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Old May 20, 2012, 04:40 PM
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Zlin Akrobat 70" Sig Fossil Found!

Found this Zlin Akrobat Fossil for FREE! From what I can surmise, itís from the late 60s to early 70s. A building manual is included written by Maxey Hester where he mentions that he now must build his 2nd Akrobat for the 1969 Nationals.

The kit is complete from what I can see though the large building prints are a bit tattered at the folds. I plan on having them scanned and do my best to document every piece of paper plus all the included decals. The balsa is dry to say the least, but was select material, so it will be an electric for sure. The prior builder (in those days) did an ok job, not up to todayís standards, but this ainít no laser cut self jigging kit mind you.

There are problems that I noticed right away Ė

1) the foam core wingís balsa skin is delaminating
2) some oil based wood putty was used at the empennageís root
3) poorly mated angles in the fuselage balsa
4) design and layout a battery hatch and tray

Not sure what the next step will be other than digging into the prints which I have already done for some 10 hours so far this last year. So this bad boy has floated to the top, and itís time has come. In the pictures you can see all the left over material. I may use it and may not, weíll see. The fuse as left unfinished, looks pretty straight actually. The wing is where I need to start making decisions. Iím just wondering what kind of glue I should use and how to clamp the sheeting back onto the foam. The foam looks in decent condition. Can you guys give me some opinions here? The wood will go back into place with about a 1mm gap on one side, the other will seam back tight.

What a pleasure this will be. Just to have an audience here at RCG and at the same time to have this old SIG kit. I hope to start in again today and do some sanding on the 1/8th inch balsa sheeting at the turtle deck as it had split on the last owner which I chased with CA last night, so back to that for now. I believe Iíll be using allot of CA with this dry balsa.

Good flyíin
DC
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Old May 20, 2012, 11:03 PM
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One thing I'm discovering is that even to hold this antique in the hand, I'm going to go over all the balsa joints again with medium CA. I believe next will be the battery hatch fab. Still studing the plans over some more.

I hope to update this thread at least once a week or more often if replies warrant.

Have a good week,
DC
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Old May 21, 2012, 02:14 PM
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i had one back in the day. It was a very nice flying plane!!!! If push comes to shove, you could trace the root and tip and cut some new cores for the wing. Maybe an e-mail to Sig would get you a new set????? I'll bet they still have the templates around someplace.

Good luck on your quest!!

Ken
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Old May 21, 2012, 09:29 PM
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Ken,

And you used to have one? Wow. I have to keep you in my file, a rare find you are Sir. May I ask, have you any pictures, or perhaps do you remember the color scheme? Did you build it from the kit? How did you finish it out? Did you use the recommended silk & dope?

The Czechs had 5ea 526AS planes as a team all different colors constructed for the world aerobatic championships. (I'm quoting from the origional building instructions written by Maxey Hester on Pg.1)

You got the juices stiring. I just sent them an email referencing this thread. So one might say the "FEDs" are watching! Addressing some basic concerns. I won't copy & paste their reply, just their bullit points.

How old that core is, is a good & valid concern. Heck, I might as well design a copy built up wing or work hard to copy a pair of the origionals cores. Lets see what they say. And thanks for your reply as well friend!

Some build data- currently the fuse weighs 14Oz. The wing 28 Oz. So far total weight is 32 Oz. Back to the shop....

DC
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Old May 22, 2012, 01:44 PM
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Originally Posted by 43851E View Post
Ken,

And you used to have one? Wow. I have to keep you in my file, a rare find you are Sir. May I ask, have you any pictures, or perhaps do you remember the color scheme? Did you build it from the kit? How did you finish it out? Did you use the recommended silk & dope?

The Czechs had 5ea 526AS planes as a team all different colors constructed for the world aerobatic championships. (I'm quoting from the origional building instructions written by Maxey Hester on Pg.1)

You got the juices stiring. I just sent them an email referencing this thread. So one might say the "FEDs" are watching! Addressing some basic concerns. I won't copy & paste their reply, just their bullit points.

How old that core is, is a good & valid concern. Heck, I might as well design a copy built up wing or work hard to copy a pair of the origionals cores. Lets see what they say. And thanks for your reply as well friend!

