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75% FT Goblin (strix nano goblin sized)

JUSS10

I like Biplanes
#1
I had posted this on the FB page yesterday and it there was quite a bit of interest. This isn't exactly a 75% FT Goblin. I made a few tweaks and changes to make it fit on a single sheet of foam. Builds pretty much the same as the FT goblin (or really any other FT plane). A few notes. I prefer to install the servos after the wing is slid in, but not glued in place. I added a small hatch on the bottom that can be opened to help with wire routing. Also I put marks on the spar as I cut the spares in half where the servos go after the spars in glued in but before I do the wing fold over. My intent was to build this to spec based on the strix nano goblin. I have a 1407 3500kv motor on order and plan to build a 2s li ion pack from some cells. AUW of the strix is 250g ready to go. I weighted this with all my electronics sitting on it and it was just over 200g. I have not flown this but I have built. I have a local flying buddy who is going to try and fly the first one I built while I wait for my electronics. I would think up to an 1806 would work well with this plane on a smaller 3s lipo.

Justin
 

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JUSS10

I like Biplanes
#3
no clue, I just scaled it down. Shouldn't have an issue balancing it in the same spot as the full size. I plan to get one all together in the next day or so.
 

JUSS10

I like Biplanes
#8
wow, that looked pretty great! I have mine built but haven't had a chance to maiden yet so its awesome to see it fly. A local flying friend built one and has run a few packs through his as well. glad to see it built ok for you!
 
#9
wow, that looked pretty great! I have mine built but haven't had a chance to maiden yet so its awesome to see it fly. A local flying friend built one and has run a few packs through his as well. glad to see it built ok for you!
Yes and thanks for the modified plans!! went together very smoothly!
 
#10
I made some 3D printed parts to mount a micro FPV camera in the nose of mine. I have yet to actually put a VTX in it yet since I am only flying with a short range park flyer receiver at the moment. I will finish the FPV install when I put a longer range one in it.
 

CapnBry

Well-known member
#12
Oh man thanks so much for publishing these plans. I lost my regular FT Goblin last week, was looking to build something similar, and have a 1407 3500kv motor so this was a perfect fit. You made it exactly to my specs too-- I didn't want to put a slide-on nose on a smaller version of this model and I was very happy from the moment I opened the plans.

Went together without any problems although I need to pick up some spraypaint and velcro to hold the battery and receiver down. Even better, you made it with plenty of room inside for my junk! Just a full-sized receiver and battery right now, but will replace with a micro receiver, add a flight controller and an AIO camera if it is as fun to fly as the regular goblin was. 75% of the size of the old FT goblin, but 45% of the weight somehow. I peeled the paper off the inside of the wings (except the elevons) and they're still super stiff, although after NAILING the 45 degree bevel freehand on one side, I made a real mess of the other side.

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Airframe itself (foam, tape, and glue only): 68g
All up weight including battery: 175g
Cobra Gold 1407 3500KV
3x4x3 propeller (185g thrust, 4.2A)
EMAX 12A ESC with 1A BEC
Battery 850mA 2S
2x 4.3g generic eBay servos
ABS 3D printed firewall and servo horns

The ESC I smoked on an older plane and repaired it, but now it no longer beeps so it was fantastically difficult to reset its settings. It may not be long for this world still.
 

JUSS10

I like Biplanes
#13
that looks great! Excited to see a few people are using these plans.

I did finally get my 1407 motor in and a small 12a esc. I just need to finish building mine and give it a shot.
 

CapnBry

Well-known member
#14
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Tried to do all the gold in one coat and got a little carried away on the nose! I think my ESC is half toast because the motor is much louder than a RS2205 on 3S. I tried hooking it to the computer to verify the blheli configuration but it refused to communicate as well. It still spins the motor though (most of the time) and I'd hate to have to remove it to replace it so we'll just ride it out until it dies mid-flight and catches fire or something.

