24" foam j-3 cub scratch build

[dgjessing]

This will be my first scratch built model airplane in about 40 years I've built lots of other stuff since then (ukuleles: http://www.wsukes.com/index.html and model engines and stuff: http://www.youtube.com/user/dgjessing?feature=mhee ), but no airplanes.

Anyway, I've always liked the looks of the Piper Cub, so I thought I'd give it a go. Got a few sheets of Dollar General foam core and have been messing around with cutting it and gluing it together in airplane-like bits. I think I'm ready to go, foam-wise.

I'm after a slow, small field park flyer that can handle a little wind.

Been reading up on radios - way too much information out there! I think I'm starting to grasp it though... I believe I'm after a 6 ch 2.4ghz unit (although the cub will be 3 ch). I'd like to get one that works with pretty cheap receivers so I can afford to do lots of scratch built planes I'm sure I'll have more questions about that soon.

A more immediate concern for the project at hand is the motor - I need to have that in hand to build the fuselage. After a few hours of reading on line I picked (and ordered) a "Firepower 250 Sport Outrunner Brushless Electric Motor" and a "HURC 10 Amp Brushless Motor ESC" to go with it. Is that kinda in the ballpark of what the proposed plane needs, or did I screw up?

Nothing to show yet, but I'll be posting pictures as I go along

[Bill G]

That power system will give you a bit of leeway. That's always a good thing, unless you really know how to hit your target weights. At .6oz it's not heavy for a 250 class motor. I tend to prefer robust builds and over time have slowly been able to maintain robustness, while considerably lightening them. Some of my earlier 2 foot span jobs require a motor that size to fly well, and especially ones with features such as sheeted fuses like this 24" Guillows Cessna 150. It has a similar sized motor.
http://www.youtube.com/watch?v=U7gyS...3&feature=plcp

[dgjessing]

Thanks, Bill - that's what I hoped to hear I'm thinking it would be good if it only needed half throttle or so for normal flight, but had some extra in reserve to fight a headwind.

[dgjessing]

OK - actually doing something now

The wing halves cut to size, laying on the plans:



Got the curve on the wing tips by placing the plan over the foam and poking through with a pin every 3/8" or so. Connected the pin holes with pencil and cut/sanded to shape. Next I cut a V groove where the wings will bend:



... and removed the paper from both sides.

I then cobbled together a jig to hold things in place while the glue dries:



Used TiteBond for this because it's hard to put down that much hot glue and keep it liquid long enough to shim things in place.

What do we paint this stuff with (if paint is desired...)?

[dgjessing]

Well here's the wing more or less complete:





22" span, about 2 degrees dihedral. Hot glued in bamboo skewer sticks and that center foam mounting plate, put some fiber tape over the center joint on top. Weighs in at .55 oz

[dgjessing]

Kinda getting ahead of myself, but I wanted to try painting foam bits. Went and got an 8 oz sample of latex paint from Lowes. Thinned it about 50/50 with water and sprayed three thin coats:



Turned out tolerably well, I think

[dgjessing]

Horizontal stabilizer and elevator so far:



The elevator halves are joined by a little hunk of oak, of all things

[quorneng]

dgjessing
Very neat so far.
I would however advise that you consider shaping the leading and trailing edges of the wing, a nice rounded shape at the front and a gentle taper to as sharp a trailing edge as you can. It does not make that much difference to the maximum lift the wing can give but it does reduce the drag considerably and hence the power needed to fly. An important consideration with electric.
Indeed if I was doing it I would not have 'bent' the foam at all, just sanded it to shape, with the bamboo skewer glued into a recess in the foam.
As the foam is a bit rough when sanded to shape I have found a strip of self adhesive tape over the leading edge results in a smooth surface and gives a bit of protection from the inevitable knocks.


Just an idea - Keep posting!

[dgjessing]

quorneng, thanks!

I *did* knock the sharp corners off the foam with sand paper prior to painting. The edges aren't totally round, but they aren't just square either.

Didn't get much time to work on it today, but did manage to get some CA hinge stuff from the LHS and do the elevator hinges.

My son came over to visit and we made a quick & dirty 11" span J-3 glider with the foam core and hot glue. I just wanted to show him the basic building technique It sort of flew a few times, until the cat caught it

[Bill G]

I try to refrain from ever critiquing a builder's methods, and at most simply offer things I've done. Seems to go over much better. In this case, I would likely have done it differently than either method, but who really cares as to each their own. FWIW, use of a rod at the LE not only gives a nice rounded edge, but still stiffens the wing and is ding resistant. With the combined support of the external struts, the wings will be robust, and may at most require a small reinforcement plate at the strut mount. Another benefit is that with a discrete reinforced LE, you can now use the strut length to set both dihedral and washout, and the washout setting will both be easier to set accurately and hold better than with a central main spar.

As for the bent thin plate undercamber section, I understand they perform nearly as well as a smooth curve undercamber section at this size of model, as well as being a good section in general for small, slow flying models.

[road king 97]

This thread caught my eye cause iam finishing a 24 inch wingspan bibe. Iam sighned up for your build. joe

[dgjessing]

Started on the fuselage today:




This hot melt glue & foam stuff is fun! The motor, servos, and ESC are supposed to be here Tuesday. Can't do much more without that stuff...

The landing gear is so comically long because the recommended prop for the motor is 6" dia. - to keep things in scale the largest prop would be about 3".

