shear webs
01-21-2009 | 05:12 PM
#27
Senior Member
RE: shear webs
This last weekend, I was cleaning out some junk from my shop and included was a 4*60 wing that was damaged. I finally decided not to try to repair it, and instead did a little off the cuff stress testing. I blocked up one end and stepped dead center. It took a fair amount of weight before it snapped. THis is after being damaged in a crash, having the LE jamed back about 1" where the the wing attaches to the fuselage, so the wing was already structualy damaged there. I then took a half wing and bridged it over the trash container, about a 24" span. I had to hammer it three or four times with my fist before it finally broke in half. I was susprised at how tough it was. I first leaned into it trying to push it in half, but that didn't work.
Wing strength has not been a problem with the 4*'s even though I saw one fail in flight. I found later that it had been crashed and repaired more than once, and it also had several hundred flights, so it was at EOL anyway.
Don
Wing strength has not been a problem with the 4*'s even though I saw one fail in flight. I found later that it had been crashed and repaired more than once, and it also had several hundred flights, so it was at EOL anyway.
Don
ORIGINAL: w8ye
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.
ORIGINAL: Gray Beard
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
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
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.
01-21-2009 | 06:04 PM
#28
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From: Woodville, WI
RE: shear webs
ORIGINAL: Campgems
[snip]
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.
[snip]
[snip]
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.
[snip]
And to answer it, I'd probably advise.. Abandon trying to make the web pieces work.. bust 'em out, make new ones, gluing onto either the front or the rear of the spars (per Mike's drawings) and call it good enough...
The wing will be much stronger because of the glue joint and surface area... Added weight will be.. oh what.. a couple ounces??? Trying to fill in all the gaps with glue or epoxy would've added that.. easy..
The "built up I-Beam" mentioned in other posts is OK, but the side shear webbing as shown in Mike's posts is even stronger, with very, very little weight gain. A piece of 1/16, or 3/32 balsa is a couple bucks.. A couple hours time...
Do it the right way.. it'll be worth the effort.
NOW.. having said all that... if there's consistently a gap between the spars and the webs.. There's probably a reason.. Like the spars not sitting "deep enough" into the ribs, because there may have been cutting waste still on the bottom of the spar saddle of the rib... Thus not allowing the spar to sit deep enough..
01-21-2009 | 07:15 PM
#29
My Feedback: (20)
RE: shear webs
ORIGINAL: Champ-RCU
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
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
Gee, I wonder why they call them Spar Slotters?
Spar Slotters -
Perma-Grit has re-introduced these very useful spar slotting tools in 3 convenient sizes.
These 6" long steel bars have the Tungsten Carbide course grit bonded to one side allowing neat, true slots to be cut into plywood, balsa or composite formers and ribs. Two sizes available, SP9C has been discontinued.
3mm or 1/8" #3c
6mm or 1/4" #6c
This guy knows a little bit about building models airplanes and he does the same thing...
http://www.airfieldmodels.com/inform...raft_wings.htm
A good read... http://www.auf.asn.au/scratchbuilder/beams.html
01-22-2009 | 10:05 AM
#30
RE: shear webs
ORIGINAL: Broken Wings
I'll throw a nickles worth in.
Gee, I wonder why they call them Spar Slotters?
Spar Slotters -
Perma-Grit has re-introduced these very useful spar slotting tools in 3 convenient sizes.
These 6" long steel bars have the Tungsten Carbide course grit bonded to one side allowing neat, true slots to be cut into plywood, balsa or composite formers and ribs. Two sizes available, SP9C has been discontinued.
3mm or 1/8" #3c
6mm or 1/4" #6c
This guy knows a little bit about building models airplanes and he does the same thing...
http://www.airfieldmodels.com/inform...raft_wings.htm
A good read... http://www.auf.asn.au/scratchbuilder/beams.html
ORIGINAL: Champ-RCU
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
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
Gee, I wonder why they call them Spar Slotters?
Spar Slotters -
Perma-Grit has re-introduced these very useful spar slotting tools in 3 convenient sizes.
These 6" long steel bars have the Tungsten Carbide course grit bonded to one side allowing neat, true slots to be cut into plywood, balsa or composite formers and ribs. Two sizes available, SP9C has been discontinued.
3mm or 1/8" #3c
6mm or 1/4" #6c
This guy knows a little bit about building models airplanes and he does the same thing...
http://www.airfieldmodels.com/inform...raft_wings.htm
A good read... http://www.auf.asn.au/scratchbuilder/beams.html
01-22-2009 | 10:40 AM
#31
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From: Mt. Pleasant,
PA
RE: shear webs
Brokenwings,
Do what you want. However removing material from a spar for any reason decreases it's cross section therefore decreases it's strength.