Some build data- currently the fuse weighs 14Oz. The wing 28 Oz. So far total weight is 32 Oz. Back to the shop....

DC
Mine was painted as depicted on the Sig box. It had a Webra .61 in it with a custom built header and muffler out of brass so everything was in the cowl. It was a tight fit!! Sorry, no photos

I used 3/4 oz. glass cloth and epoxy on all the sheeted parts and Sig coverall on the open areas with dope and then automotive lacquer paint.

I would try to recut or find new wing cores. The under camber would be easier to reproduce it the cores and wood were put back in the shucks for curing. I put servos in the wings for the ailerons as is common practice today. Robart hinge points for the control surfaces.

I'm currently, ever so slowly working on a 20% 526. It will take a 150 or so 4 stroke.

Ken
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Old May 22, 2012, 02:25 PM
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How did you acquire that thing?
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Old May 22, 2012, 05:05 PM
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Originally Posted by dedStik View Post
How did you acquire that thing?

If you are asking me (Ken) I bought it new from Sig. Gosh! I just dated myself
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Old May 22, 2012, 05:15 PM
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DC

I laminated 1/64 ply from the firewall back to the middle of the wing saddle. IMO, that joint where the rear of the nose meets the front of the wing saddle is a bit weak. You only need to do the flat part, not the turtle deck. I think I did mine on the inside as I built it. take a look see if that is still possible. If not the outside will suffice as the transition from ply to balsa will be easy to hide.

Ken
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Old May 22, 2012, 08:55 PM
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Here is Sig's reply. I did check out the links Bob Nelson kindly provided. So the wing is on hold for now untill I can document the shape & size for the 2nd link i.e Flying foam .com. they look like they can do the core, though I want the new cores to be exactly the same as the origional(s). Heck I may send them the whole wing just so they'll have in their hands. Alternativatly at this point these foam cores need to be carefully evaulated. You know if they have to come off and they are still holding their shape (which now they look ok w/ the sheeting still mostly on) fiberglassing and vacuum bagging may be a nice option. Retracts will have to be decided on, and I was considering making my own. Well here's the communication from good 'old SIG

Hi Doug,

The way the skin is delaminating from the core make me question how well the rest of the skin is bonded. I would try to slip a thin metal strip like a steel ruler under the skins and see if they come up easily and if so see if the skin can be stripped and the wing recovered. Recovering this wing without the shucks the cores came out of would be a real challenge to keep straight due to the unusual airfoils used. If the core is destroyed in the process and you need a new core there are several places that can cut them. The Core House (www.home.earthlink.net/~philcartier/) and www.flyingfoam.com are a couple that I know of and there are others if you do some research.

Good luck,



Bob Nelson
Technical Customer Service/Design Engineer

SIG Mfg. Co., Inc.
401 South Front St.
PO Box 520
Montezuma, IA 50171-0520

Tel.: (641) 623-5154 Ext.: 120
Fax: (641) 623-3922
E-mail: bobnelson@sigmfg.com
Web: www.sigmfg.com

-----Original Message-----
From: Doug Christensen [mailto:auriga@verizon.net]
Sent: Monday, May 21, 2012 8:23 PM
To: mail@sigmfg.com
Subject: 1970 Zlin Akrobat Kit


Dear SIG Manufacturing,



It is no secret that everyone in this hobby is very glad to have SIG back and going strong. I have a BIG favor to ask you folks.



You know these days of laser cut & self jigged parts, kit building has never been so easy. Seems to wonder it' s not more popular than ever.

Anyway I am the proud recipient of one of Maxey Hester's 70 inch wingspan Zlin Akrobat kits, partially assembled circa 1970 ?. It's still all there in the box, what has not already been assembled.

There is one problem however, when SIG stopped supplying the 6" wide 1/16" balsa sheeting for the 3" sheeting, and especially over the years the wing's sheeting has split / separated and delaminated. Perhaps more intact then others!



