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Tailless Biplane (Thoth)

jpot1

Elite member
#1
I was inspired by the below photo that I saw on Facebook the other day as I like planes that are a bit different (see some of my other plans in my signature link). Built a chuck glider first to find CG and then put this together. No flight video but flew great so I felt comfortable posting the plans.

Inspiration Photo:
IMG_0010.jpg


End Result:
IMG_1730.jpg
 

Attachments

jpot1

Elite member
#2
Build Log:

WIngs:
Start with the bottom wing. Do not cut out the channel for the fuse or the wing joiners (like the Baby Blender build) until the wing is glued. The channel for the fuse will go from LE to TE (dashed line does not go fully there on plans). Remove paper between the leading edge bevel and the blue dashed line in the pdf. Run that section over the edge of the table to get some curve into the foam. Remove foam for the spar notches. "C" fold for trailing edge.

Repeat with the top wing but you only need the notches cutout to receive the top of the wing joiners.

Fuse:
Standard "B" fold. Should slot into the bottom wing, some foam removal may be necessary.
Add bottom plate but leave off top until figuring out where the battery will need to go depending on your motor selection.

Wing Joiners:
Make 2 of each. The piece with the top notches goes on the outside. I used 3M 77 spray glue to glue the pieces together. Dry fit before gluing up.

Canopy:
I followed the cutouts on the side of the fuse last, added the canopy top and then did the fuse top. Using magnets to keep the canopy in place.

Feel free to ask any questions and please post pics if you build it!
 

Pieliker96

Well-known member
#3
I had a go at building it this evening. It was quite pleasant to do so - quick and simple.

20210101_192927_HDR.jpg
Lower wing completed

20210101_195603_HDR.jpg
Upper wing and pylons completed, ready for joining

20210101_200053_HDR.jpg
Fuselage going on

20210101_202130_HDR.jpg
Motor mount. Note the two sheets of foam I've used to space the firewall away from the back of the fuselage

20210101_210810_HDR.jpg
Here's how I did the linkage between control surfaces: Control horns and a small length of pushrod on both surfaces joined with zip-ties to skewers. It's connected well enough to transfer the control movements but can still slip for trim adjustments: I also reflexed the elevons on the upper wing by 3/16" at the trailing edge.

Where should the CG end up?
All-up weight is 400g with Power Pack A and an 850 3s.

Some suggestions for the plans:
-Some sort of cutout in the upper wing skin for the servo connectors
-Markings on the lower wing skin for positioning the servos
-Consider making the wing joiners one piece by attaching their leading edges with a bevel - It would look a bit cleaner but require twice the area on the plans
-Some sort of plate to cover the rear of the fuselage and mount the firewall to
-Consider extending the fuselage >= 3/8" backwards to accommodate the shorter height of motors such as the 1806 for prop clearance reasons

Will report back once I fly it.
 

jpot1

Elite member
#6
@Pieliker96 - wow looks great! On the bottom wing extend the cutouts for the fuse all the way to the leading edge and the fuse will fit more flush. I did my connections in a similar way - I wrap thread around the connection of the bbq skewer and pushrod, a few drops of super glue and then put heat shrink over.

Thx for the plans suggestions. I’ll do some tweaks. I used a bigger motor - 2826 2200 kv so did not need the offset. I need to add specs to the plans and post #1.
 

Pieliker96

Well-known member
#7
@Pieliker96 - wow looks great! On the bottom wing extend the cutouts for the fuse all the way to the leading edge and the fuse will fit more flush. I did my connections in a similar way - I wrap thread around the connection of the bbq skewer and pushrod, a few drops of super glue and then put heat shrink over.

Thx for the plans suggestions. I’ll do some tweaks. I used a bigger motor - 2826 2200 kv so did not need the offset. I need to add specs to the plans and post #1.
Cheers man! Thanks for the build tips. Whereabouts did you find the CG to be? I'll be running it at 4" back from the lower wing LE as per an online CG calculator for the maiden.
 
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Pieliker96

Well-known member
#9
I had a go at flying it today with the CG at 3" back. After I figured out how to launch it (holding it by the upper wing with the motor spooled up, then releasing it seemed to work well), It would gradually diverge to the left in yaw, eventually resulting in one wing stalling and it going in.


It could be from prop torque and the closeness of the vertical surfaces to the CG, but I think it's more likely that I've unintentionally built some toe-out into the pylons which effectively causes yaw instability, since the design has evidently flown fine in the past. To fix this, I've added two strips on the trailing edge of the pylons. They're directed outwards to provide some sort of shuttlecock effect and hopefully stabilize the craft in yaw.

20210102_164510_HDR.jpg
One of the little trailing-edge strips.

I've also dialed in a good bit of differential aileron - this causes the ailerons to deflect farther upwards than downwards, reducing adverse (and possibly creating proverse) yaw. It's sort of weird to do this on a flying wing because it then also affects pitch. We'll see how it does tomorrow.
 

jpot1

Elite member
#10
@Pieliker96 - that looked a bit tail heavy to me as shown by it pitching up at the end of several flights. My motor since it’s larger definitely sticks out farther in the back. Not sure if that is making a difference. I too had the left yaw but it was minor and I just trimmed it out. I remeasured and I would try moving your CG at least 1/4 “ forward.

Pic of my motor setup showing the distance. Unfortunately crappy weather so I won’t get to fly till next weekend but will video then.

142CB231-BE61-47FC-9BC7-E01C1797D1DE.jpeg
 

Pieliker96

Well-known member
#12
Had a successful flight today. It's now stable in yaw. I was able to get a couple circuits in before attempting an immelmann, during which it stalled out and flipped due to the thrustline's location relative to the CG, I had no altitude to recover. This is on me - I'm used to planes with control surfaces in the propwash and higher thrust-to-weight ratios. I've also noticed that a good bit of the prop disk area is blanked by the fuselage, which could easily be solved by moving to a powertrain with a larger prop - Power Pack A is not ideal for this.

Sorry for advance for the lack of contrast - white foamboard on snow is difficult to see
 

Monte.C

Legendary member
#13
Had a successful flight today. It's now stable in yaw. I was able to get a couple circuits in before attempting an immelmann, during which it stalled out and flipped due to the thrustline's location relative to the CG, I had no altitude to recover. This is on me - I'm used to planes with control surfaces in the propwash and higher thrust-to-weight ratios. I've also noticed that a good bit of the prop disk area is blanked by the fuselage, which could easily be solved by moving to a powertrain with a larger prop - Power Pack A is not ideal for this.

Sorry for advance for the lack of contrast - white foamboard on snow is difficult to see
Awesome! Boy that seems to fly slow. And yeah, the A-pack does seem to be working pretty hard to do the job. Love this build.
I think I should be building more quick & dirty experimental builds. Talk is cheap. :unsure:
 

Monte.C

Legendary member
#15
I do like that a lot. I want to see you get it flying smooth & stable. It looks like it'll be so squirrelly in pitch... The control surfaces are so close to the CG... What about a canard? I could see that happening.
 

jpot1

Elite member
#16
I do like that a lot. I want to see you get it flying smooth & stable. It looks like it'll be so squirrelly in pitch... The control surfaces are so close to the CG... What about a canard? I could see that happening.
Something does look like it’s missing from the nose so a canard is possible. However the maiden was smooth not pitchy at all. It flew kind of like a versa wing.