Looking at the power required to rotate the drum, we were running 400 rpm,
6.28 ft/rev, 45 sec to release and estimating 1000 lb line pull. I calculate
that mix to equal 102 hp.
!02 hp minus 75 hp is 27 hp. Seems to be a fair if maybe somewhat high
approximation of aerodynamic drag of glider and Spectra tow rope.
The throttle was opened to the point where the pilot reported his airspeed
to be in the desired 55-60 kt.range and which corresponded to the 2300 rpm I
was seeing on my gauge.
Again, I'm finding it hard to explain most of the discrepancy between the
altitude-corrected
180 hp
454 c.i. dynamometer data and what I'm seeing with my own eyes. Can a water
pump, an alternator (the radiator is cooled by electric fan) and mechanical
friction
eat up 180 engine hp minus102 drum hp= 78hp?
Thanks for your input but still off the rails,
Bob Johnson
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"Bob Johnson" wrote in message
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Hi Bill --
Today we towed in light wind and so turned 2300 engine RPM in the climb.
The 454 c.i. torque/hp curves show the engine was generating about 430
lb-ft torque and 200 hp at these revs.
Now here is where I tend to go off the rails. Just 75 hp is required to
lift a 1100 lb sailplane 1700 ft in 45 sec.
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"Bob Johnson" wrote in message
news:XtUnd.87541$%x.66152@okepread04...
Hi Bill --
Today we towed in light wind and so turned 2300 engine RPM in the climb.
The 454 c.i. torque/hp curves show the engine was generating about 430
lb-ft torque and 200 hp at these revs.
Now here is where I tend to go off the rails. Just 75 hp is required to
lift a 1100 lb sailplane 1700 ft in 45 sec.
I know there are some aerodynamic and mechanical losses but it's hard to
believe they amount to some 125 hp.
Bob
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"Bill Daniels" wrote in message
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xAnd.71122$5K2.10295@attbi_s03...
"F.L. Whiteley" wrote in message
...
"Jim Vincent" wrote in message
...
The drum diameter has nothing to do with the torque!!!
Sure it does. The rope has a certain amount of tension on it, usually
measured
in lbs. The rope is pulled off the drum at a certain distance from
the
center
or rotation. That distance is the moment arm. The torque is the
tension
X
moment arm, hence inch lbs or ft lbs.
Jim Vincent
N483SZ
illspam
And, in practice, constantly changing, generally increasing, depending
on
design and layup.
Frank Whiteley
The following is the summation of a couple of decades of thinking about
winch launch.
Winch drum torque is a complicated subject. It involves glider behavior,
engine torque and power curves and the winding characteristics of the
winch
drum. Drum torque cannot be described without understanding all the
other
variables.
The glider acts to demand both cable tension and cable speed. (Cable
Speed x
Tension = Power) The winch engine, controlled by the winch driver, tries
to
meet that demand while holding the glider airspeed at a value requested
by
the pilot.
Note: The winch driver can control either glider airspeed or cable
tension
but not both.
The torque on the drum shaft varies with demand and is limited by the
engines Wide Open Throttle (WOT) torque curve. (And, of course, the
breaking
strength of the weak link.)
If the winch is unable to meet cable tension demand, the glider airspeed
will decay with increasing pitch attitude. If the winch meets or exceeds
the demand, the glider airspeed will increase when the nose is raised.
If the gliders airspeed decays with increasing pitch angle, then the
glider
is rapidly approaching the stall AOA since the wing loading is also
increasing with pitch attitude. If the airspeed increases with
increasing
pitch, then the AOA will remain more nearly constant. The later is a
safer
condition.
The actual radius of the drum depends on the quantity of cable wound onto
the drum at any moment. If the instantaneous radius is one foot then the
torque in foot/pounds equals cable tension in pounds. This is a typical
mid-launch condition.
If the cable tension is to remain equal to the gliders gross weight
throughout the launch, which is desirable, then the torque at the drum
shaft
must increase with the increasing drum radius even as the drum RPM is
reduced, to maintain a constant glider airspeed.
This places heavy demands on the winch engine. Engines capable of very
high
torque at low RPM are desirable. Diesel engines typically have their
torque
peak just above idle. If the highest engine RPM is 2100 RPM then the
engine
torque capacity will increase even as the drum RPM decreases. In other
words, diesel WOT torque curves tend to match the demand of winch launch.
This explains why diesels are popular winch engines.
Spark ignition engines tend to have torque peaks closer to the max RPM
utilized by the winch. As the launch progresses, the torque capacity
declines rapidly with RPM even as the demand increases.
Sorry for the lecture and apologies to our metric friends. Now, lets
build
some winches.
Bill Daniels