source through a flow controlling venturi 30 and thence through flexible connectors 31 and duct 14 to the engine simulator the flexible connectors are of known design and allow high pressure air to be supplied to the simulator without imposing forces upon the force measuring balances 13 instrumentation within the simulator primarily sensing temperatures and pressures and lubricating oil supply are connected to outside equipment by flexible lines indicated at 29 in operation the entire apparatus is normally within a room of sufficient volume to constitute a large plenum chamber so that the surrounding air is essentially static thus no correction need be made for the momentum of the air approaching the simulator nor are there any appreciable flow velocity distortions the effect of the bellows 17 and the compensating bellows 22 substantially eliminates tare forces imposed upon the balances 13 thus the balances 13 measure the true thrust force developed by the simulator accurate determination of the mass flows involved are obviously of prime importance the flow through venturi 30 to power the simulator is readily and accurately ascertained by known characteristics of the venturi similarly the flow through the venturi group 27 at the exit is easily and accurately determined by the known characteristics of these venturis it is apparent of course that the mass flow through the fan section of the simulator is the difference between the total flow exiting the apparatus as determined by venturi group 27 and that supplied the simulator as determined by venturi 30 the features just described with reference to the diagrammatic showing of fig2 are incorporated in the exemplary embodiment of fig3 through fig6 in this configuration the chamber 40 is a tank of suitable large volume.