De Havilland decoder part one

One thing – the reference angles for all the telescoped blades are guestimates. They can be calculated precisely by tricky maths involving several factors and a couple of integrals, but I do have a life.. if ever anyone wants to make a new one, l can demonstrate.. 🙂

Now I guess it’s Rotols.. unless you fancy doing the book, Anneorac?

That’s interesting, thanks. So there could be a row going down from the 7’6″ in 3″ increments at least as far as the 51109, which is what your prop essentially was – assuming it was a DH bracket, or course.

Just a little bit of info on Mew Gull prop. diameter should be 6’9″ – I know this because back in the ’50’s Peter Clifford dinged the Mews 7’0″ prop during the Kings Cup heats whilst we were eliminated. So my father offered him use of our Proctor prop & the stop settings were changed appropriately. (32° from 28° rings in the back of my mind). The race speed for the Mew with this prop was the fastest post war. It turned out that our supposedly standard 7’0″ prop had been cropped due to tip damage & it was 6’9″ Dia.
Keith

I have finished the decoder. Turns out that the Lancaster and Spitfire prop blades were very close equivalents, tracing them back through various US references to their Hamilton Standard versions. Oddly, while the Mk.I Spitfire had essentially the same design as on the Short C-class Empire boats, the Lancaster needle-type Hydromatics, the Fairey Battle and marks of Mosquito, for the few Spitfire Mk.V’s with Hydromatics a completely different design was used. Go figure. The blade angle is not an operating angle of any kind, it is just a handy mathematical reference to describe the twist. At this hypothetical pitch at 42″ along the blade, the pitch distribution is flat. (Only it’s not where blades have been telescoped).

There are gaps, where the information runs out. Largely around the ‘home grown’ blades.

Thanks Craig.

Nice film about DH blades here: https://www.youtube.com/watch?v=PY8oQ3XNreY – Ralph, these all appear Clark-Y to me, circa 1940.

No worries, A.C. Thanks for doing that.

Ralph, I’m sorry – I don’t have any more performance data on the Spitfire than the easily-available web stuff.

Beermat
The data includes angles at different stations.

Will need to get it into a form that is easy to read as some of the photos are a bit blurry. It will be a couple of days before I can get on to it.

On a related note, Matt et al, have any of you got climb-to-height data for a Spitfire I or II fitted with a CS DH prop. The only data that appears to be easily available online are for the 2-pitch fitting or for a CS unit on the Spitfire V, which is another kettle-of-fish altogether.

Hi

That data would be gold-dust for me – thanks.

Specifically, the earlier blades from the ’55’ series are of great interest –

Blade Assembly No. P.455960, Propeller type 55/14
Blade Assembly No. P.4551172T/A-17, Propeller type 55/8
Blade Assembly No. P.4551775A-17, Propeller type 55/21, 55/24
Blade Assembly No. P.4551778A-17, Propeller type 55/12

Also this apparent early 54 series one:

Blade Assembly No. P.454300A-42, Propeller type D20/445/2, D20/445/2, D20/445/4, D20/445/5

I am ready to have the theories blown away by some actual measurement – does the data include pitch/twist?

Beermat
I could not locate any information on what the A or B suffix mean.
I did however locate blade shape data from the 1950’s with a diagram descriping how the measurements were made. Given you have got right in to doing the maths, I will let you crunch the data.

Propellers covered include:
Blade Assembly No. P.454906A-20, Propeller type D14/445/1 and 2
Blade Assembly No. P.455960, Propeller type 55/14
Blade Assembly No. P.4551172T/A-17, Propeller type 55/8
Blade Assembly No. P.4551775A-17, Propeller type 55/21, 55/24
Blade Assembly No. P.4551778A-17, Propeller type 55/12
Blade Assembly No. P.2541971A-78-1, Propeller type D29/446/1, D97/446/1
Blade Assembly No. P.2541974A-78-1, Propeller type D18/446/1, D42/446/1
Blade Assembly No. P.4542712A-42, Propeller type D16/446/1
Blade Assembly No. P.4542962A-36, Propeller type D17/446/1
Blade Assembly No. P.454300A-42, Propeller type D20/445/2, D20/445/2, D20/445/4, D20/445/5
Blade Assembly No. P.4542172A-42, Propeller type D1/445/2
Blade Assembly No. P.2012550A-57-1, Propeller type 81/313/1
Blade Assembly No. P.2011368TA-57-1, Propeller type D113/312/1, D25/312/1
Blade Assembly No. PR.1941774A-78-1, Propeller type D100/446/1, D124/446/1
Blade Assembly No. P.4543000A-42, Propeller type D87/445/1, 2, 3 and 4
Blade Assembly No. PR.1512753A-111-1, Propeller type D112/313/1
Blade Assembly No. P.4542718A-42, Propeller type D68/4451/1
Blade Assembly No. PR.2531003A-77, Propeller type D45/435/1
Blade Assembly No. PR.2531009A-77, Propeller type D47/435/1

Let me know if there are any in particular you would like to have.

