93 replies to this topic

[utd4life]

Heyy guys,

This will probably seem like a really beginners question, but obviously the higher you get in Altitude, the slower you have to fly. Usually above 30,000ft I have to lower my speed to around 280 KIAS in an Airbus. But will you still get to your destination at the same time, if you are travelling at FL350 at 280 KIAS compared to if you were travelling at FL280 at 320 KIAS?

Thanks in advance,

Danny

[ArmChairAviator]

Well if you think of track and field, if all the athletes were circling the track at the same speed, the inside runners would finish before the outside runners because they travel less distance. So by that logic an airplane circling the globe at FL280 would travel less distance than an airplane at FL350.

Edited by ArmChairAviator, 26 September 2010 - 09:52 AM.

[niteye]

The airspeed goes lower but your actual groundspeed is faster because the air is thinner up high.

[Paul Skol]

What niteye said.

What you're seeing lower is IAS i.e. indicated airspeed.  It indicates lower because the measuring device is sensing thinner air and reading this for lower speed.

But speed over the ground is what gets you from a to b, and that is higher.  Your FSX GPS will show you Ground Speed, which will be plenty fast when you see 280kts at FL300.

[ArmChairAviator]

Your IAS speed may decrease but your ground speed will increase as you increase altitude.

"The air is thinner and more dispersed at high altitudes. This causes the airspeed to be lower because there is more distance between air molecules which is what the pitot tube measures."

Edited by ArmChairAviator, 26 September 2010 - 09:59 AM.

[Flying_Scotsman]

ArmChairAviator, on Sep 26 2010, 09:37 AM, said:

Well if you think of track and field, if all the athletes were circling the track at the same speed, the inside runners would finish before the outside runners because they travel less distance. So by that logic an airplane circling the globe at FL280 would travel less distance than an airplane at FL350.

Airspeed is measured by the pitot head measuring the pressure of the air pushing against it as you fly.  The higher you go, the thinner the air.  The air is still moving at the same speed when you are high but because it is less dense, the pressure of it pushing on the pitot head is less and thus it gives a false reading and you think you are flying slower than you actually are.  
You need to use your Mach speed once you are cruising.  If your aim is to cruise at Mach 0.78 for example, then during your climb, you hold a certain air speed, lets say 300kts, as you climb, you will see your mach speed increases even though your IAS is still 300kts, at around 28'000ft, your Mach will reach the cruise speed you desire and then you switch your autopilot speed hold from IAS to Mach and then you hold Mach 0.78 from then on.  The height you switch over from IAS to Mach is called Mach Crossover Altitude.

Edited by Dr...Watson, 26 September 2010 - 10:03 AM.

[ArmChairAviator]

Dr...Watson, on Sep 26 2010, 08:02 AM, said:

Airspeed is measured by the pitot head measuring the pressure of the air pushing against it as you fly.  The higher you go, the thinner the air.  The air is still moving at the same speed when you are high but because it is less dense, the pressure of it pushing on the pitot head is less and thus it gives a false reading and you think you are flying slower than you actually are.  
You need to use your Mach speed once you are cruising.  If your aim is to cruise at Mach 0.78 for example, then during your climb, you hold a certain air speed, lets say 300kts, as you climb, you will see your mach speed increases even though your IAS is still 300kts, at around 28'000ft, your Mach will reach the cruise speed you desire and then you switch your autopilot speed hold from IAS to Mach and then you hold Mach 0.78 from then on.  The height you switch over from IAS to Mach is called Mach Crossover Altitude.

Way to over complicate things.

I have a question...if it take less power to maintain GS at altitude how come it takes more power and a longer takeoff roll at (density) altitude?

Edited by ArmChairAviator, 26 September 2010 - 10:41 AM.

[ArmChairAviator]

The edit time on this site is ridiculously short.

I have a question...if it take less power to maintain GS at higher altitude how come it takes more power and a longer takeoff roll at high (density) altitude?

