Airplanes Versus Hovercraft - A motion problem

[drdede]

Well, you know when you're in a hovercraft, and your going one direction, and you turn around, you're still going that direction, and to go the opposite direction, you have to apply a force in the opposite direction, and slowly you will eventually stop the momentum - well, that's easy to simulate. Problem is, airplanes dont work quite like that. When you're in an airplane, when you turn, your momentum turns almost simulatenously, now how does that work? How do I go about simulating that properly?

[ryan0618]

The reason an aircraft turn happens like that is because an aircraft's engine is always on. In other words, a force is always being applied, pushing the aircraft forward. The reason is that for an aircrafts wings to work, air has to be flowing over them. So, the enigne is the force you talked about here:

you have to apply a force in the opposite direction, and slowly you will eventually stop the momentum

However, if you were turn off the engine midflight, an airplane would act exactly like a hovercraft (except the airplane would be falling).

Implementation would be tough. the book Physics For Game Programmers has a section on airplane physics, although I have never tried it out, and don't know if it would be right for your needs.

Good Luck,
ryan0618

[ola]

Aircraft do not use the engine to fly turns. Gliding aircraft can fly turns just fine, they use the gravity as engine. Just look at the space shuttle, for example.

Please have a look at these pages:
http://www.auf.asn.au/groundschool/index.html
http://www.auf.asn.au/groundschool/umodule1b.html <-- Here are some nice diagrams showing the force components in various flight states
http://www.auf.asn.au/groundschool/contents.html

also,
http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html
http://www.grc.nasa.gov/WWW/K-12/airplane/glider.html
http://www.grc.nasa.gov/WWW/K-12/airplane/turns.html <--- turns!

This is also a good site full of more advanced information:
http://sites.google.com/site/aerodynamics4students/

Cheers,
Ola

[Enki]

Well, you know when you're in a hovercraft, and your going one direction, and you turn around, you're still going that direction, and to go the opposite direction, you have to apply a force in the opposite direction, and slowly you will eventually stop the momentum - well, that's easy to simulate. Problem is, airplanes dont work quite like that. When you're in an airplane, when you turn, your momentum turns almost simulatenously, now how does that work? How do I go about simulating that properly?

The basic intuition of this situation is incorrect, hence the misunderstanding. The naive hovercraft part is correct enough, but the airplane part is completely missing why the turn happens and the momentum is changing.

The airplane maneuvers by manipulating airflow, which creates forces that redirect the aircraft (turning is actually more conceptually similar to gyroscopic precession than direct action-reaction of things like billiards balls). The application of those forces from the airflow stream are a major component of an aircraft loosing maneuvering energy. The engines merely keep supplying energy to allow the airplane to maintain or gain altitude despite the effects of those several flavors of drag and gravity. Also the airplane is moving through the air at a high enough rate that the air reacts as if it is a viscous fluid rather than the mild gas/relatively-frictionless surface of the hovercraft, magnifying the reactions of control surface deflections and resulting turning forces.

I haven't looked at the book ryan0618 suggested, but it's probably OK for really basic airplane dynamics. Be careful though of getting too close to nose-up of 90 deg or you will probably gimbal lock from Euler singularities, a common problem for maneuvering aircraft sims.