Flaps on take-off and landing

"Grumman-581" writes:

Maybe in *your* plane, but *my* plane only gets a 2-3 kt decrease in stall
speed... Not worth the trouble unless you need to drop into a really short
field with trees or a power line at the approach end of it...

Why do large commercial jets seem to have huge flaps with many
settings, whereas small planes have tiny flaps with one or two
settings, or none at all?

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Mxsmanic wrote:
"Grumman-581" writes:

Maybe in *your* plane, but *my* plane only gets a 2-3 kt decrease in stall
speed... Not worth the trouble unless you need to drop into a really short
field with trees or a power line at the approach end of it...

Why do large commercial jets seem to have huge flaps with many
settings, whereas small planes have tiny flaps with one or two
settings, or none at all?


Isn't it obvious?


L.

Leonard Milcin Jr. writes:

Isn't it obvious?

If it were obvious, I wouldn't ask.

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"Mxsmanic" wrote in message
...
Why do large commercial jets seem to have huge flaps with many
settings, whereas small planes have tiny flaps with one or two
settings, or none at all?

Flaps enhance lift at the expense of drag. On a small airplane, large
complex flaps would not produce a significant enough reduction in drag
during cruise flight to justify the cost, complexity, and weight.

However, the larger and faster the airplane, the more there can be
accomplished by reducing drag significantly during cruise, especially
compared to the airfoil required to land such planes safely and within the
runways available to them (generally no longer than a couple of miles or
so).

You could land a 747 without flaps, but you'd use a LOT more pavement (maybe
double?), runway length that just isn't available. On the other hand, you
could design a 747 with an airfoil that allowed for shorter landings, but
cruise speed would suffer. The airplane is large enough and fast enough
that the extra expense and weight of flaps more than makes up for its cost
during cruise, while still allowing for reasonable landing performance.

Hopefully this one example has answered the general question of "why do
large airplanes have features not found on small airplanes?" You could
spend months asking that same question, using different features, and the
answer would always be the same: economics and usefulness.

Pete

Peter Duniho writes:

Flaps enhance lift at the expense of drag. On a small airplane, large
complex flaps would not produce a significant enough reduction in drag
during cruise flight to justify the cost, complexity, and weight.

However, the larger and faster the airplane, the more there can be
accomplished by reducing drag significantly during cruise, especially
compared to the airfoil required to land such planes safely and within the
runways available to them (generally no longer than a couple of miles or
so).

You could land a 747 without flaps, but you'd use a LOT more pavement (maybe
double?), runway length that just isn't available. On the other hand, you
could design a 747 with an airfoil that allowed for shorter landings, but
cruise speed would suffer. The airplane is large enough and fast enough
that the extra expense and weight of flaps more than makes up for its cost
during cruise, while still allowing for reasonable landing performance.

Thanks. That makes sense.

Hopefully this one example has answered the general question of "why do
large airplanes have features not found on small airplanes?" You could
spend months asking that same question, using different features, and the
answer would always be the same: economics and usefulness.

You're saying that there really isn't any technical, aerodynamic
reason why a large aircraft would require extensive flaps while a
small aircraft would not? That is, the advantages and disadvantages
from a flying standpoint are the same in both cases?

I know there are economic considerations, but since small private
planes seem to handle quite differently from large planes I was
wondering if there are fundamental differences in the aerodynamics
that might be related to scale (physical dimensions). That is, would
a giant version of a small plane, three times as big but with
identical proportions and size-to-weight ratio, fly in the same way?

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On Thu, 14 Sep 2006 07:55:05 +0200, Mxsmanic
wrote:

Similarly, I retract the
flaps completely as soon as I'm completely on the runway for landing,
so that the wind doesn't push me around. Is this the correct way to
do it?

The first plane with flaps that I flew was a PA-18 Super Cub. When I
reached for the flap lever (it has a name, but I forget), what I got
was a rocket from my instructor in the back seat. "Don't touch
anything till you stop rolling!" Of course the PA-18 is a taildragger
and more sensitive to a pilot's inattention.

