The last bit of the puzzle?

🙂 Maybe a line like “Never attempt to measure a moving propeller”..

I’ll hold off on the beermat-o-meter until I have cracked the thorny problem of why a 54350 is smaller than a 54400, but a 55260 is bigger than a 55400. It’s something to do with the 5/6 thing.. and yes, this will drive me to drink.

M

Congratulations Matt, a remarkable insight and breakthrough. Probably too obtuse for most to understand, but then I don’t understand dark matter, E=MC2, and why the missus would rather sit on the beach rather than dig through mud for corroded aeroplane parts.

I wonder if you could crowd source more data by making up a downloadable Beermat-o-meter or diagram showing key dimensions that would allow folks on the other side of the planet to feed back ‘apples with apples’ data to support your insight ? Maybe like a stick of a fixed dimension that is laid against the blade root with adjustments for different shank sizes, marks for measuring key blade dimensions and a red letter warning : “Are the magnetos isolated?” Probably also one of those stickers with a human figure getting chopped in half with a prop blade, to avoid litigation, in case a wit becomes a half wit.

Cracked it!

In case anyone’s interested – I should have saved this for the book, but I am very chuffed. That mystery third number, the ‘design number’, in DH blades.. I know what it means!

Without going into the arcane background (I have been working from AERADE and NACA reports, translating blade solidities in chords, plotting curves and looking for patterns), the simple answer is that there were only 5 basic designs of DH controllable-pitch blade going into the war, and they were scaled up and down according to their shank size.

The designs were defined by the position along the blade length of the broadest chord. The ‘base model’ for any given shank size (thousand series) had the fattest part at 50%. This would be in a case where the design number matches the shank size.

If the design number was different from the shank size, it takes the position of the widest point from the corresponding shank size ‘base model’. If the design number was higher than the shank size, it was at a point further outboard than 50%, if design number was less then it would be inboard.

This is of course then modified by the last digit, the ‘cropping number’, which trims a defined number of inches off of total diameter.

Thus:

[ATTACH=CONFIG]242171[/ATTACH]

I have prepared a little table – this might help with identification of ‘found’ blades where the drawing number has gone.

[ATTACH=CONFIG]242172[/ATTACH]

In the example above, the Spit has a number 55409. So shank is size 5, design number is 4. It does in fact have a widest point at what WOULD be 50% for a 4,000 shank base design… from the table, that’s at 46.7% along the un-cropped 5,000 series blade (from the centre of the boss). The reason why it still appears outboard of 50% is because the blade has been cropped 4.5 inches.

The Roc (55262) has a widest point at what would be 50% for a 2,000 series base design blade.. 40.6% along this one (again, from centre) and has had only 1 inch cropped from each blade.

And to help get round the problem of ‘cropping’ obscuring the blade type, here’s a handy guide to maximum widths, which give you the shank size as a starter for 10:

1,000 series 7.35″
2,000 series 7.86″
3,000 series 8.41″
4,000 series 9.0″
5,000 series 9.6″

This is still a work in progress, and I would greatly appreciate any ‘real world’ data that anyone might have out there to confirm or confound this theory!

This does not apply to Hydromatic types, and I don’t think some of the shank sizes or blade design numbers appeared in real life?

David, you have a PM.

Matt

My guess is that the aim with a carrier take- off would be to get as much thrust as possible from the moment the aircraft starts rolling. At max rpm you’d need the blade set at just five degrees for this to happen. The bracket prop could be re-set on the ground so its minimum pitch was this figure, but even with the 30-odd degrees available after the 1940 mods you’d struggle to get even a Spitfire’s economic cruise, let alone max speed, out of the coarsest pitch then available if you do this. With 60 degrees to play with, this would explain why Hydromatics might have been thought of for carrier operation?

I am guessing that yer basic Rotol wouldn’t offer this range.. what was the arrangement with Seafire I’s?

I suspect the 1942 Malta carrier trial was linked to the increase in pitch range, as the key point seems to have been take-off from the carrier. So it is possible that the 1940 change did not have this increase in pitch. Or perhaps there was a further one in 1942?

The Merlin III had two mountings low on the nose, one for a vacuum pump to replace the venturi and the other for the constant-speed mechanism. On the Spitfire the nose has a wider diameter which encloses these units. but the Hurricane required a new nose ring with small open blisters. These were present long before the cs units and props actually arrived.

To add a detail often missed, the change from variable-pitch to constant speed was not really a change of actual propeller. It had been realised quickly that the two-position prop mechanism had a ‘balance point’ in between fine and coarse settings where blade angle could be made to change with shaft torque. With the addition of a ‘Constant Speed Unit’ this could be made the basis of blade operation, such that changes in rpm caused by changes in the drag on the prop would be detected and countered by changing blade angle. The important point is that this was using the same prop as before. Bloody clever stuff, to think of this for the first time.

The engineers doing the field mods did also increase the range of pitch available to the blades, in order to make the most of this, but the prop hub mechanism and the blades remained the same.

Hydromatic props were also constant speed, but used an entirely different pitch-change mechanism.

The only major advantage of the Hydromatic over the bracket-type was that it could be fully feathered. This is why it became common on multi-engined aircraft even pre-war (you could get home on the remaining power, as long as you can lose the drag of a windmilling prop), but not on singles. The logic of attaching the heavier Hydro prop to single engined fighters is hard to explain, until you get to four blades where it becomes a production decision – as far as I know there was no attempt at producing a four-blade bracket prop by Hamilton or dH.

