Compressed Wood Propeller Blades

[303sqn]

THE USE Of WOOD FOR AIRCRAFT IN THE UNITED KINGDOM

Report of the forest Products Mission June 1944

 

Three types of propellers made from compressed wood are now in production in the United Kingdom and several other types are in various stages of development. Fixed-pitch, wood propellers of conventional type are in extensive use on various training airplanes.

 

Weybridge Blade

 

This blade, which is made by the Airscrew Company, Ltd., is carved from a blank glued from boards of Douglas-fir scarf joined, at the root end, to "Jicwood” or to similar “compreg" supplied by firms in the United States. Jicwood, supplied by the firm, Jicwood, Ltd., associated with the Airscrew Company is prepared by coating Canadian birch veneers with about 4 percent by weight of a spirit-soluble phenolic resin, drying, and then consolidating the pack under heat and pressure. The resin content and the conditions of pressing are such that Jicwood is not stabilized appreciably either by the resin or by the action of heat and moisture. The blank is rough-carved by machine, and then finish-carved and balanced by hand. Due principally to the time required for conditioning at various stages of manufacture, the elapsed time for a blade in production is from 7 to 12 weeks.

 

In one type of finishing, the blade is covered with hessian cloth cemented in place. A brass leading-edge strip is sweated to brass gauze and hammered, screwed, and riveted in place. A thick sheet of either cellulose nitrate or cellulose acetate is cemented on and the whole blade is placed in a rubber bag and put in an autoclave. In the autoclave treatment the coating is said to penetrate the Douglas-fir blade to a depth of about 1/8 inch. In the second type of finishing the blade is liberally spread with a thick lacquer and a sheet of cellulose ester is rolled into place on each face, forming laps at the leading and trailing edges. After the coating has shrunk, the laps are carefully sanded. Final balancing is done,by local scraping, and, if necessary, balancing paint is used locally.

 

These blades are repaired by splicing a new piece to the undamaged portion. Although repair of a damaged root is never attempted, the entire Douglas-fir portion of the blade may be replaced, the new scarfs nearly coinciding with the old. If the coating is not too badly injured, it is repaired locally.

 

Hydulignum Blade

 

The Hydulignum blade is manufactured by Hordern Richmond Aircraft, Ltd. One thirty-sixth-inch birch veneer is coated with approximately 20 percent of Formvar (polyvinyl formal) by weight. After the solvent (trichlorethylene and alcohol) has been driven off, the veneer is pressed into panels of specific gravity 0.95 at an elevated temperature and then cooled in the press. Two corners of the board are trimmed off and that end is then further compressed sidewise to a specific gravity of 1.3. The final board thus has a high-density double-compressed root, a transition zone, and a medium-density blade and tip. After rough patterning, several boards are assembled into a blank with a cold-setting urea-formaldehyde glue and the blank is carved in the same way as the Weybridge blade.

 

After several coats of primer containing chlorinated rubber, and of Formvar varnish have been applied, a brass leading-edge strip is riveted and screwed in place. About 14 additional coats of Formvar complete the blade.

 

A particular advantage claimed for the Hydulignum propeller is that the equalized shear strength in the root, in the two planes parallel and perpendicular to the glue surfaces, permits the use of smaller diameter hub fittings. Practical considerations requiring the use of a standard hub for all-wood blades, however, has precluded the use of a smaller hub for the Hydulignum blade.

 

Jablo Blade

 

This type of blade is manufactured by F. Hills and Sons, Ltd., and Jablo Propellers, Ltd. Veneers 0.6 mm. (1/42 inch) thick and of varying length are interleaved with phenolic-resin film glue and assembled in such a way as to give boards of 69 plies at the root and 48 at the tip. The assembly is pressed to a uniform thickness at a gradually increasing temperature reaching a maximum of about 280° F. The boards are roughly profiled, then assembled with casein glue. Carving is done by machine, followed by manual final carving. A stocking of phosphor-bronze gauze is stretched over the blade and soldered along the trailing edge. After application of a brass or steel

leading-edge strip, many coats of phenol-formaldehyde enamel are brushed on and baked so as to build up the surface flush with the metallic sheathing. After a final balancing, the blade is given a coat of grey primer and one of cellulose-acetate lacquer. (Since the return of the Mission, it has been learned that the type of finish described for the Jablo blade is no longer employed and has been replaced by a finish similar to that of the Hydulignum blade. This leaves two types of propeller finishes in use; namely, (1) cellulose acetate or cellulose nitrate (cellulose esters), and (2) "Cristofin," as developed by Hydulignum.)

