|
Brent,
You
forgot to add the disclaimer of the author..."Now I'm not an expert on
airliners, but I am happy to expose my ignorance (best way to learn!) and so
here, for those interested, is my answer:"
I wouldn't
say he's ignorant, but he certainly does not understand the purpose of
flaps. While it is all very interesting of course regarding airplanes,
none of it applies to submersibles even in the context of "speed brakes".
Your original message with the subject title "Stopping Flaps" was a misnomer
since nobody is really talking about installing "flaps" as they appear on
airplanes. Whether anyone successfully installs "speed brakes" on a small
sub is another matter, but it is a more accurate term for what has been under
discussion and will help prevent a debate on airplane flaps which has no
relevance on this list.
Thanks,
Jon
"The purpose of flaps is to enable the
aircraft to fly more slowly without
stalling. Deploying flaps increases
both lift and drag. Use of flaps when landing thus enables a steeper approach
path, better visibility over the nose and a slower touch-down speed. The
slower the touch-down speed (perhaps 120~160kts for an airliner, or around
60~70kts for the smaller aircraft I fly), then the less braking and runway
length are required to complete the landing roll before safely exiting the
runway at taxi speed. Generally speaking, full-flap will be used for landing
as this provides maximum benefit in terms of reduced landing speed and extra
drag during the landing roll. (Flaps are also commonly used for take-off in
order to decrease the speed at which the wing starts to provide lift - but
only partial flap is used for take-off to avoid having too much drag whilst
trying to accelerate to take-off speed). A simple definition of aircraft flaps
is here: http://en.wikipedia.org/wiki/Flap_(aircraft)
The purpose of
spoilers is to decrease lift, while the purpose of air-brakes is to increase
drag without altering lift. Sometimes the two effects are combined. One of the
most difficult things to do in an aircraft is to "go down and slow down" at
the same time - spoilers and air-brakes help with this. In flight, air-brakes
are used in the approach phase to slow the aircraft down (it is common for
example to have speed restrictions in busy terminal control areas e.g. below
250kts when 30 miles out say, or max holding speed of 180kts below 10,000 feet
perhaps). The second use of air-brakes/spoilers is provide additional drag and
dump any residual lift during the landing roll, as you have noticed. See
simple definition here:
http://en.wikipedia.org/wiki/Air_brake_(aircraft).
When landing an
airliner, they need to combine the effects of flap (primarily to reduce
touch-down speed, secondarily to add some drag) with the effects of
spoilers/air-brakes (primarily to add drag, secondarily to dump lift once
safely on the ground) as well as reverse thrust, and wheel braking. The
shorter the runway / heavier the aircraft / faster the touchdown then the more
of these forces will be required at once.
Confusion here seems to stem
from the idea that flaps add lift, but during the landing roll we don't want
any more lift: the answer to this is that once the aircraft has touched down,
it is quickly decelerated (by combined effects of drag, reverse thrust and
wheel brakes) to less than the the stall-speed of the fully-flapped wing;
therefore the wing (by definition) is no longer producing enough lift to be
able to fly. Airline pilots have to calculate in advance a target touch-down
speed to be just above the stall speed as they reach the landing threshold -
for a given aircraft, this speed will vary considerably depending on the
weight (load, fuel, etc), runway length, surface type (e.g. grass has less
friction than tarmac) and external conditions (headwind, crosswind, gusts,
windshear, runway condition e.g. wet/dry etc).
Landing small aircraft
is far simpler of course, but we still have to make take-off and landing
performance calculations - see GASCO (General Aviation Safety Council) article
here on this:
http://www.gasco.org.uk/upload/docs/2005_1_Flight%20Safety16-18.pdf.
Regarding
landing without flaps: this is something that we practice because we have to
be prepared for situations where e.g. the flaps malfunction for some reason.
When landing without flaps, you have to make a flatter approach, you have less
forward visibility over the nose therefore, and you touchdown say 10kts faster
- which significantly increases the landing run (i.e. runway length required);
which is as you would expect given the explanatiom above
perhaps!"
http://hardyboysblog.blogspot.com/2005/07/why-do-we-use-flaps-when-landing.html
|