One comment for Powerandpassion: The changeover from 2-blade to 3-blade variable pitch (i.e. 2-position) DH bracket props occurred early in the Hurricane production, initially only on new aircraft. There were a few 2-bladers left in service in early 1940, as seen in France, but not many. I suspect you are thinking of the change-over from the variable pitch prop to the constant speed prop, which had been planned prewar but the DH contract office were unable to agree a price with the Air Ministry. There was a change of emphasis somewhere after Dunkirk, and the oft described emergency action took place in the gap before the Battle of Britain began. I don’t know when the Hydromatics as such appeared, if they were different, but one was being tested on HMS Furious for a mid-1942 delivery to Malta, and there were enough in stock to refit the entire batch. They were standard in time for the delivery of Spitfires to Australia.

Thanks, mate. I will call E today!

No worries – I understand how all this prop stuff is like sailing through mist, blurting on the fog horn, and then suddenly you come out into sunshine, then mist again !

I sense that the nomenclature was overtaken by wartime invention and events, and those that were concerned with these things probably thought they could tidy it all up ‘after the war’. Then jets came in and there was no time or reason to conduct such an exercise, because all you really needed was to consult a chart that showed aircraft and indicated the appropriate prop/blade, why would you need anything else ?

HS certainly invested in a parts numbering system and parts catalogues that are brilliant in their structure, clarity and detail, but not ‘why’ that number.

One way to sort this out is to invent the Beermat-o-meter, the analogue version being a stick with suction cups which allows you to lay it along a blade and pickup key characteristics with cardboard templates at ‘standard’ stations. This will require dealing with stepladders and clucking and disapproving museum folk as you travel the world building up the definitive database of blades. The digital version could be one of those laser scanners that connects to an iPhone, that will allow you to capture blade models from a distance, maybe putting little dots at stations to allow comparisons, the Beermat-o-matic.

I would buy a copy of Beermats Bible, once it is published, the definitive reference source for blades and the aeroplanes they were used on, and why.

Might I suggest that you don’t mutter the words “thrusting forward” and the 23 year old secretary’s name in your dreams………

Apologies to add nothing to this interesting conversation but it made me smile anyway….

Thanks, mate.

Yep, one of those tables has been a bit of a revelation on two counts. One it confirms my suspicion that shaft size is not important… by which I mean there’s no reference to shaft/splines in the code system for DH prop blades (why should there be, when you think about it). There have been many red herrings previously here about that.. and secondly it shows that blade shank size is still indicated after the ’45’ (which just means ‘Hydromatic’) – ref our PM conversation, David C.

It also shows that there is a sort of system about rotation, at least with Hydromatics, which would probably carry over to bracket types after all. 0 or 1 means right-hand, 5 or 6 means left-hand. Does anyone know the reason both numbers were used in each case?

This now only leaves one number available to describe blade design – back to the drawing board?

I have had ‘Thrusting Forward’ (back to shaft sizes again) on order for months now. I must chase that, thanks for reminding me..

PS – on the other thing, I have been a bit cr*p, what with one thing and another – I will call E today!

******** if I know

Matt, I would make the following observations :

a) The missus is quietly relieved that you wake up muttering ‘thickness to chord’ rather than the 23 year old secretary’s name at work !

b) ‘Thrusting Forward’ might be worth chasing up to gain insight into the didactic of blade development. Described as a ‘Detailed account of the development of the Aircraft propeller; published by Hamilton Standard to commemorate their 50th anniversary in the manufacture and development aircraft propellers’.

c) I understand that deHavilland’s were a licensee of HS, and that Mr Hamilton himself was resident in Paris and concluded this arrangement. In the build up to war, the Air Ministry and dH would most likely want to adapt HS systems without becoming bogged down in reinventing parts or systems. From looking at dH Australian manufactured prop parts the part numbers are identical to US HS part numbers, so dH Aust, it seems, entirely adopted the HS parts numbering system. Therefore dH parts are HS parts, therefore design factors are HS design factors. I understand that dH GB, in seeking to describe a whole airscrew system with an ‘Overview’ number, sought to classify this through a separate British 5 number system that made more sense to them rather than adopting the US HS ‘Overview’. I understand you are seeking a logic that connects the dH GB Overview number with a design decision made by a US designer perhaps years before.

My gut feel is you will find some pieces of logic that support your hypothesis, while some drive you to drink. I think the times were so chaotic and urgent that pieces of technology were grabbed and adopted without waiting for the nomenclature to catch up, eg changing over 2 bladed Hurricanes to 3 bladed Hydromatics within ‘weeks’ during the BoB. In my own researches into Mosquito blades I have finally found that the blade selected by those perennial scavengers in the dH design office was originally designed by HS for a radial P&W 1830 engine. So how do you rationally incorporate blade design factor nomenclature for a prewar 1830 radial into a twin V12 design on the other side of the planet ? Probably by going to a pub and throwing a dart onto a board to come up with a number.

Anyway, here are some charts with some data such as ‘Activity Factor’ which I know nothing about, that might start to form some logic threads, maybe….Sorry it’s all Hydromatic :

[ATTACH=CONFIG]242076[/ATTACH]

[ATTACH=CONFIG]242077[/ATTACH]

[ATTACH=CONFIG]242078[/ATTACH]