I have recently read ‘Advance in the Air’ by Monk & Winter, approx 1935, a general descriptive text on aeronautical innovation of the period. It describes the Schwarz process of ‘case hardening’ wooden propellors, so they can be ‘hit with a hammer and no mark is made.’
I assume this refers to an innovation developed by the Schwarz Propeller Werk of Berlin, which the text states is now licensed to The Airscrew Company of Weybridge, UK. From casual observation I have a sense that German WW2 airscrews were largely wooden, and perhaps used this process to derive an effective service life equal to metal airscrews. Again, from visual observation, I note that many post 1935 British wooden airscrews were coated in a similar fashion.
The Schwarz process is generally described in the 1935 text below, with my more specific questions in brackets :
1) Airscrews are covered with a very coarse fabric (what fabric, how coarse, glued on?)
2) Over the leading edges, brass capping strips are attached, behind which are narrow strips of brass gauze. ( what gauge strip, how attached, is gauze brazed to strip, is this different to the British practice of putting leading edge brass strips on the surface of fabric covered props, after lacquering?)
3) Airscrews are covered with sheets of a special variety of non inflammable celluloid, previously softened by being hung in an atmosphere saturated in acetone. (what is the celluloid material and its gauge)
4) Airscrews are treated with an adhesive solution to ensure that celluloid makes contact with the wood. (what adhesive, sprayed on or dabbed on?)
5) Closely fitting rubber bags are drawn over the blades, the whole placed in a sealed chamber wherein it is exposed to a pressure from 50 – 100 PSI ( how long for, at ambient temperature?)
6) On removal from pressure chamber, airscrew is left for three days and finally dried off in kiln at 100 degrees Farenheit to remove remaining acetone.
7) Airscrew smoothed,polished and painted.
It is claimed that this process produced a load bearing stressed skin that allowed weight saving thinner blades suitable for variable pitch airscrews, which makes me think that this allowed high performance Luftwaffe aircraft to have wooden blades.
If this process was known and used by the British, I wonder why there was not more widespread use of wooden blades by the RAF in WW2. Perhaps there was, but the scale of US Lend Lease supply of forged aluminium blades swamped the picture. Perhaps celluloid was more strategically important as photo film stock rather than blade covering, particularly in respect of what appears to be a cruder and more practical coating process applied by the British.
In this respect I refer to RAAF Airscrew Instruction 1943 : “Manufacture and Repair of Wooden Fixed Pitch Airscrews”, using the process described below :
1) Fabric covering is linen to specification BS F1, one piece for each blade, extending from the tip to within 6 inches of the boss. Fabric to be washed in boiling water, thoroughly dried.
2) Covering to be done in room free from draughts at a temperature not less than 70 degrees F. Fabric secured to blades by glue to specification BS V10 or V11 in manner shown by drawing T5307 (this shows min 1 inch overlap on the leading edge)
3) After covering, four coats of undercoat are applied followed by three coats of cellulose laquer
4) Metal sheaths are fitted to leading edge by screws of copper rivets, rivet heads flooded with solder and smoothed, then three coats of finishing laquer applied.
The above description is necessarily simplified, but is quite specific with grades and specifications of materials. Both processes rely on a celluloid finish, the German via application of ‘thick’ acetone softened sheets and British via application of soluble cellulose lacquer. I assume that the German process was more expensive, but quicker, once systematic production was set up and utilised a thick, stress bearing celluloid layer.
I assume the British process was cheaper, leveraging of well established processes for fabric doping, time consuming but able to be accomplished in the crudest conditions and resulting in a thin celluloid layer, suitable for resisting abrasion but not stress bearing.
I would be interested to have any discussion that could shed more light on the technical aspects of the Schwarz process, answer or point the path to any answer to the questions I have about the process, its application in Britain or Germany and also the British lacquer process.
Just an additional thought on the stress bearing surface of Schwarz type props : are Luftwaffe props made out of a single piece of wood or are they a lamination of a number of planks like British props. I seem to recall images of broken Luftwaffe props which seem to show a single piece of light coloured wood, almost birch.
To give a modern example of a stress bearing surface with a non stress bearing filler the foam filled, steel clad panels used for refrigerated storehouses or site sheds illustrates the principle. I can start to understand the great advantage of the Schwarz process for the Luftwaffe if it utilised a readily available, cheap, non strategic material like timber for props instead of aluminium.
