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

[xerj]

I was trying to explain to a non-pilot why increased power is required with
altitude. She said "isn't the air thinner up there so there isn't as much
resistance?" I said "yes, but the plane needs to fly fast enough for the air
over the wings to feel like it does down low. So the speed required goes up
you get higher. More speed need more power."

This didn't really do the trick.

Can someone think of a better way of putting it without resorting to
mathematics and an explanation of IAS and TAS?

TIA

[Steve Foley]

"xerj" > wrote in message
...
>I was trying to explain to a non-pilot why increased power is required with
>altitude. She said "isn't the air thinner up there so there isn't as much
>resistance?" I said "yes, but the plane needs to fly fast enough for the
>air over the wings to feel like it does down low. So the speed required
>goes up you get higher. More speed need more power."
>
> This didn't really do the trick.
>
> Can someone think of a better way of putting it without resorting to
> mathematics and an explanation of IAS and TAS?
>
> TIA
>

There isn't as much air for the prop to grab, so it slips more.

[Grumman-581[_1_]]

xerj wrote:
> Can someone think of a better way of putting it without resorting to
> mathematics and an explanation of IAS and TAS?

Just say it is "PFM"...

Or tell her that the squirrels under the cowling can't breathe as well
at high altitudes...

Personally, I prefer to just claim it all is just PFM though...

[James Sleeman]

Here's my attempt at an easy layman explanation: "To stay up, the
plane must push down a certain quantity of air. A plane pushes air
down, by flying forwards. If there is less air, the plane must fly
forwards faster to push down the same total amount of air, so you must
add more power to do so."

[Denny]

On Feb 2, 7:18 am, "James Sleeman" > wrote:
> Here's my attempt at an easy layman explanation: "To stay up, the
> plane must push down a certain quantity of air. A plane pushes air
> down, by flying forwards. If there is less air, the plane must fly
> forwards faster to push down the same total amount of air, so you must
> add more power to do so."

Uhhhhh, I hate to bust anyones balloon, BUT, more throttle at higher
altitude does not mean you are getting more power than you were
getting with less throttle at lower altitude...

denny

[Andrew Sarangan]

The reason that explanation did not do the trick is because it simply
is not true. You don't need more power at higher altitude. You need
less power (for the same performance) because the air has less
resistance. Your non-pilot was correct.

Think about it. Your engine power (normally aspirated) drops with
altitude. But the airplane moves faster despite the power drop. If you
can maintain constant power (turbo charging), you get better and
better performance with altitude.




On Feb 2, 6:51 am, "xerj" > wrote:
> I was trying to explain to a non-pilot why increased power is required with
> altitude. She said "isn't the air thinner up there so there isn't as much
> resistance?" I said "yes, but the plane needs to fly fast enough for the air
> over the wings to feel like it does down low. So the speed required goes up
> you get higher. More speed need more power."
>
> This didn't really do the trick.
>
> Can someone think of a better way of putting it without resorting to
> mathematics and an explanation of IAS and TAS?
>
> TIA

[Mxsmanic]

xerj writes:

> I was trying to explain to a non-pilot why increased power is required with
> altitude. She said "isn't the air thinner up there so there isn't as much
> resistance?" I said "yes, but the plane needs to fly fast enough for the air
> over the wings to feel like it does down low. So the speed required goes up
> you get higher. More speed need more power."

Actually, she's right. You need higher speed at higher altitudes in
order to maintain a given amount of lift, because the air isn't as
dense. However, you don't necessarily need more power, because thin
air presents a lot less resistance to the aircraft. Airliners fly
high in part because it requires less power (and therefore consumes
less fuel). That's why they are eager to get up to high altitudes.

You may need a higher _throttle_ setting, because the engines produce
less power in thinner air. However, the amount of power required
still diminishes. To climb from altitude A to B in an aircraft, you
may have to increase the throttle from 60 to 75, but at the same time
the power produced by the engine at a given throttle setting
diminishes by 30%, so in fact you are flying with less power at
altitude B.

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[Mxsmanic]

Denny writes:

> Uhhhhh, I hate to bust anyones balloon, BUT, more throttle at higher
> altitude does not mean you are getting more power than you were
> getting with less throttle at lower altitude...

Indeed, if you work out the numbers, you'll find that you can fly with
less power at higher altitudes, even though the throttle may be set
higher.

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[Thomas Borchert]

Xerj,

> I was trying to explain to a non-pilot why increased power is required with
> altitude.

It is? I may have an idea what you mean, see point 2 below.

> She said "isn't the air thinner up there so there isn't as much
> resistance?"

Exactly right. Bright girl! That's why turbocharged piston airplanes like to
fly high.

> I said "yes, but the plane needs to fly fast enough for the air
> over the wings to feel like it does down low. So the speed required goes up
> you get higher. More speed need more power."

Uhm, not really. For a normally aspirated engine, the power output will
decrease during the ascent because of thinner air, which means fewer air
molecules per volume to burn. Thus, you need to increase the power setting to
make up for that (and you need to put less fuel in the cylinder, too, which is
achieved by leaning). At some point you will run out of throttle doing that.
There is an optimum altitude where the balance between loss of air resistance
and loss of engine power amounts to the best speed for the maximum amount of
power available. It is usually between 6000 and 8000 feet for what we fly.

--
Thomas Borchert (EDDH)

[Peter Dohm]

> I was trying to explain to a non-pilot why increased power is required
with
> altitude. She said "isn't the air thinner up there so there isn't as much
> resistance?" I said "yes, but the plane needs to fly fast enough for the
air
> over the wings to feel like it does down low. So the speed required goes
up
> you get higher. More speed need more power."
>
> This didn't really do the trick.
>
> Can someone think of a better way of putting it without resorting to
> mathematics and an explanation of IAS and TAS?
>
In a word, NO.

It is an issue of physics, and physics uses a lot of math.

To maintain the same TAS, she is right--untill IAS drops to the back side of
the power curve for the altitude at which she is then flying.

To maintain the same IAS, the power requirement will only increase linearly
in proportion to TAS with increasing altitude--until mach number becomes a
consideration (at some significant fraction of unity)

Therefore, within very finite limits, increasing altitude simply allows an
airplane to be flown at a higher TAS while holding the IAS within an
efficient range. That has the effect of only requiring the power to
increase linearly with speed--rather than as the square of the speed
increase.

