Lifespan of propellers?

[berman]

Wood propellers were assembled from laminations using low tech adhesives such as cassein and hide glue and coated with just linseed based varnish for waterproofing. Near constant exposure to the elements. Plus factor in stress from centrifugal force, air turbulence, and vibration.
Did propellers have a limited lifespan of X number of flight hours or provided that they did not sustain battle damage or landing accidents, did they last the life of the aircraft? How many hours was the limit on a propeller?

[YavorD]

The lifespan of popellers was quite short, IMHO. Besides damage from foreign objects, there was a deformation caused by thrust and centrifugal forces, causing blade shape to change.
In 1918, for example, a S.E.5a was considered worn out at about 250 flight hours, after about 2 engine replacements and using at least 5 or 6 propellers.
Regards,
Yavor

[rc.gardner]

The answer to this question is one that I have never resolved. The more I study it the less sure I am. My guess at the moment is that about ten props were made for each British WW1 aircraft in the last six months of the war. I don't mean here that ten props were fitted to each aircraft. There were spare props held at Squadron level, and at second and third line as well as in depots back in Britain. And the answer to the question is different in each year of the war and different for each type of aircraft.

Aircraft design influenced how often spare props were needed.

The props on pusher aircraft worked in more arduous conditions than tractor props. The prop on the pusher FE2B was subjected to; dust, twigs, pebbles etc thrown up ahead of it by the undercarriage wheels; and oil thrown back by the engine; and in combat by the observer's maps, pencils, gloves and by empty cartridges, although most of these were collected on leaving the breech. So pusher aircraft used more props than a tractor prop.

Similarly, naval props operated in arduous conditions whether the aircraft was a pusher or a tractor, because of the large amount of sea spray thrown up in take-off and landing. These aircraft also used more props than their land based equivalents.

The final aspect was that the requirements for the design of undercarriages and propellers were fundamentally different. As the power of aero-engines increased props tended to grow in length. Conversely the undercarriage had to be as short as possible because it caused drag in flight. So a compromise was reached where the prop ran close to the ground. Therefore pilots had to land tail-down to safeguard the prop; hence the three point landing. Even then a slightly heavy landing could use up the compliance in the undercarriage and damage the prop. This problem was reduced to some extent with both four-bladed props, which could be shorter and transmit the same amount of power; and with geared engines where reduced speed of prop rotation led to much more pitch without necessarily an increase in blade length.

I read some time back a letter written by a British pilot to his Mum where he explained he was in trouble with his Squadron Commander for breaking props; he was on his fourth in three days. Landing in blustery conditions wrote off the first. Taxi-ing over a tussock of grass damaged the second. After a severe interview with his boss he became ultra-cautious and walked across the field to select the smoothest possible run for take-off. This worked well for two or three sorties. Then he landed to refuel and rearm and had to go up again. He followed the extract track on take-off that he used on landing to discover a large fresh molehill which damaged the tip of the third prop.

The process of manufacture of propellers also had an indirect influence on the number of props available and I'll describe this in the next post, after lunch!

With kind regards,

Bob

[Jim]

Also, stone strikes were a big problem on many airfields. Remember that they were not paved like modern runways - they were literally fields. A propeller going at full power would have some similar properties to a high power vacuum cleaner as it was going down the field to take off or even just taxiing around and any stones or sticks could be sucked up into the blade creating nicks and chips. That is why wooden propellers eventually were given brass or fabric wrappings on their tips - to protect them from such damage.

[rc.gardner]

Part Two!

The process of manufacturing propellers in Britain had an indirect effect on the numbers of props used.

The records of the British & Colonial Co show that early in the war three props were provided with each aircraft built, one fitted and two spares. In the early days of 1915, B & C provided props direct to the user; so a RNAS airfield in Britain and in Malta could both order replacement props direct from Bristol. And similarly with the RFC where CFS Upavon ordered single replacement props as required.

In 1915 most contracts began to be placed centrally for props which then went to a depot. But some contracts remained open for two years or more for occasional orders for single replacement props. This suggests that the useage of props at this stage of the war was not high.

In 1916 and 1917 the supply of props became complex. More and more were needed. More woodworking firms were recruited as prop makers. I'm left with the impression that at this stage a maestro sat at the centre of a web juggling and controlling the procurement of props. Some astonishing things happened. At the level of prop manufacture there was co-operation between neighborouring makers. As examples, B & C once made a handful of props for a second maker to the design of a third. I have seen Lang designed and made props with a Tibbenham decal. Why? Perhaps a Lang made prop was repaired by Tibbenham? Or perhaps raw unfinished props from Lang went to Tibbenham to be finished due to some exigency of the service.

