Pilots and prop management: B-24

[Grittis]

I have a few questions about how B-24 pilots in WWII managed their variable pitch propellers. I have basic knowledge about the workings of the propellers used on the Pratt/Whitney (and B-17 Wright) radial engines but I am no expert. Feathering a prop to reduce drag was important for a 4 engine plane once it lost oil pressure as the pitch was controlled by that pressure. But I have some more technique type questions: Q: What is meant by having an engine "run away"? Is it a combination of high rpm and uncontrolled pitch? Q: When a pilot refers to "synchronizing his engines", how is that done? Is it just rpm? I have read that it is part by ear and some accounts of visual techniques with the outer engines; looking through the spinning inner engine prop for an interference pattern(?) The B-24 was notorious for being difficult to maintain in formation, especially at high altitude with it's narrow Davis wing. For minor adjustments in speed, did they rely on changing the boost or the rpm? Was this different on the B-17? Was this a trade off between cylinder head temp. and increased fuel consumption?

 

[cvairwerks]

An engine runaway is an overspeed condition generally caused by the prop pitch going as flat as mechanically possible and causing crankshaft rpms to exceed allowable limits. It can also be caused by allowing the aircraft speed to drive the engine over the limits, such as not reducing power while in a dive or decent. Either way, it's a mandatory inspection and possibly teardown, depending on the rpm reached.

Synchronization could be done ether by automatic controls within the power management section, or by manual adjustment using auditory or vibration sensing of the pilot or flight engineer. Out of synch engines will produce a very noticeable acoustic beat, or a airframe vibration. The pilot or engineer will set the desired power level on one engine and then adjust the others to eliminate or significantly reduce the beat or vibration. With a little practice, it's not hard to do manually, but a synchronizer system does it way better.

 

[Grittis]

You mention pilot or engineer involved in the sych of engines. Was this a task for the co-pilot? My dad was a navigator, so clear of the flight deck during take-off/landings, and in his crew they got a >green copilot before heading to the ETO. His pilot was rated in B25, B17 before getting a crew in B24s. He and his excellent engineer coached their co-pilot in his duties.
Did the pilot normally synch engines after start-up on the hardstand? Was that where the engine run-up and magneto checks were normally done?
At that point the pilot was involved with 'driving' from the hardstand to the taxiway, getting in line and heading to the runway. Eng. kept an eye on engine parameters and when the time came, reading off the ground speed during take-off (along with his many other duties, including the top turret). So what were the normal co-pilot's major duties during the pre-flight period?

 

[cvairwerks]

Grittis: Generally, in a two man cockpit, engine sync would be done by the non-flying pilot...ie whomever it was that didn't have their hands on the flight controls. With a three man cockpit, the task moved to the flight engineer, as that was part of his responsibility. Synchronization was done after takeoff, after the initial power reduction. Once at cruise altitude and power settings, it was repeated and from there on as necessary during portions of flight that require any power changes.
The B-29 and B-32 were the first a/c that I know of, that had a true three man cockpit crew. Others had a flight engineer, but his responsibilities extended beyond part time cockpit duties.

All runup tasks were carried out in the runup area. The aircraft was taxied into a position where it was pointed into the wind for best ground cooling of the engines. Mag and prop checks were carried out, along with whatever else was on the check list. During takeoff, while the pilot or copilot is on the controls, the other is ready to take control at any instant, and is heads down, watching critical flight instruments...airspeed, fuel flow, and cross check that both his instruments and those of the other pilot were alive and reading identically. The FE, would monitor engine rpms, manifold pressure, cylinder temps and fuel flow, all until the first power reduction after takeoff.

 

[Grittis]

That makes more sense. Max or near max power setting for take-off, after which power is reduced to best rate of climb or similar and at that point the engines are synch'ed. In the B-24s that was done by sound/vibration, typically by the co-pilot. To clarify, feathering a prop to stop its windmilling required oil pressure in the B-24. Any damage that dropped the oil pressure needed to be caught early and a decision made. The question of minor speed adjustments at 20k ft. altitude in the B-24 is not clear to me. RPM, boost, prop pitch can all seem to be involved. What was the best technique for low fuel consumption, low engine wear?

