# Thread: Speed relativity in flight? (physics and real life)

1. Member
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Apr 2013
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186

## Speed relativity in flight? (physics and real life)

Hi. I wonder:

When plane is very high it appears (for observant in earth) to be almost not moving, while airplane flying at 100 feet would fly above us very very quickly.

Question 1 : How does it look like from perspective of pilot? Is it the same or opposite?

Question 2:

If plane is moving with 150 nm/h at 100 feet, if given plane will fly at 100.000 feet - would his realtive (ground speed) increase because of the altitude(for pilot it would be still 150nm/h) ?

Extra question 3: does any of it works in flight simulator?

1. It's the same -- horizontally as well as vertically. It's a function of distance.

2. We have to ask whether the 150 knots (NM/hr) is indicated or true airspeed -- a big difference. The airspeed indicator is a pressure gauge, fed by air pressure through the pitot tube, so it's a good indicator of relative air density transcribed into aircraft performance, that is, stall speed, best rate of climb, etc. roughly remain the same whether near sea level or at 10,000 feet with the same indicated airspeed. But that doesn't tell your actual speed in relation to the air except under standard conditions at sea level.

True airspeed, is your actual speed in relation to the air, but tells the pilot nothing about how the airplane will fly -- it's only useful (after also figuring wind effects, giving you ground speed) to figure how long a trip will take.

So if you're discussing indicated airspeed as you climb then the ground speed will, indeed, increase with increasing altitude (assuming winds remain the same). If you're referencing true airspeed, then ground speed will not increase with altitude.

3. Yes, this works the same in FS as in real life, provided that you have your airspeed indicator set to indicated airspeed (there is a menu choice to use true airspeed, but it's useless to a pilot).

2. Member
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Apr 2013
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186

2. I'm not asking about effects like effect of pressure, wind, air resistance etc - but more "high level" effects like time dilatation, weaker gravity or advantage/disadvantage of flying in high altitudes - based on curvature of earth.

3. Well, I think I answered 1), but for the revised 2) any effects from those causes are so small, if any, that they are beyond consideration for a pilot -- negligible as far as practical flight is concerned. For any of that information you'll need a scientist with the relevant background -- it's beyond me.

4. Negligible effect on such a small scale as a single planet for this branch of physics.

6. Originally Posted by hhans

2. I'm not asking about effects like effect of pressure, wind, air resistance etc - but more "high level" effects like time dilatation, weaker gravity or advantage/disadvantage of flying in high altitudes - based on curvature of earth.
I hesitated to do this but now cannot resist the temptation ...

Last edited by FlyingHorseSindbad; 08-05-2013 at 04:10 PM.

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