Do I have the following right?:-

Jet:-
Vx IAS stays the same as altitude gets higher.
Vy IAS reduces as altitude gets higher.
Both angle and rate of climb lessen as altitude increases.

Prop:-
Vx IAS increases as altitude gets higher.
Vy IAS reduces as altitude gets higher.
Both angle and rate of climb lessen as altitude increases.

If the above is right, is there a rule of thumb for how much the various
speeds change, or is it really aircraft+powerplant reliant?

TIA

"xerj" > wrote in message
...
> Do I have the following right?:-
>
> Jet:-
> Vx IAS stays the same as altitude gets higher.
> Vy IAS reduces as altitude gets higher.
> Both angle and rate of climb lessen as altitude increases.
>
> Prop:-
> Vx IAS increases as altitude gets higher.
> Vy IAS reduces as altitude gets higher.
> Both angle and rate of climb lessen as altitude increases.

I'm not convinced that it is right.

Vx occurs at the airspeed where excess thrust is maximized, while Vy occurs
at the airspeed where excess power is maximized. Off the top of my head, I
don't see any reason to believe that for a jet, that it's a given that
maximum excess thrust would occur at a constant airspeed with respect to
altitude.

It's true that many jet installations are derated, so that the engine can
continue to produce sea-level power and thrust up to a certain altitude.
But then, the same is true for many turbocharged piston-engined propeller
airplanes. And some jet installations aren't derated and so power and
thrust falls off with altitude just as would happen for a normally-aspirated
piston-engined airplane.

Beyond the difference in combustion air compression, a jet is really not
that different from a propeller, with respect to producing thrust. Fan/prop
blades both pull air in and push it out...the jet just happens to have a
shroud around it. The similarity is especially great with respect to
high-bypass turbofan engines, where thrust from combustion is a very small
proportion of total thrust.

I admit that I haven't studied the facts surrounding your question closely.
But I think that even with a pure jet, thrust is not actually constant with
altitude (except in the case of derating, of course). At first glance, I
don't see any reason to believe that Vx and Vy behave significantly
differently depending on the basic engine technology (ignoring the question
of altitude-compensating technology like turbochargers, of course).

Pete

I agree with Peter, and haven't ever seen a difference just because of
engine type.

One minor nit.......jet engines are typically FLAT rated, not derated,
although that also happens. Flat rated means that a big engine with lots of
horsepower is limited to some lower horsepower at sea level. That engine can
then continue to extract that same horsepower as it climbs to an altitude to
where it typically temps out.

Here is a discussion of falt and derating.
http://www.pprune.org/forums/showthread.php?t=222816

Karl
"curator" N185KG


"Peter Duniho" > wrote in message
...
> "xerj" > wrote in message
> ...
>> Do I have the following right?:-
>>
>> Jet:-
>> Vx IAS stays the same as altitude gets higher.
>> Vy IAS reduces as altitude gets higher.
>> Both angle and rate of climb lessen as altitude increases.
>>
>> Prop:-
>> Vx IAS increases as altitude gets higher.
>> Vy IAS reduces as altitude gets higher.
>> Both angle and rate of climb lessen as altitude increases.
>
> I'm not convinced that it is right.
>
> Vx occurs at the airspeed where excess thrust is maximized, while Vy
> occurs at the airspeed where excess power is maximized. Off the top of my
> head, I don't see any reason to believe that for a jet, that it's a given
> that maximum excess thrust would occur at a constant airspeed with respect
> to altitude.
>
> It's true that many jet installations are derated, so that the engine can
> continue to produce sea-level power and thrust up to a certain altitude.
> But then, the same is true for many turbocharged piston-engined propeller
> airplanes. And some jet installations aren't derated and so power and
> thrust falls off with altitude just as would happen for a
> normally-aspirated piston-engined airplane.
>
> Beyond the difference in combustion air compression, a jet is really not
> that different from a propeller, with respect to producing thrust.
> Fan/prop blades both pull air in and push it out...the jet just happens to
> have a shroud around it. The similarity is especially great with respect
> to high-bypass turbofan engines, where thrust from combustion is a very
> small proportion of total thrust.
>
> I admit that I haven't studied the facts surrounding your question
> closely. But I think that even with a pure jet, thrust is not actually
> constant with altitude (except in the case of derating, of course). At
> first glance, I don't see any reason to believe that Vx and Vy behave
> significantly differently depending on the basic engine technology
> (ignoring the question of altitude-compensating technology like
> turbochargers, of course).
>
> Pete
>

> I'm not convinced that it is right.
>
> Vx occurs at the airspeed where excess thrust is maximized, while Vy
> occurs at the airspeed where excess power is maximized. Off the top of my
> head, I don't see any reason to believe that for a jet, that it's a given
> that maximum excess thrust would occur at a constant airspeed with respect
> to altitude.

