I've had a lot of fun with the Great Planes Super Skybolt ARF in the last few months. When I saw the advertised weight it floored me. I'd seen a bunch of the kit built ones and had never seen a one of them that I'd call light. So I'd never even considered building it from a kit. So when I saw it advertised at 7lb 12oz, I jumped on it. And since I'd seen how the 9lb kit ones had flown with 60size engines, I never even considered installing anything more than an OS61FX.

And mine has been excellent. Up until the other day when I was trying to do inverted flat spins and went blind in one eye. Like an idiot, I tried to blink that eye into working again and didn't bother to bring the plane out of the spin. When it dawned on me that the eye wasn't going to quit hurting and start working the SkyBolt was about 1/10 mistake high. So I let go the sticks and started to fly it out inverted. And discovered I had no idea which way it was going. So I chopped the throttle and watched it "land sorta fast" into the mature wheat field that surrounds our flying field.

No worries, that wheat has cushioned lots of off field landings that're basically flat landings.

When I got to the plane it looked as I expected, just covered in wheat. Then the motor sorta grated around in the cowl some. And I noticed the cowl screws had cracked a little of the cowl around them. But the motor seemed very loose in the otherwise perfect cowl. And when I grasped the spinner I could push it in and out. Darn, I figured the motor mount had broken. But couldn't figure why any hit that had been hard enough to break the motor mount hadn't REALLY screwed the cowling.

When I started taking it apart I was amazed.

The motor mount had "die cut" the firewall. It had almost cleanly punched out the "plywood" firewall about as cleanly as Sterling kits were diecut back in the old days.

It turns out that the firewall on my SkyBolt is two layers of what I call veneer wood. Veneer wood is one ply of some kind of wood that then has a veneer of another kind of wood glued to each side. In the case of the SkyBolt's veneers, each one is about 3mm thick. The veneers thicknesses are basically not really measurable. OK, maybe .2mm thick, but I'm guessing. They're THIN enough that I doubt they offer much more strength than the glue used to veneer the three layers together.

It turned out that the impact of the "crash" was enough to "die punch" the firewall cleanly, but not enought to break the cowling. Now that firewall construction isn't what I call sensible.

If I were to assemble another ARF SkyBolt, I would glue an additional layer of plywood to the firewall. But after what I saw inside, I doubt I'll be assembling another ARF SkyBolt.

Here are some pictures.

The first is the engine/motor mount sitting on the "die cut" piece of firewall that was punched out. The motor mount bolts were removed during disection.

The second picture shows the insides after I've removed some of the remaining firewall at the start of the repairs. Notice the foam on the inside top? The foam shapes the top of the fuselage and is sheeted with balsa that's about the thickness of the red covering that's "hiding" it. There isn't anything else up there that supports the top of the firewall or connects it back into the fuselage. The two horizontal pieces up there that stick out into the open hole look like they are missing the middle part, right? There isn't any middle part. They're the heaviest supports for the top part of the firewall. The top of the firewall is basically just freestanding as far as secondary bracing is concerned. BTW, that's the cowling in the lower left of the picture. If you've assembled one of these and wonder that the 4 cowling blocks in the picture look a bit different than you had, I added the plywood to overlap onto the fuselage sides. That insures that the blocks don't fail at the buttglue joint and increase the gluing surface.

The third picture shows the firewall piece along the "die cut" line at the top. Sorry for the blurry picture, but I took these pictures for my records and they're good enough for that. If you look hard, you can see the 5 of the 6 wood layers. From the bottom, there is a light layer, a slightly darker layer, another light layer (those three make up the forward most piece of "plywood"), another light layer that's hidden somewhat by shiny stuff that's the glue used to sandwich the two "thick" pieces of "plywood" together, and then a very dark layer, and the 6th and final layer which is light.

Notice the T-nuts protrude through the firewall. That gives an indication of how thick it was. It often happens, and is not an indictment except that this firewall isn't strong enough plywood to get by with being that thin.

BTW, I'm NOT ticked off. Nor am I angry or actually even excited about this. This is SOP for ARFs. We better get used to it. However, I'm posting this thread to inform "us" ARF users who might buy the thing in the future AND in hopes the retailers over here will hear about this thread. It's the retailers who need to QC what they sell to us. And it's way too easy for them to blow off communications to just one or two "disgruntled" customers.

