BJ-J3 Cub - NOT a Failure Story - Plans and Build Instructions

[BanditJacksRC]

I want to start off by saying- This is FT forum community is awesome. This thread is a perfect example of how the members are always willing to contribute whatever they can to help others.


This is my first attempt at making my own set of actual plans for the forum and was def. having some issues.
I started by importing a 3-View of the J-3 in to Microsoft Publisher and going from there!
View attachment 41050

My 1st beta build maiden went horribly. The plane would pretty much take off, dart to the left and into the ground. I figured this was only a thrust angle issue, and with the 2nd beta maiden I added a ton of thrust angle (down and left), but that didn't help anything.
The plane flew extremely erratically and I ended up landing about 30ft high at the top of a tree. I had to carry my huge extension ladder about 3 block to the park and spent about 45 min climbing to the top of the tree and trying to get the damned thing down... I eventually got it , but what a night!

Needless to say it was scrapped.

After a ton of help from the community, I was able to make some changes and get this plane flying great. See for yourself!

BJ-J3 Cub - Plans and Build Instructions​

PLANS:
FULL: View attachment J3 Plans - FINAL.pdf
TILED: View attachment PiperCubTiled.pdf

Specs:
Wingspan: 40"
Length: 25 1/2"
Swappable: Yes
AUW: 510 grams
(w/o battery and with landing gear)
Channel: 4 Channel

Electronics Used in Video:
Motor: NTM 28-26 1200kv 258W
ESC: 30A Turn Plush
Battery: 3s 2200mAh
Prop: APC 8x6


Building Instructions (step by steps):
Wing Build: View attachment Wing Build Instructions.pdf
Fuse Build: View attachment Fuse Build Instructions.pdf
Rudd. & Elv. Stabilizers: View attachment Rudder & Elevator Build Instructions.pdf
Final Assembly: View attachment Final Assembly Instructions.pdf


Maiden Video:

I also want to thank everyone who as helped throughout this process. I love to design and build these plans to share, and can't express how grateful I am for everyone's help.

 

[BanditJacksRC]

I forgot to add... If anybody is willing to give this build a try and see if you can have any success. Please feel free to PM me, and I can send you the PDF

Also I know the title of the plans say "FT-J-3" which is really not right. I was going to change that before releasing the plans. but I'm over it at this point and just feel like moving on to the next project.. a bloody wonder for now, and think about which plans i want to give a shot at next.

 

[SnowRocker88]

Sounds like your prognosis is correct. Your wing does look like it has a crazy profile. Why not just build the Storch wing but with the J3 profile on the tips? I think that's your only problem. Everything else looks nice! Don't give up!!

Your first attempt sounds like tip stall. Second attempt sounds like not enough aileron authority?

 

[mrstamp80]

I love the picture of the scrap in the 5 gallon bucket. Priceless! (i have numerous memories of that same picture) Atleast foam baord is only $1 a sheet! Were you ever able to get me V2 plans? I am gonna print them both out and build over the weekend. Will see if I have any better luck.
Also what motor,prop,batt were you using? And did you ever get a all up weight?

 

[mrstamp80]

Picture of plane in bucket Priceless!! Been there! I got a pusher trainer fpv style that i scratch built out of my head, that I am worried is not gonna fly worth a crap. Might end up in the trash as well!

 

[BanditJacksRC]

Sounds like your prognosis is correct. Your wing does look like it has a crazy profile. Why not just build the Storch wing but with the J3 profile on the tips? I think that's your only problem. Everything else looks nice! Don't give up!!

Your first attempt sounds like tip stall. Second attempt sounds like not enough aileron authority?

As far as using the storch wing, I have actually been thinking about that as well. I really just wanted to go smaller (with an air foil).

As discouraged as I was after this, there is no way I can just throw the towel in now. I am going to redesign a few things and just make the design bigger overall.

 

[BanditJacksRC]

I love the picture of the scrap in the 5 gallon bucket. Priceless! (i have numerous memories of that same picture) Atleast foam baord is only $1 a sheet! Were you ever able to get me V2 plans? I am gonna print them both out and build over the weekend. Will see if I have any better luck.
Also what motor,prop,batt were you using? And did you ever get a all up weight?

Yeah that picture is great, lol.

After the 2nd maiden I was just to upset to even look at the plans to make any revisions. I will most likley be re-doing them completley (bigger and better!) and will let you know as soon as they are done.

