Build it: Bitty Bomber


Written by Larry Kruse
Build it
As seen in the SPRING 2020 issue of Park Pilot




Specifications:

Model type: Foam park flyer
Skill level: Beginner/intermediate
Wingspan: 25.5 inches
Length: 14.25 inches
Wing area: 90 square inches (nominally)
Weight: 37 grams (without battery)
Wing loading: 2.4 ounces per square foot
Power: Flyzone 7 mm brushed motor and gearbox
Propeller: 4.5-inch propeller
Battery: 1S 140 mAh LiPo
Radio equipment: Tactic 1424 four-channel receiver; Tactic TTX850 transmitter
Flight duration: 5 to 7 minutes, depending on throttle management


Download free plans


Click here for full-size plans (42.5" x 33")
Click here for tiled plans 8.5" x 11"

>>One of the most iconic OT (Old-Timer) model airplanes ever designed is the Lanzo Bomber. It originated in 1938 as one of an incredible string of outstanding airplanes by Chet Lanzo. It’s safe to speculate that more SAM (Society of Antique Modelers; antiquemodeler.org) RC competitions have been won by Lanzo Bombers than any other design.

Although it is somewhat ungainly and almost stubby in appearance, its glide is unsurpassed in all categories and all sizes. The interesting thing about the Lanzo Bomber is that it was rarely mentioned in the modeling press of its day and did not achieve the same degree of popularity that many of Chet’s other designs did with modelers of that time. It could be that the original 8-foot wingspan of the airplane (scaling up or down was not permitted at that time) made it too cumbersome to haul around in the era before station wagons and SUVs.

It was only after the advent and increasing popularity of OT RC-assisted models was promulgated by SAM modelers that the Bomber surfaced as one of the best designs for Texaco and Limited Engine Run events, thanks largely to the rules that permitted the scaling of models up or down to match specific engine displacements.

Now, approximately 82 years since its origin, the Lanzo Bomber ranks as one of the outstanding model designs of that era—and a terrific choice for today’s sport modelers who want an airplane that is a good-gliding Sunday flyer, irrespective of its size.

I recently acquired some micro RC components from a fellow club mate, and it occurred to me that a foam, micro, “bitty” Lanzo Bomber might be just the airframe that these components needed. It didn’t take long to gather some Depron foam scraps and head to the drawing board.

Essentially, any micro control and power elements or their equivalents can be used to get the airplane in the air. I had on hand a Tactic 1424 four-channel receiver/servo/ESC combination board, a GPFLZA6318 7 mm brushed motor with a 6.25:1 gearbox, a 140 mAh single-cell LiPo battery, and a 4.5-inch propeller. Any of the smaller Horizon Hobby (horizonhobby.com) or Banggood (banggood.com) components that are currently available, or “harvested” components from out-of-service micro airplanes, could be used. Because it’s not a 3D airplane, mild to moderate power is sufficient.

I’m aware that the gold standard of sheet foam, Depron, is hard to find in the US; however, Great Planes Pro-Formance foam (greatplanes.com) can be found. Micron Wings (micronwings.com) in Queensland, Australia, is a reliable source of Depron if your stash has run out. I’ve been using the Micron Wings Small Size Depron Sheets packets for some time, and the shipping won’t break the bank. Delivery is usually within 10 days to two weeks.

The Bitty Bomber uses 1.5 mm to 2 mm Depron for the flying surfaces and 6 mm Depron for the fuselage pieces. Additionally, you will need a 12-inch length of .030 music wire for the landing gear, two 12-inch pieces of .020 music wire for the elevator and rudder pushrods, and a scrap of 1/32-inch plywood for the landing gear mounting plate.

I used a pair of discarded, lightweight plastic wheels that probably came from an RTF FF (Free Flight) model to complete the undercarriage. Du-Bro (dubro.com) micro control horns were used for the elevator and rudder. Trimmed-down Flyzone micro horns were used as pushrod standoffs.