Again I seem to recall a steel band at the root of blades which transfers the load from the surface of the prop into the hub. The guts of the prop would just be a lightweight space filler. Compared to British props layered from imported mahogany the Schwarz process seems far more efficient on many levels and the ultimate development of the wooden propeller.
I figure that the Schwarz process lends itself to short runs of unobtainable replica blades for ground running aeroplanes, as a minimum. Where the basic form can be made from a billet of wood and the root that engages into the hub from a small metal machining then this is a more realizable path than seeking to make aluminium forgings as the basis of short run replica props.
By: Sedbergh - 12th November 2015 at 18:08
Photos of the P-40 with Hydulignum prop blades can be seen on my website: www.haddenhamairfieldhistory.co.uk along with a potted history of Hordern-Richmond Propellers.
By: Bob - 11th November 2015 at 23:42
Just finished watching an Air Ministry/RAF film from 1943 “Squadron Repairs to Wooden Propeller Blades” made by Signet Pictures Corporation Ltd.
Seven reels, Part I – Composition of The Blades, Part II – Emergency Timber Repairs, Part III – Covering Repairs. Blade types covered are Hydulignum Board, Spruce or Fir Board or Jablo Board props.
Introduction covers the construction of various types then methods of repair of the timber following damage to edges, tip damage and bullet holes. Then repairing the blade covering. All the repairs are intended to be carried out at squadron level – more serious damage would require specialist repair.
By: powerandpassion - 19th August 2015 at 07:28
Good piece of paper to find which shows extensive use of wood type props with Merlin – Griffons :[ATTACH=CONFIG]239980[/ATTACH]
By: Flying_Pencil - 22nd July 2015 at 01:21
Very interesting stuff!
By: AgCat - 17th July 2015 at 07:22
P&P: For details of the manufacture of reproduction propellers you might want to look at Hercules:
http://www.hercprops.com/services/reproduction-propellers/
Their website carried a lot of information, especially on their coating and finishing methods.
By: tftoc - 17th July 2015 at 03:33
Is there anything wrong or to complain about this approach ? Apart from not using historically original seventy year old aluminium blades subject to intragranular corrosion dug out of a swamp.
Generally speaking, due to their structural properties, wood core blades have wider diameter shanks/butts than aluminium or steel blades so substituting the former for the later requires a new hub as well. Fortunately the provenance police only seem to be concerned with airframes and paint schemes so no worries there. If you are a member of the AEHS there is an article on the propellers developed by MT for the P-82 project in the US. I think it was presented at the 2011 convention.
Modern composite propellers (typically foam core) as found on large turboprops are a different kettle of fish. Here the fibre/matrix ‘shell’ (carbon, aramid, glass etc) is taking most of the load (as opposed to a compressed wooden core doing the work). Thinner sections (than wood) are able to be used but ringing/resonance properties are closer to those of metal blades.
By: dogsbody - 12th July 2015 at 20:08
Considering that the majority of people who might go to an airshow or aviation museum wouldn’t know a Spitfire from a Harrier, whether or not the aircraft has a historically accurate propeller is a moot point. Even those of us who profess knowledge of these machines may be unaware of this fact.
Chris
By: powerandpassion - 12th July 2015 at 11:42
Thank you
Thanks Dogsbody for that link to the pictures of the various processes on enginehistory.org; pretty much explains it. Thank you everybody else for the great links to current manufacturers of composite props, certainly not a lost art and surprising to realize that Spitfires, Hurricanes and LH tractor Ansons are all well covered. It appears that the 1930’s German technology never disappeared and is well represented in Germany and Italy today. Go Axis. I would have always assumed that these warbirds were running down stocks of original blades – the future for warbird and classic prop maintenance seems far more optimistic to me than when I first thought about it.
The replacement of original aluminium blades with modern, lighter, composite blades offers less engine wear as there is less to suspend off the crankshaft. Less destructive resonance than aluminium. These modern manufacturers claim to be able to handle up to 5,000 HP through the blades. Nothing wrong here. Struggling to find anything to complain about. Is there anything wrong or to complain about this approach ? Apart from not using historically original seventy year old aluminium blades subject to intragranular corrosion dug out of a swamp.
By: dogsbody - 10th July 2015 at 23:36
A wee bit on construction:
http://www.enginehistory.org/Propellers/Rotol/rotol.shtml
Chris
By: redvanner - 10th July 2015 at 20:36
This is a very interesting thread. I really enjoy how you put the “passion” in your name P&P 🙂
as for other companies making warbird blades I do know of this one http://www.gt-propellers.com/special-warbird-blades.html
That is Mk XVI SL721. Vintage wings of Canada.