 

Experimental Blades

 

F. Hills and Sons, Ltd., have three new types of propeller blades in development: the Norton, the Trafford, and the King. The Norton blade is made from boards each of which has approximately the same density throughout its length. The blank is glued up with four outer boards of specific gravity 1.3, and three inner boards of specific gravity 1.1.

 

The Trafford blade has a root consisting of alternate boards of specific gravity 1.3 and 0.9 scarfed to a blade of natural spruce to which are also glued densified birch leading and trailing edges of specific gravity 0.9. Both these types were designed to minimize the strain on the press Caused by eccentric loading. In addition, the Trafford is expected to be lighter and stiffer than the Jablo.

 

The King blade is a molded, uniform-density, hollow blade. Birch veneers mm. (1/8.5 inch) are tailored to a calculated shape, rolled into long tubes, impregnated with a phenol-formaldehyde resin, and dried. A large number of these tubes are loaded side by side into a steel die having a solid steel core. Heat and pressure flatten the tubes and consolidate the into a blade Of specific gravity 1.3 with a maximum wall thickness of about 3/4 to 1 inch. The core is removed after the pressing. If successful, this type of blade will have equal shear strengths in two directions, will save veneer and resin, and will be adaptable to the Hamilton standard hub.

 

F. Hills and. Sons, Ltd., is also considering the use of Zebwood, a veneer-plastic composite for propellers.

 

Molded Components, Ltd., have molded experimental blades of impregnated veneers pretailored to shape. Each of the two faces of the blade consists of nine continuous plies, a feature that avoids excessive exposure of end grain and glue lines and provides a skin running the full length of the blade.

 

None of the wood blades now in production can be fitted satisfactorily to the Hamilton Standard hub, which requires a hollow root. They are all used with the Rotol hub of British manufacture. A threaded conical ferrule is used on all wood blades and is cemented in place with "Semtex," a mixture of Portland cement and rubber latex. Considerable interest has been shown in the new lag-screw retention developed at Wright Field.

 

Although Jablo and Weybridge blades are balanced against masters, Hydulignum blades are still furnished only in matched sets and are not interchangeable.

[EwenS]

Thankyou for the above. Very interesting.

 

Recently I came came across a reference in the 681 Squadron ORB that "the Chrisofin covering stripping off the bades" of the Rotol props on their early Spitfire XIX in August 1945. Aircraft being flown that month were in the low PM5** range + PS918.

 

I have been puzzled about this as similarly engined Mk.XIV don't seem to have been affected. Going back to "Spitfire The History" I see that Mk.XIV were supposed to have Jablo blades while the Mk.XIX could have either Jablo or Hydulignum blades. So was it the latter that were causing the problem out East? So was Christofin a commercial name for the chlorinated covering being applied to these blades? And was it that that was peeling off?

 

Anyone got any answers to satisfy my curiosity?

[303sqn]

Weybridge Blade, made by the the Airscrew Company Ltd, were covered with Hessian, Jabo blades with cellulose acetate or cellulose nitrate, Hudulignum with Cristofin. Cristofin was developed by Hordern Richmond but I cannot find out exactly what it was, maybe a type of Formvar. Late in the war Jablo blades switched to using Cristofin.

[dnl42]

Great info! 

 

The full US Forestry Service document is available at 

https://ir.library.oregonstate.edu/downloads/cn69m810j

 

From https://archive.org/stream/in.ernet.dli.2015.109205/2015.109205.Dictionary-Of-Aeronautical-Terms_djvu.txt, Cristofin was a brushed-on polyvinyl enamel.

 

There's a film with further info at IWM, "SQUADRON REPAIRS TO WOODEN PROPELLER BLADES"

[303sqn]

Thanks for that. Cristofin may have been based polyvinyl acetate, perhaps with latex.

 

Formvar was the name used by Monsanto for the polyvinyl formal resins they manufactured/sold. Made by reacting polyvinyl alcohol and formaldehyde as copolymers with polyvinyl acetate and used in coatings.

[Tail-Dragon]

I just stumbled on this site that has some fascinating photo's of deteriorated Spit IX Rotol props - you have to see these!

 

http://www.skyfield.demon.co.uk/page.2.html

 

http://www.skyfield.demon.co.uk/page.3.html

 

 

[alt-92]

16 hours ago, Tail-Dragon said:

http://www.skyfield.demon.co.uk/page.3.html

 

Not to mention the rest of the stories! An early Mk.V Rotol leading to what could be one of the rare V to PR.XIII conversions?

Edited April 29, 2020 by alt-92