 Let us have a look at what method can be followed by a conceptual aircraft design team to estimate the aerodynamic lift coefficients Cl and Cd for a configuration. We assume here that some kind of a configuration and layout has already been arrived at by the design team and now they are interested to go for some analysis. So we will discuss only about the subsonic parasite drag coefficient estimation in this particular presentation and the information that is relevant to transonic or supersonic aircraft will be provided in the form of nodes. This kind of approach is mostly applicable to transport aircraft. The components of the subsonic parasite drag, we will not look at induced drag right now, we will look at it little bit later. It consists of basically skin friction drag, form drag, interference drag and others. The others would consist of miscellaneous drag due to various items which are mounted on the aircraft and they will drag due to leakages and protuberances. There are two approaches commonly suggested for estimation of the subsonic parasite drag coefficient. One method is called as the equivalent skin friction method and the other is called as the component built up method. The equivalent skin friction method essentially looks at a very simplistic approach in which the whole aircraft is replaced by an equivalent flat plate and certain coefficients are specified based on the aircraft type and using them you can get a very quick very basic first cut estimate of the parasite drag coefficient. The logic that is followed in this particular approach is as follows. The parasite drag is supposed to consist of skin friction drag and the pressure drag and the skin friction drag in a well designed aircraft would probably be 90 percent of the total drag. In a well designed aircraft the pressure drag would be kept less maybe around 10 percent. So since it is a small fraction we replace its accurate estimation by just a factor X. So the parasite drag will be equal to the skin friction drag plus a small factor of skin friction to replicate the parasite drag. We define a term called as the equivalent skin friction coefficient CFE and we assume that the CDO value of the aircraft is equal to this CFE which is a fixed number for a specific aircraft type into the wetted area ratio or SWAT by SRF of that particular aircraft. So there is a list given to what would be the appropriate value of the CFE for various types of aircraft and the list varies from high speed aircraft to preparatory sea planes which have notoriously high drag values and based on past data and experience the CFE values range from 0.0025 to 0.0060. So to calculate CDO using this method is very straightforward. Get the ratio of SWAT by SRF using the chart that Raymer has given in his textbook which we discussed about when we discussed about the initial sizing we showed that graph. So from there you get the wetted area ratio by ball parking how the aircraft looks and multiply by the corresponding value of CFE for the aircraft type. With this you will get a very very crude estimate of the CDO value. In the component built up method we go into slightly more detail we say that the total drag of the aircraft is because of the summation of drag of each component plus drag due to leakages and protuberances which we will discuss and drag due to certain miscellaneous items and for each component the drag of the component is going to be its skin friction drag plus its form drag plus the interference drag that occurs because of the proximity of various components. So for the skin friction drag we use a similar formula as we use in the equivalent skin friction the difference is that we have a different value of CF for different component of the aircraft. By component I mean only the main assemblies the wing the fuselage the tail and nacelle. So it is identical in formulation but more specific in detail. So you calculate the S-wet by S-wet for each component and you also then calculate the value of CF formulae are given for estimating CF for various types of bodies. The form drag is considered to be you can say related to the skin friction drag through a form factor. So once you know the shape of the body you can get a form factor and again there are formulae available for it and then you multiply that form factor with the skin friction drag. The interference drag is also considered as a factor applied to the skin friction drag through an interference factor Q and this factor Q is 1 when there is no perceptible interference between two bodies. This will happen when the two bodies are actually separated by a large distance or there is no flow field interaction between them or not much flow field interaction between them and when they are operating in close proximity when the drag of one the presence of one affects the drag of the other then the Q value becomes higher. And miscellaneous drag is the drag of items such as the flap the landing gear when it is unretracted the upsweep aft fuselage and the fuselage base area we will discuss all of these and then finally you have this drag due to leakages and protuberances. Thanks for your attention we will now move to the next section.