VLD and max altitude

[Chris W]

Can someone tell me if my thinking is correct here. First VLD is the
speed where there is the least drag on the plane right? So it takes the
least amount of power to maintain level flight at VLD, any slower or
faster in level flight requires more power? Now suppose you are at an
altitude where the fastest speed you can maintain is VLD, would that
mean that the plane can't climb any higher? Also would that be a
reasonable way to determine VLD, (i.e. climb as high as you can and the
indicated airspeed at that altitude would be VLD)?

--
Chris W

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[Bob Moore]

T o d d P a t t i s t wrote
> Chris W > wrote:
>>So it takes the least amount of power to maintain level
>>flight at VLD, any slower or faster in level flight
>>requires more power?
>
> This is not correct. Power required is force times speed.
> Even though force (drag) is minimum at the speed for best
> L/D, as you slow, the decrease in speed is faster than the
> increase in drag, so the product (power) continues to
> decrease.

Of course it is "thrust" that overcomes drag, not power. :-)

From Aerodynamics for Naval Aviators:

A factor more important in airplane performance considerations
is the lift-drag ratio, L/D. With the lift and drag data
available for the airplane, the proportions of CL and CD can
be calculated for each specific angle of attack. The resulting
plot of lift-drag ratio with angle of attack shows that L/D
increases to some maximum then decreases at the higher lift
coefficients and angles of attack. Note that the maximum lift-
drag ratio, (L/D)max, occurs at one specific angle of attack
and lift coefficient. If the airplane is operated in steady
flight at (L/D)max, the total drag is at a minimum. Any angle
of attack lower or higher than that for (L/D)max reduces the
lift-drag ratio and consequently increases the total drag for
a given airplane lift.

Bob Moore

[Chris W]

T o d d P a t t i s t wrote:

>>So it takes the
>>least amount of power to maintain level flight at VLD, any slower or
>>faster in level flight requires more power?
>>
>>
>
>This is not correct. Power required is force times speed.
>Even though force (drag) is minimum at the speed for best
>L/D, as you slow, the decrease in speed is faster than the
>increase in drag, so the product (power) continues to
>decrease.
>
>

I knew there was something wrong with my thinking, that makes perfect
sense, thanks. So I guess the speed I am looking for would be VLP or V
Least Power. Wouldn't that be the speed you are flying when you can't
climb any higher?

--
Chris W

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[Peter Duniho]

"Chris W" > wrote in message
news:2Ze2e.4431$Tn.2869@lakeread06...
> I knew there was something wrong with my thinking, that makes perfect
> sense, thanks. So I guess the speed I am looking for would be VLP or V
> Least Power. Wouldn't that be the speed you are flying when you can't
> climb any higher?

"V Least Power" is zero knots. It takes no power at all to stay in one
place (assuming you're on the ground :) ). It's just like "V Least Drag".

I'm a little confused about what it is you're trying to determine. You
start out asking if you can determine the lowest drag speed by climbing
until you can climb no longer. Todd explained why that won't work. But now
you seem to be looking for *any* speed that can be determined by climbing
until you can climb no longer. It's almost as though what you actually have
is a solution in search of a question.

For what it's worth, when the airplane won't climb any more, you are
necessarily flying at Vy (which is the speed where you have the greatest
*excess* power). You also will happen to be flying at Vx (which is the
speed where you have the greatest excess thrust).

But of course, climbing to the airplane's absolute ceiling doesn't tell you
anything except the absolute ceiling, and the values of Vx and Vy at that
altitude. They both change depending on altitude (Vx gets higher, Vy gets
lower, and they converge at the absolute ceiling). So getting numbers at
the absolute ceiling won't tell you much about them at lower altitudes.

Maybe if you can restate your question to more clearly indicate what it is
you want to know, you might get better help.

Pete

[yupyupxav]

>
>This is also correct, and it's consistent with what I
>posted, so I'm not sure why you posted it. It focuses on
>L/D, not power. The minimum for drag D is the same as the
>max of L/D, but the minimum of power required occurs at a
>different point. Am I missing something about why you
>posted this?
This is not true. It depends on the engine. First if the Thrust
remains constant in relation to the speed, Minimum power required
equals minimum drag. But few engines give constant thrust output vs
speed. (Turbojet, turbofan, turboprop, reciprocating prop) The engine
giving an output almost constant is the turbojet, but I don't think it
is used in aviation anymore.

