Swept flying wings

a tale of no tail

The easiest way to explain would be:

What is achieved by using sweep and twist? Consider the tail as being placed at the tips of the wings. To make you understand easily i would say that the tips provide down force to counter the moment generated by the airfoil in the middle.

But ... this info is not nearly correct. It is just a way to help as first step in understanding swept flying wings. Read on.

The angle of sweep can be shown in two ways. One is to the leading edge (used by Horten), the other is to a line, which is placed on 1/4 of the wing. Make sure, when using data of exciting models, that you don't use the wrong angle. If not mentioned which angle they use, take the one to the 1/4-line.

The twist-angle is the angle between the airfoil at the root of the wing (nearest to the fuselage) and the airfoil at the tip of the wing.

The airplane designers try to use as much lifting force as possible in a flying wing with sweep. So using a part of the wing to generate down-force would be a loss of efficiency. 

If you want the truth, you need to consider the flying wing with sweep as a tandemwing. Central part of wing and wingtips provide lift. The balance is generated by the amount of up-force and the distance between center of lift of the airfoil and the place of the center of gravity of the airplane.

I try to explain (still in as easy as possible words) how it really works using the picture above. The picture was generated by Flying Wing Designer, made by Marko Stamenovic (see below).

CG is center of gravity. The central part of the wing has more lift due to more area and a higher angle of attack (= angle between axis of airfoil and flight path of airplane). A big force, but ... the point of pressure of this force is located a bit in front of the CG (about one square in front of CG in this example). The wingtip parts of the wing have less lift due to the smaller area and the smaller angle to attack. But ... the point of pressure of these forces is located more backwards of the CG. Distance of this force is larger that the first mentioned force twoards the CG. Roughly it is even more than 2 squares behind the CG in this example.

Again, it is always a question of balance. A large force close to Cg and a smaller force at the other side but at more distance of the CG. Both can keep balance around the CG if properly designed. And the complexity of that is one of the disadvantages of a flying wing with sweep.

Advantages:

  • When using a twisted wing, the airfoils have not the same angle according to the longitude axis. This leads to good situations if you use a backward sweep. If the center section of the wing stalls, the tip airfoils are not near the angle to stall. If you place elevons on these tips, you can still control the aircraft. You can avoid getting the plane into a spin.

Disadvantages:

  • If you are planning to make a model, be prepared to see some complicated formulas in your research. But don't let it scare you. A friend of me made some good models, just be guessing the angles or using data from full-size airplanes. I have seen a site that gives a good help in choosing the right angle of sweep and twist. Just read a bit of the given theory and use the curves to determine the angles.  http://www.mh-aerotools.de/airfoils/flywing1.htm
  • Due to the sweep is construction more difficult than the flying wings with auto stable airfoils, which mostly have no sweep.

FLYING WING DESIGNER

Recently Marko Stamenovic made a very useful excell that help you a loooot when designing your own RC flying wing with swept wing. You can find all the info here.

General Aircraft GAL 56 (third version)

These two gliders of the German brothers Horten also use sweep and washout.

The lower one, the Horten Ho XIII, even has a very extreme sweep. The left one has a pilot lying on his belly. For more info about that, go see my proned pilots page. Permission to use pictures from Sharkit (www.sharkit.com). Sharkit is a firm which makes resin models of unconventional and experimental flying machines.


If you like to test a RC model of a flying wing with extreme sweep, go try the Horten H13B jet (electric ducted fan) of RBC (www.rbckits.com). Here you can see the model made by Erik Van Schaik.

Another drawing of Graham Ward.

Both engines would make this airplane uncatchable by enemy-fighters. But ... it never left the drawingboard. more detailed info at www.luft46.com

 

 

The lower video shows me inside the two-seater trainer of the SWIFT based in Beauvechain in Belgium. Contact Avia Airsports for flights in it.