Vx and altitude

B767Driver

B767Driver

New Member
I'm trying to find a good explanation of why the indicated airspeed for Vx increases as altitude increases.

I understand the reason for the indicated Vy decrease...available horsepower decreases and shifts the maximum excess thrust to a lower speed.

Obviously, Vx is excess thrust dependent. As airspeed increases...thrust drops off fast. So why is maximum excess thrust for a prop airplane found at a higher airspeed as altitude is gained.

I'm having trouble finding a good explanation for this.
 
Remember that an engine produces less horsepower (normally aspirated) as altitude is gained. In conjunction with the engine producing less horsepower, the airfoils become less effective. The definition of Vx is the speed or AOA that will produce the greatest gain in altitude within the shortest distance. Now, as we gain altitude and the effects of less dense air come into play we are going to have to hold a higher pitch attitude to achieve Vx or the speed or AOA that will produce the greatest gain in altitude in the shortest amount of time, due to a less effective airplane. Essentially, our reserve thrust is depleted. Vx and Vy change are consistently changing as you climb. Vx increases and Vy decreases until they eventually meet at the absolute service ceiling.

Hope that helped :confused: .
 
meritflyer said:
Remember that an engine produces less horsepower (normally aspirated) as altitude is gained. In conjunction with the engine producing less horsepower, the airfoils become less effective. The definition of Vx is the speed or AOA that will produce the greatest gain in altitude within the shortest distance. Now, as we gain altitude and the effects of less dense air come into play we are going to have to hold a higher pitch attitude to achieve Vx or the speed or AOA that will produce the greatest gain in altitude in the shortest amount of time, due to a less effective airplane. Essentially, our reserve thrust is depleted. Vx and Vy change are consistently changing as you climb. Vx increases and Vy decreases until they eventually meet at the absolute service ceiling.

Hope that helped :confused: .
Click to expand...


Nope...this makes no sense to me. If you hold a higher pitch attitude the IAS will decrease. Not increase.

Here's what I'm looking for that is not graphically depicted...or at least that I can find.

In the plot of thrust required vs thrust available...these curves must change with altitude...to permit the maximum excess thrust to be found at a higher airspeed. Does anyone know how these two values change with altitude?

I'm going to guess that the thrust required decreases with altitude as density drops off. This permits an increased speed to maintain Vx. But this is what I'm not sure about.
 
I've always thought of it this way. Vx is the best angle of climb, ie the most altitude over the least amount of forward distance. To keep that, at higher altitude you have a faster TAS than at lower altitude for a given IAS of Vx. Slow down the IAS and get the same "ratio" of height vs forward distance.
 
Maybe I can shed some light here...

First of all, this discussion is going to have to make some distinction between Props and Jets, because there are different effects happening.

For JET type airplanes, the aircraft's best angle of climb is at the minimum point on the drag curve (where parasite drag equals induced drag, and TOTAL drag is the smallest) This is because a jet produces relatively constant power regardless of the velocity. Therefore, the minimum drag point equals the point of maximum excess thrust. Since parasite drag decreases with altitude (due to decreasing density), Vx increases altitude in a JET due to the minimum drag speed increasing. In a PROP, the thrust produced DOES vary with airspeed, however. This is because all props have a maximum rpm limit that is reached very easily at high airspeeds. At high airspeeds the angle of attack on the propeller blades decrease and you either have to pull off some manifold pressure (in a constant speed prop), or you have to pull back the throttle in a fixed prop to keep the RPM's in line. Both of these mean that you are pulling power at higher speeds. For this reason, best angle of climb occurs at a slower airspeed than the minimum drag speed in a prop. However, the shift in Vx in a prop (Vx increases as altitude increases) occurs for the same reason as it does in a jet. Less parasitic drag at a higher altitude means you get less drag at a faster airspeed.
Therefore Vx increases with altitude because parasitic drag decreases due to decreased density. That means the point of minimum total drag occurs at a faster airspeed.


Now on to Vy. Vy occurs at the point of maximum excess POWER, not maximum excess thrust. POWER is equal to Thrust times Velocity (P=TV). As you increase altitude the point of minimum drag (or THRUST REQUIRED) moves to the right (higher velocity), but due to power being the product of Thrust AND velocity, the fact that the THRUST REQUIRED went down is overshadowed by the fact that the velocity went up. So even though the THRUST REQUIRED went down at a higher velocity, the POWER required went up. In JETs this explains the WHOLE phenomenon, but in PROPS remember that the amount of THRUST AVAILABLE is greater at slow speeds than fast speeds, the THRUST AVAILABLE went down significantly also, and therefore so did the POWER AVAILABLE. This means that in props, for 2 reasons the Vy decreases with altitude:
1. Because although thrust required decreases with an increased airspeed at a higher velocity power required actually increases.

2. Thrust and power available in a prop decrease with higher airspeed significantly.

For BOTH of the above reasons, the point at which there is maximum excess POWER (power available-power required is greatest) occurs at a slower speed as altitude increases. And Vy decreases with altitude
 
fish314 said:
Maybe I can shed some light here...

