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Author Topic: SAAB J-21  (Read 1653 times)
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Pit
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« on: April 17, 2019, 12:22:36 PM »

A particular Swedish WW II single-engined, twin-boom PUSHER fighter has intrigued me for ages and I had scoured stores and the earlier Internet for plans to build a model of it - initially for rc.  However, in 2005, I saw a photo from the 2004 or '05 FAC Nats where Pres Bruning was holding a RUBBER powered version!

I contacted him about the model and he sent me scans of the hand drawn - by none other than the late, great Nate Sturman! - that were, unfortunately, not very good quality (remember, this was 2005!) - most of the finer details were unreadable.  I started re-tracing what I could in CAD (TurboCad 4), progressing thru thre or four newer versions until I discovered that the plan had been published in the FAC Newsletter (issue 216).  I obtained a copy of said newsletter and completed a working (read: very rough) drawing and started cutting wood about ten days ago.

Even with the MUCH clearer drawings there are a number of "huh?" areas, but nothing insurmountable.  Scale is 1/24th (half inch = one foot) gives a 19" span x a tad over 17" length model.  Quite large for a "smallish" plane.  I'm using a sheet of 5.5- 6 pound '"C" grain stock that I inherited (an rc guy who didn't know what to do with such "flimsy" wood Roll Eyes) for all formers and ribs, and 6 - 7 pound "A" grain for the stringers.

I started the build on the Small Flying Arts site with more detail: http://www.smallflyingarts.net/forum/viewtopic.php?f=30&t=124
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SAAB J-21
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SAAB J-21
« Last Edit: April 17, 2019, 12:33:04 PM by Pit » Logged

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« Reply #1 on: April 17, 2019, 12:25:58 PM »

More photos:
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Re: SAAB J-21
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Re: SAAB J-21
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« Reply #2 on: April 17, 2019, 02:48:46 PM »

What an odd looking bird.  Nicely built, but definitely an odd 'un.

What size prop. were you planning to use?  My MK I eyeball estimate on that 19" span suggests it can be no larger than about 6".
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« Reply #3 on: April 17, 2019, 03:08:11 PM »

Hi Lurk!  I think that the XP-54 might have been an influencing factor, where the Swede was more successful.

No more room than for a Peck 6 incher, trimmed down to about 5.5 according to the plan and modded so that the free-wheel works.  I might make a three-blade prop for "scale fidelity", but will definitely carve a proper woodie.
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dputt7
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« Reply #4 on: April 17, 2019, 06:01:29 PM »

  Nice work, as usual.
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flydean1
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« Reply #5 on: April 17, 2019, 10:43:29 PM »

They took the same airframe, removed the piston engine, substituted a turbojet, and had their first jet fighter.
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Tapio Linkosalo
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« Reply #6 on: April 18, 2019, 05:37:07 AM »

Dean,

exactly. I just wonder if transition to a jet was in mind already when designing this prop-driven plane?

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« Reply #7 on: April 18, 2019, 06:00:37 AM »

It might have been due to the unavailability of DB engines and a drive to get jet experience. I think it was a neat bit of practical design.
Ricky
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Tapio Linkosalo
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« Reply #8 on: April 18, 2019, 06:08:40 AM »


Or maybe first test the airframe with a conventional motor, and then switch over to a jet with a tested airframe?
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« Reply #9 on: April 18, 2019, 04:10:33 PM »

That stays with the rule of not having a new airframe and engine at the same time and interesting as it also gave a short tail pipe to reduce losses on what was not a very powerful engine.
Ricky
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Pit
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« Reply #10 on: April 20, 2019, 07:48:31 AM »

Thanks for adding a bit of the background, guys!  I was going to add some (I actually have quite a bit) but it's buried on an older drive and some books.   There is a copy of the Profile Publication in the Gallery.

I've completed the rudders and sub-rudders, which came out very light.  The total weight of the completed components is at a "porky" Roll Eyes 9 grams...
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Re: SAAB J-21
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« Reply #11 on: April 20, 2019, 12:35:57 PM »

A "bern' video on his assembly/build style of a Saab  initially at least

https://www.youtube.com/watch?time_continue=7&v=Wl16MklzUbU
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« Reply #12 on: April 20, 2019, 03:47:55 PM »

A "bern' video on his assembly/build style of a Saab  initially at least

https://www.youtube.com/watch?time_continue=7&v=Wl16MklzUbU
Amazing!
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Pit
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« Reply #13 on: April 25, 2019, 08:14:24 AM »

Bern does do fantastic work!  I think the plane in the video is an XP-54, tho.

