Spar webs in wings?

[frank mccune]

     Hi Gang:

     I would like to know if adding spar webs in a wing is worth the trouble.  I have no idea as to what the correct name for these things are but they are pieces of thin balsa glued with the grain going vertical.  These connect the upper spar to the lower spar between the ribs.  I have heard of people adding these to their wings for aded "strength" but how much strength do they add? Would cutting the webs into a triangle shape be strong enough for our uses rather than using a rectangular shape of wood?

      Any thoughts?

                                                                                                                                Stay well,

                                                                                                                                Frank

[Tim Wescott]

I've usually seen them called shear webs.  They do two things:  one thing they do is to keep the upper and lower spars from buckling in toward each other.  This can happen if the ribs don't have the up-and-down compression strength to keep the spars apart; the other thing they do is to add resistance to shear -- meaning the tendency for the top and bottom spars to move spanwise with respect to one another.

The anti-buckling is obvious.  The shear strength is what makes the wing torsionally rigid if the leading edge is also sheeted -- the shear webs complete the tube that's mostly formed by the leading edge sheeting.

[frank mccune]

     HI:

      Thanks for the replies!

      Yes, they are indeed called shear webs.  I can remember these when I was working construction a long time ago.  They appeared in the structural steel that was used in construction.

      The plane that I am  thinking about is the Shark .15.  I built this plane as per the directions and it does not have any "D" tube construction.  It just has two spars, a leading edge, and a trailing edge for strength.

                                                                                                                            Stay well my friends,

                                                                                                                            Frank

[Tim Wescott]

Hey Frank:

It may benefit from shear webbing, but all it will contribute is bending strength in the up/down direction, not any torsional strength.

Leaving out the leading edge sheeting like that makes the plane much more crashworthy -- a crash that would break a sheeted wing in half will just rip up the covering and pop half of the ribs loose at the glue joints, giving you an evening's repair work to do.

[Jim Thomerson]

I have built and flown two Shark 15s, one from kit plans and one from Walter Umland kit.  I put shear webs between the spars and the trailing edge sheeting.  I flew mine with TD 09 and found them very competitive.

[Trostle]

Hey Frank:

It may benefit from shear webbing, but all it will contribute is bending strength in the up/down direction, not any torsional strength.

Actually, placing shear webbing between the main spars and shear webbing between the TE sheeting does contribute significantly to torsional strength.  I do not have any way to "measure" the torsional strength of a wing.  But I do know that as soon as I place those TE shear webs, the wing is noticeably more resistant to twist.  I use firm 1/32" sheet, vertical grain to do this with very little gain in weight and significant increase in strength.  Depending on the design, those TE shear webs are not much more than 1/2" to 3/4" deep.

Keith

[Tim Wescott]

Actually, placing shear webbing between the main spars and shear webbing between the TE sheeting does contribute significantly to torsional strength.  I do not have any way to "measure" the torsional strength of a wing.  But I do know that as soon as I place those TE shear webs, the wing is noticeably more resistant to twist.  I use firm 1/32" sheet, vertical grain to do this with very little gain in weight and significant increase in strength.  Depending on the design, those TE shear webs are not much more than 1/2" to 3/4" deep.

Keith

Are you talking about a wing with leading edge sheeting, or one without?  I was talking about one without.

The shear webs may gang up with the covering to lend torsional strength on a wing without leading edge sheeting, but I don't think they do much on wings without LE sheeting -- those are still floppy for me even after the shear webs are in.

[Trostle]

Are you talking about a wing with leading edge sheeting, or one without?  I was talking about one without.

The shear webs may gang up with the covering to lend torsional strength on a wing without leading edge sheeting, but I don't think they do much on wings without LE sheeting -- those are still floppy for me even after the shear webs are in.

