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× Welcome to the CPL Performance question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.

Slanted alpha charts

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namechanger created the topic: Slanted alpha charts

Hi Bob,

Was wondering whether you could help clear something up for me. In regards to the alpha loading charts, most of the depicted charts have straight vertical columns with varying widths, so you just move left or right the correct amount of divisions and then draw your line straight down - however I've encountered one where the divisions are slanted, plus an arrow indicating whether to move left or right.

I have attached an example with this post, and am wondering whether you should follow the slant of the line across the page until you reach the bottom of the division, or continue to draw your line straight down from the top of the division.

It seems that whenever there is a slant in loading or performance charts, you should follow the trend of the slant, but I've been told not to do this for the particular chart.

Thanks
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  • John.Heddles
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  • ATPL/consulting aero engineer
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John.Heddles replied the topic: Slanted alpha charts

Good question and one which arises periodically.

First, the quick answer.

For trim sheets,

(a) the loading lines for constant arm positions (ie everything on this sheet other than the fuel line) can be drawn either as tick mark lines or using slanted lines as in this sheet. For this latter situation, the slanted lines are NEVER guidelines and must NEVER be used as such.

(b) for the fuel line for this Type/Model, the fuel arm varies with fuel quantity. The trimsheet designer (in this case, Bill Whitney, a very experienced aircraft design and weights engineer) has chosen to set the fuel up with the varying fuel arm and the fuel grid, in this case, consists of a family of guide lines and must be used as such.

I will run up some graphics to amplify my explanation - with my current workload, this may take a little time to get on to. It is very important that we do this as the question is extremely important and it is critical that folks get the correct story on how this stuff works.

In the meantime, the trimsheet thread ( bobtait.com.au/forum/rpl-ppl/5236-aircra...loading?limitstart=0 ) is worth a read and covers the question. However, seeing you have asked it explicitly, it is well worth a specific, explicit answer to facilitate thread searches by others later on.

So far as your graphic is concerned, the first execution is wrong and it must not be done that way. The second graphic is not quite right, in that it defeats the advantage presented by the design use of the sloping lines. My next post will clear it all up and, hopefully, readers will go away with a better understanding of what the design was setting out to do here.

Back soon.

Engineering specialist in aircraft performance and weight control.
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