Angle of incidence

On Sunday, May 27, 2012 10:21:35 PM UTC-7, Roel Baardman wrote:
After speaking with an instructor about my Pilatus B4 landings, he argued that
the angle of incidence on this glider has a great influence on the landing
characteristics.
I looked it up, and it was only 1.5 degrees. With the tailplane oriented at -3
degrees.

However, I cannot figure out what other gliders have as the angle of
incidence. I have 6 degrees in my head somehow, but I'm not sure where I get
this from. Searching on Google does not give me figures, only stories about
changing it (from Ls6 to Ls8 for example).

I'm therefor wondering: can you tell me the angle of incidence on your glider
if you know it? And how does it effect take-off and landing characteristics?
For example: some people in my club argue that the Discus2 take-off
characteristics are also to be blamed on its angle of incidence.

regards,

Roel

I remember a long final glide that I made with a friend (both flying LS-6's). We were dead even, flying best L/D, we had to make it over a lake with enough altitude to clear the ridge. After a few minutes my buddy said; "Watch this" and slowly, but surely, he started gaining altitude on me. After 10 miles he was a good 50 feet above me. So, I asked, "OK, what did you do"? He replied, I went to one notch of positive flap. Why did that work? Because the wing on the LS-6 is attached at zero incidence so as to make the fuselage align with the relative wind when flying fast, but at best L/D, the nose was up and selecting +1 flaps brought the nose down a tat and alighned it better into the relative wind. Good trick to remember.
JJ
JJ

First of all: thanks for all the replies!
I find it incredibly interesting to ponder about these subjects, so more info is
better :-)
I also like the Ls-6 story!

My tendency with the B4 is to approach with too much velocity, often 100 km/h.
This gives me the feeling I have proper aileron control, so it's intentional.
When I approach the deck and slowly start to bring the nose up, I encounter the
ground effect that the B4 has. I either float for a long time before touching down
with a two-point landing, or I make a touchdown on the main wheel only (sometimes
with a slight jump after that).
What happened yesterday, was that I was so busy compensating for crosswind (I
don't want to have a traversed landing), that started floating again and gained
some altitude (1 meter roughly).
Knowing that the fault is probably in the velocity, I approached the instructor to
learn from his B4 experience.
I agree that the B4 is very friendly and benign. I will put effort in bringing my
approach speed down to 90 in the upcoming flights.

On May 28, 11:16*am, Roel Baardman wrote:
First of all: thanks for all the replies!
I find it incredibly interesting to ponder about these subjects, so more info is
better :-)
I also like the Ls-6 story!

My tendency with the B4 is to approach with too much velocity, often 100 km/h.
This gives me the feeling I have proper aileron control, so it's intentional.
When I approach the deck and slowly start to bring the nose up, I encounter the
ground effect that the B4 has. I either float for a long time before touching down
with a two-point landing, or I make a touchdown on the main wheel only (sometimes
with a slight jump after that).
What happened yesterday, was that I was so busy compensating for crosswind (I
don't want to have a traversed landing), that started floating again and gained
some altitude (1 meter roughly).
Knowing that the fault is probably in the velocity, I approached the instructor to
learn from his B4 experience.
I agree that the B4 is very friendly and benign. I will put effort in bringing my
approach speed down to 90 in the upcoming flights.

Slightly too much airspeed at the start of the flare will have a large
effect on float distance. The big culprit is ground effect which can
as much as double the glider's L/D just as you are trying to land
making it hard to get rid of that extra airspeed.

I choose a small airspeed increment over the yellow triangle according
to the gustiness of the day and use that for my "over the fence" (OTF)
airspeed. In the pattern, I'll use whatever airspeed give me a good
gust-stall margin then slow down to the "OTF" airspeed on short final.

2cents from a partially reformed sinner...

I learned to fly in Bergfalkes that have all the penetration of a well
thrown tissue. So I learned to approach with lots of speed - because it
will disappear fast once the nose comes up, and because any lack will
have you running out of energy very fast.

Transition to a big, heavy, slippery single and there is still the
tendency to want an extra few kt on the dial - and then it floats
forever in ground effect.

Have learned to bring the big Kestrel in a lot slower and the landings
are a lot less exciting. Still have a way to go. She can be landed in an
incredibly short distance if you have that approach speed nailed.

In my case the magic number is 45kts - Stall at 32-34Kt *4/3 is
42-45kt. Now that feels just WRONG but in still air it is perfect.
At 55kt (90km/h) you are carrying a massive 47% extra energy - that
takes a lot of distance to dissipate at 1:40+ - check it - set the
logger at 1s intervals, get it all stable at ~2m AGL over the numbers on
a long runway an see how long you can keep the wheel off the ground for.
Use the traces and try at a couple of approach speeds to see what your
achieved L/D is in landing configuration. At 55kt the Kestrel is still
in the air 100m later - and that is with landing flap and half airbrakes.

At low speed the ailerons are less powerful, but a little airbrake
increases effectiveness by diverting airflow over the controls so you
should have more than enough control - you are trying to fly straight -
not do aerobatics.

On 2012/05/29 3:55 AM, Bill D wrote:
On May 28, 11:16 am, Roel wrote:
First of all: thanks for all the replies!
I find it incredibly interesting to ponder about these subjects, so more info is
better :-)
I also like the Ls-6 story!

My tendency with the B4 is to approach with too much velocity, often 100 km/h.
This gives me the feeling I have proper aileron control, so it's intentional.
When I approach the deck and slowly start to bring the nose up, I encounter the
ground effect that the B4 has. I either float for a long time before touching down
with a two-point landing, or I make a touchdown on the main wheel only (sometimes
with a slight jump after that).
What happened yesterday, was that I was so busy compensating for crosswind (I
don't want to have a traversed landing), that started floating again and gained
some altitude (1 meter roughly).
Knowing that the fault is probably in the velocity, I approached the instructor to
learn from his B4 experience.
I agree that the B4 is very friendly and benign. I will put effort in bringing my
approach speed down to 90 in the upcoming flights.

