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Author Topic: Zweibox HLG  (Read 293 times)
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dosco
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« on: July 21, 2020, 05:05:39 PM »

The fun with the Lunchbox convinced me to build a Zweibox.

Here she is, not totally done yet (the incomplete paint is obvious), but she's close, posing with parts for another.

No D/T ... if Hung takes her, I'll be happy.

Cheers-
Dave
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Zweibox HLG
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dosco
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« Reply #1 on: July 22, 2020, 10:12:39 AM »

Zweibox #1 is shaping up to be about 37g with noseweight.

No sheet lead to be had, and I improvised by cutting a piece of scrap 1-5/8" OD copper pipe and fashioning a piece of "copper sheet" (by bending the piece into a flat of sorts). I'm in the process of gluing it on now, and will use plasticine for final trim at the field.

Still working on the red stripes, almost done there.

Fun!

-Dave
 
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dosco
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« Reply #2 on: July 22, 2020, 02:47:11 PM »

Here she is.

Now for some trim flights...

-Dave
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dosco
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« Reply #3 on: July 27, 2020, 09:40:52 AM »

Took her out yesterday afternoon for some trim flights.

She has a tendency to spiral dive to the left. I chose not to put a warp in the port wing, instead I added a Gurney flap ... it's not large enough in terms of 1) how far it extends into the airstream, and 2) its length in terms of wingspan. I've removed it and will install a taller and longer one.

I think the CG is incorrect ... slightly nose heavy. I noticed this with the Lunchbox design. As I mentioned I took a piece of copper pipe and flattened a piece ... it was not lost on me, as I glued it on, that it may have been to heavy. I'd rather have "not enough" copper and trim the CG with plasticine. I plan on removing about 30% of the copper.

-Dave
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dosco
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« Reply #4 on: July 28, 2020, 12:30:37 PM »

I made some modifications...

1) I removed the copper sheet noseweight from the fuselage, and cut off about 1/3 of the material. I will use plasticine to make fine adjustments, and at a later date consider adding another, small, piece of copper to the nose.

2) I removed and replaced the Gurney Flap. The Zweibox article states that the left wing washin should be 1/3 of the wingspan of the aircraft. I cut a piece of 1/64th plywood, 5 3/8" long and about 3/16" wide.

I haven't had a chance to perform any test flights. The wife and I are putting the house on the market, and the "staging" work is a PITA.

-Dave
 
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dosco
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« Reply #5 on: July 29, 2020, 06:52:14 PM »

Forecast for the next few days is bad, then I have a trip to NY ... so I scooted off to the local school field to chuck the Zweibox around.

It really wants to turn left ... even after adding a much larger Gurney Flap. The Gurney Flap is 5.5" long, made of 1/64" ply, and extends 1/8" below the lower wing surface. There is "built in left rudder" (by virtue of tapering the fuselage where the fin is mounted) as well as stab tilt. I attempted to warp in some left wing washin and right rudder. Still was gently (but firmly) spinning in to the left. I added a small blob of plasticine to the right wing, and that seemed to tame the left turn.

After a few more throws, I added a small blob of plasticine just ahead of the fin (to move the CG aft). At some point in the near future I will again remove the copper sheet from the nose and remove some of it (I will measure the mass of the plasticine and remove the equivalent from the copper sheet).

Of note, it would appear the 60% CG position may or may not be the right location. It's reasonable to assume each airplane has slightly different "as-built" wing/stab airfoils (differing from the plan), which means the actual CG position may (or may not) need to be the same as on the plan. The Zweibox plan calls for 6g of sheet lead applied to the nose ... the piece of copper I applied to the nose was 3g, and I've removed about 1/3 of it, and need to remove more. Not the end of the world. Also interestingly, with the Lunchboxes I started with no noseweight, and added a bit of clay for each flight (or series of flights) until the CG was too far forward (with the Zweibox I unintentionally reversed the process).

The last few flights before I left were quite satisfying ... I caught a few bubbles of evening lift. Interestingly, it seems I can launch the smaller Lunchboxes higher than the Zweibox (at least that is what it looks like ... not lost on me that it could be an optical illusion). Will be interesting to see if Zweibox #2 flies a bit differently with the differences I articulated below.

Also on the upside the fuselage stick seems quite robust, which is pleasing (I have memories of HLGs past where the fuselage was prone to failure after nosing in).

