SAAB Fighter

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staubkorb
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Re: SAAB Fighter

Postby staubkorb » Tue May 21, 2019 8:25 am

Used coloured note paper for the forward fuselage/wing closure (more of an experiment...) that turned out "adequate" - re-discovering the reason that I stopped using the method more than 60 years ago. Then I formed the fillets using my preferred method - balsa sanded to less-than-paper thickness (about 0.015mm) and less than 3 mg. Due to the relatively small size and multiple compound curves, they were done in two parts. Also managed to poke a few holes in the tissue during the process :x .

The open frame on the boom has been covered and the forward part will be filled with foam.

Then comes the prop... I still haven't made up my mind whether to go with a three-blade, can-formed or a normal two-blade carved prop (pusher). OR, make things easy (and heavier) using a commercial plastic one and remembering to wind backwards :? .
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Re: SAAB Fighter

Postby staubkorb » Wed May 22, 2019 1:18 pm

The tissue on the booms runs lengthwise which tremendously helps trimming the waste off and I used this characteristic to advantage when covering the remaining open frame area.

I cut a handy sized scrap of tissue, smeared glue stick on the rough side. Then smeared glue stick on the spine, the two stringers and the formers, and centered the tissue over the spine - burnishing it down. A suitable length of 1/16th wood with a smooth straight surface was then drawn across both formers - while keeping some tension on the tissue - around and down to the wing surface. A thin line of white glue or dope is laid on the wing tissue in a straight line between the boom formers and the boom tissue then pressed into the glue with the straight-edge.

Trimming the excess has to be done before the glue is dry to reduce a marked edge. I used a steel 6 inch ruler that is about 1/32nd inch thick, laid edgewise on the wing, against the boom formers. Gripping the tissue, I (carefully!) tore the excess away. As the tissue contact to the wing is at a slight deviation to the grain (airfoil!), the tear will not be as clean as some would want. To counter this, simply draw a NEW razor blade over the tissue - along the ruler - using ONLY the weight of the blade (single edged blades are best for this) and then do the tearing bit. This nicks the tissue fibers enough (usually) to enable the bias tear.

A touch of glue along the edge and lightly burnished will usually result in a near invisible join.

Four or more hands is helpful with at least two of them steady :lol:. Good lighting, a MUST!

Oh yeah... a bit of patience works wonders ;)
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Re: SAAB Fighter

Postby staubkorb » Fri May 24, 2019 7:01 pm

Went outside last night (10PM) since it was dead calm for a test toss or two. First from a kneeling position (nice) and then from shoulder height standing. It went clear across the yard :D . Humidity was high and the covering ended up very limp.

This afternoon was again near calm in the back yard so I set up the camera for some vid. I wasn't able to quite duplicate the night flights (I was too nervous with the camera running :roll: ) and a small warp may have sneaked in, but was still satisfying.

https://youtu.be/QmqX-DOL3Kc
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Re: SAAB Fighter

Postby staubkorb » Tue May 28, 2019 6:59 pm

Made up a pusher prop (fan method) for the SAAB and will try it out before fully finishing (relieving the hub). P/D about 1.4:1 using the article by Fred Hall and made it fairly wide-bladed to help compensate for the small diameter. Diameter is 5.8 inches, giving me about 3mm to boom contact - may have to shorten it a bit more. I thought I had a number of 1mm reverse "S" hooks but only one was found so I'll do the initial OtM prop testing with a '"diamond" version - to see if the prop works at all.

Will use a 4-strand 3/32nd inch motor for starters.
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Re: SAAB Fighter

Postby staubkorb » Thu May 30, 2019 2:10 pm

Mounted the prop and fed a 10'" loop of 3/32nd Tan SS (hook to peg = 9 inches) and re-balanced for a midnight test at the Dustbin Industries Proving Grounds (back yard) this last Tuesday. The rubber motor is one of the ones from my peanut Mew Gull, chosen only to give the SAAB enough oompf to determine what, if anything, needed to be corrected (thrust line), and to see if the prop worked at all.

One-hundred hand turns and an already damp saggy airplane showed that the prop definitely works and that more "down" thrust needs to be worked in. Side (right) thrust is also needed - difficult due to the prop-to-boom clearance. The model climbed at a rather alarming rate and did a near "wing-over" into a dive, landing on the stone walkway. No damage other than a bruised nose plug. The model does seem to be stable, tho, and the prop will be relieved at the hub and thinned a bit more (weight reduction).

