Why Isn't Vx The Best Rate Of Climb?

[RandyL]

Here is something that I have always wondered about, but have yet to
hear a good explanation for. Vx is the best angle of climb speed for an
aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
visualize why Vx is not the best rate of climb. To my feeble way of
thinking, Vx - the best climb angle - gets you to the highest altitude in
the shortest period of time, no? Why is that not the best rate of climb? I
hope someone can enlighten me on this. I know that there must be a good
explanation, but as I said, I just can't visualize it. Thanks...

Randy L.
--
Remember: Any landing that you can
walk away from, is a landing that you
can be fined, sued, or prosecuted for.

[peter]

RandyL wrote:
> Here is something that I have always wondered about, but have yet to
> hear a good explanation for. Vx is the best angle of climb speed for an
> aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
> visualize why Vx is not the best rate of climb. To my feeble way of
> thinking, Vx - the best climb angle - gets you to the highest altitude in
> the shortest period of time, no?

No, Vx will get you to a given altitude while covering the shortest
distance, but not generally in the shortest time. When climbing at a
steeper angle the plane will be going slower so even though Vx will get
you there in a shorter distance you'll get there a little quicker by
choosing a slightly shallower angle that lets you go faster, i.e. Vy.

It's similar to choosing whether to climb a mountain by either going
straight up the side in which case you may need ropes and make very
slow progress, or by hiking up the less steep switchback trail in which
case you'll have to go farther but will be able to go faster.

[John Gaquin]

"RandyL" > wrote in message

> Here is something that I have always wondered about, but have yet to
> hear a good explanation for. Vx is the best angle of climb speed for an
> aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
> visualize why Vx is not the best rate of climb.

Vy is time, and Vx is distance.

The key, Randy, is to look at what the definition says and consider the
words carefully. Best rate of climb considers feet of climb per unit of
time past, without considering distance travelled at all. Best angle of
climb considers feet of climb per unit of distance travelled, without
considering time at all.

>.....To my feeble way of thinking, Vx - the best climb angle -

NO. There's part of your difficulty, possibly. Stick with the proper
terminology. Don't switch words around, because they mean something. "Best
angle of climb" is not necessarily the same thing as "best climb angle".

[Peter Duniho]

"RandyL" > wrote in message
...
> Here is something that I have always wondered about, but have yet to
> hear a good explanation for. Vx is the best angle of climb speed for an
> aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
> visualize why Vx is not the best rate of climb. To my feeble way of
> thinking, Vx - the best climb angle - gets you to the highest altitude in
> the shortest period of time, no? Why is that not the best rate of climb?

I think the other two posts cover the basic definition adequately. I'll
offer a mental view that sometimes helps me understand the relationship, and
which might clarify the difference a little bit more:

Vy is about the best *rate* of climb. That is, the most vertical distance
over a period of time. Vx is about the best *angle* of climb. That is, the
most vertical distance of a horizontal distance.

That much has been explained. It may also be helpful to know that Vy occurs
when you have the most excess power, while Vx occurs when you have the most
excess thrust. Power, being a rate-based measure (that is, work (that is,
force times distance) over a period of time), winds up contributing to the
best climb performance over a period of time. On the other hand, thrust,
being a measurement of the specific force acting on the airplane (and which
is independent of time), winds up contributing to the best climb performance
when time is ignored.

I admit that the above may only be helpful if you already think in terms of
physics. But perhaps it provides some additional insight into why there
even are two different V speeds and how they relate to the definitions of
them.

