can't quite grasp the "power available" curve

I understand the "power required" curve as plotted against velocity, but the
concept of "power available" plotted against velocity is escaping me.

How is this curve derived? And why is it "curved"?

Thanks in advance.

"xerj" wrote in message
...
I understand the "power required" curve as plotted against velocity,
but the
concept of "power available" plotted against velocity is escaping me.

How is this curve derived? And why is it "curved"?

Thanks in advance.



You can analyze aircraft steady-state motions using either force or
power relationships. Two of the forces are of course drag D and thrust
T. Power required is Pr = D*V (where V is true airspeed), power
available Pa = T*V. Getting a model of thrust is quite a bit more
difficult than getting a model of drag, but it can be done. The Pa curve
is curved because propeller efficiency, as a function of V, is curved.
For a simple thrust model for fixed-pitch propeller aircraft, see
Performance of Light Aircraft, especially chapters 7 and 8.

John T. Lowry, PhD
Flight Physics

Thanks to both of you.

I'm getting closer to understanding, but a couple of things are still
eluding me:-

1) What happens to the curve with constant speed props?

2) I don't exactly know how to put this, but if a curve labelled "65%"
showed, say, 100HP at 60 kts and135HP at 120 kts, that means that the 35HP
difference is due to prop efficiency? The reason I say this is because if
the entire curve represents a 65% setting then the power shouldn't have
changed, right?

Thanks in advance, and much appreciated for the answers so far.


"Todd Pattist" wrote in message
...
"xerj" wrote:

I understand the "power required" curve as plotted against velocity, but
the
concept of "power available" plotted against velocity is escaping me.

How is this curve derived? And why is it "curved"?

In the aircraft we fly, the Pa curvature comes about from
prop efficiency and engine limitations. A prop is most
efficient when the prop airfoil is operating at it's best
L/D angle of attack. Any RPM slower or faster than the RPM
needed at the current airspeed to produce that AOA reduces
prop efficiency. Of course, changing the engine RPM changes
the power available from the engine.

At the upper speed end of the scale, the engine is operating
at redline, and the prop AOA has dropped to zero producing
zero thrust and zero power. Any faster and the engine
either overspeeds or the prop begins producing negative
thrust.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
___
Make a commitment to learn something from every flight.
Share what you learn.

With a constant-speed prop the graph of propeller efficiency vs airspeed
(or vs advance ratio) is still curved, but not as much. Constant-speed
props do NOT settle on maximum efficiency, by the way, though they're
generally fairly close to that.

On your second question, I don't understand what curve your discussing.
What is plotted on the vertical and on the horizontal axes? and what are
the other parameters describing the curve?

John.

"xerj" wrote in message
news
Thanks to both of you.

I'm getting closer to understanding, but a couple of things are still
eluding me:-

1) What happens to the curve with constant speed props?

2) I don't exactly know how to put this, but if a curve labelled "65%"
showed, say, 100HP at 60 kts and135HP at 120 kts, that means that the
35HP
difference is due to prop efficiency? The reason I say this is because
if
the entire curve represents a 65% setting then the power shouldn't
have
changed, right?

Thanks in advance, and much appreciated for the answers so far.


"Todd Pattist" wrote in message
...
"xerj" wrote:

I understand the "power required" curve as plotted against
velocity, but
the
concept of "power available" plotted against velocity is escaping
me.

How is this curve derived? And why is it "curved"?

In the aircraft we fly, the Pa curvature comes about from
prop efficiency and engine limitations. A prop is most
efficient when the prop airfoil is operating at it's best
L/D angle of attack. Any RPM slower or faster than the RPM
needed at the current airspeed to produce that AOA reduces
prop efficiency. Of course, changing the engine RPM changes
the power available from the engine.

At the upper speed end of the scale, the engine is operating
at redline, and the prop AOA has dropped to zero producing
zero thrust and zero power. Any faster and the engine
either overspeeds or the prop begins producing negative
thrust.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
___
Make a commitment to learn something from every flight.
Share what you learn.

||||----o On your second question, I don't understand what curve your
discussing.
What is plotted on the vertical and on the horizontal axes? and what are
the other parameters describing the curve? o----||||


TAS on the x axis, HP on the y axis.

