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July 10

Butterflies, part 2

Follow-up question: in https://www.youtube.com/watch?v=Kx--KBUQZ58 this video, what species (if any real species) is the butterfly on the lower left? I know the one on the upper right is Limenitis arthemis, but what's the other one? 2601:646:8A81:6070:9DD0:7736:55DB:676 (talk) 02:07, 10 July 2022 (UTC)[reply]

Limenitis arthemis var. photoshopei. Abductive (reasoning) 15:28, 10 July 2022 (UTC)[reply]

This deserves an award for 'Best Answer of the Month' or something similar. {The poster formerly known as 87.81.230.195} 90.205.225.65 (talk) 20:22, 10 July 2022 (UTC)[reply]

Good one! :-D 2601:646:8A81:6070:755E:5B1D:F097:AE65 (talk) 02:45, 12 July 2022 (UTC)[reply]

Technically possible (but maybe difficult to accomplish) or energy out of thin air?

Recently a Spanish company proposed a system to save up to 25% of fuel: adapt an airfoil to a truck (on other vehicles that's possible too) to reduce the weight of the same, and hence save energy.

Article in Spanish: [1],

Images of the prototype: [2], [3].

My first gut reaction was that the system would still need to accomplish the same amount of work.

But is this a consideration in good design? Do engineers consider to improve rolling resistance, even if this worsens the vehicle's aerodynamics?

Do we need empirically data to dismiss this or would basic physics understanding suffice?

Could this at least work in an unrealistic scenario, where the wind always blows in the same direction, and the truck always carries the same load? Bumptump (talk) 16:58, 10 July 2022 (UTC)[reply]

A weight-reducing (i.e. positive lift) aerofoil would reduce road resistance, but (particularly when encountering headwind gusts) might also reduce the tyres' traction on the road sufficiently to dangerously reduce the vehicle's braking and even steering ability. Skidding (expecially on ice and snow) and aquaplaning would presumably also become more likely. Wind can sometimes blow trucks on to their sides (a particular hazard when crossing high bridges) so the forces involved even when the 'wind' is caused by the truck's forward movement are very significant.

Unrealistically the maths might work, but in a real-world scenario with non-straight roads, varying wind speeds and directions and other vehicles in proximity, I would have thought not. However, someone has spent money developing and presenting the idea, and they presumably have some grasp of the realities.

What such a configuration might also achieve is reduction of the turbulence and drag normally caused by the discontinuity between the vehicle's cab and cargo sections, which is something designers have been doing for decades while striving to avoid creating lift: I wonder if drag reduction is actually its primary aim? {The poster formerly known as 87.81.230.195} 90.205.225.65 (talk) 20:21, 10 July 2022 (UTC)[reply]

The weight of an empty truck is already considerable; unlike a Formula-1 race car, it does not need wings to provide downforce. Much of the energy a loaded truck uses up is spent on overcoming roll resistance, the primary cause of which is the energy of deformation of the tyres, which increases with weight. This energy does not produce useful work; it is lost. Spending some energy on decreasing the weight may result in a net gain. Compare the case for mounting wheels under a sled dragging a heavy weight along a rough surface. Without doing the maths, you'll agree that this can save energy. --Lambiam 22:06, 10 July 2022 (UTC)[reply]

It is a truly terrible idea. Typical wing like objects on vehicles have an L/D of about 5 at best. That means you'd add 1 kgf of drag for every 5 kgf of weight reduction. Tires on the other hand have an L/D of of about 60 Greglocock (talk) 00:24, 11 July 2022 (UTC)[reply]

For an extra bit of fun, do it for a "driverless" vehicle. --←Baseball Bugs ^{What's up, Doc?} carrots→ 08:04, 11 July 2022 (UTC)[reply]

Lift-to-drag ratio? So keep adding tyres, and the truck will take off? --Lambiam 08:08, 11 July 2022 (UTC)[reply]

Not take off, but the adhesion weight (applies to road vehicles as well as rail) of each set of tyres might become sufficiently low that skidding (including wheel slip and slide) would become a danger: this is why such vehicles running unloaded now often raise one pair of wheels off the ground (see Tractor unit#axle). {The poster formerly known as 87.81.230.195} 90.193.130.14 (talk) 18:22, 11 July 2022 (UTC)[reply]

A slip can occur when the coefficient of friction times the acceleration of gravity is less than the acceleration you attempt to attain. Mass or number of axles don't appear in the equation, but having more wheels will smooth out variations in coefficient of friction, which may be relevant, and a higher ground pressure from having fewer wheels may prevent aquaplaning in circumstances where this is likely to happen. For forward acceleration (where a slip isn't very dangerous), the adhesion weight comes into play. During sideways (centripetal) acceleration, mass doesn't matter. For backward acceleration (braking), a lightly loaded vehicle may have its brakes adjusted so that the same setting of the brake level/pedal leads to the same deceleration (it's done on goods trains, but I don't know about road vehicles). Lifting one braked axle off the ground is one way to adjust the brakes. For forward acceleration, lifting an unpowered axle off the ground raises the slip limit as it increases the adhesion weight, which may be useful as the same engine torque can, with light loading, provide more acceleration. The other advantage of lifting an axle is that, with multiple non-steering axles, there's always some slip when cornering. Lifting an axle reduces this slip and thereby wear and resistance. PiusImpavidus (talk) 10:05, 12 July 2022 (UTC)[reply]

Our article on rolling resistance says that the rolling resistance coefficient of truck tires on a hard road is typically 0.0045 to 0.0080. Our article on gliders says that good gliders may have a lift-to-drag ratio of around 60, giving a drag-to-lift ratio of 1/60=0.017. 0.017>0.008, so even a very good wing will add more drag than the rolling resistance it saves. On a dirt road this may change.

Anyway, there's no way you can put a significant wing on a lorry without exceeding legal limits for vehicle dimensions. PiusImpavidus (talk) 08:35, 11 July 2022 (UTC)[reply]