Simple angle of attack sensor

On Tuesday, August 20, 2013 9:47:32 PM UTC-6, wrote:
On Monday, July 29, 2013 10:16:55 AM UTC-5, Bill D wrote:

The other four ports will likely trigger transition locally on the nose. They will each create about a 7-degree wedge of turbulent flow, which could/would significantly reduce the amount of laminar flow on the fuselage.


Not sure I follow this. The pressure taps would be 1mm holes. How could they significantly reduce laminar flow on the fuselage? Many gliders already have multiple static port holes on the sides of the cockpit ahead of the panel.

Jim White wrote, On 8/21/2013 6:33 AM:
At 12:49 21 August 2013, kirk.stant wrote:
On Wednesday, August 21, 2013 7:34:46 AM UTC-5, Steve Leonard wrote:


Way better than what we have now!

Kirk
66

I have the best stall warning device ever - the glider. Nose high attitude,
low airspeed, rearward stick position, shuddering, reduced effectiveness of
controls. Zero cost and complexity.

My glider does exactly the same thing, but only in straight ahead
flight; in fact, it is very difficult to force a stall while flying
straight ahead. But, in a gentle turn of 20 degrees from downwind to
base it can smoothly, quietly drop the inside wing without a
significantly nose high attitude or rearward stick position. There is no
shuddering, and the controls and "picture out the window" seem normal.
They aren't, of course, but the signs of an impending stall are very
much muted compared to flying straight ahead.

This behavior in a gentle turn is not specific to my ASH 26 E, but also
appeared in most of the other gliders I've flown. And sure enough, it's
stalls in turns where we have the most accidents, not straight ahead
stalls. This lack of sufficient warning is what drives the interest in
stall warning systems, with angle of attack (AA) being one method.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

Eric,
I am more of a novice but have noticed the same thing. Straight ahead there are all kinds of warning signs before a stall as mentioned. Yet I have been surprised while thermalling as the ship suddenly and quietly takes a bit of nose down pitch (with easy recovery). What's up with that?
Is this the reason sailplanes have a dangerous reputation for spinning in?
Why does this happen?

On Wednesday, August 21, 2013 8:37:35 AM UTC-5, Steve Leonard wrote:

For planes like your LS-6, Kirk, you might even get adequate warning with a single set point. A bit earlier warning with no flap, and a little less warning with thermal or landing flap. Planes like my Zuni, where you don't set and forget the flaps, the system becomes more complicated.



Not well done, but my comment was intended to say "It may seem simple for some cases, but it is not a simple solution that will work just that easily for all sailplanes."



I tend to add the audio airspeed (open the side scoop) when I enter the pattern. Helps with ventilation, and adds an extra airspeed sensor that doesn't require a look inside. Not AOA, but another reference.



Steve


I've mentioned this before, but I'm less concerned with stall warning that with providing an easy to use indication of the correct speed to fly when thermalling and in the pattern. The airspeed indicator is a crude way of doing that, and requires one to look at the darn thing instead of outside. Having flown aircraft with audio AOA indicators, it's a much nicer way to fly slow. For thermalling, (due to the vario audio) I would settle for some bright LEDs on the panel at the thermalling AOA (for the flap setting). But for landing (gear down) I would switch off the vario and use the AOA to give me speed cues. If you have never tried this, it may sound funny, but trust me it works great!

Of course, when approaching the stall, you can always have the lights flash and Bitchin' Betty yell at you "Stall Stall!!!".

Kirk
66

At 14:52 21 August 2013, Eric Greenwell wrote:
Jim White wrote, On 8/21/2013 6:33 AM:
At 12:49 21 August 2013, kirk.stant wrote:
On Wednesday, August 21, 2013 7:34:46 AM UTC-5, Steve Leonard wrote:


Way better than what we have now!

Kirk
66

I have the best stall warning device ever - the glider. Nose high
attitude,
low airspeed, rearward stick position, shuddering, reduced
effectiveness
of
controls. Zero cost and complexity.

My glider does exactly the same thing, but only in straight ahead
flight; in fact, it is very difficult to force a stall while flying
straight ahead. But, in a gentle turn of 20 degrees from downwind to
base it can smoothly, quietly drop the inside wing without a
significantly nose high attitude or rearward stick position. There is no
shuddering, and the controls and "picture out the window" seem normal.
They aren't, of course, but the signs of an impending stall are very
much muted compared to flying straight ahead.

This behavior in a gentle turn is not specific to my ASH 26 E, but also
appeared in most of the other gliders I've flown. And sure enough, it's
stalls in turns where we have the most accidents, not straight ahead
stalls. This lack of sufficient warning is what drives the interest in
stall warning systems, with angle of attack (AA) being one method.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

Never flown a 26. My 27 gives just as much warning at all wing loads and
angles of bank. Just ease the stick forward when it goes mushy. Maybe that
1500 hours on type helps a bit?