I was planning to get the AUW but totally forgot.

Motor: 28-26 NTM PropDrive 1200kv 258w
Battery: 2200MaH 3s 20c
ESC: 30Amp HobbyKing
Servos: Turnagy 9grams
RX: Spektrum 6100e (i think that is the nuber)
TX: Spektrum DX6i

 

[earthsciteach]

It is a great looking build. Sorry, I do not know exactly what happened the first time. From my experience, a plane behaving that erratically screams tail heavy.

In terms of airfoil, it should fly, if by no other means than simply deflecting air downwards. You may want to increase the chord a bit over scale. Just a thought.

On additional thought - what is the wing loading on the plane? Simply divide the flying weight of the plane by the flat plate area of the wing. 15 oz per square foot is a middle-of-the-road value for an RC trainer.

 

[BanditJacksRC]

It is a great looking build. Sorry, I do not know exactly what happened the first time. From my experience, a plane behaving that erratically screams tail heavy.

In terms of airfoil, it should fly, if by no other means than simply deflecting air downwards. You may want to increase the chord a bit over scale. Just a thought.

On additional thought - what is the wing loading on the plane? Simply divide the flying weight of the plane by the flat plate area of the wing. 15 oz per square foot is a middle-of-the-road value for an RC trainer.

That is great to know for the next version of the plans.. Thanks for the tip! I;m not sure what it was for this bird. I never got the AUW.

 

[earthsciteach]

It LOOKS like it should be a really good flyer. The only two things I can think of to make it so difficult are, 1) too tail heavy, or 2) too high a wing loading (simply, just too heavy).

 

[nerdnic]

The plane did the exact same thing as the 1st, BUT EVEN WORSE!! For the first 2 take offs The plane would climb about 5 or 6 feel then bank hard left, and with right rudder and ailerons, it would still just plummit to the ground. The 2nd nose bomb ripped some of the landing gear, so I ripped it off completley and went for attmept number 3.

Because the wing is so small the bottom plate looks to almost be blocking the ailerons from any airflow, which might contribute to crappy controll.

-Bandit

Designing planes is hard. Making a good flying plane is even harder, I know first hand what you're going through. Based on what you've described the issue doesn't seem to be the size of your wing or ailerons. A plane with no ailerons or rudder wouldn't even fly like you've explained. Torque roll can account for some left pitch on take off but the motor you have is very moderate and a plane that size wouldn't have that drastic of an impact.

From my experience, a plane behaving that erratically screams tail heavy.

earthsciteach is on the right track - your plane was most likely balanced tail heavy.

 

[rcspaceflight]

It's always a good idea to fudge the lines a bit. Increasing the wing chord an inch can make all the difference with wing cube loading without noticeably changing the look of the plane. (Assuming the wing cube loading was part of the problem.) Use this calc and look at the "wing cube loading" and use the chart on the bottom.

The airfoil does look tall. If it fell out of the sky when going fast, it's possible that the wings where splitting the air so much that the ailerons didn't get any air. I say scrap the spar. Keep the spacer at the back of the flat bottom part of the wing. See my Cessna Skymaster. I don't know the wingspan on this J-3 (either you didn't say or I missed it) but if it's around/under 40", you shouldn't need any spar for strength. Unless you're going crazy with it.

 

[BanditJacksRC]

-Bandit

Designing planes is hard. Making a good flying plane is even harder, I know first hand what you're going through. Based on what you've described the issue doesn't seem to be the size of your wing or ailerons. A plane with no ailerons or rudder wouldn't even fly like you've explained. Torque roll can account for some left pitch on take off but the motor you have is very moderate and a plane that size wouldn't have that drastic of an impact.



earthsciteach is on the right track - your plane was most likely balanced tail heavy.

The torque roll or P-factor is what I initally thought was the problem, but you're right- I don't think it was be as drastic as it was.

I didn't do any measurments to determine where the CG should have been exactly, but figured if I balanced it a touch nose heavy from the wing spar, I should be in good shape... Is this not a good way to determine? Do you know if I should have measured out like 33% or something like that?

 

[BanditJacksRC]

The airfoil does look tall. If it fell out of the sky when going fast, it's possible that the wings where splitting the air so much that the ailerons didn't get any air. I say scrap the spar. Keep the spacer at the back of the flat bottom part of the wing. See my Cessna Skymaster. I don't know the wingspan on this J-3 (either you didn't say or I missed it) but if it's around/under 40", you shouldn't need any spar for strength. Unless you're going crazy with it.

