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

"Frank Ch. Eigler" wrote in message
...
That only explains why one might prefer 40degrees over 30degrees of
flaps. There is still lift generated at both those settings.

Who said the planes in question are limited to 40 or 30 degrees of flap
extension?

This is why, on some small airplanes, the official short-field landing
procedure involves raising flaps on rollout.

Small airplanes don't have flaps that can be extended far enough to
dramatically increase drag. If they did, you'd probably find manuals that
recommend extending the flaps further, rather than retracting them all the
way (and few manuals actually recommend doing that, as far as I know).

Pete

From: "Peter Duniho"

says Small airplanes don't have flaps that can be extended far enough to
dramatically increase drag.

Manuals or not -- a 40 degree flap, Cessna 172 with her nose held high during
roll out will get the "large" flaps to something like 60 degrees to the air
flow. Try it and see. And it will stop as fast that way as retracting them.
When you can't keep the nose up any longer then go to flaps up and bear down on
the brakes. Now the tiny little Piper flaps may not do much in the area of
drag. Far as I can tell they don't do much of any thing. Oh yeah, something
to round out the check list. That is what Piper put them there for. Back when
Piper built planes for men-----ha ha ha----- like the Pawnee they must of been
thinking of drag cause there ain't much the Pawnee's flaps can do for lift.
Now if someone would put a brake on the nose wheel ya might get some real
braking action---weight transfer and all that. mac

"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.

"C Kingsbury" wrote in message
link.net...
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 understand that. However, if a C172's flaps were designed to extend to 80
degrees, rather than the existing 30 or 40 degree limit (depending on model
and STC), you'd find that after landing it would be desireable to extend the
flaps from the landing setting of 40 degrees to the "high drag" setting of
80 degrees (or whatever).

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

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

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