Theoretically a zero pitching moment would be optimal, but the reality is these aerofoils have significantly higher drag, cancelling out much of their benefit and making them useful only for niche applications which demand zero pitching moment such as flying wings or helicopter rotor blades. Unfortunately this causes ‘trim drag’, so there is some apparent benefit in using a low pitching moment aerofoil for the main wing. A conventional aeroplane balances the main wing pitching moment with a down-force provided at the horizontal tail. For the uninitiated, pitching moment is the tendency for a cambered aerofoil to want to rotate nose down when it is creating lift the strength of the pitching moment being dependent on the distribution of the camber. This is important for design, as an aerofoil can be selected with a camber that gives minimal drag at the cruising C L.Ĭamber not only affects lift, but the distribution of the camber is also critical for our next consideration, Pitching Moment. Looking at figure 3 it can be seen that the minimum drag of the NACA 2415 aerofoil is not noticeably higher than the NACA 0015, but it now occurs when the aerofoil is generating significant lift. The increase also moves the ‘zero lift angle-of-attack’ to a negative value, meaning that a cambered aerofoil produces positive lift even at small negative angles-of-attack.įigure 2 – Lift Coefficient vs Angle-of-Attackįigure 3 – Lift Coefficient vs Drag Coefficient With camber the C L the aerofoil delivers is increased across the whole angle-of-attack range, including an increase in the maximum value. Adding camber to an aerofoil results in a couple of effects as illustrated by the lift vs angle-of-attack and lift vs drag graphs in figures 2 & 3. The first ‘NACA 0015’ is a symmetrical aerofoil which by definition has no camber, the second ‘NACA 2415’ has exactly the same thickness distribution, but has camber added as indicated by the curved Mean Line. The top part of Figure 1 shows two aerofoils which were tested by NACA in the 1920’s. Let’s start with maximum lift and minimum drag – If you really want a wing which successfully meets both of these requirements the best thing you can do is add flaps or other high-lift devices, but for the sake of this discussion that would be cheating! So how do we get more lift? The simple answer is to choose an aerofoil profile with some curvature, or ‘camber’ in the technical jargon. There are of course other non-aerodynamic considerations such as providing the section thickness to minimise the structural weight and provide enough storage for fuel, undercarriage etc., or more subtle considerations such as the effect of aerofoil selection on control forces, but meeting the above criteria is a good start! Benign stall behaviour – Most people prefer gentle mushing to surprise aerobatics.Low pitching moment – Minimises trim drag and allows for a smaller tail.Low drag – So you can fly and climb faster or with less power.High maximum lift – For a lower stall speed or a smaller wing.You could custom design your own aerofoil, but without wind tunnel testing this is a somewhat risky proposition… and do you really want to reinvent the wheel?īefore selecting an aerofoil you need to know what you want it to do so here’s the wish list: For us mere mortals high-end software and wind tunnel testing are out of reach, so sticking with tried and tested aerofoils, for which data is already available, is the usual way to go. Getting back to aerofoils – these days the aviation industry has moved away from selecting aerofoils from catalogues, preferring to use computer modelling to design custom profiles for each project, this is followed by testing the final results in a wind tunnel to verify the computer model. It’s a superb resource not only does it have an excellent explanation of lift, but understanding its content will undoubtedly make you a better pilot too! Now I’m not going to take on the challenge of dispelling any ‘lift myths’ here, but I will implore everyone reading this article to visit and read Chapter 3, or even better the whole on-line book. This is not much of a surprise when you consider that even after more than a century of powered flight many, if not most, people still don’t have a correct understanding of how an aerofoil actually creates lift. There are few areas of aircraft design more shrouded in mystery and misunderstanding than aerofoil selection.
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