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If an airliner is flying at approximately 500-600mph, it seems to me that there would need to be a significant adjustment for altitude so as to not fly off into space.
Wikipedia says that there's an 8 inch drop for every mile because of the Earth's curvature. But, I've never heard of any airliner adjusting for the curvature. Also, shouldn't aircraft have to adjust somehow for the Earth's rotation because it varies depending on the latitude?
Aircraft altitude is measured (inferred) by atmospheric pressure. The aircraft is usually flown at an altitude that maintains constant ambient pressure (by pilot or autopilot, as the case may be). Changes in local barometric pressure (provided by air traffic control) are used to recalibrate the aircraft altimeter. As long as the aircraft is flown at a constant ambient pressure (hence constant altitude), it will be following the earth's curvature (as the atmosphere is attached to the spherical earth and has same properties at same distance from the center, in an ideal case) as the altitude is measured from the surface, which is curved, and not a plane.
There is no adjustment needed as the aircraft will naturally follow the curvature of the earth without any input from the pilot. This is because the aircraft flies through the atmosphere which also follows the curvature of the earth.
There isn't an adjustment for altitude. An aircraft flying level at a given altitude and trimmed for level flight will stay at that altitude. That means the flight path will have a gentle nose-down curve (looking from far away from the earth) as the direction of down (towards the centre of the earth) changes.
Think about the gravitational potential energy of the aircraft. To climb (which is actually flying in a straight line when you consider the curvature of the earth), the aircraft has to gain energy. In a level flight attitude, it doesn't gain any energy, so it will stay at the same altitude. A path that doesn't gain or lose altitude is an ellipse that goes around the earth.
Another way of thinking about it is to consider how "down" changes as the aircraft travels. The weight of the aircraft always acts towards the centre of the earth, and is matched (in level flight) by the lift of the wings. Imagine if you had a model aircraft suspended on a piece of string, dangled from your hand. If you hold the string and carry the model a quarter of the way around the earth, the bottom of the model will still point down (towards the centre of the earth). The model has rotated 90 degrees, without you having to rotate it by hand.
When you trim for level flight, you do so by finding the pitch attitude where your speed and altitude remain constant (or at least stable: atmospheric conditions might make them fluctuate a lot). That attitude might be a touch more nose-down than it would be if the earth were flat, but it's imperceptible.
This is more of a physics question rather than an aviation question. While other answers have addressed the question from the aerodynamics point of view, let me try answering it from a physics perspective: frame of reference.
How do you know an object is moving? The answer is you don't - there is nothing like a "fixed absolute coordinate in space". Speed is measured by referencing another object. If you think of it, every time we mention an object's speed, we always mean it's speed relative to something. The car's speed is 50mph relative to the ground below it, although we seldom say it explicitly in our daily conversations.
Most of our daily experiences of motion are related to one of the four fundamental forces of nature: gravity. Gravity is spherical - if we pick a random point on the planet, the gravitational force should be the same as any other random point (let's assume the Earth is a perfect sphere for now).
Consider me walking on the surface of the Earth. I would be unaware that the Earth is spinning without external reference points like stars and the sun. If the Earth stands still, I would walk just the same, because when I walk I am interacting with the ground below me, which is part of the Earth; I am not interacting with Saturn or any other planet.
Well, it happens that planes fly by interacting with the atmosphere. The atmosphere is affected by the Earth's gravity like the ground - the ground+atmosphere spins round and around together. Airplanes are not interacting with other stars or planets or satellites in any way - its engines produce thrust against the atmosphere (which moves together with the ground).
You chose your frame of reference as an arbitrary point in space, which is why it leads to incorrect conclusions. If you are travelling on a train and you want to move around, you don't have to factor in how fast the train is moving - you are interacting with the train, not the rail which the train travels on. Otherwise, as the train moves forward following the curvature of the Earth, you will rise higher and higher in the cabin, eventually hitting the ceiling. You are following gravity, which is spherical. Same for planes.
Is a plane fast enough to get to space? Well, unfortunately for space exploration, no, it isn’t. Let’s check with a few calculation.
First, let’s make an assumption: our plane P is flying in the void: there is no air slowing it down. Of course, this assumption is very wrong, but we only care about speed here. Air friction only slows P down. So, if P doesn’t go into space without an atmosphere, it won’t either if we add an atmosphere.
Let’s consider a reference object O orbiting the Earth at the same altitude r as our plane. Of course, we can neglect the mass of P and O, which is several orders of magnitude lesser than the mass of the Earth.
Let vO be the orbital speed of O and vP be the speed of P. If vP > vO, then P will spiral away from Earth and end up in space. If, on the other hand, vP < vO, then P will spiral down until it crashes.