Jim White wrote, On 8/21/2013 2:49 PM:
At 14:52 21 August 2013, Eric Greenwell wrote:
Jim White wrote, On 8/21/2013 6:33 AM:

I have the best stall warning device ever - the glider. Nose high
attitude,
low airspeed, rearward stick position, shuddering, reduced
effectiveness
of
controls. Zero cost and complexity.

My glider does exactly the same thing, but only in straight ahead
flight; in fact, it is very difficult to force a stall while flying
straight ahead. But, in a gentle turn of 20 degrees from downwind to
base it can smoothly, quietly drop the inside wing without a
significantly nose high attitude or rearward stick position. There is no
shuddering, and the controls and "picture out the window" seem normal.
They aren't, of course, but the signs of an impending stall are very
much muted compared to flying straight ahead.

This behavior in a gentle turn is not specific to my ASH 26 E, but also
appeared in most of the other gliders I've flown. And sure enough, it's
stalls in turns where we have the most accidents, not straight ahead
stalls. This lack of sufficient warning is what drives the interest in
stall warning systems, with angle of attack (AA) being one method.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

Never flown a 26. My 27 gives just as much warning at all wing loads and
angles of bank. Just ease the stick forward when it goes mushy. Maybe that
1500 hours on type helps a bit?

I doubt that your hours have anything to do with how the plane flies in
the situation I described. I have 3400 hours in type, and it's handled
that way since I began flying it, as have other gliders I've flown,
including the Blanik L13 our club used to train students. I'm sure it
depends on CG position, perhaps other things.

Here's how I do it to get the smooth, quiet entry into dropping the
inside wing:
- begin a gentle, coordinated turn with 15 to 20 degrees of flap in
neutral flap (flap 3 in the 26 E)
- slowly pull the stick back while maintaining that smooth, coordinated
15-20 degree bank
- as the speed diminishes, it will require more and more "top aileron",
and soon the inner wing will drop
- just moving the stick forward and centralizing the stick is usually
enough, but also moving the flaps forward a notch or so makes recovery
even quicker.

Give it a try next time you fly. A lot of pilots don't make such shallow
turns (at least not while also slowing down), and don't ever experience
this situation. It's important the bank be shallow - if it's too steep,
say over 35 degrees, the glider may not stall at all.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

At 23:19 21 August 2013, Eric Greenwell wrote:
Jim White wrote, On 8/21/2013 2:49 PM:
At 14:52 21 August 2013, Eric Greenwell wrote:
Jim White wrote, On 8/21/2013 6:33 AM:

I have the best stall warning device ever - the glider. Nose high
attitude,
low airspeed, rearward stick position, shuddering, reduced
effectiveness
of
controls. Zero cost and complexity.

My glider does exactly the same thing, but only in straight ahead
flight; in fact, it is very difficult to force a stall while flying
straight ahead. But, in a gentle turn of 20 degrees from downwind to
base it can smoothly, quietly drop the inside wing without a
significantly nose high attitude or rearward stick position. There is
no
shuddering, and the controls and "picture out the window" seem normal.
They aren't, of course, but the signs of an impending stall are very
much muted compared to flying straight ahead.

This behavior in a gentle turn is not specific to my ASH 26 E, but
also
appeared in most of the other gliders I've flown. And sure enough,
it's
stalls in turns where we have the most accidents, not straight ahead
stalls. This lack of sufficient warning is what drives the interest in
stall warning systems, with angle of attack (AA) being one method.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

Never flown a 26. My 27 gives just as much warning at all wing loads
and
angles of bank. Just ease the stick forward when it goes mushy. Maybe
that
1500 hours on type helps a bit?

I doubt that your hours have anything to do with how the plane flies in
the situation I described. I have 3400 hours in type, and it's handled
that way since I began flying it, as have other gliders I've flown,
including the Blanik L13 our club used to train students. I'm sure it
depends on CG position, perhaps other things.

Here's how I do it to get the smooth, quiet entry into dropping the
inside wing:
- begin a gentle, coordinated turn with 15 to 20 degrees of flap in
neutral flap (flap 3 in the 26 E)
- slowly pull the stick back while maintaining that smooth, coordinated
15-20 degree bank
- as the speed diminishes, it will require more and more "top aileron",
and soon the inner wing will drop
- just moving the stick forward and centralizing the stick is usually
enough, but also moving the flaps forward a notch or so makes recovery
even quicker.

