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Author Topic: Airspeed Tern, Drawing Room Scale, 34" span.  (Read 1584 times)
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RalphS
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« Reply #25 on: January 21, 2021, 07:31:28 AM »

The Colditz Cock has a low aspect ratio wing.  An old clubmate made a slope model and a full size one.  I don't know if he ever finished it.  Others have made full size versions and a TV show was made.  Not as elegant as the Tern but probably easier to get a good performance.
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dosco
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« Reply #26 on: January 21, 2021, 07:54:17 AM »

The Colditz Cock has a low aspect ratio wing.  An old clubmate made a slope model and a full size one.  I don't know if he ever finished it.  Others have made full size versions and a TV show was made.  Not as elegant as the Tern but probably easier to get a good performance.

Wish I'd learned of the Colditz Cock when I was younger. That's a great story and an interesting design!

-Dave
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TheLurker
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« Reply #27 on: January 21, 2021, 10:19:47 AM »

Nothing specific yet. But I have always fancied doing a Slingsby Gull or Petrel in this style.
Ah.  When I was discussing the Tern as a potential project that man Blackburn he was agitating for a Rhonadler (Petrel is AIUI based on the Rhonadler) and I told him it was "highly unlikely", cos the wing would be utterly beyond me at this sort of scale. I mean it's basically a stretched triangle. I also think (*warning* unsubstantiated opinion) you'd need aileron control for it, it just *looks* spin happy.   If you want an elegant pre-war shape that *might* be do-able have a look at the Professor II, not the I cos it's got that pointy, pointy wing.   Martinn Simons has interesting things to say about that pointy wing shape, see if I can dig out the quote a bit later.

The Colditz Cock has a low aspect ratio wing...
Feh! Grunau Baby knock-off.  Smiley
Having said that It's sort of hovering around at the bottom of my wish list. How to mimic stripey fabric (mattress? bedsheet?)  covering in tissue is a bit of a puzzle though.
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TheLurker
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« Reply #28 on: January 21, 2021, 03:54:51 PM »

Mr. Simons on planform.  Chapter 6 sect. 6.3

"The Rhonadler ... It had a strongly tapered wing which was efficient at low speed, but required strong washout and a change of wing profile at the tips to prevent dangerous tip stalling and spinning. At moderately high airspeeds the tips were at negative angles of attack and 'lifted' downwards, bending the wing down and ruining penetration."  Mr. Simons also notes that the wing as built was efficient, but only at one, designed, airspeed.

As far as this build goes.  Working on the wing centre section for the last couple of days.  Main structure is quickly built, but the infill over the "hump" needed some thought.  Usually with infill I rough fill and the sand down when all panels in place, but with this one the hump needs to be sanded to shape as you go to allow for the fitting of the horizontal panels on the inner section.  I honestly thought I'd get the hump & inner panel done this evening.  Will I ever learn? Smiley

The outer bays of the centre section won't be sheeted until the wing is assembled becos clamping the sections together.
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Re: Airspeed Tern, Drawing Room Scale, 34" span.
Re: Airspeed Tern, Drawing Room Scale, 34" span.
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DavidJP
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« Reply #29 on: January 22, 2021, 04:19:57 AM »

I was lucky enough to meet Pat Reid years ago - excellent bloke - and among the many fascinating things I learned was that the Glider was mainly to keep people occupied rather than for an escape.  I also asked him about his views on the German guards who were a bit rough on occasions.  His reply was very interesting. He had no hard feelings at all.  He said that the prisoners job was to be disruptive because that needed a large contingent of guards which meant less for places like the Russian front for example.  So he said if you had just come off a long shift and were feeling rather tired all you wanted to do was get into your bunk as you would be up in a few hours on duty again.  On a cold and wet night would not you be a bit grumpy at having to get out of bed to go and sort out some rioting prisoners and may feel inclined to land the occasional kick or punch?  He had too some of the things the prisoners made as escape kit - the hand drawn/painted photographs and the “rubber stamps” made from Lino were simply amazing - only close scrutiny showed they might not be real.
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Yak 52
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« Reply #30 on: January 22, 2021, 05:13:55 AM »

I have a very good profile on the Rhonaddler in the Flugzeugtypen series by Hans-Jurgen Fischer (who is by far and away the best source of 3-views drawings I've ever found)

Truly a planform to keep you awake at night. But that is kind of the point I was making - this thread is inspiring me to try some of the less sensible glider ideas  Cheesy The Gull would be slightly easier but the Petrel is such a looker.

Nice structural work on the centre section Lurk. A lot of thought has gone in to that (and the drawing of course)

There is a general indoor glider thread somewhere if you would rather we don't clutter up your build...