A year later I took Damian Delgaizo's bush-flying course in Andover
NJ. The plane was an Aviat Husky with a similar flap lever, and we
made much greater use of it. The splendid trick was to fly over a
cornfield (whatever) in ground effect, then dump the flaps as soon as
the mains crossed the threshold of the (grass) field, so that the
Husky stopped within 250 feet.

Johnson Bar


You can also use the flaps to "jump over" a fence or ditch
that you see at the last moment before a forced landing if
they are up for the glide.

But the flaps move the center of lift aft on the wing and
you need back elevator pressure to force the main gear
(trike) or tailwheel on the ground to prevent weather vaning
and skidding the wheels under braking. The flaps will tend
to lift the lift the tail and you need to follow through on
the flare to lever the balance point on the main gear.

A tailwheel airplane will use some forward elevator for a
wheel landing, but when the tail comes down [or is lowered]
you still need the stick full aft. Of course during taxi
you must use the ailerons and elevator to control for cross
and tailwinds.


"Cubdriver" usenet AT danford.net wrote in message
...
| On Thu, 14 Sep 2006 07:55:05 +0200, Mxsmanic

| wrote:
|
| Similarly, I retract the
| flaps completely as soon as I'm completely on the runway
for landing,
| so that the wind doesn't push me around. Is this the
correct way to
| do it?
|
| The first plane with flaps that I flew was a PA-18 Super
Cub. When I
| reached for the flap lever (it has a name, but I forget),
what I got
| was a rocket from my instructor in the back seat. "Don't
touch
| anything till you stop rolling!" Of course the PA-18 is a
taildragger
| and more sensitive to a pilot's inattention.
|
| A year later I took Damian Delgaizo's bush-flying course
in Andover
| NJ. The plane was an Aviat Husky with a similar flap
lever, and we
| made much greater use of it. The splendid trick was to fly
over a
| cornfield (whatever) in ground effect, then dump the flaps
as soon as
| the mains crossed the threshold of the (grass) field, so
that the
| Husky stopped within 250 feet.
|

Jim Macklin wrote:
...
You can also use the flaps to "jump over" a fence or ditch
that you see at the last moment before a forced landing if
they are up for the glide.
...
Yes and that's one lesson I'll never forget. The instructor had me do a
power-off "short approach" and told me I was *not* allowed to apply
power no matter how bad I thought it was! The pucker factor increased
enormously and I could see the edge of the hill under the runway
looming. Certain that we were going to crash into it, I was begging to
add power. The instructor calmly demonstrated how to use flaps just to
get a few more feet and complete the landing. It was a good lesson but
I think he owes me some new underwear.

Stubby wrote
The instructor calmly demonstrated how to use flaps just to
get a few more feet and complete the landing. It was a good
lesson but I think he owes me some new underwear.

Best glide distance with flaps is always less that best glide
distance flaps up. Bad demonstration...bad lesson learned.
The reason that most GA airplanes use flaps is to increase
drag and steepen the glide angle without increasing the speed.

Bob Moore

Gliding with the flaps up and about 5 knots faster than
"best glide" allows the pilot to "stretch" the glide by a
slight increase in pitch attitude bringing the speed to the
best glide speed. If you fly at best glide you have no
option to stretch the glide since any increase of decrease
in speed will steepen the glide.

Also, if there is any wind, you want minimum sink if flying
downwind and you need a faster speed when flying into the
wind. Extreme example, wind 50 knots, airspeed 50 knots,
flight path vertical with no forward progress.

Flap extension reduces the stalling speed, often only a few
knots, but lift increases as does drag. The airplane will
balloon and slow very fast, the extra distance and altitude
gained is only a few feet, but if all you need is to clear a
5 foot cattle fence or a 10 foot wide ditch, that is a
technique that is useful.


--
James H. Macklin
ATP,CFI,A&P

"Bob Moore" wrote in message
. 122...
| Stubby wrote
| The instructor calmly demonstrated how to use flaps just
to
| get a few more feet and complete the landing. It was a
good
| lesson but I think he owes me some new underwear.
|
| Best glide distance with flaps is always less that best
glide
| distance flaps up. Bad demonstration...bad lesson learned.
| The reason that most GA airplanes use flaps is to increase
| drag and steepen the glide angle without increasing the
speed.
|
| Bob Moore