Vapour trails

On Mon, 13 Dec 2004 at 03:20:38 in message
, Capt.Doug
wrote:

No, less air, because the density of the ambient air is less as altitude
rises. Less air in the front means less air out the back (though the
pressure ratio can be the same). Jet engines produce less thrust at
altitude. There is less cooling air which means that maximum exhaust
temperature is reached at a lower thrust. The efficiency gains come from the
forward speed of the engine (sort of a ram effect) and the lower aerodynamic
drag at altitude (higher true airspeed).

This interests me as it is often said, the idea of less drag at
altitude presumably comes from the idea that drag depends on air
density? Which of course it does. 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).

So since lift = weight, drag depends on weight and it reduces as fuel is
burned. The aircraft flies faster to create the lift at altitude but the
drag is presumably almost the same?

Am I wrong?
--
David CL Francis

"David CL Francis" wrote in message
...
[...]
So since lift = weight, drag depends on weight and it reduces as fuel is
burned. The aircraft flies faster to create the lift at altitude but the
drag is presumably almost the same?

Am I wrong?

Yes.

The drag is actually less. The indicated airspeed is a good way of seeing
how the airframe is currently being affected by the ambient air at whatever
density it is. Regardless of the air's actual density, the 1G stall speed
is always the same, and for constant engine power, cruise speed remains
remarkably constant (I'm not sure whether it is actually constant, but
having flown a turbocharged engine at altitudes up to 18,000' and noting an
airspeed drop only higher than 16,000', the turbocharger's "critical
altitude", I am confident in saying that, when measured by indicated
airspeed, there's practically no change as long as power is kept constant).

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.

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.

Pete

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

"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