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Drag equation

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First take a look at this (anyway theres 3 drag areas top, front and side areas of a jet)

 

but lets talk about only two top view area and frontal area.

 

 

drag.jpg

 

 

 

how should i calculate drag force since: drag = sin(angleofattack)*drag_force_of_firstarea + cos(angleofattack)*drag_force_of_secondarea  doesn't seem to work as it should.

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This is a fairly crude approach to drag force. A simpler, but equally crude, approach would be to use the area of the aircraft projected onto a plane perpendicular to the direction of motion.

 

How are you expecting drag to work?

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I calculate drag by: drag_coefficient* ( (denisty * vecolity squared) / 2.0f) * drag_area

angleof attack is angle between model pitch and moving direction

 

 

 

brucedjones

Posted Yesterday, 02:51 PM

This is a fairly crude approach to drag force. A simpler, but equally crude, approach would be to use the area of the aircraft projected onto a plane perpendicular to the direction of motion.



How are you expecting drag to work?

 

wtf?

 

 

 

CombatWombat ill take a look at it.

 

 

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doesn't seem to work as it should.

 

As a simple approach to drag, your equations appear adequate. If that doesn't "seem to work," what are you expecting (as asked above) that you're not getting?

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whenever i raise my nose (even to 0.5 degree) i get massive drag that prevents me from reaching 302 knots

float AngleOfAttack =				AngleBetweenVectors(vel,ThrustForce_vec);
float tmpangle = RadToDeg(AngleOfAttack);

LiftCoeff = sin(AngleOfAttack) / 2.0f;//+0.050;

DragForce_vec = reverse_point(vel);  DragForce_vec = Normalize( DragForce_vec ); i_drag_force = DragForce_vec;
DragForce_vec = vector_multiple(DragForce_vec,0.080f * ((dens * squareSpeed)/2.0f) * cos(tmpangle*imopi)*3.6f); //the thrust drag force

i_drag_force = vector_multiple(i_drag_force,1.280f*((dens * squareSpeed)/2.0f)*   sin(tmpangle*imopi)*38.0f);

DragForce_vec = DragForce_vec + i_drag_force;


float LiftForce = 0.50f * LiftCoeff * dens * squareSpeed * 38.0f;// <--- LIFT AREA

LiftForce_vec.x = YPRangle.AIR_MATRIX[4]; /up
LiftForce_vec.y = YPRangle.AIR_MATRIX[5];
LiftForce_vec.z = YPRangle.AIR_MATRIX[6];

		LiftForce_vec = vector_multiple(LiftForce_vec, LiftForce);


result_force = GForce_vec + ThrustForce_vec + DragForce_vec + LiftForce_vec;

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Assuming that you've stepped through your code, checking values at each line: in which line do you get unexpected values?

((dens * squareSpeed)/2.0f) * cos(tmpangle*imopi)*3.6f)

I'm not sure what all you're variables and factors represent. However, a simple drag force would be proportional to 1/2*density*v2*area. If I understand your diagram and angles correctly, the area should be proportional to the sin(angle), not cos.

Edited by Buckeye

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