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A photo in EAA's monthly magazine shows one propeller blade as a dozen distinct images rather than the usual smooth motion blur. One explanation (this answer appears to have been deleted) for this is that a propeller at nominally 2300 rpm actually varies +-200 rpm as each cylinder fires, so when it's slower, it captures more sunlight glint.

Edit: this question is about propellers, not about a photo. The photo is only a motivation, perhaps an incorrect one, for the question.

Is this plausible?

  • Piston engines have flywheels.
  • A 400 rpm change happening 2300 times per minute is a 17% change happening 38 times per second. Jerry Lee Lewis would say that's a whole lotta shakin' goin' on.
  • Onboard GoPro videos with rolling shutter artifacts show the blades "bending" from root to tip smoothly, not jaggedly.

photo from photo.stackexchange.com

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Dec 1 '20 at 7:58
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The stutter would be at 2300/min for a two-cylinder engine (or 2-stroke single). For an 8cylinder engine it would be four times that, or 150Hz or so. For a 1000HP engine the output is about 450J per ignition cycle. Estimating a flywheel effective radius of 200mm and mass 30kg the kinetic energy is around 350kJ and so the stutter would be less than 1%. I’ve skipped most of the calculations there :-)

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  • $\begingroup$ I would guess the "multiple images" are just light reflections from the water waves onto the propeller. $\endgroup$ – alephzero Nov 28 '20 at 15:50
  • $\begingroup$ @alephzero Intriguing. Feel free to answer the linked question on photo.SE. $\endgroup$ – Camille Goudeseune Nov 28 '20 at 17:51
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    $\begingroup$ Mind showing us the math? Something doesn't add up here (pun intended). Putting out 1000hp at 150 powerstrokes/second would require about 4970 Joules/powerstroke. A typo perhaps? $\endgroup$ – Jpe61 Nov 29 '20 at 14:47
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    $\begingroup$ @MikeSowsun this answer does not address why the photo appears as it does, because it answers the question asked: does a propeller stutter enough to explain the effect seen in the picture. There is question on photography.se that deals with the rest. $\endgroup$ – DeltaLima Nov 30 '20 at 8:03
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    $\begingroup$ The question asks if this is plausible. It is not, the thing would make a horrendous noise and break up. $\endgroup$ – Guy Inchbald Dec 2 '20 at 20:10
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We can count more than 15 blade-images in that short arc of revolution of the propeller, so there's no way that each one corresponds to an individual firing of a cylinder. Perhaps there is a resonant vibration involving a slight twisting of the blade, and the sun angle is such that this produces a strobing effect.

The suggestion of a 300 rpm variation between each individual firing of a piston is completely implausible.

Some sort of camera artifact seems a possibility.

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    $\begingroup$ Further, if the idea is [A] the prop is "slowing down and speeding up", it would have to be slowing down to something like 1/10th (!!!!!) or perhaps much less than that, to achieve such clear "lines" which are about 1/0th as thick as the "gaps". Does there even exist a material which would make this much deccelleration / acceleration possible?? I imagine if you , say, put a large concrete block in front of a propellor (which is the sort of decceleration we're talking about, everything would explode in shards? $\endgroup$ – Fattie Nov 29 '20 at 15:50
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    $\begingroup$ If [B] the idea is that the propellor is twisting in some way that it reflects (again, only for roughly 1/10th of each cycle) and then becomes unbent again (for the other roughly 9/10ths of each putative "reflection cycle") ... again it seems hard to believe any mechanical device could sustain such a regular twisting motion (indeed, apparently pretty smoothly along it's whole length, it seems) $\endgroup$ – Fattie Nov 29 '20 at 15:55
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As pointed out before, the variation is unlikely to be the first engine order of individual firing cylinders.

Over on the photography site they provided EXIF data, showing an exposure time 1/200s, which would lead to only 1500rpm (assuming the motion blur in the picture is 45deg), not the 2300rpm nominal.

The frequency in the picture is in the range of approx 3000Hz (1 event every 3 deg at 1500 rpm), which might be some eigenfrequency of the crankshaft-propeller arrangement. My estimation is though that the low order eigenfrequencies are lower, and the higher order eigenmodes would show nodes of zero displacement. The superposition might lead to something like in the image.

There's some (older) doc on propeller vibrations here: https://nvlpubs.nist.gov/nistpubs/jres/21/jresv21n5p639_A1b.pdf

Personally, I find some digital photography artefact the more likely explanation (some sensor-readout .. effects)

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This picture was taken using the type of LCD shutter that is essentially one large monochrome pixel. It becomes transparent when a voltage is applied.

