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Flitfire Cub

Old 06-16-2022, 01:52 PM
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Default Flitfire Cub

According to their website, the RAF Benevolent Fund “has been supporting members of our RAF Family through thick and thin providing practical, emotional, and financial support whenever they need us” for more than 100 years. To help fund their efforts, William Piper built a special Cub in 1941, painted silver instead of the customary yellow and carrying RAF livery. Dubbed the Flitfire by factory workers, they went on to produce 48 similar aircraft, all of which were donated to raise money for the RAF charity.

(If you are further interested in the Flitfires’ unique history, there are multiple online resources, including Wikipedia.)

One of those Flitfire airframes, the very first one actually, ended up at the North Carolina Aviation Museum in nearby Asheboro. A month or so ago, a friend of mine and I visited the restored airplane and took some documentation photos. Then, another friend decided to sell a NIB Goldberg Anniversary Cub and, when he found out what I had in mind for the kit, sold it and a bunch of accessories to me for a price well below market value.

So, what did I have in mind? In the fall, our club holds a fundraiser for our local food bank in which every dollar as well as can or box of food goes straight to the food bank. In other words, the club picks up all event expenses, including raffle prizes. What better way to raise money than to give away a flying model of a local airplane that itself raised money during WWII?

As a result, I am attempting to construct a Goldberg Cub dressed up as the Flitfire at the Asheboro museum. I will not obsess over scale perfection, so this build should proceed fairly quickly. I will use donated parts and labor wherever possible and plan to have this model flying early this summer so that I can showcase it at local fly-ins to promote our September 10 Winston-Salem Miniature Airshow to collect food and raise money for the Second Harvest Food Bank of Northwest North Carolina.


The North Carolina Aviation Museum's Flitfire Cub in flight.






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Old 06-17-2022, 09:56 AM
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Default Motivation

Not for me, but for the airplane. I don’t think my enthusiasm for this project will wane because the end result is not for me, but for a good cause.

This Goldberg Anniversary Club will be electric powered. I have not built or flown an ICE model for many years. Don’t get me wrong, I revel in the sound and smell of a gas or glow engine, as long as it’s someone else’s.

For aircraft in the 5- to 7-pound range, I have had success with a combination of an E-Flite Power 46 and a 4,000 to 5,000mAh 4-cell battery along with an ESC of sufficient capacity.

For this build, I will install the above-mentioned motor and a Castle Creations 80A speed control rescued from another model that experienced an inartful landing. (How’s that for a euphemism? I should be a politician.) The battery capacity will be determined by the space I end up with at the front of the fuse.


The Power 46 in place on the plans. There's plenty of room inside the cowl. Perhaps I can squeeze the ESC in here as well.

Old 06-17-2022, 01:43 PM
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Default Inventory

When I opened the box, I was absolutely astounded to find laser-cut, instead of die-cut, plywood and balsa. When you purchase an older RC wood kit, you never know what you’ll get, much like Forrest Gump’s box of chocolate. In this case, I ended up with more than I thought I had bargained for.

During inventory, I discovered only two mildly warped pieces of plywood, a fuselage side and a fuse doubler. I figured that, since they were bent in opposite directions, the warps might cancel each other during construction. Every component that should be in the kit was there.



The Carl Goldberg Anniversary Cub. What goodies might the box hold?


Shazam! Laser-cut balsa and plywood parts. Extra hardware provided by the seller. Folded (ugh) plans and a slightly crumpled instruction booklet,


All the sticks and sheets identified and labeled. Let the building begin.

Old 06-17-2022, 02:04 PM
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Default De-creasing

There’s only one thing worse than folded plans—plans rolled the wrong way so that the edges curl up, not down. IMHO, of course. However, the folded variety is not very far behind when it comes to aggravation.

With this set, I decided to try using my wife’s flatiron to get the creases out. Working on my relatively flat workbench, I pushed back and forth and watched the folds disappear. I also noticed the ink on the plans smearing. And then my wife showed up, and she noticed, with great consternation, ink smeared on her iron.

After I got it off with some solvent, I tried ironing through a thin tea towel, well past its more dignified usefulness. That worked, but didn’t flatten the creases as well. So, on to Plan C, ironing the unprinted side. The heat was just right to take out the wrinkles but not hot enough to transfer the print to the surface under it.


These plans are definitely wrinkled!


First attempt resulted in smeared plans, a dirty iron, and an upset wife.


Using a tea towel. Better result, not perfect but a happy wife.

