P-61 Black Widow 50" - RC Groups
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Oct 26, 2009, 02:16 PM
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Build Log

P-61 Black Widow 50"

This is my first build log and my first (and second) scratch built model after a 35 year break from the hobby. The P-61 has always been one of my favorite aircraft of WWII. It was first purpose built radar equipped night fighter for the US and the heaviest fighter at the time. Its contribution to the war effort was limited by its late entry (and the fact that there were few targets left by that time!)

The P-61 has good proportions for a model; nice broad wings with ample tail surfaces. Itís also a challenging build, especially the center fuselage with its complex shape and lots of glass.

I decided to build a prototype before tackling a more scale model. In the 35 years I was out of the hobby a lot has changed! It was the advent high performance electric power systems that really piqued my interest. Also, I became aware of foam building techniques and bought Keith Sparksí book Building with Foam.

The prototype is semi-profile with a full center fuselage but nacelles made from 1/2" foam board. The foam wing is covered in kraft paper. The prototype flew great right from the start, beginners luck no doubt. It looks and sounds great in flight.

The success of the prototype encouraged me to start work on the scale version.

Prototype specs:
W.S. 50" (3/4" = 1ft.)
AUW 38oz
motors 2410-09
Props 10x6EP
ESC 2x18A
Battery 2200mAH 3s1p
Last edited by pmullen503; Nov 18, 2009 at 04:46 PM.
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Oct 26, 2009, 02:52 PM
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portablevcb's Avatar
Hey, that prototype looks better than some of my planes!

Nice subject too. Love look of it in the air.

Oct 26, 2009, 03:27 PM
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I started with the fuselage. Its made by first hollowing the inside with a router. I lined the cavity with 1/16" balsa to stiffen up the sides. The prototype had landing gear for the first few flights but the fuselage sides started to break up from normal use. The problem was the shape of the fuselage and the expanded foam I used. Once the fuselage is cut for the wing saddle and battery hatch (under the cockpit), the fuselage resembles an open canoe. This shape is not very stiff and flexed easily under normal landing stresses. This time I used extruded foam and will do a few other things to strengthen the fuselage.

After gluing the halves together, I cut the side and top views with a band saw. It took about an hour to sand the fuselage to shape. I gave it a coat of WBPU to stiffen up the surface. I'll give it a final sanding and fill dings with spackle and then cover it with 3/4 oz. glass cloth and WBPU.
Oct 26, 2009, 05:57 PM
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Helihead's Avatar
The P-61 is probably my all time favorite. I have a set of plans for an 049 version that I have been staring at for years ands thinking about doing electric but it seems detined for high wing loading no matter how I figure it.

Your prototype looks great. I don't guess you would consider posting your templates for that would you? I'd love to build one of those.

In either case, I'll watch the build with interest.

Good luck!
Oct 27, 2009, 11:18 AM
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Originally Posted by Helihead
Your prototype looks great. I don't guess you would consider posting your templates for that would you? I'd love to build one of those.
Thanks, I'll post "plans" as they are finished. I really haven't sat down and written detailed plans. I've started with outlines and added bits and pieces as I go. But here's the drawings for the fuselage. It's better and stronger that what I used for the prototype and would work just fine. Adding the balsa sides to the internal cavity really stiffened it up. It also provides a stronger mount for the nose gear.

The cross sections show the simplified belly and the scale cross sections with the belly canons.

Wing plan added. See latter posts to see how I cut the outer panels.
Last edited by pmullen503; Oct 30, 2009 at 04:09 PM. Reason: Added wing plan
Oct 27, 2009, 11:38 AM
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Hereís the fuselage with one layer of ĺ oz. cloth. The blackish marks are spackling compound tinted with TransTint black dye. Its easier to see than white. The black dye works great in WBPU too. The prototype required only a single light coat of black paint because the poly was already tinted. I didnít add it to the poly on the fuselage so I could see the marks Iíd need to make for cuts later. You can also see the first big deviation from scale; Iím ashamed to admit it but I decided to eliminate the belly canon. The shape in this area is somewhat complicated and beyond my carving skills. I tried a couple times on the prototype and it just didnít look right. I still may try to produce a mold separate from the model and vacuform or make a fiberglass belly pan with the canons and graft it on later. The cross sections are shown in the drawing attached to my previous post.

