RysiuM

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Background:

I needed an engine test-stand for playing with gas-glow conversion. I got everything running fine on the bench, so I needed a plane for them. I have some ARFS in the box but I didn't want to trash them. And it does not fit right: 200+ dollars plane for 25 dollars engine. I wanted something easy, fast and cheap to build, that will be easy and fun to fly, that will glide well and survive less than perfect off-airport landings. That's why my decision was to build a SPAD. The only thing I found that would fit my needs was [link=http://www.spadtothebone.com/SPAD/corocub/Corocub_1.htm]CoroCub[/link].

Plane:

The orignal plane designed on SPAD website was to small. I wanted testbed for engine 25cc or more. So I increased the size by 25% and that would make a quarter scale Clipped Wing J3 Cub. I had only white 2mm coroplast, so I decided to make it not a yellow, but white and maybe blue (will be painted).

Plans

When drawing the plane on the sheet of 4mm coro for units I din not use"inches" but "flutes" instead. This way I was sure, that every cut wil end between the flute's wall. Fitting entire design into one sheet 4'x8' of 4mm coro I made the fuselage exactly 48 inches long. The rest fit just fine without further modifications.

Modifications

Of course there are modifications. I have that kind of problem, that I can not build a plane straight from plans and I always have to modify something. The first mod was in the fuselage. I made it with a little longer nose part so I shouldn't have a problem with balancing the plane even when I use very light engine. There is much easier to add 4 oz of lead on the tail than 1 lb at the firewall. The second and the most imortand mod was to make two wing halves. There is no way I will fit one piece 86 inch wing into my truck. This requirement change the entire wing design. And at last, but not least I used more plywood than you may expect in simple SPAD. Instead of using bamboo sticks I used pieces of flywood. This way I got extra "hard points" in the coroplast.

The building posts below.

RysiuM

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Fuselage

One piece 4mm coro made like original plans showed. The only difference in the design is the wing mounting. I will use 4 nylon screws mounting each wing half from the top instead of front mount dowels. Pieces of plywood and spruce were used to create positive connection between the wing mounts, landing gear block and the engine mount. Pictures below

RysiuM

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Engine mount.

Somehow the cutting board I was going to use did not want to stick well to CA. So I decided to use 'calssic' plywood type firewall. The rest is SPAD. Pieces of gutter pipe were used to 'link' the firewall with the landing gear block and the rest of the plane. Because my fuselage is wider than original I used additional pieces of gutter pipe instead of coroplast for that. I'm thinking of making some lightening holes in this pipe, but I need to experiment first with some spare piece to see if and how it will weaken the entire mount.

RysiuM

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Wing

This is the most complicated part so far. First I had to make my own wing tube sleves. I had aluminium wing tube left from Powerline Hobbies Edge (I used CF in there), but I did not have any sleves. This was my first fiberglas work and I'm proud to say it came out darn good. Because of the size of the wing I couldn't use just spar hoping the skin will keep it's airfoil. It is supposed to be Clark-Y not a flat Pizza box. The airfoil was to thick for one yardstick so I glued one and half side by side. I aded few light ply ribs with plywood first rib and three additional pink foam half-ribs. The leading edge was made of CF arrow (tube). Now this construction keeps wing profile and made the LE hard.

The entire rib-spar construction was glued to the 4mm coro (bottom skin) and overlapped with 2mm coro from the top. CA did not gave me enough time to work with the wing at the first attempt so after I had to cut the wing. Then the next time I used Gorilla glue (the foaming type). To streach the top skin I used strapping tape so I had a "handles" to pull the coroplast on.

So far the plane looks like on the pictuire below.

Smooth Spanky

RysiuM:
That's a sweet looking cub, very nice. Good luck with the rest of the build, and please post more pics if you get the time.

Smooth

mikenlapaz

Nice, you can never have too many clamps! Keep the data and pictures flowing. Again nice work.