I'm making it so the wing has to come off to replace the battery. Might be a design flaw in the long run, but I'll be happy if it flies at all. We can tweak things like that in future builds

[Bill G]

Originally Posted by dgjessing

Started on the fuselage today:


The landing gear is so comically long because the recommended prop for the motor is 6" dia. - to keep things in scale the largest prop would be about 3".

As for long landing gear, I laugh every time I see the the box art, on the smaller Guilllows Cub kit.

[dgjessing]

This evening I've made the vertical stabilizer and rudder, hinged them together, glued that and the horizontal stuff to the fuselage, installed control horns, and added a wire tail skid:



The entire thing (including wing) weighs in at 1.05 oz at this point. Hopefully tomorrow I'll have all the electric stuff and can start moving things around to get the balance where I think it ought to be (about 1/3 of the way back from the wing's leading edge). I'd like to make it so the battery is vertical and right on the CG so different sized ones would work OK...

[dgjessing]

OK, I've got all the electrics in house now. Besides just piddling around with the radio stuff, servos and the esc & motor, I put in a plywood firewall for the motor:



I gave it about 3 degrees of down thrust. I'm thinking the esc will fit between the firewall and the bulkhead under the wing's leading edge, then the rx, the servos and the battery will all go directly under the wing.

After test running the motor with a 6" prop I can say with absolute certainty that it's going to have plenty of thrust. If it won't fly right, it ain't going to be for a lack of thrust, that's for sure

Which brings up a question: would putting a smaller prop on the motor cause it any harm?

[dgjessing]

OK, this afternoon I fit the esc in behind the motor & drilled a hole through the bulkhead for the battery & Rx wires. After taking this picture I placed a piece of foam in there to hold the esc in place, and sheeted the bottom with foam core (the esc can't come out without cutting the fuselage open):



So here it is so far:



I went to the LHS and got that orange 5" prop to replace the recommended 6" one that I got with the motor. I had to run a 5mm reamer into the prop's bore a little ways to make it fit on the hub, but it fits quite well now & runs true.

I've got some smaller servos on order - the "mini" jobs I had are over-kill I've decided. I could make them fit, but it would be really tight.

I stuffed everything in place (including a battery), put on the wing and checked the CG - it's right where I want it to be, at the bamboo wing spars. Shear luck

It's going to weigh in at about 3 1/2 - 4 ounces ready to fly with the tiny servos. Kinda heavy... we'll see.

[road king 97]

On all of my scratch built foamys i put 2 or 3 degrees down thrust and the same right thrust for the tork. Most of my nitro planes have the same set up. joe

[dgjessing]

The new micro servos came today - here are they and the receiver in place:



I made a removable plastic panel to go on top of the servos so that the excess wires could go over them without interfering with anything - so here's everything including the battery in place:



And here's the whole thing ready for test flight (I hope!):



The whole things weighs in at 4.75 oz - definitely on the heavy side... I may get it to fly but landing is going to be pretty exciting . I glide tested it a few times into a patch of ivy - seemed to do OK, but it comes down pretty quick.

Should I hand launch or try to ROG for the first attempt? Which do you all suppose would be less likely to result in disaster?

[quorneng]

Hmmm
Looks like very little dihedral for rudder only and the rudder itself is not particularly big when compared to the wing area/span.
If you have a big really smooth surface then ROG is probably best, but its got to be smooth otherwise had launch over the longest grass you can find!

[Bill G]

Originally Posted by dgjessing

The whole things weighs in at 4.75 oz - definitely on the heavy side... I

That's not bad weight. The 24" Guillows video link I had posted has flown at over 6oz. Under 5oz for models this size is fine.

[dgjessing]

Attempted flight this morning. First I tried to ROG on a newly raked baseball field infield - as soon as it got up some speed it ground looped . Next I tried a hand launch - it immediately flipped to the right and cartwheeled in the grass - no damage though! Adjusted the rudder trim left and tried it again - this time plowed straight into the ground about 30 feet downrange, breaking the propeller (didn't bring a spare, which was dumb...). Perhaps if someone else had launched it for me I could have recovered in time.

I may make a new wing with some more dihedral before trying it again.

[dgjessing]

Tried it again a few hours later (same wing). I had taped a couple of nickles on the tail, which gave a much better hand launched glide path. Again tried first to ROG with no luck - revved it up for a hand launch and the prop flew off . I had brought an extra prop this time, but not an extra O-ring to hold it on. Sheese...

We'll try it again tomorrow

[dgjessing]

Well, tried it again this afternoon. Three hand launches, three times flipped over and hit the ground before I could get my hand back on the Tx and try to correct. Broke two props, elevator hinge popped out on the last try

I think maybe I'll scrap this thing and build some proven design trainer instead...

[dahawk]

DG,

Didn't know if you were still lookng for a radio. Here's a TX for you. I'm a Futaba guy but this baby is highly rated and uber cheap. With something like this, you could afford to go out buy a nice ARF and get on with the business of flying:

http://www.hobbypartz.com/79p-th9x-r...nel-radio.html

[dgjessing]

Thanks, dahawk, but I have bought a Spektrum DX5e, and that's going to have to do me for a while . It *would* be nice to have a plane bound to it that flies...

The J-3 has gotten a reprieve - made a new wing for it this afternoon, a little bit wider and with a lot more dihedral:



We'll try it again tomorrow!