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Roy Scott Ju88 project


David perry 1
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I'll start a new thread...

I have taken over a very old build, maybe 20 years or more, of the 1976 Roy Scott Ju88 bombermaschinen.  Most of the construction is done, some sheeting left to do and the installation.  As it stands -  I will add photos -  the flying surfaces are done. No retracts with it, a very interesting frame for the canopy and he nose glazing are all here.

 

Whilst it is designed for 2x 30 glows I shall be electrifying it which is why I was dismayed to learn the target weight is 9 1/2 lb...I thought maybe 7lb, so 800 - 1000 watts would do.  At nearly ten pounds even that kilowatt might be light...   I hope to shave weight where I can now.  I havent yet weighed it, so its hard to make an acurate stab.  Thats this afternoons job.

 

A few ideas...the pushrod and bellcrank system for aileron control is probably heavier than two mini servos.  The full length push rods for elevator and rudder ditto.  The tail wheel is more like naval plumbing and simply has to be taken out.  I will probably replace with a plstic castoring t/w.

 

Undercarriage is designed for the then standard electric retracts but todays will be lighter so I might go that route.  Alternative is fixed or dolly.  At this stage I am thinking lightweight electrics though. 

 

 

Any and all ideas and thoughts most welcome.  Photos to follow (assuming anyone is remotely interested)

 

David

 

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Of course we are interested!

I think we probably build our electric models much too heavily.    Last weekend I was examining a fellow club member's models - both from Ivan Pettigrew plans and both electric- the over 6ft  Chipmunk was very light perhaps 5 pounds and flew extremely well on a 3s 2900.  The Twin Otter at almost 6ft span weighed about 4 pounds.  Construction was minimal.    I think thats the way to go.

 

But your model is mostly built so probably cannot now be built as light as a new build could be.  What do the parts weigh?

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Aahhh. October1976. Cutting my rc teeth when this came out. See that advert for Horizon on page 6 ?. My first system. No range out of the box, brand new. Got it fixed (micro mold) and very soon swapped it for Skyleader.

 

As regards two stroke versus electric, they are the obvious choice for multi motors. A two stroke thirty on song will probably put out, 500/600 watts ish ?. Seeing as how 750 watts is one hp. My  Axi 3540 puts out 650 watts on 4s power, and seems very happy doing it. Love the 88 !!.

 

Maury

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4lb seems not too bad for the uncovered parts without any gear.   The critical part might be covering and painting which might increase weight a lot.

It looks quite well built but there seems quite lot of hardwood parts that might not be really necessary for electric and with wing servos.

My other thoughts are....

is there enough prop clearance for whatever props you might need for the electric setup?

is the almost hidden elevator joiner and horn  soldered and done nicely?

Has the wing been made with the washout incorporated?

 

Mick Reeves Models sell some epoxy sheet which can replace 1/64th ply and perhaps that could be used for the nacelles.

 

 

Note the name in the letters page at the very end of that 1976 RM......surely must be the same P Christy that still posts on this forum!

Edited by kc
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IMO I would stick with the Roy Scott and see if a WB Replica kit turns up at some point as they are significantly different from just the wings (what's better than one JU 88, two!). And to be honest, weight in the wings (around C of G) might not be a big issue as getting the weight with an electric set up far enough forwards. 

 

I have a mossie that has motors/ESC's a bit bigger than necessary, just to get the weight forwards and save adding more lead nearer the C of G (due to light weight cowls).

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My 2p's worth coming up.

 

1st give the airframe a good sanding to get rid of the old tissue that looks to be sort of attached.

Bin the rudder, make a new one the correct size and with a stick frame type construction to reduce weight. Remove and bin the rudder pushrod and use closed loop - again lighter weight. Replace wooden elevator pushrod with carbon fibre (weight reduction again), and as you stated replace tail wheel mechanism with something thinner, more modern & lighter. Use a foam wheel for the tail wheel - again light weight.

Remember with a model like this, even a few grams / ounces at the rear will require many of the equivalent at the front to counter balance.

Remove if possible the engine bearers in the nacels as they are now unnecessary. Sheet the nacels with 1/16" balsa - no need for more.

I totally agree with the removal of the bell crank / pushrod system for the ailerons.

For servos, I would look at something like HS81's IF you can keep it light enough.

Keep the servos forward, both the fuselage ones and the wing ones - as.close to the spar as you can would be good.

 

Lighter weight means it can fly slower which will need less power so lighter batteries. It also means that if you were to crash the lower energy carried into the crash will cause less damage.

 

Strangely, I believe I am the modeller referenced to earlier by KC as he has described both my models. I never knew.