Of course, I forgot to photograph the best way to do it. Suffice it to say that the plans were finally ready for construction use.
Old 06-18-2022, 03:58 AM
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I'm in....
Looking forward to following along...
Old 06-18-2022, 06:07 AM
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Thank you for following. Please chime in any time with suggestions. We all learn from each other.

Take care,

Bren
Old 06-18-2022, 04:02 PM
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Default Tail Feathers

The plans were flattened (mostly) and the kit unpacked and inventoried (completely), so it was time to start putting things together. The instructions recommended starting at the back, so I did.

There’s not much new and different to report about constructing the tail surfaces. I did scuff the burnt areas to eliminate the micro tabs that laser-cutting produce and to improve glue adhesion. Speaking of adhesion, I like to use a Top Flite Woodpecker on wood surfaces to promote glue penetration in joints.

For glue, I relied on Bob Smith Industries medium CA. A sheet of wax paper protected the plans. A preliminary block sanding took care of the slight difference in thickness of the various pieces.


I’m going to save rounding and beveling until I decide on the type of control system I’ll use.


Piece #1 pinned in place.


Horizontal stabilizer perimeter pinned and glued.


Horizontal stabilizer filled in and weighted down while glue cures.


Stab and elevator finished.


Same for the fin and rudder.


Tail feathers ready for rounding and beveling.

Last edited by GiantModeler; 06-18-2022 at 04:11 PM. Reason: Correct spelling
Old 06-19-2022, 12:09 PM
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Default Fuselage

Back in the mid-1970s, when I made my grand re-entry into the world of aeromodelling, I chose the Goldberg Eagle 63 as my inaugural effort. If I recall correctly, the fuselage of that model comprised a jigsaw puzzle of plywood formers, sides, top, and bottom that assembled without glue using rubber bands and masking tape. So too the Cub fuselage, except this time I supplemented tape and elastic with clamps. And threw in some alignment blocks for good measure.

I did encounter a minor stumbling block; the sides, top, and bottom were not laser-cut but CNC milled. As a result, the notches and tabs were not as accurately cut and all the inner corners were rounded instead of square. A few passes with a #11 blade took care of that problem and dry assembly concluded with no further difficulty.

I finished by running beads of medium CA along each dry joint everywhere except forward of the instrument panel notches. I needed to remove the firewall to fabricate the electric conversion before permanently attaching it. After allowing everything to cure overnight, I had a straight, rigid, albeit heavy, fuselage. I believe this fuse could make a serviceable cricket bat!

The one place I deviated from the instructions was installing the wing mount plates and blocks. Here I used 30-minute epoxy instead of the recommended CA.


Firewall formers laminated per instructions. Note that both surfaces were Woodpecked prior to gluing.


Here the fuselage doublers have been glued and weighted. The right fuse side and doubler bowed in opposite directions and cancelled each other out.


All the formers jigsawed into place between sides, top. and bottom. Assembly compared to plans before glue is applied.


Upper stringers glued on.



The next day, a wing hold down plate and blocked epoxied in.


I couldn't help plopping the tail feathers onto the fuse. Note that the front end is still heavily taped. Next, I need to remove the firewall and do the motor/ESC part of the electric conversion

Last edited by GiantModeler; 06-19-2022 at 02:10 PM. Reason: Another spelling error
Old 06-19-2022, 12:35 PM
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You're moving right along! Looking good Bren.
Old 06-19-2022, 06:53 PM
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Greg,

Thank you very much.

Say, have you moved back to Maryland?

Take care,

Bren
Old 06-21-2022, 10:41 AM
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Default Mounting the Motor and ESC

Before gluing the firewall permanently to the fuselage, I needed to figure out where and how to mount the electric components. Placing the motor over the plan, I noted that there should be a little over 2 inches between the firewall and the motor back plate. Normally, in an electric conversion, I fabricate an aircraft ply box that bolts to the firewall and onto which the motor is fastened. However, with this short an offset, I decided to use standoffs. Rustling around in a “leftovers” bin, I found four 2 ½” long, ½” diameter aluminum tubes.

Now, the downthrust for the Cub is built into the fuse, but the side thrust has to be created by trimming the tubes to different lengths. Consequently, the left standoffs ended up about 1/16” longer than the right producing about 2 degrees of right thrust. Using 10-24 hardware, I made up a mounting “kit,” shown below, for permanent installation after the firewall got glued in.

The ESC fit snugly beneath the motor and will be fastened to the firewall with #4 self-tapping screws. With everything mounted, I test fit the cowl and the motor shaft poked out where it should and everything else tucked neatly inside.

Time to glue the rest of the fuselage together.


First standoff temporarily installed to check motor position against the plans.


