I've got another Pukey Profile question...

With The World's Second Best Flite Streak up and flying, I am now doing dry-run assemblies of my scaled (78.5%) Impact, gauging all-up weight--read: Is this going to be worth the effort?--attempting to calculate the rearward shift of the balance point during the finishing phase.

The final pieces to fabricate are plywood fuselage doublers and the question is why can't I just use 1/64" ply instead of the heavier stuff?

Ya see, at the moment ìt appears as if the 21FP w/muffler will need to be moved back quite a lot--or at least more than I wish to see it--and this effects the looks of the model. Reference: PW's 2007 eelecktruhfried Impact and his 2008 model; the former was stubby in the nose, the latter has what we have become accustomed to seeing as the "correct" proportions.

Also, the basic fuselage is already plenty stout. I think. Central core of really light 3/8" ballzuh. Carbon plus epoxy left and right, capped with 3/16" ballzuh. After assembly the blank was shot through a planer until the fuse sides were about 1/16" thick, the fuselage 1/2" thick. I really think one could bolt a smooth-running 21FP to the maple mounts with no ply doublers at all.

And so the question remains: Why not use 1/64" ply? If not, why not? Which really asks just how much stronger is a Pukey Profile front end with doublers of 1/32" ply? Or 1/16" ply? 3/32"? 1/8"?

Noting that I am not much of a structures guy, it does seem self-evident that I need to spread the load around a bit, plywood does that job nicely, but where is the line between a nice strength-to-weight ratio and porking the sucker up with needless plywood?

Dan

Use thin ply and carbon fiber (when in doubt).

I use 1/32 on my 60 sized ships - so surely 64th is good for a measily 21.

>I use 1/32 on my 60 sized ships - so surely 64th is good for
>a measily 21.

Oh, NOW you've done it, Wynn! Even with the wimpy "smiley", DD will never let that pass . . .

Dan,
Personally, after many many hours burning up the phones with Pat, I have come to the conclusion that doublers and such are merely a method to transfer load effectively. I would say that using 1/64 on the scaled Impact would not only be appropriate but most efficient. especially since the accuracy of all components being joined has been precisely milled to a fine degree. I assume you will likely be using some form of aluminum pads under the engine? That could distribute the load concentration a bit more. I know on my front ends, I have taken to using 1/8 balsa fuse sides with a 1/64 ply doubler and bulkheads laminated from cross grain 1/8 balsa with 64th ply faces. Its all about creating a hard point to transfer the energy and prevent deformation of the nice soft balsa we love. I say its a go for it to be sure!!!

My current PA plane has about 300 flights on it and has no plywood in the nose. There is some carbon taking the place of what would be doublers, so I guess it has doublers of a kind, but no plywood. Works fine with a .65 in the nose.

Pete's Sultan, on the other hand, has no doublers of any kind. He didn't fly last weekend because the nose is getting a bit wiggly.

>My current PA plane has about 300 flights on it and has no
>plywood in the nose. There is some carbon taking the place of
>what would be doublers, so I guess it has doublers of a kind,
>but no plywood. Works fine with a .65 in the nose.

This is how many of Randy Smiths ships are build.



Greg L. Bahrman

Dan, I think 1/64th would be fine. That said, I see the purpose of the plywood as that of attaching hard maple to something harder than soft balsa. Otherwise the balsa is the weak link. No matter how good the glue joint, the hard maple will rip away from the balsa past the glue joint. Just glue a piece of motor mount to a piece of balsa and then see how easy it is to break it away.

OK, here is the next problem. If you glue a piece of thin or multiple thin layer plywood to the balsa and then glue the motor mount to it, try it again and you might be surprised in seeing how easy it is the break this loose again. (Try it). But what we are doing here in this experiment is pulling it away and delaminating the plywood. If you make a motor mount crutch and then attach this to the plywood doublers in a fuselage nose, you are now working with shear forces. You aren't pulling it away from the side (or plywood). You are trying to rip it away by pushing it up or down. Now the plywood doubler comes into its own. You have attached the motor mount to a hard surface and multiplied the surface area attached to the balsa. Using this technique 1/64th plywood would undoubtedly be strong enough even for a 75!. (Now we haven't tried it with a 75, so if you try it and it breaks loose, don't hold me responsible). But, again, I think this is the principle of what Randy Smith does with carbon fiber layered in between and used as an attach point for the motor mount crutch. It is a similar principle and does not demand a large or thick plywood doubler.

Most doublers, especially those called for on classic era ships, were much heavier than needed for the task. 1/32 is ample for most everything (when using a crutch construction in lieu of simple motor mounts attached to each side). And, certainly for the smaller motors, 1/64 should be totally adequate using proper construction techniques.