Slightly too much airspeed at the start of the flare will have a large
effect on float distance. The big culprit is ground effect which can
as much as double the glider's L/D just as you are trying to land
making it hard to get rid of that extra airspeed.

I choose a small airspeed increment over the yellow triangle according
to the gustiness of the day and use that for my "over the fence" (OTF)
airspeed. In the pattern, I'll use whatever airspeed give me a good
gust-stall margin then slow down to the "OTF" airspeed on short final.

--
Bruce Greeff
T59D #1771

At 07:16 29 May 2012, BruceGreeff wrote:

Have learned to bring the big Kestrel in a lot slower and the landings
are a lot less exciting. Still have a way to go. She can be landed in an
incredibly short distance if you have that approach speed nailed.

In my case the magic number is 100m later - and that is with landing flap
and half airbrakes.

At low speed the ailerons are less powerful, but a little airbrake
increases effectiveness by diverting airflow over the controls so you
should have more than enough control - you are trying to fly straight -
not do aerobatics.

In my 19m Kestrel used to use full landing flap and full negative flap
(separate levers!) More than enough drag from the inboard flaps plus
brakes and plenty of roll control from the outers until it came to a halt.

That is indeed the way to do it - then the negatice flap puts the
ailerons at low angle of attack and you have excellent roll control.

Landing flap and positive flap is a recipe for - don't be pointing at
something you were hoping to reuse...

On 2012/05/29 11:14 AM, Z Goudie wrote:
At 07:16 29 May 2012, BruceGreeff wrote:

Have learned to bring the big Kestrel in a lot slower and the landings
are a lot less exciting. Still have a way to go. She can be landed in an
incredibly short distance if you have that approach speed nailed.

In my case the magic number is 100m later - and that is with landing flap
and half airbrakes.

At low speed the ailerons are less powerful, but a little airbrake
increases effectiveness by diverting airflow over the controls so you
should have more than enough control - you are trying to fly straight -
not do aerobatics.

In my 19m Kestrel used to use full landing flap and full negative flap
(separate levers!) More than enough drag from the inboard flaps plus
brakes and plenty of roll control from the outers until it came to a halt.



--
Bruce Greeff
T59D #1771

On Monday, May 28, 2012 1:16:31 PM UTC-4, Roel Baardman wrote:
First of all: thanks for all the replies!
I find it incredibly interesting to ponder about these subjects, so more info is
better :-)
I also like the Ls-6 story!

My tendency with the B4 is to approach with too much velocity, often 100 km/h.
This gives me the feeling I have proper aileron control, so it's intentional.
When I approach the deck and slowly start to bring the nose up, I encounter the
ground effect that the B4 has. I either float for a long time before touching down
with a two-point landing, or I make a touchdown on the main wheel only (sometimes
with a slight jump after that).
What happened yesterday, was that I was so busy compensating for crosswind (I
don't want to have a traversed landing), that started floating again and gained
some altitude (1 meter roughly).
Knowing that the fault is probably in the velocity, I approached the instructor to
learn from his B4 experience.
I agree that the B4 is very friendly and benign. I will put effort in bringing my
approach speed down to 90 in the upcoming flights.

I would guess that you probably are not using a lot of spoiler in the round out/ flare which extends the time and distance needed to shed enough energy to land. If this sounds right, try more spoiler on final and leave them out using elevator to round out. Start the round out a bit higher to allow for steeper approach.
Good Luck
UH

On the contrary: I'm using full spoiler lately. Part of my excess in speed comes
from approaching high and putting the nose in besides full spoilers. The speed
doesn't bleed off very fast when I'm level.

On May 29, 10:57*am, Roel Baardman wrote:
On the contrary: I'm using full spoiler lately. Part of my excess in speed comes
from approaching high and putting the nose in besides full spoilers. The speed
doesn't bleed off very fast when I'm level.

Roel - I don't have any experience in a B4 so please take this as
general commentary and something to consult on before trying in a
B4:

With all the ships I've flown (both metal and fiberglass single-
seaters), the way you know you've got the right approach speed is that
when you start your flare/round-out you can bring the nose up beyond
"level" without ballooning (this leads to a decent two-point landing
or touching the tailwheel slightly before the main wheel touches
down). If you are already pulling full spoilers and you cannot bring
the nose a couple of degrees above the horizon without ballooning,
you're probably approaching too fast and you might try incrementally
decreasing your approach speed by 3-5kph (2-3mph) until you get less
float and a smooth touchdown with 50% to 80% of your spoilers deployed
(there are a couple of ships out there that benefit from closing the
spoilers most of the way just before touchdown or they kind of drop
through the flare; but again I don't know enough about the B4 to
comment).

I prefer a steep approach (for safety, in case of sink or a big wind-
gradient), and have found that sometimes you have to consciously fight
the urge to dive for the runway. It takes a bit of training to pull
spoilers *earlier* during the base-to-final turn or just after lining
up on the extended runway centerline (i.e. just as you get onto final
approach), but I find that pulling spoilers a little earlier like that
helps keep me from building up speed as I keep my pitch attitude
slightly flatter and don't dive for the runway (and if I do, having
the spoilers out BEFORE I pitch down helps the airspeed from building
as quickly - pulling spoilers *after* you dive doesn't tend to slow
you down dramatically until you get into the flare/float).

Hope this provides some food for thought!

--Noel

Thanks for the reply Noel (and others). It does provide some food for thought!