I have checked out your skymaster before, Its a pretty sweet looking model.
No spar in that wing huh..? How did you glue them together? just butting up the ends and hotglue together?
Great build

 

[rcspaceflight]

I have checked out your skymaster before, Its a pretty sweet looking model.
No spar in that wing huh..? How did you glue them together? just butting up the ends and hotglue together?
Great build

I did just butt the three wing pieces together and I did a horrible job with it. The plane eventually had polyhedral when it wasn't supposed to have any. The worst part about it is that the wings "flapped" and the polyhedral would change depending on the angle of the roll of the plane.

A two piece wing would be fine spar-less especially if you added popsicle sticks on the outside (or even inside) of the joint just for a little piece of mind.

But if the wings aren't removable, gluing the two wings pieces to the fuselage should be fine.

 

[mrstamp80]

Bandit, Thanks for the plans. Here is where I am at with some pictures and some changes. Maybe these changes will work, maybe they won't!!! Wait and see..


So I got the plans printed to scale.


After reading your trial, I have some belief along with others it may be a little tail heavy. So I know I am gonna use similar size motor (57gram) but a smaller batt. (1500 3s) So I decided to add 1'' to the length of the nose. To give me more room to slide the batt forward if need be. Also I may be wrong so correct me if I am. On the plans for the fuse it's labeled as a "B" fold but the picture on the plans is of a "A" fold. I went with the "B" fold.


Had to add another score line 2 1/2 '' back on the bottom fuse to get it to roll around, and get a good glue joint.


I decided to do a friction fit landing gear (see FT Duster). This is not the LG i am gonna use this was laying around. I will build a wider one later. Probably same wheels though.


Mock up, 2 1/2'' X 2 1/2'' plate to go over the LG wire for the friction fit.


All glued in. The Lg is easily removable. I may use a better landing gear method later, but for now to get this bird in the air this will do.

So far so good. I think the vertical stab and the horizontal stab is about 10% to small (my calculations) So I am gonna mess around with those and enlarge them. Either that or I will shorten the main wing down from 40" to 36" or 37". I will keep you updated.

 

[BanditJacksRC]

Wow, that looks awesome.
Yeah I had a page full of notes on the little things i needed to chage on the plans I hadn't gotten around to doing yet.

I only have time to create the plans on publisher when I am working at a specific office, and have 23 shift there next week. So I will be doing some updates then and attempting build # 3.

I had the same suspicion about the size of the elev and rudd. being too small as well. i think that will help alot.

Let me know how the wing build goes. My first time around i peeled the inside paper and did smooth fold back for the wings, which I think went a bit smoother and didn't bring the foil as tall as the second.

Please keep me updated with your build, it looks great!

 

[mrstamp80]

Worked on the wing tonight,


I started by taking 1'' off the inside of both wings.


I marked out the new location of the spar. and the location of the score lines for the wing fold over. Also added a 1/4'' depth to the ailerons.



Layed out on 1 sheet of foam board to get some nice clean edges to seam together.


Both wing halves cut out. Now onto the new spar.


The new spar is 23'' long and 1'' wide. Score the spar in the middle at 1/2'' and glue together.


Find the center of the spar and line it up with the center of the wing and glue into place.


You can fold and glue each half individually. But Do NOT glue the center top. After glueing the 2 halves, use the dihedral gauge and lift one side up and pack the top half with glue. I resorted back to the FT Duster build for this. Then extreme packing tape over top of it. Nice and solid.





All in all coming together well. I am gonna blow up the tail section slightly. Also i made a 1'' x 1/2'' cut out in the bottom center of wing to feed the servo wires through. Will keep you updated on the build.

 

[joshuabardwell]

Sorry for your experience. The moral of the story is this: if your plane isn't flying right, don't fight it back up into the air. Change something to try to figure out what's wrong before you try to fly again.

 

[BanditJacksRC]

Sorry for your experience. The moral of the story is this: if your plane isn't flying right, don't fight it back up into the air. Change something to try to figure out what's wrong before you try to fly again.

Yeah after the first flight I had convinced myself it was the thrust angle, which now I dont think was as big of an issue.