Debunking Glider Spoiler Turns Causing Spin Thinking

I am convinced that the airbrake is the least understood of the glider controls.
In order to give an opinion on the subject of airbrake use in turns, perhaps we should go back to their use as a glidepath control device.
I've heard many pilots explaining how the airbrakes work saying something like "When you open the airbrakes, the lift on that part of the wing is reduced or eliminated. Since you have less lift now, your vertical speed will increase and you will sink faster". Wow. And wrong, except for an instant after you open the airbrakes.
When you open the airbrakes, the lift on that part of the wing is gone, true. But only for a few moments, after that, the angle of attack, the airspeed, or both have to increase to create enough lift in the rest of the wing in order to hold the weight of the glider. Otherwise the glider would keep accelerating in the vertical direction.
If you try to keep the same airspeed when you open the airbrakes, the angle of attack has to increase. That means that you are closer to the stall, or conversely, that the stall speed increases.
Right when you open the airbrakes the total lift is reduced. This gives a nose down pitch tendency, but the extra drag causes some nose-up pitch tendency. The net effect is that some gliders tend to accelerate and some tend to deccelerate when opening the airbrakes. After a couple of seconds the glider is stable again in a steeper glidepath.
It is never a good idea to make big changes in the airbrake position on the flare because you risk a heavy landing or a ballooning.
Now, to their use in turns...

Definitely opening the airbrakes increases the stall speed. Combine it with the increase of the stall speed in a well-banked turn and your margin of safety gets reduced. However, in most gliders you run out of aft elevator before the stall in a well-banked turn (there are some exceptions).If you keep a good pattern speed through the turn you shouldn't have any problem.
The problem I see is that opening the airbrakes causes a temporary "instability", in other words, the glider may drop a few feet before getting in a stable turning configuration. That may create some problema as you try to adjust your flight in a turn, since now you have more dimensions to deal with, your curve in 3 dimensions is not as nice and stable as if you had kept the same amount of airbrake or no airbrake at all.
My suggestion is: if you really have to open or close them, by all means do.. But if there is not such a need, I think it is better not to change the airbrake lever position during the turn, or not make a big change. And by the way, just crack-opening the airbrakes is a big change of settings (as the total lift is reduced for a short period of time).
The problem as I see it is not whether the airbrakes are open or not. The problem is the additional changes in pitch and elevation in a curving descending path close to the ground, created when you make big changes in your airbrake settings. It is better to arrive to final in a stable curving configuration and then transition to a stabilized final approach where it is much easier to play with the airbrakes.

What is this - war on physics ?!

LOL
Bert TW

On Thu, 11 Jun 2015 21:26:29 -0700, Tango Whisky wrote:

What is this - war on physics ?!

Total misunderstanding of high school physics is more likely, coupled
with the sort of subject changing typical of trolls.


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

On Thursday, June 11, 2015 at 5:38:16 PM UTC-7, wrote:
The only caveat I'll add (again repeating something said already) was that I'm using G-loading to mean lift force / weight.


Maybe this is a clearer system of terminology:

Total g-loading = net aerodynamic force / weight

Lift-wise g-loading = lift / weight. This is the g-loading component that acts in the direction of the lift vector, i.e. perpendicular to the flight path. A conventional panel-mounted g-loading measures something very close to this. Excess amounts of this kind of g-loading is usually the reason pilots pull the wings off in clouds.

Drag-wise g-loading = drag / weight. This is the g-loading component that acts in the direction of the drag vector, i.e. parallel to the flight path. This component of g-loading is large in very steep dive. This component of G-loading has little effect on a conventional panel-mounted g-meter.

In unbanked steady-state flight in a glider, L D and W form a closed vector triangle. Thus the magnitude of L is dependent on the L/D ratio, and increasing the drag coefficient by opening spoilers or lowering landing gear does decrease L and also does decrease the lift-wise component of g-loading.

I don't know if the recent reference to trolls was aimed at me, but see my June 3 post and my June 4 post presenting the table of lift/weight at various bank angles and L/D loadings-- I was responding directly to Dan Marotta's question. I was not trolling for contradictory responses. Nonetheless several people disagreed with these posts, and away the discussion went in a typical internet spiral.

For high L/D ratios, we can substantially increase D and cause only a tiny decrease in L, unless the bank angle is really extreme. I never said otherwise-- the table I posted on June 4 shows it-- yet there is still SOME reduction in L, and to say otherwise is to ignore the L D W vector triangle. For much poorer L/D ratios, increasing D makes a much larger reduction in L, especially when the bank angle gets above 45 degrees or so. The terminal-velocity dive is an interesting extreme case where L/D is zero, and thus either L is zero or D is infinite. It the real world it is always the former case.

All this is kind of peripheral to looking at the immediate effects of deploying the spoilers, because the decrease in L is quite small when we deploy the spoilers starting with a typical sailplane L/D ratio, and I said as much in my June 3 reply to Dan. If folks hadn't wanted to argue, the discussion wouldn't have spiralled so far away from the original topic.

S

On Friday, June 12, 2015 at 6:16:31 AM UTC-7, wrote:


All this is kind of peripheral to looking at the immediate effects of deploying the spoilers, because the decrease in L is quite small when we deploy the spoilers starting with a typical sailplane L/D ratio,

Better would have been written "All this is kind of peripheral to looking at the long-run or steady-state effects of deploying the spoilers, because the final or steady-state decrease in L is quite small when we deploy the spoilers starting with a typical sailplane L/D ratio."

The immediate or short-term decrease in L can be much larger, as was well explained in the couple of posts by "sant..." immediately above.

S

On Friday, June 12, 2015 at 6:16:31 AM UTC-7, wrote:
On Thursday, June 11, 2015 at 5:38:16 PM UTC-7, wrote:
The only caveat I'll add (again repeating something said already) was that I'm using G-loading to mean lift force / weight.


Maybe this is a clearer system of terminology:

Total g-loading = net aerodynamic force / weight

Lift-wise g-loading = lift / weight. This is the g-loading component that acts in the direction of the lift vector, i.e. perpendicular to the flight path. A conventional panel-mounted g-loading measures something very close to this. Excess amounts of this kind of g-loading is usually the reason pilots pull the wings off in clouds.

Key point-- this is the g-loading component that is normally of greatest interest to pilots, because this is the g-loading component that affects the stall speed. When someone says that doubling the g-loading increases the stall speed by 1.4, they really ought to specify that they are talking about the lift-wise component of g-loading. We don't often use this language in actual practice. So when a pilot talks about g-loading, the listener ought to suspect that he may be talking about the lift-wise component of g-loading. As was the case in many of my previous posts.

It's really simplest to just leave g-loading out of it and simply talk about L and D and L/ weight and D/ weight. Then there is no possibility of confusion. So long as we understand the L D W vector triangle.

S

On Friday, June 12, 2015 at 6:16:31 AM UTC-7, wrote:
On Thursday, June 11, 2015 at 5:38:16 PM UTC-7, wrote:
The only caveat I'll add (again repeating something said already) was that I'm using G-loading to mean lift force / weight.


Maybe this is a clearer system of terminology:

Total g-loading = net aerodynamic force / weight

Lift-wise g-loading = lift / weight. This is the g-loading component that acts in the direction of the lift vector, i.e. perpendicular to the flight path. A conventional panel-mounted g-loading measures something very close to this. Excess amounts of this kind of g-loading is usually the reason pilots pull the wings off in clouds.

Drag-wise g-loading = drag / weight. This is the g-loading component that acts in the direction of the drag vector, i.e. parallel to the flight path. This component of g-loading is large in very steep dive. This component of G-loading has little effect on a conventional panel-mounted g-meter.

In unbanked steady-state flight in a glider, L D and W form a closed vector triangle. Thus the magnitude of L is dependent on the L/D ratio, and increasing the drag coefficient by opening spoilers or lowering landing gear does decrease L and also does decrease the lift-wise component of g-loading..

I don't know if the recent reference to trolls was aimed at me, but see my June 3 post and my June 4 post presenting the table of lift/weight at various bank angles and L/D loadings-- I was responding directly to Dan Marotta's question. I was not trolling for contradictory responses. Nonetheless several people disagreed with these posts, and away the discussion went in a typical internet spiral.

For high L/D ratios, we can substantially increase D and cause only a tiny decrease in L, unless the bank angle is really extreme. I never said otherwise-- the table I posted on June 4 shows it-- yet there is still SOME reduction in L, and to say otherwise is to ignore the L D W vector triangle. For much poorer L/D ratios, increasing D makes a much larger reduction in L, especially when the bank angle gets above 45 degrees or so. The terminal-velocity dive is an interesting extreme case where L/D is zero, and thus either L is zero or D is infinite. It the real world it is always the former case.

All this is kind of peripheral to looking at the immediate effects of deploying the spoilers, because the decrease in L is quite small when we deploy the spoilers starting with a typical sailplane L/D ratio, and I said as much in my June 3 reply to Dan. If folks hadn't wanted to argue, the discussion wouldn't have spiralled so far away from the original topic.

S

The L/D/W vector triangle is a construct of convenience, not a Holy Trinity.. It carries accuracy but no further insight than any other interpretation. Physically, it is most intuitive to think of the aerodynamic forces on a wing section as the sum of the pressures on its surface. This is mathematically complex though, and for the purposes of Newtonian motion they can be most simply represented as two vectors: a resultant force and a moment vector.. Aerodynamicists have often found it convenient to further split the former vector into two, one parallel to the incident flow (that they call Lift) and one perpendicular (that they call Drag). This is as accurate, but no more correct, than any other collection of vectors that properly sum to the result.

It is perfectly accurate to think of there being two force vectors acting on a sailplane in steady state unaccelerated flight: aerodynamic and gravitational, of equal magnitude acting through a single point in opposite directions. Any change in those three conditions results in accelerating motion.

To enumerate the changes between L and D in various flight conditions as somehow representing the Truth of Flight is an act of Faith, promoting an arbitrary construct into some sort of Enlightenment. Among other things, it leaves out the moments from wing and tailplane, AOA of the wing, and many other things which may be more important to structure and stability (for example) than any minor changes in L and D. I can fly at a glide ratio of 40 at 40 knots and at 80 knots, total lift and drag are the same but a whole lot of things are different I assure you, and opening the spoilers will have a very different effect.

Just to keep the thread going .

On 6/12/2015 10:00 AM, jfitch wrote:
Snip...
The L/D/W vector triangle is a construct of convenience, not a Holy
Trinity. It carries accuracy but no further insight than any other
interpretation. Physically, it is most intuitive to think of the
aerodynamic forces on a wing section as the sum of the pressures on its
surface. This is mathematically complex though, and for the purposes of
Newtonian motion they can be most simply represented as two vectors: a
resultant force and a moment vector. Aerodynamicists have often found it
convenient to further split the former vector into two, one parallel to the
incident flow (that they call Lift) and one perpendicular (that they call
Drag). This is as accurate, but no more correct, than any other collection
of vectors that properly sum to the result.

It is perfectly accurate to think of there being two force vectors acting
on a sailplane in steady state unaccelerated flight: aerodynamic and
gravitational, of equal magnitude acting through a single point in opposite
directions. Any change in those three conditions results in accelerating
motion.

To enumerate the changes between L and D in various flight conditions as
somehow representing the Truth of Flight is an act of Faith, promoting an
arbitrary construct into some sort of Enlightenment. Among other things, it
leaves out the moments from wing and tailplane, AOA of the wing, and many
other things which may be more important to structure and stability (for
example) than any minor changes in L and D. I can fly at a glide ratio of
40 at 40 knots and at 80 knots, total lift and drag are the same but a
whole lot of things are different I assure you, and opening the spoilers
will have a very different effect.

Just to keep the thread going .


Well stated. This (part of) this thread is a good example of the limitations
of an international-in-scope information-exchange of mathematically-complex
ideas and concepts, when limited entirely to the written word. It's even
possible some readers may've been motivated to improve their own
understandings of 'stuff' related to the O.P.'s thesis! In the spirit of
keeping the thread going...

On Friday, June 12, 2015 at 9:01:01 AM UTC-7, jfitch wrote:

The L/D/W vector triangle is a construct of convenience, not a Holy Trinity. It carries accuracy but no further insight than any other interpretation. Physically, it is most intuitive to think of the aerodynamic forces on a wing section as the sum of the pressures on its surface. This is mathematically complex though,

Hmmm.. every time I try to introduce more complexity I'm accused of changing the subject and trolling.

I agree that splitting the aerodynamic forces into L and D is in some sense arbitrary. I disagree that is not enlightening. Every time we talk about effect of G-load on airspeed at the stall angle-of-attack, etc, we are making use of that split, i.e. we are talking about the lift-wise component of G-loading. It is a useful thing to do.

I say she IS a Holy Trinity and I intend to worship at her altar again very soon!


You said some days ago: "In steady state, unaccelerated flight, Lift = Mass and the wing loading is constant. It does not matter what your sink rate is. The wing loading is the same with flaps, gear, and spoilers out as it is clean. If Lift Mass then the glider is accelerating, either up, down or by changing direction."

I say no, in an unbanked unpowered glide, in the steady-state condition, Lift is less than Weight, and Lift changes whenever the L/D ratio changes.


You said: "It doesn't matter if the glider is descending - the wing loading is the same. The only thing that will change that is accelerated flight. That is an increasing or decreasing rate of descent (not constant) or a change in velocity (that includes turns which are a change in velocity by definition). A sustainer glider will have the same wing loading in level (unaccelerated) flight as it does in a glide. "

But I say not so. I say the lift-wise component of the G-loading-- i.e. the Lift/Weight-- is slightly less in a descending glide than in level flight (all relative to the airmass of course.)


You said "It is best not to think of the "speed at which the wing stalls". That can be anywhere from 0 to beyond redline, depending on a bunch of conditions. You should think of "the angle of attack at which the wing stalls" which is for all intents invariant on a glider."

I say that's all true, and it's also true that the airspeed we see at the stall angle-of-attack is highly dependent on the square root of the lift-wise component of the G-loading, and that's why we ought to care about the lift vector or the lift-wise component of the G-loading or whatever we want to call it.

The L D W concept IS useful and enlightening, because airspeed at stall is highly related to the magnitude of L. Not to the total aerodynamic force. If we care about airspeed at stall, then we care about the lift-wise component of the G-loading.

S