3305: Ground Effect

Ground Effect

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Title text: Runners looking for aerodynamic advantage typically wear sneakers because some fancy dress shoes can create wingtip vortices.

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Explanation

This is one of 53 incomplete explanations: We don't usually have a different subsection for extra material beyond the explanation of the comic. The 'detailed discussion', while potentially interesting, is extremely digressive and not really explaining the comic. Remove superfluous detail and incorporate anything directly pertinent into a cohesive explanation. You can help by editing the explanation!

The ground effect is an aerodynamic phenomenon, which reduces lift-induced drag when an airfoil is close enough to the ground (typically within one wingspan) that circulation is significantly affected. Some vehicles (as well as many birds and bats) make use of this effect to increase their efficiency of operation.

This comic uses a different interpretation of the term 'ground effect', involving the necessity of running on the literal ground. The runners on the right, in (this definition of) 'ground effect', are making contact with the ground and can push against it, to avoid falling and generate forward motion. In order to do this, all of them must, trivially, be within their own body length of the ground. The 'runners' on the left, not within 'ground effect', float helplessly above the ground, unable to push themselves forward. This could be considered to be reducing their 'efficiency' to zero. This process has little to do with aerodynamic effects. In reality, this state would not last for long, since gravity would cause them to fall until they were in contact with the ground.

The title text is another pun. In airplane aerodynamics, wingtip vortices are longitudinal vortices (horizontal whirlpools parallel to the airplane fuselage) that a wing creates near the tip when it generates lift. These create unhelpful drag without adding lift, so modern airplanes generally have winglets at the end of the wings to make these vortices smaller. In fashion, there is a style of dress shoe called wingtips with small holes in the upper leather in a pattern that somewhat resembles a bird's wings. In reality, runners would not wear dress shoes because they would be heavy, uncomfortable, and restrictive, and they typically have soles that do not provide sufficient grip on the running surface.

Ground effect is very relevant for aircraft: excess airspeed when landing makes the aircraft float along the runway, potentially causing a late and dangerous touchdown. When taking off, the ground effect reduces lift. Without enough airspeed to create non-ground effect lift, the airplane will stall as soon as it climbs out of the ground effect cushion. When taking off from short or soft (grass or water) runways, airplanes will use ground effect like a ground effect vehicle and accelerate more quickly to flying speed before initiating climb.

Incidentally, a reverse effect is known in ships. When a ship accelerates in shallow water it is sucked towards the bottom, and its draft increases.

Transcript

[Five Cueballs are shown in the panel. At the left, two of them are floating in the air, flailing their arms and legs as they float at least 12 feet off the ground. At the right, three others are running to the right in a normal way. A dotted line is shown above the tops of these three runners' heads, and a regular line below them indicates the ground. An arrow points down toward the dotted line and another arrow points up toward the regular line behind the runners.]

[Caption below the panel:]

The "ground effect" explains why sprinters can run most efficiently if they stay within roughly one body length of the surface.

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Discussion

And as the drag is insufficient for the runners depicted above the ground to actually stay above, they would ultimately fall. 2001:4C4E:1C0F:1B00:3056:6C17:EB81:88B4 11:33, 1 October 2026 (UTC)

Ground effect in the context of racing is essentially an issue for cars. While it can be explained in terms of planes, that's a bit of a weird thing to do. F1 cars etc. exploit ground effect to increase cornering grip, which seems like a far more relevant reference point for "fast moving racing on the ground that's being described in terms of ground effect" than concentrating on something that flies through the air. Yorkshire Pudding (talk) 13:17, 1 October 2026 (UTC)

Except that the comic appears to be suggesting that less 'efficient' runners do routinely fly through the air. And the 'within one body length' bit does seem to be a direct corollary to within one wingspan. Not to say that it's not worth adding something about that as a genuine case where true ground effect is important for ground-based activities. 86.22.132.121 13:45, 1 October 2026 (UTC)

Just another case where 2 different concepts in the same field use the same term. Ground effect in cars refers to the generation of downforce by creating low pressure zones below the car. While it is a lower-drag method of producing downforce compared to traditional automotive wings, ground effect in racecars does not directly reduce drag. 2601:646:A202:38E0:EBE1:97E5:7E94:FF02 21:01, 1 October 2026 (UTC)

OK... But that doesn't change the fact that "ground effect for land-based racing" is a car thing, not a plane thing, so explaining ground effect exclusively in terms of planes, when the comic is about land-based racing and not about flight, seems to be missing the point of which of those similarly named concepts is worth mentioning.Yorkshire Pudding (talk) 00:18, 3 October 2026 (UTC)

I'd been floating in the air during my marathons till I saw this comic, very good strategy.--Death cab for me (talk) 16:01, 1 October 2026 (UTC)

In the second sentence of the second paragraph, I doubt that 'friction of the legs...on the ground' is a normal part of running. Friction of the feet (or shoes) is essential. At the end of the third sentence, I wonder about 'generate momentum'. 'Generate forward movement' seems more accurate to me. Nekoninda (talk) 16:19, 1 October 2026 (UTC)

The section 'Detailed discussion of ground effect' contradicts a few of the standard explanations of airfoil lift (explanations that may be wrong). For example, 'An anemometer (airspeed meter) sees slower airspeed above the wing' contradicts the Bernoulli Effect, which is normally given as a part of how an airfoil generates lift. In the previous paragraph, I don't understand the assertion that 'The air molecules left to hit the sloping top are those moving down and faster forward than the wing.' Classical explanations say that the wing is moving faster than most of the air molecules above and below it. Nekoninda (talk) 16:34, 1 October 2026 (UTC)