Bruce88123 nailed it 100%.
Mark
Do what you want. However removing material from a spar for any reason decreases it's cross section therefore decreases it's strength.
Bruce88123 nailed it 100%.
Mark
01-22-2009 | 11:18 AM
#32
My Feedback: (20)
RE: shear webs
ORIGINAL: Champ-RCU
Brokenwings,
Do what you want. However removing material from a spar for any reason decreases it's cross section therefore decreases it's strength.
Bruce88123 nailed it 100%.
Mark
Brokenwings,
Do what you want. However removing material from a spar for any reason decreases it's cross section therefore decreases it's strength.
Bruce88123 nailed it 100%.
Mark
I will continue to slot my spar(s) as I have never had a failure.
01-22-2009 | 12:49 PM
#33
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From: Ellsworth,
WI
RE: shear webs
wow did I start some flaim bait There is more than one right way to do something and everything has its pro's and con's. I appreciate all the advice. I think I need work on my technique for making the shear webbing true enough to not have gaps. A trip to menards for some tools, either parts to make or store bought.
So far as the building board flattness goes I did check it with a long metal ruller on edge. There was a gap in the middle of the ruler that a micrometer would have difficulty measuring. Just enough to let the table length ruler move across the builing board a little more easily than at the outside edges where the ruler was firmly touching the board. I don't know if this is too much or not. I build on homasote (avail menards, etc) which is afixed to 3/4 ply by double sided tape (sig recomendation in their instruction books. I have too old office desks, one wood one metal that are about as flat as you could hope for, one wood table thats not (bows in the middle, I use the ply to make it straight), and the a makeshift table out of hip height shelves (also with the 3/4 ply setup). It seems like all the homasote buiding boards have that imperseptable "sag" in the middle. Not perfectly flat but very close to it. Without building on metal with magnets or on glass, I don't know how I'd get any flatter. That said I'm open to suggestions[8D].
The shearing explenation was great. Can I make this work when the spars are not perfectly parallel to oneanother? The top spar follows the counture of the rib so it is at a slight angle compared to the bottom spar. Does it matter if the shear webbing has a slight bow to it? How about grain direction? Again, I probably just need some more practice, but I am a "keep it simple" kinda guy
. Thanks again. Is info on shear webs like this in the beginers forum? It might help some other folks too.
There is more than one right way to do something and everything has its pro's and con's. I appreciate all the advice. I think I need work on my technique for making the shear webbing true enough to not have gaps. A trip to menards for some tools, either parts to make or store bought.So far as the building board flattness goes I did check it with a long metal ruller on edge. There was a gap in the middle of the ruler that a micrometer would have difficulty measuring. Just enough to let the table length ruler move across the builing board a little more easily than at the outside edges where the ruler was firmly touching the board. I don't know if this is too much or not. I build on homasote (avail menards, etc) which is afixed to 3/4 ply by double sided tape (sig recomendation in their instruction books. I have too old office desks, one wood one metal that are about as flat as you could hope for, one wood table thats not (bows in the middle, I use the ply to make it straight), and the a makeshift table out of hip height shelves (also with the 3/4 ply setup). It seems like all the homasote buiding boards have that imperseptable "sag" in the middle. Not perfectly flat but very close to it. Without building on metal with magnets or on glass, I don't know how I'd get any flatter. That said I'm open to suggestions[8D].
The shearing explenation was great. Can I make this work when the spars are not perfectly parallel to oneanother? The top spar follows the counture of the rib so it is at a slight angle compared to the bottom spar. Does it matter if the shear webbing has a slight bow to it? How about grain direction? Again, I probably just need some more practice, but I am a "keep it simple" kinda guy
. Thanks again. Is info on shear webs like this in the beginers forum? It might help some other folks too.
01-22-2009 | 04:03 PM
#36
Senior Member
RE: shear webs
You started a productive thread.
Your table is OK. To get it exact, you would need a huge granate surface plate.
The fact that the upper and lower spars don't line up and are tilted with relation to each other is probably why Sig designed as they did. It would be very dificult to use true shear webs with them as they would have very little contact with the sides of the spars, and if you bend them to fit, they would have to be S shapped. The compromise was to sort of fit the beam center in the center of the spars. When I used this method on my last wing which was a full symetrical airfoil, I lapped the beam center up on the sides of the spars, not like Sig did, but more like the more examples shown. One other thing, I don't use CA here as it has almost no gap filling ability and even then it is very brittle. I use Titebond carpenters glue. It dries out and leaves a nice fillet on both sides of the joint. It is also quite tough. I do thin it by adding about 10% watter to get better penetration of the wood.
