In this book, The Making of the Atomic Bomb by Richard Rhodes (p. 705), I read:

[Paul Tibbets] eased the brakes [of Enola Gay] at 0245, the four fuel-injected Wright Cyclone engines pounding. “The B-29 has lots of torque in take-off,” he notes. “It wants to swerve off the runway to the left. The average mass-production pilot offsets torque by braking his right wheels. It's a rough ride, you lose ten miles an hour and you delay the take-off.” Nothing so rude for Tibbets. “Pilots of the 509th Group were taught to cancel torque by leading in with the left engines, advancing throttles ahead of the right engines. At eighty miles an hour, you get full rudder control, advance the right-hand engine to full power and, in a moment, you're airborne.”

I am not especially knowledgeable in aviation matters, but I believe that the torque is due to the B-29's four propellers all rotating in the same direction. Is that right? If so:

  1. Why did this problem only arise during take-off?
  2. Why did all propellers rotate the same way? Is this true for all propeller-driven aircraft with more than one engine? Why?
  3. If the 509th Group's technique was so good (and apparently not especially complex), why did only they use it?
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    $\begingroup$ Although this particular technique was not, in fact, unique to the 509th (as per @JonStory's answer), one response for as to why, hypothetically, the 509th might adopt some technique not used by other B-29 crews could be that the 509th operated Silverplate/Saddletree (nuclear-capable) B-29s, which were rather extensively modified for nuclear delivery, whereas most B-29 crews flew non-nuclear-capable aircraft. $\endgroup$
    – Vikki
    Aug 7 '19 at 22:57
  1. It didn't arise ONLY during takeoff, but it is most noticeable then for several reasons

    • The engines are at maximum power, therefore producing maximum torque
    • You've got a very good reason to need to be travelling in an exactly straight line (the runway)
    • You're travelling at fairly slow speed, so your vertical stabilizer is less effective - at higher speeds, the tail counteracts the torque more effectively (for this B-29 this occurs at 80mph, as mentioned). As soon as you get above 80mph, you can just input a little right rudder to counteract it.... B-29's only travelled below 80mph on takeoff and landing, so it wasn't a problem during flight, and during landing the throttles were at idle or very low power.
  2. It isn't true for all aircraft with more than one engine now (although it is true for some), but it does still happen and was much common back in WWII - they didn't want the extra supply line complexity of two different sets of otherwise identical engines, when it could be solved with a bootfull of right foot. Note also that the B-29 had 4 engines, most modern multi-props only have two, and they're closer to the fuselage, which reduces the effect

  3. I think you're misinterpreting the source: he's not saying this technique was unique to the 509th, just that they were taught the technique. The very fact he says they were taught it, rather than "came up with" it, suggests it was known elsewhere. The 509th are only referenced here because that's the group Tibbets was part of, it's unrelated.

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    $\begingroup$ @DaG That means one of the instructors of the 509th knew of using diff-thrust for yaw control at low speeds and passed it on to his students $\endgroup$ Feb 20 '15 at 10:14
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    $\begingroup$ It's likely that different units would have independently arrived at the same technique, it's just common sense. $\endgroup$
    – GdD
    Feb 20 '15 at 10:32
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    $\begingroup$ @JonStory "The average mass-production pilot" I read as the average B-29 pilot, who was always a bomber pilot. The thing I read is "average people are taught that in flight training, but the good pilots know to use diff thrust. The 509th only had good pilots, so we used diff thrust." $\endgroup$
    – cpast
    Feb 20 '15 at 15:59
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    $\begingroup$ Very minor point: Placing the engines close to the fuselage has no effect on the torque due to engine rotation. It does, however, make a big difference to the engine out behaviour. $\endgroup$
    – copper.hat
    Feb 23 '15 at 17:50
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    $\begingroup$ Additional note on the second question: An example for a modern multi-rotor aircraft using counter-rotating propellers is the Airbus A400M - it uses a different gearbox setup in two of the engines to achieve this. In contrast, the C-130's Allison T56 and C-130J's not quite as ancient RR AE 2100 engines turn all propellers in the same direction. The downside of the reverser gearing is, of course, additional weight in two of the engines (27 kg in the TP400 case). Source: easa.europa.eu/system/files/dfu/… $\endgroup$
    – JulianHzg
    Mar 1 '15 at 13:38

I recently had the change to speak with a current co-pilot of the CAF B-29 "Fifi". He stated that the pilot or co-pilot would typically advance the throttles for takeoff with his hand on the levers in a "twisted" position so that any given lever would be further forward than all its neighbors to the right, i.e. the tops of the levers would form a diagonal line from forward left to aft right. So differential thrust is used to compensate for a left-turning tendency. He stated that you wouldn't use full throttle at this point, because then you'd have nothing remaining to work with-- this implies that a correction to the left might sometimes be needed, since a further correction to the right could always be accomplished by reducing the right-hand throttles. He stated that once the rudder became effective, the pilot flying the airplane would then call for the flight engineer to "set takeoff power", at which point all the throttles would be advanced to the same position.

This pilot was somewhat skeptical of the idea that it would be common technique for a pilot in any group flying the B-29 to use the brakes to keep the airplane going straight during the take-off roll. He said it would be considered bad technique today in the CAF, due to the risk of skidding a tire and leaving a flat spot.