I built an I-Beamer more than 20 years ago and have not built a stunt ship since without leading edge sheeting or the entire wing is sheeted.  On these wings with LE sheeting, there are shear webs between the top and bottom spars.  Likewise, shear webbing between the top and bottom sheeting at the TE.  Same with any all sheet covered wing.  Those TE shear webs noticeably stiffen the wing to resist twist.  If you do not think it does any good, fine, but I am convinced it does and will recommend it to anyone who wants to listen.

Keith

[Tim Wescott]

Those TE shear webs noticeably stiffen the wing to resist twist.  If you do not think it does any good, fine, but I am convinced it does and will recommend it to anyone who wants to listen.

You put words into my mouth.  Yes, if you've got a plane with TE sheeting then TE shear webs would stiffen the wing.

I would not, however, recommend TE shear webs to Frank, because Frank is still crashing planes regularly.  In his case I think he still wants a wing with a Skyray-like or Ringmaster-like structure, where all of the torsional and fore-aft strength shear strength comes from the covering.  Why?  Because with a perfectly rigid, LE and TE-sheeted wing, when you post-hole the airplane, the wing shatters, possibly getting pulverized right at the center where it's least possible to fix.  With a more basic structure the covering takes the brunt of the impact while the rest of the structure is untouched or lightly damaged.  With iron-on covering, this makes repair after a crash much simpler.

Once the pilot is past the stage where they're crashing all the time then yes, by all means, use shear webs on the TE if it has TE sheeting.

[Trostle]

You put words into my mouth.  Yes, if you've got a plane with TE sheeting then TE shear webs would stiffen the wing.

Well, I was not trying to put words into your mouth.

You did state on this thread:

"It may benefit from shear webbing, but all it will contribute is bending strength in the up/down direction, not any torsional strength."

That is misleading in itself and I was just trying to explain that my experience hold that shear webbing does improve the torsional strength of the wing regardless of the hoopla of no LE sheeting or not.   Then you went on to say that you "don't think they do much on wings without LE sheeting."  Have you actually built wings to support your "thinking"?  Without the no small amount of additional explanations, your comments would at best give the wrong impression.

And I will not even get into the discussion of building airplanes to survive a crash.  I strongly subscribe to the concept that CLPA airplanes are to be built for performance.  That means they are built to be structurally sound so that twisting/flexing of the surfaces is held to acceptable limits while keeping weight under some semblance of control.  Given a proven design and properly trimmed, the lighter and stiffer the model, the easier it is to fly and the less chance that problems will be encountered that might result in a crash. 

Keith

[Tim Wescott]

"It may benefit from shear webbing, but all it will contribute is bending strength in the up/down direction, not any torsional strength."

That is misleading in itself and I was just trying to explain that my experience hold that shear webbing does improve the torsional strength of the wing regardless of the hoopla of no LE sheeting or not.   Then you went on to say that you "don't think they do much on wings without LE sheeting."  Have you actually built wings to support your "thinking"?  Without the no small amount of additional explanations, your comments would at best give the wrong impression.

Yes, I have built wings to support my "thinking", as you put it.  When was the last time you build a box-stock Twister wing?

And I will not even get into the discussion of building airplanes to survive a crash.  I strongly subscribe to the concept that CLPA airplanes are to be built for performance.  That means they are built to be structurally sound so that twisting/flexing of the surfaces is held to acceptable limits while keeping weight under some semblance of control.  Given a proven design and properly trimmed, the lighter and stiffer the model, the easier it is to fly and the less chance that problems will be encountered that might result in a crash. 

Clearly you've been flying for a long time, and have forgotten what it's like to be a beginner.  When you're just starting out, it doesn't matter how the plane is built -- you're going to crash.  Even when you're not crashing, your piloting skills simply aren't good enough to benefit from the ultimate in rigid, light, and fragile airplanes.  It's much better for the vast majority of beginners to fly something that is easy to build, rugged in a crash, and quick to repair.  Once they've gotten to the point that they're no longer crashing all the time, then they should build something that's rigid and light, to the limits of their expertise and pocketbook, and enjoy the advantages.

