Close inspection of familiar elements of our environment often reveals insights that can inform us when designing machines intended to perform in similar circumstances. Where designers have the ability to start from scratch, cumulatively selected natural structures show how an efficient arrangement can emerge from an arbitrary starting point, with a limited ‘kit of parts’. In both cases, constraints of material properties and processes influence the final shape and arrangement. In order to innovate, we must have a clear understanding of the desired outcome, and up-to-date knowledge of material properties. Looking to nature and understanding how sometimes subtle characteristics influence functionality can yield valuable lessons.
In case you are not familiar with certain terms specific to aircraft and boat geometry, you can refer to a helpful glossary here.
Case study: A feather
An Australian Raven primary flight feather from the right wing.
I recently found this feather. It caught my attention, striking me at first glance as an elegant and precise semi-elliptical wing. Examining it closely reveals some subtle features that make it a remarkable aeroelastic structure.
How will it behave as an aeroelastic structure?
The first clue is how the shaft, though running at a constant percentage of chord, is curved. It turns aft toward the tip. It maintains the elliptical planform, but gives a progressive shift aft in centre of pressure moving toward the tip.
Some built-in twist increases angle of attack (AoA) toward tip (wash-in). This may be connected with the interaction of this particular feather with those ahead of it: It operates in their downwash, so has an increased tip AoA to compensate.
However, things get more interesting when we consider the dynamic behaviour of the mechanical structure of the feather. It is truly an aeroelastic system, where aerodynamic loading deflects the structural shape, which changes the aerodynamic forces generated, and in turn affects deflection.
Since the planform has increasing sweep toward the tip, aerodynamic lift tends to reduce twist, shifting from wash-in to wash-out under load. This naturally limits ultimate bending moment on the feather: The tip will eventually approach zero AoA at really high relative airspeeds.
The shaft also curves down slightly toward the tip. This feature would also help make sure the loaded shape of the miniature wing is straighter than would be the case if the shaft started off straight and flexed up from there. In other words, the static shape happens to lead to a more ideal loaded shape.
There could be some harmonic considerations involved, where the period of energy storage and release helps improve efficiency. For example, releasing stored energy at the beginning of the up-beat may give a final ‘flick’ to maximise lift as the wingtip path is reversed into its own wake.
How does the structural shape help?
So we have an exquisitely precise elliptical airfoil, with twist and progressive sweep that have obvious aeroelastic effects. But how is it built?
The shaft is an effective structural spar, with some remarkable features that make it beautifully efficient in terms of strength-to-weight of material.
At the root (where it would have exited the skin), where the bending moment is greatest, it has maximum thickness. In fact, at this point, thickness is considerably greater than the chordwise dimension. It is taller ‘up-and-down’ than wide ‘fore-and-aft’. Thickness follows the bending moment closely, growing quickly from the inboard point to the root, and then tapering away toward the outboard tip. But chordwise taper is much more gradual. The shaft goes from being a thick and stiff beam at the root, to thin and flexible where it is in the airflow.
It appears to be hollow, with all material near the surface where it can be most effective at resisting bending loads. As well as changing global beam thickness, the section of the shaft gains a corrugation on the underside, starting just outboard of the root. The corrugation would increase stiffness, prevent local buckling, and possibly have some effect on airflow across the underside of the feather.
The vanes grow out very near the top of the shaft, making the suction side surface completely fair. The cross-section of the shaft presented to the wind is entirely below the vanes, so the pressure side is not as smooth. It is interesting to think about whether this discontinuity in the lower surface is just the least inefficient solution, or if there is a beneficial effect in trip-turbulating the flow, perhaps to discourage laminar separation given the low Reynolds numbers involved.
Toward the tip, the shaft thickness approaches the thickness of the vanes, so the overall foil section gets cleaner and cleaner as local airspeed increases.
Interestingly, the vanes near the root have some extra chord (extending aft of the fair elliptical planform). The additional length curls up, presumably to seal against the next feather where it overlaps.
The impacts on structural design
This post has primarily focused on just the shape and mechanics, but experts in avian biology will no doubt be able to dig deeper into: How the vanes ‘zip’ together to form an airtight surface, how the feathers grow and are shed in opposite pairs, and how the primaries double as wingtip devices, combining lift and thrust functions as well as control and induced drag reduction.
Looking through the eyes of a designer at something that has been naturally/cumulatively selected from an arbitrary starting point over countless generations is valuable.
At Carbonix, we are specialise in designing and building aeroelastic structures using carbon composite materials. Whether we’re applying this expertise to our drone airframes, our marine products, or another custom requirement, we find inspiration all around us from the subtle beauty of nature.