Shear webs

[Robert Zambelli]

Just looking for some opinions.
Are the webs really necessary?
I've seen people use both lead edge and trail edge webs but I cannot remember ever using them myself.
I'm presently building a second generation Chief and there is no indication of webs.

Let's discuss!

Bob Z.

[Dennis Nunes]

Hi Robert,

So far in my relatively short stunt career, I've seen 2 wings sheared off on electric stunt planes. On one electric plane the builder decided he didn't need them and left them out. The other plane had a take-apart wing and failed at the end of the hardware. I have seen Paul Walker and Orestes Hernandez make some incredibly tight turns and I have no idea what forces are put on the wing. Whatever they are it must be high.

I tend to build and fly planes with a high aspect ratio wing and use shear webs out to just past the land gear assembly (wing mounted gear) which would be a little over a 1/3 of each wing panel and use laminated balsa spars with a strip of .007" carbon fiber vertically sandwiched in between. I’ve also used 1/32” thick shear webs along the full length of a sheeted trailing edge. I like the way it stiffens the wing and prevents the trailing from sagging between each rib.

It’s an ugly sight when you hear and see a wing fold in the middle of a flight. Just my two cents, which ain't worth much these days.


Dennis

[Motorman]

There are a few things you can put in there that don't really weigh anything but give you tremendous amount of strength. Shear webs are one of those things.

[Ken Culbertson]

I have never had a wing fold and I have never used shear webs.  I have used "X" braces on the spar and TE but never shear web until now.  My new ship has both spar and TE shear webs.  I am not so sure that they don't contribute to folding.  I might be wrong about this since all of my data is from my own experience.  if you don't have a strong center section and spars that can flex some none of the force on the center joints is absorbed.  Of all of the wings I have seen fail, none of them were double planked center sections.  May have nothing to do with it!

Ken

[Dave Hull]

The use of a continuous shear web in the construction of a wing spar (say two 1/4" square pieces separated by an 1-1/2 or more) makes the two pieces act as one unit, thereby increasing the stiffness and the strength, up to the yield strength of the spar material. The section properties of the spar are a function of the depth of the spar to the 3rd power, but what really drives the strength is concentrating the areas as far apart as possible.

Two separate sticks, say 1/4 by 1/4 that are spaced an 1-1/2 apart mostly have the bending stiffness of---two separate sticks. With a shear web, the math says that you gain much, much more than a factor of two. It's cubic, remember?

The shear web also prevents early buckling of the structure. Without that, you are depending on a few rib joints and the covering to keep the geometry under control. Those joints aren't really optimal for that purpose, but they help until the ribs split along the grain.

All that said, you may want a more flexible spar system on a smaller plane that gets bounced into the grass on frequent figure 9's....

Dave

[Dave Hull]

Ken,

I do not understand your statement that "if you don't have a strong center section and spars that can flex some none of the force on the center joints is absorbed. "  Force isn't really absorbed unless you have damping, or a shock absorber, or.... Forces get transmitted from one place to another, or to a group of places. This path is called the loadpath. This may be the single most important thing to understand when you are designing structures. In general, all the weight of the fuselage (and everything it contains) is reacted into the wing.

Look at the load case from a hard pull-up.  You get a lift distribution spanwise, so the load is not constant. But no matter what (except in planes with struts) you get the maximum bending load (moment load) at the wing root. If the fuse/joint is stiff enough, you could argue that some of that moment gets reacted by the fuse sides, but you'd be better off neglecting that in any calculations. So the max bending load, which needs to be taken up by the spars and the planking happens right in the middle. No surprise there.

What you do not want is a discontinuity in the stiffness/strength of the wing structure. This is a stress concentration, and that usually means it is going to break there. How many times have you seen a wing fail right where the center planking quits?

X-braces attempt to accomplish the same thing as a shear web. They just aren't as efficient in my opinion since the brace in compression will attempt to buckle. So by all means, glue the braces together where they cross. Next, the joint geometry and glue joints need to be good. Pretty common to see the braces (and shear webs too) scabbed on the trailing edge of the spars instead of a web like in a steel I-beam. Not the best structurally, but easier to do.  Most guys find one way of stiffening up the spar system more suitable for their tools, methods, skills. And so it works best for them. As long as there is positive margin in the overall construction, you're good. But using the optimal geometry lets you shave a tiny bit of weight. Maybe not too important in a stunt plane--but crucial in a full-sized one.