[ArmChairAviator]

Dr...Watson, on Sep 26 2010, 08:02 AM, said:

Airspeed is measured by the pitot head measuring the pressure of the air pushing against it as you fly.  The higher you go, the thinner the air.  The air is still moving at the same speed when you are high but because it is less dense, the pressure of it pushing on the pitot head is less and thus it gives a false reading and you think you are flying slower than you actually are.  
You need to use your Mach speed once you are cruising.  If your aim is to cruise at Mach 0.78 for example, then during your climb, you hold a certain air speed, lets say 300kts, as you climb, you will see your mach speed increases even though your IAS is still 300kts, at around 28'000ft, your Mach will reach the cruise speed you desire and then you switch your autopilot speed hold from IAS to Mach and then you hold Mach 0.78 from then on.  The height you switch over from IAS to Mach is called Mach Crossover Altitude.

Actually to be honest I was unaware this was a function of the mach speed indicator so thx.

Edited by ArmChairAviator, 26 September 2010 - 11:00 AM.

[Flying_Scotsman]

ArmChairAviator, on Sep 26 2010, 10:43 AM, said:

The edit time on this site is ridiculously short.

I have a question...if it take less power to maintain GS at higher altitude how come it takes more power and a longer takeoff roll at high (density) altitude?

It doesn't take more power, you use the same takeoff power for any takeoff altitude.  However, the higher the airfield, the longer the takeoff roll will be as the wings don't generate as much lift in thinner air.

Edited by Dr...Watson, 26 September 2010 - 11:47 AM.

[THBatMan8]

ArmChairAviator, on Sep 26 2010, 10:37 AM, said:

Well if you think of track and field, if all the athletes were circling the track at the same speed, the inside runners would finish before the outside runners because they travel less distance. So by that logic an airplane circling the globe at FL280 would travel less distance than an airplane at FL350.

Height above the ground does nothing in relation to the distance traveled. If that were the case then if you were to drive a vehicle in hilly terrain and flat terrain for 1 mile at the same speeds, the car in flat terrain would go further.

Dr...Watson, on Sep 26 2010, 12:44 PM, said:

It doesn't take more power, you use the same takeoff power for any takeoff altitude.  However, the higher the airfield, the longer the takeoff roll will be as the wings don't generate as much lift in thinner air.

Actually it does. Turbines work by compressing air through various stages from the PT2 sensor to the PT8 sensor. The thinner the air at the PT2 sensor, the faster the engines turbines need to spin to keep up with the demands. This is also the reason why aircraft engines perform better in the winter as they do in the summer.

It's the same thing with automobiles. The higher the altitude, the faster the piston rods rotate to maintain speed due to the loss of air, and the more horsepower the car looses.

Edited by THBatMan8, 26 September 2010 - 11:57 AM.

[Flying_Scotsman]

THBatMan8, on Sep 26 2010, 11:52 AM, said:

Height above the ground does nothing in relation to the distance traveled. If that were the case then if you were to drive a vehicle in hilly terrain and flat terrain for 1 mile at the same speeds, the car in flat terrain would go further.
Actually it does. Turbines work by compressing air through various stages from the PT2 sensor to the PT8 sensor. The thinner the air at the PT2 sensor, the faster the engines turbines need to spin to keep up with the demands. This is also the reason why aircraft engines perform better in the winter as they do in the summer.

It's the same thing with automobiles. The higher the altitude, the faster the piston rods rotate to maintain speed due to the loss of air, and the more horsepower the car looses.

You set the same takeoff power, what the engines actually produce depends on many things, but its the same power setting...if you need to set a higher takeoff setting for that particluar situation then so be it.

and engines don't produce more power in winter than summer  

[THBatMan8]

Dr...Watson, on Sep 26 2010, 01:01 PM, said:

You set the same takeoff power, what the engines actually produce depends on many things, but its the same power setting...if you need to set a higher takeoff setting for that particluar situation then so be it.

and engines don't produce more power in winter than summer  

For the first part, takeoff power is rarely the same thing twice. It depends on different variables such as you mentioned. The higher the demands set on the engine, the more takeoff power you'll need.

For the second part, they do. The weather plays a huge role in the performance of the engine. They don't produce more power so-to-speak, but it becomes much easier for the engines to maintain power when the air pressure is higher, thus, giving you better performance.

If you don't believe me then try this:

Take the same airplane, and fly identical flights twice a year with the same weight configuration. One flight in the summer and one flight in the winter. You'll end up burning less fuel in the winter due to the performance hit during the summer.

