Do aircrafts fly faster when it's higher or lower?

Question

Curious because most commercial airplanes fly very high.

But i'm speculating because:

Yarn: Distance

Ball: Earth

wrap a yarn around a ball, it uses less yarn the tighter you make it (Which would be lower altitude if compared to airplanes)

But if you loosen it up, there's more yarn (Which would be higher altitude)

So is there some sort of force that could possibly make you fly faster the higher you are? Take for example does gravity affect anything?

Answer 1

Jason, me thinks it's you that needs that trip back to flight school. You have heard of things such as IAS, TAS, Mmo, Vmo right? No? OK I'll explain. Indicated airspeed is read by the airspeed indicator through the pitot tubes. True airspeed is the actual speed through the air. Now, have you ever seen indicated airspeed as fast as true air speed? No huh. You never will other than at sea level on a standard day. As you go higher, TAS is ALWAYS higher than IAS precisely because the air is thinner so jets fly faster up to a point when they climb.

That brings us to Mmo, Vmo. If a jet has a maximum mach speed, Mmo of say .85 mach, it would likely have a Vmo, maximum indicated airspeed of around 350 kts. That means at low altitude it can't get to Mmo without exceeding Vmo. As you climb though, as before, IAS becomes less than TAS and at an altitude of around 28,000 feet Vmo, max IAS, will match Mmo, max mach, if you were at maximum on both you would be going as fast as possible. Vmo was the limiting factor up to that altitude and Mmo will be limiting as you climb above it.

So Jason, yes, jets do climb higher to get better fuel efficiency as you say but they get better fuel efficiency because THEY GO FASTER ON LESS THRUST SO THEY CONSUME LESS FUEL FOR EVERY MILE THEY TRAVEL.

Answer 2

Aircraft fly faster when they're higher. My reasons are...

1. There is much less wind resistance at higher altitudes; the aircraft is able to travel at a much faster speed without overstressing the aircraft's frame.

2. Usually, there is a helpful tailwind (a strong wind that strikes the aircraft from the tail, hence the name) to "push" the aircraft faster and with much less wind resistance from the front.

3. This is less related to the question, but anyway... There is a speed limit to any aircraft that flies below 10,000 feet (this is a low altitude), which is 250 knots. However, above this height, there is no speed limit to the aircraft and it can go much faster~

Answer 3

First, let's define speed.  Speed is the rate of change in distance over a given time.  It is a relative concept.  This means that if I am moving 60 miles per hour on the earth's surface, I am moving 60 miles per hour from  a stop sign.  If I compare the 60 MPH on my speedometer as compared to the position of the Andromeda galaxy, I am actually moving at well over 100,000 miles per hour away from Andromeda even though my speedometer indicates 60 miles per hour.  Same is true from the center of the earth.  The radial distance traveled increases as I move away from the center.  As the radial distance increases in the same amount of time, the speed increases.   Now, closer to Earth,  pilots use Indicated, Calibrated, Equivalent, True, Mach, and Ground Speed.  Each of them are different.  Passengers like to see ground speed, like your car.  Pilots talk about True Airspeed, not Mach number.  Why not Mach?  Well, two airplanes flying Mach 0.85, one at 30,000 feet the other at 40,000, are probably traveling at different ground speeds.  Why?  Mach number is a function of temperature.  Different altitudes, probably different temperatures.  So, pilots like to talk about True Airspeed.  It is corrected for instrument error, temperature, and air compressibility.  Two airplanes traveling at 480 knots true airspeed are traveling through the air at the same speed regardless of altitude.

Now, airplanes, especially jets fly faster as they go higher.  But only to a point.  The benefits of flying higher are greater because of less air drag and colder air for the engines.  Colder air actually has several effects.  Colder air is more dense, making aircraft engines process air more efficiently.  Cooler engines allow more air and fuel to be burned at a lower inter turbine temperature.  This allows the engines to be run at higher rpm to create a greater speed.  BUT>>  At some altitude, different for different aircraft, the decrease in air pressure for the engines becomes more important than the decrease in drag for the aircraft.  Above this altitude aircraft performance decreases.  For many jets, this occurs between 30,000-40,000 feet.  Above this, aircraft fly slower because not enough air is available to the engines.

Answer 4

if they fly the same IAS /instrument air speed, then they fly the faster the lower the ambient air pressure is. the pressure decreases with altitude so basically the plane flying the same IAS /to maintain the same aerodynamic characteristics/ is flying faster relative to earth.

the wrapping you are talking about is working with sattelites, not airplanes. the planes are flying at altitudes up to 10 kilometers, while the Earth diameter is 6378 km /equatorial/ thus the 10 kilometers is approx 0,15 percent increase .. that is for the plane the earth is stil actually flat....

more than that the planes are not maintaining constant angular speed, which would increase the reavel speed with altitude. the plane follows its instruments, and these work regardless of the earth distance, they only measure air pressure. that is the earth distance is not critical input for the instruments. now is messed it a little bit  :)

so, draw a line of certain length. this demonstrates the IAS indicated at the aircraft. at the sea level, the line showing the actual speed /without wind effect/ would be the same length. with decreasing ambient pressure the line of TAS /true air speed/ extends, while the IAS remains the same. that means ... you are flying AERODYNAMICLY the same configuration, but you are flying faster /relative to ground - still without wind effect!/ because the air is thinner. you fly faster to compensate the decrease in pressure through the higher speed /roughly/. thats it.