WA really surprised that the tiny motor was able to make the model climb at approx 30° angle to about 15 feet before the turns ran out :shock: :o !
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drachen
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Re: SAAB Fighter

Postby drachen » Tue Jun 04, 2019 4:27 pm

Very nice work, staubkorb. The prop is exceptionally well carved. Beyond my capabilities.

Cheers,
Drachen

Sky9pilot
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Re: SAAB Fighter

Postby Sky9pilot » Thu Jan 16, 2020 5:58 pm

Pete,
Any further work done on the SAAB...looking forward to more post!!! Wishing you a Happy and Healthy New Year!
Sky9pilot

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Re: SAAB Fighter

Postby staubkorb » Fri Aug 07, 2020 4:58 pm

Yesterday was finally a decent day, after 5 PM (87°F) - not beastly hot like it's been the last week, or too windy (<2ms) - so I took this puppy out for some real flight testing.

There was an impromptu "party" at the field with about 15 flyers with their better parts, and I seemed to have caused somewhat of an uproar :) , as many were not aware of my FF activities :lol: . Lots of photos were taken, delaying my getting any "work" done - but I couldn't convince any of those present to video my test flights :( .

I had re-armed the plane with 2 strands of 1/8th x 13" (3/32nd was only for the back-yard hops) starting with 150 turns. NOT enough, but did show that a bit more weight needed in the nose. Next was 200 turns with a bit of porpoising - again too short to determine if more nose weight or "down" thrust was needed, but did expose a rather tight left turn with very little altitude gain.

Upped the turns to 400. LOTS of porpoising but got some height - maybe 30 feet - and a tight left turn! As the turns ran out, it settled into a decent glide, but the remaining height was insufficient to determine HOW good.

Next was with 500 turns, a bit of weight on the right wing tip. I moved onto the main field as many were not aware that I had been flying and wanted to be able to observe. I was already fairly confident that not much could go really haywire so I obliged. The porpoising was MUCH worse (the tight turn of the earlier flights dampened the oscillations) but the plane got up a lot higher and as the turns ran out the glide looked really nice - left under power transitioning to right in the glide.

Changing to a fresh motor, I went for (almost) broke cranking in 750 turns. I had added a small shim to the prop block to reduce/cure(?) the porpoising - wasn't quite enough, but the puppy climbed up to about 100 feet - the left turn still a tiny bit too tight - where she cruised for a bit and then went into a nice glide to the right. Touched down about 50 feet from the launch with a nice greased landing. Flight was timed by one of the women at nearly 40 seconds. We were well into dusk, so I called it quits. I hope to break in my West Wings Spitfire this weekend, but temps are expected to get up to 100°F thru Monday :shock: ! I usually don't even leave the apartment when it gets over 90!

Now to paint and add the permanent greenhouse - and possibly up the motor to 4 strands of 3/32nds. AUW of the model as tested: 22.07gm. Motor weight: 2.27gm.

Me happy!
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Re: SAAB Fighter

Postby Site Admin (Dave) » Tue Aug 11, 2020 1:57 am

Looking nice so far. :)

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staubkorb
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Re: SAAB Fighter

Postby staubkorb » Tue Sep 08, 2020 9:02 pm

Finally had some REALLY decent weather yesterday with a very comfortable 72° with light and variable "breeze" (< 2ms). Just a few days earlier and lasting over two weeks, the weather had played nasty with the temps not getting over 65 :? . Got about 6 flights in, still using two strands of 1/8th SS @12.5 inches.

Started with 500 turns again. I had added a downthrust shim of 0.5mm and the flight was decent with only a slight porpoising, with the power phase still to the left. Swapping the shim to a full millimeter, the next two flights on 700 turns were as near perfect as one could hope for with good height and a right power phase, transitioning very nicely into a flat left glide - kicking myself for not videoing this one.

Turning up the to 750 brought back the porpoising, but the glide was still great. The videos are really lousy as I had to do the work myself (tho there were a lot of of club "colleagues" present) but the gist is evident, both with 750 turns.

In this vid (https://youtu.be/KB26Zy0_QWg), one can just make out the transition into glide - view both in full screen.

This one (https://youtu.be/TkkAoQaS_p0), I think I botched the launch and the model "landed" while still under power. This was after adding another small shim...

About 45 minutes later, some curious new spectators wanted to see it fly so I started to wind... noticing the the motor was badly nicked at the knot. Against my better judgement, I continued winding but stopped at 700. Launching into a dead calm, I got another picture perfect flight of about 40 seconds with the model greasing in only 3 meters away. As usual, everything is perfect when no-one is looking or taking video :| .
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