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XL Smoothie (twin tail boom plane with extended simple cub wing) AKA Big Bird

#1
So recently, Ive been loving twin boom pusher designs and after the FT SMOOTHIE, Ive decided to use the simple cub wing as a starting point. It has a really nice rigid but light feel (box spar I guess) and has plenty of lift. It was easy enough to extend the plans because of its rectangular shape (just roughly copied and pasted). I plan to either make it 1.5 or 2 m (1.5 of I use 2 sheets, or 2m or even 2.25m if I use 3 sheets) still not sure if I'd need the extra 50-75cm but Id just like to have a really long wingspan bigger is always better right haha. Obviously I'd need to reinforce the wing with spars of some sort. Just not too sure of how much power I'd need. Also I think I might need another way of attaching the fuselage to the wing with all the weight in the nose (rubber bands might not be strong enough). Simple Cub XXL 2,25m wing full-01.jpeg
 
#2
So finally the day has come to begin work on this monster. I have glued on the plans and cut the wing out. I am using 4 35cm lengths of 4mm carbon tube in the centre to strengthen the spar. They are strong but also flex enough for the dihedral. Wingspan should be around 2.2m tip to tip, (I know, stupid right?) Pictured below is the original simple cub wing on top of the new wing. my greatest concern is the strength of the dihedral but I don't see this big boy doing any loops anytime soon so I think I will be ok. IMG_20190709_213512.jpg
 

quorneng

Well-known member
#3
duckduckgoose
Just remember that with twin boom carrying the tail and a fuselage with 'weight in the nose' the wing centre section will be under significant torsion so has to be built to handle it. The outboard wing panels will not have the same problem.
 
#4
duckduckgoose
Just remember that with twin boom carrying the tail and a fuselage with 'weight in the nose' the wing centre section will be under significant torsion so has to be built to handle it. The outboard wing panels will not have the same problem.
Good point, I might add a thicker aluminium spar in the centre or next to the FB box spar. Will see how it feels. Only Challenge is the dihedral, hence the flexible rods. Perhaps a small strip of ply?
 

L Edge

Active member
#5
Good point, I might add a thicker aluminium spar in the centre or next to the FB box spar. Will see how it feels. Only Challenge is the dihedral, hence the flexible rods. Perhaps a small strip of ply?
With a dihedral, used a very thin 1/64 basswood glued on top and bottom of wing that was 4" wide. It's like fiberglassing a wing.Some other things to watch is keeping the length of your booms so that your CG is proper. Position of the battery,esc, motor,prop etc have to be way up front. You might have to add a rudder to make it turn. Mine always fly in a nose-up attitude.
 
#6
With a dihedral, used a very thin 1/64 basswood glued on top and bottom of wing that was 4" wide. It's like fiberglassing a wing.Some other things to watch is keeping the length of your booms so that your CG is proper. Position of the battery,esc, motor,prop etc have to be way up front. You might have to add a rudder to make it turn. Mine always fly in a nose-up attitude.
Yeah I use a pretty long nose to get the CG right. I'm planning on (eventually) using a 5000mah 4s which weighs about 500g. I have been using ailerons only for my twin booms so far, but rudders might be a good idea for a plane this size. I might look into mixing rudder and ailerons together.
 
#7
carbon spars glued in (4 spread in the centre) and 5mm bamboo squewers on the outer joints as well as a heap of bamboo squewers in a zigzag pattern hopefully to combat torque on the wing centre. In hindsight I'm not sure if they would really help but I think there is enough strength as well as flex (for the dihedral). I think I'm ready to move onto glueing the wing creases.
P.S I need a longer work table!
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#9
Because the wing is so long there is no way my hot glue gun would be able to deliver enough glue let alone me being able to do it fast enough so I am using a combination of gorilla glue and hot glue, the hot glue sections should secure it in position while the gorilla glue dries
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#11
Dihedral done. Added two strips of 3mm ply to strengthen the dihedral and overall structure. It feels really solid now. It just needs ailerons. Not too sure how large to make them, probably just from the start of the outer section to the wingtip?. Onto the fuselage and tail.
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#12
Tail parts cut out and ready to assemble. Upscaled the vertical stabs by 150 % mostly because I thought it looked a little small. Might add rudder but it might add unwanted weight to the tail. Still figuring out the best tail boom length (stability vs tail weight). Might make it adjustable so I can slide it back if needed. IMG_20190712_215501.jpg
 
#13
Fuselage done. Starting to look like an airplane now. I've got to cut an access flap Into the top of the nose and I think I need to add some sort of skid plate to the front half at least (I don't have nice grassy lawns to land on, only stony gravel/dirt if I'm lucky, and rough tuffs of grass if I'm not). Might even add some sort of skids like a seaplane on the wing to help protect the wing tips on landing. Might try design a more solid wing/fuselage mount, because I'm having doubts about rubber bands keeping it straight, any ideas/methods?. I'm going back to work for a week so I'll have some time to think and design. But so far I'm fairly happy with it's progress. Explorer for size reference IMG_20190713_212246.jpg IMG_20190713_205541-01.jpeg
 
#14
Finally put some time into getting this thing done, and it's really coming together. Tail section is done (tiny trainer wing for scale) and will be attached with tubes glued under the wing. This allows adjustments to be made if the cog is too far back. Tubes will be reinforced with more FB. Still need to add ailerons to the wing and mounting rods to the fuselage. A quick 'hold together' test with the battery in the nose tells me the plane should balance without too many issues. I have built a long launching ramp out of steel pipe (from a nearby dump) and intended to make a sled out of foam to launch the plane with (to keep it centred on the rails). I also have rubber surgical tubing if I need a slingshot launch. Hopefully I'll have it near finished tomorrow (y)
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#16
Big Bird, as I am now calling it, is nearly ready for maiden. I can't say I am optimistic about it though, I just don't think the motor (SS 2216 1250kv spinning a 9x4.5) will have enough power to lift it (1.8+kg), although it does have a huge wingspan so perhaps I am wrong. If it needs more power, I have decided I will add a tractor setup on the front, which should give it plenty (I have a few 1250 kvs lying around). I added another layer of FB to the sides of the fuselage and a strip of 3mm ply to the bottom of the nose for gravel landings. It makes my explorer look small.
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#18
Maiden day! First attempt off ramp was unsuccessful, due to low airspeed and the tail resting on the ramp, causing it to nose down as soon as half the plane left the ramp (which I was half expecting). So it looks like the ramp option is off the table. Luckily I have a heavy duty welding glove I can use as protection which slides off easily so I can throw the plane then flick my hand and begin steering. Might give it some slight up trim to buy me time to get the glove off. Second attempt coming soon. IMG_20190731_111946.jpg
 
#19
Update: BIG BIRD HAS LEFT THE NEST! Successful maiden achieved on second throw (first throw failed because I used a switch to hold the elevator in a slightly up position which in the excitement I forgot to switch off) but no damage was found. Second attempt flew very nicely. I only did 2 circles and landed but it was enough to get a feel. One thing I might fix is there is a slight oscillation in the tail which is due to the booms being a little rattley in the wing mounts. Some hot glue will probably fix this. As it turned out, there was a good amount of thrust, so much that I could keep it at 1/2 - 3/4 throttle. Video coming shortly
 
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