Hi All
Firstly I know very little about propellers so if I may be asking some pretty basic or even dumb questions! Also I have tried to Google the subject but haven’t found very much so perhaps I am even asking the wrong question! Oh and I don’t know how to type the symbol for degrees on the message so I have written it longhand.
OK – The Stirling AP states that early Stirlings with Hercules XI engines either had ‘D. H. 20(degree symbol), constant-speed propellers’ or ‘D. H. Hydromatic, fully feathering propellers’. On photos of the dashboard of the Stirling you can see some with the 4 ‘feathering knobs/switches’ and some without any sign of them. OK, so I deduce that the Stirlings without feathering switches must have the DH 20 degree constant speed propellers.
But when I read about propellers it seems clear that there are occasions when you don’t want a propeller to ‘windmill’ and the most talked about option seems to be to ‘feather’ the blades so they stop rotating. But what about the propellers without any obvious ‘feathering switches’? How do you feather them? Or don’t they present a problem?
I don’t need a long explanation (unless you want to!) I’m happy to be given a link or some other reference to read or even just for someone to say ‘it is not a problem with those propellers’. It’s just that I can’t find any text that deals with that particular issue, they all seem to happily jump to variable pitch propellers. Nor can I find what the 20 degree symbol means in relation to the propeller – is that an angle or part of the product name?
I would like to know what the pilot in a Stirling would have done when he had problems with a DH 20.
Cheers
James
By: mike currill - 2nd July 2014 at 01:31
All very interesting stuff as, even with my reasonably extensive knowledge of things aeronautical, I had never understood some of that. Nice to learn something new.
By: HR339 - 2nd July 2014 at 00:54
The ‘Propellor Speed Control’ changes the RPM at which the engine is effectively governed. The constant speed unit then holds the propeller (and hence engine) RPM at that setting (as far as possible) by varying the blade angle as engine power is increased or decreased by the movement of the throttle. Google ‘Constant Speed Propeller’ for any number of better explanations.
By: smirky - 2nd July 2014 at 00:52
(on a pc) make sure NUMLOCK is on, hold down the ALT key and type 1 6 7 on the numeric keypad, then release the ALT key
By: jamesinnewcastl - 2nd July 2014 at 00:34
Hi Anon
Thanks for your reply, you’ve confirmed what was only a suspicion before, so I know that the props are not going to be windmill free. Now I’ve re-read about the Stirling controls knowing that I won’t find any direct feathering control and apparently the lowest set of levers are the Propeller Speed controls and on the non-feathering props the fully down position gives ‘positive coarse pitch’. In another pamphlet the text states that an engine failure should see the Pilot selecting PCP for the failed engine and which only minimises the R.P.M. at which it windmills. That sounds like a very undesirable condition, no wonder they went to the fully feathering type really quickly. So I assume that it didn’t matter much in a single engined aircraft with DH 20 props as a stopped engine was more of an issue than whether or not it was windmilling!
So now I struggle with the controls for the engine speed and those for the propeller speed. So a ‘constant speed’ prop would stay at a constant speed even as you varied the engine speed – but the speed it stayed at, was that set by the propeller speed control? That sounds right but implies that the prop and the engine are not locked together, I can read more about that one I think.
In any event I now have my Pilots action for a failed engine and I can be sure that lack of feathering switches means an early Stirling cockpit, cheers!
James
By: Arabella-Cox - 1st July 2014 at 22:15
Very simply: the 20 degree propellers had a pitch range of 20 degrees between coarse and fully fine. They could not be feathered even though it would be better to do so in circumstances of engine failure.
The fully feathering prop, as the name implies, could be feathered to a position in which it did not windmill the engine. A much better state of affairs.
A windmilling propeller not only makes the risk of further damage to the engine a certainty in a case of machanical failure, it also causes drag, which, in a heavy and underpowered aircraft is the second to last thing you need (the last being fire!).
Anon.
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July 1, 2014 at 8:54 pm