An elementary landing / braking doubt

"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).

"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

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

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

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