leading edge flaps

"Dudley Henriques" wrote...

The Blues bolted 'um shut. Can you imagine what would happen in a tight
diamond with an A4 if a wing position got an asymmetrical slat extension
with roll induced....say in a barrel roll? Not a pretty thought!! :-))

In over 1700 A-4 hours, I never had an asymmetric slat extension that I could
not quickly and easily control. After about 1000 hours, few of them were even
unpredictable... Careful preflight of the slats would give a VERY good idea of
which one would come out first, and any preflight "stickiness" was grounds for
rejecting the airplane or having the Airframers work on it before flight. I
believe too many TA-4s were lost in the Training Command due to students'
unfamiliarity with the airplane and instructors' failure to teach and emphasize
both preflight and recovery techniques.

That said, I fully understand why the Blues bolted them in -- their 36"
wingtip-to-fuselage clearance in some of their formation maneuvers gave quite a
bit less room for error than our nominal 3-5' wingtip-to-wingtip clearance in
the fleet.

On Sun, 11 Jan 2004 20:14:26 GMT, "John R Weiss"
wrote:

"Dudley Henriques" wrote...

The Blues bolted 'um shut. Can you imagine what would happen in a tight
diamond with an A4 if a wing position got an asymmetrical slat extension
with roll induced....say in a barrel roll? Not a pretty thought!! :-))

In over 1700 A-4 hours, I never had an asymmetric slat extension that I could
not quickly and easily control. After about 1000 hours, few of them were even
unpredictable...

I wonder if you could try to describe, as well as you can recall, what
exactly happens during an asymmetrical slat extension?

The reason I am asking such an obvious sounding question is that we
recently had a discussion concerning asymmetric slat extension of a
BF-109 in a Finnish newsgroup. While the immediate lay-man's reaction
is that the aircraft would, of course, violently roll away from the
extended slat (=extended slat up), because of the increased lift by
the slat, the situation becomes less obvious the more I think about
it.

Theoretically, from the textbook figure illustrating the effect of
trailing edge flaps and leading edge slats, one could argue that
nothing happens, since slats (unlike flaps) do not increase Lift
Co-efficient (CL) on a given Angle of Attack (AOA) but only increase
the maximum attainable CL. This at least in the case that the net wing
area does not increase when slat extends.

If the wing area increases (as probably is the case with A-4, judging
from the photo I have) as the slat extends, the aircraft would tend to
roll extended slat up. Right?

However, if the geometry is such that the leading edge moves down as
the slat extends, one could argue that the AOA of the profile
decreases causing roll towards the extended slat.

Or, if the slat extension causes a change of pressure distribution
around the aileron (the aileron snatch reported by the British BF-109
test pilots ?) moving the ailerons (probably not on the hydraulic
irreversible(?) control system of the A-4) until the pilot corrects
it, the roll could be either way.

Mikko

"Mikko Pietilä" wrote...

In over 1700 A-4 hours, I never had an asymmetric slat extension that I could
not quickly and easily control. After about 1000 hours, few of them were even
unpredictable...

I wonder if you could try to describe, as well as you can recall, what
exactly happens during an asymmetrical slat extension?

That part is easy -- the airplane rolls toward the side with the unextended (or
less-extended) slat.

The difficulties arise when the slat extension is either at greater than 1 G,
very close to another airplane, and/or in the hands of an unexperienced pilot at
the top of a loop. Higher G causes a higher roll rate. If you're in close
formation and do not correct in time, you might roll into the other airplane.
If you put in the wrong control corrections at low speed (e.g., at the top of a
loop), you may depart, stall, and/or spin the airplane. Most of the training
Command mishaps related to asymmetrical slat extension, with which I am
familiar, were of the latter variety (departure or loss of orientation and
control in "unusual" attitudes).

Normal correction was to simply "pop" the stick slightly to one side, opposite
the roll, and the other slat would extend normally. However, if (due to a poor
preflight) the second slat was sticky enough to not deploy, and the AOA was
maintained high enough to keep the first one all the way out, an unexperienced
pilot could lose control. Otherwise, the airplane was controllable with
asymmetric slats.

.. . .

If the wing area increases (as probably is the case with A-4, judging
from the photo I have) as the slat extends, the aircraft would tend to
roll extended slat up. Right?

Right.

