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Old Mar 03, 2004, 06:02 AM
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First scratch build, Ju 88A-4

hey, i'am interested in building a scale Ju-88 A4, what size should i make it to run on 2 speed 350 or 400 motors? I'am pretty new to this so i'd be happy if it even looks like a ju-88 from far :P

Thinking of balsa build up, any "must go" sites or advice on where to start? thx!
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Old Mar 03, 2004, 11:55 AM
Trampling out the vintage
Joined Feb 2002
1,892 Posts
Make it 45"

If you are new, consider some ARFS to get a feel for things. Hobby Lobby is a good choice. Theyhave some good ARF twins too.

If you want to build balsa kits, consider PR Models, Mountain Models, Bengston Aerodrome, Stevens Aero. Great kits designed for electric. Search around and you'll find the URLs. I am sure there are many other excellent ones but those are the ones I have personal experience with.
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Old Mar 04, 2004, 04:52 AM
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ive tried a kit and a number of foam planes, thinking of trying scratch building, you think i could pull off a flyable model? any plans avalaible on a ju-88?

oh and use 2 350 or 2 400s?
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Old Mar 04, 2004, 08:19 AM
All under control, Grommit!
leccyflyer's Avatar
United Kingdom, Aberdeen
Joined Sep 2000
12,588 Posts
Would a Ju288 do?

If so then RC Scale International have a plan for a Ju288 available in their plans range for 2xsp400s, designed by Dale Tattam. You should be able to get details from Traplet- www.traplet.com

Brian
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Old Mar 04, 2004, 09:39 AM
Trampling out the vintage
Joined Feb 2002
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If you are thinking of a scratch built I would say try it. If you have built a balsa kit or two you will be fine. It is very satisfying and not hard. It does require a bit of time and patience to "build the kit" first.
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Old Mar 04, 2004, 02:55 PM
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Seattle, WA, USA
Joined May 2003
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the nice thing about scratch building is that balsa is pretty cheap. you WILL invest a lot of time however! =) but as long as you are having fun, right?

some tips:

1. get a set of 3-views and drawings. many can be found online with some searching, or ordered from somewhere like Bob's. (http://www.bobsairdoc.com/)

2. use a computer with scanner/printer or even a photocopier with enlargement capabilities to scale up the drawings and make a set of "plans". they can be rough (just taped together sheets of paper, fuse bulkheads, wing ribs, etc.)

3. There are programs available for plotting and printing airfoils for making wing ribs. search the web.

4. go by what works! If you want to power it with 2 sp400s, then see what other twin 400 models span and weigh. then do something similar. same goes for construction methods, design, etc. It helps to have a bunch of plans handy to refer to. see how they construct the wings, fuse, etc. Use similar sizes of wood for spars, wing ribs, sheeting, tail, and similar construction methods. Why re-invent the wheel? =)

5. have fun! experiment, cut, glue, learn, and do it all over again. You say you will be happy with a standoff scale model --- so enlarge the wing area and tail if you want, keep things simple.

scratch building can be a blast, and is extremely rewarding. so enjoy!

-ron
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Old Mar 04, 2004, 07:26 PM
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Kilsyth, Victoria, Australia
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Good advice from Brian Cullen. I've built the Ju 288 and, even better, the Heinkel 177 from the same source and both have been good fliers. I built light and ran direct drive Speed 400 7.2V motors on Graupner semi-scale props from 7 X 1000mAh cells.
Also would recommend the ModelairTech "Stichell" as a twin trainer. Same set up but build an all sheet fuselage. Choose lightweight 3mm balsa for this. David
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Old Mar 04, 2004, 07:26 PM
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Kilsyth, Victoria, Australia
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Good advice from Brian Cullen. I've built the Ju 288 and, even better, the Heinkel 177 from the same source and both have been good fliers. I built light and ran direct drive Speed 400 7.2V motors on Graupner semi-scale props from 7 X 1000mAh cells.
Also would recommend the ModelairTech "Stichell" as a twin trainer. Same set up but build an all sheet fuselage. Choose lightweight 3mm balsa for this. David
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Old Mar 05, 2004, 07:25 AM
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Quote:
[i]
3. There are programs available for plotting and printing airfoils for making wing ribs. search the web.


-ron [/B]
Are there any programs specialy for making the plans for the plane itself? I have 3d max, would it help much if i modelled the plane in 3d max first? And what airfoil to use? profili is confusing

thx for all the replies!
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Old Mar 06, 2004, 08:26 AM
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Ok, ive decided on a Ju 88A-4.
Got my hands on a few books (pdf form) on the ju 88.