I hope this helps.

Peter

[BDS[_2_]]

> > I was trying to explain to a non-pilot why increased power is required
> with
> > altitude. She said "isn't the air thinner up there so there isn't as
much
> > resistance?" I said "yes, but the plane needs to fly fast enough for the
> air
> > over the wings to feel like it does down low. So the speed required goes
> up
> > you get higher. More speed need more power."
> >
> > This didn't really do the trick.
> >
> > Can someone think of a better way of putting it without resorting to
> > mathematics and an explanation of IAS and TAS?

TAS increases with altitude for a given power setting due to less
aerodynamic drag at higher altitudes. It does not take more power to go the
same speed at higher altitudes - at least, not in any of the airplanes I've
ever flown. Take a look at the speed/power charts for a turbo and you'll
see what I mean - if you maintain 75% power the higher you go the faster you
go.

If you're talking about altitude effects on the power output of a
normally-aspirated engine, that's a different story. At about 8,000 feet a
normally-aspirated engine will probably be putting out around 75% power at
full throttle, and it will continue to decrease as you go higher.

BDS

[Mxsmanic]

Peter Dohm writes:

> In a word, NO.
>
> It is an issue of physics, and physics uses a lot of math.

Good physicists can explain any principle of physics without resorting
to math.

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[Peter Dohm]

> > > I was trying to explain to a non-pilot why increased power is required
> > with
> > > altitude. She said "isn't the air thinner up there so there isn't as
> much
> > > resistance?" I said "yes, but the plane needs to fly fast enough for
the
> > air
> > > over the wings to feel like it does down low. So the speed required
goes
> > up
> > > you get higher. More speed need more power."
> > >
> > > This didn't really do the trick.
> > >
> > > Can someone think of a better way of putting it without resorting to
> > > mathematics and an explanation of IAS and TAS?
>
> TAS increases with altitude for a given power setting due to less
> aerodynamic drag at higher altitudes. It does not take more power to go
the
> same speed at higher altitudes - at least, not in any of the airplanes
I've
> ever flown. Take a look at the speed/power charts for a turbo and you'll
> see what I mean - if you maintain 75% power the higher you go the faster
you
> go.
>
> If you're talking about altitude effects on the power output of a
> normally-aspirated engine, that's a different story. At about 8,000 feet
a
> normally-aspirated engine will probably be putting out around 75% power at
> full throttle, and it will continue to decrease as you go higher.
>
> BDS
>
First, I stand by my remarks as mathematically accurate.

Second, you are technically correct that a given power (typically 75%) will
give a greater speed with increasing altitude. However, the increase in
speed will not be as much as many people seem to expect, but instead will be
very close to the square root of the optomists expectation.

The good news is that the graphs in the POH seem to be a good guide.

Peter

[xerj]

>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.

[xerj]

> Actually, she's right. You need higher speed at higher altitudes in
> order to maintain a given amount of lift, because the air isn't as
> dense. However, you don't necessarily need more power, because thin
> air presents a lot less resistance to the aircraft.

You need more power to maintain the same amount of lift as you get higher.
By "same amount of lift" I take that to mean angle of attack and the
resulting IAS for the same dynamic pressure. The formula is predicated on
TAS.

[xerj]

> Uhm, not really. For a normally aspirated engine, the power output will
> decrease during the ascent because of thinner air, which means fewer air
> molecules per volume to burn.

I don't mean opening the throttle to make up for the engine power loss. I
mean the fact that to maintain the same IAS you need more power as you go
up.

[Mxsmanic]

xerj writes:

> I don't mean opening the throttle to make up for the engine power loss. I
> mean the fact that to maintain the same IAS you need more power as you go
> up.

Are you sure?

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[xerj]

> Are you sure?

Positive.

Here's backup:-

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

"Let's compare high-altitude flight with low-altitude flight at the same
angle of attack. Assume the weight of the airplane remains the same. Then we
can make a wonderful chain of deductions.

At the higher altitude:
a.. the lift is the same (since lift equals weight)
b.. the lift-to-drag ratio is the same (since it depends on angle of
attack)
c.. the drag is the same (calculated from the previous two items)
d.. the thrust is the same (since thrust equals drag)
e.. the indicated airspeed is the same (to produce the same lift at the
same angle of attack)
f.. the true airspeed is greater (because density is lower)
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."

[Morgans]

"xerj" > wrote

> I don't mean opening the throttle to make up for the engine power loss. I
> mean the fact that to maintain the same IAS you need more power as you go
> up.

Why the preoccupation with IAS?

At around 6,000 feet, the power of a non turbo piston engine is around 75%.
As you go higher, the power drops off, but the true air speed goes up.

Who cares about IAS? The question was does it take more power to go faster,
right? Any non pilot will think faster means true airspeed, not indicated.
--
Jim in NC

[xerj]

> TAS increases with altitude for a given power setting due to less
> aerodynamic drag at higher altitudes. It does not take more power to go
> the
> same speed at higher altitudes

It doesn't take more power to go the same TAS, but it does take more power
to go the same IAS.

[xerj]

> Who cares about IAS? The question was does it take more power to go
> faster, right? Any non pilot will think faster means true airspeed, not
> indicated.

True, but the conversation got to how high a plane can fly. I said that
going higher did two things: limited the amount of power that an engine can
put out because of density, and that even if you had an engine that didn't
lose power, the power required goes up regardless.

Also, not that it would matter to a non-pilot, but IAS obviously matters for
keeping best range speed for instance.

[Morgans]

"xerj" > wrote

> Also, not that it would matter to a non-pilot, but IAS obviously matters
> for keeping best range speed for instance.


How so?
--
Jim in NC

[Peter Dohm]

> > I don't mean opening the throttle to make up for the engine power loss.
I
> > mean the fact that to maintain the same IAS you need more power as you
go
> > up.
>
> Why the preoccupation with IAS?
>
> At around 6,000 feet, the power of a non turbo piston engine is around
75%.
> As you go higher, the power drops off, but the true air speed goes up.
>
> Who cares about IAS? The question was does it take more power to go
faster,
> right? Any non pilot will think faster means true airspeed, not
indicated.
> --
Ok, I confess, I'd rather have an angle of attack meter to correlate more
directly with the best coefficients of lift and drag independently of
current weight. But IAS and a little math based on initial weight and fuel
consumed should work well enough for us cheap-skates.

