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Old Today, 08:09 AM   #21
Bob Gardner
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And, having finished my books on British and German WW1 aircraft propellers, I often think of writing biographies on some of the early German aviation scientists, whose books I own: Professor Reissner of Aachen University, Professor Ludwig Prandtl, Helmut Hirth. And also of some of the many propeller makers; Lucien Chauviere, Heine, Wolff etc, but right now I an going out to photograph birds.

With kind regards,

Bob
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Old Today, 08:21 AM   #22
Dbahnson
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Quote:
Originally Posted by Bob Gardner View Post
Chamfered offered by Dave is probably the best term in the Anglo-Saxon aviation litany for the shaping of the central bore-hole in the hubs of wooden propellers.


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Bob, as a woodworker I reserve the use of "chamfer" to an angled bevel cut on the edge of a piece of wood. It is not rounded, but rather is a flat cut (or bevel) between two other flat surfaces.

If you look closely at the photo in a post above you'll notice that what we are agreeing is the intrados side of the hub is where the hub plate is attached to the hub shaft (with a "fillet") . It's labeled with an arrow pointing to an "R" which I believe refers to the "radius" of the quarter round bit used to shape the intrados edges of the center bore. The use of the word "chamfer" isn't offered by me. It was in the diagram on the side PM calls extrados. On that side of the construct a chamfer is used, I believe, to simply "ease the edge" to prevent splintering of the edge as the hub shaft is passed through the center bore. (One problem with splintering is that it removes the varnish protection and can allow water to seep into the wood.) On that side (PM's "extrados") the hub plate is not fixed to the hub but "floats" in and out to allow compression of the plate against the wood.

In both cases, the shaping of the edge of the center hole serves a purpose, but on the engine side it's to allow a space for the metal hub fillet and on the "nut" side of the hub it's simply to protect the edge from splintering. It stands to reason that the wooden hub is sculpted to reduce the amount of dead space between the wood and the metal, and I can see how a rounded edge on one side (intrados, closest to the engine) and a small chamfer (under the floating metal hub plate) would accomplish that.

I'm guessing that at some point the manufacturers realized that the chamfer cut on the "extrados" was overly technical and could just as easily be a rounded edge. That saves a step in the production process, and to my knowledge all modern propellers are constructed that way.

I think we should just credit PM with the application of the term "intrados" to indicate the side of the hub that attaches to the fixed plate on the metal hub, because as he points out that can be an important distinction in identification of a pusher vs. a tractor installation, at least on some of the early propellers.
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Old Today, 01:32 PM   #23
pmdec
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Hi,

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.

Regards,
PM
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