Flaps and V-Tails of Death

"Bob Whelan" wrote in message ...
[snip]
Speed control is important in gracefully landing flaps-only gliders
(spoilers-only too, of course). What I've found - and often seen - is that
gliders' large-deflection flaps essentially 'quit working' as drag producing
devices if landed 'too fast.' True even for HP-16's. Come in too fast and
you _will_ float a long ways in flapped gliders...unless you slowly ease off
on the flaps, in which case the ship will gently settle...
[good stuff snipped]
IMHO, about the only situation I can envision where a flaps-only ship IS
worse than a spoilers-only one is that of getting low and slow on the
approach.
[snip]
Regards,
Bob W.

Hi, Bob and all.

With all humility and with the following caveats, I would like to
mildly disagree and vehemently agree with a couple of your points.
First the caveats: I am not all that experienced, and have only flown
my HP-16 for one summer, I think I am qualified to comment, but by no
means am an expert. The following comments ONLY apply to my HP-16,
N8DC, with 90 degree flaps, standard sized flaps (no flap was
sacrificed to improve the ailerons), and flown fairly CG forward.

Mild disagreement on the too fast comment. I think that the only way
to make my -16 float would be to be going too fast and then roll off
the flaps. 90 degree flaps require an impressive deck angle just to
keep the speed in the 60mph range on approach. If, when I get close to
the ground, I flare, any excess speed, and I mean ANY excess speed is
gone very quickly, and I land. Period. An approach with, say, 60
degrees of flap would indeed float if I had too much speed on, and as
such, one possible corrective action would be to roll off the flaps,
if I had slowed considerably...otherwise, more flaps is usually the
right answer. This airplane will not float with maximum flaps. There
have got to be 6 square feet of aluminum hanging perpendicular to the
airflow...we stop pretty quickly.

A minor expansion on that. It is very difficult to get the airplane to
accelerate with 90 degree flaps...If I should let the airspeed decay
on approach with full flaps, I need to push the nose down to
frightening angles...as in hanging from the straps...to
accelerate...or just roll off some flaps, which is what I do. This
presupposes that we are talking 45-60+ speed range. I would very much
not like to get much below 45 with full flaps. The aircraft's stall
characteristics are quite benign, but recovery requires a fair amount
of altitude with flaps at that level.

Conversely, speed control on appoach is quite trivial. Should I, for
whatever reason, let the speed creep up...more flaps...less speed. It
is amazingly linear...and better than any speed/lift control device I
have used, including throttle.

To agree emphatically on another point: Low and slow with flaps on is
a VERY BAD THING (tm). Too fast is no problem...roll the flaps in to
slow, roll them out to stretch. Too slow, and your options
are...well...gone. So that is an area of the performance curve that I
avoid.

I too went into the -16 wondering about all of the forked-tailed flaps
of doom talk. Turns out that my airplane is an absolute pussycat. If
yer ever planning on being around LaGrange, GA some weekend, drop me a
note. We might can work something out!

Jim

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, JJ wrote:

I rolled in 45 degrees of flaps and
everything looked just about right.
Came over the fence at 50 knots and
waited for her to settle down...

If it's floating down the runway, you just don't have enough flaps
cranked in. Dick Schreder pointed this out when he wrote the
instructions on test flying the HP-11:

http://www.soaridaho.com/Schreder/HP...t_Testing.html

which says:

: If the HP-11 is floating down the
: runway and doesn't want to slow down,
: you just don't have enough flap
: cranked down.

That was in 1963, and I'm pretty sure that it still applies. The same
advice is also repeated in the test flight instructions for the HP-14
and HP-18.

When flying my HP-11 at Air Sailing when the wind favored 21 I
generally stopped right in front of the windmill near the
intersection. Just as I cross the edge of the sage along runway 17 I
would crank on the full 90, flare across 17, and come to a halt within
less than 100 yards of touchdown. And that's landing downhill in a
ship equipped with a go-kart style band-on-tire wheel brake.

The secret to the HP landing technique is to be proactive about
increasing stick forward pressure as you crank on the flaps. If you
try to be reactive, and wait until you see the airspeed change, you've
already gotten too slow. So the technique that works well is to get
the stick moving in the right direction as you crank the flaps on or
off, and worry about refining the speed control later. Control system
engineers call this a "feed forward" system, since there's just not
time to wait for feedback before starting corrective action.

