Why Isn't Vx The Best Rate Of Climb?

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.

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

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

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

"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.

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.

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