Shorter, right standoffs up top; longer, left ones below.


The motor mount "kit."



Motor mounted, firewall vertical. Well, almost: it was off by -0.1 degrees.

There's supposed to be a photo here that I cannot get to upload despite multiple attempts. It would have shown the motor pointing 1.7 degrees upward (right when viewed from the top) resulting in a total of 1.8 degrees of right thrust.



Everything fits!


Motor shaft dead center, nothing else sticks out.

Last edited by GiantModeler; 06-21-2022 at 10:59 AM. Reason: Added pictures
Old 06-24-2022, 07:51 AM
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Default Installing the Firewall

Here I deviated substantially from the instructions. I used 30-minute epoxy instead of the recommended medium CA. Can’t have the motor flying off without the airplane, can we? Also, there’s no way that the front of the fuselage can be held together adequately with rubber bands and masking tape. So, I used clamps to hold everything together while the epoxy cured. Oh, and some tape as well.

Meanwhile, my shopping trip to Lowe’s, Home Depot, and Ace Hardware revealed that no one had 10-24 blind nuts in stock. Now, I had to have something permanently installed on the back of the firewall to receive the bolts in the motor mounting “kit.” I had no idea that tee nuts in the size were so scarce.

Taking the bolts from the “kit,” I poked them through the spacers and firewall holes and threaded a washer and nut on each, snugging them up. I then broke out the JB Weld SteelStik, kneaded up a wad, rolled it into a narrow cylinder, and smushed it around each nut and washer and up against the back of the firewall. 20 minutes later, I had permanently affixed, perfectly aligned nuts into which to screw the mounting bolts.


Here's the firewall epoxied to the fuselage, held in place with clamps and masking tape.


JB Weld SteelStik metal reinforced epoxy putty.


Home-made "blind nuts" fabricated from ordinary nuts and washers packed with SteelStik epoxy putty.

Old 06-25-2022, 01:53 PM
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Default Odds and Ends

Before I could put the fuselage aside and start working on the wing, there were a few more items to take care of. I’ll let the pictures and captions tell the story.

First, the front turtledeck:


Balsa blocks glued onto instrument panel and firewall.


To assure the smoothest possible surface, I edged glued the turtledeck pieces and fastened the assembly to the blocks with wood glue. After wetting, of course.


Here it is after drying . . .


and after the first trim . . .


and after sanding flush with the fuselage side.


A little more carving and sanding produced this.


Next, the side rails:


A relatively simple, but important, procedure. These rails support the rudder and elevator servos.


Then, the tail fairings:


After a little carving and sanding, the tail fairings are glued only to the fin at this time.


Covering tabs:


Another simple, yet important, step. Without these, the Ultracote would have very little to cling to during covering.


Finally, the landing gear:


Here are the landing gear plywood components before installation . . .


and after. I used 30-minute epoxy.


Here, the landing gear wires have been slid into the fuselage and retained by stout nylon straps.


The fuselage stands on the mains for the first time. The cowl and windshield fit acceptably.

Time to start on the wing.
Old 06-26-2022, 01:47 PM
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Default Winging It

Actually, just the opposite. While building the Flitfire wing, I stuck very close to the instruction manual. As a matter of fact, I deviated from the manual in only two instances: truing the ribs and taking some wash-in out.



First, I clamped the punched-out ribs together and trued them to each other with a light sanding.


Here they are ready for installation.


As to be expected in an older kit, the mains spars were curved. I placed them in opposition to each other, hopefully to offset the warps. In this photo, I have filled the area between the spar and TE with the bottom sheeting.


The center ribs have been glued to the main spar and bottom sheeting. The pre-notched TE has been located.


The remaining ribs and top spars have been positioned . . .


and glued, as well as the dowel leading edge. I also gently curved the bottom LE sheet up and glued it to the dowel with CA.


The top LE sheeting edges were unacceptably curved. In this picture you can see about 1/8" of daylight in the center when placed on a flat surface.


I stretched a length of sticky-back sandpaper on the flat surface and moved the balsa sheet back and forth until the daylight went away.


The top leading edge sheeting has been attached. Because this was a greater curvature than the bottom, I wet the wood.


Finally, the wingtip. As designed, with the TE sheeting flat on the board, the wingtip would have had an angle of attack greater than the rest of the wing, creating some wash-in. To compensate, I raised the TE sheeting 3/16".

Next up, the other wing half.
Old 06-27-2022, 09:55 AM
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Default Anyone Need and Extra Goldberg Cub Left Wing?

Just kidding.

