Cub Training (J-3)
- Thread starter Inq
- Start date
Inq
Elite member
Oh sure. I wasn't sure if that would be allowed on the forum. I didn't want to get banned on my first day!
It's kind of like advertising. I don't have any affiliation with them... I'm in North Carolina, USA and I think they're in the Czech Republic. I have several of their plane models - Focke-Wulf Ta 152H and C, P-38 Lightning, Spitfire Mk XVI and now... this J-3 Cub - https://3dlabprint.com/shop/piperj3cub/
Inq
Elite member
I've been printing the wings today and I've notice a good deal of wash-out built-in to them. Should make for very forgiving stall characteristics. Since I have their models from over the lifetime of the company, I've noticed how they have refined different things like internal structure and how the pieces fasten together and this very new model is really nice fitting and I really like the internal structure.
Inq
Elite member
Actually, even different from that...
I print everything in ABS normally, so I'm well versed using it. ABS is a lot lighter than PLA, but not as light as the foaming LW-PLA. ABS is also a lot tougher and somewhat cheaper than PLA and certainly the LW-PLA. I only paid $17 for the kg of Yellow, but the last time I bought in bulk around $10 per/kg. Versus... LW-PLA looks to be about $35 for 800 gm.
Anyway, I'm going cheap this round... I should be able to print two full planes... so about $8 for a finished plane. I'd think that is comparable to a foam board version cost wise. Since I can also print wheels and tires, I think I'd be cheaper over all.
I know I'll be taking a weight penalty... it's coming out at about 50% heavier than their LW-PLA specifications as shown in the plans. If that holds true, their printed weight is 308 grams and I should be around 470 grams. A regular PLA version should be about 580 grams.
quorneng
Master member
Inq
I am impressed by your ability to use ABS in such thin walled structures without distortion.
I do fear a 50% weight penalty may have a detrimental effect on its flight characteristics.
For the same airframe the power to fly is directly proportional to its all up weight so as a minimum you will need a 50% more powerful motor and prop combination to achieve the same performance. This in turn will significantly reduce the flight duration using the same weight of battery. A bigger battery would of course increase the all up weight still further.
It will be most interesting to see the finished plane.
I am impressed by your ability to use ABS in such thin walled structures without distortion.
I do fear a 50% weight penalty may have a detrimental effect on its flight characteristics.
For the same airframe the power to fly is directly proportional to its all up weight so as a minimum you will need a 50% more powerful motor and prop combination to achieve the same performance. This in turn will significantly reduce the flight duration using the same weight of battery. A bigger battery would of course increase the all up weight still further.
It will be most interesting to see the finished plane.
Inq
Elite member
Inq
I am impressed by your ability to use ABS in such thin walled structures without distortion.
I do fear a 50% weight penalty may have a detrimental effect on its flight characteristics.
For the same airframe the power to fly is directly proportional to its all up weight so as a minimum you will need a 50% more powerful motor and prop combination to achieve the same performance. This in turn will significantly reduce the flight duration using the same weight of battery. A bigger battery would of course increase the all up weight still further.
It will be most interesting to see the finished plane.
There is some oil-canning in the big slab-sides of the ABS J3. Maybe even some in the large bays of the wing. So, yes... ABS is a pain in this situation. But it certainly is no worse than dope/shrink over ribs and bodies. I did do one wing piece with some PLA years ago and it did a far better job of staying per the drawing. BUT, One hot day in the car and it'd be a deformed hockey puck in the floor. I wonder if the LW-PLA is any more tolerant of heat. I'd say not since it's basically the same chemicals.
It's difficult for me to judge what will be easy or hard for me to fly, getting re-started here. About 40 years ago, I learned to fly RC using a Sig Kadet Senior... with K&N 40 on it. Basically a blimp. IF you wrecked it, you had to be trying to!
This one will hopefully be easier to fly than the Spitfire I auger'd in. It was 38" wing span and designed for PLA... So when I made it with ABS, it was 25% lighter.It's all a big fun experiement... it's $8! I spend nearly that just driving back and forth to town.
I'm also found ONE piece of the foam board you all use at one of the three Dollar-Tree's within 30 miles of here. I have to play with that some also... although as I pointed out... its almost cheaper to 3D print.
Inq
Elite member
Not sure why the first piece I printed (front fuselage piece with all the internal structure for motor, battery, etc) was 50% heavier than their building guide said a LW-PLA version would be, but the Cub is fully printed now and the total weight of the ABS version is 23% heavier than the specified LW-PLA version.
