French Serie 540

[Dbahnson]

Here's a photo of a Chauviere prop for the geared SPAD. Regrettably, I loaned this to a person years ago so he could make a copy and I never got it back.

[pmdec]

Hi Dave,
I have pics of the same Chauvière serial 2223, but made later (around 1920) than yours:


@Dave and Bob: What is the right word to explain that the hole end is like on the engine side of a prop?

@Brokenwings: Can you post a clear pic of the "other" side of the hub? Even if you don't see anything? Some faint marks may be enough...

[Bob Gardner]

Cher Pierre-Michel,

I'm not aware that there is a specialist term. I would describe the curved edge of the central bore hole in the prop shown above as bevelled or as rounded.

With kind regards,

Bob

[Dbahnson]

The tool to make it is a quarter rounding bit, so I too just call it rounded, but I like the description "intrados", which is an architectural term applied to arches.

But I think architecturally the "extrados" refers to the top of the arch rather than the other side of the arch. In other words, when both sides of the center hole are rounded, they would both be "intrados" and the outside of edge of the hub would be "extrados".

I was mistaken in my belief that the purpose of the rounding was to make it easier to attach to the shaft of the hub. As PM has pointed out, it's actually to accommodate the metal fill where the center shaft is joined to the hub plate (welded, I presume).

[pmdec]

Hi,
Thank you for the words to be used. And about my words, sorry to not to be more explicit: "intrados" and "extrados" were just referring to the side of the blades (using same words that ones for wings) , to show that on this tractive prop the "straight" central hole was on the extrados and the rounded one on the intrados so it was not possible to use the prop on a pusher.

The joined pic is a French metallic hub from the WW1 era and it shows what Dave explained: the rear plate and the central part are joined with a "rounded" part to avoid cracks. The technical wording is perhaps "fillet"? In French, it is "congé" (same letters that the word for hollidays)

In France, until the 20s, only one side of the props central holes are rounded, so it is easy to know if they are from a tractor or a pusher. But after around 1924-25, many props have both ends of the central hole rounded.

The "rounding" (?) could be made to ways by using the concave OR the convexe part of this tool: but on quite all the props I have seen, the concave tool was used. I suspect the central hole of the first propellers was made using a hand tool, perhaps a file. The second joined pic is from a Régy serial 54 for a Blériot XI fitted with a 80 HP Le Rhône (same prop as the one of Meaux Grande Guerre museum http://www.pyperpote.tonsite.biz/lis...1111%20(1).jpg , unfortunatly without its decals).

Regards,
PM

[Bob Gardner]

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.

I have also followed Pierre-Michel's use of the terms intrados and extrados with interest. They describe curves on propellers. I have never seen their use in British aviation literature. Pierre-Michel's use of them always reminds me of reredos which describes a screen in a church.

I collect, in a minor way, technical books from early aviation but intrados and its reciprocal do not appear in Alfred Schlomann's astonishing book Das Illustriete Technische Worterbuch from 1932. The German text is 300 pages long and the translations into English, French and Italian account for another 550 pages, a total 850 pages, but nary a mention of intrados.

With kind regards,

Bob

[Bob Gardner]

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

[Dbahnson]

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.


Bob

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.

[pmdec]

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

[JR44]

Another fascinating exchange!
Where has the OP gone though?

[Bob Gardner]

Bonjour Pierre-Michel,

You are not in the least pedantic and you are a much valued contributor to this website. Please continue using intrados and extrados.

With kind regards,

Bob