Vx/Vy (am I missing something?)

AngelFuree

AngelFuree

Well-Known Member
OK bear with this question.... I remember an old post talking about glide speed and such, which at the end, the problem wasn't exactly solved, lol. I tried searching, but no luck.

Anyway, my question is....how does Vx give you the greatest increase in altitude with minimal distance, though at the same time, takes longer (in time) to reach that altitude...???

I don't know if I made sense but...umm...let's say for example...
Wouldn't climbing at 63 kts (Vx for the Warrior III) give you a higher indication on your VSI, than climbing at 79 kts (Vy)?
So what I'm thinking is...the VSI says "ft. per minute"....so doesn't that mean that having a higher indication on the VSI (higher ft. per minute) get you to an altitude the fastest? (Vx has a higher vertical speed than Vy, right?)

Agggggggggh.

Also, another question........
If Vx-63 kts, gives you the greatest gain in altitude for distance, then why do we calculate a "50 ft. barrier speed" (lower than Vx) to clear that obstacle...?

Oh geez, thanks to whoever takes the time to answer my questions.
 
Check this out:

Vx vs Vy

Also the reason for the barrier speed being less than Vx is it may take a longer distance to accelerate to Vx than it would to just accept a slightly smaller angle but get there in less total distance.
 
lol Ed, thanks man. That link is one of the first things they show us here in UND, but I'm afraid I still won't understand it...

See, I DO understand what Vx and Vy are. However, I don't understand how it turns out to be how it is....

[ QUOTE ]
Also the reason for the barrier speed being less than Vx is it may take a longer distance to accelerate to Vx than it would to just accept a slightly smaller angle but get there in less total distance.

[/ QUOTE ]
Hmm this does make some sense...
...but.....
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Thanks anyway man =)
 
Well dude if Vx was 400 knots would you just sit in ground effect for an hour to get to Vx... or would you just fly a bit slower and clear the obstacle sooner, then worry about speed?
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[ QUOTE ]
Well dude if Vx was 400 knots would you just sit in ground effect for an hour to get to Vx... or would you just fly a bit slower and clear the obstacle sooner, then worry about speed?

[/ QUOTE ]
Ha ha ha, I understand what you mean for that part now.
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The Vx/Vy question remains, however. teehee

Hmm....ok how about this....

Does the plane climb faster at 63 knots or at 79 knots?
(does the needle on the VSI point a higher rate at 63 or 79?)
I mean, the obvious answer is 63 no? but umm....

I guess that should help somewhat. The only way would be to actually try it out tomorrow. Hopefully I'll remember, lol.

OK nevermind. It's kind of hard putting it into words.
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[ QUOTE ]
Does the plane climb faster at 63 knots or at 79 knots?
(does the needle on the VSI point a higher rate at 63 or 79?)
It climbs faster at 79.

[/ QUOTE ]
Remember, if you ask which one is FASTER you are talking about altitude over TIME (Vy, 79 Kts). You are not talking about altitude over distance flown.
 
[ QUOTE ]

Does the plane climb faster at 63 knots or at 79 knots?
I mean, the obvious answer is 63 no? but umm....


[/ QUOTE ]


I dont know jack but I THINK I know this one... Imagine a runners track. Im not sure of the math but lets say the guy on the outside lane is 1mph faster. He would still tie the guy in the inside lane because the inside lane guy has a shorter distance to run. But after a few laps the faster guy would win even though he has to travel farther. For this scenario track length is angle of attack.

Vy is the fastest... But it takes longer distance to achieve
Vx takes a shorter distance to achieve but happens slower.

Again I have ZERO clue what I am talking about so someone please confirm or correct my analogy.
 
[ QUOTE ]
[ QUOTE ]

Does the plane climb faster at 63 knots or at 79 knots?
I mean, the obvious answer is 63 no? but umm....


[/ QUOTE ]


I dont know jack but I THINK I know this one... Imagine a runners track. Im not sure of the math but lets say the guy on the outside lane is 1mph faster. He would still tie the guy in the inside lane because the inside lane guy has a shorter distance to run. But after a few laps the faster guy would win even though he has to travel farther. For this scenario track length is angle of attack.

Vy is the fastest... But it takes longer distance to achieve
Vx takes a shorter distance to achieve but happens slower.

Again I have ZERO clue what I am talking about so someone please confirm or correct my analogy.

[/ QUOTE ]


You are right, that's it.
 
Tip: Unless absolutely necessary, don't climb at Vx. Why? Someone do me a favor. Takeoff, and climbout at Vx. Then, at 500 feet, pull the power back to idle. See what kind of glide you get to make a safe landing in case of an engine failure.

Climb at Vy.
 
