As in the case of propulsion by turbofan engines, the propulsion by propeller or other open rotor engines also affects the stability and control of the aircraft. Since, within their operating envelope, the propeller driven aircraft achieve higher propulsive efficiencies than turbo jet or turbofan engines, the topic of simulation of propeller effects in an experimental simulation of flight conditions remains a topic of continuous development at DNW.
The parameters that have to be correctly simulated are the thrust coefficient and the advance ratio, relating the rotor angular velocity to the free stream velocity. For a good quality simulation, it is desirable to simulate the thrust coefficient both in the aircraft frame (thrust vs. the dynamic pressure of the aircraft, Tc) as well as in the propeller frame (thrust vs. the dynamic pressure of the propeller blades, cT). In that case the advance ratio and the blade tip speed ratio are also similar to the flight conditions.
After separating the propeller forces and moments from the overall measurements with the help of the rotating Shaft Balances (RSB), the propeller induced flow effects can be quantified as a result of comparisons between the aerodynamic forces of the powered and the unpowered model. Typical results will include the measurement of interference drag and the influence of the propeller slipstream on the stabilizer. Further tests can be dedicated to the analysis of the inlet flow as it is influenced by the periodic wakes of the propeller blades.
In a manner similar to the simulation of the turbofan powered aircraft, the behavior of the isolated propellers needs to be quantified in order to be able to identify the effects of propulsor/airframe interferences. The isolated propeller tests are usually performed in a smaller wind tunnel than the tests with the fully integrated model. The propellers in both isolated as well as installed configurations are driven by dedicated air motors, suitable due to their high power density and capable of maintaining geometrical similarity in addition to the satisfying the power scaling requirements. The isolated propeller tests complement the data obtained in situ with the help of the RSB-s and the main balance.
Since the wing mounted propellers in tractor configuration produce a slipstream engulfing part of the wing, procedures to correct for the influence of the slipstream in the powered aircraft aerodynamic properties have to be considered in the evaluation.
The tests for determination of the stability and control can be complemented with aerodynamic measurements on the model (pressure) and in the flow field by means of rakes or PIV.
As a special service, the deformation of the rotor blades under high thrust conditions can be determined with the help of the non-intrusive optical measurement technique called stereo pattern recognition (SPR).