Got the nose block roughed out, most of the needed sanding and started covering. The booms are a PITA to cover and are very prone to SHS (starved horse syndrome) - have to figure out a lightweight cure... probably add two more formers or try a monocoque construction.
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Re: SAAB J-21
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« Reply #14 on: April 25, 2019, 09:35:32 PM »

Interesting subject and typical very neat work Pete.

After watching Bern's amusing and informative video- thanks Fred - a random idea crept into my mid for your booms. How about a spiral wrap of thin - 0.4 - 1/64 balsa or perhaps 0.8 x say 2-2.5 wide balsa around the booms. You may need about 3-4  evenly spaced. This should help to support the tissue without adding too much extra weight.

John

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« Reply #15 on: April 25, 2019, 10:36:53 PM »

Very nice build. I love it!
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« Reply #16 on: April 26, 2019, 12:22:16 AM »

Another way to "add" a missing former is to glue vertically-grained balsa inserts around the circumference of the fuselage between each stringer and then cut off the excess and sand smooth. This explanation is poor but look at what Tom Hallman did on his Seafire to give a base to glue the black and white invasion stripes. Study the scene at 0:25 and remember the grain goes vertically. It works really well:    https://www.youtube.com/watch?v=YSHRAdEsCBk
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« Reply #17 on: April 26, 2019, 08:48:45 AM »

Thanks for the suggestions, guys!  I looked at the vid of Tom Hallman's Seafire and then at "part 2", which showed a more detailed and annotated description of the "spider formers".  I'll probably use that method when (er, IF) I build a larger version (either 1" = 1', or 3/4 to the foot).  These booms are really too tiny and weigh only 1 gram (bare) to be worth much extra work, so I'll see how they react when fully shrunk (shrinked?).

In retrospect, I could've used stiffer wood for the keel and side longerons.
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« Reply #18 on: April 30, 2019, 11:06:37 AM »

Got a bit more done this last weekend.  Covered parts are shrunk and the starved horse issues are there, but not quite so extreme as I had feared.  After I get a coat of dope on the wing stab, I'll assemble them to the booms as a sub-unit.  Weight is climbing exponentially, but still well within my goal of 30 grams.

13.45g as shown.
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Re: SAAB J-21
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« Reply #19 on: April 30, 2019, 08:06:41 PM »

The booms don't look all that bad Pete. Nice covering.

John
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« Reply #20 on: May 12, 2019, 01:33:47 PM »

Covering is progressing, with the booms done except for the portions that go over the top of the wing (for ease of alignment). The nose portion of the booms was filled with StyroDur (green foam), which is easily shaped and then coated with thinned Titebond to shield from whatever finishing medium I decide to use and provide a bit of landing rash protection.

I was hoping to get the covering completed by this evening, but life kinda got in the way Roll Eyes Wink...
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« Reply #21 on: May 13, 2019, 02:58:31 PM »

As mentioned in the previous message, I didn't get as far as I had hoped over the weekend, but the parts are nearly ready for marriage Grin.

Fuselage is covered to the point where I can attach it to the wing.  I've left the upper rear portion uncovered to aid getting everything aligned, and the same applies to the booms.

Covering has gone on OK in strips - using a variety of methods (wet and dry) and adhesives (glue stick and thinned white glue).  Areas with sharp and/or compound curve were covered wet with the white glue (I've not had any luck with glue stick while covering wet), and ALL seams are sealed with wicked-in white glue and burnished prior to shrinking.

I just might give Eze Dope one last chance  Roll Eyes... NOT (I'll save that experiment for a general sport plane)!
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Re: SAAB J-21
Re: SAAB J-21
Re: SAAB J-21
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« Reply #22 on: May 15, 2019, 06:49:39 AM »

Hochzeitstag yesterday!

Booms joined to wing, horizontal stab attached and wing married to fuselage - all covered areas have two 50/50 coats of dope.


Balanced it out and gave it a few tosses to the bed - looks promising and then put the airframe on the scale... At first I thought the scale was in the wrong mode, but my back-up weighing utensil confirmed that the weight was correct at 14.92 grams as shown Smiley Shocked . The dope hasn't completely gassed off so it might even be a bit less.
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Re: SAAB J-21
Re: SAAB J-21
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« Reply #23 on: May 15, 2019, 12:21:05 PM »

Congratulations to you and your new wife! (which I assume is the SAAB, otherwise you have been neglecting someone)
ian
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« Reply #24 on: May 15, 2019, 01:18:36 PM »

Nice work on a very interesting model.  And at that weight I assume you'll have to tie it down to keep it from floating away.  Grin
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