Researchers attached covert feather-inspired flaps to a 3D-printed model airplane wing and mounted it in a wind tunnel to explore the role of covert feathers in bird flight. By precisely measuring how air interacts with the wing and the flaps, they uncovered the physics by which the flaps improved lift and identified two ways that the flaps control air moving around the wing. Image © Princeton University/ Lori Nichols.
Numbers coming out of a wind tunnel at Princeton University suggest a minor amend to an aircraft’s wings can massively improve lift and reduce drag whilst adding almost nothing to the aircraft’s weight. The addition, flaps inspired by the covert feathers found on every bird, offer a reduction in the likelihood of an aircraft stalling and, should it do so, a much greater ability to regain control. ScienceVega spoke to Professor Aimy Wissa who headed up the Princeton team undertaking this biomimetic research to find out why she decided on the study in the first place and what applications she feels the results may lead to.
ScienceVega: How exactly do a bird’s covert feathers aid flight?
AW: The short answer is we don’t know. There are a lot of studies on the covert feathers in terms of their structure and their placement, but they’re aerodynamic function is actually unknown.
There are only a couple studies that I know of that have studied how the coverts deploy. There are images during flight showing which feathers deploy when, but that is the extent of it; there isn’t anything about their aerodynamic functional role. In terms of how they aid in flight, this is one of the big things that motivated me to even think about these feats.
My main field is called bio-inspired design and that means you look at the biology of something, understand it and, and this is where it gets really exciting, you can see how it might help machines or mechanical systems. I want to know how to ‘translate’ it. But often, if you’re not careful, you could be copying the wrong thing. You have to really understand the physics to be able to copy it properly, and this was one of these cases where we don’t really understand the physics in birds to begin with. So my study along with others is trying to crack that.
What was interesting, our results from the covert feather-inspired flaps have sparked new conversations with biologists. We are discussing how this work may inform experiments on birds. They are taking what we saw with aeroplanes and seeing whether birds are using it, or not.
A giant petrel soaring over South Georgia in the south Atlantic. The inner and out parts of the wings are clearly displayed. Image © Brocken Inaglory, CC BY-SA 3.0 < https://creativecommons.org/licenses/by-sa/3.0 >, via Wikimedia Commons.
ScienceVega: So biologists and ornithologists are taking your research and bringing it back to their research on birds?
AW: Yes. My lab works in two fields: bio-inspired design and engineering-enabled biology. The first can be described as finding and explaining something really cool in nature, and then making something mechanical that takes advantage of it. The second thing is we can set up experiments in controlled conditions in the lab or on a computer in a model that can help us answer questions about biology or inform experiments about biology.
In the realm of ornithology, I don’t do any work on birds myself, but we’re starting collaborations with ornithologists that can help us. They are seeing these results in engineering and that raises questions for them; is this really what birds are doing? Can we measure how the covert feathers on birds deploy in a wind tunnel? Can we use bird wings in the tunnel versus a whole bird during flight? We are in conversations to inform the next set of experiments about these feather systems on birds.
ScienceVega: Are covert feathers particular to any particular species, or do all birds have them?
AW: All birds have coverts. The main thing about them is they are also known as contour feathers meaning they give the wing its shape. They are only on the leading edge at the front of the wing. Their size and elasticity or how soft and flexible they are depends on the size of the wing and the bird, because bird feathers come in all different sizes, but all birds have them.
There are also some things that we know are very distinct depending on the flight style, for instance. An example is ‘arm wing’ versus ‘hand wing’. The arm wing is the inner part that creates lift, whereas the hand wing, or outer wing helps with control and manoeuvrability. Depending on what the bird is doing, such as soaring or rapid flapping, this ratio changes.
There are other things that we can also relate to flight but for the coverts, they are contour feathers; they’re pretty symmetrical, they cover the top and bottom of the wing surface. However, scientists are now also looking closely into how the coverts and other feathers are controlled and whether there is any evidence if the actuation of the coverts differs from other flight feathers.
ScienceVega: Is your career based on biomimetics typically, or are there other areas of interest?
AW: The big theme that governs everything we work on is bio-inspired design or biomimetics, but this field is huge. People apply natural processes and properties to almost anything. Here we focus on fluid structure interaction. My training is in fluid dynamics and structural dynamics and when you put those two things together, we are looking at flexible things that move in fluid.
Postdoctoral researcher Girguis O Sedky sitting inside of the wind tunnel, showing the covert feather-inspired flaps attached to the 3D-printed model airplane wing. The flaps deploy automatically in response to changes in airflow and can help prevent planes from experiencing stall. Image © Princeton University/ Lori Nichols.
ScienceVega: What previous efforts have been made to use covert-inspired flaps to improve engineered wing design and performance?
AW: There have actually been several. Unlike in biology where there have been perhaps two or three studies that have looked at the function of coverts, for engineering, there have been a decent amount of studies that have looked at covert-inspired flaps. Mainly the studies have been for just a single flap and flaps near the trailing edge; the back of the wing.
For this study, we wondered about the dynamic role of coverts in birds. We tried to make it biologically relevant, so we wanted to focus on multiple rows of flaps and we also wanted to consider coverts near the leading edge, and at speeds and scales relevant to bird flight.
ScienceVega: Why did you specifically decide to investigate multiple sets of flaps?
