Why Cant Commercial Airplanes Go Faster? (2012)
wired.com
wired.com
1) reduce weight (fuselage, engine, wings, have the passengers diet first, ...) 2) increase engine fuel efficiency, displace more air for fewer hydrocarbons 3) (in the article) reduce drag on surfaces that don't generate lift 4) lower cross section (a longer, thinner tube), of course, still needs structural soundness
The lower cross section would have the additional advantage that it would make the plane fly faster.
So in a way, the article is "wrong". You can make a faster plane, assuming you can solve all the issues that the obvious modifications bring : longer, thinner and lighter fuselage and wings, combined with similar efficiency engines (of course they'd have to have that similar efficiency at higher speeds).
Another way would be to fly (much) higher. But then the cost of climbing high would start to offset the gains from flying faster.
Does anyone know of a similarly simple explanation of how much energy a vehicle must use relative to it's weight?
This doesn't change the conclusion; you still end up with a low spot on the graph. It is just that the graph looks more like a parabola.
To go from point a to point b at a cruising speed of v, you need to speed up to v, maintain v, and brake to a complete stop. Analyze each separately:
1. Speeding up. Since this takes so little distance (compared to the total length), let's ignore resistance. This speedup takes KE = ½·m·v² of energy to do.
2. Maintain speed. This means spending energy to resist air friction (drag) and ground friction. Drag is ½·ρ·v²·Cd·A [0]; friction is (at most) μ·m·g. Energy is force × distance, and since the distance at top speed is effectively constant, it's going to be O(m + v²) in this case. [1]
3. Braking. You can theoretically recover -½·m·v² here, but most of the energy is going to be lost to heat.
So, when you sum these up, the energy it takes to drive a vehicle a certain distance is linear in mass and quadratic in speed.
[0] The drag coefficient, Cd, is really weird. At low speeds, it's actually proportional to 1/v instead of a constant (i.e., drag is linear in velocity, not quadratic).
[1] Yeah, the article says that energy is proportional to v³. It's actually wrong--the power is proportional to v³. Whether or not the total energy is proportional to v³ or v² depends on what you hold constant: if you hold total time constant, it's v³, but if you hold distance, it's v².
Personally, I think they're facets of the same thing, but I don't know anything about it. Beyond that the piece seemed a bit of a mess to me. The content has not aged well. I usually expect Wired to do a better job of conserving the state of what they publish.
They did with limited success. https://en.wikipedia.org/wiki/Concorde
You can't fit enough people in first or business class to fill a plane on most routes, and you simply can't go fast enough to make speed premiums really worth it. Note that going to an airport usually requires on the order of 2 hours of hassle before accounting for flight time, which means that you've already lost a day of work in practice. No, unless it's a long-haul flight, there's little benefit to faster speeds, and the Concorde was never really profitable.
Should be something like "Can We Build a More Efficient Airplane? Not Really, Says Physics", not "Why Cant Commercial Airplanes Go Faster?"
This is wrong. Planes actually fly by stretching air over their wings (creating low pressure) and the higher pressure air below pushes the plane up (to fill in the void) - So planes fly by moving air up.
See Bernoulli Principle (https://en.wikipedia.org/wiki/Bernoulli%27s_principle)
PS: The reason most commercial jets cruise at ~585mph (or 85% of Mach one) is because of the huge amount of energy required to break the speed of sound.
Edit: I've you're going to downvote, you can at least comment.
The air-pressure explanation and the "throwing air" explanation is basically the same thing.
See: http://home.comcast.net/~clipper-108/lift.pdf
Or:
http://physics.stackexchange.com/questions/290/what-really-a...
The problem in these discussion is "Bernoulli" is used to refer to many things, some legitimately physical (mass continuity) and some not (equal time of passage). I like FYFD's point that what we're discussing is how to explain lift to laypeople, not how to calculate and analyze it (which is done every day in order to design aircraft): http://fuckyeahfluiddynamics.tumblr.com/post/121024503205/ev.... We (mechanical/aerospace engineers) know the equations that govern fluid motion and how to solve them, whether you abstract that with Bernoulli/Newton/Kutta-Joukowsky/etc. is up to you and not that significant.
In fact, Bernoulli applied correctly explains all of the lift. The Newton approach also explains all of the lift. They are just two different ways of looking at the same thing. Bernoulli comes from applying conservation of energy. Newton comes from applying conservation of momentum.
See this discussion from NASA: https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html
What's puzzling is that it is not at all uncommon in other areas of physics for there to be different ways to explain something, and this causes no problems.
If one person calculated the velocity of a dropped object by integrating the acceleration, and another person did it by using conservation of energy on the conversion from gravitational potential energy to kinetic energy, people might argue over which approach works better, but no one would argue that the other approach does not work and does not explain 100% of the velocity.
Or one person might calculate the response of a linear circuit to a sinusoidal signal by working in the frequency domain, and another person might work in the time domain. People might argue over which approach is easier, but again no one would say that only their approach works.