Though the dynamics may be a bit more complicated than apparent from a simple
illustration, the net result of slat extension, in all the airplanes I've flown
that had them, is an increase in lift. Contributing to the increase a

Increased wing camber
Increased effective wing area
Energized airflow through the slot delays flow separation further back on
the wing


However, if the geometry is such that the leading edge moves down as
the slat extends, one could argue that the AOA of the profile
decreases causing roll towards the extended slat.

Though the apparent AOA may decrease, I suspect the other factors prevail.
Remember that with an aerodynamically-controlled slat, the wing is already at a
relatively high AOA when it deploys. I am not an aerodynamicist, but somebody
else may be able to give some insight into the relative contributions of the
different factors.


Or, if the slat extension causes a change of pressure distribution
around the aileron (the aileron snatch reported by the British BF-109
test pilots ?) moving the ailerons (probably not on the hydraulic
irreversible(?) control system of the A-4) until the pilot corrects
it, the roll could be either way.

I'm not familiar with the "aileron snatch" you describe, but I think I can
picture it in my mind. I would guess that has to do with the point at which
flow separation occurs on the wing. If flow is instantly, and asymmetrically,
restored over the aileron, the ailerons may be pulled in that direction, causing
the opposite roll. If this is the case, the AOA at which the slats start to
deploy, and the rate at which they deploy, would likely be adjusted in the
design phase.

"John R Weiss" wrote:

"Mikko Pietilä" wrote...

In over 1700 A-4 hours, I never had an asymmetric slat extension that I could
not quickly and easily control. After about 1000 hours, few of them were even
unpredictable...

I wonder if you could try to describe, as well as you can recall, what
exactly happens during an asymmetrical slat extension?

That part is easy -- the airplane rolls toward the side with the unextended (or
less-extended) slat.

The difficulties arise when the slat extension is either at greater than 1 G,
very close to another airplane, and/or in the hands of an unexperienced pilot at
the top of a loop. Higher G causes a higher roll rate. If you're in close
formation and do not correct in time, you might roll into the other airplane.
If you put in the wrong control corrections at low speed (e.g., at the top of a
loop), you may depart, stall, and/or spin the airplane. Most of the training
Command mishaps related to asymmetrical slat extension, with which I am
familiar, were of the latter variety (departure or loss of orientation and
control in "unusual" attitudes).

Normal correction was to simply "pop" the stick slightly to one side, opposite
the roll, and the other slat would extend normally. However, if (due to a poor
preflight) the second slat was sticky enough to not deploy, and the AOA was
maintained high enough to keep the first one all the way out, an unexperienced
pilot could lose control. Otherwise, the airplane was controllable with
asymmetric slats.

. . .

If the wing area increases (as probably is the case with A-4, judging
from the photo I have) as the slat extends, the aircraft would tend to
roll extended slat up. Right?

Right.

Though the dynamics may be a bit more complicated than apparent from a simple
illustration, the net result of slat extension, in all the airplanes I've flown
that had them, is an increase in lift. Contributing to the increase a

Increased wing camber
Increased effective wing area
Energized airflow through the slot delays flow separation further back on
the wing


However, if the geometry is such that the leading edge moves down as
the slat extends, one could argue that the AOA of the profile
decreases causing roll towards the extended slat.

Though the apparent AOA may decrease, I suspect the other factors prevail.
Remember that with an aerodynamically-controlled slat, the wing is already at a
relatively high AOA when it deploys. I am not an aerodynamicist, but somebody
else may be able to give some insight into the relative contributions of the
different factors.


Or, if the slat extension causes a change of pressure distribution
around the aileron (the aileron snatch reported by the British BF-109
test pilots ?) moving the ailerons (probably not on the hydraulic
irreversible(?) control system of the A-4) until the pilot corrects
it, the roll could be either way.

I'm not familiar with the "aileron snatch" you describe, but I think I can
picture it in my mind. I would guess that has to do with the point at which
flow separation occurs on the wing. If flow is instantly, and asymmetrically,
restored over the aileron, the ailerons may be pulled in that direction, causing
the opposite roll. If this is the case, the AOA at which the slats start to
deploy, and the rate at which they deploy, would likely be adjusted in the
design phase.


Very interesting...
--

-Gord.

"John R Weiss" wrote in message

snip
If this is the case, the AOA at which the slats start to
deploy, and the rate at which they deploy, would likely be adjusted in the
design phase.

This is really good stuff, Weiss.