(OK, here is a start for another safety column. Anyone have any experience to the contrary, and, if so, what construction techniques were used?)

On your first question, I think it's not too hard to predict the CG shift due to finish, if you are using a known technique and its weight. I've done such ballpark estimates in the past, and was not too far off.
On your doublers question - (a) I would laminate plywood on a bias (i.e grain in outer layers running at 20-30 degrees to engine axis) to resist torsional loads; (b) the question is not "how thin can we go before the nose falls off in flight", but "how stiff the nose has to be to ensure consistent engine runs".

I vote for at least one (if not both) 1/32" plywood doubler, biased as above.

>On your first question, I think it's not too hard to predict
>the CG shift due to finish, if you are using a known technique
>and its weight. I've done such ballpark estimates in the
>past, and was not too far off.
>On your doublers question - (a) I would laminate plywood on a
>bias (i.e grain in outer layers running at 20-30 degrees to
>engine axis) to resist torsional loads; (b) the question is
>not "how thin can we go before the nose falls off in
>flight", but "how stiff the nose has to be to ensure
>consistent engine runs".
>
>I vote for at least one (if not both) 1/32" plywood
>doubler, biased as above.


Actually, your suggestion of biasing the plywood is a good one. We are working with shear loads here and usually attach the motor mounts in line with the outer plywood's gain. This makes it much easier to shear off. By putting it on a bias (wastes a lot of plywood, however) helps to alleviate this.

Your concern about stiffness is moot, however, if an engine crutch is used. Run the maple mount back to the wing (it can be tapered or even routed out like an I-beam behind the engine to save weight) and put cross grained balsa between the mount pieces behind the engine (makes a good tank "floor" this way) and the nose will be totally stiff. It's stiffness will not depend on additional doublers or blocks. The plywood is used only to spread the shear force and keep the motor mount attached to the balsa sides.

I've used 1/64 ply for years and years ..... back to ST60's, piped 40's, 60's, 75's, and soon 90's.

Len's right, maple to balsa isn't a winner. However, a layer of 6oz glass with some good penetrating resin or even polyester, might make a hard enough substrate for the motor mounts to attach to.

edit .... Oh .... never mind ... you were talking about a profile ... sorry.

.007 carbon would do the same thing as plywood and probably be stronger in shear. Bending might be a different story. Bending strength comes from the section modulus, so some .007 or .014 carbon strips on the mounts, inboard to outboard, would form a type of I-beam.

don Burke
Menifee, CA
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways - body thoroughly used up, totally worn out and screaming, WOO HOO, What a Ride!

Dan,
As long as this is a full fuselage model, 1/64th will work more than adequately. My really light Impact that eventually switched to electric had 1/64th doublers, and held everything from PA 40's tp PA 61's, with no problem. You'll be more than good with 1/64th. How about 1/128th? OK, that's a joke Dan.

Great. Some of your favorite e-mail items on that last line!

Your sense of balance and proportion are better than most, so I don't doubt that there's reason for concern. When you say the engine should be moved aft, how far do you estimate- an inch? There will still be room for your Hayes tank and the resulting nose shape won't be affected that much.

I agree with Paul, and others: 1/64 ply doublers are fine. Vacuum bag these (with epoxy) and you'll have the best possible bond.

Remember, there's quite a lot of weight behind the CG- like 75-80% of the wing (including those flaps and the control horn); the fuselage is rather deep for a small profile model and your stab/elevator is pretty big.

As for weight ahead of the balance point, don't you still have some of those really cool carbon LG blanks? I know where you can get more if needed. The legs could taper from 3/8 at the axle to 1/2 at the fuselage. This combined with aluminum axles and those ultralight wheels ought to save a few grams.

Also, try substituting a tongue muffler for the stock OS unit. That will save a lot of weight. . .

Derek

>Also, try substituting a tongue muffler for the stock OS unit.
> That will save a lot of weight. . .

Drop the liner and add some head shims, too, so you can get a good stunt run.

Brett

> Drop the liner and add some head shims, too, so you can get
>a good stunt run.

Not practical, Brett. If he did that, then he'd have to add weight to the piston to get the torque back up. We're trying to *lighten* the nose.
A 10-4 or 10-5 wood prop might help though. They're much lighter than those plastic APC's.
Derek

Thanks, y'all; it's a done deal and I used 1/64" ply on the nose of this Pukey Profile, albeit one with a checkered history, at least in the 100% version. Which is most definitely not a profile model.

Speaking of which, this Saturday last I saw my PP model's big daddy fly, steered around by none other than PW.