Take a quick glance at your VSI the next time you're climbing; both at Vx and Vy. That should solve your mystery.
 
I used to get confused at the same thing. I think the answer is that (and maybe someone already said it, but I'm too lazy to read all the posts
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) Vy is the speed where there is the most excess thrust (which is kind of why it is a speed near L/Dmax), which means the best climb rate. Im not sure about Vx but I'm sure that it has something to do with thrst available/thrust required and drag. Climbing at Vx will NOT give a higher indication on the VSI, but climbing at Vy will give the highest possible climb rate for the conditions.

Lets say two airplanes in the same atmospheric conditions are trying to climb 100 feet. One goes at Vx and one goes at Vy. The one that goes at Vy will make 100 feet faster, but the distance over the groud will be greater. The one who climbs at Vx will get to 100 feet after the first airplane, but will have used less distance over the ground, because it is going slower. It all has to do with speed.
 
So, it seems to me that the only purpose of using Vx would be to clear a 50-foot obstacle, correct?

The only other advantage I can see is if your engine does cut at 500 feet: if you've used Vx, you'll be considerably closer to the runway (having eaten up much less ground) and able to safely make a 180 and land, rather than if you'd used Vy and you wouldn't be a whole lot higher, but quite a bit farther out. Yes, no?
confused.gif
 
[ QUOTE ]
OK bear with this question.... I remember an old post talking about glide speed and such, which at the end, the problem wasn't exactly solved, lol. I tried searching, but no luck.

Anyway, my question is....how does Vx give you the greatest increase in altitude with minimal distance, though at the same time, takes longer (in time) to reach that altitude...???

I don't know if I made sense but...umm...let's say for example...
Wouldn't climbing at 63 kts (Vx for the Warrior III) give you a higher indication on your VSI, than climbing at 79 kts (Vy)?
So what I'm thinking is...the VSI says "ft. per minute"....so doesn't that mean that having a higher indication on the VSI (higher ft. per minute) get you to an altitude the fastest? (Vx has a higher vertical speed than Vy, right?)

Agggggggggh.

Also, another question........
If Vx-63 kts, gives you the greatest gain in altitude for distance, then why do we calculate a "50 ft. barrier speed" (lower than Vx) to clear that obstacle...?

Oh geez, thanks to whoever takes the time to answer my questions.

[/ QUOTE ]

I'll start off with your Vx vs Vy question. We have to start with induced vs parasite drag, and the total drag curve. You probably know already that drag increases as airspeed increases, (parasite drag), but another kind of drag (induced) increases as airspeed decreases. If you add up both kinds of drag to see the total drag curve, it'll look like a "U" shape, and the lowest point on this curve is called "L/D max", best lift to drag ratio. The airspeed at which this occurs is roughly Vy, most lift per drag.

Now to demonstrate this, we can go out in our airplane and pick an altitude and change our airspeed and plot our climb rate and make a graph of airspeed vs vsi. This graph will start out rising fast, then it'll level off and start to decrease gradually. The topmost point on this graph is your highest VSI, and is Vy. This also is the horizontal tangent, the point where a horizontal line is tangent to the curve.

Behind Vy we are in the region of reverse reverse command, where it is taking us more power to decrease our airspeed and maintain altitude. Vx is slower than Vy (and if you draw an ordinal tangent to the airspeed vs vsi curve, the point where it intersects the curve is Vx), and so you are needing more power to maintain an altitude at a Vx than Vy. Climbing is done by excess power or thrust, so if we are flyling along at Vx and put in full power, there is less extra power than if we were flying along at Vy (since Vx required more power to maintain than Vy). So there is less extra power to climb with at Vx, so we dont climb as fast as Vy. We are travelling slower at Vx though, so we cover less distance than Vy.

I recommend you ask a CFI to show you a drag demo to see your climb rates at Vx vs Vy vs something faster than Vy.

Now for your barrier speed question, i think i understand what you are asking. Why is better to clear a 50 ft obstacle slower than even Vx? What it boils down to is that to clear the obstacle we just need to climb 50 ft regardless of how we get there. Lets start by looking at an airplane that rotates at 70 and Vx is 82. We rotate and then accelerate to 82 and climb out at 82 to 50 ft, then transition to Vy. Now take the same airplane, but lets climb out at 75 kts, this airplanes best obstacle clearance speed. We dont climb as fast as 82, but we dont have to accelerate to 82 before we climb, so we end up getting up to 50 ft in less total distance and it took to accelerate to Vx and climb. If you've got the room, use Vx as you've got more margin for error (and further from Vmc in the airplane i normally fly), but if you need the absolute shortest takeoff distance over a 50 ft obstacle, then you want the climb asap and worry about accelerating after you clear the obstacle.
 