Even if you are operating at a speed other than best L/D, which seems mostly
reserved for Glider Pilots and Jet Jocks, reference to IAS is about the only
way (that I know of) to keep the theoretical discussion understandable

Peter
Cheapest of the cheap ;-))

[Peter Dohm]

> > TAS increases with altitude for a given power setting due to less
> > aerodynamic drag at higher altitudes. It does not take more power to go
> > the
> > same speed at higher altitudes
>
> It doesn't take more power to go the same TAS, but it does take more power
> to go the same IAS.
>
>
The way most people fly, which is well above best L/D, the same TAS will
require less power with increasing altitude.

Peter

[xerj]

>> Also, not that it would matter to a non-pilot, but IAS obviously matters
>> for keeping best range speed for instance.
>
>
> How so?

Best L/D occurs at a particular angle of attack. This corresponds fairly
well to indicated airspeed.

[alice]

On Feb 2, 3:38 pm, "xerj" > wrote:

> Here's backup:-
>
> Fromhttp://www.av8n.com/how/htm/power.html#sec-power-altitude
>
> "Let's compare high-altitude flight with low-altitude flight at the same
> angle of attack. Assume the weight of the airplane remains the same. Then we
> can make a wonderful chain of deductions.
>
> At the higher altitude:
> a.. the lift is the same (since lift equals weight)
> b.. the lift-to-drag ratio is the same (since it depends on angle of
> attack)
> c.. the drag is the same (calculated from the previous two items)
> d.. the thrust is the same (since thrust equals drag)
> e.. the indicated airspeed is the same (to produce the same lift at the
> same angle of attack)
> f.. the true airspeed is greater (because density is lower)
> 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."

What is interesting is that 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

[alice]

On Feb 2, 7:06 am, Mxsmanic > wrote:
> Actually, she's right. You need higher speed at higher altitudes in
> order to maintain a given amount of lift, because the air isn't as
> dense. However, you don't necessarily need more power, because thin
> air presents a lot less resistance to the aircraft. Airliners fly
> high in part because it requires less power (and therefore consumes
> less fuel). That's why they are eager to get up to high altitudes.

MX, common misconception here about airliners.You need to look at the
fuel required to maintain a given level of thrust at altitude for a
jet engine.
> --
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[xerj]

> What is interesting is that 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

What's false about the assumptions? He's talking about flight at the same
angle of attack at different altitudes.

[Mxsmanic]

alice writes:

> MX, common misconception here about airliners.

Hardly a misconception. The "sweet spot" for airliners is quite high,
and airlines like to be there in order to use the smallest amount of
fuel for a given distance.

> You need to look at the fuel required to maintain a given level
> of thrust at altitude for a jet engine.

I've looked that the fuel required to cover a given amount of ground,
and it's much lower at high altitudes.

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[alice]

On Feb 2, 9:46 pm, Mxsmanic > wrote:
> Hardly a misconception. The "sweet spot" for airliners is quite high,
> and airlines like to be there in order to use the smallest amount of
> fuel for a given distance.

MX,
Duh.No one is arguing that a jet uses less fuel up high.It is the
reason why that is in question.You are making a HUGE misconception
about the reason why.In fact, it could be said that you are thinking
backwards.By your reasoning, A jet would never have a service ceiling!
Explain to us what a "sweet spot" is.Why is it that you feel the
airlines dont take into account TIME when doing the preflight planing.
>
>
> I've looked that the fuel required to cover a given amount of ground,
> and it's much lower at high altitudes.

OK MX, here is the "Given amount of ground" thing again.Think real
hard about what you are saying and why you seem to think time doesnt
factor into the equasion.If you have in fact looked into the cruise
performance charts on a airliner, what did it say in the thrust
required column.In other words, ignore the fuel for a minute and you
will have your answer.
KW

>
> --
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[alice]

On Feb 2, 7:42 pm, "xerj" > wrote:
> > What is interesting is that 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
>
> 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.

[Mxsmanic]

alice writes:

> Duh.No one is arguing that a jet uses less fuel up high.It is the
> reason why that is in question.You are making a HUGE misconception
> about the reason why.

Less fuel = less power.

> By your reasoning, A jet would never have a service ceiling!

Jets have a service ceiling for several reasons. For one, eventually
the air is too thin to provide any lift, no matter how fast you are
moving. For another, eventually the air is too thin to support
internal combustion engines.

> Explain to us what a "sweet spot" is.

Greatest distance covered per unit of fuel consumed, lowest wear and
tear on the aircraft (especially engines).

> Why is it that you feel the airlines dont take into account TIME
> when doing the preflight planing.

They do, but fuel costs more than time. That's why flights are longer
now than they used to be: airlines plan for fuel economy, not speed.

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[Andrew Sarangan]

On Feb 2, 4:24 pm, "xerj" > wrote:
> >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.

That is correct, but that was not your original question.

[alice]

On Feb 3, 9:42 am, Mxsmanic > wrote:
> Less fuel = less power.

MX, I am laughing now.I cant figure out if you are really that
ignorant or if you are just messing with me.The real reson jets fly
high is for speed.The higher they go, the faster they can go.Why do
you think jets measure cruise speed in Mach?What do you think happens
to Mach as a jet flys higher?Now explain how you can move a jet (Or
anything else for that mater, like a car) FASTER on LESS thrust?For
the 3rd time here MX, you have made a misconception and you are
thinking backwards.
>
>
> Jets have a service ceiling for several reasons. For one, eventually
> the air is too thin to provide any lift, no matter how fast you are
> moving. For another, eventually the air is too thin to support
> internal combustion engines.

MX, I am laughing even harder now!Can you explain the real reason
behind a jets service ceiling and what it is a function of?
>
> > Explain to us what a "sweet spot" is.
>
> Greatest distance covered per unit of fuel consumed, lowest wear and
> tear on the aircraft (especially engines).

