Talk:Drag curve (gliders)

Dirty polar

I have never seen a polar for a glider with wet/dirty wings. Can anyone cite one? — Preceding unsigned comment added by BWDuncan (talk • contribs) 15:02, 5 September 2011 (UTC)

There is an infinite variety of wetness and dirtiness that can be found on an aircraft wing, so it is unlikely there is any authoritative information of the kind you are looking for. However, if you get hold of the book Theory of Wing Sections by Ira H. Abbott and Albert E. Von Doenhoff (1949 & 1959) you will see in Appendix IV that there is loads of information about scores of different airfoil sections. There is information related to the smooth airfoil section, and also some information about the airfoil with standard roughness. This information is obtained by testing airfoil sections that have been roughened in a consistent manner - probably by coating the test airfoil with sand that has been selected using a sieve to obtain uniform diameter of particles. Standard roughness is an approximation of wet and dirty wings.

The laminar-flow airfoil sections display characteristics that will be of interest to all glider pilots. These laminar-flow airfoils show a low-drag "bucket" at the optimum range of lift coefficients, as you would expect. (Low-drag bucket is a significant reduction in drag coefficient over a small range of lift coefficient.) With standard roughness the drag coefficient is higher than for the smooth surface, as you would expect, and the low-drag bucket is absent. Dolphin (t) 08:06, 6 September 2011 (UTC)

That's great, I'll try to find that book. Maybe I'll even try to measure the performance of my 1976 Glasflügel Mosquito! The wings have suffered gel crazing and have some cracks on the leading edge. I can't imagine getting 39:1 out of it! I agree that there are many variables associated with such polars. I always find one must read the manufacturer's published polars with a hearty scepticism.

I hoped someone would be able to point to a measured polar with bugs in Soaring Magazine or similar. Thanks!--BWDuncan (talk) 14:51, 7 September 2011 (UTC)

Measuring the performance of any glider is always a good idea. Manufacturers determine the performance of their new product, using the carefully-prepared prototype, for only one reason - to show it is better than the competition! Have fun - I haven't flown the Mosquito but I have flown the Hornet and the Cirrus 75. Great fun.

You will find a little extra information about Theory of Wing Sections by looking HERE. Dolphin (t) 22:36, 7 September 2011 (UTC)

Difference between airspeed and horizontal airspeed

The article states that the gradient of the line from the origin to a point on the curve gives the glide ratio. As airspeed is measured in the direction of travel of the aircraft - i.e. slightly downwards - this does not seem to be quite right (though it will be very close). Does anyone know whether the glide ratio is actually descent over horizontal travel vs. descent over travel down the glide? Dhlsmith (talk) 14:33, 6 July 2012 (UTC)

The glide ratio is defined to be horizontal distance traveled divided by vertical distance traveled. An instantaneous value of glide ratio can be determined by taking horizontal speed and dividing it by vertical speed. Horizontal speed through the air is true airspeed multiplied by the cosine of the glide angle so it is not the same as true airspeed. For powered airplanes with conventional wings, and gliders, the glide angle is small enough that the cosine can be considered to be one. Consequently, for these aircraft the true airspeed and the horizontal speed through the air can be considered to be the same. Dolphin (t) 06:35, 9 July 2012 (UTC)