On 30/05/2021 21:37, Vir Campestris wrote:
On 28/05/2021 20:02, Joey wrote:


"The Natural Philosopher" wrote in message
...
On 27/05/2021 20:59, Vir Campestris wrote:
but those are not really affected by any changes in engine 'efficiency'
In reality long haul airliners fly quite close to 'coffin corner'
slightly above stall speed

A long way above stall speed actually.

and slightly below mach 1 at as high an
altitude as they can because that reduces drag.
snip

I tried and failed to find a reference for that.

Given that an airliner's stall speed clean is over 150kt (easily found)
and the air pressure at cruising height is under a quarter that at sea
level (also easily found) I would expect them to be fairly close.

Andy.
In general an aircraft without flaps has around 2:1 ratio of top speed
to stall speed. Extra power in military aircraft pushes that towards 3:1
as does heavy implementation of flats and slats etc.

Its hard to get one single figure for stall speed from e.g. an airliner
as it depends on altitude, temperature, and aircraft weight as well as
what flaps etc are deployed, nevertheless a 747 lands at around 160mph
typically with all the gear out.
One may conjecture therefore that without the flappy bits out it would
be bear to its stall sped which gives a top speed of around 480 mph..at
a 3:1 ratio

(Whether or not you could fly a 747 at 480mph at ground level I do not know)

Now http://www.hochwarth.com/misc/AviationCalculator.html

tells me that at 45000 ft and Mach 0.8 the IAS is 215 knots or about
250mph, although TAS - true airspeed is over 500mph

That is not a huge amount above the IAS stall speed without flaps of say
180mph..a tight turn that increases the effective 'weight' of the
aircraft would be something to avoid

It's not the on the edge coffin corner that say a U2 used to fly at with
the difference between speed of sound and stall speed not much over
5mph! but it is a significantly smaller envelope than operation at lower
altitudes

In practice the designers do a lot of optimisation with one aim in mind
- minimising fuel and maximising income in terms of passenger miles and
high operational utility. I,e, mist passenger miles per day, since
interest on the capital cost of the aircraft accrues on a daily basis,
not on how many miles it flies!


--
In theory, there is no difference between theory and practice.
In practice, there is.
-- Yogi Berra