Wikipedia:Reference desk/Archives/Science/2019 November 27

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November 27

Helicopters spending more fuel hovering

Helicopters spend more fuel (much more actually) while hovering (per unit of time) than while flying. Is that something that happens due to helicopters being the way that they are or can we generalize this to any self-propelled flying vehicle due to physical laws? That is, is hovering always less efficient than flying, in the same way that going uphill is less efficient than going downhill? Or, is this per design, in the same way that, say, that an engine can have maximum torque at 2,000 RPM and others at 5,000 RPM? --C est moi anton (talk) 17:12, 27 November 2019 (UTC)

Who told you that Helicopters use much more fuel hovering than flying? Do you have a source for this claim? --Guy Macon (talk) 18:00, 27 November 2019 (UTC)

The graph of fuel consumption versus the velocity and altitude of the helicopter flight?C est moi anton (talk) 18:08, 27 November 2019 (UTC)

Any vehicle is likely to have an optimal fuel consumption range. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:59, 27 November 2019 (UTC)

And how many vehicles spend less fuel while moving than while quiet?--C est moi anton (talk) 19:43, 27 November 2019 (UTC)

• Note that this isn't a sudden increase when in the hover itself, it's more of a gradual shift from when below moderate airspeeds.

It's the asymmetry of the rotor moving in a moving airstream, vs. a still airstream. This depends on the airspeed of the rotor, which of course stays constant around the rotor disc (proportional to radius), during a hover in still air. If there is some forward airspeed, then the rotor airspeed (and drag) when the blade is moving sideways (forwards or rearwards position) is just the same. But at the same time, the airspeed on the advancing rotor blade is increased (rotational speed + airspeed) and the airspeed on the retreating blade reduces (rotational speed - airspeed). Near the centre of the rotor, on the retreating side, where the blade's linear speed is least, this can reduce to zero and even becomes negative. That reduces the drag on the blade, but also reduces lift. To avoid the helicopter rolling, the incidence of the blades has to be adjusted as they rotate, so as to compensate. Overall, there's a non-linear effect. Efficiency gains more from the increased airspeed on the advancing side than it loses on the retreating side, so overall there's a gain with airspeed.

There's also a smaller effect from the tail rotor. In the hover, this is needed to counteract torque reaction and avoid yaw, entirely by itself. However when the helicopter is moving forwards, the static tail surface also assists.

Other effects may apply, depending on the shape of the helicopter. Streamlining is optimised for performance in the cruise, less so below this. That's important for helos because not only is the airflow faster, but it changes direction from the pure vertical of a hover to something more astern. There may even be a dynamic lift effect for some (if there's any sort of stub wing shaping) and they can give lift from airspeed rather than the rotor disc, which tends to be a more efficient way to do it. Andy Dingley (talk) 19:25, 27 November 2019 (UTC)

I think the helicopter is designed to work best when moving, and that include

The cabin intercept and hinder the downward flow of air when hovering, more than it does moving (the airflow is then somewhat rearward

aerodynamics of the craft is designed for a forward movement

it may incluse some feature of a fixed wing, delivering better lift that the rotor (as evidenced by the better performance of planes Vs helicopters)

Now, you could design it otherwise, with best aerodynamics when hovering. It just isn't very useful

Gem fr (talk) 21:46, 27 November 2019 (UTC)

I agree with the answers above, but there is another factor which can play part in this. Many helicopters use a turbine engine. These are usually optimized to work best at a cruise speed and may work much less efficient at low inlet speeds, see Propulsive_efficiency#Jet_engines. Rmvandijk (talk) 10:51, 28 November 2019 (UTC)

You right, I forgot this obvious one: the engine produce some forward thrust, and you have to fight it back to hover Gem fr (talk) 04:40, 29 November 2019 (UTC)

