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# Cr 3 ETAS

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Brief description of the Effective TAS and why is it only used with WCA above 10º

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### Cr 3 ETAS

1. 1. CR-3(ETAS)
2. 2. WIND SIDE:The solutions for the wind problems on the computer are based on the triangle of velocities.When using a CR-3, we are not making use of the hole triangle because it would be too big to fit inthe computer, but only the small shaded triangle on the figure above. That small triangle isconstructor from the head wind / tail and crosswind components of the wind vector (thosecomponents are parallel and perpendicular to our track). Thus, placing the TC on the CR-3 (TC index)and marking the dot corresponding to the W/V we can obtain both parts.The rest of the solutions and data required in a wind problem are based on trigonometricconsiderations.Referring to the triangle above, we know that ∝ = so = ∝Both outside scales are of the logarithmic type, and the inner one is base on different values of thesine of an angle. --> That’s why we can find the WCA (α) placing the TAS of the aircraft facing theTAS index and looking for the xw component on the outer scale.If we want to calculate the GS, we have to add or subtract the tail or head wind componentsrespectively to the TAS. But for WCA greater that 10º we will always use ETAS (Effective TAS).
3. 3. In this example we could use the TAS ± tail/head wind component (which is the GS), because we canconsider AB = AF. Comentario [w1]: 2 lines are left over the lettersIn the case the WCA is greater than 10º (see figure bellow) we can’t assume that AB = AF becausethe difference BF is much bigger and the error would be too big.So, for WCA greater that 10º we have to assume that = ± /To calculate this on the CR-3 we have to understand that ∝ = and therefore, = ∝ .We have a black logarithmic scale right on the left of the TAS index (on the left because theETAS is a value always lower than the TAS value) that’s based on cosines of the WCA.
4. 4. TIME – SPEED – DISTANCEWe have on the Cr-3 (calculator side) a white outer scale called “Distance scale” (with the worddistance written next to the number 62), and an inner gray scale: the “Time” scale (the word timewritten on number 62).The inner scale can be divided into two scales: • The outer scale (Big numbers) Minutes • The inner scale (Small number) Hours (from 1h to 36h) Note that there is a direct relationship between the hours and the minutes scales as deemed logical. For example, if you look for the 1h30min you find out that it is right underneath the number 90, and if you look for 15h (the same point),it will correspond to 900min. Remember that 90 can stand for 0.9, 9, 90, 900, 9000, ...If you want to convert minutes into seconds you can use the time index and the sect arrow placedon the number 36 of the time scale. E.g.: If you want to find out how many seconds are there in4min, you can place the time index facing 40 on the Distance scale and you read what is facing the“sec” arrow (24 that, in this case means 240s).To solve a problem involving time, speed and distance, we can use both distance and time scale. = × ≫ = 1The computer is based on this to formula to give us the outcome of the date required.E.g.: Distance flown: 40nm Time taken = 12min What would the GS be?Resolution: You face 40 on the distance scale against the 12 on the time scale. Then you read out the distance corresponding to 1h (time index) = 20 200Kts