From dragon flies to fighter jets –

Pushing back the barriers on animal flight research and the speed of human flight.  

By Sabina Michnowicz

Coming up on the Naked Scientists Radio Show and Podcast we're going to be joined by experts on the science of flight. Ever wondered how insects fly when their wings are so small compared to their body size? Want to know how aeroplanes can fly at speeds six times faster than sound? You've come to the right place! This week we've got Dr Graham Taylor from Oxford University who will tell us about his work on the aerodynamics and flight control of both birds and insects. Also in the studio to discuss her work with engine for super-fast aeroplanes called “ramjets” we'll have Jenny Goodman who is also from Oxford.

Aerodynamics – introducing smokin' insects and Cossak the eagle.

Graham's research involves placing flying insects in a specially constructed wind tunnel and blowing trails of smoke across their flying wings, as modelled beautifully in figure 1. This shows how insects are able to fly with wings so small for their body size –  the flow patterns involved are much more complicated than on a fixed wing aircraft and additional lift is provided by the flapping motion of the wings. His research also looks at bird flight, as currently there is little knowledge about the dynamics of how birds fly. Trying to do this in a wind tunnel is pretty difficult so instead the bird gets to fly outside with a video camera backpack. Graham uses a Steppe eagle called Cossack (as seen in figure 2) and monitors him with a wireless video camera. By observing how the feathers are deflected by the airflow in flight, he is able to visualise what the air is doing close to the surface of the wings. His findings show that the movements of the feathers are important and have a role as automatic control surfaces, similar to the wing flaps of an aircraft.  

 Virtual reality fun for insects and why Cossack doesn't need a tail fin.

Graham's team have also built a virtual reality flight simulator for insects (how cool is that?) By projecting fast moving images on to a spherical screen surrounding the insect, he is able to immerse it in a virtual reality visual environment at the same time as varying the airflow it receives and its orientation. The insect itself is tethered to a force balance, which allows the team to measure how the lift and thrust respond to changes in what it sees and feels of its environment. This research has revealed that the control system of insects functions very differently to one in an aircraft.

Another  line of research involves birds; enter Cossak stage right. He's large enough to carry instrumentation which records his orientation, turn rate and acceleration. By combining this with synchronous video sequences (from the backpack camera) Graham's team is able to monitor how Cossak is able to make his body turn using movements of his wings and tail. This research is particularly interesting as it shows how to control flight without the need for a stabilizing vertical tail fin – something which birds lack but in aircraft it contributes substantially to fuel costs.

Six times the speed of sound – Fly from London to Sydney in under 3 hours.  

Now we've established how to learn about animal flight, what can we do to make ourselves fly a bit faster – or, as in this case, a LOT faster? Jenny is working on her PhD as part of the Sustained Hypersonic Flight Experiment (SHyFE), the aim is to create a prototype air vehicle capable of cruising at Mach 6 for 160 seconds. Mach 6 is six times the speed of sound or 2 to 3 times the speed of Concorde, anything above Mach 5 is termed 'hypersonic'. If the project succeeds then space travel will be a step closer and passenger flights could be reduced substantially in terms of flight time. The main part of her project looks at creating a ramjet (a type of advanced engine) and the trickiest part is preventing it from blowing up or melting when temperatures just 13cms from the fuel tank are hot enough to melt stainless steel in 5 seconds. Jenny's work focuses on improving the combustion (to prevent bits blowing up) using a device called a flameholder (she is photographed with it in figure 3) which helps to mix up the fuel in the combustion chamber. The intention is that more of the fuel injected into the combustion chamber is burnt before being swept out of the engine allowing the vehicle to travel further. How a ramjet works and why it differs from a space rocket.

At this point you might be thinking “sounds great, but don't we already have space rockets doing this?” Its true that currently rocket engines are used to launch nearly all space vehicles and they're able to  exceed the speeds and distances required by this project. They have one significant disadvantage however, they require that both the fuel and the oxidiser needed for combustion need to be carried on board. The SHyFE engine differs in that it only needs to carry fuel as the oxidiser used to power the engine is air drawn from the atmosphere during flight. This leads to a number of advantages, including that it will be cheaper, lighter and safer than any conceivable rocket powered engine. This would reduce the cost of launching small payloads (i.e. satellites) into space and also  allows a commercial space plane to be a viable option.

So that tell us what ramjets can do and how they're different from space rockets, but how do he actually work? Well, they're fairly similar to the gas turbines which power nearly all commercial aircraft; air is sucked into the engine and the pressure increased before being mixed with fuel and combusted, so that the thermal energy of the air is increased. The thermal energy can then be converted to kinetic energy (high speeds) by expanding the air and fuel mixture through a nozzle. The force of the high velocity air leaving the engine creates the thrust force that pushes the engine and aircraft forward. The difference between gas turbine and ramjet engines is the way that the air pressure is increased at the front of the engine. In a gas turbine engine, rotating blades are used, but ramjets can create the same pressure increase using a carefully shaped inlet section. For this to be achieved air must be forced to flow through the engine due to the vehicle's forward motion (up to about Mach 3), the efficiency of a ramjet is lower than a as turbine so its unlikely that ramjets will ever power jumbo jets. The advantage of ramjets however, is that the removal of rotating machinery to compress the incoming air substantially decreases the weight of the engine, so that ramjets are able to achieve much higher flight speeds than even the most advanced gas turbine engines.

Fascinated by flight? Intrigued by insects? Mesmerised by Mach 6? Then tune in to the Naked Scientists Radio Show and Podcast!