Using a prop size of 9 inches (3 bladed), should the size be 1/16? the wing span would be 126.45 cm & a length of 90.4cm. Directly proportional, 2.4 kg with 77.3mm main wheel.
The ju88 used a 1000 hp to 1400 hp engine, would that mean the electric motor would have to produce 182.1 watts to 254.9 watts each drawing 24.6A to 39A at 7.4V. Would a size 400 brushed motor be able to do that?

This seems logical to me but could be totally wrong Would be nice if you guys could explain if this is correct or wrong, and stuff which i can do without (the cowls seem really big), and would micro servos do?
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Old Mar 06, 2004, 08:56 AM
guillows basher!!
Pittsburgh Allegheny, Pennsylvania, United States
Joined Feb 2004
957 Posts
nothing about the hp of the real plane is going to translate to anything usefull in a electric r/c model!! are you gear drive or direct drive? 2 sp-400's can be pretty potent on a 8 cell batt pack!!
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Old Mar 06, 2004, 04:24 PM
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Kilsyth, Victoria, Australia
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In round terms you seem to be looking at a span of 48 - 50 inches which is a nice size for a Speed 400 powered twin. Just don't try getting carried away by trying to build it exact scale. You are going to have to make a few compromises. Suggest that AUW dependant mainly on cell type and count should be 40 oz. or less. The entire airframe should not account for over 16 oz. covered, painted but bare of any hardware. Cowl diameter is likely to be around 3 inches which is kind of standard for this size model regardless of origin. Personally I think you are being ambitious with your 9" three bladers. Large diameter props like that suggest a slower flying model to me. That's fine but then it means building ultra light to a bigger scale - kind of like a big parkflyer. Now I don't say this is always the case but once you start looking for performance without really powerful motors (more cells. more weight spiral) that's the way you have to head. My choice would be either keeping the weight right down and using Graupner semi-scale props on direct drive 7.2V motors or an 8" two blade on say a 2:1 gearbox using 6V motors. For aerofoil on this sort of model I stick with good old Clark Y it is honest and easy to build.
A lot of this depends on how you want the model to fly and what you expect of it once it's in the air. The most important part however is to keep weight and therefore wing loading down. If you push the weight up on this size model I might suggest you'd have to spend most of your time keeping the speed up to avoid tip stalls.
I've so far built over half a dozen successful WWII twins and the formula has always been the same. My PBY at 58" span flew smooth and too fast at full throttle on direct drive. While my 60" Heinkel 177 on the same set-up could out perform your average
.25 powered sports plane in level flight.
Servos I have always used have been good quality minis on rudder and elevator with micros on ailerons.
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Old Mar 06, 2004, 07:54 PM
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sounds good!
i was thinking 9 inch three bladers with a gearbox, would that make it slower and easier to fly? I dont really trust my piloting skills. The 2.4 kg was supposed to be with everything on, but i guess i should aim for under 2 kg?
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Old Mar 07, 2004, 01:53 AM
Single-task at best...
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Telford, UK
Joined Feb 2000
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lEMFl,

I'm really no expert, but I'd advise you to keep it all as simple as possible.

From the look of the 88, I'm guessing that an model about 45"-48" span, weighing no more 3lb (including batteries) should be do-able.

As David says, don't worry about gearboxes and big props; the S400 motors will be happy direct-driving smaller 5" -6" props at 10-12amps each, to give a reasonably agile fighter-like performance. This is about the highest draw you can go with an S400 and still expect a useful lifespan.

8 subC cells will give several minutes of duration at full throttle, and a lot more if use the throttle intelligently.

What I've listed above is the power system used by quite a few models in the size you're considering, notably the MPX Twinstar foamie of which thousands have been flown very successfully!

tim
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Old Mar 07, 2004, 03:30 AM
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Ive started on making some sort of plan for the structure.

Hmm, the 182 watts thing was really off, i was just converting the horsepower into watts and dividing by 16^3 (4096). As all of you said it doesnt work like this.

I really would rather a slower plane, i get kinda panicky when the plane gets too fast thus i'am thinking more of a geared 400 with a larger 9 inch prop, 3 bladed if possible cause the ju 88 had a 3 bladed prop. From what i understand, more thrust allows it to climb steeper? i would love to reuse my current batteries which are 2s lipolies.

As for the AUW, ill try to make it David Hipperson's recommendation of 40 oz. Also, how hard would it be to fly a model like this? It seems to have a fair amount of dihedral, I'am really scared of reducing it into a pile of very light firewood
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