That wasn't the effect I meant, I suppose that one happens too. I meant that, if we assume a fairly constant outlet speed, the efficiency of the engine goes to zero (at zero inlet speed). That would mean no suction, which is wrong and violates the conservation of mass, but the idea works for low airspeeds relative to the exhaust speed. Rmvandijk (talk) 09:23, 29 November 2019 (UTC)

The above explanation sounds expert is probably right, but I'm not surprised more fuel is used when a helicopter is still. I'd have though the air would circulate down through the rotor then up outside to the top again. If the rotor was stopped there would be a wind pushing the helicopter down until the circulatio stopped compared to what would happen if going through still air. Dmcq (talk) 12:31, 29 November 2019 (UTC)

More power is required by the rotor to hover (airspeed zero) than to fly forwards with non-zero airspeed. There are complicating effects due to proximity to the ground, so I will say more power is required to hover out-of-ground-effect (OGE) than to fly forwards slowly at the same height above the ground. Fuel efficiency is usually unaffected by a helicopter’s airspeed so I can say rate of fuel consumption is greater in the hover OGE than flying forwards slowly at the same height. The short explanation for this phenomenon is lift-induced drag.

When we investigate the power required by a helicopter rotor there are two competing processes. Firstly, as the speed of the helicopter increases up to its maximum forward speed, the parasite drag on the rotor blades and the fuselage increases, necessitating progressively increasing power. This is the same process that necessitates progressively increasing power from the engine of any road vehicle or ship.

Secondly, there is an opposing process that requires increasing power as the forward speed of the helicopter decreases! This is counterintuitive and has no counterpart in road vehicles or ships. It is due to the phenomenon of lift-induced drag. Fortunately, it can be illustrated by some simple arithmetic. Imagine a helicopter hovering out of ground effect. Let’s examine the rotor blade elements 75% of the distance out to the tip of the blade. Let’s imagine these blade elements have a speed of 300 knots due to the speed of the rotor. The angle of attack on the rotor blade is controlled by the pilot so that the lift on the rotor exactly matches the weight of the helicopter and causes the height above the ground to remain constant.

Now let’s imagine that the helicopter has accelerated to a forward speed of 30 knots. On the advancing blade our blade element at 75% is experiencing an airspeed of 330 knots; but on the retreating blade it is experiencing only 270 knots. The mean of these two speeds remains unchanged at 300 knots so it is tempting to imagine that the rotor will produce the same lift at the same angle of attack, but that is incorrect.

The lift on every blade element is directly proportional to the square of the airspeed. Assuming no change in angle of attack, the lift on our blade element on the advancing blade is proportional to 330/300 squared, or 1.21 of its original value, an increase of 21%; the lift on our blade element on the retreating blade is proportional to 270/300 squared, or 0.81 of its original value, a decrease of 19%. An increase of 21% and a decrease of 19% means a net increase of 2%. This increase in lift would cause the helicopter to accelerate upwards. The pilot wishes to maintain a constant height so she lowers the collective lever to reduce the angle of attack on the rotor blades and eliminate that 2% net increase.

Reducing the angle of attack is associated with a reducing lift coefficient; and a reducing lift coefficient causes the component of the drag coefficient associated with lift-induced drag on the rotor blades to also reduce. As lift-induced drag reduces, the power required to turn the rotor at constant RPM also reduces, and so too does the rate of fuel consumption.

As the helicopter leaves the hover and begins to accelerate forwards, the reduction in lift-induced drag is more pronounced than the increase in parasite drag. These two effects become equal in magnitude and cancel each other at the speed for minimum power. As forward speed increases even further, the increase in parasite drag on the rotor is more pronounced than the reduction in lift-induced drag. Dolphin (t) 13:12, 30 November 2019 (UTC)

How much alcohol?

The other day I was going down the production line filling up the little water bottles we use to wet sponges for cleaning soldering iron tips. Because of past experience with the water turning green I put a glug of alcohol in the water. We have Isopropyl, Methanol and Ethanol available; I usually use Isopropyl.

This got me to thinking: what is the minimum percentage of each kind of alcohol needed so that nothing - no germs, algae, mold, yeast, tribbles, politicians, etc. -- grows in the water?

