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Old Sep 02, 2011, 07:18 AM
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Pietarsaari, Finland
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Junkers Ju 88 A-4

For a long time, Junkers Ju 88 has fascinated me so much that I decided to give it a try. I didn't want anything too big, so a scale 1/10 was chosen. That will make a 2008 mm span. I think if I can build it light enough it will be able to fly, too.


I started the project by making the tailwheel unit. It is made from scarp plastic and the fork is from aluminum. As there is no room in the fuselage for a sideways turned wheel I had to incorporate some kind of a "wheel straightener" as the wheel is being retracted.

/ilkka/
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Old Sep 02, 2011, 07:32 AM
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The tires for main gear are made from some kind of a lightweight rubber foam with Sikaflex sealing compound as "rubber skin". There are stationary (non-rotating) brake back plates on both sides of the wheels. That's why the outer plates had to be fastened to the wheel collars and not to the wheels.

The main gear was a big job to make and it turned out on the heavy side. I'm not going to remake it but will try to spare weight somewhere else instead.

Below a video of the gear on a test rig.

/ilkka/

Junkers Ju 88 Gear Test (2 min 12 sec)
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Old Sep 02, 2011, 07:40 AM
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Don't forget that the main gear TWISTED as it retracted !!!

Dear I. Klemetti:

The PIPE Here...as you can see from a "growingly famous" Japanese language site about historic full-scale aircraft retract gear systems (lots of kanji characters there!) there's a page at:

http://www5a.biglobe.ne.jp/~t_miyama/ju88mleg.html

that shows you QUITE clearly how the Ju 88's main gear retracted just like a Curtiss P-40, Vought F4U Corsair or Grumman F6F Hellcat's did, by TWISTING the main gear strut through 90º to allow the main gear's wheel to be placed above the lower end of the main gear strut when retracted.

Also, if you were REALLY planning to "go all the way" and use conical Belleville spring washers for the main gear's strut springing, exactly like the Ju 88 did it for real, McMaster-Carr in the USA at http://www.mcmaster.com/#disc-springs/=dvx2no has these available IN METRIC SIZES, too...

...hope these two links help you out a small bit!

Yours Sincerely,

The PIPE....!!
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Old Sep 02, 2011, 08:07 AM
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subscribed: what plans are you working from?

Bruce
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Old Sep 02, 2011, 12:47 PM
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PIPE, thank you for the link to the Junkers gear system page. Unfortunately the original gear retracting geometry and suspension system could not be replicated in miniature format so i had to alter it somewhat.

Bruce, at first I bought a set of German plans. I knew the structures on those plans were on the heavy side but I thought I could lighten the design during the building process. When I got the plans I noticed they were too much off scale. I found a good set of three-views (actually multiple-views) with a lot of sections in a modelling magazine. I have enlarged them and I'll build on them. I have had to make some drawings for the landing gear to be able to machine the parts and ensure they would fit.

/ilkka/
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Old Sep 02, 2011, 04:51 PM
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Quote:
Originally Posted by I. Klemetti View Post
PIPE, thank you for the link to the Junkers gear system page. Unfortunately the original gear retracting geometry and suspension system could not be replicated in miniature format so i had to alter it somewhat.

Bruce, at first I bought a set of German plans. I knew the structures on those plans were on the heavy side but I thought I could lighten the design during the building process. When I got the plans I noticed they were too much off scale. I found a good set of three-views (actually multiple-views) with a lot of sections in a modelling magazine. I have enlarged them and I'll build on them. I have had to make some drawings for the landing gear to be able to machine the parts and ensure they would fit.

/ilkka/
what's the scale of this bird going to be? can't wait to see more, your off to a great start!

Bruce
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Old Sep 03, 2011, 06:52 AM
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I can see, early on, that this will be a project worth watching !

As usual, your inventiveness is showing through.
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Old Sep 03, 2011, 01:55 PM
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Bruce, this will be in 1/10 scale.

Tom, if you see any inventiveness, most of it came from your build threads. I am going to glass this ship by using your trick and use polyurethane acrylate lacquer.

Engine nacelles' frames are completed. They look like two small airplanes, don't they. The wing lower spar could not be pulled straight through the nacelle as the gear mechanism is in the way. Instead the forces had to be led via detour. The structure is mainly 2 mm plywood.
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Old Sep 08, 2011, 12:41 AM
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I chose Saito FA-40's for engines. Mainly due to the predictable twin sound I know they are not the most powerful choice but I am trying to build light. Let's see, I may have to exchange them for something more poweful on the final stage. On YouTube I have seen a Hangar 9 Twin Otter being flown with Saito FA-40's and my model Ju is about comparable size and weight.