It appears to be standard practice for firewalls and other critical bulkheads and such to be made with lightply. Overall, not always a problem. And it's not automatically even a bad practice. But in some cases, it is a bad thing to do.

I've got a Hobbico Sukhoi 31 that is fatally flawed because of lightply where real plywood should be. The bulkhead that holds the wing on the airplane is one layer of medium light ply. The wing hold down tab that is supposed to be held securely by that one piece is simply two pieces of the same. The wing hold down started failing during the first flights of that model.

I recently helped a buddy assemble his Kanke Monocoupe. Excellent and beautiful airplane. Beautifully constructed and covered. It's firewall is two layers of lightply. We chose a 90 for it before we noticed.

I've just now discovered that my Hangar9 P47 Thunderbolt's firewall is two layers of lightply. It's such a great flying and awesome looking dude in the air that my next modeling task (after finishing the SkyBolt repairs) will be to reinforce that firewall.

So what's the bottom line?
The ARF mfg's really need to use a plywood that is appropriate for the task. In the case of the firewalls, they could have easily used decent 5-ply plywood with another layer of whatever plywood they wish. Of course, that lighter ply should be strong enough to stand the compression of T-nuts it they're used. In the case of the wing hold down bulkhead, it definitely ought to be 5-ply. The use of light ply in that model in that bulkhead illustrates brilliantly the lack of understanding of the industry that is selling that model.

Will this thread get lots of "stand up guys" testifying how their ARFs have never had any problems? You betcha. I'm surprised they haven't started already. But trust me, the ARF industry needs to raise their own bar on wood selections. And the retailers need to step up and do us a better job of QC and a better job of design when it comes down to specifying wood selections.

The ARFs we're getting now are excellent deals. We're getting awesome looking models. It's amazing what we can buy. But right now, too many of them are coming across with fatal flaws that really shouldn't be there. Hope the better ARF retailers step up on this.

did this bird go straight in?

only thing that use to bother me about these arf's was not enough epoxy around the inside of the joints................
also, did it hit pretty hard?

if so, then i feel somewhat relieved in that i won't destroy a moter.

i am sorry that this happened to you though.......always a bummer to lose a good bird

It went in gliding. Like in a turn only at idle. The wingtips on one side caught the wheat and it came around to hit the spinner first. As I described above, it didn't hit hard enough to mess up the cowl. But the firewall got it's ticket punched. I've not ever seen even a bad landing where the cowl took almost no damage but the firewall failed. And failed like this one did.

Nope, not hard at all. The cowl came through. It actually stopped the engine being pushed back farther and causing any further damage. How hard could that hit have been. It was about like a couple of the bounce-in's that happened that day that broke props. And they happened on the runways without the cushioning of the wheat. Of course, the spinner was dirtied up enough to show that it hadn't been cushioned worth spit.

Don't feel sorry for me. I've got it about halfway rebuilt.... and re-engineered. It's too nice a bird to not rebuild it. I've been building and flying models since the late 50s and usually disect them completely to see how the construction fared, but only when they're not worth rebuilding. This one is certainly worth the time and energy. Also, it's about $300 worth of model, without counting the servo's etc. So it's worth the money to rebuild also. I'm actually happy that I got to see inside the nose. It's getting an additional structure or two above the tank area to support the top of the new firewall from the back. The sheeted foam is all that's up there from the mfg. Now there will be more. My airplanes usually live very long and busy lives. Now this one will have a better chance of wearing out some engines for me.

Hmmm....

Is this also a reason that the manufacturers are so quick to recommend NOT putting in larger engines?

I guess I need to go put some additional backing material into my Skybolt's firewall...

More to do with ARF's...

You know, when the Titanic was sinking, it broke in half. Was poor construction the culprit?

No, it was never designed to have the stern lifted out of the water.

Just think, if the firewall had been made of good 1/4" plywood and not broken away, you might have broken the whole nose off.

A person can now start seeing HOW they shaved the weight off this plane.

How much weight would it have added to put in a good firewall in the place of the weak one? I'd bet less than an ounce.

But now that the problem is known, that area can be beefed up easily enough.

Pretty analogy. Lousy but pretty.

