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# understanding lambda percentage changes

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Hello everyone:

Hope someone can help me with this doubt.

So here it goes; if we want to calculate the percentage difference to move our lambda from 0.80 to 0.75, i assume we need 5% more fuel to change lambda from 0.80 to 0.75. However, in one part of the course, they mention that to move from 0.80 to 0.75 lambda, we have to divide 0.80 by 0.75 and this give us a result of 1.07. This means we have to add 7% of fuel to achieve 0.75 lambda.

So the question is, why i should add 7% instead of 5%?

PS: Sorry if you have a hard time understanding my english, it is not the best.

One reason we like to use Lambda is that we can use the difference to give us an approximate % change needed. So you might say I need roughly 5% more fuel to go from 0.80 to 0.75.

However the actual calculation is 0.8 / 0.75 which is 6.667% more fuel. So the correct answer is neither 5% or 7% is correct, but both are reasonable approximations to get you close (within 1-2% of the correct value).

The closer to Lambda 1, the more the approximation is accurate (ie, it does take about 2% more fuel to get from Lambda 1.0 to Lambda 0.98).

Thanks a lot David, now all makes sense!

Now i can continue with the next course with more confidence and without that doubt hovering in my mind.