??? I gotta ask you MX, what airline do you work for?Are you saying
they use LCR or CCR charts to determine power settings?I hate to argue
with you but when I was in initial training at my fist jet job, it was
explained (very compellingly) why we dont base cruise flight on
this.If you are doing something different, Id like to know the
reasoning behind it.
>
> > Why is it that you feel the airlines dont take into account TIME
> > when doing the preflight planing.
>
> They do, but fuel costs more than time. That's why flights are longer
> now than they used to be: airlines plan for fuel economy, not speed.

OK, Ill bite MX.Why are you claiming that speed is not a function of
economy.Has it not occured to you that the LONGER a plane is in the
air, the more wear and tear it is incuring?Also, the longer a plane is
in the air, the more fuel it is burning.Which of course means the more
fuel it has to carry.Which means it is heavier.Which requires more
thrust.Tell me how this saves fuel again?
>
> --
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[Mxsmanic]

alice writes:

> The real reson jets fly high is for speed.

The real reason they fly high is for fuel economy.

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[Newps]

alice wrote:
For
> the 3rd time here MX, you have made a misconception and you are
> thinking backwards.

The third time? You must be new around here.

[alice]

On Feb 3, 11:13 am, Mxsmanic > wrote:
> alice writes:

>
> The real reason they fly high is for fuel economy.

OK MX, You are just toying with me.And, you got the last laugh.I am
kinda curious who you fly for and what they taught you in regards to
long range cruise.The only way your statements would work out might be
in a lightly loaded biz jet or something.Clue us in here.
KM
>
> --
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[alice]

On Feb 3, 11:27 am, Newps > wrote:
> alice wrote:
>
> For
>
> > the 3rd time here MX, you have made a misconception and you are
> > thinking backwards.
>
> The third time? You must be new around here.

Yes I am new. I stumbled across this list by accident.Just got
suckered in by someone who likes to argue I guess.

[Andrew Sarangan]

On Feb 3, 10:36 am, "alice" > wrote:
> On Feb 2, 7:42 pm, "xerj" > wrote:
>
> > > What is interesting is that 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
>
> > 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.


John Denker is a highly respected author and his book provides some of
the clearest explanation of the aerodynamics without resorting to
complex mathematics. This particular section is not about how
airplanes are flown in every day life, but an indepth exploration of
the factors that influence power, density, drag and AOA. Just because
you operate a jet does not make you an expert in aerodynamics.
Engineers and scientists build airplanes, and pilots operate them.
"Practice is not a substitute for understanding, nor vice versa" ,
which is eloquently stated in the Introduction section of his book.
If you are able to provide a better explanation, please do so, but it
seems to me that it is you who needs to spend some time reading up on
the basics.

[Peter Dohm]

> > > >
> > > For the 3rd time here MX, you have made a misconception
> > > and you are thinking backwards.
> >
> > The third time? You must be new around here.
>
> Yes I am new. I stumbled across this list by accident.Just got
> suckered in by someone who likes to argue I guess.
>
MX is the resident Troll on two groups that I read, and
reportedly an unknown number of others.

There was an interesting discussion about the matter a month
or two ago, and there is an excellent article in Wikipedia at
http://en.wikipedia.org/wiki/Internet_troll

In any case, please don't feed him any more than necessary.

Peter
(It's devilishly difficult!)

[xerj]

>> 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.

[xerj]

>> 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.
>
> That is correct, but that was not your original question.

Well, it's related to the original question, I think it's fair to say.

[alice]

On Feb 3, 12:15 pm, "Andrew Sarangan" > wrote:
> On Feb 3, 10:36 am, "alice" > wrote:
>
> John Denker is a highly respected author and his book provides some of
> the clearest explanation of the aerodynamics without resorting to
> complex mathematics.

Andrew, This is open to debate.I went and read some of this "Book".It
proves the old adage dont believe everything you read on the web.You
need to understand that Denker is just expressing his opinions, and
this could be why he doesnt use any math to back it up.That being
said, I can respect Denkers opinions but they are just that.I would
like to know where some of his assumptions come from.

>This particular section is not about how
> airplanes are flown in every day life, but an indepth exploration of
> the factors that influence power, density, drag and AOA.

I can appreciate the fact that Denker is using a classroom situation
to prove a point, but in order for his explination to be true, you
have to suspend certain realities about how a plane flies.Tell me how
this helpfull.

> Just because
> you operate a jet does not make you an expert in aerodynamics.

Ouch, dude I never claimed to be an expert did I?Now I will claim to
know what I need to know to fly an airplane, and I have seen Denkers
type before.He thinks you have to go deep into therory in order to be
a safe pilot, and ironically, he doesnt always get the therory
right.Mabe there is a happy medium somewhere between being able to
design a plane, and just going out and flying it.

> If you are able to provide a better explanation, please do so, but it
> seems to me that it is you who needs to spend some time reading up on
> the basics.

Better explination for what?Why do you asume that I do not have a
grasp of the basics?Are you another troll.
KM

[Jay Beckman]

"alice" > wrote in message
oups.com...
> On Feb 3, 11:13 am, Mxsmanic > wrote:
>> alice writes:
>
>>
>> The real reason they fly high is for fuel economy.
>
> OK MX, You are just toying with me.And, you got the last laugh.I am
> kinda curious who you fly for and what they taught you in regards to
> long range cruise.The only way your statements would work out might be
> in a lightly loaded biz jet or something.Clue us in here.
> KM

Here's your first clue (and it's the only one you'll really need...):

- He only flys for Microsoft Airways...

That ought to clear up a few things, eh?

BTW, welcome to the group. Feel free to post early and often.

Jay Beckman
PP-ASEL
Chandler, AZ

[Andrew Sarangan]

>
> Better explination for what?Why do you asume that I do not have a
> grasp of the basics?Are you another troll.


Explanation for why power requirement increases with altitude for the
same AOA. 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. At least you can explain
what the false assumptions are and what is your correct explanation
is. You can't just claim that something is wrong without providing an
explanation. That's what a troll is.

[Thomas Borchert]

Mxsmanic,

> Good physicists can explain any principle of physics without resorting
> to math.
>

Jeeze, and now you're a physicist, too? This is such obvious BS.

But go ahead, explain quantum physics to us without math. You coud
actually make A LOT of money writing a book about it that way.

--
Thomas Borchert (EDDH)

[Mxsmanic]

Thomas Borchert writes:

> But go ahead, explain quantum physics to us without math.

