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I was watching this Youtube video, portraying a Fokker 70 taking off. I wouldn't have noticed if it weren't for a comment, but during spool up (~ 3:24 in the video), it appears as if the engine cowling is dilating. (See image below, if you don't want to watch the video). According to the person who left the comment, this is due to the pressure changing.

I'm having real trouble accepting this — though I do see it with my own eyes, I'm finding it more intuitive to believe something else is going on (I would think something related to the camera, though it seems to be well secured and I can't see any other perspective change).

In the end, since intuition is a poor tool for discovering the truth, I decided to ask:

Do turbofan engine cowlings really dilate due to spooling up, and is it really because of pressure difference? And is it the whole cowling or only the inlet that dilates?

enter image description here

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    $\begingroup$ Can’t notice the difference, also it would be absolutely fine if it dilates although it won’t dilate too much, so much so that our eyes can notice the difference. The reason is not pressure difference but instead the fact that heating metal expands it(which again explains why it would not be easy to catch with our eyes). $\endgroup$
    – Valay_17
    Commented Jan 13, 2020 at 8:07
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    $\begingroup$ On the image, notice the end of the inlet on the bottom-left side (where the window frame visually meets the inlet). Far less of the white part of the cowling is visible on the right-hand image. Watching the relevant part on the video, you can actually see it dilating. $\endgroup$
    – user12873
    Commented Jan 13, 2020 at 8:21
  • $\begingroup$ looks like its a optical illusion, when the silver part of the cowling gets close to the window's rim it "dilates" on that corner. you can view similar effect with the base of a wine glass physicscentral.com/experiment/physicsathome/images/Lens.jpg $\endgroup$
    – chaos505
    Commented Jan 14, 2020 at 10:17
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    $\begingroup$ The pressure difference across the engine cowling is negligible. The high pressures (and high temperatures) inside the engine are inside the strong inner case of the engine, not the sheet metal (or composite material) outer cowling. In any case, the purpose of the fan is not to create pressure difference but air flow velocity. The pressure rise from front to rear of the fan blades is only 1/10 of the pressure in your car tires, for example. $\endgroup$
    – alephzero
    Commented Jan 14, 2020 at 17:08

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Now that's an interesting phenomenon! I do not believe that this is a case of an illusion of any sort, or the engine dilating. What the video probably shows, is the engine slightly turning towards the window as takeoff thrust is being applied.

Engine mounts are not 100% rigid, this can easily be observed on pretty much any passenger flight if you can see the engine. Any movement of the fuselage and/or wings, and the engines are clearly "bouncing around". The reason for this is to act as a damper between the engine and the rest of the airframe.

In the case of the plane in question, applying T/O thrust imposes a significant force on the engine mounts and the part of the fuselage that the mounts are attached to, resulting in a small inwards turning of the engine nacelles, thus making it appear as if the cowling is expanding.

This video shows the exact same phenomenon from a slightly different perspective. At approximately 2:15 mark, as T/O thrust is applied, a red mark on the outer flap mechanism fairing dissapears from view as the engine is shifted a bit.

And to further prove my point (having some extra time by chance), here are two images I've created to make this phenomenon more visible:

This is a combination of two crops from the video. Exact same location, but different time. The other is just before T/O thurst is applied, the other just before brakes are released. Image is divided into four segments, two of former, two of latter point in time. A vertical and horizontal lines are added for clarity to separate segments "Dilation" is clearly visible: dilation crop

The second image is a combination of the same original images as before, but full frame. They are layered on top of each other, and the one on the top is defined as "difference" layer in Pixlr. This creates an effect that shows areas of the images that differ from each other as bright, while similar areas are dark: enter image description here As you can see, bright area is visible around engine nacelle. Also horizon line and the runway edge are bright, these brighter lines widen towards the right edge of this composite image. This is because the aircraft tilts from the thrust being applied against the brakes.

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    $\begingroup$ If you look at the central nose cone you can see the angle has changed, which would support with this theory. Though I would expect there would also be an amount of deflection of the front cowling. Its only a cover, not a structural component so isn't likely to be all that rigid. $\endgroup$
    – Notts90
    Commented Jan 13, 2020 at 12:47
  • $\begingroup$ I'm flattered that someone thought my answer was such boulderdash that it deserved a minus one :) $\endgroup$
    – Jpe61
    Commented Jan 14, 2020 at 20:38
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The answer is unequivocally no. Not only is it not possible, but what would be the point of designing them to do that? The only thing that expands in the turbo jet engine are the turbine blades themselves, but the amount they expand is infinitesimally small.

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If you line up the two pictures using the tire tracks on the ground and the wing, it is clear the camera position has changed. Look at the outboard hinge for the flaps (with the circular dot) relative to the marks on the ground, for example. Also look at the mark on the ground near the inboard hinge, which is close to the wing in the first picture but half way between the wing and the engine in the second one.

In the engine, the fan blade tips will move forward and twist by a measurable amount (a few millimeters and a few degrees) relative to the engine casing, and the blades also increase in length because of the radial forces (that is why the casing has an abradable lining inside, so the blades can cut out their own working position to minimise gas leakage over the blade tips) but the casing itself will not deform by any visible amount.

I am dubious about the idea that the engine mounts will "move around" as well. They are designed to be free to expand and contract as the engine casing changes temperature, but that is only a millimeter or two, not something as big as the apparent difference in the two pictures.

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    $\begingroup$ between the two pictures above, the plane has moved, hence the differences in reference to the tire tracks. If you watch the video, you'll notice that the camera is steadily set in one place during the whole "dilation event" which starts at 3:27. The plane starts moving at 3:40, second screen capture timestamp is 3:41. $\endgroup$
    – Jpe61
    Commented Jan 13, 2020 at 18:30
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I don't see any dilation or change to cowling size except I could "see" dilation is in the bottom left of the window. The edge of the engine anti-ice and a line in the window changed distance. See my red rectangles in the attached photo.

I believe this is an optical illusion. I suspect the cabin pressure increased as the engine spooled up and provided more bleed air. This pushed against the window and slightly distorted it. That distortion changed the distance of those lines.

Additional the entire engine cowling could flex a bit as the thrust would cause torque on the pylon. This could also explain the change in distance of my reference points (as pictured).

enter image description here

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    $\begingroup$ Place a post-it or slice of tape to mark the top of cowling @ 3:25. Then see how the top edge moves upwards after 3:27. Also note that visual cues on the wing (small shadow + the black stripes) stay the same. You may be right about the pressure change as engines spool up, but the engine mounts and fuselage will give in a little when thrust is applied, turning the engine towards the camera. $\endgroup$
    – Jpe61
    Commented Jan 13, 2020 at 21:07
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As the aircraft starts its takeoff roll, you can see the window frame flex in and out. One possibility is that as the engine spools up, it's creating a low pressure region around the entire front cowling, causing the fuselage to bulge out slightly. Rather than the engine distorting or seeming to turn in, it is the side of the fuselage flexing.

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I think it's a camera distortion caused by vibrations: an effect you can see when some harmonics are prox to shutter frequency and can cause image distortion. In fact at 1:04 you have the same effect during pushback with engine off yet.

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  • $\begingroup$ hi and welcome. that effect is not what is present at 3:24, which is being asked about $\endgroup$
    – user14897
    Commented Jan 14, 2020 at 19:05

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