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Author Topic: Junkers 87 B-2/R-2  (Read 7014 times)
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TheLurker
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« Reply #200 on: February 15, 2020, 01:32:21 AM »

Quote from: OZPAF
If the tail plane is glued on with PVA...
Unfortunately it's not. I'm going to try the brute force and ignorance approach first, i.e. tail weight although adding it feels like far more of a defeat than having to eat three portions of green goo on toast.

flydean,  moving the peg back isn't an option short of starting from scratch and given that it's taken nearly 5* months to get to this point then, short of it not flying at all that's not going to happen.

Now what is it they say about pride and falls?  Smiley

Cheers,
A still slightly hacked off Lurk.


*More like 10 if you include the prep work.
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Andy Blackburn
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« Reply #201 on: February 15, 2020, 02:38:45 AM »


> and I am sorely, sorely tempted to bin it as a bad job.

Don't do that, whatever you do - whatever legal objections might have been submitted in a last attempt to claw back some points, it's very far from a bad job. I think it looks excellent.

Firstly, I'd try installing a slighty bigger motor which might move the c.g. back a bit - my (admittedly rathere heavier) VMC Spitfire needed 4 strands of 0.100" on a 6" VMC prop.

Secondly, If you have to add tail weight, it probably won't be very much so I'd try it to see what happens.

If that hasn't fixed the issue to your satisfaction, then all is still not lost; if you've glued the fin on with balsa cement or CA then it should be possible to soften the joint with either thinners/dope or CA debonder, in which case I think the best approach is to build a new fin and stabiliser (with a little more negative incidence) and moveable control surfaces as suggested by OZPAF. Even if you've epoxied the fin on, it might be possible to cut through the glue line, given sufficient patience.

Even if you have to add tail weight, it'll still be quite light for a KK Stuka (which, in spite of recent protestations, is what it started out as).

A.
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TheLurker
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« Reply #202 on: February 15, 2020, 03:22:00 AM »

This morning's works have managed to cut the nose assembly weight further from 7.25g to 6.88g.   Careful, very careful, removal of the spinner disk, and opening out of the existing void in the back of the nose block.  This has pushed the balance point back a bit, not much and nowhere near 1/3+ of root chord, but enough to be noticeable and it will reduce the amount of tail weight needed so better half a loaf.

abl> ... if you've glued the fin on with balsa cement or CA...

Aliphatic & CA.  Aliphatic to give me enough time to tweak alignment & touched up with a few dabs of CA to give a solid joint when the aliphatic has cured because I've had 1/16" fins work loose using aliphatic alone.

abl>  ...my (admittedly rather heavier) VMC Spitfire...

What was its eventual weight sans motor?   One finds the forum search a little less than helpful.

abl> ...best approach is to build a new fin and stabiliser (with a little more negative incidence) and moveable control surfaces as suggested by OZPAF

Aye and it would be worth doing simply to improve its chances of flying well indoors, but at 5 months or so into the build phase I'm not sure I can face another block of significant (for me) effort.
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Russ Lister
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« Reply #203 on: February 15, 2020, 06:08:38 AM »

You started this build on such an 'up' .... you can't let it finish on a low!  Smiley I hope that the Kit Scale Police haven't got to you .... I'm thinking James Cagney in Angels With Dirty Faces! Roll Eyes
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Newbie_John
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« Reply #204 on: February 15, 2020, 06:59:11 AM »

Come on, Lurk - you've got all of us watching in the background hooked on this build! You can think of something!
And if it means not eating the "A" word, even tail weight has got to be worthwhile, surely!!

Seriously, I'm sure I'm not the only one who's been watching and learning and enjoying the banter - please don't give up now!

regards,
John
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LASTWOODSMAN
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« Reply #205 on: February 15, 2020, 07:33:23 AM »

Hi Lurker.  I echo Newbie John's sentiments also.   I smashed up five nice bipes trying to get them to fly,  and each one took 4 months to build, so I am just keeping them for display.   I can relate, though.   I hope you don't bin it.