Thanks for all of the interesting input.
Andy Scott
Another one also providing composite Warbird props: http://www.mt-propeller.com/en/entw/pro_vint.htm
Michael
By: DH82EH - 8th July 2015 at 13:51
Thanks Anon for some great answers. I had no idea Hoffmann is making warbird blades. What profiles do they do ? Who else is making old profiles ? I understand Avia in the Czech Republic is making Hamilton Standard aluminium blade profiles for Mustangs. It seems like an area with a growing need in future years.
This is a very interesting thread. I really enjoy how you put the “passion” in your name P&P 🙂
as for other companies making warbird blades I do know of this one http://www.gt-propellers.com/special-warbird-blades.html
That is Mk XVI SL721. Vintage wings of Canada.
Thanks for all of the interesting input.
Andy Scott
By: powerandpassion - 8th July 2015 at 12:00
Hordern Richmond were bought by Permali in 1954 and Hydulignum is still manufactured by them today along with other resin/wood products.
No fabric was involved. Weybridge Blade, made by the the Airscrew Company Ltd, were covered with Hessian, Jabo blades with cellulose acetate or cellulose nitrate, Hydulignum with Cristofin. Cristofin was developed by Hordern Richmond but I cannot find out exactly what it was. As far as I can work out it seems to have been a type of Formvar. Late in the war Jablo blades switched to using Cristofin.
The types of blades used by modern WW II aircraft such as Spitfires were made from thin veneers of wood such as Canadian Birch, Douglas Fir etc., not blocks of hardwood such a Mahogany as in the WW I types of propellers.
Hydulignum
One 1/36 inch birch veneer was coated with approximately 20 percent of Formvar by weight. After the solvent (trichlorethylene and alcohol) had evaporated, the veneer was pressed into panels with a specific gravity 0.95 at an elevated temperature and then cooled in the press. Two corners of the board were trimmed off and that end was then further compressed sidewise to a specific gravity of 1.3. This was carried out with the top and bottom also under compression. The resulting final board had a high-density double-compressed root, a transition zone, and a medium-density blade and tip. After rough patterning, several boards were assembled into a blank with a cold-setting urea formaldehyde glue and the blank rough-carved by machine, and then finish carved and balanced by hand. Jablo and Weybridge blades were balanced against masters, but Hydulignum blades were furnished only in matched sets and were not interchangeable.
After several coats of primer containing chlorinated rubber, and of Formvar varnish, had been applied, a brass leading-edge strip was riveted and screwed in place. About 14 additional coats of Formvar completed the blade.
Gosh I love it Antoni when you put your fingers on the keyboard ! The helix of historical engineering knowledge and chemistry winding around each other is a rare form of poetry to behold. Please donate your brain to the forum when you are gone and I will happily keep it hydrated with single malt scotch if we can pull out the odd spurt of knowledge from time to time!
What I like about this forum is that there is too much to know but in one series of exchanges you can ride the elevator up quite a few levels. Now I have to find the pub quiz that asks me to spell Hydulignum !
I have always assumed that modern turbo props used aluminium blades but I will have to get up close and tap one. I wonder if modern carbon fibre has made inroads into this area. I figure that the cutting edge of blade science must be helicopter blades, simply from the span of a blade and the forces it must contend with.
I know that autogiros in the 1930’s had a tapering, cold drawn tube spar in the blades made of Nickel Chrome steel similar to biplane axle compositions and this extended into 1960s helicopter blade design. The 1930’s blades were otherwise wooden.
By: powerandpassion - 8th July 2015 at 11:46
Modern warbird repro props are made by Hoffmann of Germany to original profiles using the composite method of construction. Anon.
Thanks Anon for some great answers. I had no idea Hoffmann is making warbird blades. What profiles do they do ? Who else is making old profiles ? I understand Avia in the Czech Republic is making Hamilton Standard aluminium blade profiles for Mustangs. It seems like an area with a growing need in future years.