Your best strength would come with the webs grain vertial. On my wing I was just talking about, I though about adding a 1/16 x 1/4 strip across grain on each side of a rib and one more in the center. My rib spacing is very wide, around 5" on a 60" wing. I didn't do it and I've yet to fly the new wing, so I'm hopping my decession was right.
Don
Your table is OK. To get it exact, you would need a huge granate surface plate.
The fact that the upper and lower spars don't line up and are tilted with relation to each other is probably why Sig designed as they did. It would be very dificult to use true shear webs with them as they would have very little contact with the sides of the spars, and if you bend them to fit, they would have to be S shapped. The compromise was to sort of fit the beam center in the center of the spars. When I used this method on my last wing which was a full symetrical airfoil, I lapped the beam center up on the sides of the spars, not like Sig did, but more like the more examples shown. One other thing, I don't use CA here as it has almost no gap filling ability and even then it is very brittle. I use Titebond carpenters glue. It dries out and leaves a nice fillet on both sides of the joint. It is also quite tough. I do thin it by adding about 10% watter to get better penetration of the wood.
Your best strength would come with the webs grain vertial. On my wing I was just talking about, I though about adding a 1/16 x 1/4 strip across grain on each side of a rib and one more in the center. My rib spacing is very wide, around 5" on a 60" wing. I didn't do it and I've yet to fly the new wing, so I'm hopping my decession was right.
Don
ORIGINAL: kargo
wow did I start some flaim bait There is more than one right way to do something and everything has its pro's and con's. I appreciate all the advice. I think I need work on my technique for making the shear webbing true enough to not have gaps. A trip to menards for some tools, either parts to make or store bought.
So far as the building board flattness goes I did check it with a long metal ruller on edge. There was a gap in the middle of the ruler that a micrometer would have difficulty measuring. Just enough to let the table length ruler move across the builing board a little more easily than at the outside edges where the ruler was firmly touching the board. I don't know if this is too much or not. I build on homasote (avail menards, etc) which is afixed to 3/4 ply by double sided tape (sig recomendation in their instruction books. I have too old office desks, one wood one metal that are about as flat as you could hope for, one wood table thats not (bows in the middle, I use the ply to make it straight), and the a makeshift table out of hip height shelves (also with the 3/4 ply setup). It seems like all the homasote buiding boards have that imperseptable "sag" in the middle. Not perfectly flat but very close to it. Without building on metal with magnets or on glass, I don't know how I'd get any flatter. That said I'm open to suggestions[8D].
The shearing explenation was great. Can I make this work when the spars are not perfectly parallel to oneanother? The top spar follows the counture of the rib so it is at a slight angle compared to the bottom spar. Does it matter if the shear webbing has a slight bow to it? How about grain direction? Again, I probably just need some more practice, but I am a "keep it simple" kinda guy. Thanks again. Is info on shear webs like this in the beginers forum? It might help some other folks too.
wow did I start some flaim bait
There is more than one right way to do something and everything has its pro's and con's. I appreciate all the advice. I think I need work on my technique for making the shear webbing true enough to not have gaps. A trip to menards for some tools, either parts to make or store bought.So far as the building board flattness goes I did check it with a long metal ruller on edge. There was a gap in the middle of the ruler that a micrometer would have difficulty measuring. Just enough to let the table length ruler move across the builing board a little more easily than at the outside edges where the ruler was firmly touching the board. I don't know if this is too much or not. I build on homasote (avail menards, etc) which is afixed to 3/4 ply by double sided tape (sig recomendation in their instruction books. I have too old office desks, one wood one metal that are about as flat as you could hope for, one wood table thats not (bows in the middle, I use the ply to make it straight), and the a makeshift table out of hip height shelves (also with the 3/4 ply setup). It seems like all the homasote buiding boards have that imperseptable "sag" in the middle. Not perfectly flat but very close to it. Without building on metal with magnets or on glass, I don't know how I'd get any flatter. That said I'm open to suggestions[8D].
The shearing explenation was great. Can I make this work when the spars are not perfectly parallel to oneanother? The top spar follows the counture of the rib so it is at a slight angle compared to the bottom spar. Does it matter if the shear webbing has a slight bow to it? How about grain direction? Again, I probably just need some more practice, but I am a "keep it simple" kinda guy
. Thanks again. Is info on shear webs like this in the beginers forum? It might help some other folks too.
