Funbuild 3: Northrop Gamma Cleveland Plans, 48" - RC Groups
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Nov 05, 2009, 05:13 PM
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Funbuild 3: Northrop Gamma Cleveland Plans, 48"

The Northrop Gamma has a colorful history and several interesting variants. Frank Hawks flew the first Gamma made to a new transcontinental speed record in 1933. Three years later , Howard Hughes flew another Gamma to several new transcontinental records. The second Gamma eventually became the first aircraft to fly across Antarctica piloted by Lincoln Ellsworth. That airplane is the only Gamma still known to exist and is part of the Smithsonian’s collection. There was even a military version exported to China and used from 1934 to 1938.

I’ve ordered Cleveland plans in 1/12 scale, which will give a wingspan of 48”. I built a CL version decades ago from plans I drew up from a 3 view.
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Nov 05, 2009, 05:23 PM
It flew once before...
jofrost's Avatar

Thanks for the entry ! I have the little Dumas 30" one that I plan on tackling someday .

Nov 06, 2009, 09:55 AM
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Pat Daily's Avatar
Here is a scratch built Gamma by John Hunton--about 42 inches I think.
Nov 06, 2009, 09:57 AM
Aircraft Part Separator
Greg D's Avatar
Nov 17, 2009, 01:53 PM
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Plans Arrived

The plans finally arrived! The plans depict the first two Gammas made: Frank Hawks’ Texaco Sky Chief and Lincoln Ellsworth’s Polar Star. The Sky Chief has a better looking paint job but I think I’m going with the Polar Star due to its more interesting history. (Read about Ellsworth’s Trans-Antarctic Flight here: )

I got started on the cowling first. It’s turned from pink foam on my lathe. I glued two layers of 2” foam together to make the blank but before gluing I sanded off the hot wire cut faces. I was careful not to get glue near the surface. This allowed me to sand the joint without getting a ridge. I left a shelf to mount this dummy radial:
Nov 17, 2009, 02:36 PM
High Exalted Poohbah
planeman's Avatar
Nice work on the cowl turning. How do you plan to finish the cowl turning to prevent dings and dents in the foam? I've never worked with foam due to this problem.

Nov 17, 2009, 03:21 PM
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Originally Posted by planeman
Nice work on the cowl turning. How do you plan to finish the cowl turning to prevent dings and dents in the foam? I've never worked with foam due to this problem.

First, I brush a coat of thinned epoxy onto the sanded foam. While it's still wet, I warm it with a hair dryer. This causes air in the pores at the surface to expand and bubble up. I brush away the bubbles and repeat the cycle a couple times. This will set up to a hard crust on the foam that has some slight depth to it. I power sand that smooth and apply the first of two layers of 1 1/2oz cloth. I cut large darts near the leading edge of the cloth and wrap it down into the front of the cowl. When its hard, I'll sand down the overlaps and apply a second layer.

This sounds like it would be heavy but the cowl will weigh about as much as a vacuformed ABS copy, but it'll be much stiffer and tougher.

I put the first layer of glass on this morning. I'll post a photo when I add the second layer so you can see what I mean.
Nov 17, 2009, 06:37 PM
It flew once before...
jofrost's Avatar
Off to a nice start...
Nov 17, 2009, 08:46 PM
Two left thumbs
Signing on for the ride!

Nov 18, 2009, 11:14 AM
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More cowl work

Here I’m adding the second layer of 1 1/2oz. cloth to the cowl. After the first layer of cloth was hard, it was sanded smooth. Areas where glass overlapped were block sanded flat.

First, cut the cloth to the required shape. I wrapped a piece of paper around the foam cowl and trimmed it with a scissors to figure out the shape and size. I started at one edge and began to stick the cloth down a little at a time. Every ¾” or so I cut a dart into the cloth and stuck it down, around and inside the front of the cowl partially overlapping the previous dart. Keep the resin to the minimum needed to saturate the cloth and no more. Too much resin allows the glass to "float on top of the resin and gives a bumpy surface. Bubbles were stippled out with the cheap bristle brush I used to apply the resin.

When this resin is hard I’ll put the whole thing someplace warm for a day or two to let the resin really cure. Then I’ll sand it smooth, block sanding the glass overlaps. If I’ve cut into the cloth in a number of places I’ll add one more light coat of resin; otherwise the outside at least, is ready to paint. I’ll have to wait until I get my dummy motor to hollow out the inside. I want to leave some structure to attach the dummy motor to.
Nov 24, 2009, 11:21 PM
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More cowl pics

More work on the cowl.