In mid 1917, by when there were over a hundred firms making British props, it was agreed that some centralisation and control was essential. This was co-incidental with the establishment of the Air Board to oversee the formation of the RAF from the RNAS and the RFC. The Air Board began to experiment in the Autumn of 1917 to find the best prop for each aircraft type. Before that pilots had sought out their own private preference of prop. Thus the Air Board was able to nominate one and sometimes two or three standard props for each type of aircraft.

From around early September 1917 the Air Board ordered props in batches of 100, where a batch was described by a G number and the number of props in that batch were numbered from N1 onwards. And it mostly only ordered standard props. Thus the process of ordering and making props was simplified and the number of types in use reduced which enabled more of the best props to be made.

Increased availablity probably led to a greater turnover of props. In 1914 to 1916, props were carefully husbanded at unit level. Damaged tips were reprofiled. Wood losses in trailing and leading edges were repaired with new wood let in. Ovality in bolt holes (in the thrust plane) could be overcome for a short period by tightening the bolts above the normal amount and occasionally by introducing cork liners to the bolt hole. These may have been sacrificial.

With the increased availability of props from late 1917 onwards I have formed the opinion that the useage or props per aircraft rose and props became, in modern language, consumables.

A final aspect muddies the waters to some extent. At the armistice, the production of war materiel was brought to an end haphazardly and some aircraft and propellers continued to be made for some length of time, for contractual reasons and political reasons. These propellers skew any analysis of props per aircraft.

With kind regards,

Bob

[Shadoko]

Hi,

Thank you Bob for all theese very valuable informations.

In France, the number found everywhere for a/c made along WW1 is 55000, about half of them in 1918. For the props, the numbers vary from 120000 to 240000 from one author to another. I do think the truth is somewhere between the two. I have tried to count them in the Army archives, but data are missing for 1918, so, AMHA, no reliable precise number seems reachable.

To return to berman question, there was not, in France, established nor "mandatory" lifespan for propellers. As Bob said, from quite art objects, proudly made one by one by qualified workers in 1910, propellers became "consumables" by 1918. From 1914 to 1916, around half the contracts about propellers were for repairs. Repair seems vanishing later.

It seems the replacement of propellers was almost always the result of accident. Perhaps the lifespan of a/c themselves was not long enough for the one of prop has to be considered!

I have noted some sentences about propellers in The US. Air Service in World War 1 Maurer Maurer ed. (fully accessible on Internet). They show that the mud was the biggest ennemy of propellers... or the writer had so trouble with mud that he was obsessed with it?:

Volume 1:
p.97: The flying field itself was newly sown arid mud thrown from the wheels of the airplanes broke propellers almost as fast as they could be put on.
p.127: Up to the close of hostilities this depot had received more than 2,600 propellers and issued more than 1,600; 2,706 Liberty engines were received and 1,199 were issued.
p.132: Mud guards to prevent the breaking of DH-4 propellers by flying mud were designed and sample sets produced.
p.260 : Service flying could not be started immediately, due principally to the inferior quality of the propellers furnished by the French air park. Although it
was unwise to attempt service flying with these propellers, patrols were carried out along the lines, .../... . On the 5th of June the propeller difficulty was overcome by the receipt of a supply of another make.
p.272: Because of the excellent condition of the aviation field which the group occupied, it being well drained and covered with grass, it was possible for the planes to “take off without breaking the propeller in spite of the occasional heavy rainstorms.
p.291: The Ormeaux airdrome was in many respects an ideal terrain for an airdrome. It was large and smooth, and while standing wheat, which covered a part of it, at first interfered by damaging many propellers, the landing field was the best from which the 1st Pursuit ever operated.
p.365 : The heavy rains of the previous week had made the Amanty airdrome so muddy that the propellers of fifty percent of the airplanes were broken while taking off. This constant breakage of propellers, by the mud slung from the wheels, was a primary cause of the heavy casualties. Formations of ample strength were scheduled to carry out the missions, but it was rare that half the airplanes left the ground.
p.366: Upon the return of this mission another formation attempted to take off, but three fourths of the propellers were broken by the mud.
p.368 : Seven Breguets were assigned to the mission, but the usual toll of broken propellers allowed only three to reach and bomb the objective.
p.369 : The muddy flying field caused so many broken propellers that the formations which crossed the lines were never large enough for self-protection.
Volume 2:
p.318: High grass will injure many propellers and must be removed before flying begins.
Volume 3:
p.257: Atmospheric Conditions. Low clouds with strong winds and heavy rainstorms rendering flying conditions extremely difficult and causing broken
propellers due to the rain and accidents to shifts [?I due to very muddy aviation fields. p.332: As my propeller was hit and a spark plug blown out, I dove to within about 50 meters from the ground when the boche who were about 200 meters over and behind me gave up the fight. So I climbed up again and headed home. My propeller broke over Ourches and I was forced to land with the wind. The plane except for the bullet hole is undamaged.
Volume 4:
p.3: All these and many other things-the poor construction of radiator caps, tail skids that wore out, propellers broken because the wheels on the planes lacked mudguards, etc.,
p.20: With the arrival of the rainy season and consequent muddy fields, many propellers have been broken in taking off, and flying has had to be suspended in many cases until mud guards could be made and put on the planes.
p.187: I had to fly as low as 30 meters on the afternoon of Sept. 12, 1918, as the rain was very heavy and large drops. On that instance the pilot reduced the speed of the engine until we could just fly level to decrease the chance of the propeller being broken by the heavy drops of rain. In fact it was split anyhow but we were able to finish the mission unaware of that.


