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Oct 13, 2021, 07:57 PM
Balsa Flies Better!
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Build Log

Ace Whitman SBC Helldiver


Hi all

As some of you know, I've been on a quest to build a biplane to fly in the FAC WWII event, just so my flying buddies, Rich Zapf and Tom Nallen don't hog all the fun. This is a little silly as I haven't entered that many events with my Corsair and that airplane flies quite well. But I'm a sucker for a challenge, so I decided to build a bipe. Besides, up in Amesbury, MA, mass launch events are flown with 10% rubber. This is actually quite different than 15% which has been more universally adopted. 15% rubber allows you to use the same prop and airplane that you'd fly with unlimited rubber, but often going down to 10% yields an unflyable setup- motors that don't reach the prop hook, etc. I'm getting an idea that short coupled airplanes do better in limited rubber events- either that or you need to tweak props differently.

A kind modeler on this forum sent me an Ace Whitman kit of the SBC Helldiver. This is a 28" airplane- pretty close to the Dumas ship at 30". However, the Ace Whitman kit dates back to 1940, so I thought I'd put down some thoughts about what it's like to try and assemble a kit from this era. Well, actually, I have a pretty good idea as the EasyBuilt Corsair I recently completed probably goes pretty far back too. But this kit has some additional challenges....

Pluses- the wood in the kit was surprisingly good and seems to have aged OK. The stick can be a bit brittle, but I think they're strong enough and definitely light. Also- the fuse formers are well laid out and assembled with an eye to the grain oriented correctly to increase strength. For the top and bottom formers, most parts require you to glue together two halves and there's a nice little alignment tab. Parts fit is pretty good actually.

Minuses- the design of this kit was never completed- the airplane as drawn is unflyable. Taking a long look at the plans, you realize that the struts are often anchored in tissue- not wood. The interplane strut for the top wing is a ways off from any rib. Also- the ribs were drawn to minimize the amount of balsa needed- I've never seen a kit with parts jammed so closely together. The designers went with a symmetrical airfoil- my hunch is so that they could get more ribs out of a sheet of wood. The rib spacing is also a bit far apart for my taste.
Other issues involve scale fidelity. The bottom of the fuse has this big concave shape- presumably for the bomb that the full scale airplane carried. Only problem is that all the photos I've seen of the ship, none of them show anything close. The fuse is a standard cylindrical shape- might be a little indent. I'm not going to worry about it. Surprisingly, the wheels are oversize on the plans. I'm opting for a pair of plastic wheel halves- dremeled down some more. They weigh about 1 g each- I think I can stand that.

One of the other reasons that I think the design of the airplane was never finished is that the lower wing attachment point relies on a single former near the main spar. I guess you're supposed to rely on paper fillets for fore and aft support- and you better not put much load on the wing or something is coming off!

Here's some of the tweaks I've done on the airplane so far....

Top wing- new flat bottom ribs- added a couple, added a bottom rear spar, added a top turbulator spar, added some dihedral. I still have the emotional scars when I tried to fly a GAR F4B-4 with the top wing flat from 30 years ago. I don't think I ever got more than a lap out of that airplane indoors- it would wander and then arc down to the gym floor. I'm much happier with a flyable airplane even if it looks funny with dihedral than a hangar queen- and I don't build that pretty anyhow. Bottom wing- new airfoil- will probably add a top spar. Need to add some attachment points for the struts. I'll probably go to wire in aluminum tubing sockets for the cabanes.

Fuselage- I'm using retracts- that gear is an absolute nightmare anyhow. Needed to redraw some of the fuse formers. The box and former construction has the issue of a lot of airplanes of this era- the longerons of the box are often inside the outer shell. This means that as these longerons stand- they're useless for covering. What I'm learning in my old age is to think about how to cover the airplane as you build it. I'm definitely sweating the lower wing attachment- it's got a pretty big fillet that I think will need to be on the airplane. This suggests covering the fuse- then adding more of the structure for the wing attachment along with the fillet and essentially plugging in the lower wings.

