In the photos below, the propellers on the B-29 props all stop in the same position. Is this done by hand, it is this automatic?

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    $\begingroup$ Hmm, I suppose vertical props would pose a reduced chance of hitting one's head. $\endgroup$ Nov 17, 2022 at 19:56
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    $\begingroup$ Note that the planes visible in the background of the Enola Gay picture do not each have their propellers all in the same orientation. $\endgroup$
    – Makyen
    Nov 18, 2022 at 18:40

4 Answers 4


With the 18 cylinder radial engines in the B-29, a random shutdown would give possible propeller positions ever twenty degrees. They do not automatically stop this way.

However, for staged photos like these, where the appearance of the aircraft is important to those commissioning the image, ground crews would hand rotate the propellers into alignment -- and this was just as much the case in 1944 as it is today.

Notice that the photo of Enola Gay, despite being on a remote airfield, is staged enough that the photographer used a red filter to darken the sky and show the clouds dramatically. This isn't something that was done for just any photograph, because it required several times as much exposure (meaning the camera was most likely placed on a tripod, unlike a common press photo shot hand held).

Per comments, it was also considered a matter of pride, back in the day, to always rotate the propellers to matching positions whenever the airplane wasn't being turned around immediately for another mission. This is in the same category as always having boots polished so the sergeant can see himself, or a bunk made up so tightly a quarter would bounce on the blanket -- operationally unnecessary, but still required.

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    $\begingroup$ An “Old Timer” once told me it should be done every time, not just for photos or inspections. It was a matter of pride. $\endgroup$ Nov 16, 2022 at 18:13
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    $\begingroup$ If I'm not mistaken there are other aircrafts visible in the background of the Enola Gay which do not have their rotors aligned. $\endgroup$ Nov 17, 2022 at 15:53
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Ralph J
    Nov 19, 2022 at 15:31

The props are rotated by hand and are all positioned the same way as follows.

For 4-bladed props: when they are pointing "square" (straight up and down and horizontal) it means the engines have been turned over by hand to work the oil out of the bottom cylinders (i.e., ready to crank for startup). For 3-bladed props they leave the bottom-most blade pointing straight downwards to indicate ready to crank.

If the blades on all the engines are randomly askew it means they have not been turned over by hand and must be thus exercised before cranking to work the pooled oil out, and prevent blowing a head gasket via a hydraulic lock.


One of the pilots of “Doc”, the CAF B29 in the picture, sometimes talks about “indexing” the propellers from the cockpit when ground clearance is needed. For instance, when they taxi at a smaller general aviation airport, they have to shutdown the outboard engines before they can leave the runway. There isn’t enough ground clearance under the B29 props (18”) to not hit the taxiway lights. Before leaving the runway he shuts down the outboard engines and manually “indexes” the propellers using the starter to put them in the X orientation.

This is a consequence of tricycle gear. The B17, for instance, is a tail dragger and thus the props are higher off the ground during taxi.

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    $\begingroup$ Nice answer (I know I'm not supposed to say that but I did anyway.) $\endgroup$ Nov 18, 2022 at 3:31
  • $\begingroup$ But-- I'm having a little trouble with the geometry. Would the prop blades really be at risk of hitting the taxi lights? $\endgroup$ Nov 18, 2022 at 3:34
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    $\begingroup$ @quietflyer Yes. Take Midway for instance. The taxiway letter boxes are over 24" tall and spaced about 85' apart. The B29 has 18" of prop clearance and a wingspan of 141'. It would be really close. At my airport, a midsized corporate reliver under a metro Class B, the boxes are 60' apart, there's no way they could make it through the outboard food processors. Oshkosh taxiways are 50. Also, in northern climes our edge lighting is on 18 to 24" standards for visibility when snow is on the ground. Martin Pauley's youtube channel has a video of a CAF flight engineer taking about it as well. $\endgroup$
    – Max R
    Nov 18, 2022 at 6:01
  • $\begingroup$ Is that really necessary? Have you ever landed at Midway? $\endgroup$
    – Mazura
    Nov 19, 2022 at 21:35

It's definitely done by hand afterwards. The ultimate proof I guess would be a video of the props stopping, out of alignment, and then a video of folks aligning them later, or just a photo taken afterwards with the props all aligned. I don't have that. But I know there was no mechanism in the B-29 for keeping the props all exactly aligned in flight, so it wouldn't make sense that they would ever stop all in alignment.

Plus, even if hypothetically there was some mechanism to keep the props exactly aligned in flight, it would make no sense that they would always happen to stop in the most aesthetically pleasing orientations ("+" or "x").

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    $\begingroup$ The "ultimate proof" is in background of the OP's pic of the Enola Gay. $\endgroup$
    – FreeMan
    Nov 16, 2022 at 18:01
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    $\begingroup$ @FreeMan maybe they were rotating and the 1/1000 s shutter speed made them look like they are stopped, but out of phase (possible if different engines are at different rpms ... right? ). (I found your comment right, but I am still playing devil's advocate here, for fun and to keep the brain exercised by challenging own assumptions) $\endgroup$
    – EarlGrey
    Nov 17, 2022 at 8:54
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    $\begingroup$ Maybe, @EarlGrey, but most people don't shoot a still life at 1/1000. $\endgroup$
    – FreeMan
    Nov 17, 2022 at 12:11
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    $\begingroup$ Having all the props in perfect sync would be bad for the airframe - each time a blade passes the leading edge of the wing there would be a slight change in air pressure, adding to vibrations and affecting lift over that section of wing. Worst case, that's 4 engines x ~16 foot of wing = 64 foot of wing that experiences pressure changes, or just under half its 141 foot wingspan. Random would be smoother. $\endgroup$
    – Criggie
    Nov 17, 2022 at 21:45
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    $\begingroup$ @Criggie And this is why Mr. Monk flunked out of aircraft design school and had to become a detective instead... $\endgroup$
    – Michael
    Nov 17, 2022 at 22:17

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