> Curtis L. Olson wrote:
>
> > I see the same thing in the JSBSim c172, except that it spins down
> > rather quickly and stops.
>
> I've never shut down an engine in flight in real life, but from
> reports I've
> heard, you have to bring a 172 almost to the stall to stop the propeller
> from windmilling; once stopped, however, it will stay stopped at a more
> reasonable speed.
> David
After reviewing the propeller code *briefly* I have determined that it
*appears* as though a kludge was made to the propeller code to account for
feathering and/or thrust falloff with higher velocity than a prop was meant
to handle. For the prop_75in2f (75 inch, 2 blade fixed pitch) propeller, the
thrust coefficient table shows that the thrust coefficient goes negative at
an advance ratio of 1.2. If the prop is spinning at 600 rpm (10 rps) that
corresponds to a velocity of 72 ft/sec. At this velocity the thrust
produced by the propeller would be negative (i.e. producing drag). However,
there are these lines in the propeller code:
// Check for windmilling.
double radius = Diameter * 0.375; // 75% of radius
double windmill_cutoff = tan(Pitch * 1.745329E-2) * omega * radius;
if (Vel > windmill_cutoff)
Thrust = -Thrust;
The negative thrust produced by the propeller (as calculated from the thrust
coefficient) is being reversed to positive thrust (not drag) with the above
lines.
Again, this is a first quick look at the propeller code, so more
investigation is warranted. My thoughts at the moment based in this initial
look-see is that if we are able to come up with some good propeller config
files, then the thrust reversing code should be removed entirely.
Jon
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