I'm going to have to build up a nose section- that's pretty easy, just a couple of formers (use thick wood) and some sheet for wrapping.

I've been using the 3 Rivers Archery glue for a lot of this construction. It's not as thin as Sigment, seems to set up faster though, and forms blobs much more readily. Also- not quite as sandable. Nevertheless, I still like it a lot more than cyano...

Here are some of the photos showing where I'm at...

Sam
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Oct 15, 2021, 09:58 AM
Registered User
MKellyvich's Avatar
Good start Sam - looking forward to watching this one come together. I went back and looked at the plan, and it is kind of comical that the interplane struts are just stuck in the middle of an unsupported span of tissue...

Regarding the belly divot, I looked at the photos and drawings in the SBC In Action book and it appears that the -3 version had the divot. The larger cowl of the -4 appears to have been faired in back under the landing gear, filling in the divot.

The lower wing mount appears to have been designed so that the TE extensions would rest on the lower two stringers between formers 4 and 5 - as you said above, not a recipe for surviving any robust arrivals. The Whitman Dauntless also had the LE and TE unsupported, but had two formers that glued to the root ribs at roughly 1/4 and 3/4 span as well as a fillet plate that braced the TE against a third former aft of the wing, making the assembly pretty robust for the size and weight of the model. For the SBC I'd probably put plates on each of the fuselage sides with a sub-former between them adjacent to the LE and TE.

All part of the fun of building old kits...

Mike
Oct 15, 2021, 10:26 PM
B for Bruce
BMatthews's Avatar
Megowcoupe, have you seen this video about the SBC? It showed up out of the blue for me on YT. Proof postive that the SBC was used in WW2.

Curtiss SBC Helldiver in Pacific Action! (4 min 8 sec)
Oct 17, 2021, 02:01 PM
Intermediate Multi
Trisquire's Avatar
You've got to wonder how many of those old kits ever resulted in a successful flight back in the day. When you're a kid in 1940, kit building is more about inspiring your imagination than actually getting decent flight performance. Instructions were pretty sparse; often nonexistent altogether. I'm sure those kits all made nice looking static display models.

Bill Henn once wrote about his childhood rubber scale experiences: "The year was 1942 and I was (age) 10-11.........Built lots of scale models as a kid but none would fly. They glided OK but always looped, dove in or did some other crazy thing under power. I realize now it was thrust adjustment. The prop shaft bushings that came with the kits were wood and the fit was sloppy. Worse yet, there were no instructions in the kits about thrust offsets or any other trimming info. That was a shame because it must have turned a lot of kids off on the hobby."
Oct 17, 2021, 04:52 PM
B for Bruce
BMatthews's Avatar
I went through that even in the 60's with the "made from wood like iron" Guillow kits. I understood about the need for balancing but by the time I had enough nose weight in the plastic cowl they would more plummet than glide....
Oct 18, 2021, 05:01 PM
Registered User
I was fortunate in that my father was an aeronautical engineer and an experienced modeler. With his guidance I was able to get leftover WW2 Joe Ott kits, cardboard and pine, and Comet kits with pine stringers to fly well. After that, Guillow's kits were no problem. Before the mid 50s Guillow's kits were quite light but somewhat difficult to build (had some 1/16" X 1/32" parts and the instructions on mounting the wings were somewhat vague) but they would fly.

KF
Oct 19, 2021, 06:15 AM
Balsa Flies Better!
Thread OP
Hi KF

If anybody was able to get this airplane to fly as designed- I'll eat my hat. I know that some of the Ace Whitman airplanes did fly-and flew well, but I'll lay long odds this wasn't one of them unless somebody did some major reengineering. My dad wasn't a great model builder, but I do remember being in love with a Guillow's Lancer that we built together- I was probably 5 or 6 at the time.