The pilot's general description of the technique he used, and the specific point that some "room" was left to allow corrections to the left as well as to the right, suggests that the left-turning tendency in the B-29 on takeoff wasn't too extreme. However, since the plane didn't have a steerable nosewheel, some other form of compensation was required.

Moving on from the specific comments by this pilot, to some general observations about left-turning tendencies on take-off in tricycle-gear aircraft that sit in a level attitude until the aircraft is rotated for lift-off--

  1. In general, engine torque creates a rolling (banking) tendency to the left, but the aircraft can't roll (bank) very much when the wheels are on the ground. However the rolling tendency does increase the weight on the left tire, and thus the rolling friction of the left wheel.

  2. In a tricycle-gear aircraft that sits in a level attitude until it is rotated for lift-off, P-factor does not create a turning tendency during the portion of the takeoff run that precedes rotation. In general, P-factor creates a yawing tendency, but P-factor only exists when the aircraft is at a positive angle-of-attack, i.e. when the aircraft is in a nose-high pitch attitude in relation to the direction of the flight path. This is not the case when the aircraft is in a level attitude, with all the wheels on the ground.

  3. Gyroscopic precession does not create any turning tendency during the portion of the takeoff run that precedes rotation. In general, gyroscopic precession creates a yawing tendency whenever the aircraft is pitching, i.e. whenever the pitch rotation rate is not zero. When the propellers rotate clockwise, gyroscopic precession will create a left yaw torque when the nose is pitching up, and will create a right yaw torque when the nose is pitching down. So gyroscopic precession will make an aircraft with clockwise-rotating propellers yaw to the left as it is being rotated for takeoff, but not before or after that point.

  4. During the portion of the takeoff run that precedes rotation, the spiralling (helical) slipstream (propwash) does exert a force on the vertical fin. When the propellers rotate clockwise, the upper part of this spiral is moving toward the right, which exerts a rightwards force on the vertical fin, yawing the nose toward the left. This force will be greater at lower airspeed than at high airspeed, since when the airspeed is low, the propwash or slipstream will tend to dominate over the free relative wind created by the aircraft's motion through the airmass. Even though the vertical fin of the B-29 was not directly centered behind any one of the engines, the airflow over the vertical fin surely had some degree of left-to-right component which undoubtedly contributed to the aircraft's left-turning tendency on take-off.

Harvey S Plourde's excellent book "The Compleat Taildragger Pilot" (1991) contains a pair of graphs showing the individual and combined effects of all the different factors that contribute a left-yawing tendency during the takeoff roll in a propeller-powered aircraft with clockwise-turning propellers, for both the tailwheel case and the tricycle gear case, from the start of the takeoff roll all the way to the moment of liftoff. Different factors are dominating at different points in the takeoff roll.

Now to address one other aspect of the original question:

In the vast majority of twin-engine or multi-engine propeller-powered aircraft, all the propellers turn the same way. As another answer has noted, this simplifies production, and the supply of replacement parts. Of the well-known American-made twin- or multi-engine aircraft built in large numbers during the second world war, only the P-38 Lightning had propellers rotating in opposing directions.

Note-- the links embedded in this answer go to sections in John S Denker's excellent "See How It Flies" website.

Note-- all references to "clockwise" in this answer are from the perspective of someone standing behind the aircraft looking forward, as is the accepted convention in aviation, presumably because it also relates to what the pilot sees when looking forward through the windscreen at the propeller of a single-engine airplane of the "tractor" configuration.

Related ASE answers--

What is the influence of yaw on P-factor?

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    $\begingroup$ Torque does contribute a bit of turning tendency by putting more load on the left wheel than the right one (given a clockwise propeller rotation). More load means more rolling resistance $\endgroup$ Jun 7 '21 at 14:51
  • $\begingroup$ If the props are to blame, it might be worth noting that, in addition to more powerful engines, the silverplates had different props. $\endgroup$ Jun 7 '21 at 17:00

Something about the Tibbets statement does not ring true.

I have talked to dozens of B-29 pilots and not one of them mentioned having to do either of those things during takeoff: stepping on the brakes or advancing the left engines first. Stepping on the brakes seems like an especially horrible idea. The crews were flying overloaded planes that needed every inch of runway just to get in the air. The last thing they would want to do is slow the aircraft down by 10 mph during the takeoff run.

Also, prior to being assigned to the 509th, Tibbets was involved in training B-29 crews in Nebraska. Since most, if not all training flights would have been conducted in lighter B-29s, the tendency should have been even greater. Did Tibbets learn the technique there? Did he teach it there? (The pilots I talked to were trained in Nebraska while Tibbets was there.)

It could be that this was a problem that was unique to the Silverplate aircraft operated by the 509th because they had more powerful engines than regular B-29s.

I will check with some people who flew/fly B-29s including one pilot who flew both regular and Silverplate aircraft in 1945.

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    $\begingroup$ Right, at low speed (with little wing lift) the wheels also should have some say in where the airplane is going. Braking is not the only option. $\endgroup$ Jun 7 '21 at 14:46

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