Because O is orbiting on a circular trajectory, vO and r are correlated by the following formula:
vO = √(GM/r)
Where G is the gravitational constant and M the mass of the object we’re orbiting around.
In our case, we are orbiting around the Earth and
By using the SI units and approximating 3.99 with 4, we end up with the following relation:
vO = 2×107 r-1/2
The radius of the Earth is included in r, so we must add 6 371 km to go from altitude to r. 6 371 km is actually two order of magnitude higher than any altitude you could fly at, so let’s round it to 6,400 km and be happy with it.
So, in order to orbit at 6 400 km = 6 400 000 m from the center of Earth, we would need to go at
vO = 2×107 r-1/2 = 2×107 (6 400 000)-1/2 m/s = 2×107/2530 m/s = 7.9×103 m/s = 1.8×104 mph
That’s the speed relative to the center of the Earth, not to its surface.
Our plane P, is flying at 600 mph relative to the surface. If we suppose we’re flying above the equator, we can reach a speed of 1600 mph relative to the center; that still is an order of magnitude below the speed we need to reach to orbit at such a low altitude.
vP << vO
Our plane would crash. Fast.
Well, as we’ve just seen, the speed of the plane is not what keeps it flying. But we made a very strong assumption in our opening section: we neglected the atmosphere.
Air certainly slow our plane down. But we’ve also built it so that, when moving through air, our plane tends to go up thanks to lift.
So we have two opposing effects, here: the plane isn’t going fast enough to escape the Earth attraction; but the lift pulls up the plane hard enough that it keeps flying. But lift depends, among other things, on the density of air around the plane: the higher the planes fly, the weaker lift is. So, all in all, the plane flies in a layer of air at the same pressure, which in turn follows approximately the surface of the Earth.
The posted answers are good.
A tangent (pun intended) answer: if adjusting for curvature of earth was required, it infers that you could accidentally fly into space.
If you could accidentally fixed-wing fly into space, we wouldn't need sophisticated and powerful rockets.
It's no different than driving. If you drive absolutely straight you'll eventually leave the road. The road lines are meant to be followed.
So what is equivalent to the road lines in flight? Air pressure. The air pressure reduces the further you are from the surface. Pilots and autopilots follow the air pressure gradient, trying to keep the plane at a set air pressure. This pressure is used rather than GPS or "straight flight" because it's one of the many factors that affects flight efficiency - speed vs air resistance vs load bearing capacity of the aircraft. They are flying, or attempting to fly, in a pressure range that is going to cost the least to accomplish the various goals of the airline.
The air pressure varies according to many factors, but the main factor is height from sea level, and so by flying inside a specific range of pressures, they maintain a reasonably constant height from sea level. Since "sea level" pressure is curved along with the earth, then what you find is that they automatically follow the curvature of the earth. In other words, the planes are flying slightly down all the time.
There have been good answers, but all seem to miss one aspect.
The aircraft does not maintain a constant distance to the surface of the earth. It simply maintains a level, where the atmospheric pressure is at a set target.
To clarify:
Autopilot (or the pilot) wants to maintain a contstant reading on the altimeter. This reading is merely a calibrated difference to a reference level, which normally is set to 1013,25hPa (or 29,92inHg) on cruise flight. This means that maintaining an altitude of about 30000ft, you want to maintain a pressure level of 300hPa. I say about 30000ft, because the true altitude of this level varies greatly, affected by airmass temperature and air pressure.
Jan Hudec was close to this on his answer, though I'd like to make a few corrections. First of all, the levels aren't isobars. Isobars is by definition "an imaginary line or a line on a map or chart connecting or marking places of equal barometric pressure" source. Second of all, there's no need to over-complicate things: the (auto)pilot monitors the altimeter, and does the necessary corrections. It would do so if the aircraft was out of trim also.
I think there are a lot of good answers in regards to the curvature of the Earth, but I didn't see anyone addressing the spin of the Earth.
The atmosphere is naturally dragged along with the surface of the planet. Wind speed is measured relative to a fixed location on the surface. Therefore differences between the movement and the Earth's surface are reflected as wind. Pilots do adjust for the effect of wind on the flight path.
If the pilot is using basic navigation techniques, he would calculate the heading he had to fly with the predicted crosswind to end up at his destination. If there is any crosswind component, the flown heading will be different than the direct heading on the ground or great circle heading.
With navigation equipment these adjustments can be done in real time. for example if you want to fly a specific GPS heading, but keep drifting to the right you would turn to a more left heading to regain and keep the desired heading.
Planes don't stay perfectly level!