LASTWOODSMAN
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OH, I HAVE SLIPPED THE SURLY BONDS OF EARTH ... UP, UP THE LONG DELIRIOUS BURNING BLUE ... SUNWARD I'VE CLIMBED AND JOINED THE TUMBLING MIRTH OF SUN-SPLIT CLOUDS ...
TheLurker
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« Reply #206 on: February 15, 2020, 07:38:24 AM »

After much imitation of a flock of decapitated Gallus gallus domesticus and not a little rummaging around for the sackcloth and ashes by the design and assembly staff at Lurker Industries a plan has been decided upon.

1. No major structural revisions or redesign at this stage.
2. Test fly with tail weight to see whether or not item 1. is worth doing.
3. In the meantime get sloshed on the best sherry the Directors can turn up.

Or...
After a little further nose weight reduction effort balance is achieved at 50% +/- of root chord with 3.6g of tail weight.  Which gives a likely initial weight sans motor of 36.5g.  I started the build hoping for a ballasted weight of 35g or under so a significant overshoot.  I'm (desperately) hoping that the CG can be shunted a touch further forward to knock a few tenths of a gramme out of the tail weight.  A 14", 4 strand 3/32" motor is about 2.9g so I'm looking at a flying* weight at best a breath under 40g and possibly well over depending upon motor used.

*Now there's optimistic.

I was expecting No. 18 to be heavier than K5083 as there's significantly more additional detail and if it had been, as usual, tail heavy it would have required additional ballast up front so perhaps given the changes made this is about the best I could have expected.

For comparison
My K5083 last flew at 31.5g
Abl's Blue PR VMC Spitfire went into proving at 32.8g
Jack Plane's Pink PR VMC Spitfire at 31g ditto.
All weights without motor.

If No. 18 shows signs of flying even moderately well as it is then a return to the drawing board will be justified otherwise it'll be written off as a monument to my hubris and I will with, great reluctance, a heavy heart and a packet of indigestion remedy by my side, eat some green goo on toast. Smiley

Quote from: Russ Lister
You started this build on such an 'up' .... you can't let it finish on a low!  Smiley I hope that the Kit Scale Police haven't got to you .... I'm thinking James Cagney in Angels With Dirty Faces! Roll Eyes
Just a bit disappointed that it looks like it's going pear shaped at the very last step. I'll get over it.

Perhaps, given the potential for a,  metaphorical, shooting down in flames "White Heat", "Made it, Ma! Top of the world!" might be a more appropriate reference?  Smiley

Quote from: Newbie_John
...please don't give up now!
Don't worry, the staff at Lurker Industries have recovered a measure of equanimity and composure and will not give up until it is proved beyond doubt to be an utter Meleagris gallopavo.  Smiley

Quote from: LASTWOODSMAN
I smashed up five nice bipes trying to get them to fly,  and each one took 4 months to build,
Ouch.  Rather puts my pitiful woes with this one into perspective.
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Indoorflyer
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« Reply #207 on: February 15, 2020, 10:37:40 AM »

Lurk, what is the wing area? If you end up with a flying mass of 40 g, the incremental increase spread across that big wing may not be so bad after all. What wing loading would that give you?
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TheLurker
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« Reply #208 on: February 15, 2020, 11:15:30 AM »

Quote from: Indoorflyer
Lurk, what is the wing area? If you end up with a flying mass of 40 g, the incremental increase spread across that big wing may not be so bad after all. What wing loading would that give you?
Dunno. Too scared to work it out.  Smiley

ETA.
Using all my fingers and toeses, ignoring the wing tips and underestimating the area a bit more to allow for an/dihedral I get a single wing area of 1.77dm2 (3.54dm2 total) which at 40g gives a wing loading of 11.3g/dm2.

Further ETA.  Using http://www.ef-uk.net/data/wcl.htm which does the arith. for you that comes out at 3.7oz per square foot or a wing cube loading of 5.9.

Dunno about anyone else, but that seems worryingly high to me.

Now, given that MrsLurker does all the household accounts because, despite having done A level maths and Stinks to degree level with a garnish of Physics, my arithmetic is utterly attrocious* I would take that figure with a whole bushel of salt.

