Vx and Vy in checklist remarks, puzzles me

LongRoadBob

LongRoadBob

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In my club the checklist for our Piper Warrior II first page lists V speeds.
Under the Vspeeds are a few remarks that I have tried to figure out...

"Vx increases with altitude and GW.
Vy decreases with altitude increases with GW."

I can't figure out why altitude would have the opposite effect on each. GW I get (I think).
But why would Vy decrease with altitude?
 
LongRoadBob said:
In my club the checklist for our Piper Warrior II first page lists V speeds.
Under the Vspeeds are a few remarks that I have tried to figure out...

"Vx increases with altitude and GW.
Vy decreases with altitude increases with GW."

I can't figure out why altitude would have the opposite effect on each. GW I get (I think).
But why would Vy decrease with altitude?
Click to expand...
Maybe this explains it:
http://www.boldmethod.com/learn-to-fly/performance/vx-vy-altitude-and-where-they-meet/
 
It's great cocktail party trivia! :)

(or... how NOT to get that date you were hoping for lol)
 
LongRoadBob said:
In my club the checklist for our Piper Warrior II first page lists V speeds.
Under the Vspeeds are a few remarks that I have tried to figure out...

"Vx increases with altitude and GW.
Vy decreases with altitude increases with GW."

I can't figure out why altitude would have the opposite effect on each. GW I get (I think).
But why would Vy decrease with altitude?
Click to expand...
I dunno the details of the aerodynamics, but when they equal each other, that's the service ceiling
 
What temperature would they assume, if they specify just one number for an airplane's service ceiling?
 
tspear said:
You sure on that? I am pretty sure it was fifty (50) fpm.
Click to expand...

From the Glossary in FAA Pilot's Handbook of Aeronautical Knowledge:

Service ceiling. The maximum density altitude where the best
rate of climb airspeed will produce a 100 feet per minute
climb at maximum weight while in a clean configuration
with maximum continuous power.​
 
Pilawt said:
From the Glossary in FAA Pilot's Handbook of Aeronautical Knowledge:

Service ceiling. The maximum density altitude where the best
rate of climb airspeed will produce a 100 feet per minute
climb at maximum weight while in a clean configuration
with maximum continuous power.​
Click to expand...

Thanks, now I am curious what I am recalling that is 50fpm. Oh well, I am feeling to lazy to chase it....

Tim
 
The plane's power to weight ratio determines climb rate. As altitude goes up, power goes down. As weight goes up, well that affects power/weight ratio.

Similar situation as with cars. How fast it does a quarter mile is power/weight ratio.
 
dmspilot said:
Single Engine service ceiling of a multiengine airplane.
Click to expand...

Dam, you are right. Considering most of recent flying has been a twin; that explains why it is on the top of mind.

Tim
 
NoHeat said:
What temperature would they assume, if they specify just one number for an airplane's service ceiling?
Click to expand...
My guess is they assume a standard atmosphere. Pressure altitude and what ever the temperature would be up there.
 
NoHeat said:
What temperature would they assume, if they specify just one number for an airplane's service ceiling?
Click to expand...
It has to be standard temperature because that's where the stated altitude equals the density altitude. And density altitude is the only altitude the engine knows.
 
FWIW (from an old post):

Not very technical, but try this one:

An airplane climbs because it has more power available than the amount of power it needs for straight flight at a particular airspeed. This simply means, for example, that if you are flying an airplane that has a normal cruise speed of 100 kts at full power and you keep it at 80 knots with full power, it must climb.

Grab you book and look for the power available vs. power required chart. Vy is the airspeed at which the difference between the two curves is the greatest.

With altitude, power available decreases. As you get higher, it decreases a lot. The point at which the greatest difference exists between the curves moves down the airspeed scale.

Vx is a little different (and takes some visualization). Once you have power available for a climb, it's how high can you bring the nose and convert that excess power into thrust pointing upwards, so that you climb steeper, not faster. An airplane with virtually unlimited thrust could fly straight up and the distance it can climb within a set horizontal distance be "infinite" (very loosely speaking). Think of our cartoon view of a rocket ship - straight up to outer space with no horizontal movement at all - forward airspeed is zero. (Of course, we don't fly airplanes that go straight up, but it helps visualize the concept.) Vx is the point at which you'll get the maximum excess thrust.

