John, I can reply in two ways. Firstly in practical terms if you haven't tried
it you should. There is a really noticeable difference in how quickly you slow
down with the engine idling and the prop either in it's normal fine mode or
fully
coarse.
Alternatively looking at things from a theoretical viewpoint, the total
drag for my DG300 glider at optimum glide angle(1:42) is around15 to 20lbs,
(dividing
AUW by 42) at around 60kts. For my mono at it's best glide angle of 1:12
at 60 kts it is around 80 to 100 lbs. Now if you stick your hand out of your
car window going down the road at 60 kts/70mph and make it 'fully fine' the
force on your hand is something more than 10lbs. Imagine your hand enlarged to
the dimensions of three prop blades and you have accounted for much of the
difference
in drag of my glider and my Europa!
Having the engine idling doesn't make too much odds. At 1600 rpm a
typical
fine pitch tip angle of 20 degrees is equivalent to 9 kts. air flow. In
fact it is worse than that if you have warp drive blades or indeed most other
prop blades seen on Europas, which have low blade twist, which means that the
central and inner portions of the blades are producing significantly lower air
flow rates.
So my thesis is that an idling or stationary prop is responsible for
a high proportion of drag (admittedly somewhat lower if you have all those extra
wheels!!), and trying to slow down with the prop feathered is rather like trying
to land a glider without air brake - you can float in ground effect down
a long field until stopped by the far hedge!
Hope that all makes sense! Happy landings, David Joyce, G-XSDJ
Sent from my iPad
On 11 Jan 2012, at 08:53, "John Wighton" <john@wighton.net> wrote:
>
> I would like David to validate his last comment - field length dependent upon
prop pitch when stopped.
>
> When the prop is turning: The fine pitch gives lower residual thrust, hence
field length is potentially decreased. Feathering is not an option when engine
is working.
>
> When the prop is stopped: There is a reduction in drag when the prop pitch is
set feathered but this should not alter the field length used for landing?
Arguably
there is slightly lower drag when feathered, but this is negligible to
the total drag of the a/c in landing config.
>
> Sorry to highjack the thread but this may be important for us with feathering
props if in a forced landing situation - tinkering with the feathering switch
may not be the most efficient use of our time at that phase of the flight!
>
> I have the Airmaster AP332 with AP200 controller setup and recommend it.
>
> --------
> John Wighton
> Europa XS trigear G-IPOD
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> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=363021#363021
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