|
JU52
Junkers - JU 52 3M (Tante JU) Tri Motor – Electric
It is with great pleasure that I can now sit and write to tell of my success and pleasure with my long-standing JU 52 three motor project. Electrics have once again been the saviour in giving this project life.
Like my WW2 Lancaster Bomber before this, I have wanted to build a JU 52 Tri Motor for as many years as I can remember. The real box fuselage, splayed out wing engines and the offset flaps / ailerons and elevator have always made me feel attached to this boxy design. There are that many colour schemes, from pre WW2 and all the way through it, that the choice is endless and the possibilities amazing.
From those who know the aircraft, one of the first questions is “What about the corrugations?” I found that there was no real easy lightweight solution, so I left them off. I believe that if I tried to do them, I would have scrapped the entire project. The way I did the covering and the paint job has helped to give a feel of the full size one.
Potentially there are some stumbling blocks.
Like most of my projects, I did a lot of research on not only the aircraft, but also it’s uses and its theatre of operations. All the way through the building process, I continuously changed my mind on colour schemes, going from the Winter Camouflage used in the movie “Where Eagles Dare”, to an Air Ambulance scheme used during the Stalingrad airlift towards the end of the war. It was only days before I started painting that I settled on the “Splinter” colour scheme now used. There are 5 colours plus all the markings, which were painted on using masks. (Must be insane, however I do like the finish and feel that it was worth the extra time spent.)
CONSTRUCTION
Having obtained a set of basic plans, I had them copied and enlarged to my dimensions. From this, I created many sets of templates for foam cutting the fuselage as well as the wings. I paid extra when I purchased the foam blocks initially to have them cut perfectly square and to my dimensions, so that the mating of sections accurately could be done with the least amount of effort. I also made sure there was enough foam in the blocks so that I could set the wing templates in such a way that the dihedral angle could be cut into the cores as well as some washout at the wingtips.
Having cr eated the templates, I solicited the help of Neil Spencer (left, holding JU 52) from our Doncaster Club to assist in the foam cutting, which took longer than anticipated, however the final product was well worth it – thank you Neil!!!
Learning from experience, the front bulkhead template was done in 3mm Ply, which was left attached to the foam and has now become the former that the wing pegs plug into when assembling for flying. Another past experience incorporated, was the cutting of the wing section into the fuselage at the same time, so that the amount of time spent mating components together would be minimised.
FUSELAGE
Taking stock of all components, I was now presented with quite a few things that required assembly and this I did using 3M77 spray on glue while carefully aligning the 3 sections of the fuselage. Due to the shape and the compound curves of the front of the fuselage where the cockpit and the central motor are mounted, this needed to be final shaped by hand. Again, time taken here means that there will be time saved further down the process. Having incorporated an internal flat ridge while cutting the fuselage, I now inserted and epoxied a laminated ply tongue approx 400mm long to hold the main battery pack. Where the rear section of the fuselage mates the main body to the tail was still solid foam, due to the narrow sizing at the rear, this now had to be cored to accept the rod for the rudder leading from the servo located forward by the wing bolt on section. Using a sharpened carbon rod initially and then a round file this was accomplished with more time than expected, however being careful here has left this section quite strong without taking away too much foam.
WINGS Before joining the wings, I used a Dremel with routing attachment to cut channels in the foam for 6mm square spruce spars, which were then glued in using epoxy. At the same time using the Dremel routing attachment and using a small ball cutter attachment channels were now made for servo and main power wires. Again using 1mm Balsa sheet, the wing cores were sheeted with 3m77 and the leading edges attached and shaped to the correct form. Having cut the wing cores with washout built into them, this now became apparent and very pleasing, as they matched each other at the wing tips.
The 2 dowel pins were then inserted into the leading edge and mated to holes in the front former of the fuselage. Using rulers, string and squares, the final position was marked drilled and then mated to the fuselage via 2 x 6mm nylon wing hold down bolts to keep things secure.
Having already created the offset hinge points for the flaps and ailerons, these were now mated to the offset arms, which I then cut slots for and epoxied into the trailing edge of the wings. This certainly ended up better than I could ever have expected and I was extremely pleased with the final result.
Once again templates were made and the wing nacelles and motor mounts were created from balsa/ply mixture and mated to the wings allowing hollows internally for the wiring and motors themselves.
The centre sections at the dihedral breaks were now reinforced with fibreglass bandages. The full one-piece wing was then sanded with fine paper to smooth everything and create a good surface for covering.
FINISHING Using an air compressor, the entire airframe was blown free of all dust and dirt that might still be ingrained in the surfaces.
Starting on the wings, I created a straight line from wingtip to wingtip and sprayed 3M77 approx 2.5” wide and then used 2” wide fibreglass reinforced tape as an added strengthening spar on the under surface. Starting at the trailing edge and using Office Works 2” wide ‘Storage Tape’ which is heavier than the normal, I completely covered both the top and bottom surfaces, which really gives a strong surface with great strength properties, as well as ridges which help the corrugation look.
The entire covered structure was then sanded with 180 grit and 400 grit wet and dry paper to fully take the gloss off the tape and make keying surfaces for the paint. Looking at the specs of the Model Masters paints as well as some experience with the products, I decided to spray with no further preparation other than a rub down with Acetone.