 

[cvairwerks]

Throttle to set the desired manifold pressure and prop control to fine set the rpms. Leaning as required with the mixture control. Add in 2 speed supercharges and/or turbochargers and it gets a bit more complicated. Pull up a copy of the POH for the -24 or -17, and much of the info is in there.

 

[Joe Broady]

"The 3-bladed propellers are Hamilton Standard, hydromatic, full feathering controllable pitch, constant speed. Toggle switches on the pilot's pedestal electrically control the governors which maintain the constant speed feature." (Pilot Training Manual for the Liberator B-24)

In this photo, the prop switches are aft of the 4 levers labeled SUPERCHARGER. The switches are spring loaded to the center position. Holding a switch forward drives the corresponding prop governor in the increase direction. At full increase a limit light comes on. These lights are immediately left of the supercharger knobs and directly below the vertical speed indicator.

B-24 cockpit

The same lights illuminate when the prop governors are at full decrease. When setting power for takeoff, the book warns, "Be sure to move toggle switches forward; governor lights will also come on when switches are moved back, and this would set the governor for full low rpm."

To synchronize the left engines, "Leave the No. 2 as it is. Note the rotating shadow across the top of No. 1 propeller. If the shadow is rotating away from you the propeller is too slow." Similarly, the copilot synchronizes the right wing engines. If there's still an engine beat or pulsation, the left and right wings are out of synch. Slightly increase or decrease rpm on one wing with a simultaneous flick of both switches, visually check that they're still in synch, and listen for a change in the pulsation. By this process you get all engines synchronized. At night you can use a flashlight or the reflection from the landing lights.

"When a propeller runs away, it simply means that the propeller governors fail to hold the propeller at its constant rpm setting. Thus, before takeoff when engines are idling, propeller is in low pitch (small bite), high rpm. Sudden application of power may cause a propeller to exceed the governor limit speed before the governor has a chance to take hold and increase the pitch. This is usually the case with a runaway propeller. However, if you have complete governor failure, you may not be able to regain control with throttle alone and will have to use the feathering button."

The first procedure is to reduce throttle and operate the prop switch in the decrease direction. If that's effective, cautiously increase both controls. If not, hold the feather button in. (The 4 buttons above the center windshield in the photo.) At 2500 rpm pull the button out to halt feathering. If rpm starts increasing, the governor has failed. Continue to push and pull the feather button until at sufficient altitude to feather completely.

"The most important fact to keep in mind about a runaway propeller is not to feather it until you have tried out the 2 procedures which should give you control." The book cites a case where a propeller ran away on takeoff with a combat load. "Without trying to bring the propeller under control he feathered immediately. He was unable to maintain altitude and the ship crashed shortly after takeoff. Proper procedure would have given 15 to 50% power on that engine."

The book says nothing about power management during formation flight.

 

[PStickney]

You mention pilot or engineer involved in the sych of engines. Was this a task for the co-pilot? My dad was a navigator, so clear of the flight deck during take-off/landings, and in his crew they got a >green copilot before heading to the ETO. His pilot was rated in B25, B17 before getting a crew in B24s. He and his excellent engineer coached their co-pilot in his duties.
Did the pilot normally synch engines after start-up on the hardstand? Was that where the engine run-up and magneto checks were normally done?
At that point the pilot was involved with 'driving' from the hardstand to the taxiway, getting in line and heading to the runway. Eng. kept an eye on engine parameters and when the time came, reading off the ground speed during take-off (along with his many other duties, including the top turret). So what were the normal co-pilot's major duties during the pre-flight period?

