# Spiral design in centre of front of jet engine **URL:** \ **Category:** Factual Questions\ **Created:** [July 26, 2017, 12:22am UTC](https://boards.straightdope.com/t/spiral-design-in-centre-of-front-of-jet-engine/792125 "2017-07-26T00:22:08Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Canadjun](https://avatars.discourse-cdn.com/v4/letter/c/76d3ee/32.png) [@Canadjun](https://boards.straightdope.com/u/Canadjun)\ **Post date:** [July 26, 2017, 12:22am UTC](https://boards.straightdope.com/t/spiral-design-in-centre-of-front-of-jet-engine/792125/1 "2017-07-26T00:22:08Z")
I don’t know whether this applies to all engines on large passenger jets, or whether I tend to see mostly certain engine types on TV that just happen to have that design, but regardless; what is that little spiral for that can be seen in the centre front of jet engines? Is it just to make it more obvious when the engine hasn’t quite stopped spinning? If so, what a spiral? Why not a line or an off-centre dot? You can see what I mean in [this](http://pds.exblog.jp/pds/1/201403/17/71/f0310771_1304676.jpg). ---
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [July 26, 2017, 12:33am UTC](https://boards.straightdope.com/t/spiral-design-in-centre-of-front-of-jet-engine/792125/2 "2017-07-26T00:33:36Z")
I suspect it’s like you say, just to make easy to tell that it is still moving. But also it is good to hypnotize you into purchasing $8 beers. ---
**Author:** ![Dickerman](https://avatars.discourse-cdn.com/v4/letter/d/8baadc/32.png) [@Dickerman](https://boards.straightdope.com/u/Dickerman)\ **Post date:** [July 26, 2017, 12:34am UTC](https://boards.straightdope.com/t/spiral-design-in-centre-of-front-of-jet-engine/792125/3 "2017-07-26T00:34:36Z")
Yes, it’s an easy to see indication that the engine is spinning. A spiral is more continuously visible from different angles than a line would be. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [July 26, 2017, 8:39am UTC](https://boards.straightdope.com/t/spiral-design-in-centre-of-front-of-jet-engine/792125/4 "2017-07-26T08:39:56Z")
There are a number of reasons but it is primarily a visual indication for ground crew. The primary reason is for the safety of the ground crew but on some older models there is a mechanical reason. Some older model engines will turn like a windmill on high wind days in the direction opposite of the normal operating rotation. It is mechanically unsafe to introduce fuel to the engine when this is happening and the spiral provides a reliable way to see the direction of rotation at the low RPMs. A few manufactures also use to claim that it helped prevent bird strikes but that claim has failed to be demonstrated in actual studies. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [July 26, 2017, 8:46am UTC](https://boards.straightdope.com/t/spiral-design-in-centre-of-front-of-jet-engine/792125/5 "2017-07-26T08:46:12Z")
> [@rat\_avatar](#): > > A few manufactures also use to claim that it helped prevent bird strikes but that claim has failed to be demonstrated in actual studies. Bwuh? How the heck? Surely bird strikes only matter when the engine is running and when it’s running it’s spinning so fast that any design is going to be a blur? Edited to add: I should make clear that I appreciate that you are not advocating this position but how on earth did the manufacturer’s argue it? ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [July 26, 2017, 12:40pm UTC](https://boards.straightdope.com/t/spiral-design-in-centre-of-front-of-jet-engine/792125/6 "2017-07-26T12:40:56Z")
I think he was joking about the bird strikes. Its not possible to make the moving blades unblurry. But they do have the centre dot and bright paint and so on to help show them up. They paint the externally explosed blade part so that annyone can easily see its stopped, or moving very slowly (such as if asked to roll it over for some reason, but don’t spin it up fast ???) They can see the paint and be sure it is stopped. That way, the different is easily spotted. Only if they can see the spiral do they think that the engine is stopped. If they can’t see a spiral sitting still, they know its not sitting still. compare this to traffic lights. Green light = stopped spiral red light = anything else. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [July 26, 2017, 4:05pm UTC](https://boards.straightdope.com/t/spiral-design-in-centre-of-front-of-jet-engine/792125/7 "2017-07-26T16:05:02Z")
> [@Princhester](#): > > Bwuh? How the heck? Surely bird strikes only matter when the engine is running and when it’s running it’s spinning so fast that any design is going to be a blur? > > Edited to add: I should make clear that I appreciate that you are not advocating this position but how on earth did the manufacturer’s argue it? Cite: As it is disproven I had to resort to [archive.org](http://archive.org) > **[facts\_100.pdf](https://web.archive.org/web/20061019190934/http://100.rolls-royce.com/files/852/facts_100.pdf)** > > 1387.32 KB #10 > [@](#): > > Our aerospace engines have swirls > painted onto their spinners in > order to indicate when the engine > is rotating while on the ground. > In flight these swirls flicker as the > engine rotates at high speed, > scaring birds and allowing them > to fly clear of the engine.