Beginner: Simple 2D flyer physics with Chipmunk?

[aidynskas]

Hi,

I'm working on a very small 2d sidescroller-type flying game. Last few days I've been trying to simulate lift and drag with Chipmunk with no luck. I've looked through various resources on the internet dealing with drag and lift but still couldn't implement it correctly.

I'm having a simple cpBody with some initial impulse force applied for speed. This is how I'm trying to calculate lift and drag (every frame):

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float lift = -airDensity * speed*speed * attackAngle * liftCoefficient;
float drag = -airDensity * speed*speed * attackAngle*attackAngle * dragCoefficient;

And full code below:

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float airDensity=1.29;
float speed=cpvlength(airplaneBodyVelocity);
float velocityAngle=-atan2(airplaneBodyVelocity.y,airplaneBodyVelocityy.x);
float attackAngle=bodyAngle-velocityAngle; //attack angle is difference between velocity and current angle set by player

float liftCoefficient=1.0;
float dragCoefficient=1.0;

float lift = -airDensity * speed*speed * attackAngle * liftCoefficient;
float drag = -airDensity * speed*speed * attackAngle*attackAngle * dragCoefficient;

cpVect;
forceVector.x=drag;
forceVector.y=lift;

cpBodyResetForces(airplaneBody);
cpBodyApplyForce(airplaneBody, forceVector, cpvzero);

Any help is welcome!

[slembcke]

Assuming that the formulas are right, that looks like it should be pretty close. There are a couple potential issues though:

First of all, the following snippets are equivalent:

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cpVect;
forceVector.x=drag;
forceVector.y=lift;

cpBodyResetForces(airplaneBody);
cpBodyApplyForce(airplaneBody, forceVector, cpvzero);

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airplaneBody->f = cpv(drag, lift); // or you can use cpBodySetForce() if you find it more clear

Now, this means that lift always goes straight up and drag is always horizontal. The forces should rotate with the wing (assuming that the wing rotates). So something like this:

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airplaneBody->f = cpvrotate(cpv(drag, lift), airplaneBody->rot);

The next potential problem is this:

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float attackAngle=bodyAngle-velocityAngle;

atan() always returns results between Pi and -Pi. Using an example in degrees 170 degrees - 190 degrees should be -20. When wrapped to the range (180, -180) you will instead be doing 170 degrees - -170 degrees which is 340 degrees. While 340 degrees and -20 degrees are the same thing, 340 degrees will break your calculation.

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float attackAngle=bodyAngle-velocityAngle;
if(attackAngle > M_PI){ attackAngle -= 2.0f*M_PI }