Give it a try next time you fly. A lot of pilots don't make such shallow
turns (at least not while also slowing down), and don't ever experience
this situation. It's important the bank be shallow - if it's too steep,
say over 35 degrees, the glider may not stall at all.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

That sounds like a pretty standard spin entry technique that we use during
instruction all the time. If you kick the rudder out of the turn when it
goes mushy it will make a dramatic entry in the opposite direction. My
glider will spin reliably using this technique too, but it tells me when it
is going to stall a wing and autorotate.

Originally Posted by Soartech

Eric,
I am more of a novice but have noticed the same thing. Straight ahead there are all kinds of warning signs before a stall as mentioned. Yet I have been surprised while thermalling as the ship suddenly and quietly takes a bit of nose down pitch (with easy recovery). What's up with that?
Is this the reason sailplanes have a dangerous reputation for spinning in?
Why does this happen?

From Tom Knauff:

Here is another interesting exercise:

Roll into a modest bank angle of 30 degrees, and then pull back on the stick to bring the nose of the glider above the horizon. Watch carefully what happens.

If one wing stalls, the glider will roll as it stalls. If both wings stall, the glider will pitch nose down.
(You really must try this.)

With few exceptions, what you will observe is the glider neither rolls nor pitches, but yaws.

What happens is as the glider loses airspeed, the lift produced is not adequate to hold the glider up and it begins to fall. Since the glider is tilted, the falling glider causes the airflow to strike the side of the fuselage, causing the tail of the glider to be pushed up, causing the yawing motion.

There is no rolling. No pitching.

The wing is not stalled.

Because the wing is not stalled, the ailerons and rudder work properly.

The steeper the angle of bank, the more stall-proof the normally certificated glider becomes.

In a very steep angle of bank, (60 degrees) at a stable airspeed, the control stick may be fully back. At this attitude, how would you stall the glider? Since the angle of attack control is used up (fully back) it is not possible to increase the angle of attack and thus, the glider becomes stall-proof.

Thus, from the standpoint of stalling, steeper turns are safer than shallow turns.

Just as important: If you are performing a steeper angle of bank turn, and the glider seems to be falling/stalling, your instinctive reaction of applying opposite aileron will be OK. Since the wing is not stalled, opposite aileron works fine to level the wings.

The same instinctive reaction in a shallow angle of bank turn can create a stall/spin.

This is so contrary to what pilots are taught, that I expect lots of mail from you all.

Before you send me mail, please go try it yourself. Let me know what you experience.

Stall prevention includes:

* Know the signs of a stall in the order they occur.

* Keep the nose of the glider below the horizon.
* Fly an appropriate, stable airspeed.
* Keep the yaw string straight.
* Use steeper bank angles. (30 – 45 degrees)

All of the above is especially important during landings and all low altitude turns (rope breaks above 200 feet, low altitude thermalling.)

During training, we teach the student pilots to, “Watch the airspeed indicator.” What we mean is verify you are flying the correct airspeed by looking at the airspeed indicator every few seconds when landing or during rope break practice..

We demand the yaw string stay straight during low altitude turns

Nice discussion about impending stall recognition but that isn't really the point. Pretty much anybody from student pilots on can recognize an impending stall - if they're looking for it. You see what you're looking for and sometimes don't see what you're not looking for.

The need for a Stall warning device arises when the pilot isn't expecting a stall - perhaps when fatigued, dehydrated, hypoxic, distracted or just in over his head and overwhelmed by the pace of events. It's this pilot a stall warning is intended to save. Who among us can swear it's never happened to them? A stall warning system isn't there to take over and fly the aircraft or to diminish the skill required, it's there to watch a pilots back.

The AoA portion of the system is icing on the cake which helps a pilot an aircraft more accurately and extract more performance from it with greater confidence.

On Thursday, August 22, 2013 9:58:53 AM UTC-7, Squeaky wrote:

From Tom Knauff:


With few exceptions, what you will observe is the glider neither rolls

nor pitches, but yaws.



What happens is as the glider loses airspeed, the lift produced is not

adequate to hold the glider up and it begins to fall. Since the glider

is tilted, the falling glider causes the airflow to strike the side of

the fuselage, causing the tail of the glider to be pushed up, causing

the yawing motion.



There is no rolling. No pitching.



The wing is not stalled.



Because the wing is not stalled, the ailerons and rudder work properly.

This cannot be a technically correct explanation. And it isn't what any glider I have flown in recent memory does. If there is inadequate lift, and "the glider begins to fall" without pitching the AoA is very quickly going to exceed the stall angle. If it is sliding sideways enough due to falling that a large yawing moment is produced, the AoA is way past the stalling angle. A spin departure can produce pretty high yaw accelerations, but that ain't the mechanism.