Once the outside of the cowl was finished the inside could be hollowed. I left a step about an inch from the front edge for the dummy motor to rest on. I coated this area with WBPU with some black dye (TransTint brand) added.

On the back side I left another step to accept to cowl mount. I plan to use magnets on two matching plywood rings; one in the cowl and one on the motor mount. The weight at this point is only 20 g.

I test fit the dummy motor and it was a little too big, I had to cut off the rockers to make it fit. Ellsworth’s Gamma had a large round baffle up front which covered most of the engine so I won’t be adding rocker arms and such. The fit is good. The dummy motor snaps in and out which might be handy if the motor runs warm.
Dec 20, 2009, 04:14 PM
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I started work in earnest on the Gamma. I'm working from Cleveland plans, they are the so called "CD" series which has very high fidelity to scale. The plans seem to be designed to build a very scale static model rather than a well flying model. Mine were enlarged from a smaller master and after checking them I found they were distorted slightly. I scaned in the various components and found they match my three views exactly.

I've decided to redraw and reengineer plans to get accurate part outlines and to simplify the structure somewhat. I'm working in CorelDraw version 9. The first big change is in the fuselage. The Gamma fuselage is basically bullet shaped with a round cross section from nose to tail. The cockpit, wings and huge wing fillets are added to the basic bullet shape.

I mentioned in an earlier post that I built a Gamma many years ago. At that time, I made the fuselage by cutting circular formers and bolting them to threaded rod that ran down the center. I then planked most of the fuselage leaving enough open space to loosen the nuts and remove the threaded rod. It was a control line model so I didn't really need access inside the fuselage on the completed model once the bellcrank and elevator push rod was added.

This time I'll need more access so instead of building the model in right and left halves (with the fillets and turtle deck as part of the formers) as shown in the plans I intend to built and plank the bullet shape fuselage in top and bottom halves and add the turtle deck and fillets separately later. I've reduced the number of formers compared to the plan as well. The idea is to make a light but strong monocoque to attach the flight surfaces to.

I wanted a removable wing but since the battery hatch will weaken the monocoque fuselage I've decided to make the outer wing panels removeable. That way the the center portion of the wing and huge fillets can add the strength and stiffness that the battery hatch would otherwise compromise.

I realize that this much redesign may not be in keeping with the spirit of the contest so I've contacted the moderator and will drop out if he asks me to.
Dec 20, 2009, 04:35 PM
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The Cleveland plans show what I assume is the scale airfoil. The plans show a very thick (17%) center section transitioning to a thin symmetrical airfoil at the tips. I've decided to substitute a NACA 2412 throughout. That way I can use the free Profili software to generate the rib profiles.

I'm making the outer panels removable and I wanted to use some cheap carbon arrow shafts I'd picked up. The first photo shows how I did the shaft end. I glued a plastic 1/4" concrete screw anchor into the end of the shaft and then drilled and tapped the plastic for a 6/32 socket head cap screw. The screw's head is smaller than the shaft diameter. The bonus is I'll be able to tweak the fit latter by adjusting the screw. I clamped the shaft in vise and pulled as hard as I could; theres no way the screw is coming out!

The next photo shows the locking mechanism. By moving the lever the screw head is captured or released. I used .025 brass for the lever because I had some but I'm sure aluminum or even Formica would work. This assembly will be glued to a rib in the center section of the wing along with an aluminum tube to accept the arrow shaft. No pins or screws to lose!
Dec 20, 2009, 05:39 PM
Übung macht den Meister..
Deuce's Avatar
Regardless the outcome of the "ruling," I'll keep watching... it's such a cool plane!

That's one of the unfortunate things about the Cleveland plans, is the distortion possible from the enlarging/reducing process. I have a set of Cleveland plans for the Sperry Messenger at 1/4 scale, and the thickness of the lines is a potential stumbling block. Some prior planning would be required to ensure good fits. Still, Cleveland offers plans for a good deal of lesser-modeled subjects, and I wouldn't let those problems prevent me from purchasing in the future...

With the little latch mechanism, you could hook up a servo to it, and once you had a good head of steam worked up, jettison the outboard wing panels for some real speed!

Dec 20, 2009, 06:00 PM
It flew once before...
jofrost's Avatar
Looks good to me . Do whatever you have to in order to achieve a good flying model . You started with Cleveland plans's not your fault they're crappy and need major mods to achieve your result ( A flyable e-powered plane )


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