Umran,
Do a simple experiment with fish scales please. The force on the firewall is indeed constant, but that is not the case of the force between whole plane and rest of world.
Fprop=Ffirewall
Fplane=Ffirewall-Fdrag OR
Fplane=Fprop-Fdrag  This is what fish scales register.

Any shielding of the prop vortex blast must NOT be connected to the engine firewall system. In that case you can get rid of the drag factor.
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