An elementary landing / braking doubt

[Ramapriya]

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 :o)
Ramapriya

[Peter Duniho]

"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

[Frank Ch. Eigler]

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

[Brian]

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

[Andrew Sarangan]

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 :o)
> Ramapriya
>
>

[CVBreard]

>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

[steve.t]

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

[C Kingsbury]

"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.

[Frankster]

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 :o)
> Ramapriya
>
>

[Roger]

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 :o)

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

[Peter Duniho]

"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).

[Peter Duniho]

"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

[BTIZ]

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 :o)
> Ramapriya
>
>

[Mackfly]

>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

[[email protected]]

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