Pete

[Roger (K8RI)]

On Mon, 25 Sep 2006 23:21:44 -0700, "Peter Duniho"
> wrote:

>"RandyL" > wrote in message
...
>> Here is something that I have always wondered about, but have yet to
>> hear a good explanation for. Vx is the best angle of climb speed for an
>> aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
>> visualize why Vx is not the best rate of climb. To my feeble way of
>> thinking, Vx - the best climb angle - gets you to the highest altitude in
>> the shortest period of time, no? Why is that not the best rate of climb?
>
>I think the other two posts cover the basic definition adequately. I'll
>offer a mental view that sometimes helps me understand the relationship, and
>which might clarify the difference a little bit more:
>
>Vy is about the best *rate* of climb. That is, the most vertical distance
>over a period of time. Vx is about the best *angle* of climb. That is, the
>most vertical distance of a horizontal distance.
>
>That much has been explained. It may also be helpful to know that Vy occurs
>when you have the most excess power, while Vx occurs when you have the most

You might try the reverse order too. Look at best glide speed. In
calm air the best glide speed gives you the most distance per time
unit, but traveling either faster or slower will cause you to lose
altitude faster. Now compare best rate of climb; a speed either faster
or slower (Vx) will produce less climb versus time, so although the
best angle for clearing an object at a given distance you will be
going slower then the best rate of climb, but at a steeper angle. It's
that slower speed with less lift that gives you less altitude versus
time.




Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Stefan]

RandyL schrieb:

> Vy is the best rate of climb for an aircraft. I can't seem to
> visualize why Vx is not the best rate of climb.

Simple: The steeper you climb, the slower the airplane flyes (as long as
you don't have excessive power).

So you can climb steep but flying slow, or you can climb shallow but
flying fast. Simple calculation shows that you can optimize for shortest
distance or shortest time, but those optima need not be the same (and
usually are not).

Stefan

[Matt Whiting]

RandyL wrote:
> Here is something that I have always wondered about, but have yet to
> hear a good explanation for. Vx is the best angle of climb speed for an
> aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
> visualize why Vx is not the best rate of climb. To my feeble way of
> thinking, Vx - the best climb angle - gets you to the highest altitude in
> the shortest period of time, no? Why is that not the best rate of climb? I
> hope someone can enlighten me on this. I know that there must be a good
> explanation, but as I said, I just can't visualize it. Thanks...

You are forgetting that part of the equation for best angle of climb is
also speed across the ground. For example, say best rate is 1000 fpm
and best angle is 800 fpm. Now suppose that the groundspeed (assume no
wind) is 120 knots for best rate climb, but only 60 knots for best
angle. If you draw out the triangle you will see that the best angle
airspeed really does give you the steepest climb gradient even though
the rate of climb is 200 fpm lower than best rate.

Matt

[Steve Foley[_1_]]

"RandyL" > wrote in message
...
> Here is something that I have always wondered about, but have yet to
> hear a good explanation for. Vx is the best angle of climb speed for an
> aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
> visualize why Vx is not the best rate of climb. To my feeble way of
> thinking, Vx - the best climb angle - gets you to the highest altitude in
> the shortest period of time, no? Why is that not the best rate of climb? I
> hope someone can enlighten me on this. I know that there must be a good
> explanation, but as I said, I just can't visualize it. Thanks...


How's this:

I can get up a flight of stairs in around three seconds (best rate of
climb). It will probably take me 20 seconds to climb a ladder the same
height (I hate ladders). The ladder will get me up at a much steeper angle,
hence best angle of climb. The stairs will get me there faster, hence best
rate of climb.

[Dylan Smith]

On 2006-09-26, Roger (K8RI) > wrote:
> You might try the reverse order too. Look at best glide speed. In
> calm air the best glide speed gives you the most distance per time
> unit, but traveling either faster or slower will cause you to lose
> altitude faster.

Not quite - minimum sink (the speed at which you'll lose altitude at the
lowest rate) is slower than best glide.

--
Yes, the Reply-To email address is valid.
Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de

[Ron Natalie]

Roger (K8RI) wrote:

> You might try the reverse order too. Look at best glide speed. In
> calm air the best glide speed gives you the most distance per time
> unit,

NO! It gives you the most distance per altitude lost.

[Darrell S[_1_]]

Say there's a hill a couple miles from the runway you need to clear to stay
alive. You would choose to climb at Vx, the best angle. If the hill wasn't
there you'd climb at Vy, best rate.