I'm fairly good at vulgarizations, but others are better. Try reading
Richard Feynman's lectures, or Issac Asimov's many vulgarizations of
complex topics that include physics. Einstein could also explain
things well if needed. Hawking does well in some of his work for the
general public. Many run-of-the-mill physicists are lost when asked to
explain things, however--presumably they lack the intelligence to do
so.

The reality is that people who actually understand physics can explain
it without resorting to math. The ones who use math are those who
have learned only the math, and have no intuitive grasp of the
subject. They are all too common these days.

> You could actually make A LOT of money writing a book about it that way.

Some people have made a fair amount of money, although physics for the
masses isn't a hot topic. I'm not really interested in writing a book
at this time, although I've had stuff published in magazines. I have
some essays available for free download on my site on various topics
(not physics, currently, though).

--
Transpose mxsmanic and gmail to reach me by e-mail.

[Thomas Borchert]

Mxsmanic,

your delusional qualities never cease to amaze me.

Oh, and in case you're wondering, yes, I do have a masters degree in physics, so I know what I'm
talking about. You don't. As usual.

--
Thomas Borchert (EDDH)

[Danny Deger]

"xerj" > wrote in message
...
>I was trying to explain to a non-pilot why increased power is required with
>altitude. She said "isn't the air thinner up there so there isn't as much
>resistance?" I said "yes, but the plane needs to fly fast enough for the
>air over the wings to feel like it does down low. So the speed required
>goes up you get higher. More speed need more power."
>
> This didn't really do the trick.
>
> Can someone think of a better way of putting it without resorting to
> mathematics and an explanation of IAS and TAS?
>
> TIA
>

To fly the same IAS requires the same power. 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.

Danny Deger

[Mxsmanic]

Thomas Borchert writes:

> Oh, and in case you're wondering, yes, I do have a masters
> degree in physics, so I know what I'm talking about.

I don't recall saying anything about you.

--
Transpose mxsmanic and gmail to reach me by e-mail.

[F.Reid]

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.

[Thomas Borchert]

Mxsmanic,

> I don't recall saying anything about you.
>

I don't, either.

--
Thomas Borchert (EDDH)

[Andrew Sarangan]

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.

[xerj]

> 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.

[Danny Deger]

"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

[F.Reid]

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

[xerj]

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

Same thrust, not same power.

[xerj]

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.

[Danny Deger]

"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

[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
>
>

[Danny Deger]

"Peter Dohm" > wrote in message
...
>> I was trying to explain to a non-pilot why increased power is required
> with
>> altitude. She said "isn't the air thinner up there so there isn't as much
>> resistance?" I said "yes, but the plane needs to fly fast enough for the
> air
>> over the wings to feel like it does down low. So the speed required goes
> up
>> you get higher. More speed need more power."
>>
>> This didn't really do the trick.
>>
>> Can someone think of a better way of putting it without resorting to
>> mathematics and an explanation of IAS and TAS?
>>
> In a word, NO.
>
> It is an issue of physics, and physics uses a lot of math.
>
> To maintain the same TAS, she is right--untill IAS drops to the back side
> of
> the power curve for the altitude at which she is then flying.
>
> To maintain the same IAS, the power requirement will only increase
> linearly
> in proportion to TAS with increasing altitude--until mach number becomes a
> consideration (at some significant fraction of unity)
>

No, same IAS, same drag, same thrust, same power requirement from the engine
to generate the thrust. The statement that power is drag time velocity is
incorrect. That is the point where the error is made.

Danny Deger

[Danny Deger]

"xerj" > wrote in message
...
>> 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.

Power is net force time velocity. Thrust equals drag, net force is zero.
The energy change of the airframe overtime is zero. All energy from the
engine is going into the air. The power to move air to make the same thrust
is the same regardless of velocity. Same IAS, same engine power
requirement. Look at some aircraft performance charts.

Danny Deger
>

> 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.
>

[Mxsmanic]

xerj writes:

> That being: power = thrust x distance/time, i.e. p = t x v.

You might want to double-check the strict definitions of work, power,
and thrust. As I recall, thrust is a force. A force acting through a
distance is work. The rate at which work is performed over time is
power. So you are confusing work with power. Thrust times distance
is work. The rate at which work is performed is power.

--
Transpose mxsmanic and gmail to reach me by e-mail.

[xerj]

> No, same IAS, same drag, same thrust, same power requirement from the
> engine to generate the thrust. The statement that power is drag time
> velocity is incorrect. That is the point where the error is made.

All of the definitions of power that I have seen have been along the lines
of P = T * V, or something that equates to that.

For instance:-

"The formula for Thrust Horsepower (THP) is:
THP = D x V"

from http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page4.html.

That is wrong?

[xerj]

> You might want to double-check the strict definitions of work, power,
> and thrust. As I recall, thrust is a force. A force acting through a
> distance is work. The rate at which work is performed over time is
> power. So you are confusing work with power. Thrust times distance
> is work. The rate at which work is performed is power.

I'm not confusing work with power. Yes, force times distance is work. But
force times distance over time is power. That is why thrust horsepower is
thrust times velocity, which is distance over time.

[Mxsmanic]

Danny Deger writes:

> The power is the net force time velocity.

Power is the rate at which work is performed per unit of time. Watts
and horsepower are examples of power.

> The total force on the airframe is zero because thrust cancels drag.

Thrust and drag are both forces, as you correctly surmise.

When a force acts through a distance, it performs work. And the
amount of work performed over a given period of time is power. Power,
work, and thrust are all different. Thrust is a type of force (so is
drag).

> 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.

That seems logical. IAS decreases with altitude for a given throttle
setting because engine power diminishes. At the same time, however,
TAS can increase because air density diminishes. For a given
aircraft, there is some altitude at which the variables combine to
provide best performance (highest speed over the ground in still air),
and there is also an altitude at which variables combine to provide
best fuel economy (lowest fuel consumption per mile). These two
altitudes are not generally the same.

--
Transpose mxsmanic and gmail to reach me by e-mail.

[Mxsmanic]

xerj writes:

> All of the definitions of power that I have seen have been along the lines
> of P = T * V, or something that equates to that.

Yes. I may have misread your previous post as "distance/time" meaning
"distance or time" (not distance over time).

