Supporting/Strengthening a polyhedral wing built flat
12-09-2013, 08:22 AM
#1
Thread Starter
My Feedback: (12)
Join Date: Mar 2011
Location: Agawam, MA
Posts: 145
Likes: 0
Received 0 Likes
on
0 Posts
Supporting/Strengthening a polyhedral wing built flat
Hi All,
I have a Sagitta 900 kit and was thinking of building the wing flat and adding ailerons and flaps. I am looking for ideas or succesful similar build pictures when connecting the outer panels to the inner panels. I am assuming I will have to change the ribs at the polyhedral break to match the inner wing ribs, but am more concerned about strengthening the joint between the panels and spars. Is epoxy alone going to do the job? Any suggestions welcome, and of course any pictures will get a gold star!
Thanks!
I have a Sagitta 900 kit and was thinking of building the wing flat and adding ailerons and flaps. I am looking for ideas or succesful similar build pictures when connecting the outer panels to the inner panels. I am assuming I will have to change the ribs at the polyhedral break to match the inner wing ribs, but am more concerned about strengthening the joint between the panels and spars. Is epoxy alone going to do the job? Any suggestions welcome, and of course any pictures will get a gold star!
Thanks!
12-09-2013, 05:33 PM
#2
Fill the adjacent spar bays with verticle grain balsa to the full width of the spar and use 1/16 ply in front and rear as shear webs . If you look at Skybench .com , there's a bunch of pictures and text that are real good at making those kind of high stress joints easy and foolproof . A picture is worth at least a thousand words .
Don't forget the washout in the outer panel .
Don't forget the washout in the outer panel .
12-09-2013, 09:48 PM
#3
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,425
Likes: 0
Received 22 Likes
on
19 Posts
Due to the change in the planform you will need a break at the point where the wing tips taper due to the change in thickness. But simply using epoxy is NOT good enough. Although you are not including any dihedral you still want to use the dihedral bracing to join the spars across this joint. It's just that the joiners will be straight instead of "V" shaped. Otherwise just build the wing with the modifications needed for the flaps and ailerons.
12-10-2013, 05:32 AM
#4
Thread Starter
My Feedback: (12)
Join Date: Mar 2011
Location: Agawam, MA
Posts: 145
Likes: 0
Received 0 Likes
on
0 Posts
Thanks JetMaven and BMatthews for your quick responses. BMatthews, I didn't mean to imply I would just be using epoxy, but you did remind me about the joiners. I was wondering if it would make any difference if I used single one-piece spars from the center section to the tip (sanding down the height from the break to the tip) rather than trying to butt-glue the center to tip section (even if using joiners)?
Thanks again both of you again for your quick responses and I look forward to anyone else who would like to make suggestions.
Thanks again both of you again for your quick responses and I look forward to anyone else who would like to make suggestions.
12-10-2013, 09:29 AM
#5
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,425
Likes: 0
Received 22 Likes
on
19 Posts
Single spars would actually be the best way.
What you can do is slot the ribs to take the proper spar size from root to tip. At the panel break rib sand the slot just a hair deeper. Like the thickness of two sheets of printer paper. This will let the spars bend smoothly over the transition. Then glue on a couple of patches of paper to "wedge" the upper surface sheet back out to where it should be for the proper airfoil shape. Also if you set up TWO building boards with just enough dihedral to allow for the thickness tapering of the tip you then only need to bend one spar. The other can stay straight.
Keeping the top spar straight would look a little nicer since you would not have a break line on the top surface at the panel break. But it would be simpler to make the bottom spar straight and bend the top since you only need a flat building surface. Your choice really since the super slight amount of dihedral this implies is just not going to be enough to notice.
I'd go with the two boards propped at an angle myself. But then I normally build over two boards with the dihedral angle jigged up so I can build across the dihedral breaks all at one time. So for me and my methods angled boards is easy peasy.
What you can do is slot the ribs to take the proper spar size from root to tip. At the panel break rib sand the slot just a hair deeper. Like the thickness of two sheets of printer paper. This will let the spars bend smoothly over the transition. Then glue on a couple of patches of paper to "wedge" the upper surface sheet back out to where it should be for the proper airfoil shape. Also if you set up TWO building boards with just enough dihedral to allow for the thickness tapering of the tip you then only need to bend one spar. The other can stay straight.
Keeping the top spar straight would look a little nicer since you would not have a break line on the top surface at the panel break. But it would be simpler to make the bottom spar straight and bend the top since you only need a flat building surface. Your choice really since the super slight amount of dihedral this implies is just not going to be enough to notice.
I'd go with the two boards propped at an angle myself. But then I normally build over two boards with the dihedral angle jigged up so I can build across the dihedral breaks all at one time. So for me and my methods angled boards is easy peasy.
12-10-2013, 11:45 AM
#6
Thread Starter
My Feedback: (12)
Join Date: Mar 2011
Location: Agawam, MA
Posts: 145
Likes: 0
Received 0 Likes
on
0 Posts
Is a bend necessary or can this be a straight spar center section-to-tip? I was thinking a straight spar with possibly (or not) 1-2 degrees dihedral from the center. I agree a straight spar across the top is better cosmetically. Do I even need center dihedral? I understand this will not fly like a RES bird.