The landing gear is bent from .030 music wire and sewn and glued with CA adhesive to the 1/32-inch plywood plate, as shown.


The airplane is constructed from 6 mm Depron foam for the fuselage and 1.5 mm to 2.0 mm Depron for the flying surfaces. Full-size templates included with the plans provide guidance.


The wing’s center section and the wingtips have scrap doublers glued to the LEs to reduce flying and hangar rash. Note the undercamber in the surfaces, which should be bent in before the doublers are glued in place.


General construction notes: The accompanying drawings are templates for the full-size parts. For what few basic pieces and parts are required, the easiest method is to attach the plans to lightweight poster board with a glue stick or rubber cement and cut the parts out with scissors. They can then be used as patterns to trace onto the specified foam thicknesses using a light touch and a #2 sharp pencil. Although the lines would be more distinct, do not use a ballpoint pen to trace the parts. It leaves too much residue that shows up later as dark blotches and is hard to remove.

I’ve found that a self-healing cutting mat from a craft store works well as a base on which to cut foam. I use a #11 scalpel blade as the cutting tool, knowing that as work progresses, I will need to re-sharpen the point on it several times to keep it from tearing the foam on the bottom side of a cut. An X-Acto knife with a #11 blade would work just as well I suspect, as long as the comment about resharpening it is heeded as you go along.

After the parts are cut out from foam sheet that is the appropriate thickness, gently true up any raggedness on the edges using fine sandpaper that is wrapped around a block or attached to a T-bar sander.

Most of the airplane is built using bSi (Bob Smith Industries; bsi-inc.com) Super Gold foam-safe CA, unless otherwise noted. I also used hot glue to mount the motor and control units and 5-minute epoxy for the landing gear plate/fuselage crutch joint and the wing/pylon joint.

The wing panels require undercamber that is accomplished by rolling the bottom surfaces over a mandrel, such as this 3/4-inch section of PVC pipe. Specific instructions are in the text.


Assembling the fuselage: After the 6 mm fuselage crutch (so named because of its shape) is cut out, draw a centerline the length of the piece, both top and bottom. After the fuselage top and bottom vertical pieces are cut out, test-fit them to the centerline of the fuselage crutch to ensure that they will be at right angles to that horizontal piece, and that a cruciform shape in the front or rear profile will be maintained.

Note that the motor thrustline is set in place (both down and right thrust) by the notches in the fuselage crutch and the bottom fuselage piece. You might have to adjust the width and length for your motor choice. Check to make sure that those notches are accurately cut and that they point to the right and down—not to the left and up!

Notch out the relief cutout for the plywood landing gear plate in the bottom fuselage piece. If you want to install your control board as I did, you will need to cut a larger clearance hole in the bottom fuselage piece in the area noted on the plans. Similarly, you also might want to cut a hole in the crutch to accommodate the control unit.

Because my control board did not need the full 6 mm thickness of the crutch to clear all of its components, I carefully cut a rectangular hole in the crutch. I then removed the piece but sliced off approximately 3/32 inches of its thickness and glued that piece right back into the crutch, flush with its top surface. That made a neat pocket for the control board to be only visible from the bottom.

You can now cut out the 1/32-inch plywood landing gear plate and bend the landing gear to the shape shown on the plans. It’s easiest to bend the V-shape of the gear then the axles for the wheels (held on by a drop of Duco cement), and finally, the 45° bend approximately 1/2 inch down from the apex of the V-bend.

Place the gear on the plate and trace around the outline of the wire. This will allow you to drill small holes along both sides of the wire to sew the gear to the plate using common thread. Instead of using a sewing needle, I simply spread a bead of CA adhesive over one end of the sewing thread. When it was cured, I clipped it to approximately 1 inch in length. That made for a nice “thread-needle” that could be pushed through the holes. After the gear wire is securely sewn in place, coat it with regular CA and let it cure.

