Dumas Northrop Alpha R/C conversion build: intro - RC Groups
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Nov 08, 2003, 06:25 AM
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mu2freighter's Avatar

Dumas Northrop Alpha R/C conversion build


After reading about all of the successful Dumas 30" free flight kit conversions here I decided to do one as my first venture into mini electric IPS powered R/C...especially since Mike Molt's Hellcat is STILL UNAVAILABLE at the moment (harrumph...cough cough). I chose their kit #307, the Northrop Alpha.

Golden Age aircraft are high on my favorites list, but this plane particularly intrigues me, since it was a single piston engine, all-weather open cockpit (the poor pilot), taildragger airliner...with deicing equipment on the flight surfaces! After being displaced by the Boeing 247 and DC-3, the plane was used as a freight hauler, a subject near and dear to me, since that's just what I do for a living. Dumas' kit represents NC11Y, which currently hangs in the front hall of the National Air and Space Museum.

For the conversion, I elected to build the plane for three channel R/C, rudder, elevator, and throttle. I decided to omit the wing fillet for simplicity and weight; while the lack is evident, it's not a major feature of one's overall impression of the plane. Even the guy who built the model for the kit's box art didn't use it, which I didn't even notice till construction was well underway. I hate wing fillets, anyway...heaven help me if I decide to do Dumas' Aichi Val...

The most eye catching features of this model to my eye are the bold, Art-Deco style wheel spats, and the black rubber deice boots, which are a striking contrast to the otherwise all-aluminum finish. These were easily and lightly modeled.

I must say, I had the oddest sensation of 'doubling' in my technique as I worked on this aircraft; I've built R/C before, and I've built stick-and-tissue FF before, but this is the first time I basically did both at the same time! Not an unpleasant sensation actually

After building, my overall impression is that this is a really nice sport scale kit...but it's clearly a kit, and has a bit of that 'prefab' feel to it, for lack of a better term. Not at all a bad thing, I just think I'd like to try one soon from rubber FF plans such as modified Cleveland or similar. Somehow I feel the old creativity really comes out when building from plans.

This kit took just under a month to complete, and I found it to be an easy and fun build thanks to the laser cut parts and high quality of the wood. I thought folks interested in doing one of these might like an impression of how it went, so here are my construction notes. I decided to post these prior to actually flying the plane; somehow I suspect I'd lack the motivation if I manage to destroy it on the maiden flight...LOL
Last edited by mu2freighter; Nov 08, 2003 at 07:17 AM.
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Nov 08, 2003, 06:27 AM
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Dumas Northrop Alpha R/C conversion build: fuselage

The fuselage was built more or less normally except for substituting 1/16" square bass for the stringers, and adding some 1/16" sheet balsa around the rudder and elevator for pushrod exits. The soft balsa tail block is tack glued in for shaping and sanding, then removed for covering and stab installation. I just cut away the formers around the elevator slot so I'd be able to slide it in in one piece, tack gluing in a small chunk of 1/8" balsa in the end of the slot after removing the tail block till after covering to ensure nothing gets warped.

The cowl is removable, held in with 1/8" disc retaining magnets and keyed in place with 1/8" dowel. I went ahead and used the provided sheet plastic to skin the cowl, since the airplane has a pretty short nose and I felt it could use a little weight there. After playing around with some ideas on paper I rigged what looks to me like a strong and lightweight firewall for easy motor accessibility. It feels well secured, and provides a couple of degrees down- and right thrust as well as airflow for cooling. The spaces between cylinders have been opened up, and with the open cockpit should ensure adequate circulation. The IPS motor is secured with 3 #0 X 3/8" wood screws.

Fuse weight with cowl: 32 grams


I followed the parts locator sheet to the letter; but side former parts 9A are marked BACKWARDS relative to the actual wood parts sheet. I guess whoever set up the laser cutting managed to reverse the 9A parts...good thing I mostly build with aliphatic resin. I'd suggest laying them out over the plan to verify the correct orientation even if you do mark them beforehand as per the locator sheet. I've contacted Dumas to let them know about the error in the kit.

Later, I noticed that the wire main gear legs are also mislabeled in their diagram...refer to the plan in orienting them, lest you end up with them being reversed L&R.
Nov 08, 2003, 06:29 AM
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Dumas Northrop Alpha R/C conversion build: wing

This is a 3 channel model, and as I wasn't planning to add another spar for ailerons I wanted the wing light but strong. I substituted bass for the trailing edge, the 1/16" cap strips under each wing rib, and the 1/16" auxiliary spars.

The main spar and tips seem strong enough with the bass spars added so they were built as per the plan though the main spar was strengthened by thin CA. I wanted a strong leading edge without a large weight penalty, so after cutting the soft balsa 1/8" X 3/8" pieces I reinforced each with a strip of carbon fiber CA'd in place. The mod is visible in the attached picture. I also attached small CF straps to the rear landing gear leg supports after I took the photo (visible later in the framed up pic).