So I have a couple of questions; 1) how can I re-attach the sheeting to the old foam wing? -or- what would be your best recommendation on what should be done to repair this issue. My idea was to somehow find a glue that could not react with the foam and when applied how to hold it to the wings surface. The condition of the foam itself is unknown except at the wing tips where it was never covered seems to be in not bad condition. What's your opinion on the aging process and should it be flown with the original foam core? Should the sheeting be removed and re-installed with new sheeting? I hate to ask this next question, but....is there anyway some new cores could be produced? Note; the wing never saw any covering, this is far as the origional builder got.



Here is a link to RC Groups forum where I have started a thread where you can see some pictures of this kit as I received it. The fuselage is somewhat dried out, but with care I hope to resurrect it also. My whole outlook for this 1969 National Champ is to fly it as a mild sport class plane electrically powered of course. http://www.rcgroups.com/forums/showthread.php?t=1656111

A battery hatch is planned currently but still reading everything, guide book by Mr. Hester and the prints, still intact.



Thanks again for your expert style and technique in model airplanes.



Sincerely,



DC
College Station, TX.
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Old May 22, 2012, 09:06 PM
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Quote:
Originally Posted by dedStik View Post
How did you acquire that thing?
dedStick,

Believe it of not ot was a freebee givaway at last year's swap meet here at my local club BVRC (Brazon Valley RC) Purchases at "Al's Hobbies" in Houston, TX

Everyone was looking at th wing and it's split sheeting shaking their heads. The fuse was (is still) so dried out weighted nothing and had (see pics) bad stains of the wood filler at the rear stab roots. I was excited like a kid on Christmas morning, plus I had no idea how old this Zlin Akrobat actually. Ebay had a same half built kit going for $150 and RCU for over $200. I thought it was a P-51 or spitfire, but I knew of the potiential- for what this was as soon as I saw the box.

Back to the shop and that battery hatch which is coming out real nice, built in place too!

DC
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Old May 22, 2012, 09:14 PM
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ken,

WoW! Thanks for all the info. I need to study your notes. I just wanted to say Thanks Friend!

I'll write you back soon. Maybe include some pics of the battery mod (you mentioned a weak area!) already working the the 300 area! Good timing he he!

*EDIT* Just finished a fuse stiffener for the right side. You were absoultly correct on the weak spot, saw it "plane" as day once you pointed it out. Took some pics for post tomorrow night. The battery hatch was built in place with an extra former fore & aft. So it's going to work well I hope.

Have a great day 'yall
DC
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Old May 22, 2012, 11:25 PM
The Prez....... again
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Glad to be of help DC. You have me thinking of picking up the pace on mine but I'm 1 1/2 weeks past surgery to repair my left index finger after sticking it in a prop. It;s just a bit shorter now by about 1/4 inch. The worst thing is I cut the tendon on top of the knuckle. It's repaired but at this point the finger is just about worthless.

Ken
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Old May 23, 2012, 07:20 PM
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My friend,

I am of the same circumstances. About 30 years ago me and a missing countershaft sproket guard on my dirtbike found me inserting my middle finger into the chain/gear removing just enough to have the entire end gone. I have 3/4 or the finger nail left. Had to have the bone trimmed. I quit playing guitar, but have plans to have some extra side flesh trimmed as well. An early ESC reset itself and started on a bench coming right me & my son. Held my hand out to guard us both- 26 stiches later...I will tell you this, when I messed up my finger in my dirtbike, I felt like the dummest person that I ever met. Time passed and as in my line of work I had to get back using my hands. I remember folks telling me that "I had good hands". Since then I have enjoyed making custom jewelry, designs that I developed on the basis of impossibility. The most difficult that I could dream of. Stuff that no other jeweler would ever undertake, but it was & is still a hobby. (help get me my second wife he he! Lord please forgive me for that). Now it's Nanotechnology. I can write my name clearly in 0.20mm font! Every move I make (under high magnification) at work is a planed & cordinated movement. I just re-wound a 28mm motor with the top endbell still installed, and made 24 solder connections where one of the seinors on the therad said it was impossible. I do these things (Quote JFK) not because they are easy, but because they are hard. My body healed, but my mind forever will fight the fact that I did the dummest thing. So if anyone wants to know why I'm like this, well that's why. And I will not add the word "sadly". Never. But it took me this long, perhaps up just now (as I write you) Ken, to realize that. I'm so sorry. I had the "ghost" feeling for a year or so in the finger. But slowly faded which I pray wll be the same with you. I don't know your exact condition, but don't give up hope. Just heal for now, your body knows best. I kept banging mine around, the bone wanted to come through and it always bled. Had hair on the end because of the skin graft was taken just behind my wrist. I'm glad you mentioned it.

