Vapour trails

Corky Scott wrote:

On Thu, 16 Dec 2004 16:02:55 -0700, Newps wrote:


You don't think we have a problem?

Of course not. Why would you think we do? Why would you think that we
could change it one way or another?


I didn't say we could change it, but that doesn't mean we don't have a
problem.

Then you are not part of the global warming crowd. They believe we have
directly affected the earths temp.

On Wed, 15 Dec 2004 at 16:52:25 in message
, Peter Duniho
wrote:
As altitude goes up and indicated airspeed remains constant, TRUE airspeed,
on the other hand, goes up. Same lift (equal to weight, as you note), but
you're going faster for the same power. Obviously thrust didn't increase
(and in fact, decreased, since you get less thrust from the prop due to the
less dense air...though with a constant speed prop, much if not all of the
lost thrust can be regained using coarser prop pitch), so the only way to go
faster is for drag to have decreased.

That is true I agree but lift and drag both depend on TAS. Do you claim
that that lift does, but drag does not depend on TAS?

Since lift is constant, maximum lift/drag ratio still occurs at the
particular angle of attack where drag is minimized. But the ratio is
higher, because drag is lower. It's the angle of attack that's constant,
not the ratio itself.

But if drag is lower then lift should also be lower.

Interesting Peter but you have not yet convinced me. If you do a test on
an airfoil you will determine its lift and drag coefficients. Apart from
effects due to Reynolds number and compressibility those figures apply
to all conditions. The plot of CL against CD remains the same - why
should it change? You are effectively saying that an aircraft with an
L/D of say 12 has a much higher maximum lift drag ratio at high altitude
- how much 25 or more?

Does a high performance glider with a normal Lift/drag max of 50 have an
even higher one at high altitude?

Since lift and drag both depend directly on indicated airspeed (which
is merely a correction due to air density) for given IAS the ratio of
lift and drag will be produced at the same AoA at all altitudes. There
is a substantial difference in kinematic viscosity from sea level to
high altitude and that will change the Reynolds Number characteristics
although the higher true speeds also change the Reynolds number at least
partly in compensation - how much I am not sure but it will not change
lift and drag separately AFAIK.

Don't forget that both lift and drag depend on true airspeed and on air
density. It is the concept of dynamic pressure (0.5 *density *
velocity^2) which gives us the convenience of IAS.

I would like to get this a bit straighter in my mind. My theoretical
studies are so long ago that things may have changed and so may I!
--
David CL Francis

On Thu, 16 Dec 2004 at 03:36:14 in message
, Orval
Fairbairn wrote:

I seem to recall .002378 #m/ft3 as air density at STP.

That is in slugs per cubic foot. Both figures are correct. One in pounds
mass and the other in slugs which is a mass unit. Is the slug still
used? I used it a lot in my early days.

My old standard atmosphere data gives sea level at 0.07675 lb/cubic
foot.

Divide that by g (32.17 ft per second per second) gives 0.0023857 slugs
per cubic foot.
--
David CL Francis

"David CL Francis" wrote in message However if you fly for maximum range
than you fly close to maximum lift/drag ratio which depends only on
getting the correct alpha (ignoring compressibility effects).

If the correlation between thrust and fuel burn is fairly linear, this is
correct. A piston powered airplane with a constant speed propeller will
achieve max range at any altitude it can sustain the correct alpha angle.
Jets do not have a linear correlation. The jet I fly gets the same fuel burn
at 5000' and 250 KIAS as it does at FL350 and 440KTAS.

So since lift = weight, drag depends on weight and it reduces as fuel is
burned.

Remember that there are 2 kinds of drag- Parasite and Induced. Parasite drag
is dependent on speed. Induced drag is dependent on alpha angle (among other
things).

I suggest a book called 'Aerodynamics for Naval Aviators'. Most good pilot
shops have it.

D.

On Sat, 18 Dec 2004 at 03:35:40 in message
, Capt.Doug
wrote:
Remember that there are 2 kinds of drag- Parasite and Induced. Parasite drag
is dependent on speed. Induced drag is dependent on alpha angle (among other
things).

Yes but induced drag depends on Lift coefficient (the square of it
approximately) so if you fly at maximum Lift Drag the alpha and the
contribution of induced drag remain the same.

I have certainly not forgotten induced drag.
--
David CL Francis

Newps wrote:

You have no way of knowing that. I can just see the caveman out there
with his CO2 test kit.....

Actually, we know quite precisely how the climate was *way* before
cavemen appeared on earth.

Obviously you don't have the slightest clue of recent scientific
methods. There's nothing wrong with this, I'm sure you know lots of
other things which I don't have a clue of. Just don't make any
statements about climatology.

Stefan

wrote:

Global warming is alarmist bunk. Even if the earth's average
temperature goes up, the earth will still be OK.

Certainly, the earth will still be ok. It's just us will have some problems.

Stefan

Stefan wrote:

Certainly, the earth will still be ok. It's just us will have some problems.

Along with most of what we call lifeforms.

George Patterson
The desire for safety stands against every great and noble enterprise.