The first rubber ship I ever got to fly was a Comet Cloud Buster- all the Guillow's stuff beforehand just hopped- never got them to climb above launch height. I was probably like 10 or 11 at that point. Surprisingly, I remember getting a single magical flight with Jetex in a Jetco Cougar, a few years earlier. Never got the engine to run again, but boy that was such a trip, running underneath the airplane with it sputtering along. I had much better luck flying the Scientific "hollow logs" U-control.

I built that Cloud Buster stock with the exception of using Japanese tissue and a couple of coats of nitrate dope. Used the kit wood too. They had put in a useful power combo- that hauled the airplane up. Didn't have a great glide, but I didn't care. This was the first airplane that I built that outflew a Sleek Streek. (no winders, stock rubber). I still have a Cloud Buster lying around- it's a great airplane for kids to play with as it has a decent climb, but also you don't need a DT.

Sam
Oct 19, 2021, 07:10 PM
Balsa Flies Better!
Thread OP
I figured I should add some more build pictures. I decided to sheet the fuse where the wheels go. I thought about just gluing them on the side of the airplane -of course the lightest would be to draw them on but everytime I do that, the airplane looks like it came out of class of demented kindergarten kids so I opted for something a bit more challenging. Of course, I came up with this idea after I'd put the sheeting on the airframe- why take the easy way out and cut the holes before gluing the wood to the airplane? But I think it looks a bit more scale like with the wheels recessed.

The next shot you can see where I added a former to support the leading edge of the wing. I'm going to cover the fuse before adding the false ribs on the fuse along with the fillets. There's just no way to make the underside of this airplane look scale- the model needs a lot more incidence in the wing than the full size airplane and that's going to change the bottom of the airplane. Can't be helped.

The final two shots show that I'm carving out some of the box stringers to shave some weight. At best, I'll save about 0.5 grams. For some folks- given that this is likely to be an 80+ gram airplane, that's not worth the effort, but from my perspective, anything I can do to shave weight is generally helpful, plus it's some weight aft of the cg which may turn into a need for noseweight.

Hi Bruce- yes, I did see that video, but thanks for the link. For the purists, that's the SBC-4 and the airplane I'm building is the SBC-3- although the plans misidentify the airplane as the -4. I bought the movie Dive Bomber- and it's got some SBC's in it, but I think they're all the -4 airplanes.

I'm not going to sweat it so much- I'll probably just use some internet source for a pre- Pearl Harbor color scheme. If the guys at the FAC bellyache- well, I can pull out my Corsair which flies pretty well. The rules say between 1939 and 1945- so my interpretation is that pre-Pearl Harbor but post Sept. 1939 should be legit. There's a reason I'm not fond of the FAC's national organization.... But I'd rather do the pre-Pearl Harbor scheme- it's a lot more colorful and I have a bunch of airplanes with blue topsides. Besides, I just bought some gray Mt. Fuji tissue from Easy Built- want to try it out.

I also ordered a canopy and the peel and stick decals from Dumas. I expect the canopy to be excessively heavy, so what I'm planning on is to use the front of it- that has the compound curves and then just use some thin sheet plastic for the rest.

Sam
Last edited by Megowcoupe; Oct 19, 2021 at 07:26 PM.
Oct 25, 2021, 08:38 PM
Registered User
For anyone who's interested in the Cloud Buster, I think Guillow now sells it as the Fly Boy. They already had a different design called Cloud Buster.
Oct 25, 2021, 09:21 PM
Balsa Flies Better!
Thread OP
Hi Linc

Actually, Guillows just renamed the airplanes- the Cloud Buster became to Fly Boy, and the Fly Boy became the Cloud Buster. I've never gotten the original Fly Boy to fly- not sure I got the Cadet to fly either come to think of it. I suspect very heavy wood was the culprit.

