February 19, 2005
Induced drag reduction, the natural way.
As almost everyone must have noticed the wings of all bird have ragged outline to their trailing edges produced by the tips of the feathers but on the other hand since the early days aircraft have always had smooth trailing edges (probably because it looks more efficient!).
When early aerodynamicists were researching bird’s wings it was assumed that the saw tooth trailing edges were basically due to the requirements for the fold of the wings and nothing to do with aerodynamics. Even the oldest fossils of feathered birds show the typical ragged trailing edge. The shape does not appear to have changed much over millions of years. Darwinian theory would suggest that this feature would change over time or at least this feature would not have remain the same across all bird species if it did not have a very significant purpose. It would appear that there is an advantage in having a feather like trailing edge. Looking closely at each feather, each one is slightly different from the next (just as fingers on a hand are different). It would appear that each feather over time has developed their own particular shape for a very specific purposeand and the increase in efficiency must be significant for the system not to have canged over the millenia
Lift line theory gives a series of vortices produced along the trailing edge. The effect on a wing is that these combine downstream to produce a single large tip vortex. It would appear that the use of the wing feather ends on bird’s trailing edge is to stabilise these vortices and prevent them migrating towards the tip. They appear to act the same as wing fences but in a much more subtle way and for no extra wetted area.
Winglets are used to break up the strength of the tip vortex. Spillman tips were tried to emulate the pectoral tip feathers on birds, but they were neither long enough nor of variable incidence. There was also nothing to limit the span wise flow, so the tip vortex was already fairly powerful before it was trying to be dispersed. Recent aerodynamic developments appear to be to using small vortex generators to achieve similar a sort of result of stabilising the spanwise flow.
Looking at bird’s wings. Sea birds with large span have relatively smooth trailing edges inboard and the ends of the feather slowly gets more pronounced toward the Tip, with relatively small pectoral feathers. Large land birds on the other hand with their limited available span have much more pronounced trailing edge feathers and large pectoral feathers. These feathers have an advantage in that they are flexible and adjust automatically to the loads imposed so that they are near optimum for all stages of flight. A modern equivalent is the flexible mast and sail on high-speed sailboards. It can also be seen that birds preen their feathers frequently and the trailing edge arrangement of the feathers is important to them.
Tests on wings with stepped trailing edges have at times produced some unexpected reductions in drag, but so far there has been no systematic research on the subject.
The use of a saw tooth trailing edge on a wing should have the effect of reducing the span wise flow thus reducing induced drag and also improving the flow near the stall. An added bonus would also be reduction in wake turbulence and probably less noise.
Since this is basically a vortex system it should be more efficient at higher Reynolds numbers.
Ian Hannay, Fleet, Hants. UK 2005
Posted by timo at February 19, 2005 09:23 PM