You dunch the engines after you've gotten off the ground, cleaned the airplane up, and set climb power. After that, it's basically any time you've made an RPM change and you don't have more important things on your mind. When you're making your landing approach, you push the props up to Max Increase RPM, so that you've got power available if you have to go around. You can always tell when a DC-3/C-47 is coming in when you hear the props sped up, and the beat as they unsynch.

 

[PStickney]

That makes more sense. Max or near max power setting for take-off, after which power is reduced to best rate of climb or similar and at that point the engines are synch'ed. In the B-24s that was done by sound/vibration, typically by the co-pilot. To clarify, feathering a prop to stop its windmilling required oil pressure in the B-24. Any damage that dropped the oil pressure needed to be caught early and a decision made. The question of minor speed adjustments at 20k ft. altitude in the B-24 is not clear to me. RPM, boost, prop pitch can all seem to be involved. What was the best technique for low fuel consumption, low engine wear?

The best combination is Mixture Auto Lean, Low RPMs (As defined in the manual) and Manifold Pressure as required to maintain the proper airspeed. Note that cruise efficiency is dictated by the Power Required curve, which is airframe drag - the job of the engines in cruise is to provide the power necessary to maintain the airspeed where drag is at a minimum.
Remember that for the airplanes that we're talking about, you've got 2 types of drag. Profile Drag, which is due to the shape and size of the airplane, and Induced Drag (Drag due to Lift), which depends on how hard the wing has to work to keep teh airplane in the air. This is high at low speeds, and decreases as the airplane's speed increases. So the drag curve is sort of a "U" shape, with the lowest drag being the point where the increasing Profile Drag outweighs the decreasing Induced Drag.
When making power changes, things need to be done in order - Mixture Rich (If necessary), Prop RPMs, and Manifold Pressure when increasing power, Manifold Pressure, RPMs and Mixture when decreasing power.
When flying in formation if you're adjusting power, it's done with the throttle - (Manifold Pressure), and RPMs are kept set to whatever they're commanded to be. The Constant Speed Prop will change pitch to absorb the greater power while keeping the RPMS the same.

 

[jimh]

Interesting thread! I was lucky enough to fly "The Dragon"/"Witchcraft" for nearly 20 years, getting typed in 2008. Aside from taxiing the beast, syncing the props was the biggest challenge for the new guys. The prop governors are electric and are prone to failure at any time. Put the most fragile piece on top of a 1,200hp engine that's trying to shake itself apart...always a running joke. While we trained the F/E on how to run the systems, it was always the non-flying pilot that ran the props, mixtures, and cowl flaps. (all checklist items). After takeoff the pilot flying would control the throttles and turbos with back up being given by the co-pilot on the throttles, once clear of the ground and the gear coming up the pilot flying would bring the turbos back about half and the throttles back to METO or Climb power and the co-pilot would toggle the props down and not worry too much about them being in sync, just matched up on the RPM gauges. Once cruising altitude was reached was when the fun began. We cruised at 30"MAP with 2,0000rpm.. The easiest way to sync the props was to slide your seat back watch the shadows, left goose, right reduce. You can also do it by ear, but that took most people 20-30 hours of flying and fiddling to figure out.

A run-away prop is rare, I saw a few overspeeds on takeoff when the governor didn't catch it, but it always came back. When the governor fails the prop stays in whatever RPM was last selected, not a big deal unless you are at take off RPM, that would generally require it to be feathered. Depending on the day, hot, humid, you can use the feather switch to modulate the rpm at 2000. The copilot can push in the feather button to bring the rpm down, then release and engage as necessary. This is not a long term fix, this procedure gets you around the pattern safely when conditions dictate. A runaway prop would otherwise be feathered.

Formation flying is a chore. The aerodynamic properties of the wing allows it to fly on a 'step', much like a boat. Any variation of angle of attack and you are plowing through the air and slowing down, or diving and speeding up. I always found it easiest to use ginger amounts of elevator trim for both cruising and formation flying. It is really pitch sensitive.

Jim