Northrop XP-56 (oz545)

About this Plan

Northrop XP-56 Black Bullet. Scale model experimental flying wing pusher, designed around an electric powered contra-rotating prop drive unit.

Quote: "Northrop XP-56 'Black Bullet', by Bill Young. A project only for experienced modelers, this one was designed around an electric powered contra-rotating prop drive unit that can also be modified from pusher to tractor. A real challenge!

The US military was concerned about the advances in fighter aircraft in Germany and Japan. To encourage the US aircraft industry to experiment, the Army Air Corps opened informal competition in late 1939. Three of the proposals were selected to be built. These were the XP-54 Swoose Goose of Vultee, The XP-55 Ascender of Curtiss, and the XP-56 Black Bullet of Northrop.

Two of the XP-56's were built. The first to fly had a short upper fin and flew in September 1943. Stability problems led to the extension of the upper fin. This first airplane crashed in a landing accident. The second XP-56 first flew in March 1944, with the enlarged upper fin and some changes in the control system. The airplane was painted olive drab on top and neutral gray on the bottom. The fin tip and the numbers were painted orange-yellow. The Northrop logo was orange and black. This airplane was given to the Smithsonian and is now back at Northrop being restored.

This model is unusual in many ways and requires special consideration before building. If you have no experience with flying wings or scale, I would suggest some simpler flying wing to practice on. I built several slope gliders of the airplane to test my skills and check the basic layout. The contra-rotating propeller unit requires careful construction, or the substitution of a single-rotation unit.

I chose a fixed gear for the model at this stage of its development, as the geometry of the main gear is at least as complex as that of a Wildcat, and besides, the original XP-56 spent a lot of time flying with gear down due to problems with the gear retraction, and the difference in aerodynamics with the gear in the up and down positions. Yes, the gear could be beefed up for an IC engine. And no, don't be tempted if you plan to fly electric. You can probably tell by my selection of subjects that I prefer the unusual. This one is a prize in that nothing about it is conventional.

COUNTER-ROTATING PROPELLERS: There are many interesting scale subjects that use counter-rotating propellers. I, like many others, have shied away from these subjects because of the gearing complexity. However, I kept thinking about it and woke up one night with this relatively simple solution. There are many successful gear and belt drives on the market today for gas and electrics, Guess what? A belt drive gives same rotation and a gear drive gives opposite rotation! So each propeller is driven by a simple one-step gear or belt drive. You may gear down or not, as the case may be. One final problem is easily handled with this system. The rear propeller must run at a higher RPM or have a coarser pitch as it is running in air that has already been accelerated. This is easily solved as the belt drive can have any center distance that you may desire by using different pulley sizes and/or different belt lengths.

The photos and drawings show a unit that I built for an Astro 15 Cobalt to run in the 1-1/4 in = 1 ft model of the Northrop XP-56 Black Bulllet. The idea can be adapted to other subjects easily. While the author used a lathe for some of the parts, others were built up from tubing and collars pinned and soldered together. I think that all of the parts can be assembled the same way with a little ingenuity. Naturally, heavier gears and belts would be necessary for a gas engine. A copy of the Stock Drive Catalog can be very helpful and is a great gadget source for belts, gears, bear-ings, shafts..."

Update 24/09/2012: Replaced this plan with a better, clearer version thanks to theshadow.

Update 30/07/2013: Added supplemental drawing of drive gear, also article text pages, thanks to Pilgrim.

Supplementary file notes

Article pages, text and pics. Drive gear drawing.

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