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While driving, an unlucky butterfly was about to hit my windshield. But instead of splattering, it sort of glided smoothly upwards across the surface of my windshield. The butterfly was clearly not skillfully dodging the car because the required speed to do so would be too much for the little fellow. There was like a repulsive force between the butterfly and the screen. Though I don't have any video evidence, I guess at least some of you must have experienced this.

Butterfly avoiding untimely death

Illustrative image

How does this happen? I feel like the answer lies in the nature of airflow around the car, not sure how exactly, though. This is a Fiat punto, and it does have a fairly aerodynamic shape with the windshield about 45 degrees slanted.

AlphaLife
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3 Answers3

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While @Nick gave a good answer (“air flows up and around the car”), that answer by itself would mean no bugs ever hit the windshield - and we know that is false. So what’s the difference between a bug and a butterfly?

If we look at the problem in the frame of reference of a stationary car, there is an airstream moving towards it, and in that airstream there is a small solid object (bug, butterfly).

From the frame of reference of the object, it is in a body of air that suddenly moves up. The question then becomes - will the object move with the air stream? This depends on the size and strength of the wings and the mass of the object.

If you are a bug with small wings that you have to beat very fast to stay in the air, then most of your “lift” is generated by the motion of your wings. If the air moves a bit faster, it won’t change the lift you experience by much (because your wings were moving so fast to begin with, the extra speed of air over the wings is small). So you will go splat.

If you are a butterfly, you get enough lift without moving your wings much (because the wings are big). So if the air starts moving faster, it will tend to carry you with it.

Lucky quirk of evolution - small body with big wings will avoid fast-moving objects (though I am pretty sure that was not the main reason why butterflies evolved to have large wings...)

Floris
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You're right that the butterfly wasn't skilfully avoiding the car. It didn't need to because it was carried over your car by the air flowing around the car. If you look at the image below of the streamline pattern around a car, we see that the streamlines start to bend upwards before they reach the front of the car. If the butterfly was at the level of your windscreen a few metres in front of your car, it would happily be carried up and over your car by the airflow.

enter image description here

The exception would be if the butterfly was about the height of your front bumper. We can see from the streamline pattern that there must be a stagnation point here, in which case the butterfly would end up splatted. It might also not have a fun time being carried under your car either if it was even lower.

Nick
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    The picture's a bit misleading. Unlike a wind tunnel, In the real world the engine is running, sucking in air both for combustion and for cooling (radiator & fan). Which means that instead of a stagnation point, you have airflow into the engine compartment, so your butterfly or other insect gets splatted against the radiator. (See the million or so Google hits on how to remove bugs from your radiator :-)) – jamesqf Apr 25 '21 at 16:51
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    Good answer, but it doesn't explain why other bugs do go splat. – Dan Henderson Apr 26 '21 at 14:36
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    The other exception would be if the butterfly was less buoyant in the air and/or was carrying more momentum (ie: june bug, mayflies, crickets, etc). If you live in an area with a lot of insects, it's hard to ignore that plenty of them end up pasted to the windscreen and the face of the vehicle. – J... Apr 26 '21 at 14:56
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    @Dan Henderson: The butterfly has a larger wing area, thus more force exerted by the upwards deflection of the air. Thus the butterfly (or leaves, pieces of paper, &c) is carried along with the deflected air, while bugs with smaller wings, or the sand & gravel blowing off the dump truck you're following, don't experience enough force to keep them from hitting your windshield. – jamesqf Apr 26 '21 at 16:09
  • @jamesqf It's still "stagnation" in the vertical axis, which is what matters for getting diverted around the vehicle. – Graham Apr 27 '21 at 15:12
  • @jamesqf yes, though that picture seems to be of a volkswagen XL1, which doesn't appear to have a design that admits air into the engine compartment at the likely stagnation point. Also didn't quite understand the part where you implied a wind tunnel doesn't represent the real world; what's the point of wind tunnel testing if it doesn't? – Caius Jard Apr 28 '21 at 11:59
  • @Caius Jard: Wind tunnels AFAIK just do static testing of the aerodynamics of the body shape. For a real-world test, it'd have to include a dynamometer, so the engine could be running at the normal power levels for the speeds being tested. – jamesqf Apr 29 '21 at 02:41
  • I remain uncertain that the amount of air inducted through the engine would be anything approaching significance compared to the volume of air flowing over the vehicle. A vehicle 1.8m wide, 1.5m high, with a 2 litre engine running at 2000rpm and travelling at 40mph (17.88 m/s) will draw 4000 litres of air every minute through the engine, and have to move through a cuboid of 2,700,000 litres of air. I just can't see the induction effort significantly altering the experiment airflow or providing worthwhile thrust to the car(for comparison a jet engine shifts around 51 million litres a minute) – Caius Jard Apr 29 '21 at 04:50
  • In short, a car parked in a wind tunnel of 40mph air is to all intents and purposes exactly representative of a real world car doing 40mph through still air, regardless of things like engine state (but nothing stopping the wind tunnel people sitting the car and revving it, or indeed putting it on a rolling road if you want to turn the wheels and model eg airflow over the brakes...). The benefit of the wind tunnel being you don't have to run really fast to check if eg the wing mirror is making a buzzing noise in the real world.. – Caius Jard Apr 29 '21 at 04:55
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It depends upon what approaches the window. A piece of paper is easily carried along by the flow of air that arises when the air meets the front of the car. A stone thrown in front of the window will barely notice the flow of air and it will hit the window as if no air were present (if thrown in the right way).

Deschele Schilder
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  • I've often seen this with leafs – Mooing Duck Apr 27 '21 at 21:59
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    @MooingDuck Exactly! Mixed with butterflies and snowflakes this gives a wonderful sight! Maybe add a little mist too... Though this makes the situation a bit more complicated as the mist is a vapor and not a solid. – Deschele Schilder Apr 27 '21 at 22:01