Say.. at Vx it takes you 5 minutes to get to that hill (which you clear) by
500'. The hill is 2500' above the runway and you cross it at 3000'. Let's
say that hill is 10 miles from the runway.

If you, instead, climbed at Vy you'd fly at a higher airspeed and get to the
hill in 4 minutes (but crash into the top). If the hill wasn't really
there, in an elapsed time of 5 minutes you might be at 3500' at a point 2
miles beyond where that hill would have been. (12 miles).

So with 5 minutes of climb.. at Vx you cleared the hill and were at 3,000'.
So with 5 minutes of climb.. at Vy (without the hill) you were at 3,500'.

Vx gives you the best angle (to clear terrain) in distance.
Vy gives you the best rate (to get to cruise altitude) in time.

--
Darrell R. Schmidt
B-58 Hustler Web Site URL (below)
http://members.cox.net/dschmidt1/

"RandyL" > wrote in message
...
> Here is something that I have always wondered about, but have yet to
> hear a good explanation for. Vx is the best angle of climb speed for an
> aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
> visualize why Vx is not the best rate of climb. To my feeble way of
> thinking, Vx - the best climb angle - gets you to the highest altitude in
> the shortest period of time, no? Why is that not the best rate of climb? I
> hope someone can enlighten me on this. I know that there must be a good
> explanation, but as I said, I just can't visualize it. Thanks...
>
> Randy L.
> --
> Remember: Any landing that you can
> walk away from, is a landing that you
> can be fined, sued, or prosecuted for.
>

[RandyL]

Thanks Peter - and everybody who replied to this. It took a mighty effort,
but I think I may have finally wrapped my brain around this concept. Take
care...

Randy L.
--
"When making an emergency off-field
landing at night, turn on the landing lights
just prior to touchdown.
If you don't like what you see, then turn
off the landing lights."

"peter" > wrote in message
oups.com...
> RandyL wrote:
>> Here is something that I have always wondered about, but have yet to
>> hear a good explanation for. Vx is the best angle of climb speed for an
>> aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
>> visualize why Vx is not the best rate of climb. To my feeble way of
>> thinking, Vx - the best climb angle - gets you to the highest altitude in
>> the shortest period of time, no?
>
> No, Vx will get you to a given altitude while covering the shortest
> distance, but not generally in the shortest time. When climbing at a
> steeper angle the plane will be going slower so even though Vx will get
> you there in a shorter distance you'll get there a little quicker by
> choosing a slightly shallower angle that lets you go faster, i.e. Vy.
>
> It's similar to choosing whether to climb a mountain by either going
> straight up the side in which case you may need ropes and make very
> slow progress, or by hiking up the less steep switchback trail in which
> case you'll have to go farther but will be able to go faster.
>

[Andrew Gideon]

On Tue, 26 Sep 2006 11:29:52 +0000, Dylan Smith wrote:

> Not quite - minimum sink (the speed at which you'll lose altitude at the
> lowest rate) is slower than best glide.

Right. Which makes best glide and Vy analogous and best loiter and Vx
analogous.

- Andrew

[Chris G.]

Don't feel bad. It took me a while to grasp this concept effectively as
a student, too.

Chris


RandyL wrote:
> Thanks Peter - and everybody who replied to this. It took a mighty effort,
> but I think I may have finally wrapped my brain around this concept. Take
> care...
>
> Randy L.

[Roger (K8RI)]

On Tue, 26 Sep 2006 17:35:51 -0400, Andrew Gideon >
wrote:

>On Tue, 26 Sep 2006 11:29:52 +0000, Dylan Smith wrote:
>
>> Not quite - minimum sink (the speed at which you'll lose altitude at the
>> lowest rate) is slower than best glide.
>
>Right. Which makes best glide and Vy analogous and best loiter and Vx

Depends on what you are flying. in the Deb Vy is a tad slower than
best glide which gives 500 to 600 fpm while Vx which is a lot slower
gives the express elevator down at about 800 fpm or a tad more.
Gliding at Vx gives me a very fast and steep descent for short field
landings and a bunch of power. In the Deb Vx is definitely not going
to keep you up there longer. Maximum range under power is about the
same as Vy.