Force * distance = work
Work / time = power
Thrust = force

A constant IAS requires constant power to maintain at any altitude. A
constant TAS requires constant power to maintain at only one altitude;
if the altitude increases, the power required diminishes, and vice
versa. The power produced by most powerplants diminishes with
altitude; the thrust they can maintain at a given IAS varies directly
with the power.

I think I have that right. It's easy to get confused.

--
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[Mxsmanic]

xerj writes:

> I'm not confusing work with power. Yes, force times distance is work. But
> force times distance over time is power.

Yes, agreed. I misunderstood you.

> That is why thrust horsepower is thrust times velocity, which is distance
> over time.

"Thrust horsepower" is a misnomer. Horsepower is power, thrust is
force. That may be the source of some confusion. It's kind of like
saying "newton watts."

--
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[Casey Wilson]

"Mxsmanic" > wrote in message
...

>
> That seems logical. IAS decreases with altitude for a given throttle
> setting because engine power diminishes. At the same time, however,
> TAS can increase because air density diminishes. For a given
> aircraft, there is some altitude at which the variables combine to
> provide best performance (highest speed over the ground in still air),
> and there is also an altitude at which variables combine to provide
> best fuel economy (lowest fuel consumption per mile). These two
> altitudes are not generally the same.
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

NOTICE!!!!
Mxsmanic is NOT a pilot, has NEVER flown an aircraft and is NOT qualified to
issue competent information regarding any aspect of the operation of any
aircraft.

[Mxsmanic]

Casey Wilson writes:

> NOTICE!!!!
> Mxsmanic is NOT a pilot, has NEVER flown an aircraft and is NOT qualified to
> issue competent information regarding any aspect of the operation of any
> aircraft.

Experience in flying real aircraft is irrelevant to this discussion.
Reading a physics textbook would be a lot more useful than flying an
aircraft for this topic.

If you feel compelled to point out who is flying a simulator and who
is flying a real aircraft, at least save yourself some trouble and
mention it only when it actually has a bearing on the discussion at
hand.

Some of the people in this discussion are right, and some are wrong,
and frankly I don't see any correlation between who is right and who
is wrong and who is a pilot and who isn't.

--
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[Roger[_4_]]

On Fri, 02 Feb 2007 11:51:34 GMT, "xerj" > wrote:

>I was trying to explain to a non-pilot why increased power is required with

Increased power is not needed and not normally obtainable at higher
altitude with a normally aspirated engine. It takes less power to
maintain speed at altitude compared to lower. If you just maintain
power you go faster than you do down lower.

>altitude. She said "isn't the air thinner up there so there isn't as much
>resistance?" I said "yes, but the plane needs to fly fast enough for the air
>over the wings to feel like it does down low. So the speed required goes up
>you get higher. More speed need more power."
>
>This didn't really do the trick.
>
>Can someone think of a better way of putting it without resorting to
>mathematics and an explanation of IAS and TAS?

"I think" you are confusing the difference between IAS and TAS at
altitude versus power at altitude, or as Dennis already suggested,
throttle position compared to power.

>
>TIA
>
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Casey Wilson]

"Mxsmanic" > wrote in message
...

>
> A constant IAS requires constant power to maintain at any altitude. A
> constant TAS requires constant power to maintain at only one altitude;
> if the altitude increases, the power required diminishes, and vice
> versa. The power produced by most powerplants diminishes with
> altitude; the thrust they can maintain at a given IAS varies directly
> with the power.
>
> I think I have that right. It's easy to get confused.
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

NOTICE!!!!
Mxsmanic is NOT a pilot, has NEVER flown an aircraft and is NOT qualified to
issue competent information regarding any aspect of the operation of any
aircraft.

[alice]

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.

Here is a good way to look at it. Posters on this thread referenced
Denkers material several posts ago and yet we still dont have an
answer.Is everyone on this thread stupid, or was Denker not being very
clear.Can you explain what Denker was talking about?
>
> 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.

I think the reason his book is free is because no one would pay for
it.Think about it, it is too tecnical for some, and for others it is
not tecnical enough (or accurate enough).I think he would have a
pretty slim market if he were to publish it.Another big detraction is
the way Denker jumps back and forth between aerodynamic theory, and
giving dual instruction.This iritating and I think he should stick to
one or the other.
As far as "benifiting greatly" from this book, I gotta ask you, are
you a pilot?Not that it really maters, but it would explain why you
dont understand peoples issue with this material.

> If you think my asking him to provide an explanation is
> an insult, then I don't know what to say.-

I think your sugesting I am a know it all is an insult.I think your
sugestion I go back to the basics is an insult.Why dont you START with
the basics.

[Mxsmanic]

Casey Wilson writes:

> NOTICE!!!!
> Mxsmanic is NOT a pilot, has NEVER flown an aircraft and is NOT qualified to
> issue competent information regarding any aspect of the operation of any
> aircraft.

And you, I presume, are not a physicist, a mathematician, or an engine
mechanic.

--
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[xerj]

> Increased power is not needed and not normally obtainable at higher
> altitude with a normally aspirated engine. It takes less power to
> maintain speed at altitude compared to lower. If you just maintain
> power you go faster than you do down lower.

TAS most definitely increases. In a round about way, I was talking about
IAS. My understanding, and I'm pretty sure of it although I've been told
otherwise here, is that to maintain the same IAS (and thus dynamic pressure)
at a higher altitude, you need more power. I don't mean throttle position --
for the sake of the argument I am leaving density effects on engine power
output aside.

[Danny Deger]

"xerj" > wrote in message
...
>> No, same IAS, same drag, same thrust, same power requirement from the
>> engine to generate the thrust. The statement that power is drag time
>> velocity is incorrect. That is the point where the error is made.
>
> All of the definitions of power that I have seen have been along the lines
> of P = T * V, or something that equates to that.
>
> For instance:-
>
> "The formula for Thrust Horsepower (THP) is:
> THP = D x V"
>
> from http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page4.html.
>
> That is wrong?
>

You can certainly define a term called Thrust Horse Power as thrust x
velocity. And this link definition of Brake Horse Power is correct (torque
times RPM). But there is no reason to think these terms are equal in an
aircraft. A great deal of the power out of the engine (all of the power if
in steady state level flight) goes into the air and not the airframe. It is
my understanding that for a given thrust at a given IAS (actually Equivelant
Air Speed, EAS, is the better term), the engine power requirement is
basically the same for different altitudes. I wish I had a good aircraft
performance handbook to confirm this.