Jon
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TheLurker
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« Reply #31 on: January 22, 2021, 06:00:19 AM »

Quote from: Yak 52
Truly a planform to keep you awake at night. But that is kind of the point I was making - this thread is inspiring me to try some of the less sensible glider ideas  Cheesy
"Less sensible"  I can think of less diplomatic ways of putting it and on your own head be it. Smiley 
Mind you... if you do get a Petrel flying I'll be first in the queue for the plan

Quote from: Yak 52
... if you would rather we don't clutter up your build...
Clutter away.  I do like a discursive build thread, not quite as good as having a natter over a coffee or a pint but in times such as these...
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lincoln
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« Reply #32 on: January 22, 2021, 09:17:32 AM »

For a free flight model, it's probably fine to have just one efficient airspeed, as long as it's trimmed to that speed. It's not really an airspeed, but an angle of attack, so it may be possible to position the tow hook for that angle, assuming the fin size and dihedral allow stability at that angle. However, I'm not entirely certain about the tow hook.
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TheLurker
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« Reply #33 on: January 23, 2021, 04:40:09 PM »

Quote from: lincoln
For a free flight model, it's probably fine to have just one efficient airspeed, as long as it's trimmed to that speed.
Aye, I'm hoping that will prove to be the case.  I think this one, if it flies, is going to be a light winds, fair weather only job.

The centre section is as near done as it can be until all wing sections are done and glued together.  Stupidly I though it'd only take me a couple of evenings to do this. Ha ha ha.
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Re: Airspeed Tern, Drawing Room Scale, 34" span.
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TheLurker
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« Reply #34 on: January 24, 2021, 02:18:40 PM »

One wing.  Need some dry, bright weather (slightly warmer would be nice as well) to shape the TE & LE and do some other bits of fettling 'cos there's only so much sanding I can get away with in the dining room and the light in the garage is not good enough for anything but rough work.  When that's done it can be put together & the centre section sheeting can be finished off.

Time to wander off and ponder on the fuselage and the best way of tackling it. I may be some time....

Images.  Making sure stbd washout matches port, wing plan elev. & front elev. showing dihedral.



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TheLurker
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« Reply #35 on: January 30, 2021, 08:53:23 AM »

The Production Engineering Dept. at Lurker Industries has been beavering away trying to turn the back of an envelope sketch of the fuselage from the Draughting Office into something that can actually be built.  The Dept. has, at long last, some ideas on general construction and has just submitted its proposal for ballasting to the Board for approval and has, as one man, shot off to the Four Ale bar of the Dog & Duck* so that they may drown the memory of the rubbish they received from the Draughting Office.

*Poor devils will find no solace there as The Dog & Duck has been closed by order of HMG.

Or....

  Mostly thinking. Partly about general fuselage construction and also about the ballast box.  Have a solution for ballasting.  Don't want to infill top stringers and put a hinged lid in as it will detract from what little scale appearance I can give it so noseblock will be removable with access hole in F1 behind NB.  Did think about small magnets, but haven't got any and am in a parsimonious frame of mind so will use a keyed hole. The rear of F1 will have thin ply faces / wedges to take up the slack as the NB is rotated into position. It may need a second (piano) wire pin at the base to stop the NB rotating and it's going to be interesting to see how well (if) it stands up to hard arrivals with any degree of shear stress. 


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Yak 52
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« Reply #36 on: January 30, 2021, 09:22:24 AM »

Took me a minute to catch up with you, as in gliders 'ballasting' refers to weight added at the CG to increase wing loading and therefore speed for a given Lift/Drag ratio. From the context I presume you mean 'balancing' with nose weight.

On my Tandem Tutor, as I recall, I established the practical CG position in flight using blu-tack on the nose. I then drilled a 5 mm hole up into the nose block from below, into which I could shove scraps of lead to maintain the same balance point. These were secured with a splash of thin CA and the hole covered up. Perhaps not so elegant a solution but it worked ok. I did have the skid think about which made a removal nose block rather tricky.

Jon
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TheLurker
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« Reply #37 on: January 30, 2021, 09:46:04 AM »

Quote from: Yak 52
... 'ballasting' refers to weight added at the CG to increase wing loading ...

I didn't know that, ta.   However I'm sloppy and use "ballasting" simply to mean any weight adjustment.  FWIW, last time I flew a glider (gosh, 20+ years ago) it also referred to adding weight forward of CG to make up for a lightweight pilot, I know this 'cos I was only borderline heavy enough* for P2 in a Ka13 and had the conversation about additional "ballast" several times with the CFI.

I'm intending that the bulk, say 75%-80%, of the "ballast", Smiley, will be fixed in the NB.  The box in the fuselage will let me tweak it to suit wind conditions on the day.  As for the CG. I have an estimate using a geometric method and a (very, very) rough tail vol. calc suggests that it's about right.  I may yet make up a sheet profile model and establish the CG empirically.  I did that for the Horsa and it worked very well.  Much less fraught than heading off to the field with an untried and yet to be balanced model.