Well back to the Zlin, yes I addressed the weak spot! Check out the white paper cutout. Tonight I'll install the left & right side fuse stiffner that you helped define. It will have to conform to a curve however. I hope that the old balsa will take it as this will be the best test so far of the structural integrity of the "old stuff", hey- I'm talk'in about the balsa, not me!

I laminated 1/64 ply from the firewall back to the middle of the wing saddle. IMO, that joint where the rear of the nose meets the front of the wing saddle is a bit weak. You only need to do the flat part, not the turtle deck. I think I did mine on the inside as I built it. take a look see if that is still possible. If not the outside will suffice as the transition from ply to balsa will be easy to hide. I see what your saying. You know I have to fit a battery tray of some sorts, so I did add a 1" x 3/8ths thick piece of balsa to the bottom nose. I hope the fuse siffeners will make (see pic where pen is pointing) the nose section more mechanicaly sound as there are no spars going all the way through, just a bunch of 3/8ths balsa glued in front of the wing section (400 section).

Covering the turtle deck has already be done...w/ 1/8th balsa. No wonder why it was all split. But it good now after filling w/ some glue & balsa dust, then sanded.

SIG's response was so loaded with good information I said what the heck and posted Bob's entire email.

DC
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Old May 23, 2012, 09:31 PM
The Prez....... again
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DC, That is the spot!!! I think????? I followed that angled piece of balsa stick between F3 & F4. 1/8 is not needed. 1/64 is plenty strong. You just need to tie all the bits and pieces together. Maybe even remove some of it in the open bays areas as you won't have to contend with vibration like a glow motor produces. You could use a long skinny balloon to hold the ply in place while the glue dries. I'm thinking out loud on the balloon thing

come to think about it, I added some 1/64 to the rear of the wing saddle as it looked "weak" to me. Use your own judgement there.

Look good so far!!

Ken
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Old May 27, 2012, 06:36 PM
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Well, Mr. Ken, I read your recommendation of not using the 1/8th inch plywood after I had already CA'ed it in. One thing for sure, it will be good & strong!

I'm also looking at that rear wing section. I believe I can see a week spot where the fuse stiffeners terminate at their rear with the "stick" part of the fuse continuing on. I'll take a picture. Not sure exactly what the fix will be, so waiting on your recommendation Sir.

Good news-
The wing is fixed. I tried a trick. Placed the wing in the sun wth some wet paper towels on the top (the side w/ the splitting & delaminating 1/16" balsa sheeting. The sheeting expanded, closing the gap(s). What ever the origional builder used to stick the sheeting to the foam core is still tacky when warmed in the sun. It all went fairly quickly. I realized that the sheeting wanted to re-stick to the foam, the glue is kind of a contact cement looking and acting compound. I guess it is "foam-safe". So when I realized this, and the gaps were closed for the most part, got out the medium CA and away I went. I have to say that due to the dryness of the balsa on this old kit, CA soaks up really well. It's all sanded and looking promising with no blisters, all stuck down. The wing is cooled off now for about 3 hours & still looking good. I'm thinking of brushing on a urethane clear coating or something that will retard the balsa from "breathing", and keep it in it's current condition. On the other hand it might be fine, but it's 40+ y/o sheeting. Making some end tips. Man the aerofoil is wierd with an undercut on the underside TE. Bob Nelson @ SIG was right when he mentioned "unusual airfoils used". I'll post a pic of the wing tip cross section from the plans as it's quite interesting looking.

More on the wing: The TE has a concave radius that accepts the aileron's LE. The hinge point will have to be inset into the aileron's LE. On the print, SIG shows using a stacked design assembled hinge from 2mm nylon sheet. I'll include that pic as well.

Thinking monocoat instead of the silkspan. It will be lighter. Opinions??

More soon-

DC
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