For this reason, best angle of climb occurs at a slower airspeed than the minimum drag speed in a prop. However, the shift in Vx in a prop (Vx increases as altitude increases) occurs for the same reason as it does in a jet. Less parasitic drag at a higher altitude means you get less drag at a faster airspeed.
Therefore Vx increases with altitude because parasitic drag decreases due to decreased density. That means the point of minimum total drag occurs at a faster airspeed.
Click to expand...

This is exactly what I've been trying to validate. Most texts will not delve into this.

Good job.
 
Yeah. Most of the texts for pilots won't go into this. I majored in Aeronautical Engineering, and some of the basic Aero Eng. texts cover this stuff.
 
B767Driver said:
This is exactly what I've been trying to validate. Most texts will not delve into this.

Good job.
Click to expand...


I kinda see it a different way. Thrust required increases as altitude increases because of an INCREASE in induced drag. The coefficient of lift must be increased to compensate for the decrease in air density. So, in order to get to the minimum drag point again we must increase airspeed (and reduce AOA) to reduce induced drag. It is a balancing act between induced and parasite drag. An explanation of Vx using only one type of drag is incomplete. If you can understand why the graphs move the way they do then you are 99% of the way there.

Once you have that straight in your head, start working on what Vx and Vy do in terms of True airspeed.:rawk:
 
vabantha said:
I kinda see it a different way. Thrust required increases as altitude increases because of an INCREASE in induced drag. The coefficient of lift must be increased to compensate for the decrease in air density. So, in order to get to the minimum drag point again we must increase airspeed (and reduce AOA) to reduce induced drag. It is a balancing act between induced and parasite drag. An explanation of Vx using only one type of drag is incomplete. If you can understand why the graphs move the way they do then you are 99% of the way there.

Once you have that straight in your head, start working on what Vx and Vy do in terms of True airspeed.:rawk:
Click to expand...

Vabantha, your analysis here is shows insight, and is also correct. The induced drag would need to increase if we kept that same airspeed, and increasing induced drag also causes the point of the min drag curve to shift right (towards greater velocity). Therefore, for both reasons (increasing induced drag and decreasing parasite drag) the point of min drag moves to the right (larger velocity). So Vx increases b/c the point of min drag increases, and therefore the point of maximum excess thrust increases.
 
fish314 said:
Vabantha, your analysis here is shows insight, and is also correct. The induced drag would need to increase if we kept that same airspeed, and increasing induced drag also causes the point of the min drag curve to shift right (towards greater velocity). Therefore, for both reasons (increasing induced drag and decreasing parasite drag) the point of min drag moves to the right (larger velocity). So Vx increases b/c the point of min drag increases, and therefore the point of maximum excess thrust increases.
Click to expand...


Good job, guys. Clear, concise and accurate. Thanks.
 
fish314 said:
This is because all props have a maximum rpm limit that is reached very easily at high airspeeds. At high airspeeds the angle of attack on the propeller blades decrease and you either have to pull off some manifold pressure (in a constant speed prop), or you have to pull back the throttle in a fixed prop to keep the RPM's in line.
Click to expand...
You might want to rethink the part which deals with constant speed props...
 
ananoman said:
You might want to rethink the part which deals with constant speed props...
Click to expand...

Yeah, I was a bit confused when I read that as well. My understanding is that if you have an overspeed condition the prop governor will allow oil to flow out of the hub and increase pitch on the blades.
 
Yeah, you're probably right. My experiences with constant speed props is pretty limited. Flew em about 8 years ago, but only for about 15-20 hours. Most of my experience is with jets. For this question, though, it should be immaterial, however, because if constant speed props don't lose the ability to produce thrust as velocity increases, they would still suffer a reduction in Vy with altitude due to the shift in the power required curve. In other words, the "JET" explanation above would be the same phenomenon in a constant speed prop, but a fixed prop would have 2 factors affecting it.

Oh, well. That'll teach me. Think before you speak (or type!):sitaware:
 
fish314 said:
Yeah, you're probably right. My experiences with constant speed props is pretty limited. Flew em about 8 years ago, but only for about 15-20 hours. Most of my experience is with jets. For this question, though, it should be immaterial, however, because if constant speed props don't lose the ability to produce thrust as velocity increases, they would still suffer a reduction in Vy with altitude due to the shift in the power required curve. In other words, the "JET" explanation above would be the same phenomenon in a constant speed prop, but a fixed prop would have 2 factors affecting it.

Oh, well. That'll teach me. Think before you speak (or type!):sitaware:
Click to expand...


I have been looking at my explanation for Vx in an earlier post and I realized I should take my own advice and look at the graphs. It's tough to justify why Vy would decrease by looking at the excess power graphs because they move up and to the right like the excess thrust graphs do. I think it's kinda important to note that in terms of true airspeed, both Vx and Vy increase with altitude. However, Vx increases at a greater rate than Vy. Vy increases about 1% per 1000ft, but when you convert TAS to IAS (2% per 1000ft), it shows a 1% decrease. Oh well, so much for simplicity.
 
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