[ QUOTE ]
Tip: Unless absolutely necessary, don't climb at Vx. Why? Someone do me a favor. Takeoff, and climbout at Vx. Then, at 500 feet, pull the power back to idle. See what kind of glide you get to make a safe landing in case of an engine failure.

[/ QUOTE ] Depending on the airplane of course, why would speeding up about 5 knots as opposed to slowing down about 5 knots to get to best glide make such a difference? Besides, at 500' at Vx you'd be much closer to the runway than at Vy. Heck, at some airports you'd still be over the runway and might have a few more landing options.

But I think we agree on the ultimate difference between the two. Use Vx when you need to climb to a certain altitude within a certain =space= and Vy when you need to climb within a certain =time=..
 
[ QUOTE ]
I used to get confused at the same thing. I think the answer is that (and maybe someone already said it, but I'm too lazy to read all the posts
smile.gif
) Vy is the speed where there is the most excess thrust (which is kind of why it is a speed near L/Dmax), which means the best climb rate. Im not sure about Vx but I'm sure that it has something to do with thrst available/thrust required and drag. Climbing at Vx will NOT give a higher indication on the VSI, but climbing at Vy will give the highest possible climb rate for the conditions.

Lets say two airplanes in the same atmospheric conditions are trying to climb 100 feet. One goes at Vx and one goes at Vy. The one that goes at Vy will make 100 feet faster, but the distance over the groud will be greater. The one who climbs at Vx will get to 100 feet after the first airplane, but will have used less distance over the ground, because it is going slower. It all has to do with speed.

[/ QUOTE ]
Bingo! Thanks! Exactly what I was looking for. I knew it had to do something with those drag curves.
Goootcha.

Thanks to all who've replied as well =)
 
[ QUOTE ]
Also, another question........
If Vx-63 kts, gives you the greatest gain in altitude for distance, then why do we calculate a "50 ft. barrier speed" (lower than Vx) to clear that obstacle...?

[/ QUOTE ]I can help you with this one. The published Vx is the best angle of climb speed =clean=. The barrier speed is best angle of climb speed with the flap setting recommended on the takeoff performance chart.
 
[ QUOTE ]
I'll start off with your Vx vs Vy question. We have to start with induced vs parasite drag, and the total drag curve. You probably know already that drag increases as airspeed increases, (parasite drag), but another kind of drag (induced) increases as airspeed decreases. If you add up both kinds of drag to see the total drag curve, it'll look like a "U" shape, and the lowest point on this curve is called "L/D max", best lift to drag ratio. The airspeed at which this occurs is roughly Vy, most lift per drag.

Now to demonstrate this, we can go out in our airplane and pick an altitude and change our airspeed and plot our climb rate and make a graph of airspeed vs vsi. This graph will start out rising fast, then it'll level off and start to decrease gradually. The topmost point on this graph is your highest VSI, and is Vy. This also is the horizontal tangent, the point where a horizontal line is tangent to the curve.

Behind Vy we are in the region of reverse reverse command, where it is taking us more power to decrease our airspeed and maintain altitude. Vx is slower than Vy (and if you draw an ordinal tangent to the airspeed vs vsi curve, the point where it intersects the curve is Vx), and so you are needing more power to maintain an altitude at a Vx than Vy. Climbing is done by excess power or thrust, so if we are flyling along at Vx and put in full power, there is less extra power than if we were flying along at Vy (since Vx required more power to maintain than Vy). So there is less extra power to climb with at Vx, so we dont climb as fast as Vy. We are travelling slower at Vx though, so we cover less distance than Vy.

I recommend you ask a CFI to show you a drag demo to see your climb rates at Vx vs Vy vs something faster than Vy.

Now for your barrier speed question, i think i understand what you are asking. Why is better to clear a 50 ft obstacle slower than even Vx? What it boils down to is that to clear the obstacle we just need to climb 50 ft regardless of how we get there. Lets start by looking at an airplane that rotates at 70 and Vx is 82. We rotate and then accelerate to 82 and climb out at 82 to 50 ft, then transition to Vy. Now take the same airplane, but lets climb out at 75 kts, this airplanes best obstacle clearance speed. We dont climb as fast as 82, but we dont have to accelerate to 82 before we climb, so we end up getting up to 50 ft in less total distance and it took to accelerate to Vx and climb. If you've got the room, use Vx as you've got more margin for error (and further from Vmc in the airplane i normally fly), but if you need the absolute shortest takeoff distance over a 50 ft obstacle, then you want the climb asap and worry about accelerating after you clear the obstacle.

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Awesooooooooooome! Thanks so much!!!!!
Excellent explanation, all clear now.
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