12-12-2013, 10:02 AM
#8
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,425
Likes: 0
Received 22 Likes
on
19 Posts
With the Saggita wing you have a straight constant thickness inner panel and a tapered tip panel. The tip panel tapers both in chord and thickness. So while slight it means that one or both of the spar caps would need to bend at the transition point.
If using carbon for the upper and lower caps instead of spruce then you would have the option of stacking layers of thinner carbon strip so that the bend at the transition can be a tighter radius to the point where you would require little or no filler to make the shape nice and sharp.
Why does the spar need to curve inward you ask? (I'm assuming you are wondering) That's because if you don't let the curve move in slightly then the spar material will bulge out to either side of the bend beyond the desired line. Which is why I'm saying to ease the slot in the rib at the transition to allow the spar cap to curve inward so that when it returns to straight it's at a tangent to the desired shape instead of bulging out and then back inward again. But by letting the spar cap curve in slightly we technically need to add some filler to extend the straight lines to either side up to the transition line. That make sense?
Having flown some models with no dihedral and some with some dihedral I'm a fan of keeping some dihedral. With the angle provided by the orginal center line joint of the Saggita you'll get just enough dihedral that it'll aid with having a more groovy and self compensating model in turns. That means it'll reduce the amount of "top aileron" you require to hold the model from wanting to drop into a spiral dive. A little dihedral will also make the model self level the wings without needing any pilot input when flying way out at speck like distances. That'll make the flight path a little more efficient by avoiding long term unwanted side slipping. And finally it also aids in keeping the wing tips from snagging in the grass during landings.
And having some dihedral will also balance the big fin and rudder area which will also be trying to drive your model into a spiral dive. Which raises another good point. The vertical tail volume coefficient is a factor in achieving the optimum amount of spiral stability. Too little means that the model does not track well and tends to side slip with the tail low in turns. But too much vertical area tries to tip the model off into a spiral dive unless corrected. But polyhedral models tend to have oversize vertical tails both to aid in balancing the side area of the wing and provide the model with good tracking in the turns as well as allow for big oversize rudders to generate the yaw angles desirable for RES flying. But when the tip panels are lowered to be inline with the inner panels the tail area now becomes far too big and issues with wanting to tighten the turn becomes a factor.
This increases the work load on the pilot by demanding that some top aileron be held all through the turns. It also reduces the efficiency of the model by adding drag due to the constant surface deflection.
All in all I'd suggest that you will want to cut down the rudder or both rudder and fin by about 25 to 30% to bring the vertical tail volume coefficient in line with the new flatter wing.
If using carbon for the upper and lower caps instead of spruce then you would have the option of stacking layers of thinner carbon strip so that the bend at the transition can be a tighter radius to the point where you would require little or no filler to make the shape nice and sharp.
Why does the spar need to curve inward you ask? (I'm assuming you are wondering) That's because if you don't let the curve move in slightly then the spar material will bulge out to either side of the bend beyond the desired line. Which is why I'm saying to ease the slot in the rib at the transition to allow the spar cap to curve inward so that when it returns to straight it's at a tangent to the desired shape instead of bulging out and then back inward again. But by letting the spar cap curve in slightly we technically need to add some filler to extend the straight lines to either side up to the transition line. That make sense?
Having flown some models with no dihedral and some with some dihedral I'm a fan of keeping some dihedral. With the angle provided by the orginal center line joint of the Saggita you'll get just enough dihedral that it'll aid with having a more groovy and self compensating model in turns. That means it'll reduce the amount of "top aileron" you require to hold the model from wanting to drop into a spiral dive. A little dihedral will also make the model self level the wings without needing any pilot input when flying way out at speck like distances. That'll make the flight path a little more efficient by avoiding long term unwanted side slipping. And finally it also aids in keeping the wing tips from snagging in the grass during landings.
And having some dihedral will also balance the big fin and rudder area which will also be trying to drive your model into a spiral dive. Which raises another good point. The vertical tail volume coefficient is a factor in achieving the optimum amount of spiral stability. Too little means that the model does not track well and tends to side slip with the tail low in turns. But too much vertical area tries to tip the model off into a spiral dive unless corrected. But polyhedral models tend to have oversize vertical tails both to aid in balancing the side area of the wing and provide the model with good tracking in the turns as well as allow for big oversize rudders to generate the yaw angles desirable for RES flying. But when the tip panels are lowered to be inline with the inner panels the tail area now becomes far too big and issues with wanting to tighten the turn becomes a factor.
This increases the work load on the pilot by demanding that some top aileron be held all through the turns. It also reduces the efficiency of the model by adding drag due to the constant surface deflection.
All in all I'd suggest that you will want to cut down the rudder or both rudder and fin by about 25 to 30% to bring the vertical tail volume coefficient in line with the new flatter wing.