Danny Deger
>
>
>

[Danny Deger]

"Danny Deger" > wrote in message
...
>
P.S. I have a Master's in Aerospace and have worked in the industry for many
years. I will admit most of my schooling and experience was with jets and
rockets -- not pistons and props. But I do recall the equations and
techniques to calculate engine horsepower required for various flight modes
of a prop plane was VERY complex. I am CERTAIN equating thrust horsepower
(thrust times velocity) to brake horse power (torque time RPM) is wrong.
Anyone have an aircraft performance chart to look at the IAS for 75% power
at sea level and at altitude?? I am not going to say it will be exact, but
I think it will be close.

Danny Deger

[F. Reid]

> TAS most definitely increases. In a round about way, I was talking about
> IAS. My understanding, and I'm pretty sure of it although I've been told
> otherwise here, is that to maintain the same IAS (and thus dynamic pressure)
> at a higher altitude, you need more power.

X, I hate to sound discouraging, but you may not find an answer here.I
looked on two websites and referenced the book Aerodynamics for naval
aviators, and they kinda contradicted each other.I think you are
looking for a real world answer to a hypothetical situation.The IAS or
dynamic pressure on a plane WILL decrease with altitude.Take a look at
a typical plane doing 300 KIAS at 10 thousand.The TAS will be within
about 40 KTS of this.Now climb up to FL350 and the KIAS will be about
230 with a TAS of about 475 (Roughly). Now you do need more power but
the point about IAS is mute (Or hypothetical) because you cant
indicate 300 KTS at 350.The part about maintaining the same AOA isnt
gonna happen either.I hope someone can explain this better.

[xerj]

> P.S. I have a Master's in Aerospace and have worked in the industry for
> many years. I will admit most of my schooling and experience was with
> jets and rockets -- not pistons and props. But I do recall the equations
> and techniques to calculate engine horsepower required for various flight
> modes of a prop plane was VERY complex.

Yeah, those damn eggbeaters hanging out the front make it all pretty
complicated. I most certainly DON'T have a Master's in Aerospace. I find it
slightly comforting that a guy that does says it's complex.

Thanks for taking the time to answer.

> I am CERTAIN equating thrust horsepower (thrust times velocity) to brake
> horse power (torque time RPM) is wrong. Anyone have an aircraft
> performance chart to look at the IAS for 75% power at sea level and at
> altitude?? I am not going to say it will be exact, but I think it will be
> close.

Do you mean working back from TAS to get an IAS?

I looked up a Navajo information manual. There's a chart True Airspeed vs
Density Altitude. I chose the line for 260 BHP which is around 75% of the
350 BHP engines.

At sea level the TAS is shown as around 207 MPH (have to interpolate, it's a
grid that goes up in 10s). That is obviously the IAS as well.

At 20,000, the TAS is close to 250 MPH. The inferred IAS is 184.

Any thoughts?

[Danny Deger]

"xerj" > wrote in message
...
>> P.S. I have a Master's in Aerospace and have worked in the industry for
>> many years. I will admit most of my schooling and experience was with
>> jets and rockets -- not pistons and props. But I do recall the equations
>> and techniques to calculate engine horsepower required for various flight
>> modes of a prop plane was VERY complex.
>
> Yeah, those damn eggbeaters hanging out the front make it all pretty
> complicated. I most certainly DON'T have a Master's in Aerospace. I find
> it slightly comforting that a guy that does says it's complex.
>

Jets and rockets are actually much easier to do design work on than prop
planes. The jet produces thrust, which is the thrust used to propel the
plane. Calculate the thrust required then it is a simple step to calculate
fuel flow from the engine to get the thrust. With a prop, exactly what
happens as you convert rotation power into thrust is complex, complex,
complex.

> Thanks for taking the time to answer.
>
>> I am CERTAIN equating thrust horsepower (thrust times velocity) to brake
>> horse power (torque time RPM) is wrong. Anyone have an aircraft
>> performance chart to look at the IAS for 75% power at sea level and at
>> altitude?? I am not going to say it will be exact, but I think it will be
>> close.
>
> Do you mean working back from TAS to get an IAS?
>
> I looked up a Navajo information manual. There's a chart True Airspeed vs
> Density Altitude. I chose the line for 260 BHP which is around 75% of the
> 350 BHP engines.
>
> At sea level the TAS is shown as around 207 MPH (have to interpolate, it's
> a grid that goes up in 10s). That is obviously the IAS as well.
>
> At 20,000, the TAS is close to 250 MPH. The inferred IAS is 184.
>
> Any thoughts?
>

See my other posts. I stand corrected. For a given engine power, IASI
drops off with altitude. For a jet, IASI does not drop off for a given
engine thrust as the plane climbs. Maybe that is an inherent reason jets
are faster at altitude than a prop.

Danny Deger

[Orval Fairbairn]

In article >,
"Danny Deger" > wrote:

> "xerj" > wrote in message
> ...
> >> No, same IAS, same drag, same thrust, same power requirement from the
> >> engine to generate the thrust. The statement that power is drag time
> >> velocity is incorrect. That is the point where the error is made.
> >
> > All of the definitions of power that I have seen have been along the lines
> > of P = T * V, or something that equates to that.
> >
> > For instance:-
> >
> > "The formula for Thrust Horsepower (THP) is:
> > THP = D x V"
> >
> > from http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page4.html.
> >
> > That is wrong?
> >
>
> You can certainly define a term called Thrust Horse Power as thrust x
> velocity. And this link definition of Brake Horse Power is correct (torque
> times RPM). But there is no reason to think these terms are equal in an
> aircraft. A great deal of the power out of the engine (all of the power if
> in steady state level flight) goes into the air and not the airframe. It is
> my understanding that for a given thrust at a given IAS (actually Equivelant
> Air Speed, EAS, is the better term), the engine power requirement is
> basically the same for different altitudes. I wish I had a good aircraft
> performance handbook to confirm this.


That is incorrect! A classic problem in sophomore aero engineering is to
determine the maximum altitude at which an aircraft will fly,
simplifying the problem by assuming turbosupercharging to allow constant
power and discounting compressibility effects, given its stall IAS and
lift/drag curves.