The "Figure 1" on this PDF probably illustrates it best:-

http://www.nar-associates.com/techni...ht/weight1.pdf

The graph you referenced involves three different power (throttle)
settings, all at sea level. Keep in mind the airplane will be in level
flight ONLY at the two intersections of the Pr and Pa curves; in between
(Pa Pr), it will be climbing. (Outboard of those intersections, it
will be descending, unless stall conditions intervene, in which case it
will REALLY be descending!) Keep at it; this stuff will eventually make
common sense.

John Lowry
Flight Physics

"xerj" wrote in message
news
Thanks to both of you.

I'm getting closer to understanding, but a couple of things are still
eluding me:-

1) What happens to the curve with constant speed props?

2) I don't exactly know how to put this, but if a curve labelled "65%"
showed, say, 100HP at 60 kts and135HP at 120 kts, that means that the
35HP
difference is due to prop efficiency? The reason I say this is because
if
the entire curve represents a 65% setting then the power shouldn't
have
changed, right?

Thanks in advance, and much appreciated for the answers so far.


"Todd Pattist" wrote in message
...
"xerj" wrote:

I understand the "power required" curve as plotted against
velocity, but
the
concept of "power available" plotted against velocity is escaping
me.

How is this curve derived? And why is it "curved"?

In the aircraft we fly, the Pa curvature comes about from
prop efficiency and engine limitations. A prop is most
efficient when the prop airfoil is operating at it's best
L/D angle of attack. Any RPM slower or faster than the RPM
needed at the current airspeed to produce that AOA reduces
prop efficiency. Of course, changing the engine RPM changes
the power available from the engine.

At the upper speed end of the scale, the engine is operating
at redline, and the prop AOA has dropped to zero producing
zero thrust and zero power. Any faster and the engine
either overspeeds or the prop begins producing negative
thrust.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
___
Make a commitment to learn something from every flight.
Share what you learn.

Thanks, John!

I'm making a conscious effort to finally understand stuff that I should have
had a better grasp on. Knowing ~why~ things happen is much better than just
knowing that they do.

"John T Lowry" wrote in message
k.net...
The graph you referenced involves three different power (throttle)
settings, all at sea level. Keep in mind the airplane will be in level
flight ONLY at the two intersections of the Pr and Pa curves; in between
(Pa Pr), it will be climbing. (Outboard of those intersections, it
will be descending, unless stall conditions intervene, in which case it
will REALLY be descending!) Keep at it; this stuff will eventually make
common sense.

John Lowry
Flight Physics

"xerj" wrote in message
news
Thanks to both of you.

I'm getting closer to understanding, but a couple of things are still
eluding me:-

1) What happens to the curve with constant speed props?

2) I don't exactly know how to put this, but if a curve labelled "65%"
showed, say, 100HP at 60 kts and135HP at 120 kts, that means that the
35HP
difference is due to prop efficiency? The reason I say this is because
if
the entire curve represents a 65% setting then the power shouldn't
have
changed, right?

Thanks in advance, and much appreciated for the answers so far.


"Todd Pattist" wrote in message
...
"xerj" wrote:

I understand the "power required" curve as plotted against
velocity, but
the
concept of "power available" plotted against velocity is escaping
me.

How is this curve derived? And why is it "curved"?

In the aircraft we fly, the Pa curvature comes about from
prop efficiency and engine limitations. A prop is most
efficient when the prop airfoil is operating at it's best
L/D angle of attack. Any RPM slower or faster than the RPM
needed at the current airspeed to produce that AOA reduces
prop efficiency. Of course, changing the engine RPM changes
the power available from the engine.

At the upper speed end of the scale, the engine is operating
at redline, and the prop AOA has dropped to zero producing
zero thrust and zero power. Any faster and the engine
either overspeeds or the prop begins producing negative
thrust.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
___
Make a commitment to learn something from every flight.
Share what you learn.