As far as getting sucked into a cloud, the only time it happened to me
it was scary, but other than that rather tame. I cranked on about 75
degrees of flaps, put out the wheel to silence the warning buzzer, and
trundled out the bottom in a 45-degree dive at about 50 knots. The
important thing is to get the flaps out before you exceed the Vfe
speeds.

On another branch of this thread, Eric observes that one reason that
flaps are an issue is that there are so few two-seaters with 90-degree
flaps. I agree; this is one of the primary reasons that the next HP
kit will not be developed with 90-degree flaps. If anybody really
wants one, I can set the wings up for it, but the primary
configuration will have airbrakes. Personally, I really do prefer the
flaps over airbrakes. However, I do not have the resources to
prosecute an ideological battle on their behalf. If anybody wants to
know the depth to which I've sold out in pursuit of a successful
career as a sailplane developer, feel free to let this be the measure
of it.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24

Bob Kuykendall wrote:

On another branch of this thread, Eric observes that one reason that
flaps are an issue is that there are so few two-seaters with 90-degree
flaps. I agree; this is one of the primary reasons that the next HP
kit will not be developed with 90-degree flaps. If anybody really
wants one, I can set the wings up for it, but the primary
configuration will have airbrakes. Personally, I really do prefer the
flaps over airbrakes. However, I do not have the resources to
prosecute an ideological battle on their behalf. If anybody wants to
know the depth to which I've sold out in pursuit of a successful
career as a sailplane developer, feel free to let this be the measure
of it.

Are there engineering or manufacturing issues that make spoilers a more
desirable choice these days? For example, a fiberglass wing might be
more flexible than a metal one, which would make a 90 degree flap harder
to implement. The early ASW 20 had problems this way with it's 60 degree
flap setting.

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Eric Greenwell
Washington State
USA

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

(Mark James Boyd) wrote in message news:3fbc8231$1@darkstar...

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.

All flaps will lower stall speed at small settings - even split flaps.
That's why you usually have a takeoff setting for soft and/or short
fields.

Boy, I havn't flown an AA-1 (the original, American Aviation Yankee,
with the "hot" wing) in a LONG time (32 years!) but I seem to remember
it's flaps didn't do much at all - at least compared to any Cessna.
Sure was a fun little thing to buzz around in, though!

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

This is a bit of a myth, I think. You still need the same tail volume
regardless of tail configuration, so you end up with basically the
same wetted area. If you cut down the size of the tails, you start
having stability problems (the Bonanza was originally a bit marginal,
I think), which is probably how V-tails on gliders got their bad
reputation in the first place. HPs apparently got it right from the
start. The theoretical saving is in reduced interference drag due to
fewer intersections (less of an advantage compared to T-tails) and
reduced weight (a big advantage compared to T-tails). Then there is
the rudder-elevator mixer issue...

They do look nicely retro, though. And the Fouga Magister proves that
the V tail can work beautifully in a relatively high performance fully
aerobatic jet trainer.

Finally, let's face it, V-tails were a fashion for a while in the late
40s and 50s (Bonanza, Magister, SHKs, Sisu, HPs, etc.) Now, T-tails
are the "cool" tail - which probably has a lot to do with why the poor
little PW-5 is dissed so much.

Kirk

Kirk Stant wrote:

They do look nicely retro, though. And the Fouga Magister proves that
the V tail can work beautifully in a relatively high performance fully
aerobatic jet trainer.

And I'm told the Salto does quite well as a Vee tailed aerobatic glider.


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Eric Greenwell
Washington State
USA

From: (Kirk Stant)

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

This is a bit of a myth, I think. You still need the same tail volume
regardless of tail configuration, so you end up with basically the
same wetted area. If you cut down the size of the tails, you start
having stability problems (the Bonanza was originally a bit marginal,
I think), which is probably how V-tails on gliders got their bad
reputation in the first place. HPs apparently got it right from the
start. The theoretical saving is in reduced interference drag due to
fewer intersections (less of an advantage compared to T-tails) and
reduced weight (a big advantage compared to T-tails).

Right. Also, according to Dick Schreder, the V-tail shares the T-tail's
advantage of being up out of harm's way if landing in tall crop. The
conventional tail suffers there. Dick also spoke of the advantage of the
V-tail for the metal home builder, which obviously applies to the HPs. Just
build 2 identical parts, mirror imaged.

-Doug

( "Flaps and V-Tails of Death", you gotta love that subject title)