The weather here is awful but I couldn't wait any longer. It is blowing over 10 knots, gusting 15-18 but at least it stopped raining. The little teensy plank in that wind was a real handful, but the fun kind of handful. A gust of wind would come and the Goblin would shoot straight up and suddenly just be flying a couple of meters higher or turning smoothly then quickly cut into the turn. I started with the CG in the recommended position but it seemed too far forward. I kept moving it back and eventually went way too far (especially considering the conditions) and it turned into a flip monster and crashed it. The nose accordioned in a little, but no damage that needs fixing. NICE! I ended up with the CG at 22-23mm back from the leading edge. I feel the same way about the full size FT Goblin as well though, that its suggested CG is way too far forward.

That said, it handled really well given the conditions. It flew down to 40% throttle and was still controllable but the wind really messed with it at low speeds. When I cranked it up, it accelerated in an instant and zipped from one side of the field to the other. I had my aileron rates down at 70% and still could roll it wicked quick. Definitely a lot of fun to be had here, and given that it goes together even quicker than the FT Goblin, and is only one sheet for a whole plane, these plans should be getting more attention.

I did forget to mention that I actually did run into a small problem with the build in that my servos are thinner than the thickness of the foam so the control horn hole ended up even with the wing surface which would allow no throw. I shimmed the servo with a 1.6mm piece of corrugated cardboard which seems to be plenty. That gave the servo a little "floatyness" in the mount until I covered it with a piece of tape. Is there a better way to shim servos up higher? I just sort of looked around where I was standing and scanned for something of the appropriate size because a layer of foam was too much.
 

CapnBry

Well-known member
#15
This thing is also crazy efficient on battery. I flew around on the 1407 3500KV, 3x5.2x3 prop, and a 2S 800mAh battery that I think wasn't even fully charged. I crashed and smashed the nose half off at the 11 minute mark (6mins TH%) and was still at 3.80V/cell so I probably had another 3-4 minutes left on it. I'll put a little reinforcing strip of plastic where the fuselage meets the wing like I did on the full-sized FT Goblin the next time I build one (this one will fly again after a little hot glue) but I think this is a much more durable configuration. I think my favorite CG position is 19-20mm now though, after flying it on a nice calm day. 25mm was flyable but really too agile for my skills.
 

CapnBry

Well-known member
#16
I lost my plane :( It flew around the backside of a stand of trees and didn't come out the other side. No problem, I'll just walk around and fish it out of the tree or maybe finding it in the nearby pond. NOPE, completely vanished without a trace. I spent 30 minutes staring into the backs of the dozen or so trees but couldn't see it anywhere. I also spent another 30 minutes wandering around the area and still no dice. I started to wonder if these things float if they hit the water doing 20-30MPH because maybe it sunk into the pond before I could walk over there? I know there are FliteTest float planes but what if the plane went in nose-first?

Because this design is so quick to go together and I already had the plans cut out, I just made another one. Just most the wing and fuselage out of some scrap, and then filled it with coins in a ziplock to make it weigh the same. Even got the CG right! Total build time was 8 songs on Spotify, including the time to weigh out the coins and tie a string through it.
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And yeah it floats just fine. I threw it as hard as I could into the middle of a pond and let it sit there for 5 minutes. The nose flap bit came off but the rest of the plane stayed in surprisingly good shape. I pulled it in and gave it another hard landing in the pond for good measure, but my bag of coins fell out which will now be preserved for generations to come. When someone finds it in 2379 they're going to wonder "Why did the penny exist if it is almost literally worthless and cost more than a penny to make?" I'm not sure either, FutureMan. I'm not sure either.

float-plank.jpg
 

CapnBry

Well-known member
#17
I flew another plane into the Bermuda Tree Triangle today, about chest-high, and when I went to retrieve it it had vanished. I thought maybe I had shot through underneath the lowest limbs so I walked around back and starting poking around from that side. One spider to the face later, a glint of bright blue caught my eye. It was Princess Plank, the 75% Goblin from above that I lost over a month ago in that stand of trees.
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The inside was growing moldy and the foam board pieces just came off in my hand as I opened it up. The battery inside was toast obviously, but I hooked it up to a bench power supply and the receiver fired right up and connected. Upped the current a little bit and the servos tested out too. Turned up the power again and armed the motor... CONTACT! It spun right up. It sounds a little less smooth than it did originally, but I think a quick rebuild with clean bearings might bring it back up to spec. Even the prop is fine, I am pretty ecstatic. These parts will fly again in a new 75% Goblin!