Dave

[Bob Hunt]

I would advise to always use shear webs in a CL Stunt wing. In an inside maneuver the top of the wing is in compression and the bottom of the wing is in tensile. Just the opposite in outside maneuvers; the top is in tensile and the bottom is in compression. This forces tend to want to make the top and bottom spars move in opposition; I believe the technical term is "racking." In essence the top and bottom spars want to "clap hands" with each other. Installing vertical grain shear webs between the spars prevents the movement of opposition, and the wing doesn't fold - or more accurately collapse - under cornering loads.

Bob Hunt   

[Ken Culbertson]

I can see that, as usual, I am in the minority.  I will, and have, put shear webs on my latest toy.  What has me puzzled is why we always made sure our sailplane wings would flex and not snap under the load of a zoom launch which puts considerably more stress on a much thinner wing than we do.  Another of life's mysteries.

Thanks for the beatdown, that is how you learn things.

Ken

[Mike Alimov]

I would add that for modern full size stunt ships, balsa spars do not yield enough tensile strength, and I would recommend replacing them with spruce (say, about 1/4" square at the root and tapering towards the tip) or basswood, or with some carbon tow laminated to about 1/3 span. Plain balsa spars were adequate for classic era ships that weighed 40 oz, not today's beasts that are in the 65-70 oz range. Structural failures at this year's Nats were proof of that.
And yes, shear webs are essential.

[Dan McEntee]

I can see that, as usual, I am in the minority.  I will, and have, put shear webs on my latest toy.  What has me puzzled is why we always made sure our sailplane wings would flex and not snap under the load of a zoom launch which puts considerably more stress on a much thinner wing than we do.  Another of life's mysteries.

Thanks for the beatdown, that is how you learn things.

Ken

     If you flew R/C sailplanes, then a lot of the ships you flew with built up wings had shear webs. Look at the photos that Mr. Hunt posted and note the grain direction as it is vertical when installed. The only thing I remember differently about sailplane wing construction was that the webs were glued to the face of the spars for ease of construction. The critical thing was to have them fitted tight against the ribs. The grain direction will still allow for some flexing and that is controlled by the stiffness of the covering and finish. If I am building a model that has them designed into the wing, I sure would not leave them out. If I am building a model that doesn't have them, I would not hesitate to add them if I thought they were needed.
  Type at you later,
   Dan McEntee

[Ken Culbertson]

     If you flew R/C sailplanes, then a lot of the ships you flew with built up wings had shear webs. Look at the photos that Mr. Hunt posted and note the grain direction as it is vertical when installed. The only thing I remember differently about sailplane wing construction was that the webs were glued to the face of the spars for ease of construction. The critical thing was to have them fitted tight against the ribs. The grain direction will still allow for some flexing and that is controlled by the stiffness of the covering and finish. If I am building a model that has them designed into the wing, I sure would not leave them out. If I am building a model that doesn't have them, I would not hesitate to add them if I thought they were needed.
  Type at you later,
   Dan McEntee

You are right to a point.  I checked the plans for one of the planes I flew, a Sagitta 900, and it had a solid spar out to the end of the spoilers using 1/4" in-between 1/4" spruce spars.  About 1/2 of the wing.  The rest was open.  Most of the others were not kits, at least not at the time.  I think my Pantera had the same spar structure.  The rest were foam.

Ken

[Brett Buck]

You are right to a point.  I checked the plans for one of the planes I flew, a Sagitta 900, and it had a solid spar out to the end of the spoilers using 1/4" in-between 1/4" spruce spars.  About 1/2 of the wing.  The rest was open.  Most of the others were not kits, at least not at the time.  I think my Pantera had the same spar structure.  The rest were foam.

Ken

  There's a difference between "stiff" and "tough/strong".

      Brett

[Dennis Adamisin]

There is not much doubt that shear webs will greatly increase the strength and stiffness of the wing spar system. But there are other benefits,  If you build a C-tube wing in a jig, it can still warp when you take it off the jig.  If you add shear webs while it is still on the jig, it will stay straight.  (Of course if it was crooked in the jig THAT will be preserved too!)

[Ken Culbertson]

I am going to give in.  Hands Up - Don't Shoot!  I am convinced, you need shear webs.  I have never used them except out to the LG.  I turn some pretty tight corners and I have never had a wing fold.  Brett has it right.  Stiff and strong are two different things.  I have always felt that putting a small vertical strip on the ribs to keep the center from compressing was enough and having clearance for leadouts precludes even that on much of the inboard wing.  Since I have never had a failure, it must have been enough, but, I am going to use them going forward.

