# Aerospace Altitude change in straight flight

1. Nov 21, 2012

### pbaumer

Hi,

Assume an aircraft starts flying to east with zero pitch and roll, would this aircraft slowly move away from earth surface due to earth being a sphere ?

Or would changing gravity direction or some other factor cause this aircraft to fly an arc like path which is parallel to underlying earth surface?

Thanks!

Paul

2. Nov 21, 2012

### Staff: Mentor

Welcome to PF!

The curvature of the earth is so gradual it has no impact here. Straight and level is straight and level.

But I suppose if it mattered, the changing direction of gravity would help, since that is the reference point for "level".

3. Nov 21, 2012

### AlephZero

The question obviously "matters" in terms of geometry, for long distance flights.

But air traffic control "flight levels" are defined in terms of local air presssure, not distance above the ground level. For example "Flight level 100" corresponds to 10,000 ft above sea level for the International Standard Atmosphere, but at any moment in time it can vary from about 9,000 to 11,000 feet over different parts of the earth's surface, depending on whether it is summer or winter, or day or night, at a particular place.

The basic reason for defining it that way was historical, since all "mechanical" flight instruments work with reference to air pressure. It stiill makes sense even when location systems like GPS are available, because the aircraft's performance (fuel consumption, etc) depends on the air it is flying through, not its distance above the ground.

So "level flight" actually means "keeping the air pressure outside the plane constant", not "flying along a mathematical straight line".

4. Nov 21, 2012

My vote: the aircraft would rather quickly fall towards the ground. Zero pitch means zero lift.

5. Nov 21, 2012

### RandomGuy88

The wings can be at angle of attack relative to the airflow but the aircraft is not pitched.

6. Nov 22, 2012

### Staff: Mentor

Since a plane is constantly pitching, yawing and rolling by a small amount, any amount due to the geometry of the earth is lost in that noise. Increasing the distance doesn't change that.

7. Nov 22, 2012

### Staff: Mentor

As well, if the zero point for angle of attack is geometrically defined, the wing will have lift even at zero angle of attack.