This Titanic bumped into the dock when coming into port at All Ahead Slow. And it hit so softly that the bow of the ship wasn't hurt. But the Titanic's engines were broken off their motor mounts.

Just think, if the firewall had been made of good plywood the engine wouldn't have punched a clean little hole in the firewall. Since the cowl was strong enough to stop the damage after that centerpunch, wonder why the whole nose didn't break off. I'd suggest that if the cowl was strong enough to stop further damage, there is an excellent argument that the impact wasn't great enough that is should have caused the firewall wood to fail. But since it did, there's a pretty good argument that this Titanic's motor mounts were from inadequate materials.

I realize that, and I'm not trying to be a smart *****, I'm just saying that if the mount had failed while doing what it is designed to do (IE, Fly the plane) I'd say, "Ok, it was too weak to do it's job". But you weren't there to see what weird thing it may have done as it hit. Whatever it was, the firewall was not designed to stand up to that.

It WAS designed to pull the airplane, and to be as light as possible as it does so. It did it's job until something it wan not designed to do happened.

It's kind of like back in the 70's when they started to make cars that crushed on impact to save lives. Everyone thought it was a great idea. Then as soon as they had a fender-bender that cost $3000 to fix, they all complained about the "Crappy way they build cars nowadays"

Good interpretation.

And you know what. I've got the sucker rebuilt. Actually, the epoxy is curing as we speak. I'll have the cowl on this evening. And guess what I'm going to do. Weigh the sucker. And we'll know how much weight it gained.

BTW, I replaced the firewall with a sandwich of plywood much like the original. The original was two pieces of ~3mm liteply glued together. My repair uses one piece of liteply sandwiched with a piece of 5-ply Aircraft Plywood. Since my two plywoods gave exactly the same thickness as the original, I decided to put 5-ply "pads" on the backside, to protect the liteply from crushing by the T-nuts. The main problem with using liteply in this situation (with T-bolts holding on a motor mount with a compact foot print) is that the liteply stands up to the compression poorly. Five-ply doesn't crush nearly as easily as liteply when compressed. Liteply could crush during installation, but who'd be able to tell? And the compression could be slight enough that it didn't obviously damage the veneer, yet degrade the integrity of the firewall enough that it'd fail later. Heck, liteply might fail from flight loads if you really hossed the airplane around. Heck, you wouldn't even need to hoss it. Just do some snaps.

But you're doing such an excellent job of it.

I'm not the least bit convinced that it did not fail while flying. From my autopsy, it looks to me like the liteply was breaking down. And the airplane hadn't even had a single hard landing. But know of no way to prove or disprove either way. BTW, the veneer layer had been imprinted by the lettering on the back of the motor mount. It wasn't strong enough to stand that. And the veneer was obviously impressed by the edge of the mount all around.

I'm not entirely convinced that most ARFs are designed as "classically" as were the earlier models. This one in particular is really just an iteration of a very popular model from a few years ago. The way it performed for me, I'd suggest that the mfg didn't do any design beyond duplicating some measurements. When his "design" came down to choosing the components, I'd suggest he didn't do anything close to the rather exhalted idea that he figured out how much stress two sheets of very crappy liteply could take and decided that would be perfect for providing "crush protection in the event of auto accident". You seriously believe that stuff?

And today our cars are designed to crush appropriately on impact to save lives. Darned if they didn't change some details along the way. Changed some materials. And darned if I'm not suggesting that the SkyBolt would benefit from a slight change up in the strength of some materials. Just like my Sukhoi needed. And the Monocoupe. see a trend here? Those failed in "normal flight".

OK!!!

Really, I was only making a point.

I recently replaced the OS 91 I had in mine with a YS 91.

Along the way I managed to pop out one of the blind nuts and I had to remove the tank to replace it. During the process, I had a good look at the firewall, and saw nothing to cause me any concern - YET

We'll see how well it holds up to the YS.

One thing I DID notice however was that after sitting over the winter the stopper on the tank had become very loose, so that's another thing to keep an eye on.

BTW, I got two flights over the weekend with the YS 91 and it's a screamer!

Now wait a minute!!!! That's a 4stroke right. Ain't they just for sounding nice and for when you want 25% excess weight in the nose? and of course, less horsepower? and not to forget conspicuous expense???? grin...............

NOOOOOOOO!!!!

