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Can't trim out roll (sailplane) SOLVED

duckduckgoose

Well-known member
#1
TLDR: powered glider keeps rolling either way and can't be trimmed out
20210224_183739-02.jpeg

BACKGROUND: Hi, been a long time since I've been in here but I have a rolling issue with my latest build, a twin boom sailplane. The plane has an upscaled tiny trainer wing (lengthened and flaps installed as well) and has a wingspan just shy of 1900mm with a slight dihedral. I am basically rebuilding a previous twin boom that was hugely successful.

THE ISSUE:So the issue is that I can't seem to trim out a rolling tendency that was completely absent from the previous build. The previous plane was so stable, that many times, I could put the controller down once the flaps were engaged.and just watch it go around in lazy circles (until I lost track and it 'wandered' into the woods). This plane starts off straight, then starts leaning to the left or right, and just keeps rolling tighter and tighter as it turns. I have tried trimming but it seems to just then roll the other way, there seems to be no middle ground. Should I be adding polyhedral wing tips? Does anybody know what might be going on? The tail booms are the same length as the old one, and the vertical fins are slightly larger to compensate for the slightly longer wing.
Thanks for any feedback!
 
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#2
TLDR: powered glider keeps rolling either way and can't be trimmed out
View attachment 193515
BACKGROUND: Hi, been a long time since I've been in here but I have a rolling issue with my latest build, a twin boom sailplane. The plane has an upscaled tiny trainer wing (lengthened and flaps installed as well) and has a wingspan just shy of 1900mm with a slight dihedral. I am basically rebuilding a previous twin boom that was hugely successful.

THE ISSUE:So the issue is that I can't seem to trim out a rolling tendency that was completely absent from the previous build. The previous plane was so stable, that many times, I could put the controller down once the flaps were engaged.and just watch it go around in lazy circles (until I lost track and it 'wandered' into the woods). This plane starts off straight, then starts leaning to the left or right, and just keeps rolling tighter and tighter as it turns. I have tried trimming but it seems to just then roll the other way, there seems to be no middle ground. Should I be adding polyhedral wing tips? Does anybody know what might be going on? The tail booms are the same length as the old one, and the vertical fins are slightly larger to compensate for the slightly longer wing.
Thanks for any feedback!
Dihedral, my friend. Dihedral.
(in case you don't know what it is, basically it makes the plane not roll in flight.)
 

duckduckgoose

Well-known member
#6
Looking at it now, I think you may be right about the dihedral. The booms or the rubber bands mounting it might be flattening the dihedral out if it (There was a dihedral before I added the tail booms). I might try adding a polyhedral bend out towards the wing tips as there is a carbon spar inside the center section of the wing.


20210225_011117.jpg
 
#8
Looking at it now, I think you may be right about the dihedral. The booms or the rubber bands mounting it might be flattening the dihedral out if it (There was a dihedral before I added the tail booms). I might try adding a polyhedral bend out towards the wing tips as there is a carbon spar inside the center section of the wing.


View attachment 193542
Yes you need much more dihedral than you have. the wingtips should be 1-1.5 cm or 0.4-1 inches higher than the root of the wing.
assuming p = polyhedral and d = dihedral and s = square inches of wing and f = sucsessful flight, then
if (d*s)>(p*s) then f.
Basically
degrees of dihedral times square inches covering the dihedral area should be bigger than degrees of polyhedral times square inches of polyhedral area.
 

duckduckgoose

Well-known member
#9
If you don't have ailerons you need dihedral. For a high wing 5° is good enough. You can cut the wing and rise the outer panels.
It has ailerons and flaps. But for some reason, I can't seem to trim out the rolling tendency, but yeah more dihedral should probably do the trick, thanks.
 

duckduckgoose

Well-known member
#10
T
Yes you need much more dihedral than you have. the wingtips should be 1-1.5 cm or 0.4-1 inches higher than the root of the wing.
assuming p = polyhedral and d = dihedral and s = square inches of wing and f = sucsessful flight, then
if (d*s)>(p*s) then f.
Basically
degrees of dihedral times square inches covering the dihedral area should be bigger than degrees of polyhedral times square inches of polyhedral area.[/QUO
t
Yes you need much more dihedral than you have. the wingtips should be 1-1.5 cm or 0.4-1 inches higher than the root of the wing.
assuming p = polyhedral and d = dihedral and s = square inches of wing and f = sucsessful flight, then
if (d*s)>(p*s) then f.
Basically
degrees of dihedral times square inches covering the dihedral area should be bigger than degrees of polyhedral times square inches of polyhedral area.
Thanks, I'll make some adjustments and let you know how it goes.
 

