Skip to main content

TEst of Tilt-Rotor Air intakes

Final Report Summary - TETRA (TEst of Tilt-Rotor Air intakes)

Executive Summary:
The TETRA project is in the frame of CleanSky Project and particularly in the frame of the GRC2 subproject (Reduced drag of airframe & non lifting rotating parts) of the Green Rotorcraft (GRC) Integrated Technology Demonstrators.
Due to the expected growth of the rotorcraft traffic for passenger transport, the rotorcraft (including tilt-rotor) contribution to environmental impact, negligible today, would become more significant in next decade unless a major initiative succeeds in keeping it under control. An important aspect for the energy efficiency of an aircraft (and consequently for the reduction of the fuel burn and CO 2 emissions) is the behaviour of the engine air-intakes.
The present project pertains to the wind tunnel testing of the air-intake of a tiltrotor European platform (ERICA). The aim of the activity is the evaluation of the effectiveness (mainly in terms of pressure losses and flow uniformity at the AIP) of the shape optimisation performed by GRC consortium on the intake duct. Both original and optimised configurations were tested to assess the optimisation effectiveness by comparison.
The project was based on wind tunnel testing activity. The tests were carried out on a model appositely developed in the frame of the present project. Two different model of internal ducts were produced to compare the original and the optimized shape. The complete air-intake configuration models (including nacelle and external wing) were tested with the original and the optimized duct at Politecnico Wind Tunnel (GVPM), in order to evaluate the effectiveness of the new intake design optimised by GRC2. The wind tunnel tests included the use of internal PIV technique to evaluate some physical details.
The model was designed and produced by Revoind with collaboration of Politecnico di Milano. The control system and the test instrumentation was provided by Politecnico di Milano that also carried out the test activity.

Project Context and Objectives:
The objective of the project is the assessment, by means of wind tunnel testing, of the effectiveness of the optimization carried out by GRC2 on the internal duct shape of the ERICA tilt-rotor air intake. The tests were carried out on a very complex model of the nacelle, including a part of the tilting wing and a rotating hub with the blade stubs. The air suction corresponding to the required flow-rate, both at the AIP (Aerodynamic Interface Plane) and at bypass exit, were provided by air movers. Beside the classical pressure and flow measurements, also some endoscopic PIV (Particle Image Velocimetry) surveys were carried out to get some important details of the flow field. This activity integrated the information about the pressure and velocity fields measured on the AIP to evaluate the behavior of the internal flow inside the baseline and optimized ducts.
In the first part of the project the activity of this first period was mainly addressed to the model design and manufacturing and to the preparation of the experimental setup.
The air-intake model was a quite complex system and although the main technical solutions were already defined in the project proposal, an accurate definition of appropriate solutions for some mechanical problems was performed. The model has been detailed design considering all the requirements due to the expected operating conditions and considering all the particular systems (electrical and hydraulic motors, pressure scanner, endoscopic PIV) to be held with all the corresponding piping, cabling and optical access. All the CAD/CAM design of the model was developed and the production started from the internal ducts to start with the check and calibration of the measurement system.
The 3 and 5 hole probes to be used inside the duct were manufactured and calibrated as well as the suction system provided by air-movers was also tested to calibrate the flow rate measurement at both AIP and by-pass. A very important aspect of the project was the preparation of the endoscopic PIV system set up for the surveys in the internal ducts.
When all the parts of the model were manufactured, the activity was focused on the model assembly in the wind tunnel test section considering all the configurations planned (hover, cruise and conversion). All the pneumatic and electrical connections for the measurements were checked before the tests as well as the correct functioning of the hydraulic motor driving the rotating hub with stubs.
A comprehensive test campaign was performed in the Politecnico di Milano wind tunnel to compare the aerodynamic performances of the ERICA engine air-intake. Indeed, several operating conditions were tested i.e. cruise, conversion and hovering, with and without blade stubs for several values of flow rates of the principal and by-pass duct.

Project Results:
Cruise was the flight condition considered in the optimization carried out by GRC2. As a matter of fact, a remarkable total pressure saving was found with the stubs-off model. The total pressure drop at AIP was reduced of about 13% with the optimized duct. On the other hand, in terms of flow uniformity (as measured by the parameters DC60 and SC60) the results were very similar for the baseline and optimised ducts.
Unfortunately, the presence of the rotating hub significantly reduced the gain. In fact, the total pressure is 6% higher with the optimized duct, but it must be considered that the stub blade makes much more disturbance that the real complete one as the tip of the stub , and consequently the tip vortex , is very close to the duct inlet. This could be the reason of the very apparent differences in the flow at AIP for the two configurations. It must be also observed that the AIP flow behaviour is quite similar to the predicted one only when the stubs are not present, although the rotor effects were included in the simulation by means of an actuator-disc boundary condition. This again leads to the conclusion that a truncated rotor is not a good model of a rotor for these purposes, especially when the blade stubs are so short that their wake significantly interferes with the air intake. In any case these results give an important indication about the importance of a proper design of the rotor hub.
In the tested conversion conditions the optimized duct resulted to produce a total pressure increase in both stubs-off and stubs-on cases. Again the pressure drop was reduced of about 13% and the flow behaviour was not very different in the two cases probably because at the angle of attack tested the duct inlet was less interacting with the hub wake.
In the hovering conditions, not included in the optimization process carried out by GRC2, a rather unexpected result was found. A loss increase of 2% was found without stubs and a 14% increase was obtained with them. In any case, it can be said that in this condition,that does not influence so much the fuel consumption of a mission, the flow inside the duct resulted to be rather regular with and without stubs.

In general, the wind tunnel tests results confirmed that the optimized duct offers significantly improved performance with respect to the baseline configuration for the cruise and conversion conditions, in particular in terms of total pressure drop saving. Moreover, the experimental investigation highlighted that a careful choice of the blade stubs length has to be carried out to avoid significant interferences between the wake of the stubs and the air-intake.
Finally, state-of-the-art endoscopic PIV surveys enabled to investigate the flow behavior inside the duct showing that this technique is both feasible and very promising to investigate the flow properties in region of interest where other measurement techniques fail or cannot provide such a level of detail.

Potential Impact:
The tilt-rotor is a quite promising concept that conjugate the possibility to take off and landing vertically with performances typical of a real aeroplane. There is a real need of aircraft like this for many purposes as, for example, the fast rescue service in case of environmental disasters. Also the connections between cities and big airports could be efficiently carried out by aircraft of this typology. Of course an improvement in the efficiency of the air-intakes (and consequently of the engines) will increase the flight range (so important for rescue) and reduce the pollution (so important for the environment).
The experimental data allowed to validate the optimisation process so that it will be applicable to the NextGen (CleanSky2 tiltrotor) development with aim of more efficient engine producing higher engine power efficiency, less fuel burn, less emission, less pollution.
All the achievements of the project will become also part of the cultural background of Politecnico di Milano that is teaching to a wide number of students coming from Europe and the rest of the world.
The main dissemination activity consists in the production of scientific publications.

List of Websites: