Debunking Glider Spoiler Turns Causing Spin Thinking

On Thursday, June 4, 2015 at 11:34:35 PM UTC-5, wrote:

As an experiment try this at altitude: trim to the recommended approach speed with full airbrakes or recommended approach speed found in the AFM (or if it's not specified use the formula you were taught for choosing an approach speed), roll the glider into a good coordinated 40 degree bank turn then move the elevator progressively back. In every glider I've flown the elevator hits the stop without provoking a stall. I tried this after reading a Derek Piggott article suggesting it.

What is the result of the same test with airbrakes closed?


What do you need to do with the stick to hold a constant airspeed as you open the spoilers in wings-level flight?

Is the heart of the matter simply that for a given airspeed in wings-level at any given bank angle, including zero degrees bank, the stick needs to be positioned further aft with the spoilers open than with the spoilers closed?

S

On Fri, 05 Jun 2015 06:35:11 -0700, platypterus101 wrote:

What do you need to do with the stick to hold a constant airspeed as you
open the spoilers in wings-level flight?

That's dependent in what glider you're flying:

- If you're in a Puchacz you give the stick a healthy shove forward as
you open the brakes. If you don't, it will loose at least 5 kts almost
instantly.

- if you're in a Libelle, SZD Junior or an ASK-21 there's little
immediate speed change.

- in a Grob G103 you pull back a bit because these drop their nose and
pick up speed when you open the brakes.

Is the heart of the matter simply that for a given airspeed in
wings-level at any given bank angle, including zero degrees bank, the
stick needs to be positioned further aft with the spoilers open than
with the spoilers closed?

No. The main effect of well-designed airbrakes is to reduce effective
lifting surface, but, as a side effect is to add drag, most gliders will
need the stick to be eased forward a little to keep the airspeed
constant. However, the wind gradient and turbulence will probably be more
significant: you'll think you're correcting for these factors rather than
for the extra drag from the brakes. But, as I said above, it does depend
on the glider. For the G103 to behave as it does, its drag must reduce
along with the lift as you open the brakes. Conversely, the Puchacz has
huge speed-limiting upper and lower surface brakes: so much so that it
would be surprising if shoving those out in the breeze didn't show it
down.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Saturday, June 6, 2015 at 7:59:13 AM UTC-5, Martin Gregorie wrote:

No. The main effect of well-designed airbrakes is to reduce effective
lifting surface, but, as a side effect is to add drag, most gliders will
need the stick to be eased forward a little to keep the airspeed
constant. However, the wind gradient and turbulence will probably be more
significant: you'll think you're correcting for these factors rather than
for the extra drag from the brakes. But, as I said above, it does depend
on the glider. For the G103 to behave as it does, its drag must reduce
along with the lift as you open the brakes. Conversely, the Puchacz has
huge speed-limiting upper and lower surface brakes: so much so that it
would be surprising if shoving those out in the breeze didn't show it
down.

One could steer this conversation in the direction of transient versus steady-state effects. For example, if we start with a typical sailplane L/D ratio (say 30:1), and then we deploy some device that doubles the drag coefficient and halves the lift coefficient, we'll experience a temporary deceleration due to the sudden drag, but as the flight path curves downward, it must be the case that we'll finally come to equilibrium at a much higher airspeed than we started with.

Because "scaling up" the L and D vectors, by increasing the airspeed, is the only way to create a closed vector triangle of L D and W.

Assuming that angle-of-attack is held constant throughout.

When we open airbrakes without moving the stick, there's no reason to assume that angle-of-attack stays constant. We're killing the lift over one part of a wing which has twist (washout), so we're making a change in the average incidence of the "working" part of the wing. And we may be changing the airflow over the tail as well.

So a question of interest remains-- let's forget about airspeed entirely-- in wings-level flight or at some shallow bank angle, does the stick need to be further aft (closer to the aft stop) to induce a stall with airbrakes open than with airbrakes closed?

S

We can beat up theory all day, but what about practice?

I don't give a sh!t what I do with the stick when I open the brakes, nor
what is the arc hyperbolic cosine of the angle of the dangle, nor what
the glider does in response. When I deploy the boards, I also operate
the flight controls to make the flight path do what I want.

Or you can be debating theory as you proceed merrily towards the ground.

On 6/6/2015 7:32 AM, wrote:
On Saturday, June 6, 2015 at 7:59:13 AM UTC-5, Martin Gregorie wrote:

No. The main effect of well-designed airbrakes is to reduce effective
lifting surface, but, as a side effect is to add drag, most gliders will
need the stick to be eased forward a little to keep the airspeed
constant. However, the wind gradient and turbulence will probably be more
significant: you'll think you're correcting for these factors rather than
for the extra drag from the brakes. But, as I said above, it does depend
on the glider. For the G103 to behave as it does, its drag must reduce
along with the lift as you open the brakes. Conversely, the Puchacz has
huge speed-limiting upper and lower surface brakes: so much so that it
would be surprising if shoving those out in the breeze didn't show it
down.

One could steer this conversation in the direction of transient versus steady-state effects. For example, if we start with a typical sailplane L/D ratio (say 30:1), and then we deploy some device that doubles the drag coefficient and halves the lift coefficient, we'll experience a temporary deceleration due to the sudden drag, but as the flight path curves downward, it must be the case that we'll finally come to equilibrium at a much higher airspeed than we started with.

Because "scaling up" the L and D vectors, by increasing the airspeed, is the only way to create a closed vector triangle of L D and W.

Assuming that angle-of-attack is held constant throughout.

When we open airbrakes without moving the stick, there's no reason to assume that angle-of-attack stays constant. We're killing the lift over one part of a wing which has twist (washout), so we're making a change in the average incidence of the "working" part of the wing. And we may be changing the airflow over the tail as well.

So a question of interest remains-- let's forget about airspeed entirely-- in wings-level flight or at some shallow bank angle, does the stick need to be further aft (closer to the aft stop) to induce a stall with airbrakes open than with airbrakes closed?

S

--
Dan Marotta

On Sat, 06 Jun 2015 06:32:10 -0700, platypterus101 wrote:

On Saturday, June 6, 2015 at 7:59:13 AM UTC-5, Martin Gregorie wrote:

No. The main effect of well-designed airbrakes is to reduce effective
lifting surface, but, as a side effect is to add drag, most gliders
will need the stick to be eased forward a little to keep the airspeed
constant. However, the wind gradient and turbulence will probably be
more significant: you'll think you're correcting for these factors
rather than for the extra drag from the brakes. But, as I said above,
it does depend on the glider. For the G103 to behave as it does, its
drag must reduce along with the lift as you open the brakes.
Conversely, the Puchacz has huge speed-limiting upper and lower surface
brakes: so much so that it would be surprising if shoving those out in
the breeze didn't show it down.

One could steer this conversation in the direction of transient versus
steady-state effects.

Kindly stop changing the subject. You asked about transient effects when
the brakes are opened. I passed on personal experience of flying gliders
with a range of behaviour when the brakes are opened, which is what you
asked about.

There is only valid answer: "what happens depends on which glider you're
flying", i.e. there is no single universal answer. Live with it.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Thursday, June 4, 2015 at 11:34:35 PM UTC-5, wrote:


As an experiment try this at altitude: trim to the recommended approach speed with full airbrakes or recommended approach speed found in the AFM (or if it's not specified use the formula you were taught for choosing an approach speed), roll the glider into a good coordinated 40 degree bank turn then move the elevator progressively back. In every glider I've flown the elevator hits the stop without provoking a stall. I tried this after reading a Derek Piggott article suggesting it.

In other words is the heart of the matter that the airbrakes make the glider trim to a lower angle-of-attack, for a given elevator position? Possibly because of changes in airflow over the tail?

I guess I'm simply suggesting that in wings-level flight, the glider may stall with the stick further aft with airbrakes open than with airbrakes closed. Yes? Airbrakes probably decrease downwash over the tail, causing the glider to trim to a lower a-o-a.

The need for progressively more aft stick to command a given angle-of-attack (e.g. the stall angle-of-attack) as we progressively increase the bank angle is not unexpected, but I've never done a side-by-side comparison of airbrakes closed vs airbrakes open. Perhaps you have the same demand for extra back stick to maintain a given angle-of-attack as you increase the bank angle in both cases, but in the airbrakes-open case, you are starting with the stick further aft, so you and up hitting the stop sooner.

S

On Thursday, June 4, 2015 at 11:34:35 PM UTC-5, wrote:

As an experiment try this at altitude: trim to the recommended approach speed with full airbrakes or recommended approach speed found in the AFM (or if it's not specified use the formula you were taught for choosing an approach speed), roll the glider into a good coordinated 40 degree bank turn then move the elevator progressively back. In every glider I've flown the elevator hits the stop without provoking a stall. I tried this after reading a Derek Piggott article suggesting it.

In recent tests (happened to be in a Ka-6) I couldn't tell that the airbrakes had any effect on the stick position and stick force at the stall angle-of-attack, in wings-level flight as well as in turning flight. The stick had to be much further aft to bring the wing to the stall angle-of-attack when the glider was substantially banked, as expected.

On the other hand I seem to recall noting once that the airbrakes had a powerful nose-down trim effect on a 1-26. I.e. that the glider tended to trim to a much lower angle-of-attack/ much higher airspeed. It's been a while since my last flight in one so check it out for yourself rather than taking my word for it...

S

Bruno must have had a laugh using the term "Debunking" on RAS.
Jim

On Monday, June 1, 2015 at 8:09:34 PM UTC-7, wrote:

Thanks for your insights and comments to try to help clarify the confusion.
Bruno - B4

"What is the result of the same test with airbrakes closed? "

About the same - you run out of elevator authority before the ship stalls. Ships I've personally tried this on: Grob 102 CS-77, Grob 102 Standard III, Grob Twin Astir (RG), Grob Twin II Acro, Let L-13 Blanik, LET L-23 Super Blanik (short wingspan version only), LET L-33 Solo, PW-5, Standard Jantar 1, DG-505 (20 and 18 meter configuration), Twin Lark, Schempp-Hirth Duo Discus, Schweizer 1-26, Schweizer 1-23, Schleicher ASW-15B.

Boiled down: if spoilers being extended during causes a problem during a base or final turn you've already seriously screwed up by flying the circuit and approach at far too low a speed and not using well banked turns. For more information go to Knauff or Piggott. They've got a hell of a lot more experience and knowledge than almost anybody.

"Holding your breath does the same thing.
Try it, you'll feel lighter.
Jim "

Wow! That really works, just tried it hear in my chair!