An elementary landing / braking doubt

Big airplane, lots of weight. Notice that the big guys have a HUGE
difference in their takeoff weight compared to their max landing
weight.

So with spoilers, they kill off the excess lift while making use of the
drag of the flaps (excess lift becomes an issue in ground effect). This
lessens the load on the gear as they touch down (as opposed to the solo
student slam down ;-) ). And if they really have to stop, I'm told they
don't touch the brakes until after full reverser deployment (I am
assuming here that from their touch down speed of 120-180 knots, they
kill off 30-50% via reverse thrust).

But, I'm still learning and I may not have all of this correct. But it
is how it all appears to me in thinking about the physics involved.
Later,
Steve.T
PP ASEL/Instrument

I subscribe to the theory that after slowing below stall speed, like during
the landing roll, flaps do not add any lift to speak of, mainly drag.

-Frank

"Ramapriya" wrote in message
oups.com...
Hi folks,

I've always seen spoilers and flaps being deployed fully upon landing.
While the reason for spoilers is straightforward, I haven't yet figured
why flaps are deployed too.

Isn't the landing roll the time when you'd be wanting all the load of
the craft to be on the main wheels, which is where the brakes are,
instead of creating lift whereby the load gets transferred onto the
wings and possibly lessening the braking effect? I know the plane would
be decelerating all the time with the engines throttled back fully and
even the forward thrust depolyed, perhaps, yet why create any lift
possibility at all? Wouldn't braking be more effective with no flaps
deployed? Or does the drag produced by the flaps compensate for the
lift?

I suspect I've missed something really fundamental )
Ramapriya

Don't make the mistake of thinking stalling speed rather
than angle of attack. The wing is generating lift while the airplane is
rolling on the runway even well below stall speed, simply because the
attitude is below stall angle. Flaps can add a lot of lift below stall
speed, as I could prove by getting a 172 off the ground with full flap
at around 40 MPH, in ground effect.
The 172's flaps reduce the stall speed mostly in the first 20
degrees, and the last 20 add mostly drag. There's only about 1 knot
stall difference between 20 and 40 degrees, so it's best to leave them
hanging out. If they're electric they retract too slowly to do much
good, and might actually hurt the stopping effort by removing drag and
contributing lift as they pass through 20 degrees. If it's an old 172
with manual flaps, dumping them on touchdown can help a lot.

Dan

On 24 Dec 2004 03:37:31 -0800, "Ramapriya" wrote:

Hi folks,

I've always seen spoilers and flaps being deployed fully upon landing.
While the reason for spoilers is straightforward, I haven't yet figured
why flaps are deployed too.

Isn't the landing roll the time when you'd be wanting all the load of
the craft to be on the main wheels, which is where the brakes are

I find that in the Deb, full flaps help shorten the roll out.
A short field landing is a steep approach with substantial power and
relatively slow. Vso for me alone and partial fuel is only 55 MPH ( ~
48 knots) at roughly 2700#. This is for a pretty slipery airplane.

In the Deb, as soon as the mains are on the runway, let the nose down,
get on the brakes and full up elevator. If you don't get on the
brakes first it's going right back up in ground effect which could
prove to be more than a little inconvenient.

instead of creating lift whereby the load gets transferred onto the
wings and possibly lessening the braking effect? I know the plane would

In many planes with electric flaps there is little if anything to be
gained by raising them as you are slowing to the point where they have
pretty much lost their effect well before they are all the way up.

be decelerating all the time with the engines throttled back fully and
even the forward thrust depolyed, perhaps, yet why create any lift
possibility at all? Wouldn't braking be more effective with no flaps
deployed? Or does the drag produced by the flaps compensate for the
lift?

Aerodynamic braking works great.

OTOH, flying an old Hershey Bar Winged Cherokee 180 with the Johnson
bar flaps, raising the flaps on touchdown on a short sod strip made a
considerable difference.


I suspect I've missed something really fundamental )

I think basically it depends on the plane.
Worked on the Cherokee, doesn't on the Deb.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
Ramapriya

flaps are used to change the shape of the airfoil.. allowing flight at
slower speed for approach and landing... lower speed on landing, less wear
and tear on the tires, less braking action required and less runway length
required. And with flaps, creating more drag, more power on approach is
normally required to fly a std glide path.

FWIW, my aircraft, standard approach configuration, leading edge slats, and
full flaps, approach speed was about 150knts depending on weight. A flaps
up, no slat approach was about 210knts.. again depending on weight of the
aircraft (how much fuel remaining).. and if the approach were carried
through to landing, would require double the runway and possible hot brake
fire. If no fire, a tear down of the braking system on each tire (8) would
be required to ensure no heat damage from high braking temperatures.

no flap approaches would be practiced, but to a missed approach.

BT

"Ramapriya" wrote in message
oups.com...
Hi folks,

I've always seen spoilers and flaps being deployed fully upon landing.
While the reason for spoilers is straightforward, I haven't yet figured
why flaps are deployed too.

Isn't the landing roll the time when you'd be wanting all the load of
the craft to be on the main wheels, which is where the brakes are,
instead of creating lift whereby the load gets transferred onto the
wings and possibly lessening the braking effect? I know the plane would
be decelerating all the time with the engines throttled back fully and
even the forward thrust depolyed, perhaps, yet why create any lift
possibility at all? Wouldn't braking be more effective with no flaps
deployed? Or does the drag produced by the flaps compensate for the
lift?

I suspect I've missed something really fundamental )
Ramapriya