kargo

OK Gang, a question after fighting with the "factory cut" shear webs on my 4 star 40 kit. The 4 star has shear webs between the bottom and top front spars. Middle of the bottom spar and middle of the top. I have had an enourmasly difficult time getting these things to fit nicely. I need a good wood to wood joint for strength and wind up with gaps everywhere... Maybe there is a tool or some technique I don't know to get these things to true up (or at least as true as my model is thus far)... I trouble with this on my LT-40 too. So far as buidling experience goes I've built the LT-40, a HOB Decathlon, and am part way through a Kadet Senior.

Ok so here is my question. The instructions for the kadet senior my son and I are building show the shear webs glued to the edge/aircraft front of the spars. Could I do this with any future model that has them in the middle of the top/bottom spars or is this a bad idea?

Thanks in advance

Tony Hallo

Many planes have the shear webs glued to the face of the spar, provides for more glue surface. The edge glued joint is the weakest joint, if you have the equipment you can route grooves in the spars that allows the shear web to fit inside, this allows for some error and additional glue surface. However I would simply glue to front or back as you suggested.

krayzc-RCU

My Feedback: (31)

man i fogot these things once on a build about 8 years ago. The plane held up without them until i banked and yaked it and crash close to the ground. I use to fly back then with only the right stick and that left stick was always buried at full throttle

foodstick

try going over the glue joints with some gorilla glue/ultimate glue, sort of paint it in the joint lightly with a disposable brush, q-tip/It will expand and fill the joint very strong and light, usually not pretty. Try not to get it anywhere you will have to sand it off, its tough...Keep an eye on this type of glue when first using it. It tends to have a life of its own. I LOVE the stuff! especially for wreck repair and arf REGLUING when new...



P.S. almost all kits had the webbing on the back of the spars back when I started buildinggrain running up and down, I think its the easiest way ....

carrellh

Most Great Planes kits, and my Tower Hobbies Uproar 60, called for gluing the shear webs to the front or back of the spars. The exception was my PT-60 and it had grooves running the length of the spars to build an I beam structure.

kargo

Thanks guys, I'm probably stuck with the in the middle thing for now as I got the shear webs that way on the right wing. Although it would be an interesting experiment to do it on the outside on the left one and see which wing fails first I can't believe I just said that...

I'll try the gurilla glue, I used epoxy to fill the gaps this time, very heavy... Its what I did with the LT-40 and I'm positive 20% plus of the weight on that airplane is E-P-O-X-Y!

jrcaster

If you are talking about the gaps between the shear webs and the ribs, don't bother filling them in. All you are going to gain there is weight.

Campgems

I've built two 4*60's and the shear web/main spar fit was not a problem. In fact, I've coppied that wing design onto three other planes without problems. The main thing to keep in mind is that even with the laser cut parts, there are nubs that need to be sanded off to keep things right. The shear web/main spar should fit tightly between the upper and lower spars. Check for extra material on the tabs ( where the laser jumps a bit to keep the sheet one piece) and sand as necessary. The design of the 4* series is very good and the parts can almost be assembled without any glue, only the covering holding things together. I wouldn't try it, but things do fit very well, right out of the box. On my first, I had problems with applying to much pressure on the top spar near the root rib, colapsing the shear web/main spar. I ended up having to shave down the dhideral brace to fit as I put to much preasure on the top spar prior to gluing (CA) it down. My second went much better given the lesson of the first build.

Sand things to fit prior to applying adheasive, and then don't over clamp them.

Don

Broken Wings

My Feedback: (20)

Perma-Grit makes a spar slotter. http://www.bvmjets.com/Pages/Catalog/permagrit.htm

I make a shallow slit on the spar using the table saw.

It is a pain trying to get them even...

Champ-RCU

My Feedback: (132)

My 2cents.

DON'T SLOT ANY SPAR!!! THE MATERIAL YOU REMOVE DECREASES IT'S STRENGTH.

The spar slot tools from perma grit are not to slot the spar but the ribs were the spar fit into the rib.