The ***TAS*** would definitely increase, but the drag/thrust curve moves to
the right rather than to the right and up like the power required curve. As
far as I can tell, and I'm basing it (probably WAY too much) on the
interactive program found at this page
http://www.professionalpilot.ca/aerodynamics/performance/climb_jet.htm, the
***IAS*** remains almost constant for a jet.

Confusing stuff.

"xerj" > wrote in message
...
> The ***TAS*** would definitely increase,

We were talking about IAS. I didn't change that.

> but the drag/thrust curve moves to the right rather than to the right and
> up like the power required curve. As far as I can tell, and I'm basing it
> (probably WAY too much) on the interactive program found at this page
> http://www.professionalpilot.ca/aerodynamics/performance/climb_jet.htm,
> the ***IAS*** remains almost constant for a jet.

Got an example that doesn't require the installation of an ActiveX control?
I have, against my better judgment, already succumbed to Java and Flash, but
I draw the line at adding Shockwave to the mix.

Also note that "remains almost constant" isn't the same as "stays the same".

> Confusing stuff.

It's only confusing to those who haven't studied it enough to have an
intuitive sense of how things work.

Of course, that describes most of us (including myself). :) But still, it
seems to me that in aerodynamics particularly, most analysis winds up being
based on some fairly simple principles. The trick is knowing how to apply
them.

Pete

No, I don't have an example without the need for shockwave.

I'll trawl around and see if I can can up with something.

>The engines do have significantly different >characteristics.

Yup. And that is where I arrive back at one of my original questions.

Does Vx IAS stay the same for a jet (in theory at least) based on the fact
that excess thrust will diminish with altitude, but that the point on the
graph will be the same IAS as the drag curve moves right?

"Peter Duniho" > wrote

> The problem is that this analysis assumes constant thrust. I've seen
> nothing to indicate that, as a rule, jet engines maintain constant thrust
> as altitude increases.
>
> It may be a convenient simplification, but jet engines aren't exempt from
> the same rules of air density that piston engines abide by.

I thought that the constant thrust part of the jet engine, is that at a
constant altitude, it produces constant thrust with varying speed. Is that
correct?
--
Jim in NC

"Morgans" > wrote in message
...
>> It may be a convenient simplification, but jet engines aren't exempt from
>> the same rules of air density that piston engines abide by.
>
> I thought that the constant thrust part of the jet engine, is that at a
> constant altitude, it produces constant thrust with varying speed. Is
> that correct?

AFAIK, yes. But as altitude changes, so does (potentially) the thrust the
engine produces, as the air density affects how much fuel can be burned.

Pete

"Morgans" > wrote in message
...
>
> "Peter Duniho" > wrote
>
>> The problem is that this analysis assumes constant thrust. I've seen
>> nothing to indicate that, as a rule, jet engines maintain constant thrust
>> as altitude increases.
>>
>> It may be a convenient simplification, but jet engines aren't exempt from
>> the same rules of air density that piston engines abide by.
>
> I thought that the constant thrust part of the jet engine, is that at a
> constant altitude, it produces constant thrust with varying speed. Is
> that correct?
> --
> Jim in NC

The jet I flew(LR24B) definately did not maintain a constant thrust with
varying speed. Faster was better. More ram air = more thrust. Best rate
below 10,000' was about 220kias. (best glide was 180) Best rate over 10,000'
was more like 270kias. Above 30,000 use .75 - .82(the redline), faster was
better.

Al G

"The problem is that this analysis assumes constant thrust. I've seen
nothing to indicate that, as a rule, jet engines maintain constant thrust as
altitude increases"

The thrust definitely decreases with altitude, but it is constant across all
speeds at the same altitude. Unlike a power available curve, it is a
straight line.

The drag curve shifts to the right with altitude, indicating that the TAS
for minimum gets higher. The thrust line moves down towards the drag curve
as altitude increases, but the point where the biggest difference between
the two occurs is the same IAS.

"T o d d P a t t i s t" > wrote in message
...
> The constant thrust assumption only needs to apply with
> respect to changes in speed at a fixed altitude, not changes
> in altitude.

Okay...I get it now. Except for the other post that points out that thrust
is *not* constant even at a given altitude. :(