yorisimo's response seems ok, i guess you can also use:

phent_addforcelocal (ENTITY* entity,VECTOR* vForce,VECTOR* vPos );

where vPos is the location on the plane you want to add a specific force, may be able to use it to push the front down a bit.. but really why do you need this? if you want the front to appear heavy, cant you just keep tilting it down a bit and increase speed based on the tilt angle?

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