The same loss of power with altitude means that there isn't as much excess thrust available. So, you have the lower the nose. And that means airspeed has to increase.

As altitude increases and power and thrust both decrease, Vy will decrease because there's less power available. Vx will increase because there's less thrust available.
 
midlifeflyer said:
FWIW (from an old post):

Not very technical, but try this one:

An airplane climbs because it has more power available than the amount of power it needs for straight flight at a particular airspeed. This simply means, for example, that if you are flying an airplane that has a normal cruise speed of 100 kts at full power and you keep it at 80 knots with full power, it must climb.

Grab you book and look for the power available vs. power required chart. Vy is the airspeed at which the difference between the two curves is the greatest.

With altitude, power available decreases. As you get higher, it decreases a lot. The point at which the greatest difference exists between the curves moves down the airspeed scale.

Vx is a little different (and takes some visualization). Once you have power available for a climb, it's how high can you bring the nose and convert that excess power into thrust pointing upwards, so that you climb steeper, not faster. An airplane with virtually unlimited thrust could fly straight up and the distance it can climb within a set horizontal distance be "infinite" (very loosely speaking). Think of our cartoon view of a rocket ship - straight up to outer space with no horizontal movement at all - forward airspeed is zero. (Of course, we don't fly airplanes that go straight up, but it helps visualize the concept.) Vx is the point at which you'll get the maximum excess thrust.

The same loss of power with altitude means that there isn't as much excess thrust available. So, you have the lower the nose. And that means airspeed has to increase.

As altitude increases and power and thrust both decrease, Vy will decrease because there's less power available. Vx will increase because there's less thrust available.
Click to expand...


********...I do not know how to thank you. Your explanation put the puzzle pieces in place for me. I had read countless explanations about "power available" but though I was on the verge of getting it looking at the graphs of power vs. drag, etc. I was never quite there. I couldn't wrap my mind around "but physics doesn't care about how much extra you have that isn't in play now!" And the idea of excess power. How did the universe know how much I have in reserve. Dumb...I know, but I couldn't quite get around that, until now.

That one sentence (primarily, the rest also was a huge help) about "if you are flying at normal cruise with full power" was the key I needed.
Again, with all the rest of your post, it put it into place for me. Thank you so much!

It really did cement the theory for me. I get too why Vx works that way. That last sentence was perfect. Why don't they have that in the manuals?

For the first point where cruise full power is 100 KIAS but you keep it at 80 KIAS the only way I know to do that is to raise the nose...but in a way I'm understanding that raising the nose (which non pilots think will make the plane climb magically) works because there is excess power which means lift...right? If I had decreased power, I wouldn't climb, or depending, either nit climb as fast or even descend.

There is also the issue of KIAS vs. TAS, but the main point was all about KIAS.
The biggest help too is all about "converting" and then I am back into understanding. It just threw me the way the textbooks describe it without saying what you did.
 
Last edited:
Longroad, just finish up and start flying already, will ya???
 
Never forget: Pitch+Power=Performance, that's what everyone here is saying.
When you're approaching pattern, you can pull down power, if you don't pull the yoke, you will descend. Eventually slowing a plane down becomes second nature, and you know how much pitch you need to add when you reduce power, to maintain altitude - resulting in straight and level flight, but slower speed.
 
eman1200 said:
Longroad, just finish up and start flying already, will ya???
Click to expand...

Believe me, I'm trying. I have to get the exam out of the way to keep learning to actually fly. I'm almost done. I have to check with the flight school, but only have psychology to go.

Then a 20 hour classroom instruction to go. Trying to find out when I can take it so it all works out.
It's different here, I think there are more requirements here. Not sure. But it's a lot.

I just need to get over the hump and get flying again.
 
If U.S. pilots had to take psychology, PoA would be a ghost town!
 
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