The left engine thrust line is 2 deg to the left and the right one points 3 deg to the right.

I pondered for many days how to get the wing tubes aligned correctly. The answer was obvious. I glued them to both nacelles simultaneously and used the aluminum tubes as guides.

/ilkka/
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Old Sep 08, 2011, 04:57 AM
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I like Saitos for their power and dependability. I think you will be happy with your choice.
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Old Sep 11, 2011, 07:16 AM
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Big Saitos big power. Small Saitos small power. My success with the small Saitos depends on my ability to build light. Time will tell.

I cannot imagine more lightweight and simple system for attaching the wings than traditional rubber bands. They are quick to install on the field, too. Actually the only thing you have to build are the hooks.

For testing the gear I used a pair of old but presumably unused Hitec HS-422s. I had hacked them to rotate 180 deg and one to rotate backwards. Although the gear works the power of the servos is marginal. I might switch to more powerfil servos or use these with 6 volts.

Anyway, aluminum servo arms are to be used on landing gear.

/ilkka/
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Old Sep 11, 2011, 09:56 AM
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And here's a WW II Ju 88A photo showing its landing gear...!!

Dear Ilkka:

The PIPE Here again - I've GOT to get back to getting everything going with my pair of online COLLEGE courses this Sunday, as I'm starting on the "concluding half" of my eight college courses to get a two-year business degree earned by June of next year.

I remembered you were saying that your Ju 88's main LG doesn't "quite" retract the exact same way as the Japanese site's animated GIF file was showing, but from that last "I'm standing" photo, when compared to the photo I've attached from Wikipedia's "Commons" section, it sure LOOKS like everything that "shows" looks QUITE like the original aircraft, so at least you've got the general look of your RC Ju 88's main LG just about right!

Don't forget that there are FOUR main LG doors for each landing gear on a Ju 88 - two strut doors, and two wheel doors - and much like a P-51 Mustang's wheel doors, whenever the main LG on a Ju 88 is at either extreme of its movement (fully retracted OR extended), those wheel doors ARE shut tight.

I'm just glad that the scale 90º rotating main LG struts sure DO look "prototypical" so far on your Ju 88, especially on the exposed parts...and by the way, I've got a single Saito FA-40a myself for a complete re-do of the small Bücker Jungmeister whose plans are available at http://www.rcmplans.com/index.php?ma...oducts_id=1670 , with the first one I built from those plans viewable at http://www.buecker-museum.de/modellb.../bue133_02.htm from way back in the early 1980s, and my new one IS going to have fully shock-absorbing, sprung main LG struts that I'm designing myself in CAD on my home PC.

In clising, here's a pair of videos of other people running up their Saito FA-40a four stroke "mills" on their test stand setups, for a small bit of inspration for both of us...

Saito FA 40 glow engine test stand run (1 min 47 sec)


Saito FA-40a End of Break-in (3 min 44 sec)


...hope you enjoy those!

Yours Sincerely,

The PIPE....!!
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Old Sep 11, 2011, 02:29 PM
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PIPE: You know what a scale model is all about. It's a fake. It looks like the real thing but it isn't. This applies to my model's gear, too. The original retracting mechanism is not suitable to be driven by a servo. It was designed for a very compact and powerful hydraulic cylinder. The rear pivot point in the nacelle was at the front edge of the retracted wheel. I had to move the pivot point further back above the wheel. After thus changing the mechanism the original idea for rotating the wheel could no more be used so I had to change that, too. However, these changes are inside the nacelle and cannot be seen from outside.

The Saitos in the videos sound good. A Saito in a Jungmeister is a sound idea.

/ilkka/
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Old Sep 11, 2011, 04:55 PM
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I. Klemetti: I don't care what anyone says your work is an inspiration to me and I'm sure many others keep up the good work! I'm very interested to see what you have in store for the rest of your build.

Bruce
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Old Sep 11, 2011, 05:08 PM
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Quote:
Originally Posted by I. Klemetti View Post
PIPE: You know what a scale model is all about. It's a fake. It looks like the real thing but it isn't. This applies to my model's gear, too. The original retracting mechanism is not suitable to be driven by a servo. It was designed for a very compact and powerful hydraulic cylinder. The rear pivot point in the nacelle was at the front edge of the retracted wheel. I had to move the pivot point further back above the wheel. After thus changing the mechanism the original idea for rotating the wheel could no more be used so I had to change that, too. However, these changes are inside the nacelle and cannot be seen from outside.

The Saitos in the videos sound good. A Saito in a Jungmeister is a sound idea.

/ilkka/
If it looks right, it is right. Just keep building. You're doing a fine job !
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