So, for Zweibox #2:
1. I will sand the TE of the wing much thinner (I left it a bit thick, definitely thicker than the plan).
2. I will warp in a boatload of washin. I may even cut an aileron and glue it into the "down" position.
3. The fin is cracking because of my attempts to warp in adjustment at the field; I will sand the next one thinner.
4. The horizontal stab may be a bit heavy (and I did try to warp in a bit of "up elevator" ... and there was some cracking); I'll sand the next one thinner as well.

-Dave



EDIT: added some comments about the Gurney Flap (and wing warp) as well as CG position.
« Last Edit: July 29, 2020, 07:35:10 PM by dosco » Logged
OZPAF
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« Reply #6 on: July 29, 2020, 08:18:10 PM »

What a pretty looking glider Dosco.

I have had that strong turn tendency on a couple of my gliders - even with wing panels close in weight. Invariably if the rudder offset wasn't too much, it was caused by a the wings being slightly skewed on the fuse! It doesn't take much. To my eyes this could be your problem looking at the plan view of your build.

It will take a lot of inboard wing washin to control this - as you are discovering. Perhaps a better solution would be to remove the fin and replace it with much reduced or even no, left rudder offset.

Happy flying.

John
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dosco
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« Reply #7 on: July 29, 2020, 08:26:52 PM »

What a pretty looking glider Dosco.

I have had that strong turn tendency on a couple of my gliders - even with wing panels close in weight. Invariably if the rudder offset wasn't too much, it was caused by a the wings being slightly skewed on the fuse! It doesn't take much. To my eyes this could be your problem looking at the plan view of your build.

It will take a lot of inboard wing washin to control this - as you are discovering. Perhaps a better solution would be to remove the fin and replace it with much reduced or even no, left rudder offset.

Happy flying.

John

John:
Thank you very much for the kind words.

I agree, I did notice some wing asymmetry after I assembled her (drat! I was hoping to get away with it ... lol!) ... I think the panels are slightly different in shape by virtue of aggressive sanding of the dihedral joint, but I'm not 100% certain. When I made the wing blanks, I made them separately and then glued them together at the root. Instead of using acetone to separate them (which probably would've been the smart thing to do), I used my Zona saw. The wing blank for #2 is a single (spanwise) piece.

I'll continue to fly #1 (I think I'll name her "Stripey" and the next one will have stars painted on the wings). I like the idea of altering the fin. One option would be to mount it on the port side of the fuselage stick (the fin is mounted to the starboard side, and the taper is sanded into the starboard side). Other option is make and mount a smaller fin in the location specified by the plan (starboard side).

John, how much smaller would you think? 20%?

Cheers-
Dave

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OZPAF
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« Reply #8 on: July 29, 2020, 08:44:45 PM »

I use a couple of small pieces of double sided tape to hold the separate wing panel blanks together while shaping the outline to ensure they are very close. I also use jigs to sand the dihedral and poly joints which help to give stronger joints and more accuracy. I started down the jig road as i was working with kids and wanted to ensure that they had a decent chance of flying.

If you reduce the fin height - I would suggest that you just shave say 1-1.5mm and radius it and then reshape the fin leaving the width the same at the root. Take small steps. Moving it to the other side of the fuse may be a good idea.

I use a very small fin which I arrived at many years ago by this sort of trial and error. Have a look at the Jikah AL300 CLG in the plans gallery.

Hope some of this helps Dave and good luck with the move.

John
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dosco
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« Reply #9 on: July 29, 2020, 08:57:33 PM »

I use a couple of small pieces of double sided tape to hold the separate wing panel blanks together while shaping the outline to ensure they are very close. I also use jigs to sand the dihedral and poly joints which help to give stronger joints and more accuracy. I started down the jig road as i was working with kids and wanted to ensure that they had a decent chance of flying.

If you reduce the fin height - I would suggest that you just shave say 1-1.5mm and radius it and then reshape the fin leaving the width the same at the root. Take small steps. Moving it to the other side of the fuse may be a good idea.

I use a very small fin which I arrived at many years ago by this sort of trial and error. Have a look at the Jikah AL300 CLG in the plans gallery.

Hope some of this helps Dave and good luck with the move.

John

John:
Thank you for the advice. Will start there and after making alterations to the noseweight I'll make some test flights.

Also, thanks for the good word on the move. The house is ready, and we'll have a showing tomorrow at lunchtime. We'll see what happens, the market is very tight right now (not many houses for sale, and surprisingly strong demand). The upside is that we may be able to sell quickly. Downside is that houses are getting snapped up very, very quickly. We have a place in mind in Annapolis, which has many benefits (one is a full basement with a suitable "work area" (i.e. "model building area")), but it could get bought at any moment.

These things always work out ... so only time will tell!

Regards-
Dave
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