Piotrsko

Elite member
#11
I'm going in a different direction here, but either im getting waaayy too old or something in the picture looked crooked, like the left boom/rudder. Based on the flight description, sounds like a leading edge droops a wee bit on the side it turns to.. or you have sloppy aileron flaps. Never mind, the left wing has a different droop in the front on picture. if you can live with the nasty yaw before the turn starts, you can survive flat wings. Or you can add tip plates, or do a really fast tight loop... and stop using 4 ea #64 bands to hold the wing on you need # 32. #64's like you have will crush the wing.
 

duckduckgoose

Well-known member
#12
I'm going in a different direction here, but either im getting waaayy too old or something in the picture looked crooked, like the left boom/rudder. Based on the flight description, sounds like a leading edge droops a wee bit on the side it turns to.. or you have sloppy aileron flaps. Never mind, the left wing has a different droop in the front on picture. if you can live with the nasty yaw before the turn starts, you can survive flat wings. Or you can add tip plates, or do a really fast tight loop... and stop using 4 ea #64 bands to hold the wing on you need # 32. #64's like you have will crush the wing.
I think the fuselage was a bit off centre from the last landing, or it could be the wide angle Lens of my phone camera. The wing droop sounds about right, I might have to experiment with polyhedral or try adding some wing tips. The rubber bands do seem a bit tight stretched over a big wing, but so far I've never had too many issues, I just put some popsicle sticks on there to spread the load. I'll have to get onto those #32s thanks!
 

PsyBorg

Wake up! Time to fly!
#13
I'm going in a different direction here, but either im getting waaayy too old or something in the picture looked crooked, like the left boom/rudder. Based on the flight description, sounds like a leading edge droops a wee bit on the side it turns to.. or you have sloppy aileron flaps. Never mind, the left wing has a different droop in the front on picture. if you can live with the nasty yaw before the turn starts, you can survive flat wings. Or you can add tip plates, or do a really fast tight loop... and stop using 4 ea #64 bands to hold the wing on you need # 32. #64's like you have will crush the wing.
I with that something looks off. To me it looks like the leading edge for the right wing points straight or even up but the left wing is pointing down.
 

duckduckgoose

Well-known member
#14
I with that something looks off. To me it looks like the leading edge for the right wing points straight or even up but the left wing is pointing down.
Here's another picture with I in a table(I was trying to hold it one handed so the photo wasn't the best. I think it drops down both sides the same?

20210225_074954-01.jpeg
 

Piotrsko

Elite member
#15
Dont look the same to me, or its a really bad fisheye lense. Put it on a table that's been checked and known plumb just like in the picture. Take a straight edge big enough to span the wing cross section (aka chord) and use a level to see if the straight edge on the other wing is plumb also. You can also use a doorway, but those may not be plumb. Dont got a level?? There's a free phone app for that to download. If they aren't level, you need to tweak one so they are level then add some diagonal tape to keep them the same.
 
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PsyBorg

Wake up! Time to fly!
#16
Here's another picture with I in a table(I was trying to hold it one handed so the photo wasn't the best. I think it drops down both sides the same?

View attachment 193596
Something still looks a tad off with the inboard part of the left wing that is not symmetrical with the right. I know there are issues with dihedral as you stated and the wing is flatter then expected after mounting.. Anyways I dont know I am not there live to see it and pictures do skew may things perspective wise without super expensive special lens.

To me it looks like the right side has a slight saggy bow to it but the other side is more straight but has a twist or something close to the inboard side.. Again old eyes and pictures... good luck figuring it all out. Quads are more my thing.
 

duckduckgoose

Well-known member
#17
It's hard to show in photos but they have a gull wing type sag. I think the camera angle was too high and too close to the right wing, but thanks for the help nonetheless. Its possible that I may be able to 3d print a sort of 'tent pole' type pillar to stick in the centre, then use string,or wire to pull the booms (and with them the wings) upward and toward the centre to try and regain the dihedral.
 

duckduckgoose

Well-known member
#18
It's not the prettiest solution but it did the trick. Flies quite a bit more stable now (though it's a bit gusty at the moment so I can't be sure). The strings pull the wings together an push the centre down, giving it a bit more dihedral. The knots are all CA glued so they should stay good for a while at least.
20210225_140512.jpg


20210225_140516-01.jpeg


BEFORE

20210225_074954-01.jpeg

AND AFTER
20210225_141220-01.jpeg
 
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duckduckgoose

Well-known member
#19
Decided to go all in and bend up the wing tips a bit as well, and I am seeing a big improvement on stability. It was a bit windy when I flew and it did try to turn a bit, but this was just the wind. Once turned around, it leveled out and then went with the wind. Video below
20210225_164900-01.jpeg