nnChipmunk v2

CapnBry

Elite member
This has been on my to build list for the past 6 months now, but I was worried about doing the split rear tail with the two pushrods and if I could get that to work. Turns out, that part was super easy, barely an inconvenience. The only part that was a bit of trouble was attaching the posterboard along to the tail. This build has about 40% of it just barely attached, and the other 60% not attached at all. The bottom of the posterboard is on real good though, so it isn't going anywhere. It just doesn't stand up to close scrutiny. Speed wing build with a 3D printed PLA box spar. (my cat for scale)

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I'm a big fan of using a hot iron to melt the edges of the FT planes, so I thought I'd give it a try here too instead of cutting it all out with a knife to form the contours for the wing and tail pieces. It worked okish on the wing, where following similar instructions to Nick's I ironed the inside before folding. I had a hard time keeping the trailing edge from splitting open when gluing so there's a bit of some gap on one side. For the tail surfaces, I ironed the contour in after making the folds. This worked great. I also tried it on a spare wing half (oops I made two left wing halves) and that's definitely the way to go. Just fold the wing in half and glue it shut, then come back after with the iron and shape it.

Because I had two thicknesses of paper in the center of each fold (I did not remove any paper from the inside), I decided to make my hinges there, cutting off 45 degree bevels on both the top and bottom of the control surface and leaving the double paper as a hinge. This also worked great, but remember to run glue along the edges and also where the new edges will be once the control surface is cut out. If you don't, the hinge will develop a small gap between the top and bottom inner paper layers and it will act like a spring returning the surface back to center. This puts a little extra strain on the servo, but it doesn't appear to be a problem in the one aileron I neglected to glue properly.

Waiting to maiden for the paint to fully cure. You may notice the wing filet are missing. I thought these were going to be hard to get on but they were super easy too. I did minwax the whole plane before I tried to put them on though so when I came back after letting them cool, they were just sitting on top of the wing with the hot glue fully cooled and flat but not sticking at all. I'll try again once minwax dries-- the solvent in it breaks hot glue down like denatured alcohol does.

FlySky FS-iA6B receiver
30A BLHeli_32 ESC
FliteTest Radial 2212 1050KV motor (B pack)
9x4.7 SF prop (all I have in 9", but will try it and maybe go with 4S on an 8x6)
4x 9g SG90 knockoff servos
3S 1400mAh battery
All up weight 565g (115g battery)

Thanks to Nic for the plans and stellar 2 hour long build video I've seen like 19 times now. Looking to replace my many-times-destroyed-until-floppy FT Mustang as my long term go to aerobatic flyer.
 
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nerdnic

nerdnic.com
Mentor
Wow what a great post! Thank you for sharing your build with us. I like what you did with the ironed edges. I've played around with that technique and I love how stiff it makes the edges. The main challenge is that you can't do complex shapes very well (wing tip) but for straight edges it's a nice finish.
 

CapnBry

Elite member
You got that right! I've got probably 10 models' worth of experience doing it so my technique is pretty tight, but it's a real one-way operation. Either you get it right on the first try or you don't-- there's no fixing any mistake. Luckily, we're not sending a man to the moon with these builds so a little slop here and there doesn't make any difference. At least that's what I tell myself!

Florida weather in the winter is a bit of a mixed bag. On the one hand, there's only like a 5% chance of it being too cold outside to stand out there for 20 minutes, but the wind really picks up and gets gusty. I did a lot of waiting for a good day and eventually got impatient. My field is pretty sweet but only 100m long at its longest stretch, surrounded by trees, so everything happens pretty fast.
  • Maiden - Perfect easy launch, was getting it trimmed out and looked down at my transmitter for an instant and could not figure out my orientation fast enough so I had to ditch before I went into the woods. Total flight time just 3 minutes, cracked the wingtip at the servo, 30 second repair.
  • Second day - Much better conditions! Plugged in my BEC backward and fried it.
  • Third day - Very windy, 10kt gusting to 20kt. Spent a long time trying to decide if I should fly. Launched, model was immediately blown backward, I stared directly into the sun and couldn't see and ended up cartwheeling, broke the fuselage aft of the skewers. Another few minutes of hot glue and tape repair for 15 seconds of flight.
  • Fourth day - Still windy but I flew anyway. Everything went according to plan finally and I flew 8 minutes on my 1400mAh 3S with the 9x47 Slow Fly. Handled well considering the wind but I needed a lot of throttle to keep it going where I wanted, the FliteTest Radial 2212 1050KV is just a little underpowered in this setup. It has plenty of thrust to fly sort of high alpha too, but control was a tough in the wind and I wouldn't call the model very sporty except at 90-100% throttle.
  • Fifth Day - Swapped in an 8x6E prop and 4S 1300mAh battery, all up now 620g. Oh man this is heaven!
Let me break out here and describe how this model actually flies, now that I've had such a string of bad luck to start with and finally had a good day. Oh man oh man this thing flies SOOOO GOOOD. Cruise at 50% throttle (3-3.5A) or less, flies straight, turns quickly, stalls like it has a gentle parachute on. I've never flown a model that is as good at flying upside down as this. I'm so used to having to give a bunch of elevator and messing with the throttle, but if I did that I'd actually climb easily. I only needed a slight pressure on the elevator and no extra throttle. It can really get up and go too, I'd say it does 50mph/80kph easily (and pulls only ~13A to do it). My elevator throws were way too high for that speed though so I was wobbling through the sky. I was already dialed down to 80% throw so I'm going to mechanically lower that to get better control with a low rate profile for higher speeds. Rolls aren't super snappy quick but I wouldn't call them lazy either. They're deliberate. Really the only thing I had problems with is a hammerhead / yaw stall turn. Just could not get it to flip around cleanly. Landed a few times but a bit long on all of them. I guess I am coming in kinda fast and it does float a while at 0 throttle. Total flight time 13 mins / 7:10 Throttle % time, and landed with 3.75V/cell.

