant spar.
I measured the angle of the rib top at the leading edge. It is 8 degrees.
I also measured the angle of the rib on the drawing and that too was 8 degrees. The top of the front spar measures at 12 degrees, and the nose rib looks to be about 17 degrees.
So my guess it to add a bevel to the top of the spar at 12 degrees then create a nose piece to properly align with the spar. I can sand or plane a curve to the top of the spar to transition from the rib to the nose rib.
Sorry for the confusion. I did not mean to cut anything on the ribs.

[Vince Carucci]

It sounds like you have a good understanding of the problem and are working on a well thought out solution. That puts you ninety-something percent on the way to success. Good luck and let us know what you ended up doing (angles, measures, process, etc.)

Vin

[scottiniowa]

I'll try one more time, as I noticed I said rib when I meant spar.
I measured the angle of the rib top at the leading edge. It is 8 degrees.
I also measured the angle of the rib on the drawing and that too was 8 degrees. The top of the front spar measures at 12 degrees, and the nose rib looks to be about 17 degrees.
So my guess it to add a bevel to the top of the spar at 12 degrees then create a nose piece to properly align with the spar. I can sand or plane a curve to the top of the spar to transition from the rib to the nose rib.
Sorry for the confusion. I did not mean to cut anything on the ribs.

Peterros,   as noted, you have got it pretty close 

As you have calculated, each person that has built a XL wing or any other in the family- has come up with slightly different numbers on spar heights, rib heights, nose rib spread. And at the speed for flying this family of aircraft- this is zero problem.
One thing I would like to point out.  A spline line (and that is what the top curve of a wing nose, spar, rib profile) has NO flat spot, thus NO X degree of angle,  to have any measure-able angle, you need two flat lines to calculate from.  On this wing  you only have the flat bottom or the cord line.  The other line is what your measuring from. And it would be safe to say, that if you gave three people the task to try measure this, and even with a electronic level, you would easily get three different measurements. It all depends where you hold a flat bottom measuring tool to a curved line.

Would your 8, 12 and 17 degree measurements be close to the curved spline line?- yes they would be. But can they truly be measured in degrees? no.  Draw any line under a curve-spline-circle in CAD and then try to measure the angle between the two, and it will not give you back a measurement at all, simply can't be done.

So I would call any of the cuts on a degree for your top spar cap,  "something that is close, but still outside the spine curve" so you can sand down to what you need.  I believe you have stated this and are good to go.

Best of success!

[Peterross]

Thanks for the added info. You are correct, as the surfaces I measured are curves. I measured the best I could right at the end.
I plan on cutting then sanding to get the desired curve to blend the rib into the spar into the nose rib for a nice spline curve.

[Dan_]

I'll try one more time, as I noticed I said rib when I meant spar.
I measured the angle of the rib top at the leading edge. It is 8 degrees.
I also measured the angle of the rib on the drawing and that too was 8 degrees. The top of the front spar measures at 12 degrees, and the nose rib looks to be about 17 degrees.
So my guess it to add a bevel to the top of the spar at 12 degrees then create a nose piece to properly align with the spar. I can sand or plane a curve to the top of the spar to transition from the rib to the nose rib.
Sorry for the confusion. I did not mean to cut anything on the ribs.

If you must have a number...

Give me the height called out on the plans of the back of the spar (where the ribs attach), give me the height of the front of the spar called out on the plans.  Then give the the thickness of the spar cap.   An angle can be found.    If this is not on the plans give me the height of the rib, the thickness of your d-tube plywood.  Rib height minus plywood thickness will give the height of the back of the spar.  Then give me the height of your nose rib.  The front of the spar must match them.

If you have your spars built --build the ribs to match them.  If you have your ribs built, make the nose ribs --build the back of the spars to match the ribs, build the front of the spars to match the nose ribs accounting for the plywood thickness...  We don't need no stinkin angles...

It's never going to be perfect.  Shoot for perfect, but take what reasonable time constraints, tools and materials will give you.  Measure it with a micrometer, mark it with a crayon, and cut it with an axe...  ;-) 

When you stand back and take in the big picture --it's going to look good.

[Dan_]

According to someone I know who always has the plans handy, Steve...  The angle of the top of the spar is given on page 43 as 10.5 degrees.  

Your original question about the spar height needing to accommodate the plywood is Yes the spar has to be shorter that the rib by 2X plywood thickness so that there is not a "step" where the ply meets the rib at either the top or bottom.  

I would wait on cutting the spar until you make sure the nose rib is going to jive, and when you do cut it leave it proud and sand it in like Scott advised...

Perhaps you can use the 10.5 to draw things out on the workbench for now...

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