Folks, please feel free to criticize or suggest better ways of doing things. As I mentioned in the thread starter, this is my first scratch built model in 35 years working in a medium I have no experience with. So if I'm doing something obviously stupid..........
Oct 27, 2009, 12:36 PM
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Opening up the fuse.

I got a lot done last night. I cut off the top of the fuse to install the wing mounts and cockpit hatch liners. The fuse at this point is MUCH stiffer than the prototype. The wing mount is 1/32" ply and the cockpit is lined with 1/16" balsa. I liked having the balsa sides to attached to.

I did discover my first mistake. While the battery fits below the bottom of the cockpit floor, the wires will not. This model will have a clear canopy and unlike the prototype, I can't use that space for batteries and wires. I fashioned the tool shown in the photo to cut out more foam. It's 1/32" wire and it fits between the balsa fuse sides. It's set on the wood block to remove an additional 3/4". I heated the wire with a torch and plunged it into the foam and pulled it along. I could cut a couple inches before I had to reheat the wire. Plenty of room now.

I used my table saw to cut a slot for the nose gear mount (1/8" ply). I cut a "T" shape with the leg sized to fit snugly between the balsa sides, slid it in and marked where to cut to match the fuselage outside. The nose gear mount is three 5/32" LG straps and 5/32" brass tube. The nose gear strut will be 1/8" piano wire. The balsa sides give me something besides foam to attach to. I tried to twist the fuse this morning and it's really stiff now.

Thinking about the front and rear canopies it seems like the smartest thing to do would be to form the front canopy and fuse top as one piece. I think I'll have to do the rear canopy in two pieces. Some type of bulkhead will separate the two which means I'll have to notch out the wing TE.

Any opinions or suggestions?
Oct 28, 2009, 02:19 PM
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When I built the prototype, I had to learn how to cut wings with a hot wire. I had a lot of trouble cutting the tapered outer panels using tip and root templates. I had come across a method using a hot wire with a fixed point and movable end with a single template to cut tapered panels. Since I planned to use that method to cut out the nacelles, I figured I try it on the wings. The trouble with the method is how to incorporate washout? After some experimentation, I came up with a way to twist the foam blank in the opposite direction so that after the the wing core is cut it will relax to the proper washout. The photo shows a little block of wood twisting the blank, I actually ended up using a 2 degree wedge I cut with a miter saw.

The wire was 60 lb stainless fishing leader. Theoretically, the pivot point should have been 32.4" from the root template but in practice not only did I increase the pivot point to 36", I had to drop the pivot 1/4" to make the bottom cut to give a wing core with the 1/8" trailing edge I wanted.

Wing has a 1/16" ply spar running the full depth of the wing. I cut the panels in half with the hot wire. You can see in the photo how I cut wiring channels using a couple "C" shaped templates.

The bottom of the spar is flat. The P-61 has 4 degrees dihedral in the center section and 2 degrees at the outer panels. By keeping the bottom flat I'll still have the polyhedral look when viewed from above and it's a lot easier to built a nice straight wing. I glued the (6) 1/2 panels together (2 for the center and 4 for the outer panels) with the spar flat on the work bench before I covered them.
Oct 28, 2009, 02:40 PM
Beware the Axis of Weasel.
thewildweasel's Avatar
Looking good so far. I've just about got a 32" version finished, just need to finish rigging the controls and fitting the turret. Here's a link to my build thread.https://www.rcgroups.com/forums/show....php?t=1119710
Oct 28, 2009, 02:49 PM
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portablevcb's Avatar
From the looks of it you tension the cutting wire by hand? Nice work for a rank "beginner"

Oct 28, 2009, 02:55 PM
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Originally Posted by portablevcb
From the looks of it you tension the cutting wire by hand? Nice work for a rank "beginner"

It doesn't show in the photo but there's a metal ring to hold on to on the movable end of the wire. The cutting board is clamped to my cabinet saw so I can lean back to maintain tension on the wire. Kinda like water skiing.