Building the right wing flowed smoothly, especially because I managed to avoid the minor, but frustrating, errors I made on the left. It, too, came out straight, strong, and (gulp) heavy.


As with the left wing, the main spars were somewhat curved. I installed them with the warps opposing each other in hopes of them cancelling out.


If gluing the spars to the ribs didn't completely solve the problem, then all these shear webs did.


At this point, I installed the LE sheeting wing tip extensions, both top . . .


and bottom. The instructions called for installation after the wing halves were joined, but I thought there was less risk of hangar rash if I did it while the halves were separated.


And here they are, ready to join.

Last edited by GiantModeler; 06-27-2022 at 09:58 AM. Reason: Correct spelling
Old 06-27-2022, 10:00 AM
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Looking good Bren!
Old 06-27-2022, 10:03 AM
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Greg,

Thank you. And thank you for looking in. I hope to start covering in the next day or two.

Take care,

Bren
Old 06-28-2022, 04:28 AM
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Another day or two and you will be flying it......

Wish I could build them that fast.
Old 06-28-2022, 01:31 PM
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Rick,

Thank you for checking this thread. I appreciate the encouragement.

If you didn't live so far away, I'd invite you for the maiden.

Take care,

Bren
Old 06-28-2022, 04:03 PM
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Default A Time To Join

In my lifetime, I have joined many things: the Army, my wife in marriage, my local RC club. But the thing I dread joining most is wing halves. I’m much more comfortable building aircraft with wings that plug in. It seems to me a lot easier to set incidence and dihedral.

Speaking of dihedral, what is it for a full-scale Cub? According to calculations done from actual measurements, it’s 0.64 degrees for each wing half. Now, what dihedral occurs if one follows the Goldberg directions? Let’s see.


With the kit-supplied dihedral risers in the prescribed position, the upward angle for the right wing half measured 2.4 degrees. In this, and the following two pictures, the left wing half is flat against my level bench.


When I moved the supports to the outermost rib, the angle decreased to 2.0 degrees, still too much.


I removed the Goldberg risers and slid an aileron under the last rib. The result? A 1.2 degree upward angle. Divide that by 2 and you get 0.6 degrees, close enough!


Once I had determined the proper amount to elevate the right wing tip, I immobilized both wing halves with weights and pins and went searching for the plywood center braces. Having found them, I glued them onto their respective spars with 30-minute epoxy instead of the specified CA, clamped them, and walked away.


All the center reinforcements glued in place. Note the carpenter's level clamped to the trailing edge to assure alignment.


And this is what I saw as I walked away to take a nap.


Upon return, I finished sheeting the upper center section and called it a day.


The top center sheeting in progress . . .


and completed.

Old 06-29-2022, 12:59 PM
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Default Aileron Installation

Centrally controlled strip ailerons and I generally do not get along. At least not as well as separate built-up pieces that are individually powered. When fabricating an aileron from bits and pieces, you can better manage its shape and fit with the rest of the wing. And the linkages involved are usually just straight rods between the servo and control surface.

Take the Cub torque rod, for example. It’s threaded on one end and bent at a right angle. After insertion into a nylon tube, the other end is supposed to be bent at an angle that would, in my opinion, produce adverse aileron differential; that is, more down than up. I chose to make the ends orthogonal (perpendicular to each other), creating a 90-degree angle with the bottom of the wing at the servo end when the aileron is neutral. With a straight cross arm on the aileron servo, this should result in zero differential. But I plan to install a vee-shaped arm that will make the rising aileron travel further than the drooping one (I hope).

Let’s get groovy. Strip ailerons like the ones on the Goldberg Cub require grooves in both the back of the wing and two filler pieces into which the linkages nestle. In the past, I have tried, with varying degrees of success, to carve these by (shaky) hand with a ball-shaped burr on a rotary tool. Now, I have a such a tool with a handle that can lay flat against the workbench. So, I shimmed each filler piece, as well as the rear of the wing, and made several passes with the burr and produced straight, perfectly centered grooves.


Cutting a groove in a filler block.


The block finished along with aileron torque rod.


Corresponding groove cut in rear of wing.

Then there’s the ailerons themselves. Miraculously, two of the three provided blanks were straight enough to use. The third got cut into shorter lengths for fillers where slight warps do not matter. Time to bevel and hinge.

Beveling manually was time consuming, but I did enjoy using my new hinge slotter. Line it up with the centerline, rock and push, and you get a slot that snuggly hugs a standard CA hinge. The slotter set (aerobroach.com) comes with two other blades, one each for standard and heavy-duty plastic pre-pinned hinges. I highly recommend it.