By: antoni - 7th July 2015 at 20:12
Hordern Richmond were bought by Permali in 1954 and Hydulignum is still manufactured by them today along with other resin/wood products. Beech is certainly used today but as far as I know birch was used in WW II. The Use Of Wood For Aircraft In The United Kingdom Report of the forest Products Mission June 1944 lists the types of wood being used in aircraft production:
“Aside from birch, the principal veneer used in plywood for aircraft frames and in propellers, the following species are used in plywood: Gaboon, English beech, hard maple, yellow poplar, and red gum. Large quantities of birch, a part of which is cut in England and a part in America, and small quantities of hard maple go into aircraft frame plywood and propellers. Small amounts of American-cut yellow poplar and red gum are made into plywood for lightly stressed aircraft parts. Gaboon and beech are used chiefly for boats and War Office supplies.”
No fabric was involved. Weybridge Blade, made by the the Airscrew Company Ltd, were covered with Hessian, Jabo blades with cellulose acetate or cellulose nitrate, Hydulignum with Cristofin. Cristofin was developed by Hordern Richmond but I cannot find out exactly what it was. As far as I can work out it seems to have been a type of Formvar. Late in the war Jablo blades switched to using Cristofin.
The types of blades used by modern WW II aircraft such as Spitfires were made from thin veneers of wood such as Canadian Birch, Douglas Fir etc., not blocks of hardwood such a Mahogany as in the WW I types of propellers.
Hydulignum
One 1/36 inch birch veneer was coated with approximately 20 percent of Formvar by weight. After the solvent (trichlorethylene and alcohol) had evaporated, the veneer was pressed into panels with a specific gravity 0.95 at an elevated temperature and then cooled in the press. Two corners of the board were trimmed off and that end was then further compressed sidewise to a specific gravity of 1.3. This was carried out with the top and bottom also under compression. The resulting final board had a high-density double-compressed root, a transition zone, and a medium-density blade and tip. After rough patterning, several boards were assembled into a blank with a cold-setting urea formaldehyde glue and the blank rough-carved by machine, and then finish carved and balanced by hand. Jablo and Weybridge blades were balanced against masters, but Hydulignum blades were furnished only in matched sets and were not interchangeable.
After several coats of primer containing chlorinated rubber, and of Formvar varnish, had been applied, a brass leading-edge strip was riveted and screwed in place. About 14 additional coats of Formvar completed the blade.
The normal choice of bonding resin for compressed and non-compressed wood laminates would be phenol formaldehyde better known as Bakelite (hence the name sometimes used – Bakelised wood) . This because, unlike other synthetic resins, Bakelite bonds with the cellulose in the wood. It this that which provides the strength. Bakelite is thermosetting plastic, which does not mean that is requires heat to harden it as many people think (Bakelite will harden quite happily at ambient temperatures but takes a longer time. Brief heating to a high temperature is used to hasten the hardening process, the condensation reaction.) , it means that once fully hardened it will not soften again when heated. This is because the cross-linking is in three dimensions. Thermoplastics can be re-softened many times by heating because the cross-linking is only in two-dimensions.
In the case of Hydulignum Bakelite could not be used because of the requirement to compress the wood twice. Another, thermoplastic, had to be found. After researching various vinyl resins Formvar was chosen as having the most suitable properties. Formvar was registered trade name for polyvinyl formal produced by the Monsanto Chemical Company, which is a family of polymers formed from polyvinyl alcohol and formaldehyde as copolymers with polyvinyl acetate, also called modified polyvinyl acetal resins.
A particular advantage claimed for the Hydulignum propeller was that the equalized shear strength in the root, in the two planes parallel and perpendicular to the glue surfaces, permitted the use of smaller diameter hub fittings. Practical considerations requiring the use of a standard hub for all-wood blades, however, precluded the use of a smaller hub for the Hydulignum blade. A contemporary article in Flight Journal describes how, in order to demonstrate the feasibility, of making wooden blades with narrow roots, some damaged blades from Curtiss hollow steel electric
airscrews, which had very small diameter roots were obtained. The steel shanks were sawn off, heat treated, and pressed into a conical shape. They were then used as an adaptor into which the wooden blade root was screwed. After ground testing a full set of blades were made and flight tested on an American aircraft. This seems to explain the role of the P-40.
References
The Use Of Wood For Aircraft In The United Kingdom Report of the Forest Products Mission June 1944
Flight: http://www.flightglobal.com/pdfarchive/view/1942/1942%20-%202476.html
By: Arabella-Cox - 7th July 2015 at 18:25
I borrowed a book (printed by the A.M., I think) from someone, many years ago, which described in detail the construction of the Rotol wooden blades. I am still in touch with that person so I’ll ask if they can find it and loan it to me again. I will scan and post the required pages.
