On Tue, 17 Jan 2006 22:48:09 -0800, "skyloon"
wrote:


Kershner states that the maximum thrust force occurs when the plane is
standing still (at a fixed throttle setting, I guess), and decreases as you
go faster. I do not understand this. Is it beacese AOA is largest? I am
trying to see how this relates to power. Power would be force*distance/time
or force*velocity. Maybe the thrust decreases slowly with airspeed, but the
power still goes up as you go faster.

This is just a hand waving argument. Please, anyone who knows more, feel
free to correct this picture.

Dave

I'll pick up on this one. There's a mechanics equation which is
specially straight-forward. It says if you apply a constant force to
an object,
and it moves in the direction of the force, then the work done is the
product of force times distance.

As expressed in the SI system, it's specially simple: F X D = W
gets the units of
F in Newtons times Distance in Meters equals work in joules
Even more interesting: F X V = P
force times speed = power.
In SI units again:
force in Newtons times speed in meters per second = power in Watts

OK that was the engineering/physics.

Now the application:
An airplane with a constant power recip prop engine.
Lets say the engine is putting out 90 HP say a C-152
90 HP = 90 X 746 watts = 67kW

Lets check the numbers at 10 mph, 50 mph and 100 mph
10 mph = 4.5 meters/sec
50 mph = 22.4 meters/sec
100 mph = 44.7 meters/sec

The unknown in the following equation is F
F X V = P or F = P/V

Now force is the same measure as thrust, so now we can
check available thust at these three speeds:

10 mph
F = 67000 W/4.5 M/Sec = 14890 Newtons
A newton, like a small apple weighs a quarter pound about.
So 14890 Newtons = 3340 pound That's a lot of thrust!

Now 50 mph
F = 67000/22.4 = 2990 Newtons or 671 lb.

Now 100 mph
F = 67000/44.7 = 1500 Newtons or 336 lb.

Or the general rule: the faster you go with constant power, the less
the thrust available.
Same applies to boats.

But think about planes with (some) jet engines,
these can be constant THRUST.

That means, the faster they go, the more HP they put out!
(A reason why jets on slow planes is not a great idea)

Brian Whatcott Altus OK