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Old 04-19-2019, 02:32 PM   #23
Forum Contributor
Join Date: Nov 2006
Location: France
Posts: 555


Sorry to be pedant, but I have some difficulties to explain my words...

In French, the extrados is the upper side of an aircraft wing (when the aircraft flies "normally"!) and the intrados the lower side.
The "extrados" is rounded / domed / curved / ...
The intrados could be flat or slightly curved, but the opposite way vs the extrados (chiselled?).

On the extrados surface, the pressure is less than the ambiant (static) pressure, because the path from the leading edge to the trailing edge is longer than a straight line, so the air speed is higher, thus the pressure is lower (Bernouilli).
On the intrados surface, the pressure is slightly higher than the ambiant pressure because the intrados plane has some positive incidence vs the path of the plane (the "AoA", Angle of Attack) and some dynamic pressure appears.
And it is why the plane flies: The wing is suctioned by its upper side and pushed up by its lower side (about 3/4 to 2/3 suctioned and 1/4 to 1/3 pushed up).

As a prop blade works like a wing, we use the same words: extrados for the curved face, which is always the forward side (vs direction of the plane), and intrados for the flat face, which is always the rear face (tractor and pusher props have the same face turned forward).

I erroneously thought that the same words could be used in English, but I was wrong...

The metallic hubs for tractors and pushers are the same, but for tractors it is the "intrados" side which is against the engine and for pushers it is the "extrados" side which is against the engine. If only one of the end of the central hole is rounded, the side rounded allows to know if it is a pusher or a tractor prop.

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