But with lift, people seem to like to latch onto one approach to the point of rejecting the very idea that there might be a different way to get the same result. It makes no sense to me.
The Newton approach is easier to teach reasonably correctly, so that's probably best up through, say, first year college physics and for pilot training.
We still don't understand areodynamics fully, so you can't arbitrarily state that difference between pressures above and below wings is the main force.
And no, I didn't downvote you.
The way that this happens comes from a number of effects, from vortexes at the tips of the wings to the angle of attack, but in the end it all comes down to F=MA. And the only thing that can be accelerated by a plane is air. Ergo, air is moving downwards so that the plane will move upwards.
But, as everyone else responding to the parent has pointed out, the forces generated in this way are not nearly enough to keep the plane in the air on their own.
The compression of the air below the wing moves it downwards, more or less. There are no ways to get around Newton's laws of motion (barring relativity, and planes aren't fast enough for that).
Anyway, I'm not saying you're wrong, I'm just saying that thinking about the individual forces involved leads to more complexity than necessary (this is usually the case). It's much simpler to just consider the energy going into the system and derive the net work produced from that.
Then, there would be no lift generated, since there is no acceleration of mass.
For example, from Newton one can state that the whole lifting force F_lp acting on the plane is opposite to gravity force F_gp acting on the plane, so plane flies horizontally (in the simple case): F_lp + F_gp = 0. Next, from third law of mechanics the lifting force F_lp acting on the plane is opposite to F_da force acting on air downward: F_lp = -F_da. From the second law of mechanics force F_da acting on air causes the air momentum to change: F_da = dp/dt, which means that either the plane pushes a lot of air down or the plain pushes air down really fast, just like article says.
Looking into the picture from Bernoulli perspective, we can talk about flows around the wing. If say we have a wing which is flat underneath and curvy on top, the air is initially pushed upwards until the top point of the wing is reached. From there air continues along the wing profile, which goes down, so air has a chance to accelerate - this is effectively an empty space which becomes available to the air, and air goes from points of higher pressure to points of lower pressure. The air speed increases, the pressure remains less than in (unchanged) air below, so we're getting the Bernoulli effect - lower pressure above the wing.
If the plane is upside down, it won't fly horizontally. It has to fly somewhat upwards - in which case flows on both sides of wings change, with net effect the same - the air is pushed down, pressure under the wing is higher than above, both Newton and Bernoulli explanations work.
I guess you can also see it from conservation of momentum. Gravity acts on the plane, so each second there is a certain amount of downwards momentum added to the system. But the plane is not in fact moving downwards, so the all the downwards momentum has to end up in the air, so it has to move downwards. The net "lift" force is equal to the rate at which momentum is added to air.
If I understand it correctly, you can calculate the lift on the wing either in terms of the pressure on the two sides, or in terms of the downwards acceleration of air--it should be two equivalent views giving the same result. Bernoulli's principle gives a way to calculate the pressures if you only know the velocity of the air (this is a typical situation in a wind tunnel, where you make movies of moving smoke puffs), but it's not a separate effect from the air motion.
In aggregate a force is applied to all the air in a way that is equal and opposite to the force applied on the plane. However "Ergo, air is moving downwards so that the plane will move upwards." is not quite complete. One need only look at an airfoil in a windtunnel to see that the action of an airfoil is not the simple "air based rocket" you would have us envision.
If what you said were true, the most efficient wing design would just be a flat plank strapped in at a 45 degree angle of attack.
But it says nothing about the how a wing is throwing air down.
So why would it be that the air on top of the wing has to go faster than the air below the wing?
Exactly. This is one of the fallacies that are explained in the best reference I know about this subject: http://www.av8n.com/how/ It's even illustrated in the title picture... ;-)
Then, to your PS, If a huge amount of energy is required to break the speed of sound, you've explained why commercial jets don't exceed mach one, not why they don't get within 100mph of it.
So why can planes fly upside down then?
I think this has a lot to do with the wing's angle of attack (most planes' wings have a slight upward tilt; when upside-down, that tilt goes downward instead). It would be more interesting to see the effect on something without an AoA bias.
Even if your premise is accurate (I'm not qualified to say 100%), your conclusion does not follow. Water holds up the hull of a boat, but you wouldn't say that a boat floats by moving water up.
Basically, the idea that the Bernoulli Principle is the primary factor in airplane lift is a popular lie. For some reason it continues to be broadly taught, so you can't be blamed for not knowing better. The main factor is actually angle of attack--when you hold your hand flat out a car window, tilting the leading edge up drags your hand upwards. That's the core principle of powered flight.
Most conventional planes have the wings installed with an upward angle, so they can achieve lift in level flight. Fighter and aerobatic planes have wings parallel with the fuselage, which means they have to keep the nose slightly up to fly level, but in exchange they can fly upside down almost as well as rightside up.