 So, let us see the difference. So, this is what you will be calculating now, okay. So, the problem that we saw earlier, we assume dry air. Now we say no, it is not dry, it is 80 percent relative humidity. So, let us see what is the error you get in the calculations. So, to calculate this error, first of all, you will have to find out what is the value of E at sea level. Because in this case, the airship is flying at sea level and that to under IOSA conditions. So, which means you will have to get the value of, so apply this particular formula, get the value of ES for C of 15 degrees Celsius. Yeah, do not look at me, please do the calculation. So, C is not 0 in this case, C is the ambient air temperature in degrees centigrade. So, under sea level IOSA conditions, what is the temperature? 15 degrees centigrade. So, you put C equal to 15 and get the value of ES, 1695. Yeah, it was around 1700, correct. Units will be Pascals, units will be Pascals. So, the value of ES is around 1695. So, ES is 1695 and we have E as Rh by 100 into ES, where Rh is expressed in percentage. So, E will be 80 by 100 into this number or 0.8 times that number, 0.8 times 1700, okay, 1356. Now, let us go here straight. So, calculate the value of rho A with E included. PS is going to be ambient air pressure at sea level IOSA conditions which is 101325 Pascals. The value of E just now you got, multiply that value with 0.378. Subtract it from 101325, multiply this 0.003484 and divide by the ambient air temperature. In this case, the value of temperature is not centigrade. This is in Kelvin, 1.209, okay, 1.209, okay. Now, you have to verify what is the value of density you get if you ignore the value, ignore this correction. It should come to 1.2256. Let us check. Check what to do, that is known. So, no need to calculate anything. You can just do P by RT. If you do P by RT, you will get rho. P101325, R is 287 kg per degree Kelvin, T is 288.16. You will get 1.2256. So, against 1.2256, you got 1.209. So, what is the percentage error? So, if it is 1.219, what is the percentage error between that and 1.2256? 0.44 here depends on what you take as the baseline. Baseline should be 1.2256. It should be less actually. It should not be so much. 0.4%, yes, 0.36 or 0.4% is what I know. It should be 0.4%, right, correct, 0.4%. So, nothing great. We are worried about 0.4%. All these huge derivations Dalton law and Amagat law, blah, blah, blah, blah, blah. 0.4%, but that is only true at sea level and that 2 ISA conditions with humidity of 80%. Of course, humidity cannot be really more than 80, 90 versus 100% then it will condense. So, it may seem right now that this is the exercise in wilderness, but when you do this calculation for let us say ISA plus 30 degree centigrade, you will find this error will be 3.5%, 4%. And 3.5% error in the basic, see we are for airships the only source of lift is static lift. Dynamic lift is only a gift, only when you start moving. So, if you start with the 3.4% error on the basic calculations it is not acceptable. So, therefore, when you want to calculate the parameter that is important that you have exact numbers with you. So, this we have done now. Next concept very quickly to be done is the static lift. Static lift is you know difference. So the gross lift is rho A into V envelope into G, we have already seen this. And we also know from our, with all the errors in E which I will correct when I upload this slide, rho A can be related to this number. So, therefore, LG will be just put the value of rho A, it will be that same expression into V and V into G. So, this is how you can calculate the gross lift. It is nothing the same formula with you know V, E and V into G added at the end of it. Now, K is the static constant. This is different from the previous one because now we have G also. So, it will be 10 times more. It is a constant. So, you can say that the net or the gross not net gross lift is PS minus 1 minus RDW V into E upon TA times K into V ENV where K is a constant. So, if another volume is known to you if the relative density is known to you that also is known to you 1 minus RD is actually a fixed number for water vapor. So, it should not be a problem. So, let us take two very simple cases. Dry atmosphere gross lift is nothing but multiplication of these three things. Rho A we have already seen before. So, the gross lift is very simple. In dry air ignoring humidity, the gross lift is only PS by TA into K into V ENV. Easy to calculate. But when you have IFCA conditions then formula is the same but Rho becomes Rho S which is the standard ambient temperature and condition. Now, this is known to some students who have done aircraft design course and performance courses that density of air up to 11 kilometers in atmosphere can be assumed to vary because temperature varies linearly. There is a loss of 6.5 degrees per kilometer altitude. So, therefore, T by T naught to the power 4.259 gives you the pressure temperature ratio to the power will give you the density ratio. So, Rho S can be easily calculated for any operating condition in ISA. C level value Rho 0 which is 1.2256 multiplied by 1 minus L. L is the lapse rate. I am taking lapse rate as positive. Actually, L is negative. So, you can say 1 plus a negative number but be very careful. If you blindly apply formula, you will make a mistake. The value of L is 6.5 degrees last per altitude but it is taken as plus because the formula already has negative. So, by this you can calculate the value of Rho S. If Rho S is known to you, V envelope is known to you multiplied by G, you get the value of Rho G. This is a condition of dry ISA conditions and we are not assuming any value in a present. So, this is what we did also. This is what you calculated few minutes ago. Now, the last problem that I wanted to do today is I wanted to calculate the percentage loss in gross lift. When it is operated at an air field with pressure as 1 0 2 0 0 Pascal, temperature 27 degrees centigrade and an R H, this will be temperature. So, now to calculate this, what are the steps you will follow? First thing you will do is you will calculate the value of density of air at this condition. So, now the ambient temperature is also going to play a role and the ambient pressure also is not 1 0 1 3 2 5 but it is some other number. This is some air field where we read the values as 1 0 2 0 0 and we calculate the temperature. So, what steps will you follow? So, first thing you calculate is the saturated vapor pressure that is the value of ES. So, for that the same formula as before the value of C will be 27 degrees. So, first thing I need is how many Pascal did you get? 3 5 4 8 seems to be okay. You have used the approximate formula. The exact formula gives 3 5 6 6 you get 3 5 4 8, negligible difference. So, we will accept it. So, first thing you will do is you will calculate the pressure. Then let us see, can you now calculate the value of E which is the actual vapor pressure? 50 percent of this. Then now let us calculate the value of lift using this simple formula. So, what is the envelope volume you are taking? 7000 and what is the T A or T S you are taking? 300. So, you get 80.8 seems more appropriate 80.8 80.7 80.8 kilo Newtons. Now, what about the gross lift? This is in Newtons. So, suppose I want the force in kilograms is divided by the value of 9.807.