# Altitude change in straight flight

by pbaumer
Tags: altitude, flight, straight
 P: 1 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
 Mentor P: 21,897 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".
Engineering
Thanks
P: 6,031
 Quote by russ_watters But I suppose if it mattered, the changing direction of gravity would help, since that is the reference point for "level".
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".

P: 1,408

## Altitude change in straight flight

 Quote by pbaumer 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?
My vote: the aircraft would rather quickly fall towards the ground. Zero pitch means zero lift.
 P: 355 The wings can be at angle of attack relative to the airflow but the aircraft is not pitched.
Mentor
P: 21,897
 Quote by AlephZero The question obviously "matters" in terms of geometry, for long distance flights.
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.
Mentor
P: 21,897
 Quote by RandomGuy88 The wings can be at angle of attack relative to the airflow but the aircraft is not pitched.
As well, if the zero point for angle of attack is geometrically defined, the wing will have lift even at zero angle of attack.

 Related Discussions Classical Physics 2 General Physics 11 Calculus & Beyond Homework 5 General Astronomy 0 Introductory Physics Homework 1