The only other modification was to increase the dihedral of each wing panel from the recommended 1 1/4" to 1 1/2" as this yields just about 8 degrees dihedral. I figured this should be adequate for a low wing model with a flat center section using rudder for turns. 1/4" washout each tip was added after covering.

Wing weight, with wire main gear legs: 21 grams.
Nov 08, 2003, 06:32 AM
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Dumas Northrop Alpha R/C conversion build: tail group

The horizontal stabilizer is designed for the stab/elevator to plug in in halves. Can't do that in the R/C conversion; the elevators are tied together with a 1/8" dowel and covered and hinged to the modified stab to make one unit, this just slides into the slot at the rear of the fuse.

I used 1/8" square bass for the leading edges, 1/8" by 1/4" balsa for the stab trailing edge/elevator leading edge, and the one 1/8" square stick of nice hard C-grain balsa in the whole kit for the elevator trailing edges. I attached the points of two toothpicks to the bottom of the rudder to aid in locating it and strengthening the joint a bit, with corresponding holes drilled in the fuse top.

For control I used simple homemade 1/32" thick plywood horns extending out about 5/16" for the rudder and 1/4" for the elevator. These took about 15 minutes to make both, from initial drawing to filing to shape. The control surfaces are slotted to accept thin strips of ordinary CA type hinge material. My only worry is that the rudder's a bit on the small side...but each time I think this I look at how small the REST of the plane is!

Weight of tailfeathers with hinge material: 8 grams
Nov 08, 2003, 06:34 AM
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Dumas Northrop Alpha R/C conversion build: assembly

The wing's mounted with a dowel through the leading edge into a balsa block, and like Mike Molt and others have done, I used an 8-32 nylon bolt in a threaded brass insert mounted in a block attached to the fuse to secure it. I took the pic right before I added the wing hardware, but with everything stuck together it started looking unsettlingly like an airplane, so I couldn't resist.

To check my progress, I piled everything on the scale; AUW with wing hardware and wheels added, electronics, motor, and Kokam 340 2S lipoly battery was 5.4 ounces at this point. Encouraging, thinks I. With covering, wheel pants, and the extra wire bits and tailwheel I'm estimating just over 6 ounces.
Nov 08, 2003, 06:43 AM
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Dumas Northrop Alpha R/C conversion build: covering and finishing

Okay, those of you who have written here about the ease of covering with Solite/Nelson Litefilm are guilty of massive understatement! This stuff is simply amazing. I was expecting good results, but I'm still shocked...this covering is easily the most versatile, best-behaved film I've ever worked with, bar none. The only tool used was a Hobbico sealing iron.

After seeing how well it covered the tailfeathers, for example, I attempted to cover the round, tapered fuselage with one piece of film. Almost worked, too; the only hitch was Solite's well documented habit of sticking to itself when using large pieces. The long sheet required ended up bunching up and terminally sticking to itself. No big deal, I ended up covering it with just 3 pieces. Wow. Wouldn't matter if you used 20 pieces, this stuff goes over itself so well. You have to squint and move the plane around in the light to (barely) see the couple of patches added after the inevitable minor post-covering mishaps.

Remember how Monokote used to sometimes bunch up in corners and get little, hard wrinkles that simply would not come out? I got a few of those on the wing with Solite, very minor ones. I thought I'd have to live with them...until I turned up the heat a notch (to about 250) and they came right out!

I liked the results so well, I even used Solite to cover the stiff paper cockpit cover, pilot's headrest fairing, and wheel spats. Went on ridiculously easily, it's stronger than paint, and obviously a perfect color match. Heck, next time I may even cover my wooden control horns with it...in a word, Solite ROCKS!

I have never used press on Mylar markings before, and must say that I'm pleased with the results I obtained. For looks and to save weight, all of the larger numbers were cut out and applied individually. I trimmed the smaller ones as closely as I could in one piece, and I think they look acceptable. It's actually hard to see the clear film on the larger passenger door outline, which is the piece I was most worried about. I was happy to find that any small wrinkles and bubbles smoothed out with a couple of pinholes and passes with the iron.

The deicer boots are black trim Monokote, traced, cut out and applied. This is the only part of the covering I'm not entirely happy with, as there are a few tiny wrinkles that wouldn't shrink out. I'll deal; they're not very obvious. Next time I'll use Solite!

The cowl is airbrushed with water-based Polly Scale Aluminum with some Oily Black for the engine cylinders. The pushrods are 1/16" aluminum tubing. I also airbrushed the GWS prop with Aluminum, as bright orange just wasn't going to cut it. If all goes well and I stay with this prop/motor, I may paint the prop tips blue, white and red to match the one at NASM. The red Guillow's tailwheel provided was colored black with a large permanent marker, and I used an ultra fine Sharpie for the aileron lines.
Last edited by mu2freighter; Nov 08, 2003 at 07:11 AM.
Nov 08, 2003, 06:49 AM
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Dumas Northrop Alpha R/C conversion build: equipment installation (A)