Cheers,
Lurk

*IIRC 10 stone dead (140lb).
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Yak 52
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« Reply #38 on: January 30, 2021, 10:06:25 AM »

Yeah, I guess 'trim ballast' is an appropriate term. (I've been influenced by radio gliding somewhat.)

Yours,
Repentant Pedant
Jon  Grin
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TheLurker
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« Reply #39 on: February 03, 2021, 03:31:38 PM »

You are unlikely to be surprised to hear that the pictured approach to fuselage assembly is, ahhh, now, how shall I put it?  "Experimental"?   The plan (You have a plan? Really? Ed.) is to fit the stringers and then sheet the sides with soft(ish) 1/32".  There may be cause to rethink this approach.  We shall see.  Smiley
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Re: Airspeed Tern, Drawing Room Scale, 34" span.
Re: Airspeed Tern, Drawing Room Scale, 34" span.
Re: Airspeed Tern, Drawing Room Scale, 34" span.
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Yak 52
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« Reply #40 on: February 08, 2021, 07:44:31 AM »

Looks good Lurk. I've been looking at the pretty little Scheibe Specht but there isn't an obvious straight reference line you could use to build it on a board. So I'll be watching how you get on with this central tube method.

Jon

https://s3.eu-west-2.amazonaws.com/abpic-media-eu-production/pictures/full_size_0317/1476491-large.jpg
Airspeed Tern, Drawing Room Scale, 34" span.
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TheLurker
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« Reply #41 on: February 09, 2021, 03:45:15 PM »

Quote from: Yak 52
... I'll be watching how you get on with this central tube method.
.
Badly.  Smiley
I think it will work if the keel is sufficiently wide to prevent flexing and or rotation.  The keel on this one isn't.  Some balsa was ruined and a number of bad words were uttered, sotto voce of course.  The obvious alternative is to use square section tube, but I didn't have any to hand and didn't fancy a trip to a DIY barn.  The good thing about using a cylindrical tube is that you can use an offcut to cut exactly the right size holes. Never mind.  For this one am reverting to the same sort of construction as I used for the Willow Wren. See the side frame pics.  Part way through a new set of formers to go with them.

Quote from: Yak 52
...pretty little Scheibe Specht ...
.
It certainly is and striking scheme that looks easy to do as tissue only.  Lots of nose weight needed though?



The other experiment that didn't produce the desired result was the keyed peg fixing for the noseblock.  It will work, and I'll keep it in mind as a technique, but not at this size as you take so much wood out of F1 that it's left quite fragile even with the shortest possible retaining pegs. I could face it with 1/64" or 1/32" ply, but then it starts to get needlessly complicated. See the pic.

One experiment that did work, andbetter than I expected, was the way I tried of applying wax to the joint areas on the first frame so I could lay the second over it.  Take one tealight, light it.  When a pool of wax has formed about the wick use a chisel ended cocktail stick to drop & spread a thin layer of the liquid wax over each joint on the lower frame.  You need to scrape, gently, the wax off the lower frame when you separate them and it may need the very gentlest sanding as well, but I found it much easier than faffing about with cling film or greaseproof paper, both of which I've tried in the past and made a complete horlicks of.   Of course if I were using 3/32" or even 1/8" strip I wouldn't bother I'd just take a razor to the gap, but 1/16"?  Ehhhh, p'raps not.
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Re: Airspeed Tern, Drawing Room Scale, 34" span.
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TheLurker
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« Reply #42 on: February 13, 2021, 04:44:51 PM »

Fuselage mostly done, built it in much the same manner as the Willow Wren.  Sides yet to be sheeted (1/32", soft) and tail platform to be fitted, but that'll have to wait for a day or three.  Decided that the noseblock will be held on with a dowel push fitted into F1 possibly with a magnet for security.  I toyed with the idea of a slide fit with 1/16" ply rails, but again think that F1 is too small for it to be practical.

That's most of the tricky (for me) bits out of the way now, but need to sort out some fillets at the rear wing roots of the centre section

All components so far 15g by the kitchen scales.
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Re: Airspeed Tern, Drawing Room Scale, 34" span.
Re: Airspeed Tern, Drawing Room Scale, 34" span.
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TheLurker
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« Reply #43 on: February 15, 2021, 03:14:59 PM »

My dimness knows no bounds, I am a truly dim bulb,  In the words of the Goon Show joke,  "Only 20 watts?  Not very bright."