Lurk.

*True story.  Maths class.  Teach is dishing out the jotters having marked the homework when he comes to mine, "Boy Lurker!", "Sir!", "What's three threes?" Quick as a flash I answer, ... "Six"  Whereupon I am struck upon my, non-bald** but utterly dense mid teenage, bonce.

**Hard to believe I know, but it is true, there once was hair upon this hairless pate. I have photographic evidence from a time before Photoshop.
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Andy Blackburn
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« Reply #209 on: February 15, 2020, 01:14:40 PM »

> Dunno about anyone else, but that seems worryingly high to me.

Executive summary : It'll be fine.

Arithmetical Evidence:

You can work out the wing area from Wikipedia; since we know that the KK Stuka is 19" span and the full size spans about 45 ft 3.5 in, the scale factor must be ((45 x 12) + 3.5)/19 = 28.6 . The wing area of the full size is about 343.37 sq ft, so the area of the model wing must be (area/(scale-factor x scale-factor)) = 343.37/(28.6 x 28.6) = 0.419 sq ft =~ 60 square inches. I think the difference is partly because the "standard" wing area always includes that bit of the wing covered by the fuselage.

That means that the wing loading at 40 grams is 40/0.419 = 95 grams sq ft. The equivalent figure for my VMC Spitfire at 46 grams was 113 grams sq ft. I think the best that managed on about 800 turns was about 27 seconds.
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billdennis747
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« Reply #210 on: February 15, 2020, 02:16:42 PM »

I once worked in agricultural research and we had a machine that measured area. You fed grass leaves through it and up came the area. I seem to recall once cutting out a paper  tailplane of a model I was worried about and feeding through the machine. Eventually I realised that small tailplanes are rarely a problem and stopped worrying. Only a Ryan S of St L  was tricky and a Halberstadt impossible
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TheLurker
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« Reply #211 on: February 15, 2020, 03:01:56 PM »

Quote from: Andy Blackburn
... the area of the model wing must be (area/(scale-factor x scale-factor)) = 343.37/(28.6 x 28.6) = 0.419 sq ft =~ 60 square inches.
 I think the difference is partly because the "standard" wing area always includes that bit of the wing covered by the fuselage.

Working back from the area taken from the plan and approximating 1 sq ft to 9dm2 I get 0.4 sq ft for the wings ignoring fuselage and 0.42 sq ft treating the area of the fuselage as a part of the lifting surface.  Point 4 gives an upper limit of 100g/sq ft.

Quote from: Andy Blackburn
...equivalent figure for my VMC Spitfire... 800 turns was about 27 seconds.
You have no idea how just how cheering your figures are.  If I can get anything over 20s from this I'll be very pleased. However...

... I think I need to go back to the plan and see what can be done to limit the tendency to nose-heaviness.  With that chin and UC I think it's always going to tend that way, but a few things strike me that can be done without destroying the look of the end result.

1 - Lose the infill on the nose forward of the cockpit.  If you scallop the frames* you'll still get a decent curve without it. That should knock about 0.05g/panel out forward of the CG. Saving somewhere between 0.7 and and 0.9g.

2 - Don't try for the open rad. inlet.  It doesn't work and adds the dead weight of the ply reinforcer.

3 - Don't fit a ballast plate.  There's plenty of space on the back of F2, which is hidden by the rad. gills, for any ballast that may be needed.

4 - Use 1/32" sheet for the spinner plate and cut it out when the nose assembly is completed.  You could use 1/16", but 1/32" will be easier to remove without damaging anything

5 - Lose the short doubler strips fitted to provide a hardpoint for the exhaust stacks.  They aren't necessary.  The top line of the exhaust stacks stack coincide with a stringer run.  See the pic.

6 - Build voids into the noseblock laminations.  Amusingly enough I considered doing this to start with but then I thought, "Nah it'll be tail happy and I'll need the weight up front."  Hah!

7 - Use a square nose plug, 12mm nominal as per VMC's MfM kits, rather than the rectangular plug I used.  The rectangular plug was because I had originally planned to fit a thrustline adjuster.

