AIRCRAFT PERFORMANCE- A BRIEF INTRODUCTION

In our previous post we have discussed about what is aerodynamics and how its study with respect to air crafts is divided into two main categories; the subsonic and supersonic. In this post we will discuss about what is aerodynamic flight and how it affects the performance of an aircraft flying. But before going so deep into aerodynamics lets first understand how actually aerodynamic forces act on a body and how it affects an aircraft performance characters. There are two primary aerodynamic forces the lift and the drag, the lift is the component of the resultant aerodynamic force in the direction perpendicular to the air flow while the drag is the component of aerodynamic force parallel to the direction of flow. The lift is induced by the flow happening over an aerofoil inclined at an angle( angle of attack) to the flow direction. For flow to happen a forward pusher force is required which is the thrust provided by the engines of an aircraft. When an aircraft is flying steady level and straight then the aerodynamics forces hold the the following relations with the thrust and weight of the aircraft.
Thrust = Drag    

Weight = Lift
These would be the primary fundamental parameters that would help us analyse the performance of an aircraft. All our discussions on aircraft performance in future will deal with these conditions i.e. straight, steady and level flight. Now that we know the elemental conditions for analysing the performance characteristics of an aircraft let us start to understanding how various aerodynamic parameters such as lift, drag, velocity, thrust required, etc. vary under various conditions. Before doing that we need to know how the lift and drag coefficients are related. They are given by the drag polar which is
Cd = Cdo + K(CL)^2
where Cdo is the drag at zero lift,
          K is a constant given by 1/(3.141*Oswald efficiency*aspect ratio),
          CL is coefficient of lift and
          Cd is drag coefficient.

flaps

Now that we are done with some basics  let us now try to understand what happens to the lift and drag when HIGH LIFT DEVICES(HLD)are used. The HLD's include the flaps and slats which changes the camber of

slats

the wing aerofoil of an aircraft and hence generates a greater lift coefficient. but as we understand from the relation between the lift coefficient and drag coefficient, the drag coefficient increases with an increased lift coefficient. An increased drag would mean that the maximum distance the aircraft can fly under steady straight level conditions is now reduced because of greater fuel consumption to generate thrust which supports the increased drag. a natural question in any body's mind would be why do we need a device that will cause us  a disadvantage??? Well don't think too much the answer is quite simple, the devices as their name suggests are high lift devices which means they generate a greater lift at a given velocity when deployed compared to when not deployed. This in turn means that we can generate the lift required for the aircraft to be airborne at a lower speed, which on ground means the length of the runway required for the aircraft to attain that minimum velocity to be airborne is reduced, because velocity is proportional to distance. Now you must be having a clear idea as to when and why these HLD's are used. YES!!! They are used only when an aircraft wants to take off and also wants to land. Wait a second!!! Why did I mention the landing distance???? Well the answer is again simple, in fact you have already read the cause for the usage of HLD's in landing. Still guessing???? remember that the drag increases when lift is increased...???? Yes the relation between the lift and drag coefficients is what causes the drag to be increased and hence reduce the landing distance.

the flaps deployed for landing

Now that we have discussed about the performance of an aircraft when high lift devices are used we believe we have served our purpose of giving a small introduction to aircraft performance.Hope you enjoyed reading our blog for updates on our blogs please add us on +1 or follow the author on google plus. Do share it if you found the blog informative.

AERODYNAMICS: SUBSONIC Vs SUPERSONIC

Aerodynamics is the core of aeronautical engineering which deals with the study of air flow over a body and the behaviour of flow characteristics over the body. This has been a very interesting subject for many people from various fields proving to be one of the most important for their respective profession.

aerodynamics used in cars

The god of cricket Sachin Ramesh Tendulkar had attended crash courses on aerodynamics and flow over a ball just to understand how a cricket ball spins and learn to use it to his advantage. there have been similar other sportsmen who have learnt aerodynamics in sports such as golf, football and hockey and a wide variety of engineers who have been trying to make the most out of this wonderful subject. since our blogs are related to aeronautical engineering lets try understanding how well aerodynamics is applicable in the flying of of the huge beautiful metallic birds.

The aerodynamics or flow of air over an aircraft is generally divided into two main fields of study. Namely; subsonic aerodynamics and supersonic aerodynamics. The subsonic aerodynamics as the name suggests deals with the study of flow of air over an aircraft when the aircraft is flying at speeds lesser than mach 1 or less than the speed of sound. In this type of flow the air is considered to be inviscid, i.e. non viscous, and incompressible. Though air is practically a compressible fluid its compressibility effects do not show up much

in the subsonic regime only when the speed of flow reaches a value close to mach 1, i.e. around mach 0.8, the compressibility effects of air becomes significant and is taken into considerations for studies. On the other hand supersonic aircraft deals with flights involving speeds greater than mach 1 which also includes hypersonic flights (speed greater than mach 5).Supersonic aerodynamics aircraft takes into the consideration the compressibility effects as well as the viscous effects of the flow and the affects of these effects at such high velocities is what makes the base for supersonic aerodynamics. As soon as a flow gets changed into a supersonic from a subsonic flow it gives rise to what is known as shock waves

which generate a sudden and steep change in the drag induced. This is caused due to travelling of the aircraft at the speed of sound. Since the source of the sound generation travels at the speed of sound, sound waves ( which is nothing but a pressure disturbance itself) accumulate together with the aircraft giving rise to a very high and unnecessary drag. Due to this the aircraft's generally prefer travelling at speeds which are far from the speed of sound (either high supersonic or high subsonic). This region where the drag increases drastically is called as the transonic region in aerodynamics. We will deal with supersonic and subsonic flow phenomenons in our future posts.

Now that we know there are two flow type studies in aerodynamics we should also understand that both these types of flows can be generated in a laboratory by the help of experimental setup.

fig. a subsonic wind tunnel schematic

Such setups used to study the flow of air over a given body is called as a WIND TUNNEL. There are both subsonic as well as supersonic types of wind tunnels which are being used actively to study the slow of air over aerofoils and air crafts.

fig. a schematic of blow down supersonic wind tunnel

While subsonic wind tunnels are all continuous and run for a long duration of time, their supersonic counter parts run for a significantly lesser time lasting only up to a few seconds to a few minutes. But supersonic wind tunnels are used only in large research centres and universities where research is being carried out on supersonic flight. Do get back in future to read more on wind tunnels and their working in our future posts.

          

a subsonic wind tunnel

a supersonic wind tunnel

This was our post on aerodynamics-an introduction, hope you liked it. Add us on +1 and follow us to keep getting updated on our posts and blogs in future.