An elementary landing / braking doubt

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

"Ramapriya" wrote in message
oups.com...
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.

Flaps only improve lift to a point. Even on the little planes, past that
point they simply add drag.

So yes, what you're seeing is the use of flaps to create a lot of drag. The
spoilers accomplish that too, and at the same time "spoil" the lift the wing
might otherwise create (with or without the flaps).

How much of each does what, I can't say. Suffice to say, with spoilers and
flaps fully extended, there's LOTS of drag, and very little lift.

Pete

duniho wrote:

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. [...]

Flaps only improve lift to a point. Even on the little planes, past that
point they simply add drag. [...]

That only explains why one might prefer 40degrees over 30degrees of
flaps. There is still lift generated at both those settings.

This is why, on some small airplanes, the official short-field landing
procedure involves raising flaps on rollout. That way, lift is
reduced and maximum weight is applied to the main wheels where the
brakes are. People flying retractable-gear airplanes are sometimes
taught not to bother, in order to avoid playing with the gear selector
instead by mistake.


- FChE

The answer to you question is primarily that the flaps allow the
aircraft to gly and touch down at a lower airspeed due to the
additional lift they produce.
As a result if the airplane touches down 10 mph slower then that is 10
knots less that it has to decelerate on the runway and a lot less
energy that the brakes and tires have to absorb.

Brian
CFIIG/ASEL

What you are saying is true. It is for this reason that some people retract
flaps after touchdown when doing short field landings. However, the lifting
effect of flaps after touchdown is pretty minor. The lift decreases as the
square of the airspeed. So lift drops off very fast as you decelerate.



"Ramapriya" wrote in news:1103888251.673617.173970
@f14g2000cwb.googlegroups.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

As a result if the airplane touches down 10 mph slower then that is 10
knots less that it has to decelerate on the runway and a lot less
energy that the brakes and tires have to absorb.

----------------------------

Agree.

Energy is a function of velocity squared, so touching down at, say, 40K ground
speed instead of 50k results in about 35% less energy to dissipate on rollout -
dramatically shortening the landing roll (and wear-and-tear on the machine).


Engineer and Former CFII

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

"Peter Duniho" wrote in message
...
"Ramapriya" wrote in message
oups.com...
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.

Flaps only improve lift to a point. Even on the little planes, past that
point they simply add drag.


Just a nitpick here- there's a huge difference in design and efficiency
between the simple flaps you see on most small planes and the fowler jobs
found on transport jets. If you put slats and fowlers on a typical GA
plane's wing you'll get a STOL monster like the Helio Courier.

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

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