Mark

TFF

An ok fit is well, ok. Gluing on the outside is ok too. The webs job is to transfer the stresses from the most stressed spar to the least. When glueing outside the forces are having to go through a glue joint in shear. Not eligant and only as strong as a good joint, but most of out planes are over designed and understressed; a full scale plane doing what we do would have a G meter and the pilot would have to fly under the limits or loose a wing we do what we want without worry. Inbetween the spars, the glue is really there to keep it from popping out; the stresses are transfered directly by contact although enhansed by glue.

MinnFlyer

My Feedback: (4)

You can easily glue to the front or back of the spars - even on the left wing after doing it the other way on the right wing.

The only thing to be careful of is at the root - be sure you don't add a web that will interfere with the wing joiner.

kargo

Thanks for all the input guys... It must be something I'm doing incorrectly. When I get the shear web in, it always seems to show daylight at the bottom where it touches the bottom spar. The left and right on the bottom of the shear web touches the bottom spar, but there will be a gap in the middle. The instructions allways have you (sig) glue the things in and then trim the tops (shear webs) so the top spar will fit. That seems like it would make it difficult to get a true edge on the top of the shear web... It seems like it should be a lot easier. The top spar on the 4*40 is tilted backward to match the counture of the rib, something I didn't take into account before, so maybe shear webs on front and/or back of the spars won't work as well as I thought. Thanks for the advice from experience Minnflyer, its good to know where the "edge of the envelope" is. Maybe I'll head down to Menards and look for some tools to help me with making these things a little more "true."

Thanks again

Campgems

How flat is your build table? Touching on the ends and air gap in the center sounds like your table isn't flat and you are fighting a bow in the wing. It is very dificult to build a wing flat on a bowed table. The

I've found that if you select them carefully, the aluminum door jams at the hardware make a great straight edge. The selection is needed because they get bent and when I picked up my last on, I went through about 15 pieces before finding one that was straight. You also need to drill the stamped in contersink holes as they will keep it from laying flat.

Once you have your straight edge, using it on edge, check corner to corner to detect any bow in the table. Two checks, back left to front right and then front left to back right will tell you if you table is OK. You should be able to trap two sheets of printer paper the full length of the straight edge.

You can also use your straight edge to check the shearweb/spar center. Don't worry if the spars themself are not flat, most sticks will have some bending to them. Just select an upper and lower that have very close to the same bend the flip one over so the bends are fighting each other. You can even pull some bow out of the shear web/beam center. If your table is flat, then the bend is pulled out of the bottom spar and when you put the top spar over the shear web, it will complete the I beam and ends up very flat.

Don

MinnFlyer

My Feedback: (4)

The way I do it is as follows:

1) Lay the web material against the spars and mark the top with a pen or pencil

2) cut the web slightly smaller than the mark. This will ensure that the top of the web will be below the top surface of the spar so it won't need to be sanded down for covering or sheeting.

3) run a bead of glue on the top spar, and the bottom of the web. This prevents glue on the bottom spar from running down onto the plans, and leaves the top of the web free of glue so you can hold it without gluing your fingers to it.

4) lay the bottom of the web against the bottom spar, and rotate the top in place.

Works like a charm!

Gray Beard

My Feedback: (-1)

Here is a question that I have wondered about for a while. I build most of my planes from plans and mostly older designs. I was taught years ago that if you sheet a wing you can forget about the shear webs because the wing gets it's strength from the sheeting. On almost all of my older plans any wing that is sheeted there are no webs shown. Just wondering if anyone else has noticed this and what are there thoughts?? I have never had a wing fail but when building this is always at the back of my mind.
Gene

qldviking

I have noticed the same, but I always put a web in, even if its only 1/16 balsa. I am paranoid about the fitting of webs too, I make sure they fit nicely all 4 sides and glue them in well. Properly fitted webs will stiffen the wing up noticably.

w8ye

My Feedback: (16)

Very true but I have seen both in a wing.