In sequence, epoxy the landing gear plate to the bottom of the fuselage crutch, use hot glue to fasten the control board in its place, then glue the bottom fuselage to the bottom centerline after tapering the back end to a 2 mm thickness by sanding it to match the rudder thickness. After everything is cured and dry, you can add the top fuselage piece, again tapering the rear portion to fair smoothly into the rudder.

When the fuselage cruciform is complete, trial-fit the motor and gearbox into the slots in the foam, making sure to maintain the built-in down-thrust and right-thrust angles.

After you have confirmed the fit, hot glue the unit in place and hold it until it cools and cures. I found that it was easiest to set the hot-glue gun to its highest setting, squeeze out a puddle of glue, then use a toothpick to apply the glue to keep it out of the motor and gear mechanism.


These photos show how the control board, landing gear, and motor assembly were mounted. The 1/32-inch plywood gear plate was epoxied into position on the bottom of the fuselage crutch, while the control board and motor assembly were set in place using spots of hot glue applied with a toothpick. Note the required weights that were epoxied in a forward position ahead of the landing gear plate. The loop portion of the hook-and-loop battery mount is hot glued to the left bottom side of the fuselage.


Wing construction: With the fuselage essentially complete, the wing is next. For those who might not be familiar with the characteristics of thin Depron sheet, some basic information might be useful.

Depron has a grain that can be seen by holding it up to the light. The grain usually runs lengthwise on a stock rectangular sheet. The foam also has a shiny side and a dull side. For purposes of bending and molding, the shiny side should always be placed downward.

Because the Bitty Bomber’s wing is undercambered, it will be necessary to create a curved surface to make it more amenable to following the curves of the wing ribs as they are glued in place. It will also be necessary to remove all rings from your fingers before you begin the process of setting the camber, to avoid denting the material on the top side!

To get the required undercamber, a 12 x 3/4-inch outside diameter piece of PVC pipe or a similar form can be used as a mandrel. All you need to do is place the foam blank, shiny side down, over the mandrel and roll the foam back and forth, using your palms and the flat of your hands for pressure against the mandrel. It won’t take much to induce the required curved surface.

Curve the main center section and the two wingtips identically then glue the LE (leading edge) doublers to all three surfaces when you are satisfied with them. The two thinner dihedral ribs can be glued to the center panel roots, and the two 6 mm dihedral ribs can be glued to the wingtip roots with approximately 1 mm of each rib protruding to assist in the dihedral sanding process.

Take care that all three assemblies are constructed flat and square, with no twists built into them. Any twists or warps will give you the opportunity to cut some replacement pieces and try again!

The 3-inch wingtip dihedral is accomplished by propping each respective wingtip up to its required height and sanding the 6 mm dihedral ribs to a 90° angle, much like a Hand-Launch Glider. Because there’s not much material to work with, sand slowly and carefully, checking frequently for fit.

When both wingtips fit satisfactorily with no gaps in the dihedral joints, hold the center section flat with scraps of balsa pinned over the LE and trailing edge to the work board to clamp the piece down. Install each wingtip individually by propping it up to its required 3 inches. Glue it in place with foam-safe CA. Work slowly and carefully to avoid inducing any warps or uneven surface joints. When the unit is cured, sand the LE doublers to a rounded surface, tapering the wingtip doublers from full depth at the root to nothing at the tips.

The wing dihedral joint is comprised of a 1.5 mm to 2.0 mm rib mounted to the center section root rib, and a 6 mm rib mounted to each wingtip. The wingtips are then sanded to a shape that is similar to a Hand-Launch Glider wing to arrive at the correct angle for the 3-inch dihedral in each wingtip.


Tail surfaces: The stabilizer/elevator and the fin/rudder can individually be cut as one piece then separated at their hinge points. It will be necessary to join the elevator pieces at the rear with a small dowel then notch out the rudder to clear the dowel. I sanded a barbecue skewer to the required thickness and epoxied it in place. Both the elevator and the rudder should be sanded to a chisel shape at the hinge line to allow for adequate control surface movement.