The camera manufacturer probably used a square-wave (2 kHz is often recommended) to avoid damaging the shutter with a persistent DC bias. The inescapable deviations from the ideal waveform would be enough to cause the shutter to vary in transparency, probably between fully transparent and slightly dimmed.

Another possibility is that the clearing voltage contained a ringing artifact, a common result of attempting to apply a square wave to an electronic control.

The registering retina might have been either photosensitive film or a photocell array. The stuttering-image illusion is due to the nature of the shutter and independent of the recording tech.

I assume that the stuttering-image illusion will appear only when using this camera to photograph a fast moving brightly lit object, in this case a specular reflection of the sun on the polished blade. Note how the other propeller blade, which is not reflecting the sun, does not seem to stutter.

EDIT:

The numbers match.

Suppose the shutter is driven by a square wave oscillator (a DC bias might damage it). Each oscillator cycle produces two square waves, one in each direction, with a slight gap between due to the inevitable deviations from the ideal waveform. This would produce two propeller images per cycle.

At least one LCD shutter vendor recommends a 2KHz AC square wave. This would produce 2000 * 2 = 4000 images per second, or 240000 images per minute.

I count 12 separate propeller images in the photo, in an arc that appears to be about one-tenth of the complete circle. One engine rotation therefore would produce 120 images.

Suppose the aircraft engine is running at 2000 RPM. This would produce 120 * 2000 = 240000 images per minute, as expected.

These numbers are all ballpark guesses. 2000 RPM is a low power setting for most GA planes, but if the engine was running faster and the square wave oscillator was set to 3KHz instead of 2KHz, the numbers would still match.

This does not prove my theory but it does invite further investigation. Maybe I could get a grant.

EDIT:

The photographer, Jim Busha, reports that he used an unmodified Nikon D850, which definitely does not have an LCD electronic shutter. The D850 is perfectly capable of achieving all the electronic shutter functionality it needs simply by turning the image sensor on at the appropriate time.

My theory is disproved.

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  • $\begingroup$ Interesting theory but wouldn't that be one super flawed camera? Professional photographers use slow shutters for aesthetic effect all the time and this kind of artifact is not acceptable. $\endgroup$ – Roman Nov 29 '20 at 8:14
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    $\begingroup$ @Roman: If the camera shutter is made slow on purpose the ringing will be on such a short timescale it won't matter. $\endgroup$ – Kevin Kostlan Nov 29 '20 at 8:47
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    $\begingroup$ @A.I.Breveleri , just to clarify, you state that you know it was taken with a camera using that type of shutter. Do you mean (A) somewhere it states which camera was used for the photo, and you know about that camera model, or do you mean (B) you have no idea which camera was used; but from looking at the photo you know that type of shutter must have been used. Can you please clarify or B? $\endgroup$ – Fattie Nov 29 '20 at 15:44
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    $\begingroup$ interesting idea but such cameras don't exist... $\endgroup$ – szulat Nov 29 '20 at 18:24
  • $\begingroup$ @Fattie: B. I know that such LCDs are manufactured (by e.g. Pimoroni Ltd, Meadowlark Optics, Inc., LC-Tec Displays AB) and sold to camera makers. I don't know what makes and models of cameras use them. $\endgroup$ – A. I. Breveleri Nov 29 '20 at 22:58
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Purely regarding the fact that the color is different on each side of the prop...

(Remarkably, even a couple of the current answers on the photography site are "puzzled" by this.)

Every photo, ever taken, of a propellor has a different reflection on one side than the other .... since .... props bend one way on one side, and the other way on the other side.

enter image description here

enter image description here

enter image description here

enter image description here

(I simply googled "photo of propellor" and (obviously) literally every single photo looks like this. These are the first four.) (I guess this confirms there are no "flat" propellors, heh!)

I appreciate the question here is about engine stutter, but, a number of people have been puzzled by the different color on each side.