8 - Don't use so much paint on the nose block.  Yeah, I know obvious, but there are more coats on this than there should be because of that dud tin of yellow.  I may yet sand back and repaint. It won't save a lot, but it will save something.

All told I think those changes would be good for a reduction of at least gramme forward of CG with a corresponding (hopefully larger) reduction in tail weight.  If my estimate is any good at all that would bring the flying weight with down to about 38g with a 14", 4 strand 3/32 motor which gives a wing loading of ...

  10.8 g/dm2
  3.4 oz/ sq. ft.
  ~90 g / sq. ft.

Another marginal weight reduction might be to cut voids in the two in-most UC leg lamina, as per the central ply core. That would probably net another 1/5 to 1/4 g, again forward of the CG.

Cheers,
Lurk.

*I meant to say some time ago. I scalloped the frames on this build (a Lurker Industries first) and was very pleased with the result when it came to covering.
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TheLurker
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« Reply #212 on: February 15, 2020, 03:08:30 PM »

Quote from: billdennis747
You fed grass leaves through it and up came the area.
Now that's interesting.  How did it work?  Analogue or digital computing?
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billdennis747
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« Reply #213 on: February 15, 2020, 03:19:33 PM »

Quote from: billdennis747
You fed grass leaves through it and up came the area.
Now that's interesting.  How did it work?  Analogue or digital computing?
This was back in the 70s. I think it had a row of tiny lights and receptors that recorded when light was blocked off, linked to the speed of the feed through. It was a Heath Robinson device, probably made in the workshop. It was effective though
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TheLurker
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« Reply #214 on: February 15, 2020, 03:26:10 PM »

Quote from: billdennis747
Quote from: TheLurker
Quote from: billdennis747
You fed grass leaves through it and up came the area.
Now that's interesting.  How did it work?  Analogue or digital computing?
This was back in the 70s. I think it had a row of tiny lights and receptors that recorded when light was blocked off, linked to the speed of the feed through. It was a Heath Robinson device, probably made in the workshop. It was effective though
Ahh.  I like that.  Lot to be said for straight-forward solutions.
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kkphantom
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« Reply #215 on: February 16, 2020, 05:37:08 AM »

Have you considered using a balsa prop? Certainly lighter and probably more efficient than a plastic one.
Gary
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TheLurker
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« Reply #216 on: February 16, 2020, 09:48:08 AM »

Quote from: kkphantom
Have you considered using a balsa prop? Certainly lighter and probably more efficient than a plastic one.
Very briefly.  I have always considered prop. carving to be deep magic and far, far beyond my wood-butchering skills.  There's a reason I use laminations so very much for all my builds. Smiley

Just out of interest what would be a likely pitch and diameter to replace the 6" plastic paint-stirrer used for this build? And what sort of weight would you expect it to be?  For reference an unbalanced 6" VMC supplied prop. averages 2.5g.  I've never bothered to weigh a balanced prop. There never seemed to be any point with most models being tail rather than nose heavy.

In other news the final planned bits fitted;  the elevator mass balances.  A tweezers & beta-blockers job and one I'm glad I didn't try to cram in last weekend.  Fiddly? I'll say. They're slightly overstated, too deep, but I'm content to let that ride.

Now a case of cutting some matching lead tail-weight (*sobs*) to glue just ahead of the rudder post, making up a fresh motor and then waiting for reasonable trimming weather.

Given my stocks of strip I'm planning a 14" to 15" 4 strand 3/32" (0.094") as used in my Camel which flies at about 42g.  Anyone care to offer an opinion as to the wisdom or folly of this choice?

Lurk.
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Andy Blackburn
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« Reply #217 on: February 16, 2020, 11:08:31 AM »

> Given my stocks of strip I'm planning a 14" to 15" 4 strand 3/32" (0.094") as used in my Camel which
> flies at about 42g.  Anyone care to offer an opinion as to the wisdom or folly of this choice?

Personally, I'd go perhaps a little bigger for outdoors. Maybe 4 strands of 100 or 109 thou...?