Mike

kbear

Very Nice. Can't wait to see how she flies. If it's not too much trouble, when you get to the engine install, could you give us a rundown on what you've done as far as the Gas Glow. I follow that thread here and there, but it's just so much to read through for anyone who might be seeing it here for the first time.

RysiuM

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Wow, I did not expect such interest

So here are more pictures. Either I don't understand the SPAD instruction or something is wrong. I'm talking about the closing wingtips - it did fit fine all around but at the leading edge the fold was bad. I just left it open and then I filled them out with Gorilla glue than made a foam filling. See the picture .

Now I'm putting together ailerons. They are the same construction as the wing: balsa ribs and LE and coroplast covering. You can see balsa blocks for Robart hinges and for control horn. This is the picture just before I put the top covering (it will be 2mm coroplast)

As for the engine I'm going to test Poulan 25 running gas-glow mix (5 gas, 1 15-18 fuel) with glow plug powered all the time. Standard mix doesn't work well for my carb and transition is quite bad so I had to reduce the amount of methanol. To get maximum performance I need to keep glow plug under power all the time. I did a bench test and the engine performed very well (7650 rpm on MA 16x8 Classic). The fuel consumption is the best part - I run it for 15 minutes WOT on 8 oz fuel tank. CoroCub will have 24 oz tank

kbear

How goes the build? Any chance you'll get video of the maiden?

RysiuM

My Feedback: (1)

It is going. Unfortunately I can not spend as much time on it as I would like so it is going slowly. The wing is finished in ARF state, so I started to work on the fuselage. The tail section is going by original instruction from SPAD site. The only change I did was mounting servos on the tail (rudder and elevator will get HS-475HB each), and mount the tail wheel bracket. I also prepared hard points on the elevator and the rudder for control horns. Even now, not glued together, the whole tail is stiff and strong. I am surprised that so far the fuselage is very light - as for the size of the plane.

If I will have a camera and someone to operate I will provide a video. I hope it will fly like a very big trainer. I simulated it in RF G2 with all parameters and in G2 it flies just great. And so far RF G2 is very accurate for the flying envelope of models I built. I simulated my H9 Edge, Powerline Hobbies Edge, Giant Wilga and even GWS C-47. So I don't expect any surprises here.

Pictures below showing the servo tray on the tail, blind nuts for tailwheel bracket, tail section together with many bamboo sticks and finally me showing the size of the plane.

RysiuM

My Feedback: (1)

The tail is finished. All went exactly like they were described on SPAD page. This is the first time I made control horns of gutter pipe (I used electrical box before). They so easy to make, custom size, cheap as nothing and better than DuBro

I also made my own pushrods. Thi is CF tube with insterted 4-40 rods on both ends. It takes about 5 minutes to make one. Here is the instruction I invented
Materials:
1. Use 5mm CF tube (exact measurments is OD=4.95mm, ID=3.10mm). This is very common size. In example at [link=http://nyblimp.com/superior/carbon-tubes.htm]Air Dynamics[/link] it is $3 per 3 ft.
2. Use 4-40 all threaded rod. Available everywhere. In example at [link=http://www.servocity.com/html/threaded_rod.html]ServoCity[/link] it is $1.75 per 1 ft.
3. 1/4" heat shrink tubbing. I used electrical black, but can be anything. In example Fry's has one for $1.79 per 3ft.
4. Nylon or silk thread. I used single thread extracted from the string from old window blindes. But you can use anything, that can be soaked in CA and is strong enough.
5. Medium and thin CA

