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push vs pull vis a vis rudders
nice, accurate response, Keith.
From: Keith Hughes
Date: 3/27/2004 7:07 PM Eastern Standard Time
Message-id:
Derek Rowell wrote:
Would you all agree that in areas of dispute the truth may be revealed by
an
experiment?
Yes, if the experiment is setup properly, and the data is
*accurately* analyzed and interpreted.
Please try the following:
Take a fan, say a large house cooling fan (that's your propellor). Take a
flat surface, for example a stiff lightweight book (thats the rudder).
Turn the fan on and hold the rudder at an angle on the outflow side
(transmission in forward). Does the flow exert a torque (turning effect)
on
the rudder? Let go one corner and see. Is there a sideways thrust that
you have to oppose to keep the rudder in position?
Clearly. f=ma, i.e. force is the product of Mass times
acceleration. In this case the mass of accelerated air will apply
disproportionate force to the 'book' as a function of aspect
ratio. Basically, the book will "weathervane" until the force
applied to each side equalizes.
Repeat the experiment with the "rudder" on the inlet side of the fan
(transmission in reverse). Is there a turning effect (torque) or not?
Is
there a sideways thrust on the "rudder"?
You tell me - I just did it. The answers to all four questions is yes.
And...irrelevant! YES there is force on the 'vane', just as there
was on the downstream side. Less due to the the wider flow pattern
on the suction side, but still significant.
However, in the context of the boat model, this force is virtually
irrelevant. In the first case, water is forced over the rudder at
an angle to the boat centerline. It's a simple vector equation.
Water moves to starboard, reaction force is thus to port, the boat
turns starboard.
In REVERSE, the water flow, *irrespective of rudder position* is
along the centerline of the boat, thus the reaction force is
parallel to the keel line, and the boat moves straight back
(ignoring the precessional forces that result in 'prop walk' that
is). It's only when the boat moves through the water that the
rudder can have an effect - another simple vector equation. There
must be additional non-parallel force applied in order to produce
a turn, and that is caused by *additional* water flow past the
rudder caused by boat movement.
Aero/hydrodynamic lift/drag is determined by the flow patterns over
surfaces
(Bernoulli effects, etc), not by the simple minded pseudo-science that is
being thrown around here.
Lift and drag are irrelevant. Consider, what is the effect of lift
on the rudder? Heeling action, not turning action. You appear to
be confusing hdyrodynamics with simple force/vector equations.
It's a VERY complex situation.
Not at all. Oh the precessional effects are definitely complex,
all the more so when hydrodynamic effects are added in the
equation, but precession, while important to why your stern always
drifts one way in reverse (i.e. force applied to a spinning object
- the propeller - will be translated 90° in the direction of
rotation, creating a turning force), is not a factor in why the
rudder is ineffective in reverse.
We all agree
that in practice the effect is much, much weaker in reverse but it is still
present. (The reason that it is weaker is that only a small fraction of
the in-flow to the propellor actually passes over the rudder in reverse.)
Sorry, but that's just not accurate.
Keith Hughes
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