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powerandpassion
07-05-2015, 01:31 AM
Hello, I am new to the forum, based in Melbourne, Australia and would appreciate any technical assistance in respect of interwar props. The long term intent is to remake airworthy examples for British strip steel aircraft 1925-35, such as examples shown on www.silverbiplanes.com

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 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 and 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 shwn 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, 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 resulted 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.
Thank you,
Ed

powerandpassion
07-05-2015, 07:57 AM
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 mahogany the Schwarz process seems far more efficient on many levels and the ultimate development of the wooden propeller. Ver gut.:D