The governing principle is Newton's Third Law of Motion: "Every action has an equal and opposite reaction", so the force of the exhaust being directed upwards causes a reaction force driving the exhaust pipe downward.The equation is F = MA, so what's needed to compute it is the exhaust mass and the acceleration (from zero vertical velocity to whatever). Mass will be ingested air plus fuel, or 14/13 x CFM x 0.076 lb., but acceleration is a little trickier as it depends on exhaust gas velocity, which in turn depends on density and exit pipe diameter. For example, compared to a typical collector, a proper rocket nozzle would increase thrust at the expense of power lost to backpressure.
I have a policy of not doing math in public, so someone else can take it from here...
Felix, qui potuit rerum cognscere causas.
Happy is he who can discover the cause of things.