A.
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TheLurker
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« Reply #218 on: February 16, 2020, 12:55:47 PM »

The stores wallahs tell me that all they have to hand are the items listed below and given the expenses incurred for both materials and overtime on this project already discussed in these memoranda it is unlikely that Mr. Chisholm will approve any further expenditure.  

1/4"    0.25
3/16"   0.19
1/8"    0.125
3/32"   0.09
1/16"   0.063

Or...

OK, proposed motor is a little under-powered but not ludicrously so for initial flight trials.  If it gets through the 12 second barrier, a story soon to be rendered for the Kinema by director David Lean and starring many of Britain's finest actors and actresses with staff from The Lurker Industries Kinematography Club in supporting rôles, then we shall explore the possibilities of an uprated motor.
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Rhys
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« Reply #219 on: February 18, 2020, 01:03:56 PM »

Wonder if anyone has ever employed a small device wheras you could check the fuselage nose/tail balance as you build?  A simple frame with withdrawing metal pins of size on each side. Just a feeble thought.

 Sturzkampfflugzeug looks Fantastisch!!!

Cheers
Rhys
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Graham Banham
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« Reply #220 on: February 19, 2020, 05:32:55 AM »

Lurk: apologies if i’ve missed something but have you actually tried to fly it other than bed glides (which will usually tell you nothing in practical terms)?

You may be worrying needlessly as the solution could be as simple as a couple of acetate tabs bent up on the tailplane, or nothing at all once it’s powered up.
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TheLurker
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« Reply #221 on: February 19, 2020, 11:39:49 AM »

Quote from: Rhys
...looks Fantastisch!!!
Aye, well.  Until and unless it flies it doesn't matter how good (or bad) it looks, and I'm still deeply narked I dropped summat on the stbd wing, nonetheless the compliment is appreciated.  Thanks.

Quote from: Graham Banham
...have you actually tried to fly it other than bed glides...
I drew back the curtain, to see for certain, what I thought I knew... aye, it's still blowing half a gale and raining stair-rods, or καρεκλαποδερα for the more cosmopolitan members of the parish, and forecast  to do much the same for the foreseeable.  Nope  Smiley 

It's a teeny bit annoying; had I managed to finish the build anywhere near my planned end date I would have had 2 or 3 days of calm, warm(ish) still weather towards the end of Jan. to test it.

Short of removing and resetting the stab with more neg. incidence the acetate tab idea is well worth considering.  Ta.
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Squirrelnet
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« Reply #222 on: February 19, 2020, 01:39:24 PM »

Quote
Short of removing and resetting the stab with more neg. incidence the acetate tab idea is well worth considering.  Ta.

or even adding a gurney flap to the top surface of the tail Huh

Personally I still think your black pudding is safe....
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Andy Blackburn
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« Reply #223 on: February 19, 2020, 04:02:30 PM »

...

Personally I still think your black pudding is safe....

It pains me to say so, but I agree. I think you've got this one in the bag, Lurk. Protests notwithstanding, of course.

A.
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TheLurker
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« Reply #224 on: February 23, 2020, 02:55:52 PM »

The Met. wallahs keep piling on the misery here.  Absolutely no chance of flight trials for heaven knows how long so I've been getting on with correcting the "dud" formers so that the after part of the fuselage looks right.  Well, less wrong.

While I'm here.  It has been suggested, see discussion above on nose-heaviness, that the peg position be moved back to bulkhead No 9.  Now this strikes me as a nice idea, but given the width of the fuselage at that point I'm not sure that it's terribly wise.  It looks to me that F9 can't be opened up enough to give good access for a stuffing stick without making everything too fragile to take a fully wound motor.  See the progression pic for F9, the test formers are laid on a 1" grid

Question is; if, hypothetically, you were building a model to this plan would you risk opening up F9 so that the motor peg could be moved back?

Pics. 
Progression through formers to "correct" shapes,  test stringer fitting to be sure that the lines are OK and the fuselage of the current incarnation showing the boxy aft section for comparison.
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