I am guessing that for Ethanol the percentage will be about that of the strongest available wine. I believe that windshield wiper fluid is 50% Methanol but I suspect that is for the antifreeze effect. --Guy Macon (talk) 18:11, 27 November 2019 (UTC)

Who told you tribbles grow in water? --C est moi anton (talk) 18:44, 27 November 2019 (UTC)

You forgot about the politicians. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:59, 27 November 2019 (UTC)

Politicians grow everywhere like weeds.C est moi anton (talk) 19:32, 27 November 2019 (UTC)

I recall reading about Nicolae Ceaușescu who, because of pathological fear of poisoning or infection later in life, asked his hands to be sanitized with 90% alcohol, presumably ethanol. Perhaps the real minimum percentage needed to kill all germs is somewhat lower. Brandmeister^talk 21:43, 27 November 2019 (UTC)

Antiseptic concentration vs. killing power is not a linear or direct relationship. For example, 60-70% isopropyl alcohol is a more effective disinfectant than 90-95%. And nothing will kill everything. - Nunh-huh 00:27, 28 November 2019 (UTC)

You don't see a lot of things growing in whiskey bottles do you? (smile.) Bottles of isopropyl alcohol also tend to be notably lacking in thing growing in them. --Guy Macon (talk) 05:26, 29 November 2019 (UTC)

If things didn't grow in whiskey, there wouldn't be any alcohol in it. And bourbon makers need to be aware of which bacteria are contaminating their product. Pediococcus, lactobacillus and other bacteria change the flavor of the end product in different ways. And it's best not to read about the alcohol tolerant features of E. faecium before drinking... - Nunh-huh 23:08, 29 November 2019 (UTC)

I linked that microbe in your post, to help others not-read about a meaning of "shitty beer" other than what I drank when I was a poor college student. DMacks (talk) 23:12, 29 November 2019 (UTC)

You poor dear! Pabst? -Nunh-huh 04:41, 2 December 2019 (UTC)

Re: "If things didn't grow in whiskey, there wouldn't be any alcohol in it". see Distillation.

Re: E. faecium and alcohol "In preliminary experiments, various concentrations of alcohol and E. faecium inoculum sizes were assessed. At 'full strength' isopropanol [70% (v/v)], killing was complete and resulted in greater than 8 log10 reductions in broth culture and an inability to detect differences between isolates. However, by lowering the alcohol concentration in a stepwise fashion, we were able to identify a dynamic range in which we observed marked differences in the time-kill curves between isolates." Source:[1] (the study goes on to conclusively show that 23% doesn't reliably kill and that the tolerance is getting higher from year to year.) That's a frighteningly high alcohol tolerance for an organism that causes lots of trouble in hospitals and it's getting worse. We haven't quite gotten to the point where something can grow in a bottle of 70% isopropyl alcohol though. --Guy Macon (talk) 06:38, 2 December 2019 (UTC)

Here, when you have a strong drink outdoors in summer, especially in a cafe (where there's always a healthy supply of alcohol drinkers), don't be surprised when tiny flies arrive to feast on what you didn't drink (or if you're drinking too slowly). 60% brandy doesn't seem to faze them at all... 93.136.178.2 (talk) 21:30, 29 November 2019 (UTC)

You're talking about beverages created, or at least bottled, under sterile conditions as sterile products. You don't generally see much growing in bottled water if bottled under the same conditions, and that's 0% alcohol. What you are observing is more the fact that spontaneous generation doesn't exist, so a bottle sealed under sterile conditions with sterile contents won't grow anything. 70% ethanol (or isopropanol) is what is used in biosafety laboratories as a disinfectant. That and 10% bleach. --OuroborosCobra (talk) 22:43, 29 November 2019 (UTC)