Edited by THBatMan8, 26 September 2010 - 01:47 PM.

[Flying_Scotsman]

THBatMan8, on Sep 26 2010, 12:06 PM, said:

For the first part, takeoff power is rarely the same thing twice. It depends on different variables such as you mentioned. The higher the demands set on the engine, the more takeoff power you'll need.

For the second part, they do. The weather plays a huge role in the performance of the engine. They don't produce more power so-to-speak, but it becomes much easier for the engines to maintain power when the air pressure is higher, thus, giving you better performance.

If you don't believe me then try this:

Take the same airplane, and fly identical flights twice a year with the same weight configuration. One flight in the summer and one flight in the winter. You'll end up burning less fuel in the winter due to the performance hit during the summer.

I know, I was just thought it would be safer to say that engines produce more thrust in colder conditions that hot conditions.  Assuming summers are warmer than winters is not something Scottish people do  

[THBatMan8]

Dr...Watson, on Sep 26 2010, 03:48 PM, said:

I know, I was just thought it would be safer to say that engines produce more thrust in colder conditions that hot conditions.  Assuming summers are warmer than winters is not something Scottish people do  

Well that makes sense.

You may already know this, but to those who don't:

The reason why aircraft engines perform better in colder weather is due to air pressure differences. Air molecules rise in the warmer months making the atmosphere thinner, and air molecules condense in the colder months making the atmosphere thicker. The thicker the atmosphere, the better performance you'll get from the engine.

[ArmChairAviator]

THBatMan8, on Sep 26 2010, 09:52 AM, said:

Height above the ground does nothing in relation to the distance traveled. If that were the case then if you were to drive a vehicle in hilly terrain and flat terrain for 1 mile at the same speeds, the car in flat terrain would go further.

Both cars would travel the equivalent of 1 mile but if you measured their lateral distance the car that drove the flat would be farther along.  I thought this was a theoretical question at first so with the planes...the one at FL300 travels farther (as it circles the globe) than the one a FL280 so it must have a higher GS to arrive at the same time.

BTW in your example speed is irrelevant...a mile is a mile. Now if your were to say "travel the same speed for the same time" then again the car travelling the flat would be farther along.

Edited by ArmChairAviator, 26 September 2010 - 04:48 PM.

[ArmChairAviator]

Here is a photo two shoe laces of the same length ( 1 mile) displaying the paths of both cars. While both cars have travelled 1 mile...the one on the flat is clearly farther along than the one that drove the hills.



Now if you drove a car around the Earth and drove another car around Jupiter..the Jupiter car would have to be travelling faster to arrive at the same time as the Earth car. The same for the planes.

Edited by ArmChairAviator, 26 September 2010 - 05:08 PM.

[THBatMan8]

ArmChairAviator, on Sep 26 2010, 05:34 PM, said:

Both cars would travel the equivalent of 1 mile but if you measured their lateral distance the car that drove the flat would be farther along.  I thought this was a theoretical question at first so with the planes...the one at FL300 travels farther (as it circles the globe) than the one a FL280 so it must have a higher GS to arrive at the same time.

BTW in your example speed is irrelevant...a mile is a mile. Now if your were to say "travel the same speed for the same time" then again the car travelling the flat would be farther along.

For the first part, not necessairly. Altitude does not dictate the distance you travel. Nautical miles are measured the same regardless of what altitude you're at. It might take you longer depending on what variables are at play due to the workload of the engines, but your distance traveled will not increase or decrease with altitude.

For the second part, that's partially true. It's not due to more real estate that you have to cover but due to the performance of the engine. The engine RPM's are more at higher altitudes then at sea level due to the loss of air pressure as I already mentioned.

[ArmChairAviator]

THBatMan8, on Sep 26 2010, 03:37 PM, said:

Altitude does not dictate the distance you travel.

It does if you are travelling in a circle around the earth.

Edited by ArmChairAviator, 26 September 2010 - 05:46 PM.

[THBatMan8]

ArmChairAviator, on Sep 26 2010, 06:41 PM, said:

It does if you are travelling in a circle around the earth.

Not even then. In that case what makes a difference is the locations you fly to/from. Obviously, flying along the Equator will equate to a greater distance if you circled the globe via the polar regions.