Vy and power available curve

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MrManH

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MrManH
Hi everyone,

As seen in this image, the power available curve increases with TAS and I'm not sure I can explain why other than mathematically:
https://www.boldmethod.com/images/learn-to-fly/performance/vx-vy/power-available-animation.gif

Power = thrust * TAS so obviously if TAS increases more than thrust is reduced, power still increases. However, this is by no means a practical explanation.

Does anyone have an explanation that doesn't point to the formula?

Even though engine power is what allows a prop airplane to produce thrust, this chart isn't directly related to engine power right?

Thanks!
 
Horsepower available is the power output of the engine. RPM * Torque. With a fixed pitch prop, RPM increases with airspeed which gets you into higher power outputs.
 
Bobanna said:
Did you get this at a Chicago Public School?
Click to expand...

https://www.boldmethod.com/learn-to-fly/performance/vx-vy/

Horsepower available is the power output of the engine. RPM * Torque. With a fixed pitch prop, RPM increases with airspeed which gets you into higher power outputs.
Click to expand...

The article in which this chart was posted makes no mention of RPM or torque. It looks like they're trying to imply something that applies to all aircraft with reciprocating engines.
 
Yep I'm aware of that, I wrote "so obviously if TAS increases more than thrust is reduced, power still increases" to imply that thrust decreases.

I think my confusion comes from the fact that the chart is talking about horsepower with figures that resemble a conventional GA engine's BHP (<= 160HP) and yet this isn't about the engine, or at least not directly.

Thrust decreases with airspeed but thanks to a greater TAS, power still increases. It's not the power of the engine that increases but simply the rate of doing work. The only thing the engine has to do with this is that it drives the propeller and influences its thrust. Does this sound right?
 
It's about DRAG. For any given airframe at a specific weight and balance, there will be a sweet spot from which either slower or faster will cause more drag and require more HP in relation to the speed. I'm sure somebody will come along to explain it better or prove me wrong though!
 
The curve is a 2-D picture. Therefore it replicates a constant altitude situation for a given plane. As power output increases, velocity increases and drag decreases until Vy because part of the thrust is used to maintain the altitude. After Vy, drag increases as the square of the velocity and so more horsepower is required to gain airspeed. The power available increase with velocity as the engine runs more efficiently at higher airspeed.
 
Some planes may also experience a bit of ram air into the induction system allowing it to make a bit more power. If it can get more air, it can be used to burn more gas, and make more power. It likely would not amount to a lot of power, but more is more.
 
MrManH said:
Hi everyone,

As seen in this image, the power available curve increases with TAS and I'm not sure I can explain why other than mathematically:
https://www.boldmethod.com/images/learn-to-fly/performance/vx-vy/power-available-animation.gif

Power = thrust * TAS so obviously if TAS increases more than thrust is reduced, power still increases. However, this is by no means a practical explanation.

Does anyone have an explanation that doesn't point to the formula?

Even though engine power is what allows a prop airplane to produce thrust, this chart isn't directly related to engine power right?

Thanks!
Click to expand...

Why isn't it practical?

Power is referring to propulsive power, not engine output at the crankshaft. Your explanation seems to match what's given in Aerodynamics for Naval Aviators. Check out pages 145 to 156 or so.
 
MrManH said:
Power = thrust * TAS so obviously if TAS increases more than thrust is reduced, power still increases. However, this is by no means a practical explanation.

Does anyone have an explanation that doesn't point to the formula?
Click to expand...

I don't think it can be explained completely except mathematically.

Capt. Geoffrey Thorpe said:
Horsepower available is the power output of the engine. RPM * Torque. With a fixed pitch prop, RPM increases with airspeed which gets you into higher power outputs.
Click to expand...

No. In this context, as in all aircraft performance calculations, we're talking about thrust horsepower: THP = TV/550, where T is thrust in lbs and V is airspeed in feet/sec (or TV/375 if you want to use mph). While rpm may increase with airspeed, the torque curve peaks at a lower rpm than the HP curve. So just like the relationship between thrust, power, and airspeed, the engine's power (talking shaft horsepower here, or SHP) increases with rpm until the dropoff in torque is greater than the increase in rpm.

Propeller efficiency, which will vary with speed but always be less than 100%, equals SHP/THP.
 
Other than ram air pressure, there is no intrinsic connection between power available and airspeed. The chart is making the assumption that RPM (and hence power) is increasing with airspeed. It would have been simpler, and conceptually still correct, to keep the power available as a constant value.
 
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sarangan said:
Other than ram air pressure, there is no intrinsic connection between power available and airspeed. The chart is making the assumption that RPM (and hence power) is increasing with airspeed. It would have been simpler, and conceptually still correct, to keep the power available as a constant value.
Click to expand...

No, it would have been incorrect. As the OP and others said, while shaft (engine) horsepower is more or less constant, thrust horsepower, which is what matters here, equals thrust times airspeed, so THP increases with speed until the thrust falls off faster than the speed increases.
 
MrManH said:
The article in which this chart was posted makes no mention of RPM or torque. It looks like they're trying to imply something that applies to all aircraft with reciprocating engines.
Click to expand...
Two things happen with all recip engines. Max power and torque vary with engine speed. With fixed pitch props the engine speed will vary with airspeed. Also, propeller efficiency varies with airspeed - a constant speed prop has a flatter efficiency curve with airspeed, but a curve none the less.
 
Dana said:
No, it would have been incorrect. As the OP and others said, while shaft (engine) horsepower is more or less constant, thrust horsepower, which is what matters here, equals thrust times airspeed, so THP increases with speed until the thrust falls off faster than the speed increases.
Click to expand...

So if the shaft horse power is constant and the thrust horsepower falls off, where is the extra power being spent?
Which is the relationship that governs thrust and airspeed?
 
sarangan said:
So if the shaft horse power is constant and the thrust horsepower falls off, where is the extra power being spent?
Which is the relationship that governs thrust and airspeed?
Click to expand...
"The propulsive power available, Pa, is the product of the propeller efficiency and applied shaft horsepower."
 
Thanks everyone for the replies. I think this image summarizes the idea.

horsepower-graph.png
 
sarangan said:
So if the shaft horse power is constant and the thrust horsepower falls off, where is the extra power being spent?
Click to expand...
It's lost due to the prop being less efficient.

Which is the relationship that governs thrust and airspeed?
Click to expand...
Thrust decreases as airspeed increases, because the prop blade AOA decreases.
 
As others have said, brake horsepower available is a constant - 160 bhp for a Lycoming O-320 for example. But thrust horsepower, something propeller aircraft need, changes with airspeed due to propeller efficiencies at different airspeeds. When you start your takeoff roll, the aircraft is at a relatively low RPM even with full throttle, as you reach rotation speed, the RPM increases - power produced by the prop has increased.

At higher and higher speeds, the efficiency of the propeller drops off. This is why constant speed props are ideal - at higher speeds, the prop should take a bigger bite of air.

Drag doesn't come into play here, since that only affects the power REQUIRED curve, independent of the power available. The interplay between available and required gives us where Vy is.

Other aircraft like jets and rockets don't concern themselves with power available - they produce thrust directly.

Dan, CFI
 
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