WING PROFILE WITH VARIABLE FLAP March 2009
INTRODUCTION Flaps are something that can be found on a majority of aeroplanes. That they fill an important purpose in affecting the aircraft s performance is quite obvious (otherwise they would not be used) but why and how they do it is something this experiment hopefully will bring clarity to. On all commercial aircraft today these so called high lift devices comprise of advanced constructions where both trailing edge and leading edge flaps are used. The trailing edge flaps usually consist of several elements, see pictures 1 and 2. In this experiment we will take a closer look at a more simple type of flap which nevertheless basically has the same function as the more sophisticated flap arrangements. Picture 1. Boeing 747 in landning configuration. Picture 2. MD-11 in landing configuration. Laboration vingprofil med variabel klaff 2
DESCRIPTION The wing profile used in this experiment is called NACA 2412. NACA (being the predecessor to NASA) is one type of standard used on profiles and the numerical combination 2412, simply put, tells the shape and form of the profile. This type of profile is typical for smaller general aviation aeroplanes, such as the Cessna 172. The wing profile is equipped with the simplest form of high lift device, called a plain flap. On an aeroplane it is placed on the wing s trailing edge close to the fuselage, see the Cessna in picture 3 below, and un-deployed it holds the shape and form of the wing. The plain flap was the first kind of high lift device used on an aeroplane and just because of its simplicity (synonymous with low weight, which is very important in aviation) it still is very popular. Today it can mostly be found on General Aviation aircraft such as Cessna, Piper etc. Figures 3 and 4 show two types of aeroplanes that use the plain flap. Please notice the extension of the flap on the plane in picture 4. Picture 3. Cessna 172 with plain flap. Picture 4. Vought F-4 Corsair with plain flap. Laboration vingprofil med variabel klaff 3
DESCRIPTION, contd. A plain flap is, just as the name implies, a very fundamental construction. The flap itself is hinged and can only be angled down. As a comparison, the flap devices that can be found on larger commercial aeroplanes (see pictures of the Boeing 747 and the MD-11) not only are angled down but also extended backwards to increase the wing area (these are called Fowler flaps). THE EXPERIMENT What this experiment seeks is basically to examine how different flap extensions affect the lift and drag produced by a wing, and thereby give you an understanding on how high lift devices work and why they are used. The practical part of the experiment is made using the university s eminent wind tunnel. The wind tunnel is equipped with a measuring device that can register lift and drag unfortunately (for you) not both at the same time. The measurement device is then connected to a computer adjacent to the tunnel where values are presented. There is also a smoke generator that enables visual demonstrations of the flow around the profile. PRACTICAL In the practical part of the experiment you are to make measurements on three different flap extensions: 0, 22,5 and 45 degrees. For every flap angle you are to register how lift and drag varies in relation to angle of attack. Start with the flap un-extended (i.e. zero degrees) and position the profile at an angle of attack of 5 degrees. Turn on the tunnel, read and register the value. When registered, turn of the tunnel and re-position the profile at an AoA of, e.g. 3 degrees. Repeat this until a AoA of approximately 15 degrees is reached, or the profile stalls. Do the same with the flap extensions 22,5 and 45 degrees. Please observe that the measurement equipment only can register one force at a time and that lift and drag therefore must be measured seperatly. A more extensive briefing on the tunnel and how to operate it is given on location. Laboration vingprofil med variabel klaff 4
THEORY For practical reasons it is simpler to use the dimensionless coefficients for Lift and Drag, C L and C D respectively. These can be calculated using the equations below. Coefficient of Lift: L V C L 1 2 2 S Coefficient of Drag: D V C D 1 2 2 S The coefficients C L and C D are to be more specific functions of angle of attack (AoA), Mach number and Reynolds number but since both the Mach number and Reynolds number are pretty much constant they can be ignored. Note that the denominator for both equations are the same. PRESENTATION OF RESULTS The results from this experiment are to be summarised and presented in a written report. The report should include: - A description of the actual experiment (execution, purpose etc.) - Presentation of measurement data - Graphs of C L och C D plotted against angle of attack - A discussion on how lift and drag are affected depending on flap extension. The report should also discuss the following: - What function does high lift devices have? - How does the use of flaps influence the maxium value of C L (dvs. C L,max )? - More advanced flap systems also increase the wing area. How does this influence C L? Laboration vingprofil med variabel klaff 5
Table 1 Flap extension: 0º Amb. Pressure: mbar Wing area, S: m 2 Amb. temperature: º C Air density: kg/m 3 Angle of attack (º) Air flow speed (m/s) 1/2*ρ*V 2 *S Lift (N) Drag (N) C L C D C L /C D Laboration vingprofil med variabel klaff 6
Table 2 Flap extension: 22,5º Amb. pressure: mbar Wing area, S: m 2 Amb. temperature: º C Air density: kg/m 3 Angle of attack (º) Air flow speed (m/s) 1/2*ρ*V 2 *S Lift (N) Drag (N) C L C D C L /C D Laboration vingprofil med variabel klaff 7
Table 3 Flap extension: 45º Amb. pressure: mbar Wing area, S: m 2 Amb. temperature: º C Air density: kg/m 3 Angle of attack (º) Air flow speed (m/s) 1/2*ρ*V 2 *S Lift (N) Drag (N) C L C D C L /C D Laboration vingprofil med variabel klaff 8