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Author Topic: Airspeed Tern, Drawing Room Scale, 34" span.  (Read 903 times)
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TheLurker
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« on: January 06, 2021, 01:55:03 PM »

Ladies, Gentlemen, Girls & Boys it's about time for another Lurker Industries Aviation Co. Ltd. Build Thread.

Although repairs to the Ju87 and the Chiribiri No. 5 ought to take priority it has been decided by the Board of Directors that a new project will be undertaken in the interests of staff morale because the workforce really aren't quite as chipper as they could be. The urgency to repair the existing aircraft is also somewhat reduced as neither indoor flying nor meetings with the company's preferred consultants in the matter of trimming rubber powered aircraft are possible because of certain inconvenient public health problems.

Why the AS Tern? Any number of reasons.  It's quite important in British aviation history as the founding aircraft of a company that gave us a number of important technical innovations, most notably the retractable UC on the Courier, it held a number of UK gliding/soaring records, it was (to my mind) the first "modern" sailplane produced in the UK , and last, but not least, I like the look of it and have wanted to build one since reading Mr. Norway's autobiographical monograph, "Slide Rule" which, if you haven't yet, you really ought to read.

After some consultation with Mr. Blackburn, of this parish, a plan has been drawn up of a model prototype.  The intention being to build a Free Flight version first and if that proves viable to investigate an RC assisted version.

A few random thoughts.

- Given the strongly tapered wing and (relatively) high aspect ratio it feels like it might be a bit "borderline" as a flyer to me at this scale, but material costs will be well under a tenner and possibly under a fiver so it's got to be worth a bash.

- It's more "kit scale" than "scale", but I'm not a rivet counter. Smiley

- Aerofoil is Eiffel 400. Not scale.

- I'm nicking Peter Smart's double sided tow hook with offset as used on his Willow Wren rather than use an auto-rudder.  Should simplify launching (one can hope) and will let me retain the skid and give more of a scale feel.

- Stab and fin are both about 5% over scale.

- Wing will have dihedral of about 3 degrees (AS Tern had none) with about 3 degrees of washout.

- The fuselage sides will *probably* be soft 1/32" sheet. Top & underside tissue. Topside antique/linen (ish) with a tan underside to approximate plywood. A full tissue finish cannot be completely ruled out.

- Full formers will be 1/32" laminated balsa with weight reducing voids. It is *possible* that barring F3, 4 & 6 which I think do have to be full formers that I'll use the same construction as the Willow Wren with horizontal spacers and part formers to give the hex cross-section rather than full formers throughout.  Still swithering about it though.

- Still haven't decided how to incorporate a ballast chamber.

- The band on empennage is intended for the shakedown build only.  When (if!) I get it sorted the plan will be redone with the experimentally determined setting baked in and the prototype will lose the pegs and have the empennage fixed permanently in place as I've done with my VMC Osprey.

- Hope to start cutting the wood for it this weekend and it'll be the usual slow burn.

You'll need to click on the thumbnails to see them.  Sorry about that.  I swapped to a Linux machine over Xmas and have still to find a simple bitmap drawing tool that I like.
Attached files Thumbnail(s):
Airspeed Tern, Drawing Room Scale, 34" span.
Airspeed Tern, Drawing Room Scale, 34" span.
« Last Edit: January 06, 2021, 02:08:34 PM by TheLurker » Logged
lincoln
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« Reply #1 on: January 06, 2021, 03:30:56 PM »

 Great project! A subject like the Tern has much more character than a super smooth, carefully calculated composite sailplane.

If I'm not mistaken, from some of the drawings and photos out there, the airfoil on the Term was mostly flat bottomed.

At the small size you're using, my guess is that it would fly much better with thinner foils, especially at the tips.
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strat-o
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« Reply #2 on: January 06, 2021, 05:10:45 PM »

I agree with Lincoln's airfoil advice of keeping the thickness down! 

Marlin
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USch
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« Reply #3 on: January 06, 2021, 05:46:10 PM »

Why not a Clark Y thinned to 9%, reasonable thin for good flying, reasonable thick for a scale glider. I would also argue against the wash-out of 3°; 2° should be enough, but add a bit to the dihedral, say 5° per half-wing.

