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.

Al wrote..
The Ventus Strut is pushing up.
Part of the preflight on my Ventus B is to push down on the inboard flap
both sides to feel the gas strut pressure.

So, what is the purpose of the air strut?
JJ Sinclair

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.

Let me ask you this, Robert. Put yourself 5 feet above ground, 1 knot
above stall in any glider. Flaps, spoilers, trailing edge air brakes,
drag chute, or whatever. It is probably not going to be a pretty
landing, is it? About all you can do is try and get the nose down a
bit and then rotate one last time to reduce the smiting the ground
will give you. But you asked a good question.

I personally like my flapped ships. If you do put yourself in the
situation (botched flare, bounce, etc) you can also roll on a bit more
flap just before the smiting and soften the blow a bit. Assuming, of
course, you weren't already at full flap. I have had to do this twice
in my 1100 hours of Zuni flying. Once in an off-field landing on a
gusty day with a strong wind shear encounter at about 50 feet (I think
I lost 15 to 20 knots, but I was too busy looking outside to know for
sure). I stopped with the tail less than 10 feet into the field. I
have pictures to prove it. And nothing was even so much as scratched.
The other was over a runway, again involving a bit of wind shear.
Works nice. Pull and crank.

I also have flapped time in an HP-16, and an HP-14. Spoilered time
includes Ka-6 and 604, among others.

Interesting safety note for you all to think about: We have all seen
incidents involving spoilers openning on tow. I bet you have never
seen an inadvertant flap deployment on tow, have you?

Steve Leonard
Second Highest Time Zuni Pilot
(Bob Whelan still has more hours in them than me)

To expand on the earlier statement I made that flaps essentially 'quit
working' if a glider so-equipped is landed 'too fast,' even a short-span
(55') HP-14 with original (long) length flaps exhibits the behavior.
Understand we're talking a rather fine point here when considering HP
gliders (which generally have LOTS of flap compared to other flapped gliders
with which I'm reasonably familiar...Zuni's, Concept 70's, Nuggets and
PIK-20A/B's).

At the time I sold my '14 I felt I could easily put it into any field an
experienced 1-26 pilot would go for...and some unavailable to 1-26's
(because of the HP's steeper approach capability). May not have been
correct, but that's the confidence I felt in the bird. (Prior to the HP, I
flew a 1-26; it, too, is a ship that helps pilots build confidence in the
basics.)

I'd encourage any driver of a flapped ship to experiment at the home field
to convince himself to what degree his or her ship exhibits the 'quits
working' behavior. Once you're comfortable and consistent making
_full-flap_ approaches, roundouts and touchdowns under reasonable
conditions, start adding a speed increment coming downhill. Use the same
aiming point as for your normal landings. For example, if you're consistent
at 45 knots, pick up the next landing to 50 knots, then to 55, etc. Do the
flare carrying the extra speed, so's to end up nose-down a foot or so off
the deck, waiting for the speed to bleed off. Not 'too far' above your
normal approach speed, you'll find you're amazed at how much farther the
bird will float before 'drag overcomes downwash.'

The weaker the flaps, the more noticeable the effect.

The only reason I mentioned the effect is because discovering it on a
short-field outlanding made in dicey conditions that may encourage the pilot
to carry some extra speed (e.g. gusty winds, conditions conducive to wind
shear, etc.) is likely to set off some more ill-considered 'flaps are spawn
of the devil' stories!

For the record, I never experienced an indicated airspeed change due to
windshear in my HP-14 when making full flap approaches. I _have_
experienced them in my Zuni making full flap approaches. I'm reasonably
satisfied the difference lies in the flaps...the HP's were draggier,
necessitating a considerably nose-lower attitude. Get nose down enough, and
horizontal wind shear speed deltas in essence decouple from the glider's
airspeed. Imagine being able to do a vertical approach - your touchdown
spot will change due to horizontal windshear, but not your indicated
airspeed.

Since not all windspeed changes occur in the horizontal plane, a prudent
pilot may opt to carry extra speed despite powerful flaps, but the way to
shoehorn into a small field under those conditions is to begin bleeding the
extra speed off earlier/higher in the flare.

