Debunking Glider Spoiler Turns Causing Spin Thinking

Re "one last try" immediately above-- please take a moment to re-read my previous post. It is the lift-wise component of the G-loading vector-- closely related to what your panel-mounted G-meter reads-- not the total G-loading vector-- which is determined by the L vector, or the L/D ratio, whichever way you want to look at it. In previous posts I spent some time discussing the difference betweeb the total G-loading vector, and the lift-wise component of the G-loading vectorm

I stand by the tables I posted today and on June 4. Do you have a specific correction to offer?

S

Phone-phart, disregard second-to-last, will repost. S

"In the last case, where thrust equals weight, setting the wing to the zero-lift angle-of-attack yields a steady-state climb regardless of airspeed, so the concept of stall speed becomes meaningless.*"

Sorry, that was a bit sloppy. If thrust=weight, no thrust is available to counteract drag, and so airspeed must be zero in the steady-state case. Angle-of-attack is undefined, so the wing cannot be considered to be stalled. Progressively higher T/W ratios (greater than 1:1) allow for progressively higher airspeeds, with the wing held at the zero-lift angle-of-attack, as the aircraft climbs straight up. Obviously we have abandoned the concept of a 40:1 L/D-- we have shoved the stick forward to unload the wing to zero lift as we climb straight up, so L/D is zero.

"In the last case, where thrust equals weight, setting the wing to the zero-lift angle-of-attack yields a steady-state climb regardless of airspeed, so the concept of stall speed becomes meaningless.*"

Sorry , I was a bit sloppy there. If thrust = weight and l/d 40:1, airspeed must be xxx. If we hold thrust = weight and unoaqd L to zero, so L/D becomes 0, ....

Substitute the words "table of June 4" for "table of June 6", sorry.

S

A bit more concisely now--

See Dan Marotta's question of June 3, and the discussion of whether or not L/D ratio affects "wing loading" in subsequent posts.

My table of June 4 was directly relevant to this discussion.

The take-home message of the table was as follows:

If we are holding L/D constant, then then L, D, and Na (net aerodynamic force) are fixed in proportion to each other.

If we are holding L/D constant and degrading the glide angle by banking, the magnitude of L, D, and Na (net aerodynamic force) must all increase as the bank angle increases, but the increase in these force vectors is much smaller if the L/D ratio is very poor than if the L/D ratio is very high.

Whether you define wing loading as N/W or L/W, there is much less increase in wing loading with increasing bank angle when the L/D ratio is very poor, than when the L/D ratio is very high.

For typical sailplane L/D ratios-- even with spoilers open-- the effect is likely too small to ever be noticed-- as I said to Dan-- but it is a real effect.

If we are not only holding L/D constant, but we are also holding angle-of-attack and Cl and Cd all constant, then we can see that for any given angle-of-attack-- including the stall angle-of-attack-- as we degrade the glide angle by banking, the airspeed must increase, because the airspeed is proportional to the square root of L, or the square root of D, or the square root of Na. (Remember that L and D and Na are fixed in proportion to each other.) The increase in airspeed with increasing bank angle will be much less when the L/D ratio is very poor, than when the L/D ratio is very high.

Again, for typical sailplane L/D ratios we'll probably never notice this, but it is a real effect.

The extreme case of a very poor L/D ratio is a terminal-velocity vertical dive, in which case the bank angle is undefined (or defined to be zero?) and thus has no effect on airspeed, L, D, and Na at all.

Any suggestion that opening the spoilers will reduce the stall speed, by degrading the L/D ratio, was in error-- when we open the spoilers, we change Cl.

We can't draw conclusions about the how stall speed varies as we move vertically (rather than horizontally) in the table I posted on June 4, because we haven't specified why the L/D ratio is changing-- whether due to changes in Cl, Cd, or both. But we can still see how the wing-loading (L/W or Na/W) varies as we move vertically through the table.

Here is the table one more time, but I've modified it to included Na/W (net aerodynamic force / weight) as well as L/W at various bank angles. Take your pick of which one you prefer to call the "G-loading". I could have also added the D/W, but you can easily calculate that from L/W. I've also added a line for the L/D =0 case.

Gliding flight (no thrust.) A table of (lift / weight), followed by (net aerodynamic force / weight), at various bank angles and L/D ratios:

Bank angle, L/W, Na/W:
L/D Infinite--
0 deg 1.000,1.000 30 deg 1.155,1.155 45 deg 1.414,1.414 60 deg 2.000,2.000
L/D 10:1--
0 deg .995,1.00 30 deg 1.147,1.153 45 deg 1.400,1.407 60 deg 1.966,1.976
L/D 5:1--
0 deg .981,1.00 30 deg 1.125,1.147 45 deg 1.361,1.388 60 deg 1.869,1.906
L/D 2:1--
0 deg .894,1.00 30 deg 1.000,1.118 45 deg 1.155,1.291 60 deg 1.414,1.581
L/D 1:1--
0 deg .707,1.00 30 deg 0.756,1.071 45 deg 0.817,1.155 60 deg 0.894,1.264
L/D 0/1
0 deg 1.00,1.00 30 deg 1.00,1.00 45 deg 1.00,1.00 60 deg 1.00,1.00

S

****Re-posted-- the only change is in the last line of the table****

A bit more concisely now--

See Dan Marotta's question of June 3, and the discussion of whether or not L/D ratio affects "wing loading" in subsequent posts.