At very high altitudes a plane will fly very fast at low IAS (min porew
required speed/alpha.

The power = speed*thrust is valid and is a basic tenet of aero
engineering.

[Orval Fairbairn]

In article >,
"Danny Deger" > wrote:

> "Danny Deger" > wrote in message
> ...
> >
> P.S. I have a Master's in Aerospace and have worked in the industry for many
> years. I will admit most of my schooling and experience was with jets and
> rockets -- not pistons and props. But I do recall the equations and
> techniques to calculate engine horsepower required for various flight modes
> of a prop plane was VERY complex. I am CERTAIN equating thrust horsepower
> (thrust times velocity) to brake horse power (torque time RPM) is wrong.
> Anyone have an aircraft performance chart to look at the IAS for 75% power
> at sea level and at altitude?? I am not going to say it will be exact, but
> I think it will be close.
>
> Danny Deger

Danny,

Go back to "Airplane Performance Stability and Control," by Perkins &
Hage, John Wilet & Sons, NY, London (1949).

[Danny Deger]

"Orval Fairbairn" > wrote in message
...

snip

> The power = speed*thrust is valid and is a basic tenet of aero
> engineering.

How much power is a B-29 generating during a full throttle run-up before
takeoff :-)

Having said this, I agree now that the equation accurately calculates the
increased power required to fly the same IAS at a higher altitude.

Danny Deger

[Morgans]

"Danny Deger" > wrote

> How much power is a B-29 generating during a full throttle run-up before
> takeoff :-)

That goes back to the basic definition of HP. You have to do work on an
object, since HP is the amont of time required to lift an {whatever weight}
object to {whatever} height.

The noise and heat being produced, and the fuel being burned suggests
something else, though. :-o <g>
--
Jim in NC

[RST Engineering]

I'd suggest a REALLY GOOD flight review next time you need one with a REALLY
GOOD instructor who knows the relationship between power, altitude, and true
airspeed.

Or did ..maniac assume a pseudonym?

Jim



"xerj" > wrote in message
...
>I was trying to explain to a non-pilot why increased power is required with
>altitude. She said "isn't the air thinner up there so there isn't as much
>resistance?" I said "yes, but the plane needs to fly fast enough for the
>air over the wings to feel like it does down low. So the speed required
>goes up you get higher. More speed need more power."

[Matt Whiting]

Morgans wrote:
>
> "Danny Deger" > wrote
>
>> How much power is a B-29 generating during a full throttle run-up
>> before takeoff :-)
>
>
> That goes back to the basic definition of HP. You have to do work on an
> object, since HP is the amont of time required to lift an {whatever
> weight} object to {whatever} height.

No, horsepower is not an amount of time. It is a rate of doing work. I
can lift 100 lbs 10' in one minute or I can lift 100,000 lbs 10' in one
minute. Same time, but vastly different amounts of HP required.

Matt

[xerj]

"RST Engineering" > wrote in message
...
> I'd suggest a REALLY GOOD flight review next time you need one with a
> REALLY GOOD instructor who knows the relationship between power, altitude,
> and true airspeed.
>
> Or did ..maniac assume a pseudonym?

As you can probably see by the length of the thread, this one has gone on
for a while. I was talking about trying to explain the phenomenon of how to
keep the same IAS you need higher power as you increase altitude, but
without actually referring to concepts like IAS and TAS. I wasn't talking
about TAS increasing with altitude. I should have made that clearer in the
initial post. The whole thing started when a non-pilot friend of mine asked
how high planes could fly. I said that it depended on a few things, one of
them being the fact that engine power decreases with altitude because of air
density. At some point you'll hit an altitude where it can no longer
generate the power required for level flight. HOWEVER, even if you have an
engine that was turbocharged so well that it didn't lose power with altitude
all the way up to space, you'll still probably hit a limit (unless the
engine was extremely powerful) because the actual power required goes up
with altitude as well. Is there anything unreasonable in what I have just
said there? I don't think so. Where I went wrong is in not explaining my
question well enough, and people very reasonably thinking that I either
meant TAS or how open the throttle has to be.

As has been established, I am now certain that my understanding of power,
altitude and TAS was and remains correct. I was merely seeking a good layman
non-mathematical explanation of it. Hardly a major sin, I think.

[Danny Deger]

"xerj" > wrote in message
...
>
> "RST Engineering" > wrote in message
> ...
>> I'd suggest a REALLY GOOD flight review next time you need one with a
>> REALLY GOOD instructor who knows the relationship between power,
>> altitude, and true airspeed.
>>
>> Or did ..maniac assume a pseudonym?
>
> As you can probably see by the length of the thread, this one has gone on
> for a while. I was talking about trying to explain the phenomenon of how
> to keep the same IAS you need higher power as you increase altitude, but
> without actually referring to concepts like IAS and TAS. I wasn't talking
> about TAS increasing with altitude. I should have made that clearer in the
> initial post. The whole thing started when a non-pilot friend of mine
> asked how high planes could fly. I said that it depended on a few things,
> one of them being the fact that engine power decreases with altitude
> because of air density. At some point you'll hit an altitude where it can
> no longer generate the power required for level flight. HOWEVER, even if
> you have an engine that was turbocharged so well that it didn't lose power
> with altitude all the way up to space, you'll still probably hit a limit
> (unless the engine was extremely powerful) because the actual power
> required goes up with altitude as well. Is there anything unreasonable in
> what I have just said there? I don't think so. Where I went wrong is in
> not explaining my question well enough, and people very reasonably
> thinking that I either meant TAS or how open the throttle has to be.
>
> As has been established, I am now certain that my understanding of power,
> altitude and TAS was and remains correct. I was merely seeking a good
> layman non-mathematical explanation of it. Hardly a major sin, I think.
>

I would just leave it that the engine looses power with altitude and not
mention the fact that for a propeller aircraft the power required also goes
up. Keep in mind for a jet the thrust required does not go up with
altitude. More output being required is strictly for props.

Danny Deger

[Danny Deger]

"Danny Deger" > wrote in message
...
>
> "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.
>

Don't listen to me. I was wrong. Same IAS at altitude requires more power.

> Danny Deger
>
>
>