AW: When you look at how the coverts are arranged on a bird’s wing, there are multiple rows. Biology classifies them into lesser, median and greater. They are not necessarily very organised but for simplicity, let’s say there are three rows at the front on the leading edge and this interaction had not been studied in the literature, especially not at the Reynolds number, which in this case is basically the speed and the scale that matter in terms of bird flight for the bird that we are studying.
ScienceVega: Did you come up with an optimum number of flaps to deploy or did different numbers have different effects and benefits?
AW: In the study we show that there are two ways these flaps can improve lift; one if they are near the front of the wing and one if they are near the back of the wing. We also show that the mechanism for the front of the wing is additive, so if you add more flaps you get more lift. For example one front flap gives you 5% lift, two will give you 7% and three might give perhaps 8%. We were getting in the region of a 50% improvement in lift and 30% reduction in drag, and that’s really impressive.
In terms of the optimal number, that is really tricky – it requires some mathematical formulation and we have not yet done that. What we have done is reach a point of diminishing return meaning if you add more flaps you don’t get twice the benefit – instead one and a half times the benefit and so on.
The other thing to mention is the number of flaps and the length of the flaps are coupled. What really matters is that you design the flaps such that they interact with particular flow features, and that also depends on the air speed.
And this raises another interesting thing in that if you have smaller birds versus larger birds and you look at the length of their coverts, is there any evidence that the length of their coverts is such that it can interact with the specific flow features? That is a really cool question to think about.
ScienceVega: How easy could it be to use this research to develop future aviation design?
AW: I think it’s easy. We did fly one of these configurations on an aeroplane that did really and we saw really incredible benefits. These flaps add less than 1%, perhaps not even 0.2% weight, and they require no power, no actuation, and no sensing, so it’s very easy and it’s worth putting it on the on the aircraft. I can see it definitely being applied to smaller general aviation and perhaps small-scale radio-controlled aircraft.
As far as larger aircraft are concerned, I don’t see anything in the physics that limits it. Perhaps the specific configuration will change in terms of integration and material properties and so on, but I don’t think it’s very hard to integrate these flaps on vehicles, especially at the smaller scale.
ScienceVega: The improvements in lift and reduction in drag the minimal additional weight sound amazing. How confident are you in this?
AW: So you’re saying it’s too good to be true! I know that’s not what you meant!
We followed the scientific method, we did many repetitions, we have done error analysis and we have tested this at many Reynolds numbers and at many speeds with different configurations. What we are seeing is definitely true.
The 50% improvement in lift and the 30% reduction in drag we saw in the wind tunnel, so things were really controlled and we knew exactly the air speed and so on. But we took it to flight and we saw evidence that the aeroplane was stalling less and even on the stalls, that stall is much more controllable. We wanted to make sure that we had significant statistical analysis to make sure what we were getting was not an artefact, but it’s physics that’s repeatable. I encourage people to repeat our experiment. If they can repeat the experiment under their conditions, then I encourage them to do so. That’s the point of science. I’m very confident in these numbers and in the results of the study.
That being said, and this is important when you’re doing design, this was designed for a particular Reynolds number for a particular vehicle and the materials of the flaps were designed to work in a certain way. If someone comes to me and says they have a full-scale airplane in their backyard and they want to put flaps on, there has to be design consideration to make sure that things are scaled properly.
ScienceVega: Have you had any interest from other groups wanting to build on this research?
AW: We already have an industry collaborator who’s working with these flaps in a different way. We’re collaborating with Toyota Research Institute in North America who are interested in inflatable airborne wind energy harvesting kites. Basically, it is a really big inflatable kite with no tail that has to go to very high altitudes to be able to harvest wind energy from jet streams. Because there is no tail, one of the things they would need is control authority to be able to turn the kite in roll and yaw. That is especially important when you’re harvesting energy because to harvest energy, there are specific manoeuvres the aeroplane has to do. The coverts we designed for them have sufficient control authority; they can change lift and drag significantly, and when you use them asymmetrically on the left and right wing, they become control devices, like ailerons and elevators.
We have done this in our collaboration where we have implemented similar flaps on an airborne wind energy harvesting platform. What we did was successful; they were able to harvest energy. They were able to stay up for more than 12 hours in their initial test and the main control surfaces were the coverts.
Ahmed K. Othman, Girguis Sedky, Hannah Wiswell and Aimy Wissa (left to right) equipped the wings of an RC plane with covert-inspired flaps, and flew it at Princeton’s Forrestal Campus to prove that the flaps help prevent airplanes from stalling. Image © Princeton University/ Lori Nichols.
ScienceVega: What research will you be undertaking next?
AW: There are two areas we’re looking into. The first one is that everything that was done here, where the aeroplane was flown in a high angle of attack with respect to the oncoming velocity and then we deployed the flaps passively or actively, was done in static conditions. In reality, if you’re flying, you’re doing something that’s dynamic. You could be pitching really fast or you get hit by a sudden gust. Therefore the first thing we are working on is trying to see if we can expand the design space to be able to use these devices, not just in static conditions, but in dynamic conditions.
The second thing is that for the work that was published in the study, the stiffness of the feathers and the hinge of the feather were all the same. We used the same covert-inspired flap for the leading edge and we put it on the trailing edge flap. We are now trying to think a little bit more about if there are different stiffness levels – can you design the stiffness and tune it so that you are able to improve things further depending on where they are on the wing and what kind of flow features you want them to interact with?