I don't think it's at all reasonable to recommend to someone who will crash that they fly a design that's going to be destroyed, or take a month of rebuilding, every time they hit the dirt nose first.  Not in a world where they could fly a Skyray that takes about an hour to repair after a similar crash.

[Trostle]

Yes, I have built wings to support my "thinking", as you put it.  When was the last time you build a box-stock Twister wing?

I don't think it's at all reasonable to recommend to someone who will crash that they fly a design that's going to be destroyed, or take a month of rebuilding, every time they hit the dirt nose first.  Not in a world where they could fly a Skyray that takes about an hour to repair after a similar crash.

Tim,

This thread was started with a straight forward question about shear webs.  Based on your first response, you gave information that was a bit misleading and I tried to explain that shear webbing added considerable strength with little weight penalty.  Then you defended your statements by qualifying the information you gave in defense of building airplanes that were repairable after a crash which has nothing to do with the original question.   Lighten up.

Keith

[Tim Wescott]

This thread was started with a straight forward question about shear webs.  Based on your first response, you gave information that was a bit misleading and I tried to explain that shear webbing added considerable strength with little weight penalty.  Then you defended your statements by qualifying the information you gave in defense of building airplanes that were repairable after a crash which has nothing to do with the original question.   Lighten up.

What part of "shear webs add torsional rigidity if there is leading edge sheeting" is inaccurate?

Either I'm wrong about what you're trying to say, or you're mistaken.  Saying that shear webs add to torsional rigidity in a wing without sheeting to complete the tube is inaccurate.  Any small increase in torsional rigidity in the structure is still drowned out by the torsional strength of the covering.  Yet you seem to be claiming that a wing with no sheeting will be magically made rigid by the addition of sheer webs -- which simply is not so.

[Tim Wescott]

Then you defended your statements by qualifying the information you gave in defense of building airplanes that were repairable after a crash which has nothing to do with the original question.

My statements about repairs after crash damage was motivated by the fact that I know Frank to still be in the "crashes a lot" stage (he's asking about bent needle valves in another thread), and from personal experience, I don't believe that you want to start building wings with LE sheeting until you've at least gotten to the point where you're only crashing occasionally.

[Trostle]

What part of "shear webs add torsional rigidity if there is leading edge sheeting" is inaccurate?

Either I'm wrong about what you're trying to say, or you're mistaken.  Saying that shear webs add to torsional rigidity in a wing without sheeting to complete the tube is inaccurate.  Any small increase in torsional rigidity in the structure is still drowned out by the torsional strength of the covering.  Yet you seem to be claiming that a wing with no sheeting will be magically made rigid by the addition of sheer webs -- which simply is not so.

I do not think I said anything that  "shear webs add torsional rigidity if there is leading edge sheeting" is inaccurate?

As I mentioned earlier, I  only have experience with one I-Beamer from more than 25 years ago and have no experience or claims about shear webs on wings with no sheeting.   However, the sheeting between the top and bottom "beams" of the I-Beamer serve as shear webbing.  In fact the entire strength of those wings (other than the covering), comes from that internal I-Beam.   However, a wing with no sheeting and some sort of a box spar utilizing vertical shear webs will be stronger (in bending and twist) than a wing without that box spar, regardless if there is LE sheeting, no LE sheeting or a completely sheeted wing.

Maybe your lectures to beginners on building to survive crashes could be taken off line or placed in a different thread.  The question regarding shear webbing has nothing to do with the survival rates of beginners models.

Way too much attention to an off topic subject and sensitivity is being shown here.

Keith

[Bill Johnson]

     Hi Gang:

     I would like to know if adding spar webs in a wing is worth the trouble.  I have no idea as to what the correct name for these things are but they are pieces of thin balsa glued with the grain going vertical.  These connect the upper spar to the lower spar between the ribs.  I have heard of people adding these to their wings for added "strength" but how much strength do they add? Would cutting the webs into a triangle shape be strong enough for our uses rather than using a rectangular shape of wood?

      Any thoughts?

