The XB-42 Propeller System:
Independent control of coaxial propellers — How did they do that?

by Tom Fey
Memorial Day, 2011



The Douglas XB-42 was a sleek experimental bomber powered by two Allison V-1710 12-cylinder (initially V-1710-103E (E23)) engines that drove contra-rotating pusher propellers in the tail of the aircraft. The design was an attempt to improve upon the speed and maintain the range of a B-17, but in a more efficient package using two engines and a crew of three instead of four engines and a crew of ten. Two prototypes were built and flown, with one additional static test airframe. Eventually, two wing-mounted jet engines were added, resulting in the XB-42A, which survives to this day. Somebody needs to write a book on the XB-42 (two piston engines turning), XB-42A (two turning, two burning) XB-43 (America’s first jet bomber; survives intact) as they are not well known and completely fascinating in design and execution.


The engines were buried side-by-side behind the cockpit in a stainless steel tub, canted inward towards the tail and inclined 20°, and drove the tail-mounted, modified V-3420-B reduction gearbox via dual, 6-piece, 6-intermediate bearing, 29-foot long drive shafts. Allison truly was the master of complex shafting as this assembly, essentially P-39 components, gave very little trouble during the life of the aircraft. Allison’s expertise in this field continues to this day with the V-22 and F-35.


The XB-42 gearbox had a reduction ratio of 2.77 to 1, which was unique to the XB-42. Amazingly, two gearboxes survive; one resides in the NASM XB-42A (now at Wright-Patterson Air Force Museum for restoration) and one in storage at the California Aerospace Museum in Sacramento, California. The gearbox weighed approximately 390 lb and had its own oil cooling system. The weight of the propellers, gearbox, drive shafting, and cruciform tail, as well landing gear that pivoted rearward into the empennage so far behind the wing required the engines to be mounted ahead of the CG to provide proper balance to the aircraft. The gearbox oil cooler ducting is on the port side of the aft fuselage, the intake geometrically camouflaged by aft edge of the star and the exhaust shutter at the aft edge of the bar.


One of the many unique features of the XB-42 was that each engine drove one of the propellers, with the port engine (rotating counterclockwise) driving the inner prop in a clockwise direction when viewed from behind the aircraft, and the starboard engine (clockwise turning) driving the aft prop in a counterclockwise direction. Each power system had independent control of the propeller pitch, including feathering capability for both props, and reverse pitch for the aft prop. Synchronizing capability was also installed. This is an amazing feat of propeller control, and it has taken me 10 years of intermittent research to figure out. Last year’s AEHS trip to the NASM Garber facility where I got to view the partially disassembled XB-42A, produced the final pieces to the puzzle.


The propellers are each 3-blade Curtiss-Electric units, variably described as C(57) 62SP-B1 (used on Allison test stand) or 836-17C2-18 and 837-17C2-18 (as described in NACA Report L4K09), or 3-C-2 (Dan Whitney). The forward prop had a diameter of 13 feet 2 inches and the aft prop 13 feet, most likely to avoid the impact and vibration of the tip vortices coming off the fore blade tips if they were to be in the same plane as the aft prop. I have been unable to locate technical documents on these propellers, but this is not too surprising since these props were unique to this application and never achieved production status. The blades were of Curtiss two-piece, welded, hollow steel construction, and two types of elongated spinners, one rather long and pointed, the other somewhat shorter and blunted, were used to fair the props to the fuselage.


Actuation of the propellers, like all Curtiss constant-speed props of that time, was by electric motors and reduction gearing in the hubs of each prop. Curtiss was uniquely qualified to take on the task of independent control of coaxial, contra-rotating props since it already manufactured a hollow bore, electric propeller (C6315SH) to accommodate the nose cannon of the P-39 Airacobra My best guess is that this basic design, adapted to a 70 spline shaft, was used to actuate the inner propeller. This is supported by the general configuration seen on the XB-42A pictures, including the semi-circular housings found 120° apart around the circumference which house the planetary/bell gear reduction train. I presume a “standard” Curtiss multi-lane brush block assembly would be found attached to the face of the gear box to provide controlling current to the inner prop hub and actuating motor.

But how was the aft propeller independently controlled? Pictures taken at the Garber facility of the cruciform tail-less XB-42A (aft prop removed and unavailable), and data from the recently acquired maintenance and erection manual show that the hollow bore of the inner prop shaft (supports aft prop) was filled with probably an insulating material which held 2 or more electrical conduits. The picture below right shows what I believe are two electrical posts in the bore of the shaft. My best guess is that a brush block assembly was mounted on the back side (engine-side) of the gearbox that mated with a slip ring assembly that routed conduit through shaft bore. The aft prop, based on pictures and video of a spinner-less XB-42 (, seems to show a rather standard looking 50-spline Curtiss Electric prop assembly like that found on a P-38, P-51A, or P-40. My guess is that the hub was modified to provide electrical connectors between the prop shaft bore terminals and the prop hub. The hub likely had insulated pins that completed the circuit to the centrally-mounted electric motor and brake assembly. Thus Curtiss was able to use proven designs and technology to achieve independent control of coaxial, contra-rotating propellers.





XB-42 Data Sources


Original autograph of XB-42 test pilot Gen. Robert Cardenas from the collection of Tom Fey obtained by personal correspondence, 2008
Email correspondence with Maj. Gen. Fred Ascani, 2008-2010
XB-42 Maintenance and Erection Manual
XB-42 Pilots Manual
Rolls-Royce Heritage Trust - Allison Branch, Indianapolis, IN
Pictures taken by author at the NASM Paul Garber Restoration Facility, Silver Spring, MD, during AEHS convention visit in August 2010 and the California Aerospace Museum during the AEHS convention visit in July 2008.
Internet sources as noted


Vee’s for Victory, Dan Whitney, Schiffer Military Publishing, 1998
Elegant Failure, America’s Forgotten Wings, Air Classics Review #1, 1994
The First, The Last, and The Only XB-42/42A/43, Walt Boyne, Airpower Magazine, Vol. 9, #6, 1979
The Douglas XB-42 Bomber of 1944, Robert F. Dorr, Aviation News, April 1985
Design and Development of the XB-42, Carlos Wood, American Aviation Historical Society Journal, Vol. 47, #3, 2002

NACA XB-42 model performance reports

A5J12 Aerodynamic characteristics of a 1/8 scale (XB-42)…
A4K04 Estimated flying qualities of (XB-42) from 1/8 scale…
L4K09 Investigation of tail-prop interference (XB-42) …

The End


A copy of this article with large images is available in the AEHS Member's Section