They're for when you want you plane to have some CLASS!

sheesh.......i bought three of these birds.(yes, coming back and i'm hooked).........maybe i should buy a fourth??

really though, the firewall has plenty of mass for the recommened size engines, and of course there have been guys putting double that as in one of the topics here,

http://www.rcuniverse.com/forum/m_42...tm.htm#4236365

last post on the first page............a 1.20AX for petes sake............and some guys have mounted OS 1.20's and YS1.10's...............no failures.

what i'm really suprised at with your plane Rock, is the cowl........it took me many emails and actually 2 purchases to get perfect cowls (uncracked) for each one i own and 2 spares.
the one i have finished right now was clear coated with Varathane, and that was to get a good gloss finish.

I see no reason to avoid the 4th. I would suggest that you layer that firewall with some decent 5ply. 3/32 would be about right. That's what I plan to do if I ever need another one.

Mass isn't the only characteristic to look for in a wood component like a firewall. The same mass of balsa wood still won't be as strong as liteply. And the same mass of liteply won't be as strong as 5-ply Aircraft Plywood.

The original ARF package I opened had been packaged with the hardware sack placed loose inside the cowling. And that cowling had been beat all to hell. I got a replacement from Tower as soon as they became available. It's been good since then. Of course, since starting to assemble ARFs, I've found that I almost always reinforce the insides of all the recent cowls where they are going to see stress. This one had 2oz glass cloth painted inside where the 4 hold-down screws go through. Basically 1"x1" squares of glass cloth. Of course, the cowl took all it's abuse in this hit from the spinner backplate and if there had been a bit more energy would have crushed forward of those 4 points.

To clarify my outlook on this firewall.....

If I had been building this thing from a kit and seen a two layer liteply firewall, I probably would have used it as a pattern. You've got to remember that the center top of the firewall has virtually nothing behind it either supporting it or connecting it to the front of the fuselage. Yes, there is the top of the fuselage that butts up to the top of the firewall, but that fuselage top is foam, sheeted with balsa wood (looks about 1/16" balsa). The sides of the firewall have adequate connection to the side formers of the fuselage, but the center top has really almost nothing. So I would have wanted the firewall to have adequate strength from side to side.

One of the first modifications I'd have made to a KIT of these ARF parts, would have been to place some kind of bulkhead across the inside top that would have given the firewall some support. It could be another piece of liteply. That's what I've put into mine during the repairs. As it comes in the ARF, the firewall is supported in basically a "U". Down the sides and across the bottom is nicely connected to fuselage sides, bottom and bulkheads. And a liteply firewall just isn't going to do much of a job being supported on 3 sides only.

I would suggest to the mfg, that he not sandwich two layers of liteply, but at least replace one with good 5-ply. And add some support for the entire front top of the fuselage to bridge that gap. Go back and look at the middle picture. All that open area is still open area behind the new firewall, just like it was behind the liteply firewall that had it's center punched out. A crossbrace or two??? Like between those two tabs that stick out into the opening. I was amazed that those two "tabs" were there. I can't imagine any reason whatsoever to have cut out the intervening wood. It would have helped stiffen that firewall, and was simply waste when cut out. (Or maybe some other part was taken out of that area, who knows.)

Personally, I don't think that happened in the crash...

I think the Slybolt just spit that 2-stroke off in disgust.

I'm just busting on you. But I see your point, a decent piece of plywood would have been a big plus. And a better bracing system would have helped. But I also stick to my guns in saying that today's planes are completely re-engineered from the way they were 10 years ago.

You have a legitimate complaint about the firewall, but someone else may have broken a stab and said, "If it were fully sheeted like the KIT version was, it wouldn't have happened". And someone else might say, "If they had beefed up the landing gear area, etc, etc."

A few years ago when 3-D came into vogue, the industry went "Light Crazy" (Just as now they are going "Electric Crazy" ). Look at how planes are engineered nowadays with their lite ply interlocking framework. It's strong, yet light and totally different from the way we used to build. But it has to give up one thing to gain another.

So yes, they could have made a stronger firewall, and that would satisfy you, but it wouldn't help the guy who ripped his landing gear off. If they beefed up all of the stuff that everyone had a problem with, we're back to a plane that weighs a pound heavier.

