S-shaped profile
S-shock profiles ( English reflexed airfoil s ) are profiles for wing , particularly at tailless aircraft , flying wings , winglets and rotor blades of helicopters are used.
The name describes the course of the skeleton line that touches or intersects the profile chord in the rear area of the profile and then points upwards again in an S-shape to the rear edge of the profile. However, this peculiarity does not necessarily mean that an S-lay profile must be externally recognizable, as on the adjacent drawing. For example, the NACA-230XX profile series has an almost straight line on the top in the rear area, while the bottom is convex. A concave contour on the top side of the profile is therefore not absolutely necessary in order to identify a profile as an S-lay profile.
description
Normal profiles create a torque around the transverse axis due to their curvature (usually measured at 25% of the profile chord), which tries to turn the nose of the wing downwards (negative profile torque). This then has to be compensated for with an additional horizontal surface (usually a horizontal stabilizer or duck wing ). A conventional horizontal stabilizer generates downforce throughout the flight in order to counteract the profile moment of conventional profiles and the center of gravity in front of the center of lift.
The S-flap means that such profiles either have a significantly reduced negative profile moment or no profile moment at all (like symmetrical wing profiles) or even fly inherently stable with an even stronger S-flap. However, the maximum buoyancy of the so-called " pressure point-resistant " profiles (for example hoarding) is higher than with a symmetrical profile drop of the same thickness and with inherently stable profiles roughly the same as that of comparable symmetrical drops. The zero lift angle of such inherently stable profiles also corresponds roughly to that of symmetrical profiles at 0 °.
If the S-stroke is increased further or the intersection of the skeleton line and chord is moved further forward, the zero lift angle is even positive, i.e. the profile only generates lift when a clearly positive angle of attack is reached. Such profiles - turned on their backs - are used as so-called supercritical profiles in many commercial aircraft.
properties
S-flap profiles have a lower air resistance than "normal" profiles in flight with low lift coefficients, comparable to a flap that is raised for high-speed flight on a normal aircraft, but without the kink caused by the flap deflection. However, due to the upward trailing edge of the profile, S-flap profiles have a lower lift capacity and a higher resistance in slow flight.
commitment
S-flap profiles are particularly important in the construction of flying wings , as they offer stability around the transverse axis , which would otherwise require a tail with a horizontal stabilizer . In helicopters, they offer the advantage of increased lift capacity compared to the frequently used symmetrical blade profiles.
Various war planes from the Second World War also received a variant (Clark YH) of the well-known Clark Y profile with a slight S- flap. These included, for example, the Hawker Hurricane and the Ilyushin Il-2 . Messerschmitt be used in the series Bf 109 , Bf 110 , Bf 161 / 162 as well as the Me-321 / 323 The American NACA 2R1 with thicknesses from 11 to 18%. A slight deflection of the flaps made it possible to restore the high-lift properties of the original profile, but with the kink that now exists in slow flight. However, performance losses in slow flight were irrelevant for combat aircraft. The Focke-Wulf Fw 190 , Vought F4U Corsair, Grumman F6F Hellcat fighter planes used profile rails made of profiles from the NACA-230XX series, mostly NACA 23015 at the root and NACA 23009 at the edge curve .
The companies Beechcraft, Robin and Fournier also mainly used profiles of the (so-called five-digit) NACA series 230XX or 430XX, since the best possible performance in cruise flight was the main focus of the designs.
history
Friedrich Ahlborn was the first to examine the flight characteristics of the Zanonia seed ( Alsomitra macrocarpa ) and recognized the S-beat in the profile as one of the reasons for its flight stability. However, the Flugsamen and the technical implementation developed by Ignaz Etrich in 1906 does not have a continuous S-blow profile, but is also aerodynamically twisted .
The first such profiles were tested by Georges Abrial in Gustave Eiffel's wind tunnel and the implementation of a tailless aircraft with an inherently stable wing without tail unit and without swept wing ( board only wing ) was carried out by René Arnoux (Arnoux Stabloplan or Stablavion) and Abrial (e.g. Abrial A12 "Bagoas") itself. The best known inherently stable wing profile is probably the Fauvel F2 with 17% thickness of the AV.36 by Charles Fauvel , which is also based on a profile by Georges Abrial.
Stable profiles made by yourself
In earlier days there was a rule of thumb that said that a profile can safely fly with its own stability when the intersection of the skeleton line and the profile chord is between 75% and 80% of the profile depth. The method was used by Chuck Clemens and Dave Jones to derive the CJ profile series from existing NACA profiles. For any normal profile (starting from X0 Y0) a new chord was drawn in, which intersected the previous skeleton line at 75%. Then the rear part of the profile was changed using a curve ruler so that it ran harmoniously up to the end of the new tendon at X 100. A mixed form (strak) of the newly created and the original profile then also represented the moment-free (pressure point-resistant) variant of the "new" profile family. The most suitable for this somewhat rustic method were biconvex starting profiles (colloquially also known as semi-symmetrical profiles) from slight bulge (about 2-4%) and 10-12% thickness. In particular, the Ritz profile series and especially the Ritz 2-30-12 were ideally suited for such modifications. The illustration on the right shows the Ritz 3-30-10, which, in its modification as AR 2610-S80, was very popular in model building circles in the 1980s.
literature
- Reinhard H. Werner: "" Flying wing sailors remote controlled "- Neckar-Verlag 1984- ISBN 3-7883-0194-5
- Hans-Jürgen Unverferth: "Fascination Nurflügel" -Publisher for technology and craft-1989- ISBN 3-88180-026-3
- Nickel / Wohlfahrt: "Tailless aircraft. Their design and their properties" Birkhäuser-1990, ISBN 3-7643-2502-X
Web links
- Website with explanation and drawings
- René Arnoux website
- NACA profiles - Sections 3.5.1 and 4.2
Individual evidence
- ^ Warren F. Phillips: Mechanics of Flight . John Wiley & Sons, Hoboken, New Jersey 2004, ISBN 0-471-33458-8 , pp. 346 ( limited preview in Google Book search).
- ^ Peter Garrison: Refugee from Compromise . In: Flying Magazine . September 2003, ISSN 0015-4806 , p. 92 ( limited preview in Google Book search).
- ↑ Ulrich C. Hallmann u. a .: Hundred years of patent office: Festschrift. German Patent Office, Heymann, 1977, ISBN 3-452-18307-6 , p. 238.
- ^ Friedrich Ahlborn: The stability of the flight organs. 1897.
- ↑ Hermann Dingler : The movement of the plant flight organs. 1889.
- ↑ Ingo Rechenberg : On the nature, value and development of bionics. ( Memento of the original from September 23, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.1 MB) Script, TU Berlin, 2000/2001.
