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%MAC Question
Sunny11
Topic Author
Sunny11 created the topic: %MAC Question
Hi,
I need help with a question about calculating %MAC.
The forward limit of the centre of gravity range of an Echo a/c at a gross weight of 2625kg expressed as a percentage of the mean aerodynamic chord is ?
This is a three step question.
First you must calculate the forward limit at 2625kg. Use the forward limit formula for the Echo. 2625 - 2360 x .27 + 2400 = 2471mm.
Now get the mean aerodynamic data from the Supplement on Page 17 para 5.3.
The centre of gravity forward limit is presently at 2471mm and the leading edge of MAC is at 2190mm so the forward limit is located 281mm into MAC. Express 281 as a percentage of 1900. = 14.79%MAC.
Hi Bob,
I know this post is old but I would like to ask you some other questions:
1. When you calculate the Forward Limit Formula is it always (Your Gross Weight)-2360x.27+2400 ? Where did you get .27 from?
2. I have the following example but I cannot figure it out:
Echo: BRW 2894kg 748.21 IU CG = ??
748.21 x 10k / 2894 = 2585.38
LEMAC = 2190 aft datum. 2190 – 2585.38 = - 395.38
MAC = 1900. 395.38 / 1900 x 100 = 20.80% MAC√
The answer for this question is 20.80% MAC but how can you find out the CG if the INDEX UNIT is not given?
3. The centre of gravity range for an Echo aeroplane at a gross weight of 2600 kg is closest to:
a 2680 mm
b 215 mm
c 32.3 mm
d 2517.8 mm
What is the answer for this one? How can you calculate CG range if you do not have Index Unit? I know the equation is Weight x Arm = Moment but we only have the Weight on this particular question.
The formula for the forward limit for the Echo is explained on Page 177 of the Performance book.
To find the centre of gravity range all you need to know is the position of the forward limit and the position of the aft limit and find the difference between them. To find the forward limit, use the formula for a weight of 2600 kg. The aft limit for the Echo is constant for all weights at 2680 mm. You don't need to know the moment.