Does anyone know what the ‘A’ and ‘B’ suffixes meant? Was A post- 1938 twist revision? and B, like the Spitfire, meant twisted by telescoping?

Here it is:

[ATTACH=CONFIG]256499[/ATTACH]

I couldn’t get the Spitfire geometry as shown in the RM to make sense. But then I discovered that if you use the US convention of centering everything on the 42 inch station along the blade regardless of blade diameter or 0.7 R/r or whatever..

Everything snaps into place. Here is a diagram showing how a constant pitch variation with length – that gave ‘butter-cutting’ twist at 25 degrees at 42 inches – was completely messed up by de Havilland in their keen-ness to give the Spit blade butter-cutting ability at 15 degrees.

This the new blade had – around the 0.7r mark, which everyone agreed was all-important. However, the blade now had two pitch distributions, an inboard terminating at the beginning of the telescope, around 33 inches out, and an outboard one that met the new thinking about the correct angle of ‘constant pitch’ being a lot less than actual constant speed blade angle – 15 degrees less.

EDIT – THIS IS WHERE I WANTED TO PUT THE KILLER DIAGRAM, BUT THE FORUM UPLOAD PROBLEM HAS STRUCK

Douglas Adams would have loved this.

The answer is…. 42

Will explain shortly.

Just to say, though, that I have the 55100 sub-series at 0.7 radius angled for flat distribution at 25 degrees and with efficiency optimised for 170 mph with a Wright Cyclone on 16/11 ratio (the cruise of the DC-1 for which it was designed). It also worked for a Pegasus-engined Empire ‘boat at 150 mph.

By extrapolation it looks like 60 mph increments, such that a 55200 sub-series blade would be ‘flat’ at 31 degrees at 0.7, optimum efficiency meant for 230 mph, intended max speed of a Roc. 55300 is an unknown, but probably 38 degrees intended for a 290 mph optimum (with a Merlin on standard reduction, for example).

All of this is wrong in that this happened without understanding the ’15 degrees-off’ rule from 1938.

Do that, and a Lancaster – say optimised for 230 mph cruise – would be a 200. Old system would put it at 31 degrees. New system would have it at 31-15= 16 degrees, at 0.7r. HS130B table has the 6353 as.. 16.8 degrees at 0.7r.

Not bad. Not bad at all.

So, Goshawk = evaporatively-cooled Kestrel, Peregrine = downdraft – oops, downdraught Kestrel?

I’ll do some diagrams at some point – I want to check the maths first – or even better get Trak-tor to do it!

Yes, he’s lost me now to. Pictures, I need pictures and diagrams, my brain works better that way.

Goshawk was updraught

Thanks a lot. That’s my kind of reading before falling asleep…
And I have a degree in maths, so it should make some sense.
😉

Propeller theory yes – https://babel.hathitrust.org/cgi/pt?id=uc1.$b80327 is one of them, also try Aircraft Propeller Design (New York and London: McGraw-Hill Book Co. 1930), by Fred E. Weick, which is also available online. They both get into pretty heavy maths, but are useful to check a lot of the web-based info against.

What blades DH and Hamilton actually made and why – no. Not yet, anyway 🙂

It’s really interesting, though most of the time I have only distant idea what are you talking about…
But I like it.
Is there some book/manual to study this?
Thanks.

Back on topic (ish).. I have had a bit of a lightbulb moment which I should have had a week ago. The Hamilton Standard tables that I have talk of a reference station – normally at 42″. The angle at this station is the angle at which the pitch change with distance is constant along the blade – ie. a flat distribution. This is the angle at 42″ which would enable butter-cutting (but as above isn’t the best angle for air-cutting, as it turns out).

So, the 6353 seems to have a ‘reference angle’ of 19.9 degrees.

I couldn’t work out why HS didn’t just use the UK method of putting a zero at 0.7 radius, and working back from there. I realise now that this doesn’t tell you enough about the geometry of the blade without complex maths, while the US version puts it on a plate.

Doing the complex maths (actually not, cheating by drawing it on a piece of paper) shows the Spit blade has a reference angle at 0.7 radius (about 48 inches) of 25 degrees inboard and 15 degrees outboard – it has two foci. This is because DH twisted it when they telescoped it. The average is around 20 degrees – which works out more or less the same as the later Lancaster blade, but done less elegantly by simply twisting half the blade twice as much.

All that remains for me is to untwist the Spitfire blade to get an original 5,000 series twist – and then work out where it fits on the ‘hundred’ sub-series. Are we having fun yet?