I may make a 18650 3000mAh 2S battery to power the next one? I should also say I found the other plane I had gone to retrieve today, it was like 6 inches above my head sitting on a branch.
 

CapnBry

Well-known member
#18
I did make another one, with an even lower effort paint job!
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I made a few changes to the design.

ABS Fuselage Doubler - For one, I used 3D printed ABS doublers. They're 3mm thick and under 6g for the two of them. I removed the paper from the nose sides to compensate and used white gorilla glue to attach them so there's ~2g of difference in weight but a lot more strength. Now on a nose-crash, the energy is transferred back to the wing. I haven't had any really bad nose-ins yet to fully test it but it feels a lot stiffer. I did some snap tests on the bench and the weak point now is where the nose bit transitions to the long thin section, which could be reinforced in the 3D model.
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Wingspan - I also extended the wingspan by 0.5" on each side, so 1" total addition, which adds 5% more effective wing area. I would have liked to have gone a full inch on each side, but the servo wires on my tiny 4.3g servos juuuust barely reach now and I didn't want to make new cables just go go a tiny bit further.

Reduced Vertical Stabilizers - Size reduced to 90% (just freehand cut 10% off the back and top). It still gets blown off the wind in a gust so I might reduce these to 80% size, but it has been so windy here that I'm waiting for more of a calm day to test before I say for sure.

Tail Taper - The motor end of the fuselage I reduced the width by 5mm to bring it to more of a point. It was like 39mm wide which is insanely too much for holding a 1407 motor and means 51% of the propeller is blocked by the firewall. This gives it a little a little cleaner air to spin in. In addition, because the gap at the back is narrower, there's less flex in the firewall from thrust (not that that is really an issue). I also cut the bottom off the firewall so it is only 26mm tall, again to allow better airflow to the prop. If I make another one of these, I'd reduce the taper further down by about 7mm, which is where the sides start to run into motor wires. I'd try to keep it this size for an 1806 motor.

Baked in Reflex - I cut the area between the fuselage and the vertical stabilizers, scored it, and bent it up so there's some reflex built into the wing itself rather than relying entirely on the eleveons. Having the reflex closer to the center of the craft also reduces roll induced by not having them exactly right, which is harder when the control surfaces are further out. I only raised them 4mm but I still needed a lot of reflex in the elevons so either this needs to be a lot more bent up, or it isn't very effective. (The left one was done first before I thought to score the bottom so it bent a bit weird).
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Same setup as before except with a new larger battery.
FlySky FS-iA6 receiver
Cobra 1407 3500KV motor
3x4x3 (5A) or 3x5.2x3 (6.1A) prop
EMAX 12A ESC with 1A BEC
950mAh 2S battery (old was 850mAh)
2x 4.3g eBay servos
187g all up weight

After flying the Simple Cub for the past month, getting back to this tiny plank puts a grin on my face because it is a wholly different flying experience. I swear this thing can do a 180 degree turn in less than a meter of space which provides a lot of confidence that you can get out of virtually any jam as long as your fingers can tell the plane what to do fast enough.
 

CapnBry

Well-known member
#19
Finally the wind died down here! This thing is the greatest. It flew so well that I decided to try my first inverted flight. A couple of attempts later I was upside down flying like a pro. My confidence level was through the roof. Unfortunately, that skill was short-lived because I went inverted and had brain problems and dove it directly into the ground. The reinforcement worked-- well, sort of.
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The plastic broke on the inside and the foam sheared away from the support, but this little plank will fly again after a little glue and tape. I wish I had some better video but my hat camera is pretty garbagey. I record in 2.7K then crop out a 720p section that contains the plane because 170 degree FOV is not the best for this application. Still, I have posted it for posterity and hopefully not the last time I fly inverted (at 3:52).

Something was up with my ESC. It was having a hard time starting the motor spinning and you could hear it desyncing and stuttering now and again. I was hitting max throttle round 80% of the stick too. I tried re-calibrating the throttle but no difference. The motor wires all look good and still fully insulated as well. I also tried changing the timing, frequency, brake, and startup power, no difference. I ended up resetting the ESC to default then re-calibrating it and seems to be ok now. Maybe the ESC that spent a month in a tree is not in fresh-from-the-factory working condition?