Regards,
Bob W.


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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

"JJ Sinclair" wrote in message
...
Al wrote..
The Ventus Strut is pushing up.
Part of the preflight on my Ventus B is to push down on the inboard flap
both sides to feel the gas strut pressure.

So, what is the purpose of the air strut?
JJ Sinclair

Maybe it has no real purpose. The Ventus 2bx does not have a air strut on
the flap linkage (at least I have not seen one). Maybe S-H decided that one
was not needed.

Duane

On 20 Nov 2003 00:32:06 GMT, (JJ Sinclair) wrote:

Al wrote..
The Ventus Strut is pushing up.
Part of the preflight on my Ventus B is to push down on the inboard flap
both sides to feel the gas strut pressure.

So, what is the purpose of the air strut?
JJ Sinclair

So the flap pushrod can shorten when you open the brakes. It is much
the same in the Mini Nimbus and Mosquito and gas struts in gliders are
a disaster waiting to happen to you. When they fail it is usually with
no warning. Try getting one while at a contest in the boonies. I found
that the one from the hatch of a VW Golf (Rabbit in the US)fitted.

Mike Borgelt

I think that some folks got it right on the money
when they say that flapped ships are just different.
At some flap deflection (maybe 20, 30, 45 deg) the
wing may be better than 0 flap for floating down the
runway. So 0 flap or 90 flap may be great
for landing while something in between may cause
lots of floating.

Seems to make sense intellectually, but may take a little
getting used to. I'm glad someone mentioned
the PIK-20, we have one at our club and we
have a newer pilot getting used to it, but things
were quite different from the Blanik (our only flapped
trainer).

I was always under the impression that Fowler flaps
reduced stall speed, but didn't think plain flaps
did much other than just add drag. On the
Katana, AA-1 Grumman, and Tomahawk (all power planes)
they seem to do nothing but add drag. Interesting
to hear these experiences, and I'll certainly
look at the next HP I come across more carefully.

As far as V-tails go, anything to reduce wetted area
is good, right? ;-P

"Mark James Boyd" wrote in message
news:3fbc8231$1@darkstar...
As far as V-tails go, anything to reduce wetted area
is good, right? ;-P

I was under the impression that V tails don't reduce wetted area......
There is a number called tail volume coefficient. Distribution
of this area in a V ot T planform results in the same wetted area.
You might make the case that the V tails are of a higher aspect ratio
and "better" but you also have to account for crosswind performance.
The max crosswind you can land in is determined by rudder effectiveness.
(I'll skip the wing low attitude/long wing problem) A V tail reduces up
elevator authority as the crosswind component grows. So it appears that
a V tail gliders minimum approach speed go's up as a function of the
crosswind. There may be insufficient "elevator" area or pitch authority
to flare as opposed to straight line flight where both elevons/elevators
will be moving "up".

Scott

I was under the impression that V tails don't reduce wetted area......
There is a number called tail volume coefficient. Distribution
of this area in a V ot T planform results in the same wetted area.
You might make the case that the V tails are of a higher aspect ratio
and "better" but you also have to account for crosswind performance.
The max crosswind you can land in is determined by rudder effectiveness.
(I'll skip the wing low attitude/long wing problem) A V tail reduces up
elevator authority as the crosswind component grows. So it appears that
a V tail gliders minimum approach speed go's up as a function of the
crosswind. There may be insufficient "elevator" area or pitch authority
to flare as opposed to straight line flight where both elevons/elevators
will be moving "up".

Scott
I would like to add,
If the V tail has a fixed stab the moving elements have to be large, because
of that, the airfoil can not achieve its aero dynamic potential. In the
case of
the HP 18, the hinge line at the root is at 45% and at the tip at 55%.
With
an all flying V tail this could be improved.
Over all the T-tail is the most effective, as each element can be optimized
for the function in term of size and aero dynamically generally and
specifically
having different airfoil for the Horizontal and vertical stab.

Udo