Regards,
PM

PS: @berman: casein and hide glue were (and are) not low tech! I have two WW1 props (one walnut hide glued and one beech casein glued) with their blades blown up in accident: the broken lines are not the glue lines but inside plain wood.

[berman]

Thank you all for your replies. To Bob Gardner: When will your complete book series on German propellers be available for sale?

[rc.gardner]

Hi Berman,

The first book, is a primer describing the Idflieg etc rules and regulations of prop making; the makers; how to decipher the data on the prop; the woods used; the results of British blockade of German ports and the concomitant reduction in imported woods like mahogany and American walnut. It's finished but the publication process will take some months.

Part One, about 190 pages, describes makers from Adastra to Garuda.
Part Two, about 170 pages, describes makers from Geest to LZ Propeller.

Publication of these three books will be around September 2013.

Part Three describes the remaining makers and is some months from completion.

These books will have soft covers and therefore will be about thirty percent cheaper than the books in my series on British props.

With kind regards,

Bob

[vossiewulf]

Originally Posted by Shadoko

PS: @berman: casein and hide glue were (and are) not low tech! I have two WW1 props (one walnut hide glued and one beech casein glued) with their blades blown up in accident: the broken lines are not the glue lines but inside plain wood.

Low tech doesn't necessarily mean "bad". There are lots of examples where low tech remains the best solution to particular problems.

Japanese katanas are decidedly low tech, the best ones still made from steel created in smelting furnaces whose design hasn't changed in 1000 years, and then are entirely hand hammered, hardened and tempered in a process that again hasn't changed in hundreds of years.

If I gave you any technology, machinery, and materials you asked for and told you to make a slashing weapon better than the output of those low tech processes, you couldn't do it.

And an even more extreme example, the single sharpest edges which can be produced by individuals remains a good flint-napped edge in a piece of obsidian, and THAT tech goes back about 70,000 years.

And how many of us pilot types wear leather jackets? We do because there's nothing better for the purpose and leather tanning hasn't changed appreciably in hundreds of years at least.

I think this qualifies as preaching to the choir

[Jim]

[QUOTE=vossiewulf;633978]Low tech doesn't necessarily mean "bad". There are lots of examples where low tech remains the best solution to particular problems.

Japanese katanas are decidedly low tech, the best ones still made from steel created in smelting furnaces whose design hasn't changed in 1000 years, and then are entirely hand hammered, hardened and tempered in a process that again hasn't changed in hundreds of years.

QUOTE]


Hi Vossiewulf.

I totally agree with you, though I would say that some of the old tech is actually better than anything that can be created today. Japanese swords, which are called Nihonto are such an example. They were worn in a set of two called a daishō which consisted of one longer than 60 cm which is called a katana and one that is between 30 cm and 60 cm called a wakazashi. The katana was used when you were outside and with nothing near you so you could swing such a long sword, while the wakazashi was used when you were indoors, or around obstacles where the longer sword could not be used without interference. The wakazashi was the one that was used more frequently. Sometimes they also carried a tanto which is a blade less than 30cm and is more like a knife. I am fortunate to own a 14th century wakazashi from a known smith. These swords are still made today, but modern ones are simply not of the quality of the antique ones. The secrets of the old smiths have been lost for hundreds of years now and even with the best technology of today the quality of the old ones has never been matched.