R/C equipment installation was straightforward. The servos are mounted with foam tape to balsa plates glued to the frame, and I used .05" carbon fiber pushrods with wire ends to hook them up, as per the neat method detailed on Air Dynamics' how-to page here . Simple, elegant, reliable, and lightweight. I like it.
Nov 08, 2003, 06:50 AM
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Dumas Northrop Alpha R/C conversion build: (B)

The Pico receiver is foam taped to one of the balsa servo mounting plates, and the Kokam Li-Poly 2s pack is Velcroed to the lower motor mount plate. With all this done, the AUW is 6.4 ounces, including the 1/12 scale Williams Bros. pilot. I'm pleased, I think this weight will provide good performance with some reserve in the likely event that I want to use higher capacity batteries. The ship balances at just about exactly the CG position shown on the plans; any fine tuning will be little chunks of A Line lead weights, if necessary, taped to the battery. Control throws are set to 1/2" for the rudder and 3/8" for the elevator. Lastly, I added 1/4" washout to each wing tip by blocking up the tip's trailing edge and reshrinking the covering. Did I mention that Solite ROCKS?!? LOL
Nov 08, 2003, 06:53 AM
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Dumas Northrop Alpha R/C conversion build: the plane's done

Now comes the really hard part...taxi tests followed by (or maybe including) the maiden flight!


AUW= 6.4 oz.
GWS Pico receiver
E-Products Jim McPherson's Litenna SF
2X GWS Pico servos
GWS IPS-DX-S2 (3.5:1 gearing) with GWS 8 X 4.8 prop
Castle Creations Pixie 7P ESC
2s Kokam 340 (for now) Li-Poly battery
Last edited by mu2freighter; Nov 08, 2003 at 07:06 AM.
Nov 08, 2003, 06:56 AM
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Dumas Northrop Alpha R/C conversion build: another view

The underside; I didn't mess with trying to recess the wing bolt so as not to weaken the wing. Not too awfully noticeable...
Nov 08, 2003, 07:05 AM
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Dumas Northrop Alpha R/C conversion build: final preflight

Thanks to these fine vendors whose quick service and great items helped me complete this aircraft:

Penn Valley Hobby Center: Dumas kit

Hobby Lobby : Lithium polymer batteries & Kokam LIPO-402 charger

Todd's Models: GWS Pico receiver, DX-A motor & props (I'll be using this on my next project...)

Bill Birmingham at Park Flyer Motors: Solite covering, McPherson Slowflyer Litenna

Aeromicro: IPS-DX-S2 Motor & props

Air Dynamics: Carbon fiber pushrods, misc. small aircraft hardware

Forcefield: 1/8" rare earth disc magnets for securing cowl

Happy trails!

Last edited by mu2freighter; Nov 08, 2003 at 07:12 AM.
Nov 08, 2003, 08:58 AM
Departing at 'O' dark 30
Cass's Avatar

hows your hearing, flying that noisey MU2?

Good luck with the maiden. i guess i better get off my butt and start building my StevensAero RV-4, only problem is i have to wait 4 months to maiden it with winter setting in
Nov 08, 2003, 11:14 AM
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Gonzo007's Avatar
Really Nice

Nov 08, 2003, 02:28 PM
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Martin Hunter's Avatar
That's beautiful, mu2! I do fear, however, for the controllability of your plane. With so little dihedral and being low wing to boot, I doubt that the rudder will provide decent roll coupling. Even the Molt Models Hellcat, with it's much larger amount of dihedral, is nearly uncontrollable using the rudder only. I've tried this on mine out of curiousity. I hope she works out for you, though. Keep us posted.

Nov 08, 2003, 06:07 PM
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mu2freighter's Avatar
Thanks for the feedback, folks!

Martin, I quite agree about controllibility with a configuration like this.

Call me a snob, but I don't think of a model without full 3 axis control as primarily an R/C airplane...rather, I view at as free flight with R/C assist.

I want an airplane to fly in a scale manner; in the Northrop's case, ROG capability, decent level flight and wide turns are all I'd ask for. I am fortunate enough to have a large open field near me for e flight/sailplane use, so as long as I can steer it in the general direction desired to fly a traffic pattern I'm a happy camper.

This airplane is designed to be self-righting enough to fly freely, hopefully the few degrees of dihedral I added will help. Other than a slightly increased wingloading this plane is in FF configuration. If the designers at Dumas are earning their pay and putting out models that fly as intended, all should be well

Besides, your own rudder/elevator Herr Ryan was one of the first FF conversion threads I read here. Looked like it had a similar layout to the Alpha, albeit with a little more dihedral.

This build was a great way to get my sea legs back for building after more than 4 years out of the hobby, plus it's a test bed for this new (to me) micro R/C & Li-Poly stuff. I enjoy building and photographing my planes a lot, and if it flies reasonably well I'll be pleased.

What the heck, if it flies poorly or I squish it I'll just stuff the gear in something else. Speaking of which, I do have a Sterling/Estes Sukhoi enroute to me as we speak...love that eBay...

thanks for the input!

Last edited by mu2freighter; Nov 08, 2003 at 06:14 PM.

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