One 1/2-inch piece of Du-Bro Electric Flyer Hinge Tape was applied to the top of the stabilizer/elevator joint and the flat side of the fin/rudder and it worked well to hinge the surfaces.

Final assembly: Final assembly consisted of gluing the wing, elevator/stabilizer, and the rudder to the fuselage. Probably the easiest method is to first glue the tail sections to the fuselage crutch while constantly checking to see that they are at right angles to each other and not skewed in any direction.

The wing is next, and because I wanted to have a little bit of a fudge factor to ensure it was square with everything else, I marked the center of the main panel, used a smear of 5-minute epoxy over the top of the pylon, and put the wing in place. As it cured, I was able to make minute adjustments in its angle and position for a perfectly square fit in all dimensions.

After it cured (as one of the photos shows), I used two 6 mm ribs rounded on one side as the braces for the wing/pylon joint and placed them carefully in position, one at a time on each side, trying not to tilt the wing in the process. After one was dry, I installed the other.

If you want to add any color to your Bitty Bomber, this would be the time to do it. I used—and would recommend—Design Master Colortool Spray from Michaels craft stores (michaels.com). It is a fine spray that is easy to control in terms of the depth of color. It dries fast and is safe for all foam applications.

The Bitty Bomber has just one light spray coat on the wingtip panels, the aft part of the fuselage, and the tail surfaces. The weight penalty was just half a gram. The vinyl lettering was custom cut by my club mate, Dan Nicar, especially for the little airplane.

All that’s left now is to hook up the control surfaces, but you will want to fire up your radio to find out which servo is attached to what control stick before you drill any control horn holes in the tail surfaces—particularly if you have the receiver board and servos mounted inverted. Having the transmitter’s rudder stick attached to the elevator servo is not what you want to do. Don’t ask me how I know.

It should be noted that practically all FF model aircraft from the late 1930s had short noses because the ignition batteries, motor, tank, and timer sat far forward to balance the airplane. The Bomber is no exception, so don’t cringe when you have to add more weight than you think you should to the nose.

The Bitty Bomber required 9 grams of nose weight fastened to the underside of the fuselage crutch to balance at the indicated center of gravity, inclusive of the battery and the hook-and-loop battery strap. Without the flight battery, the airplane weighed an acceptable 37 grams, given its 251/2-inch projected wing area.

Flying the Bitty Bomber: Subscribing to the adage, “It’s not an airplane until it flies,” I waited for a fall morning with acceptable temperatures and a suitable amount of wind. When the airplane launched on its maiden flight at my local field, I flew it from the area where gliders are typically flown—out and away from the runway.

I needed to add just two clicks of down-trim after a straightaway hand launch. Two club mates who were flying powered aircraft from the runway area took it to be my big, electric Lanzo Bomber that I usually fly. They were amazed to see how small it was when it landed.

The best way to sum up its flying characteristics is to state that it flies like a bomber. It has a nice, gentle climb to altitude, requires slow, smooth turns to place it where you want it, and has a cruising flight that will easily last more than 7 minutes with some battery left.

Landing consists of chopping the power any time you are ready to land and watching the propeller gently turn over as it glides lightly down to land in front of you with a final flare. In all, the Bitty Bomber turned out to be a delightful spur-of-the-moment project that I hope you will enjoy as well.

The 3/4 views of the Bitty Bomber show its diminutive size, simple construction, and the OT lines of Chet Lanzo’s original Bomber from 1938.






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5 comments

i like the article is great i made the plane into a rubber band powered one.the only problem is that you didint say the wing dihedral angle.

Check the text and the plans. The wing requires you to prop up each tip 3" measured at the outermost point of the tip.

The download link supplies a pencil version of the plans without the scale square. Where can I download the inked (final) version shown in the article?

Does it fly as good as it looks?

As a matter of fact it does! Be careful of flying it when warm air and thermal conditions are present. I've come very close to losing it on two occasions!

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