Hehe.... You? busting on me? Naaahhhhh................
But yeah, I understood that but your Titanic allegory (it's rhetorical heat puts it beyond analogy into allegory at least) really deserved a bit of trimming.

Thanks for judging my complaint as legitimate. chuckle..... As for what someone else may or may not say..... not really of much importance. And that's true of almost everything. And in this case, I've simply recounted and documented what happened and my take on it. Which if you thing about it, is about all that anyone can do and then leave it for comment. And hope they are sensible enough not to wax eloquent about something else. Like turn of the century ship wrecks.

Yeah, but I think everyone understands pretty clearly that this Skybolt isn't a 3-D, nor is a two liteply firewall either adequate for a decent service life, nor is it probably adequate for a medium one. It's light yet not what you'd call strong. And it's not supported worth spit. And everyone nowadays has the inclination to hang oversized engines on the sucker and the trend toward 4strokes means that design (from "a few years ago"? right? so it's outdated) is also going to have to stand up to even greater vibration and backfire potential.

What we've got here is speculation of the greatest sort. You've speculated that the ARF mfg's are doing some pretty magic redesign. And speculated that the landing gear strength has something to do with landing gear?????? And surmised that if they used a layer of 5ply and placed a brace to close off what's obviously a large open hole that the airplane would weigh a pound heavier.

Sorry, but you can't win the argument by guessing that what I'm suggesting is going to add a pound of weight and make the landing gear fall off and associate all that speculation with what I'm suggesting. Nor is the issue on the order of the sinking of the Titanic.

Ok, I'll stop arguing about the skybolt - but the Titanic analogy is perfect in any era.

It broke in two because it did something it was not designed to do.

Now fix that firewall and put a 4-stroke on it!

Guys, let's put this into perspective.

I'm suggesting that the design is weak. As weak as the firewall......... grin

And I've also recounted what I've done with my repairs.
Well, the sucker is back together. And I've weighed it. And I've got a problem. Turns out I'm a bit shy to report the weight.

You see, awhile back, I used a really good Triner 10lb capacity scale. But I knocked it off the counter a week ago and busted that sucker into many many little parts. (And no, I didn't then suggest to Triner they make it out of cast iron. I don't expect a scale to last forever when subjected to that kind of use. And I do not have 50years experience building scales.) So I was using my backup scale that isn't nearly as accurate. But a day or so ago I bought a really accurate 11lb digital that is "USPS accurate".

So when I weighed the repaired fuselage last night the new scale reported virtually the weight that the old one showed months ago. And that's simply not possible. There is a new liteply horizontal bulkhead behind the new 5ply firewall. I did cut out some lightening holes in it, so the liteply firewall isn't going to add weight but the new 5ply should weigh something more than the old liteply layer. So the new firewall is one layer of liteply and one layer of 3/32" 5ply. So you'd expect that to be heavier than two layers of liteply.

So what I'm guessing is that the old Triner wasn't as accurate as .......................... oh wait.................. I think I know what's happening.

When I was doing the repairs, I pulled the old throttle servo and put a Hitec225 in it's place. The old servo was a 1980s Airtronic and I bet the sucker was 1.75 oz. The new HT225 is only .9oz

That's gotta be it. That and the probability that the two scales differed somewhat too. whewwwwwww......

Too bad that scale didn't fall into a wheat field

(Sorry, you KNOW I couldn't resist LOL)

Ain't it the truth.

Our field actually has been one of the safest I've ever flown at. And it's simply because the farmer usually plants corn. And there is about 100 acres around our runway. Last year, we had the normal number of "crashes" and when the corn was up about a foot until it had been harvested, almost none of the models took any appreciable damage. If you had any problem with your model and could get it to at least go in somewhat flat, it was almost guaranteed that when you found it, there wouldn't be any damage at all.

When the farmer went to wheat, it looked like we weren't going to have our "padded" fields. Turns out that the stuff is just as cushiony if the airplane goes in level. It doesn't do spit for the ones that're hauling butt or almost vertical. But that stuff is better than plowed ground.

Only problem with the corn was finding the suckers when the corn was tall.

Lucky you! I dropped a favorite plane into a cornfield once and I remember thinking, "I'll bet the corn stalks softened the blow"

Nope, they shredded the plane []

I never would have suspected that much damage, but it was totaled!