Winglets (Eng. winglet «a small wing») represent a special aerodynamic form of the wingtip which is designed to reduce inductive resistance produced by the vortex coming from the wing tip.
In order to understand the operation principle of winglets, it is necessary to look at the aerodynamics theory. So far as it is known, the total force of the wing aerodynamic resistance consists of three components:
- drag force which is determined by the form and thickness of the wing section with a fixed attack angle and air speed;
- friction force which depends on the roughness of the section surface;
- inductive resistance which will be discussed below.
Inductive resistance appears when the air on the wingtips flows from the high-pressure area under the wing to the low-pressure area above the wing (Fig. 1). Since the wing is in free air, such flow induces vortex cores and additional flow angularity on the section. The development and swirl of vortex cores needs energy; it is withdrawn from useful power. Flow angularity on the section caused by vortices reduces the lift force. As a result, the aircraft is subject to additional resistance when moving forward.
It is considered that due to the flow and pressure balancing around 5% of the wing lifting surface which is exactly its tip part cannot work effectively without winglets, which indicates the reduction of the effective wing span.
In view of this, there is a reasonable wish to exclude or at least minimise the possibility of such unproductive airflow. Winglets, specially designed aerodynamic surfaces on the wingtips representing a mechanical and aerodynamic obstacle for vortices, are the solution to this problem (Fig. 2).
Fig. 2. Schematic representation of the intensity of vortices formation on a regular wing and wing with a winglet.
There are different forms and names of such wingtips (Fig. 3). If we assess the effectiveness of using winglets on passenger airplanes with respect to fuel efficiency, its value ranges from 1.5% to 7%.
Why do we give examples of using winglets on wings? We should talk about propellers… It is simple: propeller blades are the same wings, spinning wings which are subject to the same aerodynamics rules.
The DTpropeller propellers’ peculiarity and difference from the propellers of other manufacturers is the presence of aerodynamic wingtips of a complex form on the blades (Fig. 4). It was determined in the process of numerous tests that the most effective form is a combination of a winglet and a raked wingtip.
Fig. 4. DTpropeller wingtip.
In order to assess the performance efficiency of winglets, we performed comparative tests. We manufactured two propellers with absolutely identical parameters: diameter, attack angle, form, and blade section. However, one propeller had regular wingtips and the other one had winglets.
2-bladed carbon fixed-pitch propellers 125 cm in diameter with the Moster 185 engine with a reduction of 1/2.68 were tested. The tests were performed multiple times on different days and in different weather conditions with the change of order of installation of propellers to the engine. In order to minimise the measurement errors, the periods between the tests did not exceed 20 min. Therefore, it is appropriate to state that the results below were obtained in identical conditions: the same engine, pressure, temperature, humidity, air speed.
As it was mentioned before, the blades of the two propellers had an identical attack angle which let the engine develop the maximum engine rpm speed allowed by the manufacturer, that is 8300 rpm. During the testing of the propeller without winglets, the engine developed a speed of 8300 rpm and the maximum static thrust was 74-75 kg. Then the propeller with winglets was installed onto the same engine. The engine speed turned out to be significantly higher and reached the level of 8500 rpm. Moreover, the thrust increased by 2-3 kg. The higher engine speed with the propeller with winglets proves the ability of such wingtips to reduce airflow resistance, thus, release a part of the engine power which was used to overcome airflow resistance when using the propeller with identical parameters without winglets. In other words, blades with winglets have lower aerodynamic resistance, hence, higher aerodynamic efficiency.
In order to explore the influence of winglets on performance efficiency, we performed a new series of tests with the results presented below. The information obtained shows that with identical engine speed and in identical weather conditions the propeller with winglets helps receive higher maximum static thrust. Two 2-bladed carbon fixed-pitch propellers 125 cm in diameter with the Moster 185 engine with a reduction of 1/2.68 were tested. The first propeller had no winglets, its attack angle let the engine revolve with a speed of 8300 rpm. The second propeller had winglets, its attack angle was increased by one degree so that the engine could not exceed the maximum allowed speed of 8300 rpm. The period between the tests of the two propellers did not exceed 20 minutes. The engine with the propeller without winglets with a speed of 8300 rpm developed a static thrust of 74-75 kg, whereas the maximum static thrust with a speed of 8300 rpm of the propeller with winglets was 76-77 kg.
Another positive point of winglets is the reduction of aerodynamic noise by way of reduction of vortex formation on wingtips. With the propeller speed of 2140 rpm (engine speed of 5700-6000 rpm) the level of noise of the propeller with winglets 125-130 cm in diameter is lower than that of the propeller without winglets, on average, by 2 dB (test video: https://www.youtube.com/watch?v=Fawof1QqjMA).
The tests performed proved the effectiveness of the wingtips we designed. Now the blades of all carbon propellers produced by DTpropeller have this element regardless of the fact that the production of blades with winglets is more intensive and results in higher production costs.