The wings I've seen fail was a Four Star where the guy was being foolish and the wing breaks just where the center sheeting ends. Four Stars have webbing

The other plane was a Avistar were the guy was diving as fast as it would go and then pulling up and the wing broke at the dihedral brace.

squeakalong

My Feedback: (8)

Hi kargo,

If you, or anyone else, are interested here is where I explained how I have made shear webs over the years for my RC model aircraft:

Go to: Vintage & Antique RC
Go to: Vintage style airplane project What is it?
Go to: Posts 19, 21, 26 & 27 (has pictures)

Hope this helps.

Joe

vmsguy

I have to agree with Mike's "Shear webbing in 4 easy steps" a few posts back..

I would only add a couple things.

1. Try to get glue on the ribs too. I use a Q-tip to spread glue.. Then again I build almost exclusively with yellow wood glue.

2. Clamp, clamp, clamp... ANY glue joint will be stronger if you clamp it while it dries. I have a few dozen of the smaller quick-clamps. The plastic ones available at your neighborhood Menard's, Home Depot, Lowe's are work wonderfully.


If the shear webs in the plan aren't the right size, don't be afraid to make new ones. Or try different webs from the kit, there could be differences in cutting that some pieces are slightly bigger/smaller than others.

Personally, I wouldn't use gorilla glue or any other glue as a filler. If I was worried about the gap, I'd take scrap, maybe 1/8" square, and as long as needed, and glue into the corners between the web and the rib. It'd probably take 5 minutes or so to glue/fix the entire wing.

MinnFlyer

My Feedback: (4)

Well, I'm not an engineer, but the definition of "shear" (at least the one we're interested in) is "to become fractured along a plane as a result of forces acting parallel to the plane", or in other words, two parallel planes moving in different directions.

In this case, the two planes are the top and bottom of the wing (Or the top and bottom spar)

As pressure lifts the wing, it bends it (or at least, it TRIES to bend it). As anyone who has bent a phone book can tell you, the inner and outer cover move at different rates and slide along the pages between them. This is what the two spars are trying to do - and WILL do unless we prevent it.

Imagine building a square out of 1/8" sticks. Now "Shear" two sides of the square. It doesn't take much force to break the joints and move them. But add a diagonal brace and it becomes MUCH stronger (This is why we build diagonal braces into built-up items like stabs or stick fuselages)

Add a top or bottom to the square with sheeting and it's stronger yet.

This is the purpose of shear webs - To keep the top and bottom spar from shearing.

w8ye

My Feedback: (16)

Some kits do not have nor specify tight fitting shear webbing. The webbing only covers about 80% of the space between the ribs

bruce88123

The "C" beam shown in post #5 is better than the "I" beam method due to increased glue area. In steel girders it must be remembered that the vertical shear web is an integral part of the beam when it is formed and NOT an attached piece. "I" beam composite floor joists are used in some housing but they are carefully assembled to demanding engineering specifications.

Rodney

Minni flyer has given you some very good advice in his above posts. His methods work very well and are easy to implement. Having a good glue joint between the top and bottom spars is quite important. What the shear webs do is prevent the buckling of the spars (the forces are at a 45 degree angle to the span) where the one in compression tends to cave inward under stress. The 45 degree angle is why it makes no difference if the webbing is vertical or horizontal in grain, both are equally strong for doing the job that shear webs are meant to do. That is why the X bracing shown in his last post does nearly as good a job as a fully sheeted shear web, better under some conditions.

Campgems

Mike, the four star doesn't really have shear webs, at least the vertical grain ones, but rather a built up I beam. Note the schetch on opening post on this topic. On the 4*60, the center of the beam is a laser cut piece with knotches for the ribs. They egg crate together. The build sequence is to lay down the lower spar, pin the beam over it, place the ribs on the beam and CA the ribs to the beam, and the beam to the lower spar. Then the upper spar is installed and CAed to the beam and ribs.

As I read his opening question, and his follow on, he is having trouble fitting the parts and as a result, there are some air gaps between the center of the beam and the spars. This would either be caused by a work table that is not flat, or the beam center not being cut straight.

I think this topic has taken off on a different corse than answering why he is having problems, although there has been a lot of good information shared.

Don