A Study of Hybrid Airfoil Design Method

Section 1. Icing does harm to the aerodynamic performance of aircraft and it is necessary to evaluate the aerodynamic performance of the aircraft in icing conditions. Icing wind tunnel test is the main method to study icing and anti/de-icing at present, but limited by icing wind tunnel test section size, sometimes it is impossible to carry out full scale model test in it. Instead of scaling the whole airfoil, a designed hybrid airfoil with a shorter chord length is usually used in icing wind tunnel which is composed of a leading edge geometry identical to that of the full scale leading edge and a shorter aft section. The aft section is usually designed to provide full scale flow field and droplet impingement on the leading edge. A newly developed method is to design the aft section to provide full scale airfoil surface pressure coefficient. This paper examined the “same surface pressure coefficient” design method based on icing similarity.

Section 2. According to the icing scaling method, the icing process consists of six similarities: (1) Geometric similarity (2) Flow field similarity (3) Drop trajectory similarity (4) Water catch similarity (5) Energy balance similarity (6) Similarity of surface water dynamics. If all of the similarities are achieved the icing processes would be the same. This chapter examined each similarity and found main flow flied parameter to each similarity. Finally it found that, based on the basic hybrid airfoil design theory, if the velocity field near the nose section of the hybrid airfoil is the same as that near the full scale airfoil, the parameters such as β, freezing fraction and so on will be the same. And then all of the similarities would be the same, and so as the icing process.

Section 3. A 1 m chord length NACA0012 airfoil model is chosen as a research model, which is also known as full scale airfoil model. The limits of water droplets impingement on the full scale airfoil leading edge will be predicted using numerical method. That part of the full scale airfoil is fixed for the hybrid airfoil, named nose section. The aft section of the hybrid airfoil is then designed to provide full scale airfoil surface pressure coefficient on the nose section of the hybrid airfoil. Flow field is calculated by solving Navier-Stokes equation and the catch efficiency distribution was acquired using an Eulerian method, and the prediction of the ice accretion shape is based on Messenger, multi-island genetic algorithm and gradient algorithm are used in designing. This paper assumes that if the percentage difference of the pressure coefficients between the two airfoils is within 10% (or within 0.05 in absolute value) at the grid points of the nose section, the design of the hybrid airfoil meets the requirements. Finally a 0.5 m chord length hybrid airfoil is acquired.

Section 4. In order to compare the velocity fields between the hybrid airfoil and the full scale airfoil, the value of velocity and the angle of incidence of the velocity from 7 planes are compared respectively. The chosen planes are in front of the leading edge of airfoils. Results shows small differences of velocity and angle of incidence of the velocity between hybrid airfoil and full scale airfoil. Only when it is very close to the surface of the airfoils, the difference is only a little more. Results shows that the water droplet catch efficiency and the water droplet impinging limit trajectory of hybrid airfoil are almost the same with those of full scale airfoil. And comparing the ice shapes of hybrid airfoil and full scale airfoil, it shows that this hybrid design method is good for the ice accretion simulation. It can be concluded that the velocity field near the nose section is the same when the surface pressure coefficient is the same, and then the hybrid airfoil ice accretion would be the same with full scale airfoil.

Section 5. Three conclusions are drawn from this study: