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Propeller design...

Old 3rd Feb 2019, 15:34
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Propeller design...

...can someone explain why the props on the Allison powered CV580 are cut off square, whilst other props were almost pointie (like on a Viscount 700)? Aerodynamics no doubt...but why?
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Old 3rd Feb 2019, 16:13
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They were like that on the Dart-powered F27s as well. OK, on the -400 version, at least. But the Darts on the Herald had pointed props, as I recall.

I always understood that it was to maximise thrust while keeping tip speeds subsonic. I have no idea if that's right or not.

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Old 3rd Feb 2019, 19:20
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Try this: Prop shapes
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Old 3rd Feb 2019, 19:54
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Just to add to the confusion, the prop tips on the F27-100 were rounded, while on the -200 and following they were square. The -100 had lower-powered engines, albeit still Darts, so perhaps a factor.
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Old 4th Feb 2019, 00:38
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On the Lockheed Electra, there was a choice of two prop manufacturers - Aeroproducts (part of GM) or Hamilton Standard (part of United Technology). The Aeroproducts props were square tipped, the Hamilton Standards were rounded. Don't think it made any difference to the airplane although the majority of airlines picked the Aeroproducts prop.
Aeroproducts are now extinct. With the use of composite materials the shape of props has become quite bizarre - notably the Airbus A400 contraprops.
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Old 4th Feb 2019, 08:33
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Flew the square tipped Aero Products Electra only, but we leased a Hamilton Standard prop Electra that some of our crews flew, and they reckoned it was 10 to 15 knots faster in the cruise.
Of course this may have been due to the condition of the wing, it may have had a lot less "Boiler Plate" patches causing drag.
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Old 4th Feb 2019, 09:35
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I have this from someone who I consider one of the most knowledgeable people in this area, although he qualifies it by saying his memory is dragging it out from 1969!
"The rounded tip produces less drag and is mainly employed on the lower powered engine whereas the square tip allows a wider chord, a smaller radius and, whilst generating more drag, produces/permits/absorbs (not sure which) more power before the tip goes supersonic".
As he says, a long time ago, so file under "not wholly reliable"
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Old 4th Feb 2019, 11:41
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Propeller design is a black art, usually incorporating moonlight seances, invocations to various dieties and human sacrifices. Having said that, many earlier (pre-composite/computer design era) props are only partly designed, the other part being a compromise between knowledge and production limitations, in my view. We've also managed to lose track of a lot of knowledge on how to design props for higher-powered engines once we switched to using gas-turbine engines without props.

When you look at 21st century prop design, you see that the trend is for blades to be scimitar shaped, with a more swept, narrow-chord bit near the tip as opposed to the inboard section. Essentially, this copies knowledge from high-speed wing design but it wasn't possible to build these props until we came up with composite blades in which we are able to counter the effects of blade drag and blade twist with strategically designed and applied material properties. For the engines under discussion, at that point in time there was a need for props to absorb more engine power, for which the immediate solution was to either increase the number of blades, or their span. Both are limited by either hub construction and complexity, or geometric limitations, tip speed and the associated loss of efficiency. They then had to move to increasing the blade surface by building broad chord blades. This then necessitates the use of a more high speed profile on the outboard ends and I think that the decision to use squared off tips is connected to either a construction limitation or the outcome from testing that pointed towards a higher overall efficiency from these tips as opposed to rounded ones. Sorry, not a very cohesive answer I'm afraid.
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Old 4th Feb 2019, 13:46
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Some time ago I asked about the propeller design on the Bristol Britannia. Here is my question and the answers:-
Prop Tip Question
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Old 4th Feb 2019, 21:39
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I was filling in for the manager in KDCA and was asked to take a US Navy Squadron Commander out to look at some of our Air Shuttle L-188 Electras. He was flying P-3A Orion, a direct descendant of the Electra. He took one look at the numerous boiler plate patches on the wings and asked me how many missions it had flown over Hanoi. Pardon the drift.

Last edited by tonytales; 5th Feb 2019 at 23:10. Reason: corrected typo
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Old 5th Feb 2019, 17:15
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A British registered Sea Fury now sports an R2800 with a broad bladed square-tipped prop apparently off a Grumman Guardian - yes, I know! - but it doesn't half sound good at speed! A German collector has acquired a Greek-based A-26 Invader which also has square-tipped blades; hopefully we may get to see it in the UK later in the year.
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Old 5th Feb 2019, 23:09
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Of course the A-26 Invader originally had round tipped Ham Standards for it's B-Series R-2800's. Then for counter-insurgency work in places like Viet Nam they were upgraded with C-Series R-2800 and were equipped with what looked like very much like the square-tipped Ham Standards from the DC-6.
Some years before I had seen A Martin B-26 Marauder converted to executive use which also had been upgraded with C-Series R-2800 and the large paddle-bladed square-tipped Ham Standards.. -
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Old 7th Feb 2019, 00:50
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ISTR from reading a book on the development of the Viscount the prop tips were squared to give more area so as to be able to absorb more power as the engines were uprated as the models got bigger.. I think they were broader chord and bigger diameter as well. I read this isn't entirely accurate.
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