Release time

This comic is listed as a Wednesday comic, but it was published on Thursday... (UTC, EDT and EST timezones)--1234231587678 (talk) 00:52, 2 October 2026 (UTC)

i fixed it and added a trivia section. Someone should probably double check that this was actually posted on September 30 and not October 1. The explain xkcd bot created this explanation on October 1 but from the web archive you can see that the previous comic was till on the site on October 1st, so this comic seemingly hadn't come out. How do we treat time zones here? FaviFake (talk) 18:10, 2 October 2026 (UTC)

The Archive list for the comics lists the date as 2026-09-30. So unless the true intention is hidden by some pesky pre-automated system used by Randall presuming it should fill the scheduled slot, I'd pretty strongly assume that it was just a Wednesday comic that was delayed for trivial reasons and Randall still added deliberate data such that he considers it entirely a Wednesday one. 92.23.3.72 20:45, 2 October 2026 (UTC)

could someone check if any of the other comics posted outside of the main Monday Wednesday Friday schedule also have had this kind of data added to them? FaviFake (talk) 20:48, 2 October 2026 (UTC)

More to the point, check the other comics that were already noted as being "A very late <release-day> comic". The last one I noticed/remember was not more than a couple of months ago.

But a quick selection of the comics that are currently noted as 'off the MWF cycle', going back as far as I wanted to before getting bored is:

• 961: Eternal Flame (Thursday) - follows a Wednesday, followed by a Monday, making it effectively an 'early Friday'.
• 1037: Umwelt (Sunday) - April 1st. Came after a Friday comic, the next was Wednesday, so an 'early Monday'.
• 1350: Lorenz (Tuesday) - instead of the Wednesday comic (published between Monday and Friday) as a complex dynamic comic
• 1551: Pluto (Tuesday) - forward-shifted from Wednesday for topical-event reasons?
• 1642: Gravitational Waves (Thursday) - follows a Wednesday, followed by a Monday, making it effectively an 'early Friday'.
• 1608: Hoverboard (Tuesday) - Replaced that week's Wednesday, probably with book-promotion schedule in mind.
• 1975: Right Click (Sunday) - Another April 1st that took the form of a 'deliberately early Monday'.
• 1733: Solar Spectrum (Thursday) - came after a valid (and HUGE) Monday comic, with a Friday being the next one. Trivia 'explained' this in both this and the Friday comic.
• 2198: 2198: Throw (Tuesday) - actually between Monday and Friday, and a dynamic comic, so a potentially early 'complicated release', unusually.
• 2289: Scenario 4 (Saturday) - Unusual addition.... Friday had an April 1st dynamic comic, so perhaps this was considered a 'delayed normal Friday comic'?
• 2300: Everyone's an Epidemiologist (Thursday) - preceded by a valid Monday, followed by a valid Friday. Given an "unclear if this is a late Wednesday" Trivia.
• 2445: Checkbox (Thursday) - an April Fool that followed a valid Wednesday and was followed by a valid Monday, so was really more lie "an intentionally early Friday" that actually shifted the schedule.
• 2672: What If? 2 Flowchart (Tuesday) - Publishede between a Monday and a Friday, probably brought forward from Wednesday for its book-promotion.
• 2717: L6 Lagrange Point (Tuesday) - Actually between the Monday and the Friday of a Christmas Week, so 'early Wednesdaay'?
• 2986: Every Scientific Field (Tuesday) - In line with being a 'late Monday', but an example of officially being noted by Tueday's date
• 3241: Horizontal Stabilizers (Tuesday) - A 'late Monday' that is actually noted as officially Monday, by date.
• 3286: Particle Physics Equipment (Tuesday) - Between a Friday and a Wednesday, but with a "Note: Monday's comic will go up on Tuesday this week." on the Website, so was actually acknowledged as late.

...so many different treatments/situations, and acknowledgements. I leave it up to you to check the official xkcd.com Archive Dates.

But this doesn't count the ones that were just given the presumed actual MWF spot that they were (typically) slightly too late for. You're welcome to scan through page Histories to identify those possible examples (excepting where the BOT wasn't active, or other reasons delayed the check-and-upload, of course). You may have to account for Randall being on the eastern edge of the US, timezone-wise. Though I think when he was book-promoting in Europe he occasionally released a day's comic at a time that was his current morning, sometimes slightly before most of the US was that day. Possibly even more so if he ever went east (or west!) to Australia/etc. I've no idea if he's ever posted 'correctly' but from, say, the Hawaiian TZ that he might have been in for a trip.

But you already should have known that the release-times are very variable. Even the 'normal' mid-range seems to vary between early in his mornings and late in his evenings, and "so late that it's technically tomorrow" isn't unknown, but could still be possibly before he finally goes to bed, post-midnight.

As someone who does some of my own best work (and, to be honest, some of my worst work, too) after midnight at the end of a long day, good luck properly and accurately classifying each and every 'slippage' to the satisfaction of all concerned. 92.23.3.72 22:28, 2 October 2026 (UTC)

Yeah alright. Thanks for looking into it. I don't think I want to start thinking about comic release dates and timezones right now (or ever). For all intents and purposes, this was a wednesday comic. FaviFake (talk) 22:45, 2 October 2026 (UTC)

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i personally feel we should remove the extra section, and just link in the corasponding Wikipedia article to the word ground effect 73.132.167.116 14:11, 3 October 2026 (UTC)
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