[Chris W]

Peter Duniho wrote:

>"Chris W" > wrote in message
>news:2Ze2e.4431$Tn.2869@lakeread06...
>
>
>>I knew there was something wrong with my thinking, that makes perfect
>>sense, thanks. So I guess the speed I am looking for would be VLP or V
>>Least Power. Wouldn't that be the speed you are flying when you can't
>>climb any higher?
>>
>>
>
>"V Least Power" is zero knots. It takes no power at all to stay in one
>place (assuming you're on the ground :) ). It's just like "V Least Drag".
>
>
I meant the least power needed to maintain level flight. I'm not what
the correct nomenclature for that would be.

Right after I posted that I realized what I was thinking was wrong.
Because at a high altitude the true air speed would be higher than if
you were close to the ground and flying at the same indicated airspeed
and a higher true air speed requires more power. So the airspeed that
would let you fly with the least amount of power would be different at a
low altitude, or any other altitude for that mater.



>I'm a little confused about what it is you're trying to determine. You
>start out asking if you can determine the lowest drag speed by climbing
>until you can climb no longer. Todd explained why that won't work. But now
>you seem to be looking for *any* speed that can be determined by climbing
>until you can climb no longer. It's almost as though what you actually have
>is a solution in search of a question.
>
>
Oh I'm just playing around seeing how high I can get various planes to
fly in FS2004 and it got me to wondering about the speed I should be
flying when at max altitude.

>For what it's worth, when the airplane won't climb any more, you are
>necessarily flying at Vy (which is the speed where you have the greatest
>*excess* power). You also will happen to be flying at Vx (which is the
>speed where you have the greatest excess thrust).
>
>
If you can't climb any higher, how could you have any excess power?


>But of course, climbing to the airplane's absolute ceiling doesn't tell you
>anything except the absolute ceiling, and the values of Vx and Vy at that
>altitude. They both change depending on altitude (Vx gets higher, Vy gets
>lower, and they converge at the absolute ceiling). So getting numbers at
>the absolute ceiling won't tell you much about them at lower altitudes.
>
>
The more I think about it, that's pretty much what I am realizing.

>Maybe if you can restate your question to more clearly indicate what it is
>you want to know, you might get better help.
>
>
I'm not sure there was specifically something I wanted to know (sort of
thinking out loud) except maybe what speed you should shoot for to get
to the max altitude. If you increase the AOA too much, and go too slow,
you won't reach the highest possible altitude. From what you are saying
I guess Vy is the speed you want to maintain, but as you said that
changes with altitude.

--
Chris W

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give the gifts they want
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[Peter Duniho]

"Chris W" > wrote in message
news:DXj2e.4478$Tn.3559@lakeread06...
> [...]
> If you can't climb any higher, how could you have any excess power?

The key here is "greatest excess power". At the absolute altitude, your
excess power at Vy is 0. But at any other speed, the airplane actually
requires more power than is available; "excess" power is negative. 0 is
larger than any negative number, thus you are still at the "greatest excess
power".

> [...]
> I'm not sure there was specifically something I wanted to know (sort of
> thinking out loud) except maybe what speed you should shoot for to get to
> the max altitude. If you increase the AOA too much, and go too slow, you
> won't reach the highest possible altitude. From what you are saying I
> guess Vy is the speed you want to maintain, but as you said that changes
> with altitude.

You should start at Vy, and gradually reduce your airspeed as you climb. As
a very rough guess, average Vx and Vy at sea level and aim for that at your
final altitude (so, it will help if you have at least a rough guess as to
the absolute ceiling of the airplane in question, so you know how to adjust
your airspeed as you climb). Vx and Vy don't actually change at the same
rate, so the final Vx and Vy that are the same isn't actually their sea
level average. But it's not far. :)

The question of what airspeed to fly at maximum altitude will be answered
when you reach maximum altitude. There is only one airspeed at maximum
altitude at which you'll stay at maximum altitude. Any other airspeed will
result in a descent.