And then I the same model fly again on Sunday. Most illuminating...

Responses given as deemed appropriate:

DennisM: I have tried a couple mock-ups--on another model--incorporating both ply and carbon fiber. It ought to work fine; it does not work for me. I think my materials (Commie Fiber) and epoxy resin (System III) are good, my techniques at least passable. But when the whole mess was passed across a gram scale it went in the trash.

Ralph: The comment of "a measily 21" (sic) is hardly enough to get me juiced up. Heck, PW made a joke out of much the same comment concerning his development of the 40VF and he has stretched the joke out for about two decades now.

Mark: Yep. My current Impact, now being used to decent effect after a bit of a lay-off, has doublers of 1/64" ply. Even though it was built around a Saito 72, this rather hurriedly replaced with a Saito 56, now being powered with a RO-Jett 61 on pipe. The latter is a real sweet engine, but I sure wish the bottom panels of the wing had been done in Etch-a-Sketch as "Powered by Pond Scum" is now just so wrong on so many levels...

Still, the doublers on that model are spaced quite a bit farther apart than those on a PP with a .500-inch fuselage.

Randy and Greg: Yes, I know of the technique and that Randy Smith has incorporated it into many of his kits. Again, I am not the best structures guy, but that interface of balsa and maple mounts makes me wonder a bit. Still, if it works, it works...

Len: Agreed on all counts. I kinda wonder if early CL kits weren't designed more for the low-skilled modeler, using what we now view as crummy glue, not to mention really annoying mis-matches in wood thickness. Maple mounts just .010 thicker than the balsa fuselage would raise havoc when trying to build light and strong. That's gotta be the reason for such heavy doublers on so many profile kits from the early days.

Mike: The biasing of the ply is a good idea, although I did not take advantage--the doublers had already been scissored to the proper outline. And my concerns are the same as yours, that being consistent engine runs.

Paul: I have actually wished for 1/128th" ply for some applications, although not this one. And has everybody noted that 1/64" used to be, still is, sold as wing skin material? I'll bet it's been many, many years since this material has been used as intended by whomever first brought the stuff into the country. Foam wings only whizzed at in lightening them, plywood to wrap around the foam, contact cement to hold it all together. Whee! And to think that I actually was given, and flew with gusto, just such a wing. Mounted in an RC Pattern model, 60 two-stroke on pipe, 35% and 40% fuel. Very strong, very fast. Also very heavy. But Lordy! Lordy! was it spectacular the day I punched it in, straight down from nearly OOS, the impact zone a mere 40-50 feet in front of me. Uh, that was not the bad part; the point of total destruction took place probably another 60-70 feet to the pit side of the line from which were all supposed to fly.

Derek: I ended up easing the 21FP back 5/8". It was a compromise as I want to use to stock muffler but also to fiddle with the O.S. O.E.M. header/coupler/pipe I told you about. In either case ballast will probably be required. Alas.

Uh, my recollection is that you kinda/sorta offered CF gear blanks, I didn't have a suitable application at the time and took a pass.

I have an application now, however.

Or maybe not. As you note, the fuselage on an Impact is real deep and I would not have a lot of use for gear which put the model in a VTO attitude.

Brett and Derek: Look, I simply cannot monitor you guys all the time. So it's time to 'fess up, letting me know **all** of the real tricks to the 21FP. Here I am struggling with bog-stock--anybody else still miss Henry Manney--21FPs while youse guys are keeping all the high-tech tricks to yourselves. Yrobnsvnt is humbled and dismayed.

Dan

PS: First "word," last sentence, done from memory. It does not look right--anybody have the actual spelling? dan

>
>Speaking of which, this Saturday last I saw my PP model's big
>daddy fly, steered around by none other than PW.
>

And then not fly...


>And then I the same model fly again on Sunday. Most
>illuminating...
>
At a slow speed...

Flew it again Wednesday morning, with lots of new parts, and all is well. Flies just like it used to!

>
>Brett and Derek: Look, I simply cannot monitor you guys all
>the time. So it's time to 'fess up, letting me know **all** of
>the real tricks to the 21FP. Here I am struggling with
>bog-stock--anybody else still miss Henry Manney--21FPs while
>youse guys are keeping all the high-tech tricks to yourselves.
>Yrobnsvnt is humbled and dismayed.

I am sorry, Dan, it forgot that it was all a secret. By the way, did you try my suggestion to use a banjo grinder and an extra-coarse rat-tail file instead of a milling machine? And just leave the shavings in the engine, it will help break-in. I did try the 11-7 prop you sent me, it works great, has a lot of bite. Launch revs are ~6000.

Brett