Sitting at the controls of the mahogany bomber today letting George do all the work* when I (think) I finally twigged how to do the push-fit noseblock.  Make the NB in two parts.  Use a bit of 1/4" sheet fixed to the front of F1 to provide a decently secure socket form the after part of the NB.  The front part plugs into the afterpart.  See sketch.

Lurk.


*I.e. sitting at my desk slack jawed & staring vacantly out of my study window instead of beavering away at the day job.

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Rich Moore
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« Reply #44 on: February 15, 2021, 03:22:05 PM »

Lush model. I still need to find that winder...
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Russ Lister
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« Reply #45 on: February 15, 2021, 03:58:23 PM »

Looking great .... a particularly attractive centre section you have there!  Smiley
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TheLurker
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« Reply #46 on: February 16, 2021, 02:46:25 AM »

Quote from: Russ Lister
Looking great .... a particularly attractive centre section you have there!  Smiley
Thank you.  One does try to take care of one's figure, but middle aged spread will creep up on a chap.  Ohh, you mean the aeroplane...  Cheesy
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TheLurker
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« Reply #47 on: February 20, 2021, 02:47:11 PM »

Plodding along slowly.  Fuselage sides are now sheeted and the coaming was fitted today. The main part of the coaming is soft 1/32" sheet with tissue reinforcement on the reverse.  I've simplified the line of the coaming at the rear of the cockpit, but not a huge amount. Mainly to allow for my limitations.  The blocks forming the rear of the cockpit coaming / forward wing root also serve as reinforcing braces for frame 4. I'm not planning to have any hard arrivals, but they do happen and my West Wings Swallow needed the equivalent frame repaired a couple of times where the wing came forward hard on impact.  The bit of foam serving as the pilot's head rest is temporary and just to check it all looks about right.

I'm beginning to think that it might be possible to simplify the fuselage construction by just using 1/16" sheet sides rather than a 1/16" frame with 1/32" sheet sides.  I opted for the frame and sheet to keep weight down - those wings! - but so far the weight, 8g for the fuselage, isn't a worry.

I'm also debating sheeting at least the front of the underside, it'll depend on the final AUW and how well (if) it flies.  This would be more like the prototype and save me a good deal of time in running repairs as the grass in my usual flying field is pretty coarse and does so like to rip holes in tissue undersides.

Gratuitous picture showing ballast chamber arrangement.

Noseblock tomorrow and work on the empennage next week.
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Re: Airspeed Tern, Drawing Room Scale, 34" span.
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TheLurker
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« Reply #48 on: February 21, 2021, 01:57:29 PM »

Plug in NB works surprisingly well. The cyano lining to the hole makes it a nice squeaky tight fit and a little cyano reinforcing of the edge of the removable portion is also called for so it doesn't flake with use, but other than all seems to be good. Some fine sanding, which I didn't spot until after I'd taken the pics., still to be done.

Fuselage now looks like Tern and as it weighs 9.1g without skid, hook & windscreen am going to go with tissue rather than sheet undersides.  Best guess at the moment is for a bare bones weight about 20-22g. Might be a bit more, depends on whether or not I find any decent light 1/16" sheet in my woodpile to finish off the wing sheeting.

Cheers,
Lurk
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Re: Airspeed Tern, Drawing Room Scale, 34" span.
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Re: Airspeed Tern, Drawing Room Scale, 34" span.
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TheLurker
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« Reply #49 on: February 27, 2021, 03:37:14 AM »

That's all the major sub-assemblies done, so time for the traditional "bones" pic.  As pictured 21g, but there's a few bits & bobs like skids, pegs some more sheeting and fillets and profiling of LEs & TEs to be dealt with so the true bare bones weight is probably going to be more like 25g.  Allowing 10g for tissue & dope and (by comparison with the Baby M.) about 5g of noseweight probably looking at an AUW of around 40g. Anything under that will be accepted with gratitude.

I've embedded some pics as well as the usual thumbnails because summat odd has happened to my camera which means the thumbnails I generate don't expand as they used to.  If you click on the embedded pics you should see a reasonably large pic. The embedded images will evaporate in time as I clear down the files I post at imgbb, but the thumbnails will be available as long as HPA is.


https://i.ibb.co/88J1KJb/Fin-Blank-Done.jpghttps://i.ibb.co/8mZHDk5/Stbd-Rear-Quarter.jpghttps://i.ibb.co/ysKnfsR/Stbd.jpg
https://i.ibb.co/sCZdFyr/Port.jpghttps://i.ibb.co/483vWhk/Port-Forward-Quarter.jpg
Airspeed Tern, Drawing Room Scale, 34" span.
Airspeed Tern, Drawing Room Scale, 34" span.
Airspeed Tern, Drawing Room Scale, 34" span.
Airspeed Tern, Drawing Room Scale, 34" span.
Airspeed Tern, Drawing Room Scale, 34" span.
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