My table of June 4 was directly relevant to this discussion.

The take-home message of the table was as follows:

If we are holding L/D constant, then then L, D, and Na (net aerodynamic force) are fixed in proportion to each other.

If we are holding L/D constant and degrading the glide angle by banking, the magnitude of L, D, and Na (net aerodynamic force) must all increase as the bank angle increases, but the increase in these force vectors is much smaller if the L/D ratio is very poor than if the L/D ratio is very high.

Whether you define wing loading as N/W or L/W, there is much less increase in wing loading with increasing bank angle when the L/D ratio is very poor, than when the L/D ratio is very high.

For typical sailplane L/D ratios-- even with spoilers open-- the effect is likely too small to ever be noticed-- as I said to Dan-- but it is a real effect.

If we are not only holding L/D constant, but we are also holding angle-of-attack and Cl and Cd all constant, then we can see that for any given angle-of-attack-- including the stall angle-of-attack-- as we degrade the glide angle by banking, the airspeed must increase, because the airspeed is proportional to the square root of L, or the square root of D, or the square root of Na. (Remember that L and D and Na are fixed in proportion to each other.) The increase in airspeed with increasing bank angle will be much less when the L/D ratio is very poor, than when the L/D ratio is very high.

Again, for typical sailplane L/D ratios we'll probably never notice this, but it is a real effect.

The extreme case of a very poor L/D ratio is a terminal-velocity vertical dive, in which case the bank angle is undefined (or defined to be zero?) and thus has no effect on airspeed, L, D, and Na at all.

Any suggestion that opening the spoilers will reduce the stall speed, by degrading the L/D ratio, was in error-- when we open the spoilers, we change Cl.

We can't draw conclusions about the how stall speed varies as we move vertically (rather than horizontally) in the table I posted on June 4, because we haven't specified why the L/D ratio is changing-- whether due to changes in Cl, Cd, or both. But we can still see how the wing-loading (L/W or Na/W) varies as we move vertically through the table.

Here is the table one more time, but I've modified it to included Na/W (net aerodynamic force / weight) as well as L/W at various bank angles. Take your pick of which one you prefer to call the "G-loading". I could have also added the D/W, but you can easily calculate that from L/W. I've also added a line for the L/D =0 case.

Gliding flight (no thrust.) A table of (lift / weight), followed by (net aerodynamic force / weight), at various bank angles and L/D ratios:

Bank angle, L/W, Na/W:
L/D Infinite--
0 deg 1.000,1.000 30 deg 1.155,1.155 45 deg 1.414,1.414 60 deg 2.000,2.000
L/D 10:1--
0 deg .995,1.00 30 deg 1.147,1.153 45 deg 1.400,1.407 60 deg 1.966,1.976
L/D 5:1--
0 deg .981,1.00 30 deg 1.125,1.147 45 deg 1.361,1.388 60 deg 1.869,1.906
L/D 2:1--
0 deg .894,1.00 30 deg 1.000,1.118 45 deg 1.155,1.291 60 deg 1.414,1.581
L/D 1:1--
0 deg .707,1.00 30 deg 0.756,1.071 45 deg 0.817,1.155 60 deg 0.894,1.264
L/D 0:1-- (vertical dive) (bank angle could also be considered to be undefined)
0 deg 0.00,1.00 30 deg 0.00,1.00 45 deg 0.00,1.00 60 deg 0.00,1.00

I think the issue was made murky by stipulations. Any aircraft IN A TURN that stalls is prone to spin - much more so if it is uncoordinated. The turn to base or final is the last place you want this to happen. Not adding flaps while turning was part of my pilot training in the 70's. The combination of turning (higher G loading) increases stall speed, and if you add drag while doing it, you can inadvertently loose too much airspeed for the bank angle, add to this the possibility of an uncoordinated turn. To error is human, to error close to the ground offers the answer to one of mankind's greatest debates; what happens to your soul when you die!

In the USA a pilot must consistently recover from a stall with less than 50ft altitude loss in order to get a license. Do we do stalls at 150ft? Heck no; we do them at 2000ft. We know a mistake close to the ground will kill us. The same is true for screwing with the aircraft while it is in a turn.

On Friday, January 5, 2018 at 1:32:08 AM UTC-5, wrote:
I think the issue was made murky by stipulations. Any aircraft IN A TURN that stalls is prone to spin - much more so if it is uncoordinated. The turn to base or final is the last place you want this to happen. Not adding flaps while turning was part of my pilot training in the 70's. The combination of turning (higher G loading) increases stall speed, and if you add drag while doing it, you can inadvertently loose too much airspeed for the bank angle, add to this the possibility of an uncoordinated turn. To error is human, to error close to the ground offers the answer to one of mankind's greatest debates; what happens to your soul when you die!

In the USA a pilot must consistently recover from a stall with less than 50ft altitude loss in order to get a license. Do we do stalls at 150ft? Heck no; we do them at 2000ft. We know a mistake close to the ground will kill us. The same is true for screwing with the aircraft while it is in a turn.

Could you please provide the reference from which you got the 50 foot standard.
UH

As a currently appointed glider DPE I can say with some certainty the there is no documented minimum altitude loss in the current US FAA Glider Practical Test standard.

The current standards were published in 1999, prior to that time there were many elements that are no longer tested or for which the tolerances have changed.

Mike