Flaps and V-Tails of Death

Michael wrote:

Robert Ehrlich wrote
Except for the presence of the runway instead of unlandable terrain below
the glider, this is exactly the situation of my question, and so the answer,
as I suspected, is that you are out of options.

I think that's rather overstated. Unless you have botched the pattern
so throroughly that you are at 30 degrees of flap and coming up short,
you do exactly what you do in a spoiler-only ship - you retract the
flaps. The difference is this - you can't just retract completely, as
you would spoilers, because retracting that last 30 degrees is going
to cause a transient drop you won't like. However, going from 90
degrees to 45 causes no loss of lift at all, and a huge loss of drag -
thus causing airspeed to increase, which allows lifting the nose and
dramatically flattening the approach. This was certainly the case in
my HP-11, and I can't imagine it would be much different on other
flaps-only gliders.

This will work on approach, when you have the nose down attitude
corresponding to the flaps setting but not in the situation of my
initial question, i.e. just after a botched flare, a few feet above
the runnway and no nose down attitude. In this case retracting the
flaps will cause a loss of drag, but no so huge, i.e. the drag will
much more than in zero flaps or zero spoilers configuration and there
is no gain in lift and no change in the stall speed, you don't have
the altitude that you can convert into speed, so I think that you are
going to fall on the ground, with a slighly increased delay compared
to what would happen if you didn't retract some flaps.

Earlier, Robert Ehrlich wrote:

This will work on approach, when you have the nose down attitude
corresponding to the flaps setting but not in the situation of my
initial question, i.e. just after a botched flare, a few feet above
the runnway and no nose down attitude. In this case retracting the
flaps will cause a loss of drag, but no so huge, i.e. the drag will
much more than in zero flaps or zero spoilers configuration and there
is no gain in lift and no change in the stall speed, you don't have
the altitude that you can convert into speed, so I think that you are
going to fall on the ground, with a slighly increased delay compared
to what would happen if you didn't retract some flaps.

I have gone from plus 60 degrees to full negative, and then back to
plus 90, all within a few feet of the runway. It is rarely necessary
to do that (it sure wasn't when I tried it), but it can be done. All
it takes is speed, distance, careful modulation of pitch attitude and
a flare for the dramatic.

I think that what it boils down to is that everybody who has flown a
flapped aircraft knows that "sinking feeling" that you get when you
reduce the flap deflection, and most of them think that there's
nothing they can do about it. And that's wrong. If you apply the right
pressure to the stick in the right direction at the right time, you
can go from zero to full flaps and back again with no added or
subtracted vertical acceleration. This is, of course, slightly limited
by the longitudinal inertia of the aircraft; but is basically true for
all practical flap application rates. All it takes is practice.

Flaps are not magical lift-conjuring and lift-disappearing devices.
They are just a way of modulating the effective camber of the airfoil
section so as to change its L/D polar. All you need to do with the
stick is to adjust the angle of attack so as to maintain a constant L
value as you modulate the flaps to achieve the desired D value. And
that value can get very large indeed.

The only odd corner to flapped ships is that they, like most gliders
(all that I know of) stall at a higher speed with the flaps retracted
than with flaps deployed. So it is true that if you are just above
stall speed and you bring the flaps up, you could stall even without
changing the speed or loading. For example, the book values show the
HP-18 stalling at 35 mph at flaps 60, and 40 mph at flaps 0 (both at
770 lbs gross). However, it is expected that when you're operating
near the ground you have much more margin over stall than that 5 mph
difference. Gusts and/or wind gradient effects will often account for
more than twice that.

Bob K.
http://www.hpaircraft.com

Robert Ehrlich wrote
This will work on approach, when you have the nose down attitude
corresponding to the flaps setting but not in the situation of my
initial question, i.e. just after a botched flare, a few feet above
the runnway and no nose down attitude. In this case retracting the
flaps will cause a loss of drag, but no so huge, i.e. the drag will
much more than in zero flaps or zero spoilers configuration

I do not concur. Imagine, for example, a similar situation involving
a glider with approach flaps and spoilers, such as the Blanik L-13.
It is normally landed with full flaps, and spoilers are used for
glideslope control. It is also a common primary trainer, so the
situation you cite comes up with some regularity. The solution is to
retract the spoilers - but you still have flaps hanging out. How is
this any different from retracting Schreder-type flaps to 30 degrees?

and there
is no gain in lift and no change in the stall speed, you don't have
the altitude that you can convert into speed, so I think that you are
going to fall on the ground, with a slighly increased delay compared
to what would happen if you didn't retract some flaps.

Actually, you will fly onto the ground, and if you're not careful you
will float. With the flaps retracted to 30 degrees, the HP-11
certainly had a tendency to float even when brought in at very low
airspeed. In fact, after some experimentation I came to the
conclusion that if my flap mechanicsm were to ever fail with the flaps
at less than 30 degrees, I would need about 2 miles of runway to stop.

