Why Isn't Vx The Best Rate Of Climb?

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.

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.

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

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.

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

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

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

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.

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