That's the long and short of it. As these 600 mph machines go that 1 mile forward, both gravity and air pressure force the plane down those 8 inches. More, actually; these massive metal monstrosities have to put up a fight to be able stay up in the air! That's what the elevators, rudders, engines, airfoils, etc. are all there for.
Of course, if it were going sufficiently fast (at least 25,020 mph), it would overpower gravity, leave the atmosphere, and enter orbit. I hope it has engines that don't rely on air, and a body made of reinforced carbon-carbon!
Lots of good answers, but nobody has addressed the simple mathematical aspect of the question.
Assuming the “8 inches per mile” is accurate, that means you’d have to “nose down” to a pitch of -1” per 7920” flown, or a pitch of -0.007 degrees. That’s pretty much level flight. The pilot nor autopilot would even be able to discern the difference. You can’t even eyeball 1 degree let alone 7/1000ths of a degree.
An important flight instrument is the Attitude Indicator, which has an artificial horizon line. This instrument tells the pilot if the wings are level and whether the nose is pointing above or below the horizon.
Rather than never taking the curvature of the earth into account, when a pilot is using this instrument to keep his flight level, he is actually constantly adjusting for the curvature of the earth.
EDIT: The following link is to a question on how attitude indicators are kept accurate. A particularly acrobatic flight might cause it to tumble and become useless until it is reset. However it easily maintains accuracy during normal flights, including sustained turns.
How are attitude indicators kept accurate
Short version: Yes, a gyroscope would have problems with earth curvature, but another part of this instrument constantly maintains a "local down" for the instrument.
Your question is basically "What makes the flight change its direction continuously?"
Let's consider the case of a plane flying overhead right now. If we are told that the plane is in level flight, the vertical, downward pull of gravity is equal to the vertical, upward lift. Further both forces are perpendicular to the fuselage and cancel out.
Imagine what happens a very very short time later when the plane has moved a very small distance ahead in a straight line. Since the plane has moved in a straight line, the lift is still pointing upwards and perpendicular to the the fuselage, but gravity is pointing towards the center of the Earth. So now only a part of the lift is countering the gravity, and there is a resultant gravitational force that will pull the aircraft down towards the center of the Earth.
This is very similar to how you can tie a stone to a rope and whirl it. The stone will go around in circles. The force of the string (tension) keeps the direction of the stone changing continuously without affecting the speed.
I'm missing one thing mentioned in the answers. The point is: They say that the gravitational force and the lift force's vertical component have to be equal to make the plane fly levelled. This is not entirely true. The plane is also turning, which needs some force. Let's see how much it is, for a plane at $10\,{\rm km}$ flying $900\,\rm{km\,h^{-1}}$:
Force needed for turning: $-mv^2/r = -m\cdot (250\,{\rm m\,s^{-1})^2} / 6388000\,{\rm m}=-m\cdot0.00978\,{\rm m\, s^{-2}}$
($m$ is the weight, $g$ is the gravitational acceleration, $v$ is the speed and $r$ is the turning radius)
So the force needed for turning is $1/1000$ of the gravitational force. This is negligible in the planes power and other settings, and also much smaller than any irregularities such as varying air density, wind speed etc.
Conclusions: the adjustment for "turning down" is so little that it's negligible.
I am not really qualified to answer, but did so on account of the many answers implying that some form of pilot control is necessary to keep an airplane following the curvature of the earth. Input from a real aerodynamicist would be appreciated.
Several answers state that when an airplane is flown to a constant altitude, it implicitly follows that curve. True enough, but the question asks whether pilot control is necessary. Are there physical constraints relevant to the question?
If an airplane were to follow a tangent, rather than the curvature of the earth, it would experience at least two effects: the direction of the earth's gravitational field would change, causing its weight vector to rotate rearwards, and the local air density would decrease as the aircraft's distance to the earth increases.
As the weight vector rotates backwards, it increasingly retards the airplane, and if it is longitudinally stable, it will, as a result, tend to pitch downwards. The effect of decreasing density seems more complicated, on account of its effects on power as well as aerodynamic forces, but there seems to be a general tendency for a trimmed, longitudinally stable airplane to be stable in density altitude - see What does it mean for a plane to be aerodynamically stable?. It is certainly true that every airplane has a maximum achievable altitude, determined by physics.
Both of these effects (longitudinal stability with respect to the local gravity field, and stability in density altitude) will tend to guide the airplane to follow the curvature of the Earth without pilot input.
For reasons that are explained in several other answers, there is no need for an aircraft pilot to make any special calculations or adjustments of the controls in order to "drop" the aircraft eight inches per mile and thereby follow the Earth's curvature. Instead what pilots actually do is to keep the aircraft at a constant pressure altitude, which (approximately) follows the curvature of the earth. The path followed by an aircraft in this fashion actually can rise or fall quite a bit due to weather-induced variations in pressure, so it's not guaranteed to drop 8 inches each and every time the aircraft flies a mile, but it will follow the curve of the Earth much more closely than it follows a straight line over any long-distance flight.