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    $\begingroup$ This addresses the question on the photography stack exchange, and commendably so, but, as it states, it doesn't address this question on the aviation stack exchange. (Are the two sites' cultures more different than I'd guessed? Is the overlap, ahem, blurrier?) $\endgroup$ – Camille Goudeseune Nov 30 '20 at 2:29
  • $\begingroup$ @CamilleGoudeseune , you're right, but the whole question (on both sites) is confusing beyond belief! Notice for example the other answer on this site which is (frankly nonsensical) which deals only with the photography aspect, has numerous upvotes and comments that it "finally explains!" the effect. My own answer which trivially explains the whole thing (every photo ever taken digitally of a prop has the two effects, repeats and bending), has downvotes! On the photo site there's now a wholly astonishing discussion of how it's, no really, reflections of waves,.... $\endgroup$ – Fattie Nov 30 '20 at 13:02
  • $\begingroup$ (which magically change, or something unexplainable, at 10kHz), and then there's folks stunned at how the prop can be different colors on each side, and there's the original "pistons firing" thing which would be like confusing the heat of the sun with an icecube :) So .. I just thought this answer might clear up one aspect anyways :O $\endgroup$ – Fattie Nov 30 '20 at 13:04
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I have never heard of a propellor stuttering in this way, at least not in normal flight. The calculations in the various answers to date put the vibration in the audio mid-range, to which our ears are most sensitive. The magnitude shown in the photo would be clearly audible if it were real, and the pilot would be throttling back and looking for an emergency landing place, not lazing around posing for a photo-shoot!

Now, I know this question is not about digital cameras, but the evidence supporting the question certainly is; the things are a menace when it comes to this kind of shot.

So it's pretty much a slam-dunk: the prop is not stuttering, the camera is.

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    $\begingroup$ Torsional vibration of the blades is also a possibility. $\endgroup$ – Mark Nov 30 '20 at 21:02
  • $\begingroup$ @Mark The argument applies to all vibration modes, torsional included. $\endgroup$ – Guy Inchbald Dec 1 '20 at 5:43
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It's nothing more than the usual "time aliasing" of any (digital) photo ever taken of any propellor.

There are 1,000 detailed explanations of this online ...

http://resourcemagonline.com/2015/11/why-your-camera-makes-airplane-propellers-look-so-weird/60421/

(Note that if you look carefully at the photo of the blue plane, you can see the typical "bending" as well - as with every single digital photo, ever taken, of a propellor you get two effects (a) bending and (b) repeats. Exactly as in the photo here. Like every single digital photo, ever taken, of a propellor.)

Please note that the answer over here: https://photo.stackexchange.com/a/121533 (which the writer states is a "theory") is "unbelievably incorrect".


Here's a photo that perfectly shows the "bending effect" will be either more or less depending on the geometry domain.

https://www.dpreview.com/forums/thread/3692327?page=2

enter image description here

This perfectly shows that the "bending effect" can be either extreme or gentle - gentle seen on the blue plane image.

You can see in a zillion photos that you get repeats of the blade (more than the actual number of blades!) This is completely commonplace and applies to literally every photo, ever, taken digitally of a prop.

http://sbiii.com/aviatn-7.html

enter image description here

The image just above is literally identical to the blue plane image, except the time domain is a bit different.

(For the record, the idea that the propellor could be revolving in a step-wise fashion is bonkers.)

Every single digital photo even taken of a prop shows

A - multiples

B - the bending effect

the blue plane photo is wholly unremarkable (indeed I instantly googled dozens of images where the "multiples are close together" as in the blue plane image).

BTW I realize the headline question here is "how much does a propellor stutter" but the idea is so silly, and the facts on that have already been noted in the other answers.

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    $\begingroup$ The effect pictured is very different from the rolling shutter effect described in the linked article. $\endgroup$ – Mark Nov 28 '20 at 19:32
  • $\begingroup$ hi @Mark, I am sorry but you're completely wrong! I've added some more info $\endgroup$ – Fattie Nov 28 '20 at 19:33
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    $\begingroup$ "literally every photo, ever, taken digitally of a prop." is an unnecessary and certainly wrong hyperbole. $\endgroup$ – Bergi Nov 28 '20 at 19:47
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    $\begingroup$ You can get multiple propeller images close together from a rolling shutter, but you cannot get multiple propeller images overlapping. Further, the image in the question shows clear evidence of a long exposure time, while increasing the multiplication effect requires a short exposure time. $\endgroup$ – Mark Nov 28 '20 at 20:08
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    $\begingroup$ @Fattie, Modern pocket cameras and cell phone cameras don't have mechanical shutters, but digital SLR cameras and other "pro" cameras generally do have. My own personal camera gives me the option either to use the focal plane shutter or, to operate in a purely electronic mode. $\endgroup$ – Solomon Slow Nov 30 '20 at 20:50

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