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Jan 16, 2020, 05:44 PM
Balsa Flies Better!
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Build Log

RCM Wildfire - Balsa Workbench Short Kit

Hi all

A couple of months ago, I picked up a bunch of short kits from Rob at Balsa Workbench.

One of them was the Wildfire which he did a brief build thread on this site. What I've realized is that Rob is a better builder than me and he can look at a bunch of parts and visualize an airplane. Well, I can't- I need help.

So I'm starting this build log. Please don't pester me for pictures- I have to use a camera (dumb phone) and download the files from the chip onto my computer (which is probably going into the shop anyhow tomw.) so it's a bit of a PITA and I don't want to bother unless I've got a few. Besides, others are much neater builders than me.

About the Wildfire- I think it's about 350 square inches- scaled down sport airplane from a .40 size design. Rob put an OS 26 Surpass in his which was plenty of oomph while the airplane lasted. It's a comfy size for me- 42" span, originally called for a .10 to .15. I guess that would work- but I think my poor Webra .10 would have a hernia lugging the airplane around, but my Enya .15 would have done OK. Rob's came out at 3 lbs which is a bit heavy for my taste. It's pretty close in size to my Herr P-51.

I've got a new field to fly at relatively close to home- <15 min. drive. The downsides are that the pilot areas face the sun at a bunch of times of day and there are some pretty tall trees within the boundaries of the field. It's a grass field- so I suspect the wheel pants will be put away for some other project. (yeah, right.) I'm not happy flying my faster, heavier airplanes there yet, so I want something easy to drag out and low EDF. (Emotional Debt Factor) Hence, the Wildfire seems to fit the bill.

It's definitely a glow ship. One of the things I've decided is that I like older glow ships- especially when they were reasonably designed. Modern electrics are often fragile. Yes, they fly well at reduced weights, but often you have to be careful with handling them. Older glow ships are more, well, knock around for lack of a better term. With modern electric power systems, they fly fine- there's no weight penalty compared to glow anymore.
However, I still can't stand adding extraneous weight- it's a sickness, I know.

The Build Log
So the first thing I did was I opened up the size of the center cutout in Former F3. Be careful- I mixed up the top and bottom of the former and I'd have probably left a bit more beef where the wing dowels go although I suspect it'll be fine.

Next up- I laminated the ply doublers. Here was an opportunity to shave weight (could have cut out some of the centers)- but I just didn't bother. I used Titebond 2. While I'm happy with the strength and flexibility of the bond- I'm not happy with the warps it induces. May have to try some of the thicker CAs or look around for a contact cement. I've used Gorilla Snot- very strong- but way too stiff. Epoxy tends to be heavy and also pretty stiff. Maybe I should just use a couple of coats of Sigment?

Add the rear reinforcing pieces of 1/8 x 1/4" next. You need to use these along with F-5 to line up the wing doublers- otherwise you may have a gap. (Ask me how I know- or better yet- don't!) Then use F-5 to line up the interior wing doublers. Use F-3 to line up the nose doublers.

I glued the fuse up starting with F-3 and a bunch of clamps- then added F-4 and F-5. It's a simple rectangular box. This is where I've gotten to so far.

The included firewall is balsa- I'll go to ply instead. This airplane needs to use a motor with a firewall mount, I think the cowl is basically going to be along for the ride.

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Jan 30, 2020, 06:31 PM
Balsa Flies Better!
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Some pix from this stage....

One of the issues I have is that I occasionally change my plans as I build. This airplane is no exception.

Oh yeah- what I should point out is that my Herr P-51 weighs in at 30 oz- has about 200 watts on an Astro 19 swinging a 8 x 4.5 APC drone prop. (Don't tell the prop it's not on a multi rotor- it won't know the difference.). This is a very comfy power setup- the airplane goes close enough to straight up and is fast enough to get into trouble. The Wildfire is a bit larger- has a thicker airfoil, also semi symmetrical- and more tail moment as well as area. I'm thinking the Wildfire is going to come in at 2 to 2 1/4 lbs- the balsa framework, canopy, and a couple of servos but no landing gear or power system comes in at 15 oz so far. So I'm thinking that a Mega 22/10/5 which I pulled out from deep in a drawer should do about right on a 2s pack (not my favorite setup)- probably need a 3000 mAH pack because on an 8 x 6, I think it'll draw about 35A or so- should be around 250 watts.

The change in plan? (The above is a digression.) I went to a nosering mount rather than a firewall mount. I decided that with the top deck running forward, I'd have enough beef to get away with it and it made the whole removable cowl issue disappear.

Jan 30, 2020, 06:39 PM
Balsa Flies Better!
Thread OP
These were shot today which shows the progress on the airplane. I fully admit that my wing construction is a bit schizophrenic with regards to bottom sheeting, but it turns out that the TE stock I used was a bit thicker than I thought. The wing has plenty of torsional rigidity. I'm not Rob, I'm happy enough with balsa spars, but I think he prefers spruce.

I haven't put in webbing- I don't think the airplane really needs it. I've gotten so used to using webbing that I'd forgotten how nice it is to be able to glue the top sheeting in place without it. Have to go back to adding the webbing after the sheeting- I've been using it to line up rib spacing.

I didn't mind the setup of two wing dowels off on the sides- I think it's a bit stronger than a single center dowel. I just would have liked a bit more meat below the dowels on the bulkhead though- one of the reasons I'm glueing in the bottom cowl piece.

I used a bit of a different shape on the tips than shown on the plan- I did the old trick of going at a 45 degree angle from the bottom of the rib to the top surface. Of course, I'd hollowed out the tips a bit planning on the fully rounded version, so these tips have a bit of a cut out.

I glued in the aileron horn with Zap a Dap- no ply needed. I'm a little worried about my aileron setup- with the aileron servos glued in that way- access is surgery. If the airplane has a lot of adverse yaw, it'll be annoying, but I think the ailerons are close to having the horn on the hinge line. If not, oh well, isn't that "character" in an airplane?

Canopy is from Sparky- it's pretty light which made it not too hard to cut out. Haven't figured out who's going in the cockpit yet though- thinking about a yellow/orange/red color scheme, i.e. Wildfire.

I did carve in some cylinder heads a la a 30's racer. I think it adds a nice esthetic touch. I'm planning on adding the wing fillets as well.

Gear is coming in from Kirk at New Creations- he's bending up an Al blank for me.

Last edited by Megowcoupe; Jan 30, 2020 at 06:46 PM.

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