I find it easier to cut this way compared to using a bow and the panels come out perfect.
Oct 28, 2009, 03:06 PM
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Originally Posted by thewildweasel
Looking good so far. I've just about got a 32" version finished, just need to finish rigging the controls and fitting the turret. Here's a link to my build thread.https://www.rcgroups.com/forums/show....php?t=1119710
Oct 28, 2009, 04:27 PM
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Wing covered

Here's a couple photos of the covered wing. It's covered with kraft paper in three pieces; center and 2 outer panels. The method I use is to first cut out the paper to fit the panel: I wrap around the trailing edge and overlap the LE about 3/4". I wet the paper throughly and blot it on some bath towels. I brushed a light coat of WBPU (with some dye) on the sanded core with a foam brush. I brush a heavier coat on the wet paper and wrap (WBPU toward wing) it around the wing over lapping at the LE. I brush a final coat of WBPU on the outer surface, blot any wet spots with paper towels and let it dry. I did the center panel first, then the outer panels. The paper shrinks a bit as it dries making a smooth strong wing.

The second photo shows the wing cut in half with my table saw. The block placed under the tip set the 3 degree dihedral angle. Notice the leading edge is pink foam. I had planned to use balsa but my LHS didn't have what I needed. The pink foam worked great. So other than the spar there is no wood in the wing. The prototype was built this way except with a 1/8" spar. That wing has proved durable, surviving about 100 belly landings and an impact with a steel post so forceful it snapped both booms off!

When I made the prototype I had MANY rejected wing panels to play with and I was curious to see which covering method was best. I tried 1/16" balsa, kraft paper, lightweight gift wrapping paper, and 3/4 oz. fiberglass. The results surprised me somewhat. The attached document has a graph of the weights of each covering extrapolated for the entire wing.

Balsa (with 3M 77 adhesive) made the strongest panel and easily covered imperfections in the foam core but would add about 7.5 oz to the wing plus whatever finish was required to fill the grain.

Kraft paper with WBPU also made a strong panel and covered all but the worst imperfections in the foam. (It bridged the 1/8" grooves routed into the prototype's wing for wiring). It added only 3.5 oz to the wing AND had a very smooth finish that needed only light sanding and a couple coats for WBPU to be ready for paint.

Gift wrap paper added only 2 oz to the wing but the pebbly texture of the foam was noticeable. The skin dented easily with a fingernail.

Fiberglass cloth was the lightest adding only 1.5 oz. but every imperfection in the foam showed. It also dented easily. Using epoxy instead of WBPU would have stiffened up the surface but it would have been heavier too.

All 4 methods yeilded a wing that I think would be strong enough for a model of this size. It took a lot of force to destroy them .

I picked the kraft paper because it gave the quality of finish I wanted at half the weight (and expense) of balsa.
Last edited by pmullen503; Oct 29, 2009 at 12:34 PM.
Oct 28, 2009, 07:48 PM
The "pro" in procrastination
Steve85's Avatar
Very cool. . My brother built an .049 powered version of this for control-line back when we were teenagers. It was around 30-32" inch wingspan, fully sheeted and flew very nicely. Good luck with your build and keep the pictures coming!

Oct 29, 2009, 12:31 PM
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I started work on the nacelles. The nacelles are round from the firewall to the wing trailing edge then narrow to an oval shape at the rudder. The shape can be broken down to a cone with slices taken off the sides to form the ovoid cross section at the tail.

The cutting jig consists of a board secured to my table saw with a screw driven in to act as a pivot for one end of the hot wire. The foam block is mounted (with double sided tape) to a triangular wood spacer also screwed to the board. The spacer is narrow enough to allow the hot wire to enter and exit the foam block without interference. The wire is steel fishing leader and the power supply is a 12-volt battery charger. One lead is clamped to the moving end and the other clamped along the wire to give the right temperature. The moving end has a ring to hold on to. The template was cut from Formica on my lathe.

I powered up the charger and leaned back to tension the wire. I cut the outside first then affixed the rectangular guide shown to slice the sides of the nacelle to form the oval shape near the tail. You can see in the photo I had to move the pivot point in about 6" to get the right angle. Finally the inner cut was made to complete the nacelle half. I made four identical pieces and glued the pairs together. It only took about 10 minutes to sand both to their final shape. I gave them a quick coat of WBPU before remembering I still had to cut them for the wings. As it turned out, it was not a problem. I made sure I rotated the nacelle before I cut it to accommodate the dihedral AND remembered to make one right and one left nacelle. Iíll have a chance to correct any error in the angle when I glue the rudders on.

The walls are about 1/2" thick tapering to about 1/4" on the sides at the tail. Weight is 32g. each.

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