Please submit the proposal to the tech office of LIA Ldt, I don't want to shout against the wind  Wink

Urs
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lincoln
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« Reply #4 on: January 06, 2021, 08:18:56 PM »

I guess the Clark Y wouldn't look nearly as non-scale as a thinner airfoil at the root. At the tip, I think a much thinner airfoil would be good. It would also provide some aerodynamic washout, requiring less twist.

If tbe RC version is to be larger, the aerodynamics will be better and the penalty for even a full thickness Clark Y at the root would be mild.
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flydean1
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« Reply #5 on: January 06, 2021, 09:03:39 PM »

If decreasing the thickness is out of the question, then an increase in thinness is a viable alternate. Grin
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Andy Blackburn
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« Reply #6 on: January 07, 2021, 03:03:49 AM »


Without getting into a religious debate about airfoils, it seems that Lurk does like Eiffel 400 for gliders of this size.  Smiley

And in any case, surely the important thing here is to ensure that the fuselage carries the vital text of "Airspeed Limited, York"...

A.
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lincoln
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« Reply #7 on: January 07, 2021, 04:34:37 AM »

The vital text should only be applied if the model flies well. ;-)

The only thing that's religious about it is one's relative emphasis on appearance vs performance. The performance is the kind of thing that can be objectively established. There's a lot of data out there about this sort of thing. And then we have Xfoil. For Re(sqrtCl) = 40,000   Profili/Xfoil likes the 9 percent Clark Y a lot better than the Eiffel without a turbulator, and someone better with turbulators on each. If you don't care what the bottom of the wing looks like, a turbulated MVA 123 is better than either of the above. Unturbulated, it's comparable to the 9 percent Clark Y. However, where things get really hairy is at those skinny little tips. THere, they all look better with less camber and less thickness (except for the MVA 123, which is quite thin already. Or, at least, I didn't thin it, only reduced camber. )
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Yak 52
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« Reply #8 on: January 07, 2021, 12:08:16 PM »

I took the liberty of doing an analysis of the Tern planform using Hepcats CG_Calc and Suter's Sailplane Calc. I took the dimensions from the drawings by Vincent Cockett:
https://cevans.me/VINTAGE/Documentation/Airspeed-Tern/Images/Tern.pdf

There are some concerns which you are obviously aware of but I think a prototype to 'suck it and see' will be worth it and I await the flying with interest  Cool

Firstly the good news: The longitudinal stability looks pretty decent. By which I mean that the scale tailplane volume is better than expected. Tail volume is about 0.5 and the tail moment arm is 3.2 times the mean chord. My 'red flags' for these parameters are Vh below 0.4 where static stability becomes problematic and less than 2x the chord where dynamic longitudinal stability gets awkward and you can expect phugoids and difficult trimming. So a 5% enlarged tailplane is a safe size and shouldn't be an issue.

Now the bad news:

1) Taper ratio of 0.33 is really very high and you can expect tip stall issues. This is going to happen at slow speed/high lift coefficients - especially on a bungee launch. Trimming a bit fast may be needed to be safe from wing drops.

2) Spiral stability. The main issue here is not fin area but the extremely short fin moment arm to wing span ratio. This comes out at 0.29 of the span. (I would be happy with 0.5 or more and concerned about less than 0.4)

To be clear the vertical tail moment arm is part of the mechanism that makes the dihedral actually work. It's like a dihedral multiplier: a good long fin moment arm will amplify whatever dihedral effect is there. If you have tons of dihedral it's not an issue but if dihedral is marginal it's quite important. High aspect models have acquired a bad rap for this reason but it's actually because they tend to have a low fin moment arm/span ratio.

I ran the numbers in Sailplane Calc for the Blaine Rawdon 'B' Parameter which quantifies this effect. If B is greater than 5 you should have positive spiral stability. With your 3° dihedral B=1.74 - not so good  Undecided

To achieve B=5 at slower speeds you will need 7-8°. That said you probably have some high winger effect that will add 2 or 3 degrees of effective dihedral. Even so, assuming this total effect is worth 6° you would still have B=4.35.  I do suspect you will have some moderate spiral problems at 3° dihedral.