>analogous.
>
> - Andrew
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Roger (K8RI)]

On Tue, 26 Sep 2006 08:33:23 -0400, Ron Natalie >
wrote:

>Roger (K8RI) wrote:
>
>> You might try the reverse order too. Look at best glide speed. In
>> calm air the best glide speed gives you the most distance per time
>> unit,
>
>NO! It gives you the most distance per altitude lost.

Yur right.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Sylvain]

T o d d P a t t i s t wrote:
> Vx and Ivy are also engine/propeller dependent. Vx
> corresponds to excess power.
....
> Similarly, Vy is "analogous" to best L/D (same as best
> glide) expect that we look at thrust, not power.

don't you have it backward here? thought Vx was
determined by available thrust while Vy by available
power?..

--Sylvain

[John Clonts]

All have provided some very good answers, and I will add this: for
planes with Vx < Vy (which is most planes I believe), when taking off
into the wind, the wind will improve you climb angle w.r.t. the ground
in the same manner and direction. That is, flying slower will allow
you to clear obstacles more easily. In IFR this comes into play when
following obstacle departure procedures, which specify a minimum climb
*angle* (specified in feet per nautical mile).

Of course, the wind direction does not affect Vx per se.
--
Cheers,
John Clonts
Temple, Texas
N7NZ

Darrell S wrote:
> Say there's a hill a couple miles from the runway you need to clear to stay
> alive. You would choose to climb at Vx, the best angle. If the hill wasn't
> there you'd climb at Vy, best rate.
>
> Say.. at Vx it takes you 5 minutes to get to that hill (which you clear) by
> 500'. The hill is 2500' above the runway and you cross it at 3000'. Let's
> say that hill is 10 miles from the runway.
>
> If you, instead, climbed at Vy you'd fly at a higher airspeed and get to the
> hill in 4 minutes (but crash into the top). If the hill wasn't really
> there, in an elapsed time of 5 minutes you might be at 3500' at a point 2
> miles beyond where that hill would have been. (12 miles).
>
> So with 5 minutes of climb.. at Vx you cleared the hill and were at 3,000'.
> So with 5 minutes of climb.. at Vy (without the hill) you were at 3,500'.
>
> Vx gives you the best angle (to clear terrain) in distance.
> Vy gives you the best rate (to get to cruise altitude) in time.
>
> --
> Darrell R. Schmidt
> B-58 Hustler Web Site URL (below)
> http://members.cox.net/dschmidt1/
>
> "RandyL" > wrote in message
> ...
> > Here is something that I have always wondered about, but have yet to
> > hear a good explanation for. Vx is the best angle of climb speed for an
> > aircraft. Vy is the best rate of climb for an aircraft. I can't seem to
> > visualize why Vx is not the best rate of climb. To my feeble way of
> > thinking, Vx - the best climb angle - gets you to the highest altitude in
> > the shortest period of time, no? Why is that not the best rate of climb? I
> > hope someone can enlighten me on this. I know that there must be a good
> > explanation, but as I said, I just can't visualize it. Thanks...
> >
> > Randy L.
> > --
> > Remember: Any landing that you can
> > walk away from, is a landing that you
> > can be fined, sued, or prosecuted for.
> >

[Marc Adler]

RandyL wrote:

> I can't seem to
> visualize why Vx is not the best rate of climb.

I think what's confusing about it is that it's always presented in
terms of getting over obstacles at the end of the runway. You (as in
"everyone") always think of getting over those obstacles as quickly as
possible, which is why Vx seems "faster." (At least, that's the way it
seemed to me.)

So forget time for a second and think about it in terms of horizontal
distance: Vx will get you to a higher altitude over a shorter distance
than Vy. It's a steeper slope. And that's it. If you add time into the
equation, it'll take you longer to reach the same altitude along Vx
than Vy, but that's because you can go that much faster at Vy than at
Vx.

Hope that helps.

Marc