Michael

Michael wrote:

Robert Ehrlich wrote
This will work on approach, when you have the nose down attitude
corresponding to the flaps setting but not in the situation of my
initial question, i.e. just after a botched flare, a few feet above
the runnway and no nose down attitude. In this case retracting the
flaps will cause a loss of drag, but no so huge, i.e. the drag will
much more than in zero flaps or zero spoilers configuration

I do not concur. Imagine, for example, a similar situation involving
a glider with approach flaps and spoilers, such as the Blanik L-13.
It is normally landed with full flaps, and spoilers are used for
glideslope control. It is also a common primary trainer, so the
situation you cite comes up with some regularity. The solution is to
retract the spoilers - but you still have flaps hanging out. How is
this any different from retracting Schreder-type flaps to 30 degrees?

and there
is no gain in lift and no change in the stall speed, you don't have
the altitude that you can convert into speed, so I think that you are
going to fall on the ground, with a slighly increased delay compared
to what would happen if you didn't retract some flaps.

Actually, you will fly onto the ground, and if you're not careful you
will float. With the flaps retracted to 30 degrees, the HP-11
certainly had a tendency to float even when brought in at very low
airspeed. In fact, after some experimentation I came to the
conclusion that if my flap mechanicsm were to ever fail with the flaps
at less than 30 degrees, I would need about 2 miles of runway to stop.


OK, I have no experience on this kind of ship, I was just trying to
figure how to handle this situation. What you are saying is that
in the range 30-90 degrees on this ship, flaps work like spoilers
on usual (from my point of view) gliders, i.e. retracting them to
30 degrees brings the drag at a level similar to that of a non-flapped
ship with spoilers closed, without change in lift. So the important
thing in order to be able to recover from the 3 most common mistakes
is to have the flaps at more than 30 degrees when you begin the flare.

Someone mentioned that landing with T or V tail is
better than conventional tail in tall crop.

I'd like to know of someone who has landed
in crop with a conventional tail. Any
stories out there?

It strikes me that the yaw of crop hitting a
wing is the real problem. A conventional tail,
if it grabs the crop BEFORE the wing does,
might actually keep the glider straighter,
kind of a keel effect?

Maybe in 2-3 foot crop this means the
conventional tail is damaged but T-tail isn't,
but in very tall crop, the conventional
tail is damaged and the glider
pancakes in straight in vs. yawing
and rolling T-tail (since a wing hits first).

Any opinions about this? Anyone
have anecdotes? I've wondered
the same thing about high-wing vs. low-wing
water landings in power aircraft...

Don't know about conventional tails. But I have been helping rebuild a
DG202 (T-tail) that landing in a tall wheat feild. It caught a wing
tip and turned sideways breaking nearly every part in the fuselage.
(Major Damage) Don't know if a conventional tail would have helped.

Brian

The trick with landing in tall growth is not flaring at the top of the
'crop' but continuing to fly down to ground level. I flared early and
'landed' at 1.5 metre above the deck. The resulting 'loud bang' was the
ASW20's main wheel being pushed up into the fuselage. Thankfully the
structure absorbed the impact and I got away without any back injury.

Of course the problem with flying into the tops of uneven tall growth is the
potential to catch a wingtip and 'groundloop' at 1 metre with the resulting
potential for 'major' damage and resulting injury. In hindsight I'm not sure
that I didn't actually inadvertently do the right thing.

My incident happened when I changed my mind on finals just to be closer to
the field access road. Dumb, dumb, dumb, I was closer to the access road but
the damage took 6 weeks to repair!

Ian



"Brian Case" wrote in message
om...
Don't know about conventional tails. But I have been helping rebuild a
DG202 (T-tail) that landing in a tall wheat feild. It caught a wing
tip and turned sideways breaking nearly every part in the fuselage.
(Major Damage) Don't know if a conventional tail would have helped.

Brian

Mark wrote
It strikes me that the yaw of crop hitting a
wing is the real problem.

You're completely right about this. I got my LS-6 after its previous owner
tried to land in 8 foot high corn. The fuselage broke in 3 places, before
anything hit the ground. The major problem is catching a wing tip and then
performing a flying ground loop. My wing took the sudden reversal in direction
very well and was just scraped up a bit. The fuselage (mainly the tail mass)
wasn't able to turn that fast and failed (in compression) on the inside, of the
very rapid left turn that was commanded when the left wing tip hit the corn.
That's a massive moment arm (25 feet). The spar is tremendously strong when
loaded in this way (for and aft) but the fuselage isn't. The ASH-25 has an arm
of 43 feet and one must never place ones wing tip in any crops. Plowed ground
is my choice, because DIRT DON'T HURT.


JJ Sinclair

In article ,
(Michael) wrote:

Imagine, for example, a similar situation involving
a glider with approach flaps and spoilers, such as the Blanik L-13.
It is normally landed with full flaps

It's a few years since I've flown a Blanik, but I've flown 5 different
L-13s a total of 110 flights. I'd estimate that at least 105 of those
landings would have been with zero flap.

-- Bruce