How to do it:
1. Cut CF rod to size - about 2.5 inches shorter than the distance between the servo and control horn.
2. Cut two pieces of 4-40 threaded rod about 2.5 inch long each.
3. Wrap the nylon thread around the 1.75 inches length of the threaded rod so it fits inside the CF tube - snug but not tight fit. I did it by running the thread in three loose layers. Make sure your wrapping ends at the unwrapped end of the rod. Cu not cut the thread yet.
4. Spread medium CA on the wrapped part of the rod - it should soak also the nylon threaad.
5. Now slide the rod inside CF tube - all the wrapped part. Do it quick before CA kicks of. You should have only unwrapped part of the treaded rod exposed (3/4 inch long) with the loose end of nylon thread hanging out of the CF end. Also you should have excess of CA in a form of little drop at this point.
6. Wrap the nylon thread around the threaded rod where it exits form CF tube (and where the excess of CA formed a drop) and then continue wrapping the CF tube as well. CF tube should have tight wrapp on the length of least 0.75 inch.
7. Soak the nylon threaad with thin CA. It will harden in seconds.
8. Slide a piece of heat shrink tube on the wrapped CF tube and shrink it using heat gun. Heat from the gun will speed up curing medium CA inside the tube too.
9. Repeat steps 3-8 on the other end.

Pictures will come soon.

Right now I'm showing pitures of the tail section of my CoroCub. The tail wheel bracket (leftover from some ARF) is a piece of junk and I will replace it soon. Now the tail is RTF. The only mod I did was adding the vertical fin support at the front-bottom corner inside the fuselage as shown on the picture.

I started on working on the nose part. First is the mounting of the powerplant - Poulan 25cc converted for gas-glow.

MrGreenSpeed

Great craftsmanship and building techniques.
Can't wait to see the finished product [8D]
Thanks for all the build pics.
Jeff

RysiuM

My Feedback: (1)

Here is the picture of the mounted CF easy pushrod. When I will be making ones for ailerons I will taka a picture of each step as well.

kbear

Looking good. I'm watching with interest because I'm thinking of getting into the gas/glow thing. I have a Homey that has been dismantled and cleaned that the magneto was no good on. It's just been sitting for a long time (oiled) and needs a carb. Since I have to get a carb for it, what would you suggest? It's a 26cc I believe. I want to put it on an air frame that is known to fly well so that the engine is the only variable.

RysiuM

My Feedback: (1)

I think you will find more answers in the thread [link=http://www.rcuniverse.com/forum/m_1267448/tm.htm]gas fuel with glo plug no ignition[/link], as this is SPAD Forum

kbear

Thanks

RysiuM

My Feedback: (1)

I have a little problem with this plane. I entered all data from that model to RF G2 again this time including actual weight, control surfaces, incidence ect - whole nine yards. And model came out a little squirly - not pich stable . For the sake of experiment I inreased the span of horizontal stab and all problems went away.

I checked the numbers again and compared them to original Piper J3 Cub and CoroCub from SPAD website. Here are the numbers:

My CoroCub:
Wing: Span 86 in, Chord 15.5 inch, area 1333 sq inch
Horiz stab: 22 inch, Chord 11 inch, area 242 sq inch
Wing to horizontal stab area ratio : 5.5

SPAD CoroCub:
Wing: Span 76 in, Chord 12 inch, area 912 sq inch
Horiz stab: 22 inch, Chord 9 inch, area 198 sq inch
Wing to horizontal stab area ratio : 4.6

Piper j3 Cub
Wing: Span 422 in, Chord 65 inch, area 27430 sq inch
Horiz stab: 117 inch, Chord 48 inch, area 5616 sq inch
Wing to horizontal stab area ratio : 4.8

So I guess I screwed up on the horizontal stab span - I did not recalculate it [:@]. It should be around 26 inches. When I put that number in RF G2 the Cub few very stable. I wonder if I can just add 2 inches of coroplast at each end of the stab using bamboo sticks and tape (or maybe replace stab tips with a wider ones) or should I just replace the entire stab. Or maybe RF G2 is wrong and the plane will fly just fine.

Edit: I corrected my mistake about Lateral stability. What I ment was pich stability (Def: Longitudinal stability is pitch stability, or stability around the lateral axis of the airplane).

spad4mebaby

Do read up on the paints that Might stick to coro. at spadw0rld
Sign vinyl or shelf paper fom a hardware store . Most wally-worlds only have pink bunnies or daisies.
I also bought white coro paint will not stick for long. Rusteoleum is supposed to be best.

