[jhb] Re: Getting Too Real
- From: "Fossil" <fossil@xxxxxxx>
- To: <jhb@xxxxxxxxxxxxx>
- Date: Tue, 28 Jul 2009 17:51:17 +0100
Prop theory is even more complicated than aerofoil theory but you can learn
some interesting things.
A prop is just a wing going round an axle. It develops "lift" in exactly the
same way a wing does and in the same way as a wing can stall if the AoA is
15 degrees or so. This is why a coarse prop can be semi stalled if you sit
at the end of the runway and open up the power - it is only when you start
gaining forward speed that the AoA reduces and it becomes more efficient.
The optimum AoA for a wing is about four degrees (best L/D ratio) and the
same is true of a prop. The AoA of a prop changes with forward speed so it
is at its maximum when the aircraft is stationary and reduces as speed
increases. Eventually you reach a speed where the prop is operating at four
degrees AoA (maximum thrust) and the aircraft cannot go any faster because
from that point onward the prop will become a drag disk. Fine props reach
max thrust AoA sooner than coarse props which is why they give you a lower
top speed.
A V/P prop is better because as you accelerate towards max thrust you can
change the pitch and gain more thrust than a fixed prop. You can only push
this so far though as coarsening the prop adds load to the engine.
Prop pitch is measured in inches rather than degrees because the latter
varies - a prop blade twists towards the tip. The blade twists because you
need to obtain even lift across the blade surface for maximum efficiency. As
the tip is going much faster than the blade section at the root it will
develop more lift so, to even things out, we apply normal aerofoil logic to
even out the thrust. As you move from tip to root you will see that a normal
prop will gradually change in both thickness and pitch. This ensures the
inboard section of the prop is developing as much lift as the outer section
despite it rotating at much slower speed.
Tip speed can become critical and it is important to avoid the tip reaching
sonic speeds as it will stop producing thrust. In most light aircraft the
red line limit of the engine is about 2700rpm and at that speed it is easy
enough to work out the maximum prop diameter to make the tip sonic. At sea
level the speed of sound is 660kts or roughly 1125fps. A C172 has a 6.25ft
prop which will have a tip travel of 19.63 feet per revolution so at 2700rpm
it will have a speed of 6.25 x PI x 2700 = 53014 feet per minute - about 523
kts. On a C185 it's a bit different with a 7.33ft prop and red line at 2850
which pushes tip speed right up to 648kts which is Mach 0.98! Indeed the
C182 is known for its tip speed howl (as was the Harvard) when flown by
pilots who don't reduce rpm after take off.
Pitch angle is calculated from the given pitch value divided by the
circumference. As the tip circumference is large the angle is quite low but
as you move towards the hub the circumference reduces and the pitch angle
gets larger. There is quite a good article on this at
http://ma3naido.blogspot.com/2008/11/basic-propeller-principles.html
bones
bones@xxxxxxx
-----Original Message-----
From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On Behalf
Of Peter Dodds
Sent: 28 July 2009 15:15
To: jhb@xxxxxxxxxxxxx
Cc: pdodds@xxxxxxxxxxxxx
Subject: [jhb] Re: Getting Too Real
I knew there would be an essay on the subject forthcoming. Excellent info,
Bones, TVM.
Peter
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