Ken

[John Park]

There is not much doubt that shear webs will greatly increase the strength and stiffness of the wing spar system. But there are other benefits,  If you build a C-tube wing in a jig, it can still warp when you take it off the jig.  If you add shear webs while it is still on the jig, it will stay straight.  (Of course if it was crooked in the jig THAT will be preserved too!)

Precisely.  Surely the point of shear webs between top and bottom mainspars is to create a full D-tube LE structure, in order to promote torsional stiffness and resist warping.  Given the thick wing sections we use in stunt, I don't see the value of shear webs for any other purpose.

[Ken Culbertson]

Precisely.  Surely the point of shear webs between top and bottom main spars is to create a full D-tube LE structure, in order to promote torsional stiffness and resist warping.  Given the thick wing sections we use in stunt, I don't see the value of shear webs for any other purpose.

You have hit on why I have probably never had a wing, on anything, fold in 60+ years of doing this.  Even in the 60's my designs had 2 1/4 to 2 1/2" thick wings.  I use noting but straight grained wood in the wing, stiffer and larger TE and diagonal braces from the spar to the TE.  Using those braces keep the wing from warping and provide a gap for the leadouts on the inboard.  They provide some crush strength but not much.  Finding enough wood to build this way is getting harder with today's balsa situation.  I am not arguing, even a tiny bit, against webs.  Maybe I have just been lucky.  I am using them on my next plane which will have my first "geo" wing.   1000 words (pix) attached.

Ken

[Ara Dedekian]

   the webs were glued to the face of the spars

    That's how I install them in my builds. Is there a benefit to installing them between the spars as in Bob H's photos?

     Ara

[Sean McEntee]

    That's how I install them in my builds. Is there a benefit to installing them between the spars as in Bob H's photos?

     Ara

      I've built in sheer webs on models using both methods.  The facing method is a bit faster and easier, while the "sandwich" method calls for more attention to fit.  However, I imagine the latter is stronger.  Alot of modern FF gas and electric models call for vertical grain balsa installation between spars for the same reason as in CL stunt.  Wings on powered FF models endure alot of stress during launch and the result is the same if the wings aren't built stiff enough to handle it.

[Bob Hunt]

   the webs were glued to the face of the spars

    That's how I install them in my builds. Is there a benefit to installing them between the spars as in Bob H's photos?

     Ara

Hi Ara:

Yes, there is a good reason to install them between the spars and the ribs.

I fit my shear webs to be a slight to moderate press fit between the spars and the ribs. This allows me to set each wing panel (and I make them in halves and then join them...) in its respective lower cradle half in the Lost-Foam system and check for any warps. As Dennis mentioned, and I totally agree, this allows you to ensure that there are no warps in the panel before you glue the shear webs in place. The snug fit of the shear webs "holds" any twist adjustments you make to the panel. It's easy to set the wing panel in the cradle, look for any warps (rocking of the panel root to tip, and front to back). If there is a warp, lift the panel and twist it slightly in the direction required to remove the warp, and then check it again (and again, and again, and again...). Sometime it takes several tries to get the panel to set perfectly in the cradle. When it is perfectly warp free, it is a simple matter to very carefully lift the panel from the cradle and, using thin CA glue applied with plastic tubing, wick some glue into the joints between the first shear web and the adjacent spars and ribs. Then set the panel back in the cradle to ensure that you have not let anything "shift" when you picked in up. If everything is still good, then wick glue into all the shear web/spar/rib joints. Trust me, that panel will never warp in the future. And, as Dennis pointed out, if you have allowed a warp to be in the panel and have glued the shear webs in place, that warp is there for the duration as well.

This, along with the benefit of the shear webs not allowing the upper and lower spars move in opposition to each other, they yield a warp free and very rigid wing indeed.

I have a photo illustrated manual that depicts the entire process of making a Lost-Foam wing, and there is a very detailed section in that manual dealing with the fabrication and installation of the shear webs. That manual is free (as are all my building manuals), and if you would care to have a copy, please send me a PM with your email address. I'll send along all my manuals in PDF form.   