The machines referred to are free flight type stick / frame construction with shaping pieces outside the frame then sheeted. Both designed (by Ivan Pettigrew) at the time when electric flight was buggy brushed motors and heavy lower power / short duration NiCads.

 

Good luck with this project, I shall be watching with interest.

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Hi  David,

I have built both the Roy Scot and Warbird's Ju 88, I was going to send you the original R.M. article but you now have it. By going electric the nacelles will need to be modified with the motor mount moved further forward and enough room left to house the battery if like the Warbirds one you are fitting them in the nacelles. The alternative is to mount them in the cockpit but then you can't add any detail. 

On mine I can just squeeze in  a 4s 3600 battery behind the motor, it is powered by two Racestar 4250 800Kv motors from Banggood. Due to the high Kv I have to use 11.5" diam. props it performs very well on these I did have 14"  but they drew too much current. (no problem with clearance). I don't think the 88 ever had 4 blade props.

The rudder looks fine for the bomber version.

As the wing is unskinned it should straighten when you put the sheeting on 

As Andy said 1/16" sheeting would be fine for the nacelles as there is not as much stress with electric. 

Cheers Eric.

 

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Hi David, 

on page 15 of the Warbird Ju88 there is a short vid. of mine flying and on page 14 a picture of the model after it's maiden. If you enlarge it you can see the hatches for the batteries. this picture will give you a better view of the nacelle construction. 

I did not cut the shafts down on the motors where they protruded into the battery compartment and did not realise one was rubbing on the battery. on take off it was dropping a wing then recovering I then found why too late, it was picking up on the plastic cover of the battery and slowing the motor, as this was intermittent I did not find out until it was too late. after repairs the motor shafts were shortened and it has been fine ever since. Just waiting for the strip to be cut and some fine weather to fly it again.

The Roy Scot one flew good too but when an engine cut it crashed not too bad but one engine was damaged so I modified the front to take a 61 two stroke and it flew like that for a long time. I shaped the nose to look like the early night fighter and the prop was not apparent once flying.

Eric. 

_20210711_074106[2343].JPG

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Getting interesting.  Im using the retracts from  another, sadly now gone, model which will do the job but as I try to fit them I find the previous owner (and builder) hasnt really factored retracts in.  A lot of work to be done to fit them, all do-able of course.  It's making the modelling very interesting indeed as Im having to assess what can be removed, what must be removed and what needs to be left or replaced to support the retracts.  Nothing to actually show yet but in a few days there should be.  Digits crossed!

 

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Quick question before I fall foul of Ye Olde English over-engineering bug...

 

I'm prepping the retracts for the Ju but as the original build wasn't made for them I have had to cut out some skin and rib and so am left with what you see.  My intention is to add a ply plate between the motor bearers which stretches back to the aft extent of said bearers.  This will be well glued in even possibly glassed around the bearers.  On the other side, upper surface,  I am not yet sure: I can add further lite ply ribs or struts or just pour in some poly foam and watch it expand!

 

My question really is this: as the retracts came from a foam spit (big one, 70inch and 6s) and were only bonded to the foam and yet worked, is my plan here sufficient, overkill, or right (or just wrong).  Size wize, the wheels are 3.5in and the legs are 5mm piano wire.

 

I know it will work...but at what cost!  I'm after experience - giants shoulders on which to stand if you like.  Remember that this is a resurrection build...it was started 20 odd years ago and it is what it is.

 

Ta

 

D

20220212_111345.jpg

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Hi David, 

I had a discussion about this recently with Richard Wills  where I remarked on the foam models having the retracts bolted into plastic boxes moulded into the foam and we came to the conclusion the loads must be transferred throughout the foam, also they are generally lighter than built up models. I am currently building a 71" span Mosquito from foam board following the success of my Me 110. out of the same material. I feel tempted to try something like that for retracts but if the expanding foam does not work it will be a hell of a job to remove it. 

It may be worth a try to use a block of foam that the foamies are made from and mount the retracts in that and bolt it in the nacelles.

The full size Ju 88 had twist and turn retracts but I would not like to try and get them in . One of our club members has an FMS  Corsair foam model and he has a lot of trouble with them.

      Eric.

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IMO, don't beef it up to much and ask yourself will it take normal landing forces compared with balsa/foam models?

 

If you go over the top what will break if there's pilot error or something untoward happens (another failure causing it to crash), better in my book to rip the retract out or break the mounting plate rather than rip the entire nacelle out of the wing.

 

Have a look at Richard's design's, but in the end its a box in a foam veneer wing for all the single motor models and that works just fine.

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