                                                                                                                                Stay well,

                                                                                                                                Frank

Hi, Frank. I'm not an "engineer" but have a couple years experience in structural repairs and modifications so I'll take a stab at this.

Adding shear webs between the upper and lower spars creates an I beam-type spar. This design is much stronger and resistant to compressive load failures. They are marginally more resistant to twisting loads but that is a secondary effect and, as pointed out by other members, dependant on other design features.

The triangular idea would not be beneficial. Assuming a full bay width span on, say the lower spar, would leave a single attach point on the top spar which would probably concentrate stresses, which is what you do not want. So square shears webs, full span within each bay (rib-to-rib). It doesn't take much in the way of material. 1/32 balsa would work, I believe. In some Boeing aircraft, I've worked on spars that, with upper and lower spar beams pretty darn massive, had shear webs as thin as .032". They had a few stiffeners to keep them from bending but as long as they are flat, carry and transmit loads very effectively.

[Brett Buck]

It doesn't take much in the way of material. 1/32 balsa would work, I believe. In some Boeing aircraft, I've worked on spars that, with upper and lower spar beams pretty darn massive, had shear webs as thin as .032". They had a few stiffeners to keep them from bending but as long as they are flat, carry and transmit loads very effectively.

    I have used GM silkspan as shear webs on older built-up models and it was extraordinary how much more rigid it was for no more weight than it added.

    Brett

[Chris Wilson]

I don't really see the wisdom in running the shear webs the full length of the span since it was pointed out to me that there is very little flexing going on near the wing tips.

[Douglas Ames]

If your looking for torsional rigidity, sheet the last bay of the wing next to the tip, and butt it to the aft edge of the LE sheeting.

Shear webs reduce flexing of the spars  --->  under load.
                                                     <---

[Chris Wilson]

If your looking for torsional rigidity, sheet the last bay of the wing next to the tip, and butt it to the aft edge of the LE sheeting.

Shear webs reduce flexing of the spars  --->  under load.
                                                     <---

Hi Doug,
              two things here - how much 'load' is there in the last wing bay?
And if the front lead-out happens to intersect the shear webs near the tip leaving a compromised web, how much does it matter?

My point is that the wing root or the 'armpits' carries far more of the load and the tips next to nothing so knowing that makes me think that placing shear webs past the undercarriage is more of a reflex action - I know that it stiffens the wing more if runs full span but I ain't going to sweat it if a tapered wing that uses consistent material thicknesses misses the last few bays.

Cheers.

[Douglas Ames]

Hi Doug,
              two things here - how much 'load' is there in the last wing bay?
And if the front lead-out happens to intersect the shear webs near the tip leaving a compromised web, how much does it matter?

My point is that the wing root or the 'armpits' carries far more of the load and the tips next to nothing so knowing that makes me think that placing shear webs past the undercarriage is more of a reflex action - I know that it stiffens the wing more if runs full span but I ain't going to sweat it if a tapered wing that uses consistent material thicknesses misses the last few bays.

Cheers.

You misunderstood me. Two different responses.
First one was in regard to wing twist in a previous post regarding LE sheeting, etc.
Second one was reiterating the function of shear webs. I run mine out about 2/3 the span. Yes, I agree. There's no reason to run them full span.

[Howard Rush]

Where's a stress man when you need one?

[Tim Wescott]

Where's a stress man when you need one?

I'm a man, and I'm feeling kinda stressed right now -- does that count?

The part of the ambiguity in this thread that's not covered by people not knowing their mechanical engineering, or not agreeing on terms, is the fact that not everyone is looking for the same sorts of strength in a wing.  So one person's "just strong enough" may be another's "excess structure".

[Howard Rush]

I was just poking you-know-who to comment.  It's sorta an inside joke.

[Chris Wilson]

You misunderstood me. Two different responses.
First one was in regard to wing twist in a previous post regarding LE sheeting, etc.
Second one was reiterating the function of shear webs. I run mine out about 2/3 the span. Yes, I agree. There's no reason to run them full span.

Thanks Doug.