So, one way you could do this is to start at Vy and climb. When the
airplane stops climbing, reduce your airspeed by a knot or so. If the
airplane starts climbing again, repeat the exercise each time it stops
climbing. Once reducing the airspeed after you've stopped climbing results
in a descent rather than a continuation of your climb, you've hit the
absolute ceiling of the airplane.

Note that all of the above is for real life situations. While I think MSFS
gets more crap for its flight model than it deserves, it's true that some
odd things sometimes happen near the boundary cases. I can't guarantee that
everything will work exactly as it does in real life. :)

Pete

[Andrew Sarangan]

You need two separate plots to discuss this issue. One is drag vs speed.
This is where the minimum L/D is defined. The other is power vs speed.
This is where the minimum power to maintain level flight is defined.



Chris W > wrote in news:o462e.4423$Tn.2163@lakeread06:

> Can someone tell me if my thinking is correct here. First VLD is the
> speed where there is the least drag on the plane right? So it takes
the
> least amount of power to maintain level flight at VLD, any slower or
> faster in level flight requires more power? Now suppose you are at an
> altitude where the fastest speed you can maintain is VLD, would that
> mean that the plane can't climb any higher? Also would that be a
> reasonable way to determine VLD, (i.e. climb as high as you can and
the
> indicated airspeed at that altitude would be VLD)?
>

[John T Lowry]

"Chris W" > wrote in message
news:o462e.4423$Tn.2163@lakeread06...
> Can someone tell me if my thinking is correct here. First VLD is the
> speed where there is the least drag on the plane right? So it takes
> the least amount of power to maintain level flight at VLD, any slower
> or faster in level flight requires more power? Now suppose you are at
> an altitude where the fastest speed you can maintain is VLD, would
> that mean that the plane can't climb any higher? Also would that be a
> reasonable way to determine VLD, (i.e. climb as high as you can and
> the indicated airspeed at that altitude would be VLD)?
>
> --
> Chris W


Has anyone mentioned that you can't GET to maximum altitude with finite
fuel and finite time?

John Lowry
Flight Physics

[Mike Rapoport]

"John T Lowry" > wrote in message
ink.net...
>
> "Chris W" > wrote in message
> news:o462e.4423$Tn.2163@lakeread06...
>> Can someone tell me if my thinking is correct here. First VLD is the
>> speed where there is the least drag on the plane right? So it takes the
>> least amount of power to maintain level flight at VLD, any slower or
>> faster in level flight requires more power? Now suppose you are at an
>> altitude where the fastest speed you can maintain is VLD, would that mean
>> that the plane can't climb any higher? Also would that be a reasonable
>> way to determine VLD, (i.e. climb as high as you can and the indicated
>> airspeed at that altitude would be VLD)?
>>
>> --
>> Chris W
>
>
> Has anyone mentioned that you can't GET to maximum altitude with finite
> fuel and finite time?
>
> John Lowry
> Flight Physics
>
>

Max altitude is reached with minimium fuel right before the engine quits.
You certainly can't reach max altitude starting with zero or infinite fuel.

Mike
MU-2

[John T Lowry]

"Mike Rapoport" > wrote in message
ink.net...
>
> "John T Lowry" > wrote in message
> ink.net...
>>
>> "Chris W" > wrote in message
>> news:o462e.4423$Tn.2163@lakeread06...
>>> Can someone tell me if my thinking is correct here. First VLD is
>>> the speed where there is the least drag on the plane right? So it
>>> takes the least amount of power to maintain level flight at VLD, any
>>> slower or faster in level flight requires more power? Now suppose
>>> you are at an altitude where the fastest speed you can maintain is
>>> VLD, would that mean that the plane can't climb any higher? Also
>>> would that be a reasonable way to determine VLD, (i.e. climb as high
>>> as you can and the indicated airspeed at that altitude would be
>>> VLD)?
>>>
>>> --
>>> Chris W
>>
>>
>> Has anyone mentioned that you can't GET to maximum altitude with
>> finite fuel and finite time?
>>
>> John Lowry
>> Flight Physics
>>
>>
>
> Max altitude is reached with minimium fuel right before the engine
> quits. You certainly can't reach max altitude starting with zero or
> infinite fuel.
>
> Mike
> MU-2
>
>
Von Mises, p. 416: "It can be seen by simple reasoning that the ceiling
altitude cannot be reached within a finite time."

John Lowry
Flight Physics