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Nov 14, 2019, 05:49 PM
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Build Log

Yet another Goldberg Sailplane


I have loved the look of this airplane since I first saw one . Hardly a straight line on it - beautifully curvey. Fortunately I ran across Tandy Walkers now famous "Master Class" Buildlog for his sailplane - nearly 200 pages of exceptionally helpful detail. Without this, I would certainly have given up.

There are a number of guys selling short kits and plans for this airplane. However - as most folks reading this may already know, the 1940 Goldberg Sailplane sold by Comet was a free flight airplane designed for maximum performance . It was build extremely light as performance was a more important design goal than survivability. The original design gross weight comes out to be around three pounds. RC conversions, even masterful jobs like Tandy Walkers, come in at around 5 pounds. The plans are 80 years old, and the parts drawings may or may not have matched the plans all that well. Also, when getting a short kit, it is a good idea to find out how "short" before starting to stick parts together. Mine was missing a few ribs, duplicated some others and only came with half the formers for the sides. It is easy to cut out new ones using the ones you have as templates, but not after they are part of the half-completed airframe. So, when committing to this project one is committing to a fair amount of improvisation. There are other build logs for this airplane that are also quite helpful like the one from jodini and Low and Slow1.

SO, here we go...
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Nov 14, 2019, 11:27 PM
I'd rather be flying.....
JeffMac's Avatar
Nice project, Watching.....

Best regards,

jeff
Nov 15, 2019, 02:25 PM
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Looking forward to your build . Mine is currently hanging up so as to not get damaged . Super slow flying airplane. If I was to do it again I would have scaled up the vertical stabilizer with more rudder. It sure doesn't turn like a Quaker. Mine was a Bob Holman short kit and for the most part was pretty close to the plans.
Low and Slow
Nov 18, 2019, 01:12 PM
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More Yet...


Thanks Jeff and Slow/Low

I am doing the rudder much like the way you did yours - using the vertical stab spar as the divider for the rudder hinge. With your advice, I might just take a Zona saw to what I have done on the vertical stab/rudder and extend it. Easier now than later.
Nov 18, 2019, 02:26 PM
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Continuing.....


The wing work goes fast - not much different than any other kit, although the previous build logs for this project certainly helped speed things along...
Nov 18, 2019, 04:09 PM
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Had to cover the wing before I destroyed it


In hanging the wing up I snagged a rig and nearly broke it...had to decide on covering. I liked Tandy Walkers approach in using the Sig polyested covering and then silk over that, but jeeze - it involved about 20 coats of dope in prep, and actual covering. Also, I have no experience with silk and silk is expensive. I did some experiments with heavy silkspan, and though it looks nice, it is not strong enough...and would require about 15 coats of dope to look right. So, I decided instead to use the new Oracover. It is pretty heavy and I wouldn't want to use it on anything much smaller than this. But since this airplane has a reputation as being a slow floater, even at 5#, and, I am likely to be rough on it, this is what I used.

This is probably the easiest covering I have ever used...just iron it on like monocoat but much easier and it looks soooo much better. And never puckers with time.
Nov 18, 2019, 06:18 PM
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OraTEX not OraCOVER


I keep mixing these up...Oratex from either Balsa USA or Tower - about 60 bucks for 5 meters
Nov 18, 2019, 07:42 PM
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one of my favourites, I'll be watching as well.
Nov 19, 2019, 01:28 PM
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Your Sailplane is looking like it is going really well. You commented on the elevator and it looks like you have plenty of it. You shouldn't have any problems with pitch control. I am not sure what tucking under is though. you have made great progress in such a short time . Looking great .
Nov 19, 2019, 08:38 PM
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Thanks Gents


So, thanks for the evaluation of the elevator size...as to "tucking under" , I have read that this beast tends to enter a steepening uncontrolled dive if it first enters a steep descent. If you have ever read Ernie Gann's "Fate is the Hunter", this is what happens with "elevator unporting" ...nasty.

I wonder if this sort of thing might be fixed by adjusting the decalage...but then I have never read what that is supposed to be for this airplane. I think I'll go with zero degrees and hope for the best!

With regard to the vertical stab/rudder - what experience did you have that convinced you to increase the size? I have read that reduced - size rudder/stabs are a really bad idea. I of course noticed that the sweep of the rudder is restricted by the elevator, so making it bigger seems like a good idea.

AS far as rapid progress - well, Ive been working on this for a couple of months, so I have been filing stuff as I get to it.
Nov 20, 2019, 02:21 PM
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A few years back someone in a discussion brought up dutch roll. So as the discussion went on the guy was talking about a Goldberg Zipper I think. He was saying that he modified his with more fin and this helped. It seems that a lot of the designs back then had big verticals but for some reason it probably wasn't necessary due to the low power engines and it was free flight. Now a days extra power is easy and still light weight. But that was something that stuck out in my mind that after it was flown. I should have built a bigger fin and rudder. Most of my flying is motor off prop brake on. It has no trouble turning with power on. This is all my opinion as I have never read a NACA manual. LOL. As for elevator unporting I may have to get that NACA manual.

Low and Slow
Nov 20, 2019, 07:55 PM
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L and S


Speaking of the Zipper - there is a wonderful piece of 8mm color movie on YouTube showing Carl Goldberg flying a Zipper. Easy to find - "Carl Goldberg Sailplane Movie" should do it. I guess the Zipper was difficult to trim - although I have never had one...But watching Mr Goldberg in this clip is magic - First, he is dressed in a natty shirt and tie at a flying field in the dust of Bakersfield, CA. I can't recall the last time I wore a tie to the field..... Second, he nonchalantly starts the Zipper, and just tosses it in the air.....where it proceeds to do a steep 180 degree climbing turn, motor shuts off and it transitions into an easy glide. That free flight airplane was under more control than many RC airplanes I've seen!!!! I think this movie was part of what media there was to document what was essentially a victory tour for Comet Models in 1941.

Thanks for the encouragement - advice.


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