Infamous Downwind Turn
John T. Lowry
response on the same subject in rec.aviation.piloting.
For me, the convincing reason there is NO DIFFERENCE
between a downwind turn and an upwind turn is: **all the relevant
FORCES in the turn (except gravity, which doesn't change) are
aerodynamic.** With those forces being only between the airframe
or control surfaces or propeller and the AIR, what possible effect
could anything else have? It's only forces which make
accelerations which are changes in velocity. This of course
assumes the exact same air mass motion (no gusts or wind shear),
the same control inputs, etc., throughout the two (to downwind or
to upwind) turn maneuvers.
Now the airplane DOES slow down (in air speed) in a turn,
due to increased induced drag due to larger lift due to bank, but it
doesn't slow down any MORE in a downwind turn than in an
upwind one.
Phil's insistence on a **coordinated** turn is very
important in a practical sense. Say you turned by kicking hard
rudder, with a real big rudder, so that you skidded around 180
degrees in (say) two seconds. (Strange to say, one airline pilot told
me another airline pilot told him that this slewing-around skidding
turn is just the thing to do!) THEN you've got a problem all right.
You've then got (almost) your original 50 knots of ground speed,
in the same direction (now backwards), plus a 20 knot tailwind.
Seventy knots air speed, but a NEGATIVE 70 knots air speed with
reference to the direction the airplane is pointed. You'd "fall out of
the sky." However, there is no difference in this foolish maneuver
whether it's a turn to downwind or upwind. In the other case,
where you'd initially have 70 knots air speed and 90 knots ground
speed, an instantaneous skidded reversal would leave you at 90
knots ground speed backwards and a 20 knot headwind, net result
still the exact same negative 70 knots air speed with reference to
the direction the airplane is pointed. So still a real bad idea to skid
around, but the SAME bad idea whether to downwind or upwind.
There's no AERODYNAMIC difference between turns to
dowwind or to upwind. The only differences are between their final
ground speeds, and in the perception of the pilot who unwittingly
slows his air spseed when he notices, by looking at the earth,
increased ground speed during and after his turn to downwind.
Hope this clarifies. Sometime later on I'll address the
question "Where does the additional kinetic energy (with respect to
the earth, an (approximate) inertial frame of reference), after a
downwind turn, come from?" It DOES come, even though all the relevant
forces, as argued above, are the same. It's a subtler issue.
John Lowry.
--
John T. Lowry, PhD
Flight Physics; Box 20919; Billings MT 59104
Voice: 406-248-2606
Trevor Fenn
"John T. Lowry" wrote:
> Voice: 406-248-2606]
So who the hell is Gavan Cook and why dredge up this tired old topic again.
It's been done to death.
Trevor Fenn
trev...@erols.com
mee...@hotmail.com
Perhaps better to look at the topic in relation to the ILLUSIONS
created during low level turns downwind and upwind.
The apparent skid can be a problem.
Meeka69.
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Greg Scholes
>There is only a problem when the pilot performs this manoeuvre with
reference
>to a point on the ground. If it's a constant rate co-ordinated turn there
can
>be no change from head-wind to tail-wind, or vice-versa.
>
>That's where the arguments about it being dangerous fail (nearly said "fall
down").
In theory this may be true but I can assure you it does not hold true in
practice. There is a period of transition where the groundspeed remains
constant and the airspeed changes when a turn is made in strong wind. A
good example is approaching RW34 in Melbourne via overhead Essendon with a
strong northerly. When you turn final the IAS will kick up commensurate
with the 'new' headwind component with no change in groundspeed INITIALLY.
This is not theory; I see it on a normal days work.
What is being forgotten in this theoretical argument is the transition
period. When the aircraft has a groundspeed of say 200 kts with a TAS of
200kts and then turns into a headwind of 40kts, the groundspeed WILL stay
constant for a period because of inertia. The aircraft does not 'instantly'
slow to a groundspeed of 160 kts. The IAS will go up commensurate with the
increase in wind and how quickly it changes (rate of turn). If the wind
changes at a slow rate, inertia will not be obvious. If it changes
quickly, inertia will be very obvious.
The mass of the aircraft is a key factor. A lightie at 2000 kg and 90 kts
is going to notice inertia a lot less than a heavy at 200,000 kg and 200
kts.
A B747 once lost 65kts of IAS on final into SYD due to windshear. The only
thing that saved them was they were flying a precalculated minimum
groundspeed in anticipation of a shear. Had they been flying at Vref they
could have had a very serious accident.
Greg Scholes
Wayne
you are getting windshear mixed up with the common myth of turning ing a
constant wind.As you say the jumbo on approach suffered windshear ie,meeting
different wind velocities as it continued it's approach. Windshear is a
different circumstance to your theory. Take note of a previous poster when
he mentioned a number of continuous 360 turns in a strong constant wind. You
don't honestly believe the IAS will fluctate if doing continuous orbits in a
jumbo while in a 180kt jetstream [orbit = constant angle of bank turn ] The
reference groundspeed procedure you mention is for the effects of windshear.
You say it does not hold true in practice because of some transition period
,I don't think that is the case. If you are on base with a 65kt tailwind, no
IAS is lost during the turn onto final except that which can be put down to
the extr drag of the turn, OR if the is windshear present. The impression I
have is that you fly heavies, so next time you have a large track change
while in a jetstream,watch the last digit of the machmeter....you will note
no change
regards
Wayne
Greg Scholes wrote