RysiuM

My Feedback: (1)

For the paint I know this one works. I painted my Corostang with it, and glow fuel did not touch it. It is also tough so you have to bounce the model of the ground to get a scrach
The paint I'm talking about is the cheapest spray-paint you can find in Walmart. It used to be 92 cents a can (10 oz) but now it is $1.02. Look for blue cans with the name:

ColorPlace
interior/exterior
FAST DRY
spray paint

(the rest is in Spanish)

In example Gloss Black has UPC 0-78742 03225-2

This is WalMart brand, dries overnight (I think that's what WalMart calls "Fast")

I'm going to try also a paint for garden furniture. It may works too. I will let you know.

For now here is my Corostang painted in green with black/white stripes

GravityTester

My Feedback: (25)

keep the posts coming

spad4mebaby

I feel your pain about the price increase. WallMart Bamboo kabob sticks used to be $.77 per 100 as soon as I started to use alot of them $1.34 per 100. Every thing else seems to be climbing pricewise also.
I have never tried that paint. Its 115 in the shade here in the summer. But 82 degrees in the dead of winter, nice about 9 mos of the year. I could not get any paint to stick to my coro Regal plastics 3mm and 2mm white. Heat then ac plus glow fuel= peling paint for me. Do you prep the surface ?

RysiuM

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Only clean it well with paint thinner (mineral spirit). Maybe sanding with 600 sandpaper will help a little too. I found sanding the surface with 80 grid paper helps gluing it with CA - no flame flushing needed. BTW another tip for using CA for gluing coroplast - this was proved to be the strong bond, it will tear the coroplast not the joint.

Here is the tip. Prep both surfaces with 3 steps:
1. Clean with mineral spirit
2. Run woodpecker
3. Rough with 80 grid sandpaper

Put small drops of medium CA every 1/2 - 3/4 inch. Press it and do not move it for at least 10 minutes. Do not use kicker, water or anything that supposed to speed up. Usually I let the glue set overnight.

When gluing wood to coroplast (plywood or balsa) I use the same prep for coroplast. For places, that will not work hard (in example servo tray) I skip woodpecker step.

One more information. When I use Gorilla glue with coroplast the surface prep is exactly the same (don't skip woodpecker step). Additional step is that I wipe the surface with wet cloth after sanding. I pre-mix a small amount of Gorilla glue with few drops of water - it will start foaming and expanding almost immediately. I use brush to spread that glue on both surfaces. Work fast because the glue will start setting up in about 5 minutes. Gorilla is more forgiving than CA because you can move parts after putting them together for easy adjustment. CA may not let you do it as it can bond solid without warning. Gorilla glue will be hard (full strength) in about 1 hour.

RysiuM

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Unfortunately, I can not post pictures of making CF pushrods from my ailerons. Servos are so close to ailerons that pushrod (including clevises) is only 2.5inch long. Clevis is 3.4 inch long, so 1 inch is left between them. I just used 1-1/2 inch long all threaded rod (no need for CF tube). Now with such short pushrods and with servo horns moving horizontally and ailron horns moving vertically, I had to use Dubro Swivel Ball Link 2-56. For that size 2-56 is enough.

The wing is mounted with four 1/4 -20 screws. I wonder, what is the strenght of 1/4 nylon screw - can I use them or should I use metal? I estimate that RTF plane will be between 12 and 15 lb (two 6-inch wheels with aluminium LG is over 2 lb itself).

Villa

Hi RysiuM
In your 10-1-07 message you mentioned the design was squirly--not stable lateral. I have always had difficulty understanding lateral. Do you mean the plane is unstable in roll or unstable in pitch. I think unstable laterally means unstable in the roll axis.

RysiuM

My Feedback: (1)

Thanks - stupid me [:@][:@][:@] I ment Longitudinal stability. I will edit it, to not create more confusion.[:@] Sorry.