Here, the aileron blank has been marked for beveling.


My slotting tool which turns this process from a dreaded chore to almost fun.

To put everything together, I started with the control linkage. After roughing up the nylon tubing, I spot glued it with CA into its groove on the back of the wing. I followed up with epoxy on the facing piece. I managed not to slop much glue into the linkage, so after curing it worked freely.


The control linkage tubes have been spot glued to the back edge of the wing.


Left aileron torque rod installation complete. Filler block epoxied on.

In went the hinges. This helped precisely locate the wing tip fillers which got CAed on as well.


Left aileron hinged and temporarily placed on torque rod. It'll all get permanently put together after covering.


Phew! I am relieved to have completed this phase and I hope not to have to do it again soon.

Last edited by GiantModeler; 06-29-2022 at 01:55 PM.
Old 06-30-2022, 07:15 AM
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Default Mechanical Differential on single servo ailerons

Worth mentioning, another way of creating some mechanical differential for a single servo driving the ailerons would be to cut servo horns as shown in this picture.

In this case - looking from the bottom of the wing on a top wing plane the two remaining horns would be toward the front of the airplane. As the servo travels (in degrees) it would create more push (in distance traveled) therefor causing more up travel in the ailerons than down.

Needless to say, one thing to watch for on these setups is hitting the bulkhead in the fuse that generally sits right behind the wing saddle.




Old 06-30-2022, 07:51 AM
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Rick,

Thank you for the suggestion. This is the type of feedback I had hoped for when I started this thread.

To find out if I took your advice, and if it worked, please read my next reply, which I should post this afternoon.

Take care,

Bren
Old 06-30-2022, 01:51 PM
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Default It Worked!

I prepared a servo horn with two arms forward, as Rick suggested. But I went one step further. Rick's picture shows an included angle of 120 degrees between the arms. I figured the closer the arms, the better. So I cut my horn so that they were perpendicular. (In actuality, my servo did not come with a six-arm horn and I was too lazy to root around for one in my stash.)

Anyway, here's the results.


Note that the torque rod arms are perpendicular to the wing bottom and the servo arms are 90 degrees to each other and face forward.



Here's the maximum downward deflection . . .


and the up. About twice as much up as down. (The wing is upside down.)

Mission accomplished. Thanks Rick.

Oh, and there's no interference with anything in the fuselage.
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Old 07-01-2022, 09:00 AM
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Default Bits and Pieces

I spent most of yesterday taking care of mini projects for the Flitfire. Once they are completed, I can prep the major components for covering and painting.

First, leveling the wing:


Lateral angle measured on stabilizer plate.


Wing lateral angle. I think a difference of 0.1 degree is acceptable. I only lightly sanded the wing seat.

Installing the wing hold-down bolts:



I thought briefly about using 1/4" nylon bolts instead of the steel bolts and blind nuts supplied. However, with the scant width of mounting blocks, I though the larger holes might weaken them unacceptably.

Installing the wing locating dowel:


Using a long 1/4" bit, I drilled through the fuselage and glued in the appropriate length stick.

Tail wheel mount:


A large cut-off wheel made the perfect size slot.


Tail wheel mount in place; to be permanently affixed later.

Fuselage pushrods:


I also considered replacing the kit pushrods with more modern versions. Then I remembered how long my Eagle 63 flew without any control problems. Here, I have bent the aft wires to the shape on the plans.


Here, the tip of the 3/8" square bass rod has been drilled, grooved, and sanded, ready for wire insertion and fastening.


The pushrod end is finished: glued in, wrapped with carpet thread, and glued again.


Both pushrods finished.

With the wing and fuselage wrapped up except for final shaping and sanding, it was time to finish as much as I could on the tail.

Control horn plates:


I don't particularly care for control horns that bolt directly through balsa. After awhile, they tend to loosen as the balsa compresses under load. So, I try to add plywood plates where the control horns fasten and use self-tapping screws that do not protrude through to the other side. Here, I was chiseling a recess at the base of the rudder . . .


into which I epoxied this plywood plate.


I did the same for one elevator half.

Joining the elevator halves:


Using my newly acquired grooving skills, I carved 1/4" diameter recesses into the leading edges of both elevator halves. Once I had epoxied the joiner dowel, I positioned the assembly against the horizontal stabilizer and weighted everything down.

I had hoped to bevel and hinge the tailfeathers, but I ran out of time (and steam).

This project will pause for a few days so that I can prepare for and attend a local flying event.

See you next week, and thanks for looking in.

Last edited by GiantModeler; 07-01-2022 at 09:04 AM.

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