Hollow steel blades were developed to production standard by Curtiss and used on many wartime types (Thunderbolt, Marauder, Wildcat, in UK). They also continued development post-WW2 and the de-H company took out a licence. They didn’t pursue the (unreliable and temperamental) electric pitch drive, which Curtiss used but instead designed hybrid Hydromatic pitch change hubs combined with the steel blades. These were fitted to Beverley, Britannia and S.R. Princess.
Oddly, Rotol took the electric pitch drive units and fitted them to some of their hubs and these were used on late Wellington.
The brass strip on the blade LE was an outer covering to prevent minor damage and peeling of the blade covering material. It was black painted but this soon wore off to expose the brass strip, especially near the tips.
Anon.
By: Graham Boak - 7th July 2015 at 17:39
Hydulignum is quite correct. Mea culpa.
My comments came from a quick dash through the Rotol book, particularly Appendix four. However, although Rotol did look at hollow steel blades they did not consider production using Duralumin, as DH were already using this material and it was foreseen to be a possible supply problem. Rotol magnesium blades were fitted to one squadron of Spitfires by the time of Dunkirk.
Heine is a German company, like Schwartz and VDM. I don’t know the differences between Jablo, Jicwood and Hydulignum, but they are all resin-impregnated compressed wood veneers. I can add that the cover on the Hydulignum blades was called Cristofin, and that the brass strip at the leading edge is shown as the outermost material, as opposed to being covered over on the others. Or so the diagram shows – it may of course be partial.
By: Arabella-Cox - 7th July 2015 at 16:59
For Hydrolignium it should read, I believe, HYDULIGNUM.
Rotol airscrews, and the VDM airscrew range of Germany, were designed to enable the use of various materials for the blades including aluminium alloy, magnesium and wood. The broad-chord large paddle-blade airscrews of the later German high engine-powered aircraft types (Junkers bombers, Heinkel bombers, FW-190 and derivatives, etc, etc) used wooden blades pretty much as standard. Wood being judged there, as here, as a non-strategic material.
Modern warbird repro props are made by Hoffmann of Germany to original profiles using the composite method of construction. Whilst they could, no-doubt, make these modern old-pattern blades from timber, P&P, the composite method is preferred due to higher strength/weight ratio and more consistent results. It is also current propeller production technology.
A very interesting subject.
Anon.
By: powerandpassion - 7th July 2015 at 07:23
Schwartz blades using Jablo and later Jicwood with a Rayoid cover. Rotol looked at Schwartz and Heine blades, with the use of Jablo, Jicwood and the later Hydrolignium with a Rotoloid cover. The difference in the covers being mainly in the chemicals used to soften the cellulose nitrate sheet. This outer covering appears to be mainly for weather and wear protection rather than being a significant load bearer.
Graham, thank you for a quick and informative reply. As Sgt Schultz would say, “Hogan…I know nuthink” about these types of props. Can you explain what the trade names are – Jablo, Jicwood, Hydrolignium, Rotoloid, Heine. How generally thick is the outer celluloid covering? Did Rotol use the same manufacturing steps as the Schwarz process described ? Are these processes still used today by anybody ? If not, why not ? So many bloody questions!
Thanks, Ed
By: Graham Boak - 6th July 2015 at 13:10
Britain did use large numbers of wooden props, notably from Rotol. Some details of their process can be found in the book Rotol: The History of an Airscrew Company 1937-1960.
To describe British props of the period as “layered from imported mahogony” is setting up a false scarecrow and based on a misunderstanding. I suspect it is true of the earlier propellers, and propellers in use for low-power engines. However, the majority of British wooden props for powerful aircraft were manufactured using the same principle as the German ones – the interior is not non-stress bearing and light but high-density compressed resin-impregnated birch veneers. Prewar, study had been made of German techniques with The Airscrew Company of Weybridge having a licence for the Schwartz blades using Jablo (as the Germans did) and later Jicwood with a Rayoid cover. Rotol looked at Schwartz and Heine blades, with the use of Jablo, Jicwood and the later Hydrolignium with a Rotoloid cover. The difference in the covers being mainly in the chemicals used to soften the cellulose nitrate sheet. This outer covering appears to be mainly for weather and wear protection rather than being a significant load bearer.
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July 6, 2015 at 11:05 am