Vertical Tail volume (Vv) is 0.021 which is really low but I'm not sure you'd want to increase the area. The effects of fin area are complex. On many models, especially with props, decreasing area helps. It's simpler on gliders though and area has less effect, in fact where gliders without fuselage/prop effects interfering, the area increase will work in your favour. Definitely in experimental territory here but I would start with scale area.

....

After writing the above I thought it would be worth analysing my Slingsby Tandem Tutor and it turns out it has very similar numbers ie B=1.70! It has 2.5° but maybe a bit more keel effect. (Assuming total effective dihedral is 6° gives me B=5.2 which matches up with the positive but marginal stability.) Tail length is slightly better at 0.34 of the span. The fin volume is scale at Vv=0.19.

This model was spirally stable at low angles of bank (I believe you've seen the videos) but I've never flown it outdoors. The fact that the numbers are so close is encouraging however?


I would be tempted to try the Tern with scale empennage. I think the dihedral increase is wise though and I would be tempted to sneak in some extra width in the wing tips. I think the model would be stable indoors but might well be a wing dropper outside - with a fair amount of height lost in recovery from odd attitudes (if at all) The truth is there are a fair number of assumptions in these numbers but it's interesting to compare. I really think the Tern could be a success even if a little on the tricky side to trim.

I'll come back on airfoils and fire up X-foil at some point.

I look forward to more...


Jon
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TheLurker
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« Reply #9 on: January 07, 2021, 12:58:56 PM »

Thank you everyone for your suggestions.  One of the nice things about this forum is that people want to see others' projects succeed.

On aerofoils.
Abl's right.  For highly irrational and unscientific reasons I do like Eiffel 400.  ClarkY or similar would be much closer to the prototype's section, see also the Horsa, but for this one, which is only a tiddler, I suspect it won't make a huge difference.  I'm quite content to be proved wrong though.

On wing thickness.
The wing is about as thin as I can make it while still retaining a modicum of scale appearance and knowing that I'll be able to build those thin and oh so narrow wing tips.

On dihedral.
Thank you for the extensive analysis Jon.  I had hoped to keep the dihedral on the low side combined with the 3 deg washout for scale appearance sake, but if the number crunching shows it to be quite definitely an AAIB report in waiting it had better be bumped up a bit.  BTW, it was Vincent Cockett's drawing I used as my starting point so your numbers are almost certainly trustworthy for my attempt at it.

I do need to redo the stab. It has been noted that a full width spar maybe beneficial.

There will now be a short delay while the (electronic) pencil is sharpened and number of alterations are made.

If it does prove to be a turkey it won't matter too much as I get most of my fun out of building the models and the rest of you will get to enjoy some hard earnt and well deserved schadenfreude. Smiley
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Squirrelnet
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« Reply #10 on: January 07, 2021, 01:23:55 PM »

What a great build, I'm enjoying this one already.... pulls up chair  Grin
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Russ Lister
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« Reply #11 on: January 07, 2021, 02:39:02 PM »

After your Minimoa I'm really looking forward to this  Smiley
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Yak 52
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« Reply #12 on: January 08, 2021, 05:08:07 AM »

I had hoped to keep the dihedral on the low side combined with the 3 deg washout for scale appearance sake, but if the number crunching shows it to be quite definitely an AAIB report in waiting it had better be bumped up a bit. 

Just to clarify. When I said:

I think the dihedral increase is wise though...

I meant the increase over scale that you had already planned, ie 3°

It might be safer to go to 4° or 5° but I'd quite like to see how it performs with 3° Smiley but then it's easy to egg you on when I'm not the one investing my time Cool It's really a guess as to how much high wing effect you'll get.