Later - Bob Hunt 

[Lauri Malila]

[quote author=Ken Culbertson
  What has me puzzled is why we always made sure our sailplane wings would flex and not snap under the load of a zoom launch which puts considerably more stress on a much thinner wing than we do.  Another of life's mysteries.
Ken
[/quote]

Not really a mystery. I have made thousands of launches with otherwise identical gliders, just with spars made from different modulus of elasticity carbon. Results are quite clear; stiffer spar = higher launch. Also, a model with stiffer wing (or stiffer everything) behaves in a more predictable and logical way. L

[Ken Culbertson]

[quote author=Ken Culbertson
  What has me puzzled is why we always made sure our sailplane wings would flex and not snap under the load of a zoom launch which puts considerably more stress on a much thinner wing than we do.  Another of life's mysteries.
Ken


Not really a mystery. I have made thousands of launches with otherwise identical gliders, just with spars made from different modulus of elasticity carbon. Results are quite clear; stiffer spar = higher launch. Also, a model with stiffer wing (or stiffer everything) behaves in a more predictable and logical way. L

I would agree with your assessment 100% but in 1982, my last year flying sailplanes they were almost all wood.  Locally, a few were starting to experiment with carbon, fiberglass and foam.  The "zoom" launch we used was more of a springboard with flapping wings.  If the wing was too stiff it would bend the joiner rod.  My last plane was a Pantera which pretty much tells you how long ago it was.

Ken

[Lauri Malila]

I would agree with your assessment 100% but in 1982, my last year flying sailplanes they were almost all wood.  Locally, a few were starting to experiment with carbon, fiberglass and foam.  The "zoom" launch we used was more of a springboard with flapping wings.  If the wing was too stiff it would bend the joiner rod.  My last plane was a Pantera which pretty much tells you how long ago it was.

Ken

It was also an era without accurate altimeters, that can easily prove that there is no height gain from this ”springboard” effect. L

[Dan Berry]

      I've built in sheer webs on models using both methods.  The facing method is a bit faster and easier, while the "sandwich" method calls for more attention to fit.  However, I imagine the latter is stronger.  Alot of modern FF gas and electric models call for vertical grain balsa installation between spars for the same reason as in CL stunt.  Wings on powered FF models endure alot of stress during launch and the result is the same if the wings aren't built stiff enough to handle it.

They endure greater stress on the DT. The stab when it pops and the wing when the plane slams the ground. That's where the shear webs help the FF planes.

[Ken Culbertson]

It was also an era without accurate altimeters, that can easily prove that there is no height gain from this ”springboard” effect. L

I can only speak for local fliers back then but if it didn't work, why did everybody who could use it?

[Mike Griffin]

Guess I will weigh in on this for what it is worth (probably nothing).  I have built wings with and without shear webs.  I can't remember ever putting them in the trailing edge if it consists of two pieces of 1/16 that sandwich the trailing end of the ribs between them.

When I did put shear webs in, it was on the leading edge and I just face glued them to the side of the spars which transformed the C leading edge to a D leading edge. By the way, I usually use spruce spars.  I never tried cutting shear webs to size and slipping them in between the top and bottom spar, I just face glued them to the spars. 

I never had a wing fold on me for lack of shear webs or any other reason , however I would think they would make the wing more rigid and certainly could not hurt anything by using them. 

Sometimes I think we get a little carried away trying to over engineer something and by doing so make matters worse and certainly we can all a lot of unwanted weight by doing certain things that may not be necessary. 

All that being said, I tend to build smaller models using less power than most of the modern era stunters that are using PA.75 engines and the like. 

Mike

[Robert Zambelli]

WOW!
I had no idea that my simple query would generate so much useful information, not to mention so much controversy!

Thanks to all - I’m enlightened.

Bob Z.

[Steve Helmick]

I've used spar webs since I was 14 or 15 years old. Personally, wouldn't consider the use of diagonals instead. IF I did, I would glue diagonals to both sides of the spars, forming X's...no doubt a lot heavier.

The webs keep the spars in their assigned places, AND keeps them from shifting spanwise, relative to each other.  Steve

[Dave_Trible]

I usually put webs in about every other bay for the forward spars,  either none, or all bays for the trailing edge sheeting.  There I'm more concerned about warp-resistance.  It should give enough rigidity and cut the weight.  I've had two wing failures-both at Team Trials ironically.  One was a take-apart airplane where the assembly hardware tore lose from it's mounting to the wood.  Also I will usually have a 1/4" carbon tube spar fore and another toward the trailing edge extending out past the landing gear to help support the wing in hard maneuvers.  On that one I discovered I had removed those tubes and they weren't in the wing when it blew off.   The other was a fully sheeted wing and my first use of dots of white glue to stick the sheeting to the ribs.  When that one let go it had about 300 flights.  Most all the glue dot joints had broken leaving the sheeting largely unattached to the wing ribs inside.  Back to Sigment!  I didn't use any shear webs on this, figuring the full sheeting WAS enough to prevent flexing....

Dave