On advice from the hobby shops, the correct colour paints were purchased as well as thinners for airbrushing. The main base colours went on really easily and quickly, but then the masking for all the patterns of the splinter had to be done and matched to both the wings and the fuselage. This is a time consuming and tedious job made easier with painters ‘low tack’ masking tape and newspaper for the masks. The actual spraying is approx 1/20th of the time compared to masking and setup.
The nacelle rings were proving to be a huge problem in obtaining something off the shelf and I really did not want to go down the path of making moulds. The answer finally presented itself in the form of 1.5 litre water bottles of the correct dimensions, but with ridges going around. I have used them and found that the final product is fantastic, finally painted in yellow with black facings simulating the engine cylinders. All markings were done via templates including the windows using silver paint to give the glass reflection look. The cockpit was done in silver as well, using trim stripe for the metal frame detail.
FLYING After finishing, the first taxi tests were done on a concrete driveway at home. Results were pleasing with easy response to control surfaces and power. Next logical step was naturally off to the field same day.
In subsequent flights I have tweaked the CG, moving surface throws and flap settings. As I have to fly the aircraft more to complete the final setups, I am sure that with a minor amount of work, I will have more than I could ever have hoped for. At the moment the low speed handling and stall is pretty scary. I really am looking forward to changing the landing gear to get that classic WW2 tail in the air roll out and landing as well as the really slow realistic flights. Scratch building has challenges, but also gives me the greatest feeling of accomplishment after so much work. This project has been incredibly rewarding.
August 2007 – Replaced the 3 brushed motors with 3 low price brushless outrunner motors and 3 speed controllers from United Hobbies. Main power is now via a 3 cell 6000mah Li-Poly battery. The change in weight, performance and general handling has dramatically improved and I can now say that I am totally satisfied with the end result. The aircraft no longer scares me.
FINAL SPECIFICATIONS – JU 52 Tri Motor
Ø
* Span: 2.1 meters Colin Kahn
|
|
*** 10 January, 2010 10:26 PM +1000 *** |
|
|
|
Home | About | Print | Feedback | Contact Us | Site Map | Disclaimer | © 2010 |
I had
decided to use brushed motors as this is not an aircraft that was going to
be flown all the time and the price difference between 3 brushless motors
including 3 brushless speed controllers was very different from my eventual
configuration. Also having a 70-amp Schultze speed controller sitting there
doing nothing ended up clinching the deal, as I am using one speed
controller for all 3 motors.
Extra time
spent on accurate templates means that there is significantly less time
being spent in finishing. To this end I photocopied extra sections of the
plan or my drawings and then glued them directly onto 3mm craft wood and
then cut the outlines using a scroll saw. Finishing off done by mostly
circular sander and of course hand finishing using a Dremel and sandpaper.
It was
always on the cards to sheet with 1mm Balsa, but the compound curves and the
tight corners became a little daunting, as I did not want to use a planking
method. I kept looking and finally acquired some really great quality
flexible balsa sheet that I thought would work. It took months before I
actually figured out a way of sheeting without adding much extra weight and
still be time effective. Having joined a few sheets together to create a
panel, I sprayed 3M77 on both the foam and the sheet and let them dry for
approx 3 minutes before wrapping it tightly around the fuselage. At all
curves and corners where the sheet kinked, I simply slit the balsa and did
an overlap. Using a new and sharp blade I simply cut away the excess overlap
and then sanded with wet and dry to get it smooth. Although the sandpaper
clogs with the excess glue in the overlaps, it really did not take long to
get a great finish with no requirement to fill in gaps. I will certainly use
this process in the future!!!
Using the
Dremel router again, I created 2 channels to take the spruce for the fixed
landing gear. These I glued in using a liberal amount of 12-hour epoxy, as I
really want them to stay in place. With the help of Rod King of Hobby and
Model Warehouse,, the landing gear was bent and basically finished awaiting
final assembly. Once again Neil Spencer assistance was sought for the final
joining and soldering.
As the
trailing edge moving surfaces are independent of the wings these were
created by sanding hard balsa to the correct wing sections required (flat
bottomed fortunately), then split for ailerons and flaps. The long taper
towards the tips created some problems in retaining strength, straightness
and accuracy of section. Using a plastic clevis fork, these were inserted
into pre-drilled holes, which will then become the hinge points for the
offsets from the main wing. Taking extra time again has proven its value for
easy assembly at the end.
This
method was then applied to the elevator as well with the same successful
results.
Using
the same concept including the fibreglass-reinforced tape, the fuselage was
done as well. Reinforcing here was down each side of the fuselage. The end
result was a reasonably lightweight and strong structure.
With a fully
charged pack and placed on the grass runway, full power for take off only
resulted in lifting the tail and digging the central prop into the spongy
grass. With some push assistance, the first roll on grass was obtained but
needed full up elevator to stop it from nosing over once it got moving. This
however then created the first unintentional flight of about 10 meters. The
results looked fairly stable and the second push taxi resulted in the first
full flight. Take off was reasonably easy with a moderate headwind, but
ailerons were less than effective for my requirements. The roll rate was
really sluggish with the aircraft tending to roll to the right. After
approximately 4 adrenalin pumping minutes I landed safely and reset the
aileron on the left slightly up to counteract the roll tendency, and
electronically mixed aileron to flap thereby creating full span ailerons if
required. On inspection, the landing gear is smack on the C of G point,
hence the nosing over. This is to be rectified ASAP.