so if you are to maintain the travel speed of 300 KTS IAS you maintain the thick arrow on the 300 value and let the air flow... you are only interested in the TAS when calculating the wind drift and estimated times of arrival, in which cases you need the value relative to earth and that is the TAS /tas equals the speed relative to ground in no-wind conditions and is one of the inputs of winddrift triangle/

notice... the wrapping. the sattelites have to keep a certain angular speed to maintain in position relative to earth which enables them to maintain the zero gravity /ballanced centrifugal and gravitational forces./ thus the higher you want the sattelite to fly, the faster it MUST move to remain in the proper position in the sky. that is- only one ANGULAR speed provides the sattelite with no need for engined maneuvering. if the sattelite is flying too low equaling too fast, it would fly away to higher orbit... if it is too slow /like during decelerating in the upper atmosphere/ then it continuously decreases until it reaches stable lower orbit or it descends to the atmosphere uncontrolled which means its destruction. - note the MIR station -- it was slowing down too much and eventually hit the upper atmosphere. should the Russinas want it to survive ,they would need to manage to launch another prolleling engine that would push the station back to highr orbit and push it forwards to have the angular speed.

Answer 5

The air is thinner so there is less resistance, and it is more fuel efficient. Thats why speed of sound changes as the altitude is increased. The more efficient the more fuel you can put into thrust therefore more faster. This was one of the first things they teach you in flying school.

Answer 6

the higher one goes the lesser the air density is but as one says the speed is constant so the speed will be the same unless there is a tail wind which gives additional thrust to the airplane. But the real thing there is the saying that the  shortest distance between two points  is a straight line.Since the globe is a sphere traveling straight up will shorten your distance than traveling straight around the globe. Yet because of aircraft structure a plane can go only on certain heights. So it is true the higher one go it's less distance with a constant speed.

Answer 7

No matter what altitude you fly you cant over speed the acft. for example if your acft can only fly lets say .8 mach no matter what altitude you fly at you can only push the acft to .8mach with out damage.  now at higher altitudes it takes less energy (gas) since there is less resistance on the acft in all directions. gravity, drag, lift. so there for you will have to throttle back to keep from over speeding your acft.

Answer 8

Gravity affects air density, so there's less friction from wind resistance up there. Less stuff to crash into too, ie. : mountains, birds, inclement weather...

Answer 9

Theres less resistance forces the higher you go so it takes less energy to maintain a specific speed high up

Answer 10

(for one of the people who answered altready the mach number does not change with air density, but with temperature.)

As you climb, the air gets cooler (more dense), but as there is less overall air, the net is the air is less dense.

While the less dense air decreases the avaiable power to the engines, the side effect is also that (1) there is less drag and (2) the fuel/ air ratio for combustion is less.

So, while there is less power available at high altitudes, less power is required to fly a speed and less drag is there so the fuel burn is significantly less.

In terms of speed, the aircraft can fly a  "faster speed" the higher it goes as the power it looses it less than the drag it looses with less dense air, but it is so small it most cases that it is not the reason we fly higher.  

I just ran some numbers for you and between the surface and 18,000ft the speed capabilities are different but above thay the difference in speed is pretty small.

For reasons that don't need great explanation for this quesiton, the "indicated airspeed" will have a larger difference to the "ground speed" due to air density, but the ground speed at 35,000ft will closely match the ground speed at 20,000ft.

Secondary reasons, as someone else stated is that most weather is located 35,000ft and below so the higher you fly the better chance you can get above a thunderstorm and not have to go around it enroute.

In addition, the higher your aircraft can go the less traffic you will encounter so the less potential enroute delays.

So... in an way too long answer to your excellent question, aircraft fly higher for better fuel burn and secondarily weather and ATC delays.

Answer 11

Lots of bad answers there...wow go back to flight school guys.

Aircraft speed at higher altitude is limited by the speed of sound.  As air density decreases, the speed of sound (Mach 1) decreases.  Therefore, as an airplane goes up, the limiting factor decreases as well.  Its as if that speed was a wall.  The higher you go, the closer that wall gets to you.

If an aircraft has a published limiting speed of Mach .85, that speed will be higher at a lower altitude.  It will decrease at a higher altitude.  In plain English, jet airplanes go slower the higher they go.  The main reason for altitude increases is fuel efficiency.  Less air density does decrease drag and make the airplane more fuel efficient.

Gravity does not vary significantly from the surface to an airplane's cruising altitude.  Gravity acts as if it was at a point at the exact center of the earth (because there's just as much stuff exerting gravitational pull on both sides of that point).  That point is about 3000 miles below your feet.  Cruising at an altitude of 35000' is about 6 miles.  Theres not much of a difference between 3000 miles and 3006 miles when it comes to gravity.  In fact, there isn't even much noticeable difference in gravitational pull between the surface and the altitude that satellites and space shuttles orbit.

Answer 12

you go faster at 30,000 feet because the air is less dense (thinner).  It takes less work to move because there is less resistance on the aircraft, so you can go faster.  Also, they fly above most weather, allowing them to move faster because they do not worry about rain or wind