I've weighed all my gear to go in it. My battery is considerably larger than they specified at 2200 mAh and my total weight should be around 710 grams.
Using the articles Tench745, FoamyDM, I've gone through the calculations...
It looks like I'm WAY over my piloting capabilities!!! Should I do something to de-rate it so it's down in the trainer 60-75 wtt/Lb ratio?
Thanks,
Inq
I've weighed all my gear to go in it. My battery is considerably larger than they specified at 2200 mAh and my total weight should be around 710 grams.
Using the articles Tench745, FoamyDM, I've gone through the calculations...
- My budget motor is rated at 180 watts.
- Weight 709 grams => 1.56 lbs
- Watts / pound ratio = 115 Wtt/Lb
- 80% usage at full throttle flight time = 2.2*11.1/180 = 0.136 hr => 8.1 minutes
- If I only use throttle down in the "Trainer" level = 2.2*11.1/(70 * 1.56) = 0.224 hr => 13 minutes
It looks like I'm WAY over my piloting capabilities!!! Should I do something to de-rate it so it's down in the trainer 60-75 wtt/Lb ratio?
Thanks,
Inq
Inq
Elite member
????
I'm sure the weight will go up. The only thing I don't have accounted (I think) is the reinforcement. It has slots above and below the built-in printed spar: "Press in and glue a piece of PolyAir, PLA, PETG or 1.5 mm carbon rod into the top and bottom opening to create a wing spar and improve the rigidity of the wing." I'll use fiberglass tow I have laying around. I've noticed over the years how they've improved the design of the assembly. This one was easy to assemble and everything looks to be true. Even the very slight dihedral and washout looks to be consistent for left and right wings. I thought it was real clever how they use the filament (1.75 mm ABS in my case) as the hinges for the ailerons, rudder and elevator. It just slides in and can easily be removed and replaced.
quorneng
Master member
It will be interesting to see how the all up weight compares when it is in a 'ready to fly' state. If you can indeed keep the excess down to the 72g from using ABS you should be fine although it will fly that bit faster and crash a bit harder.
Snap! on using filament for hinge pins. The working printed sprung undercarriage on my Antonov AN2 uses it.
Snap! on using filament for hinge pins. The working printed sprung undercarriage on my Antonov AN2 uses it.
Inq
Elite member
Impressive! So... those vertical struts have springs in them? If so... where do you get springs for something like that?
Inq
Elite member
In the close-up of the Cub landing gear it works like the real plane's by using shock cords in tension. In this case... a rubber band. But what really surprised me was how small the diagonal printed members were... especially if in PLA. I'm used to beefy steel piano wire. But then... being a beginner pilot I'm probably a bull in the china store.
quorneng
Master member
Inq
Yes the struts have springs in them. The springs came from ebay. Quite soft to give over an inch of vertical travel. At that point the prop would also touch the ground.
I do agree a printed undercarriage is comparatively weak to a spring steel version.
I found the biggest issue is not the strength of the strut itself but the strength of its 'joint' assembly at the fuselage. Most spring steel undercarriage it is secured over an area in the fuselage reducing the point loads to an acceptable level.
However you will likely find with an all printed plane it is not only the undercarriage that will need 'delicate' landings.
Yes the struts have springs in them. The springs came from ebay. Quite soft to give over an inch of vertical travel. At that point the prop would also touch the ground.
I do agree a printed undercarriage is comparatively weak to a spring steel version.
I found the biggest issue is not the strength of the strut itself but the strength of its 'joint' assembly at the fuselage. Most spring steel undercarriage it is secured over an area in the fuselage reducing the point loads to an acceptable level.
However you will likely find with an all printed plane it is not only the undercarriage that will need 'delicate' landings.
Inq
Elite member
Inq
Yes the struts have springs in them. The springs came from ebay. Quite soft to give over an inch of vertical travel. At that point the prop would also touch the ground.
I do agree a printed undercarriage is comparatively weak to a spring steel version.
I found the biggest issue is not the strength of the strut itself but the strength of its 'joint' assembly at the fuselage. Most spring steel undercarriage it is secured over an area in the fuselage reducing the point loads to an acceptable level.
However you will likely find with an all printed plane it is not only the undercarriage that will need 'delicate' landings.
I was just re-looking at your spring containing struts and realized that the geometry alone won't keep them together when flying especially with the added weight of wheels and tires. What keeps the "piston" from just coming out and dropping the spring? Is the spring melted/glued at both ends or some other stops?
Similar threads