So the answer to the first part of your question is, yes, pilots do adjust the aircraft's flight path to allow for the curvature of the Earth, and this is how they do it. There is no explicit "adjustment for curvature" term in the pilots' (or autopilots') calculations, however, because they keep the aircraft on the desired (curved) path by observing the pressure altitude, much like the way you would keep an automobile in its lane on a highway by observing the lane markers, which does not require you to know anything about the radius of curvature of each curve the highway engineers laid out. If the curve is gentle enough you might not even notice that the road is curved, yet you would still be following it.
A notable difference between highway lanes and the altitude of an aircraft, however, is that the aircraft does not have the ability to fly above a certain altitude at a certain setting of the controls, due to the low density of air at higher altitudes--so at higher altitudes the aircraft will tend to descend--whereas at much lower altitudes the aircraft's engines will generate more than enough power to keep the aircraft aloft and so the aircraft will tend to climb. The physical properties of the atmosphere and the aircraft therefore tend to deflect the aircraft along a path that (approximately) follows the curve of the Earth, and they absolutely prevent the aircraft from continuing along a straight tangent line into outer space. A large part (one might say the most important part) of how pilots make the aircraft stay at a given pressure altitude is by adjusting the "trim" of the aircraft (including throttle) in order to make it tend to fly at the desired altitude.
As for the Earth's rotation, since the atmosphere generally rotates along with the rest of the Earth, and aircraft fly in the atmosphere, the aircraft do not have to fly faster or slower to overcome the Earth's rotation, any more than you have to be able to run 500 mph in order to make your way from the aft restroom forward to your seat. (Some bits of the atmosphere do sometimes go a little faster than the Earth's rotation, sometimes a little slower, depending on which way the wind is blowing at each place; pilots do to account for the wind in order to fly the desired path over the ground and to know how long it will take to arrive.)
On the other hand, pilots very much do have to take the curvature of the earth into account when planning the lateral direction in which to fly. For example, to fly from New York to London, the shortest path (fewest miles over the ground or water) heads out of New York on a course of about 51.4 degrees. The return flight starts out toward New York on a course of about 288 degrees. Similarly, from London to Tenerife is a course of 213.5 degrees, 23.1 returning, and from Tenerife to New York is 300.5 degrees, 85.8 returning. If you take those three flight paths as the three sides of a triangle, it has angles of 34.4 degrees at New York, 74.5 degrees at London, and 82.6 degrees at Tenerife, which adds up to 191.5 degrees, which is impossible for any triangle plotted on a flat plane. Pilots and other people who plan the routes of aircraft do so by calculations (nowadays generally done in software) that take into account the approximately spherical shape of the Earth to do so (and nowadays even take into account the few miles' difference between the diameter of the Earth measured from pole to pole and the diameter measured across the equator).
I think that the aircraft should fly straight, not following the Earth curvature if nothing is done. This means, it should have a slight tendency to raise up but this may be only noticeable when it flies half way around the globe.
This effect may be much smaller than other factors that would alter the altitude. Anyway, if the aircraft is set to fly at some fixed altitude, it would compensate for the Earth curvature as well without doing anything special.
Assuming we are flying "on the front side of the power curve"--
For a given power setting, there is only one angle-of-attack that will allow a given aircraft to maintain a constant altitude.
For a given angle-of-attack, there is only one power setting that will allow a given aircraft to maintain a constant altitude.
So your question is essentially, "is the required angle-of-attack, or power setting, different than it would be if the earth were flat?"
The answer is "yes-- very very slightly."
If your question is also meant to ask "is this something that a pilot must consciously take into account?", the answer is certainly "no". Having a few pounds of dust accumulate in the nooks and crannies of the aircraft would probably make more difference to the required angle-of-attack or power setting than the fact that the earth is round rather than flat.
Likewise for effects due to the spin of the earth and its surrounding atmosphere.
If your question is also meant to ask whether an ARHS reference system or other similar system must specifically take into account the curvature of the earth to remember which way is "level" on a flight covering a long distance or whether this is automatically corrected for by other built-in functions of the system-- that is a much more complicated question but the answer is generally the latter. The same is generally true about remembering which way is "level" over a long duration of time, as the earth rotates about its axis.
For self-contained inertial navigation systems that operate completely independently of any GPS-derived position information, the answer is quite different: in this case effects related to the spherical shape of the earth, and effects related to its rotation about its axis, are very important.