So a bit of a rocky start but worth it in the end. Just some minor tweaks to perfect my build and now hours of fun to follow. I have no notes on the design! If folks are using the Radial 2212 it might be worth starting with the 8x6E + 4S combo, or the Radial 2218 with a 9x6E on 3S will work well too I think.
 
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CapnBry

Elite member
Despite the initial breakage, I've had nothing but good luck with this model over the past few weeks. It does all the tricks! With the 4S and 8x6 prop I feel like there's no situation I couldn't get out of with a liberal use of power, and it is the first plane I've enjoyed flying fast. Between my poor old man eyes and small flying field, I've traditionally preferred more gentle slowfly stuff, but this I can just point it and go so I rarely feel that things are about to get away from me.

I finally moved over all the electronics from my trusty FT Mustang and bid it farewell, so now I've got more data to share. People love numbers as much as I do right?
  • Full throttle is about 20A on the ground, about 18A flying when you punch it, and drops to between 15-16A once the model accelerates and the prop unloads a bit.
  • Max speed today was 105kph/66mph downwind, 84kph/52mph into the wind for an average of about 94.5kph/59mph at around 230W input power.
  • Cruising around at 40-50kph is about 2-3.5A (30-50W) and the 1300mAh battery lasts around 20 minutes. Flights with a lot of vertical are above 10 minutes, with 13-15 mins being my average.
  • Max roll rate is about 250deg/s and pitch rate around 180-200deg/s in my setup, at 60kph. The pitch rate is so fast you can almost flip around and fly backward. Still need to mechanically dial that down, but currently just set to 80% rate.
I'll say it again, super fun and my favorite fixed wing in the hangar!

EDIT: Here's the iNav guts. Omnibus F4 Pro V3, with current sensor, beeper, BN-180 GPS, Flit10 receiver. The RX antenna is in a not great place, but the GPS has no problems. There's small cutouts in the base plate of the hatch for them to fit through. There's also a slit cut on the side fuselage to be able to access the USB and microSD slot.
DSC06806.JPG
 
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andrej

Member
I crashed my nnnchipmunk. I'm thinking of building a new one. what do you think with the scorpion motor SII-3014-1220KV (V2) prop 7x6 and 4s battery speed will be about 150km/h?
 

andrej

Member
If you believe this data chart https://www.scorpionsystem.com/files/i1,074_data_chart.pdf, 8x8 overkill, no more than 8x5.
For experiment,tried a motor 1806 2300kv prop 7x4 3S. And the plane flew with this tiny engine, and not full but half of the thrtotle.
Remember, CG about 6 cm from the leading edge of the wing? with such CG, the nose turned out to be heavy, and I had to fly holding a stick of the elevator. Or I glued the tail a little at the wrong angle...
 

nerdnic

nerdnic.com
Mentor
You can move your CG back a little bit. I had to trim my elevator up a little after my first flight also.
 

andrej

Member
such a misunderstanding. The indentation under the spar does not go in a straight line as in the photo. Is that how it should be, or is there something wrong with my plans?
is it necessary to bend the spar? (I'm not talking about dihedral)
 

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Ratcheeroo

Legendary member
such a misunderstanding. The indentation under the spar does not go in a straight line as in the photo. Is that how it should be, or is there something wrong with my plans?
is it necessary to bend the spar? (I'm not talking about dihedral)
There is a spar for each wing side, not one long one to span both wings, only one shown on plans but you need to cut 2 and lay one on each wing
 

andrej

Member
here, too, the spar does not go along the line of the foam spar. it looks like the aluminum spar should just be pressed against the foam. I just think that airfoil won't go bad ...
 

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Ratcheeroo

Legendary member
Yeah, I get it, but I was talking about the aluminum spar. it consists of one piece
One side gets glued to the side of foam board spar , wait for it to dry, then fold other wing up to match spar dihedral and glue to side of foamboard spar on that wing. Should really watch the video first.
 

andrej

Member
I watched the video more than once. but English is not my native language, and I don’t understand all the subtleties of translation
 

SSgt Duramax

Junior Member
@nerdnic what power pack should I use? This looks awesome btw
You want to use a power pack C. He has the motors laid out on his site. I think his base motor is a NTM 3530 and goes all the way up to a Tacon Bigfoot 10 for the 120 mph plane. But I flew the V1 on a C Pack and it had plenty of power. It would probably fly on a B Pack, but not optimally.
 

andrej

Member
Looks like I didn't build my nnchipmunk very well. The airfoil thickness is thinner. That is, when inserting the wing into the fuselage, there is an empty space between the wing and the fuselage.
The problem is that the plane does not fly in a straight line. He flies down. I trimmed the Elevator up, it helped a little, but not much. Even if you move the CG back by 20-30mm, the plane still flies down. although with such a tail heavy, on the contrary, he should turn up his nose. I think that my wing or elevator is glued at the wrong angle. Tell me in which direction to lower the wing to raise?
 

Ratcheeroo

Legendary member
Looks like I didn't build my nnchipmunk very well. The airfoil thickness is thinner. That is, when inserting the wing into the fuselage, there is an empty space between the wing and the fuselage.
The problem is that the plane does not fly in a straight line. He flies down. I trimmed the Elevator up, it helped a little, but not much. Even if you move the CG back by 20-30mm, the plane still flies down. although with such a tail heavy, on the contrary, he should turn up his nose. I think that my wing or elevator is glued at the wrong angle. Tell me in which direction to lower the wing to raise?
You don't move the CG point, you shift weight (your battery) to achieve balance at your CG point. My guess is you are still nose heavy, try positioning your battery further back.