I don't know the OP is necessarily talking about sealed bottles. I think it's true that even if you open a bottle of whiskey, let alone propan-2-ol even if you're careless and allow contaminants you're not going to get that much growing, although it's not likely to be nothing either. I wouldn't try this with UHT milk. Of course even with water you're probably not going to get that much in a lot of cases, in part because while it may not kill stuff, plain water is not a great growth medium either. Whiskey would actually be far better in that regard. There are obviously lots of factors at play here, like what can grow in the medium, how fast, how easy it is for the those to be picked up from the environment, etc/ 19:00, 1 December 2019 (UTC)

I have a lot of experience with containers of water with a small "squirt" opening on top. Dozens and dozens of them, used to wet soldering sponges on a production line. Use tap water and the water turns green in days. Use distilled water and it takes weeks. A little alcohol (maybe 5% to 10%) added to either and the water stay clear indefinitely. I figure that the algae can't live on nothing but 100% pure water but that it doesn't take much exposure to the environment for a little dust to fall in and add needed minerals. --Guy Macon (talk) 06:38, 2 December 2019 (UTC)

Why does this peripheral drift illusion stop working when rotated 45°?

Primrose field optical illusion

When viewed fullscreen, moving waves appear on this checkerboard. Yet when I rotate it 45 degrees, such as on a mobile phone, the illusion stops working for me. It works again when rotated 90 degrees. Is there a reason for this?

Thanks,
cmɢʟee⎆τaʟκ 20:05, 27 November 2019 (UTC)

For me, it even stops if I turn the head about 45 degrees. Very interesting. I second this question ;-). --Stephan Schulz (talk) 20:40, 27 November 2019 (UTC)

Chirality is conditioning most everything we experience (test yourself on the picture with shells at the bottom of the article) --Askedonty (talk) 21:04, 27 November 2019 (UTC)

Try doing this closing one eye. Gem fr (talk) 21:25, 27 November 2019 (UTC)

Rather reading Popeye. It's Screen reading and Vision span (which could be improved). we are highly trained to exercise viewing along recurring patterns. --Askedonty (talk) 21:30, 27 November 2019 (UTC)

Works just as well for me when I rotate 45 degrees. Dmcq (talk) 11:59, 29 November 2019 (UTC)

It seems to work best on me when the picture is moving, like if I'm zooming in with my head or scanning around with the eyes. When I'm simply staring at the center of the picture it only seems to ripple when my stare wanders a little involuntarily. At 45 degrees only the connections between perpendicular lines ripple. 93.136.178.2 (talk) 21:34, 29 November 2019 (UTC)

Thanks so much, everyone. I think movement causes attempts to match left and right eye views to vary between correct and wrong matches, causing the waving. When rotated 45°, it always matches correctly, so I stop seeing the illusion. cmɢʟee⎆τaʟκ 22:06, 30 November 2019 (UTC)

What the hack, after reading this and tilting my head, trying with one eye closed then the other, now it's not rippling no matter how I look at it! אילן שמעוני (talk) 04:08, 2 December 2019 (UTC)

after 3 minutes it's back to normal. Interesting. אילן שמעוני (talk) 04:13, 2 December 2019 (UTC)

What are these creatures?

Small animals in a mangrove forest

I saw these little animals in a mangrove forest in Shenzhen. They're maybe 2-3 cm long and look something like fish, but I didn't see them swimming, just sitting on the rocks, using their short fins to crawl, and skipping across the surface of the water. I couldn't get a picture from up close, because they jumped away when I approached. Any idea what they are? —Granger (talk · contribs) 23:49, 27 November 2019 (UTC)

To my untrained eye they look to be in the family that includes Mudskippers. Other editors may have a more accurate link for you Granger. MarnetteD|Talk 23:53, 27 November 2019 (UTC)

That looks right—thank you! If anyone can identify a genus or species that would be great, but I'm also satisfied with the general identification. —Granger (talk · contribs) 00:10, 28 November 2019 (UTC)

Hard to be sure, but barred mudskipper looks likely. Mikenorton (talk) 08:37, 28 November 2019 (UTC)