How easy would it be to design it so you could add more later?
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USch
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« Reply #13 on: January 08, 2021, 08:29:19 AM »

Lurker,
a similar project is described in Free Flight Quarterly #78:
Taking An idea To A Kit, One Modeller's Approach, by Paul Bradley, Independence, KY, USA

The subject is a German ultra light glider ULF-1, short coupled, nearly no dihedral, airfoil similar to RAF32 or Eiffel400. Have a look if not done already.

Urs
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lincoln
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« Reply #14 on: January 08, 2021, 08:35:31 AM »

Of course the ULF-1 doesn't have the taper or the swoopiness.
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TheLurker
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« Reply #15 on: January 08, 2021, 03:38:51 PM »

Quote from: Yak 52
... I'd quite like to see how it performs with 3° Smiley but then it's easy to egg you on when I'm not the one investing my time ...
How easy would it be to design it so you could add more later?
This looks like a fairly easy wing to build so it'll probably be as quick to build a new wing with more dihedral as it would be to design something adjustable although Abl was arguing in favour of a one piece fuselage with plug-in wings.  However the Luddite faction held out for a one piece wing held on with laccy bands.

Quote from: USch
The subject is a German ultra light glider ULF-1...
Urs, you are a bad man. Smiley  Now I've got a full size project on my wish list and I have no idea how the blazes I'm going to build that on the dining room table!

Thanks though, that's a new one to me.  Is there an electronic copy anywhere of the article you mentioned?  I'd quite like to read it.

Quote from: lincoln
Of course the ULF-1 doesn't have the taper or the swoopiness.
More swoopiness is always a good thing.  Mind you the ULF is still a charming wee beastie.

It took 10 minutes to modify the stab. plan.  An hour to work out how to get a decent image for use here.  Hey ho.
Attached files Thumbnail(s):
Re: Airspeed Tern, Drawing Room Scale, 34" span.
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USch
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« Reply #16 on: January 08, 2021, 04:54:27 PM »

Urs, you are a bad man. Smiley  Now I've got a full size project on my wish list and I have no idea how the blazes I'm going to build that on the dining room table!

Thanks though, that's a new one to me.  Is there an electronic copy anywhere of the article you mentioned?  I'd quite like to read it.

Sorry, trying to do my best  Wink  and get a bigger dining table, your wife will be happy and you can have a lot of new friends at dinner!

Here the contact addresses for FreeFlightQuarterly, the driving man behind is Sergio Montes from Tasmania, but he has also a English contact person:

Website: www.freeflightquarterly.com/Wordpress
Editorial Team
Andrew Longhurst
45 Pavillion Way, Eastcote, Ruislip,
Middlesex, HA4 9JR, UK
e-mail: [email protected]

Maybe you know him already?

Urs
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lincoln
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« Reply #17 on: January 08, 2021, 07:56:32 PM »

Mr. Lurker,

To keep the Tern project moving, you should avoid ulf-1.com . You should also avoid the information and plans which are on line for the Woodstock sailplane. If you aren't especially large or heavy, then you should by no means look up the info and plans for the Carbon Dragon. Similarly, if you like engineering, but aren't as keen on the building work itself, don't look into omiiting the engine and moving the pilot forward in a Sky Pup. Otherwise, we might not hear from you in years.

As far as dihedral changes, I imagine you could design the joint at the center so that heat would soften the glue enough to reposition things. You might have to replace a couple of ribs and some tissue.
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TheLurker
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« Reply #18 on: January 09, 2021, 08:17:58 AM »

Urs,
Thanks for the contact info.  Someone (no names, no pack drill but I am very grateful) has kindly sent me an extract with the article you mentioned.  It's interesting to see how I've ended up with much the same process purely by trial & error even though I'm using a 2D drawing package rather than a proper CAD application.  

A paragraph on image tracing that particularly caught my attention was, "...that matches the image being traced. You can attempt to manually move the cursor along the line but that rarely produces an accurate tracing. Fortunately it is easy to adjust a curve created by tracing a line with your cursor. An alternate method when using graphics software is to simply draw several connected straight lines with their ends touching the curve being traced. Once the rough straight line based curve is created,the line segments can be converted to curves and then adjusted to fit the curve being traced. A little more work than when using CAD software but certainly very workable if that is the software you have available."   If anyone else is using a graphics package to do this (as I am) rather than a CAD application check to see if it has a bezier curve tool.  The bezier tool (in Inkscape at least) allows you to trace curves extremely easily and quickly. Briefly, draw the line from start to end point, convert from object to path and drag the line to roughly fit the curve then use the "handles" on the start and end-points to adjust for a more precise tracing at high magnification.


