Increasing power required with altitude.. what's a good plain english explanation?

On Feb 3, 11:51 pm, "Andrew Sarangan" wrote:


Explanation for why power requirement increases with altitude for the
same AOA.

Drew, I think you are not looking at the big picture.Read section 7 of
Denker's material again.
You said "this author comes up with the right
answer, but he uses some false asumptions.Its obvious he hasnt spent
much time in a real airplane". The latter part I find rather insulting
of the author, but I will leave it at that.
I dont think Alice was trying to unsult the guy.You should understand
that Denker is not an aerdynamisist by schooling or by trade, he is
just another pilot like you and me who has read the same books you and
I have.Denker could do a better job of relating how many of his
theories and formulas relate to the actual operation of an aircraft
(He could also stand to correct some of his terminology errors).This
is where the experience level plays in because more real world
experience might help the relevance of his material.

You can't just claim that something is wrong without providing an
explanation. That's what a troll is.
I think all Alice was trying to do was provide some insight as to why
airliners cruise at high altitudes.All you have done is post insults.

Mxsmanic,

I don't recall saying anything about you.


I don't, either.

--
Thomas Borchert (EDDH)

On Feb 4, 1:04 pm, "F.Reid" wrote:
On Feb 3, 11:51 pm, "Andrew Sarangan" wrote:

Explanation for why power requirement increases with altitude for the
same AOA.

Drew, I think you are not looking at the big picture.Read section 7 of
Denker's material again.You said "this author comes up with the right
answer, but he uses some false asumptions.Its obvious he hasnt spent
much time in a real airplane". The latter part I find rather insulting
of the author, but I will leave it at that.

I dont think Alice was trying to unsult the guy.You should understand
that Denker is not an aerdynamisist by schooling or by trade, he is
just another pilot like you and me who has read the same books you and
I have.Denker could do a better job of relating how many of his
theories and formulas relate to the actual operation of an aircraft
(He could also stand to correct some of his terminology errors).This
is where the experience level plays in because more real world
experience might help the relevance of his material.

Great. But the poster said Denker had made some incorrect assumptions.
I am still anxiously waiting to hear what those assumptions are.

I don't know Denker personally, but I have read the book, which he
gives to the world for free, and I have greatly benefited from his
insights. Many people in this group have repeatedly cited his book. If
someone is challenging his views and calls him as someone who "hasn't
spent much time in a real airplane" the least he can do is explain
where Denker might have gone wrong. Otherwise it can only be construed
as an insult. If you think my asking him to provide an explanation is
an insult, then I don't know what to say.

To fly the same IAS requires the same power.

You mean the same *thrust*. The same IAS at a higher altiitude will be a
higher velocity, but the same thrust. The same thrust will give the same
dynamic pressure, which is basically what the ASI shows calibrated in speed.
However, thrust does not equal power. Power = thrust x velocity.

The drag curve (which is the same as the thrust curve in straight and level
flight) shifts to the right. The power curve shifts to the right AND up.

To fly the same TAS, requires less power. Because the air is thinner, you
need a higher throttle setting to get the same power out of the engine.
Maybe you are getting throttle setting confused with power.

No, I'm not talking about how open the throttle is. I'm talking about the
effect above. I was trying to think of a way to explain it without neeeding
to refer to IAS and TAS and power curves. Still not sure how to do that.

"xerj" wrote in message
...
If you
can maintain constant power (turbo charging), you get better and
better performance with altitude.

The TAS will increase, but say you want to hold a specific angle of attack
and its attendant IAS (maybe for range), you will need more power to do
that as you get higher.


This in not true. You will need the same power for the same IAS regardless
of altitude.

Danny Deger

On Feb 4, 12:11 pm, "Andrew Sarangan" wrote:

Great. But the poster said Denker had made some incorrect assumptions.
I am still anxiously waiting to hear what those assumptions are.
And I am anxioudly waiting to hear why you are being such a jerk,).

I went and read a couple of sections of this book after I read this
thread and I saw more than a few errors.I would sugest you check
Denkers material against some other books before you make your
condisending posts.
Andy, I cant understand why you are STILL having such a hard time with
this, and you are probably just trolling, but take a look at 7.5.5.Now
look at where it says "an airplane needs more power (After he said
power stays the same elswhere) to maintain a given CAS at
altitude".But an airplane doesnt maintain a given CAS at altitude now
does it Andy.To state that it applies to props and jets equally kinda
blurs a distinction about why jets fly higher than props.Why does he
throw CAS into the picture on the last step when we are talking about
TAS ?Now take a look at the bullits following the "At the higher
altitude line".Denker is ignoring certain realities about what is
happening to a plane as it climbs (And how we fly in real life).Some
of these bullits are true, but not all at the same time.For example,
what happens to your IAS as you climb?What happens to your drag?If the
power required is greater, why would the thrust be the same?Do you fly
based on CAS TAS or IAS (Or Mach)?Think about why this really relates
to needing more power.
Another thing that I noticed was that he gives the wrong definition to
certain terms like coffin corner and penetration speed.I have a little
bit of understanding of sailplane aerodynamics because I race
sailplanes, so I read with interest the parts of Denkers book that
pertain to gliders and you guessed, he got alot of it wrong (Although
he did accuratly describe the theory behind some of the
aerodynamics).

I don't know Denker personally, but I have read the book, which he
gives to the world for free, and I have greatly benefited from his
insights.

It is worth what you paid.(kidding)

Many people in this group have repeatedly cited his book.

That makes him an expert

This in not true. You will need the same power for the same IAS
regardless
of altitude.

Same thrust, not same power.

In that section, Denker is not talking about what is done operationally.
He's discussing the physics. They're not in doubt. That being: power =
thrust x distance/time, i.e. p = t x v. That's what started this whole
shebang in the first place. I was looking for a non-mathematical way to
explain WHY p = t x v.

"alice" wrote in message
oups.com...
On Feb 2, 3:38 pm, "xerj" wrote:

Here's backup:-

Fromhttp://www.av8n.com/how/htm/power.html#sec-power-altitude

snip
g.. the power required is greater (since power equals drag times TAS)
The last step is tricky. Whereas most of the aerodynamic quantitites of
interest to pilots are based on CAS, the power-per-thrust relationship
depends on TAS, not CAS.

This means that any aircraft requires more power to maintain a given CAS
at
altitude. This applies to propellers, jets, and rockets equally."


The power is the net force time velocity. The total force on the airframe
is zero because thrust cancels drag. This is to say you are not putting
energy in or taking energy out of the airframe. All energy is put into
moving the air.

The thrust is the same at high and low altitudes for the same IAS, if you
look at the power required to spin the propeller to generate the same thrust
at the high and low altitude, you will find that the power to spin the
propeller is the same. Same IAS, same power.

Danny Deger

"xerj" wrote in message
...
What's false about the assumptions? He's talking about flight at the
same
angle of attack at different altitudes.

Are you serous?
First, take a look at his opening statement.We dont fly planes like
this in real life.It seems he has made the deductions first, and then
came up with the opening statement.Also, not all of these deductions
can be true at the same time.

The purpose of that section is not an operational guide. It's merely to
illustrate a point -- that being the effect of flying at the same angle of
attack at different altitudes. This leads to the last point: to do that
you need more power.

I don't see which one of the deductions isn't true given the parameters.

See my earlier post. It is that power equals drag times velocity. The net
force on the plane is zero, so the power with reference to the airframe is
zero. That is to say the energy in the airframe is constant with time.

You need to calculate the power (energy per unit time) going into the air.

Danny Deger