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A “tail” of better aerodynamics: NEXTTRIP

23 February 2021

The empennage, also known as the “tail” of an aircraft, provides much-needed stability and directional control to the plane. But these familiar structures could be optimised to improve their aerodynamics, reducing drag and weight, thereby increasing the overall efficiency of an aircraft. 

The Clean Sky NEXTTRIP project answered that challenge, developing two new designs – the V-tail and the T-tail – which will feed into the final tail for the Next Generation Civil Tilt Rotor – Technology Demonstrator. Both designs will be lightweight and reduce drag, while retaining or improving the aerodynamic characteristics, stability and the directional control needed both in forward flight and in transition between hover and airplane mode.

In addition to the wind tunnel test, a CFD-based aerodynamic optimisation was performed on the aircraft V-tail configuration, aimed at further enhancing aircraft stability and reducing drag. The main outcome of the optimisation was a reduction of V-tail drag by 2-4%.  Stability characteristics were also improved, and the low-speed pitch-up phenomenon during transition from hovering to forward flight was confirmed to be of low magnitude. 

The existing 1:5 scale, full-span NICETRIP model was modified to accommodate both T-tail and V-tail geometries.

A new set of rotor blades with a 40% bigger radius was also developed to better capture rotor inflow effects over the wings and empennages, improving the aerodynamics of the aircraft and reducing drag.  

New visualisation technique developed

The V-tail was wind-tunnel-tested in DNW-LLF, and a new technique was used to visualise the flow field around the NEXTTRIP V-tail. Particle image velocimetry (PIV) holds that a fluid seeded with tracer particles is assumed to faithfully follow flow dynamics, and is illuminated so that the particles are visible. This new approach involved the employment of large-scale PIV with helium-filled soap bubbles (HFSB) as particles. 

HFSB are both lighter and larger than conventional particles and they enhance light scatter. The technique yielded excellent flow visualisations, and was tested successfully up to 60m/s with a 3x3 seeding rate. 

The project also optimised its use of the wind tunnel, by using state-of-the-art remote controls on the model to operate all possible settings of the individual components independently, without the need to enter the wind tunnel to make model changes.

New heights

NEXTTRIP will be presented at several conferences over the coming year: the AIAA Aviation Forum (Washington DC, USA, 7-11 June 2021) and the 47th European Rotorcraft Forum (Glasgow, UK, 7-10 September 2021); 

The results will be used in a follow-up project, HIGHTRIP. While NEXTTRIP successfully characterised the new empennages at low speeds, HIGHTRIP will build on those results to create a new tail that is optimised for higher speeds. The final tail will then be integrated into the NGCTR - TM demonstrator.

This project received € 2 582 756.90 in EU funding under the Horizon 2020 programme as part of Clean Sky, while the overall project costs were € 2 777 222.50. The topic manager was Leonardo Helicopters and the project partners were: NLR, DNW, DLR, HIT09, and Fokker.