## Does an airplane have to nose down in order to follow a curve physics forums rahal e gas card

I understand the aerodynamics of airplanes and their mass distribution in relation to the center of lift or pressure. Sure, the CG ahead of the CP allows for a stable aircraft when near stall, at the cost of more overall drag and a higher stall speed. However, I’m just trying to see how this is a force that allows for the airplane to follow the curve of the earth. if you had a plane that had its CG very far rearward, it would still fly, be more unstable, but would follow the earth curve by just flying level (parallel to the relative wind) The nose would never have to be controlled down because the aircraft would always be following a consistent air pressure region. (assuming there are no variables such as air currents, temp change, density changes). i mean, an blimp travels around the earth , with no "nosing " of the craft downward to follow the curve…. nor does a submarine. this is the argument im trying to fight, and there is a physicist, that seems that he "knows" that the airplanes, have to "nose down" ever so slightly , to follow the earth curve. again, i disagree and am looking for more logic from the board to either learn if im wrong or help correct this misconception.

Here is a response to me by the physicist on a discussion of the topic. This seems incorrect, but im basing my view on the fact that the atmosphere is curved around the earth and the force of gravity which is perpendicular to the flight direction, should naturally follow the curve with no adjustment. he disagrees , adamantly

the video he refers to is by a pilot. also i think he has it wrong. however, he has a video that shows the pendulous vanes compensating for earth curvature at a rate of 20 degrees over 8 mins, much faster than a 500mph plane flying around the earth which only would require 1 degree change (correction) every 8 mins.

>>>>>>>>>>>>>>>>>>>>>> I have re-viewed Wolfie’s video: "Do aircraft change attitude to follow the curvature of the Earth?" and I find absolutely nothing wrong with his analysis. No, his autopilot really is incorporating a tiny correction for sphericity into the normal trim changes for correction for other variants. It has to. It doesn’t know it’s flying around a globe, it is set to correct barometric altitude, and as that barometric reading changes as the plane flies into higher, lower pressure air, it must adjust trim slightly downwards, even though it is a much tinier component of trim than any others. Planes have inertia of forward motion, they do not magically stay at a specific altitude or magically bend their path. they are NOT buoyant and the atmosphere doesn’t hold them up. The autopilot finds itself, due to travel around the earth, in an inadvertent climb, and adjusts to correct it by trimming down. You see, an autopilot, or any pilot for that matter, changes trim periodically, not continuously. This makes the plane’s path through the air a series of successive straight line paths that only approximate a circle around the earth. IF you could adjust trim perfectly, you would NOT need to keep adjusting it, but that is a pipe dream. The inertia of motion of an aircraft is in one direction, not a curved direction. It is NOT in orbit where everything is balanced. It is flying an orbit within the atmosphere, if you will, by brute force. This means it has to steer downward as the earth falls out from under it due to travel. The autopilot/pilot does not know it/he is supposed to fly an always down-curving path. It is simply told to keep the barometer steady with what it can control, the trim tabs. And it doesn’t respond continuously, but periodically, which makes the circular flight around the earth into geometric approximations of a circle, using straight paths. If you graphed the motion of the trim you would find a train of semi-sinusoidal deviations up and down to correct for flight, but you would find they were riding on an inclined step-wise straight line ascending to the right that accomplishes the change in direction over the sphere segment traveled. This graph for a flat earth would find that inclined supporting component to have zero slope.

If it is unstable then it always needs to be actively controlled to do anything . That is essentially what it means to be unstable. It doesn’t make sense to do the thought experiment with an unstable aircraft. An unstable aircraft, when uncontrolled, doesn’t neatly follow the curve of the earth, it veers off randomly and crashes!

I am literally talking about above and below, in the vertical direction. Stability in all directions is important, but here we are talking about stability in the pitch axis. Yes, unstable aircraft exist, but they must be actively controlled anyway. To ask about following the curve of the earth without control you have to have an aircraft that can fly long distances without control. That requires a very stable design, including a center of gravity below the center of lift.unstable is different than conditionally unstable. a aircraft with its cg at its center of pressure, or even behind it, can be very stable at normal flyiing speeds. infact have less drag as well. But, near stall, can be much more unstable and unpredictable. I dont think we are talking about that..

So, with a stable or aircraft with its CG at its center of pressure, why would it have a "natural torque"? that makes it nose down? this is the part i was looking to understand. it seems to me, if the plane is flying level. (not straight due to the earth curve), and there is no active control to follow the curve. But, in reality, the plane is rotating about its axis, so is it the atmosphere that causes this rotation?