Working on the tail feathers now. Enlarged the stab a bit, just for a little insurance. Actually, the stock version doesn't look too small, but I think the fudge is pretty unobtrusive and I doubt it'll hurt.

Still have to monkey with the cabanes- have the aluminum tubes in the airplane- need to bend the wires. Well, haven't put the tubes in the top wing- not sure it's necessary.

The box from Dumas came today- press on decals- a bit heavy, but I think I can get away with them- they worked on my Super Solution- as well as a plastic cowl. Not very helpful for this airplane as its way too big- I wonder if the Dumas airplane is closer to the -4? Gave Dumas a heads up that I was hoping for a canopy- which is what the paperwork said. I guess somebody just mixed the order up. Dumas was actually pretty good to deal with in the past. I once requested some lighter wood for the F-82 kit as one sheet was really heavy- and the other sheet wasn't. Didn't even charge me.


Sam
Last edited by Megowcoupe; Oct 25, 2021 at 09:27 PM.
Nov 15, 2021, 11:56 AM
Balsa Flies Better!
Thread OP
Not surprisingly, the Dumas canopy was on the "portly" side- I think it was 9 grams. Anyhow, I basically cut it back to the windscreen which is about a gram or so- I can live with that. I'm still monkeying with the cabanes- I may tissue the fuse, add the cabanes, set up the top wing, then cover the top wing after I know that the cabanes work. Going to have to cover the rear underside of the fuse first so I can begin setting up the lower wing mount. This airplane is a real challenge to not build yourself into a box....

Sam
Nov 16, 2021, 06:31 PM
Balsa Flies Better!
Thread OP
I've discovered one of the "joys" of these kits. I put on a first strip of tissue, applied wet, and as it shrunk, wrinkles developed! I found out from a good OFB that the Mt. Fuji tissue I used has a much more aggressive shrink than the Esaki I'm used to. I applied the tissue wet- that may have been a no/no. What I think is going on is as the tissue shrinks, it's distorting the stringers so wrinkles develop. On a more modern fuse with lots more formers and a more rigid structure, I think this is less of an issue. But on this fuse, with some pretty soft wood and not a lot of support- it's an issue.

I'm going to try applying pieces where if the tissue shrinks aggressively will be problematic dry and see how that works. The Mt. Fuji feels thicker and stronger than Esaki- might be a problem on really lightweight airplanes, but I don't think it's going to be a big deal on this one. Plus, I'd rather have a little extra puncture protection...

Sam
Nov 16, 2021, 07:07 PM
Intermediate Multi
Trisquire's Avatar
Maybe pre-shrink the Mt. Fuji?
Nov 17, 2021, 07:25 AM
Registered User
MKellyvich's Avatar
The SBC is coming along nicely Sam. I used Mt. Fuji tissue on an Embryo last fall. Tried to cover the wing wet with unshrunk tissue and wound up with some very interesting but decidedly non-aerodynamic contours... Built a new wing, pre-shrank the Mt. Fuji and applied it damp, all was well. It does have a pretty aggressive shrink, but it's tough and the colors are great.

Mike
Nov 17, 2021, 09:16 AM
Balsa Flies Better!
Thread OP
Here's the thing....I typically am pretty careful about shrinking tissue on aerodynamic surfaces and I often prefer to have a few wrinkles than something drum tight as I build pretty light. But on a fuselage? My thought process is that if a fuselage can't handle the forces of shrinking tissue- then it's not going to handle forces of a rubber motor- especially during an "arrival".

I'll try the trick of using the 70% IPA for shrinking the flight surfaces rather than water. I'm not fond of pre-shrinking, although I have done it in the past. The other alternative is to try some of the domestic tissue I've got on the flying surfaces- which might make some sense, although sooner or later, I probably better figure out how to use this Mt. Fuji tissue. I may have to try the Flying Bird tissue from Wind It Up- maybe that's a bit lighter and shrinks less, but doesn't have the color range. I've still got a semi decent stash of Esaki though.

Sam


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