Author: shiprv8@... Time: Fri Mar 12, 2004 5:29 pm PST Link
After examining the prop that broke during testing, it appears the stresses
were caused by ground proximity, i.e. the effect of the shock wave created when
a prop runs close to the ground in a static position.
The mfr. will develop a minimum ground clearance height for this particular
prop. The test stand will be raised higher off the ground for testing this
type of prop.
This prop is for aircraft NOT exceeding 120mph, i.e. not RV's
The engine itself suffered no damage to the crank or any other rotating
components.
Ship
[Non-text portions of this message have been removed]
Author: "midniteoyl2" <midniteoyl@...> Time: Fri Mar 12, 2004 8:22 pm PST Link
Just how close was this prop to the ground, and is it something to
worry about if, say, I had one on a 'light sport qualified' plane?
Jim
--- In vansairforce@yahoogroups.com, shiprv8@a... wrote:
> After examining the prop that broke during testing, it appears the
stresses
> were caused by ground proximity, i.e. the effect of the shock wave
created when
> a prop runs close to the ground in a static position.
>
> The mfr. will develop a minimum ground clearance height for this
particular
> prop. The test stand will be raised higher off the ground for
testing this
> type of prop.
>
> This prop is for aircraft NOT exceeding 120mph, i.e. not RV's
>
> The engine itself suffered no damage to the crank or any other
rotating
> components.
>
> Ship
> [Non-text portions of this message have been removed]
Author: shiprv8@... Time: Sat Mar 13, 2004 5:29 am PST Link
the prop tip was running about 9" from the ground...very low. this was also
a large prop @ 74" diameter with a sheared square tip and a thick airfoil
section. given that we were running at 2700rpm, the consensus is that the tip was
going supersonic as it passed through the large vortex that forms on the
ground in a static run and slamming into it's own shock wave at the bottom of the
prop arc. this makes sense given the type of noise and sound pressures we
were experiencing during the runs.
we ran appx. 10 hours at full power static before it separated. in normal
flight operations, running at 2700rpm static is rare if ever, and only for a few
seconds at most. normal runup is around 2000rpm for short period. on
takeoff runs, 2700rpm is not in a static condition, thus no major vortex
interference.
our test represented extreme conditions. we were using this prop to load the
engine for endurance runs at full power. the prop mfr. recommended this prop
for this purpose, as had several other engine mfrs. now we know one of it's
limitations.
as for light sport, i couldn't say for sure although there's probably little
chance of seeing 200hp and 74" props on LS aircraft. i suspect that most LSA
will use lower power and smaller diameter props.
another consideration is that this is an adjustable f/p prop....it would be
pitched for around 2200rpm at static....nowhere near the test conditions.
in real life this prop would be operating on tall landing gear....and a
taller test stand :-). even a few inches higher from the ground would make a huge
difference.
ship
[Non-text portions of this message have been removed]
Author: "midniteoyl2" <midniteoyl@...> Time: Sun Mar 14, 2004 3:50 am PST Link
Thanks ship. Just wondering since you said several times that it was
a prop made for 120hp or less... So ya, I can see why it shredded at
200hp while giving the ants a haircut :)
Jim
--- In vansairforce@yahoogroups.com, shiprv8@a... wrote:
> the prop tip was running about 9" from the ground...very low.
this was also
> a large prop @ 74" diameter with a sheared square tip and a thick
airfoil
> section. given that we were running at 2700rpm, the consensus is
that the tip was
> going supersonic as it passed through the large vortex that forms
on the
> ground in a static run and slamming into it's own shock wave at
the bottom of the
> prop arc. this makes sense given the type of noise and sound
pressures we
> were experiencing during the runs.
>
> we ran appx. 10 hours at full power static before it separated.
in normal
> flight operations, running at 2700rpm static is rare if ever, and
only for a few
> seconds at most. normal runup is around 2000rpm for short
period. on
> takeoff runs, 2700rpm is not in a static condition, thus no major
vortex
> interference.
>
> our test represented extreme conditions. we were using this prop
to load the
> engine for endurance runs at full power. the prop mfr.
recommended this prop
> for this purpose, as had several other engine mfrs. now we know
one of it's
> limitations.
>
> as for light sport, i couldn't say for sure although there's
probably little
> chance of seeing 200hp and 74" props on LS aircraft. i suspect
that most LSA
> will use lower power and smaller diameter props.
>
> another consideration is that this is an adjustable f/p prop....it
would be
> pitched for around 2200rpm at static....nowhere near the test
conditions.
>
> in real life this prop would be operating on tall landing
gear....and a
> taller test stand :-). even a few inches higher from the ground
would make a huge
> difference.
>
> ship
> [Non-text portions of this message have been removed]
Author: shiprv8@... Time: Sun Mar 14, 2004 12:29 pm PST Link
220hp prop.....pitched for 120mph......i.e. flat twist
[Non-text portions of this message have been removed]