Lincoln,
You are in very great danger of being removed permanently from my Xmas card list. Smiley

The plan (at the moment, things may change) is to build two wings.  One with 3o and one with  4.5o of dihedral.  Current regs. forbid leaving the house for anything other than "exercise" once a day and the weather is foul and likely to be so for weeks yet to come so there's no rush.  

Righty ho, time to get on with cutting some ribs and other part profiles. I may be gone some time...
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TheLurker
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« Reply #19 on: January 17, 2021, 01:46:59 PM »

* TheLurker said ....it'll be the usual slow burn.

More like a smoulder.  So far we have one half of the main-plane.  Three degree dihedral, 3 degrees washout.  LE & TE yet to be sanded to profile.  Not too many issues turning 2D into 3D.  Wing tip is a bit "meh" but for this build I'll go with wing tip construction as is.  If this thing ever flies well enough that the plan can be shared it'll need to be rethought.

Images.  Jigging up for washout.  Notching spars to take account of the "waffer thin" outer ribs.   Clamped to the gash centre section to check the sub-assemblies will match nicely. 
Attached files Thumbnail(s):
Re: Airspeed Tern, Drawing Room Scale, 34" span.
Re: Airspeed Tern, Drawing Room Scale, 34" span.
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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Squirrelnet
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« Reply #20 on: January 17, 2021, 02:01:43 PM »

Nice smouldering Lurk  Grin
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Yak 52
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« Reply #21 on: January 17, 2021, 04:51:44 PM »

Nice!

Unfortunately one of your embers has landed in the tinder-dry pile of scale glider ideas that I have stacked in my head  Smiley I've been going through the books and three views again....  Roll Eyes
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TheLurker
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« Reply #22 on: January 18, 2021, 05:17:57 AM »

Quote from: Yak 52
...the tinder-dry pile of scale glider ideas that I have stacked in my head.
You have piqued my curiosity.  Do tell.
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Yak 52
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« Reply #23 on: January 18, 2021, 08:59:17 AM »

Nothing specific yet. But I have always fancied doing a Slingsby Gull or Petrel in this style. That was where I was heading after the Tandem Tutor before they cancelled indoor gliding at the Nats.
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lincoln
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« Reply #24 on: January 21, 2021, 05:19:24 AM »

Yak52: There are some gliders with lower aspect ratios that look very good and are probably much easier to cope with as flying models.

Castel-Mauboussin Jalon:
https://www.j2mcl-planeurs.net/dbj2mcl/planeurs-machines/planeur-fiche_0int.php?code=2757

Lo-100:
https://www.j2mcl-planeurs.net/dbj2mcl/planeurs-machines/planeur-fiche_0int.php?code=1576

Horten Hx-a:
https://www.j2mcl-planeurs.net/dbj2mcl/planeurs-machines/planeur-fiche_0int.php?code=3224  (I suppose it's a matter of taste.)

Horten Parabel:
http://nurflugel.com/Nurflugel/Horten_Nurflugels/horten_nurflugels.html

IS-4 Jastrzab: (aspect ratio creeping upwards here)
https://www.j2mcl-planeurs.net/dbj2mcl/planeurs-machines/planeur-fiche_0int.php?code=735

ok, the Schneider ES-54 Gnome is cute, not pretty:
https://www.j2mcl-planeurs.net/dbj2mcl/planeurs-machines/planeur-fiche_0int.php?code=1253

I would argue that some versions of the so called Pou Planeur were pretty, though now we have too MUCH wing area:
http://claudel.dopp.free.fr/Les_planeurs/Descriptions_planeurs/Cosandey%20Pou%20Planeur/Cosandey_Pou-Planeur.htm
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