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Lift and Drag question

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zhezhizhang created the topic: Lift and Drag question

If you increase AOA of a symmetrical aerofoil from 1 degree AOA, to above the stalling value, at constant IAS, how will lift and drag vary?

A) Lift will increase then decrease. Drag will increase then decrease
B) Lift will increase then decrease. Drag will increase only
C) Lift will increase. Drag will increase
D) Lift will increase then decrease. Drag will decrease then increase

The correct answer given is B
If it was asking about a typical GA wing, I would go with D. As at 4 degree drag will be lowest.
But because it's a symmetrical aerofoil. Is it normally assumed at 1 degree the total drag is minimum hence B)drag will only increase?
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  • John.Heddles
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John.Heddles replied the topic: Lift and Drag question

If you have a look at experimental data (e.g. www.osti.gov/servlets/purl/6548367/ and www.preprints.org/manuscript/201610.0095/v1/download ) you will find that min drag for a symmetric aerofoil tends to be a little bit past zero incidence.

What I suggest is that the way the question is styled reflects this. The only sensible answer is (b). The pedants might opine for (d), again as one can find data to support that option. However, I suspect the examiner will be looking for (b)

There are plenty of papers available on the net for you to delve into to get a handle on what's going on. You need to keep in mind that pilot theory training endeavours to simplify stuff to make the general understanding a bit easier. Unfortunately, sometimes that can confuse the issue a bit if you look into things a bit deeper. I'd be a bit careful making statements like "as at 4 degree drag will be lowest" .. I can pull out experimental data which shows quite different numbers. Again, just one of the problems of trying to simplify and generalise stuff which should have lots of qualifications and caveats ...

Engineering specialist in aircraft performance and weight control.
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