Debunking Glider Spoiler Turns Causing Spin Thinking

On Sunday, 7 June 2015 17:05:05 UTC+2, OG wrote:
Hi Surge, which club and instructor was that? It certainly is not normal practice in SA

Hi Oscar

It was with two very experienced instructors at MGC (Orient) but I'll refrain from mentioning their names as they're not on this newsgroup to defend their point of view.

Exactly-- in turning flight, the flight path is curved, and therefore the relative wind (direction the airflow would come from if not disturbed by glider) is curved too-- at any given instant, the nose of the glider is moving in a different direction than the tail. In the real world there is downwash etc but the horizontal tail is still meeting the air at a different angle in a turn than it would in wings-level flight. So is the vertical fin.

For an extreme case of curvature of the relative wind in the yaw dimension, just visualize the relative wind at different points on a glider that is flat-spinning! Different parts are moving in different direction and experiencing different relative wind directions.

S

On Sunday, June 7, 2015 at 10:00:08 PM UTC-5, George Haeh wrote:
Piggott pointed out that the airflow over
the tailplane in a turn comes at an angle
that requires more elevator than normally
available to stall the glider.

On one of my spin checks, we found that
the Puchaz will spin out of a 45 degree
banked turn.

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

On Fri, 5 Jun 2015 06:17:43 -0700 (PDT),
wrote:



Lift does not equal weight in unbanked gliding flight, unless L/D is infinite.

It does.
Otherwise a glider would fly a ballistic arc, not a straight line.


Regards
Andreas

On Tuesday, June 9, 2015 at 8:05:23 AM UTC-5, Andreas Maurer wrote:
On Fri, 5 Jun 2015 06:17:43 -0700 (PDT), platypterus101
wrote:



Lift does not equal weight in unbanked gliding flight, unless L/D is infinite.

It does.
Otherwise a glider would fly a ballistic arc, not a straight line.


Regards
Andreas

Still on with this misconception? See my previous posts, especially the table of G-loading at various L/D ratios and bank angles. Multiply the G-loading values by the glider weight and you transform the table into lift force in pounds, for various L/D ratios and bank angles.

Take a moment to learn how to draw the L - D - W vector triangle for unbanked gliding flight, and then you'll see the light...

Here's an illustration of L D W vectors in an unbanked glide, though it would be clearer if they were re-arranged into a closed triangle:

http://www.recreationalflying.com/tu...descent_forces

This SHOULD be near the front of any good book on learning to pilot gliders..

Wish we were addressing this in a new thread instead of confusing the airbrake discussion with this...

There's really not much to say about this that hasn't already been addressed...

At normal sailplane glide angles the lift vector is ALMOST as large as the weight vector, in unbanked flight, but not quite. If you don't understand why, then you also don't understand why the L/D ratio is the same as the glide ratio (forward distance / vertical distance or horiztonal speed / vertical speed) in still air.

A glider pilot ought to know these things-- or at least ought to know enough to avoid posting statements to the contrary on a sailplane discussion forum!

We ought to change the SSA logo to a picture of the L-D-W vector triangle, since it is what makes gliding flight possible...

All in the spirit of a good discussion...

S

Hi "S",

On Tue, 9 Jun 2015 07:11:21 -0700 (PDT),
wrote:


On Tuesday, June 9, 2015 at 8:05:23 AM UTC-5, Andreas Maurer wrote:
On Fri, 5 Jun 2015 06:17:43 -0700 (PDT), platypterus101
wrote:



Lift does not equal weight in unbanked gliding flight, unless L/D is infinite.

It does.
Otherwise a glider would fly a ballistic arc, not a straight line.


Regards
Andreas

Still on with this misconception?

No misconception, but probably simply a different use of terms.

In my opinion "Lift" is not the force that is generated perpendicular
to the ldirection of flight, but lift (read: the force that keeps the
glider in the air on a straight, non-accelerated flight path) is the
sum of the "lift" vector created by the wing and the drag vector,
hence the "Resultant Force" in the drawing you mentioned:

http://www.recreationalflying.com/tu...descent_forces



This SHOULD be near the front of any good book on learning to pilot gliders.

In Germany it is one of the most important chapters in the books, but
explained a lot simpler than on the website you mentioned.


Wish we were addressing this in a new thread instead of confusing the airbrake discussion with this...

There's really not much to say about this that hasn't already been addressed...

At normal sailplane glide angles the lift vector is ALMOST as large as the weight vector, in unbanked flight, but not quite. If you don't understand why, then you also don't understand why the L/D ratio is the same as the glide ratio (forward distance / vertical distance or horiztonal speed / vertical speed) in still air.

Don't worry, I understand that pretty well since a couple of decades.


A glider pilot ought to know these things-- or at least ought to know enough to avoid posting statements to the contrary on a sailplane discussion forum!


We ought to change the SSA logo to a picture of the L-D-W vector triangle, since it is what makes gliding flight possible...

Germany here, I don't care about SSA...


Greets
Andreas

Re the above post: Andreas, if you define lift as the NET vertical aerodynamic force, then you have no room left for drag.

What you are really doing, is defining "lift" as the vector sum of what most people call "lift", and what most people call "drag".

That is the dashed vertical line in this diagram http://www.recreationalflying.com/tu...descent_forces .

By your system of definitions, there is can be no drag vector (unless it happens to equal zero in magnitude), and the glide ratio is no longer equal to arctan (D/L). The L/D ratio ceases to be a meaningful concept.

So... maybe you want to rethink that?

S

Mostly very helpful but isn't atan(D/L) the glide angle rather than the glide ratio? Maybe once again it's largely a matter of vocabulary. I certainly don't know. Thankfully, being fairly dumb about these things has not seemed to diminish my flying skills - wishful thinking?

On Wednesday, June 10, 2015 at 12:06:04 PM UTC-5, Jim Lewis wrote:
Mostly very helpful but isn't atan(D/L) the glide angle rather than the glide ratio? Maybe once again it's largely a matter of vocabulary. I certainly don't know. Thankfully, being fairly dumb about these things has not seemed to diminish my flying skills - wishful thinking?

It's all good. I still say my answer to Dan Marotta's question
"How come nobody ever states that these G loading increases are for level flight? Since the glider is always descending, wouldn't it be better to include something about the descent rate being maintained? What about a climbing turn? Maybe some trig including the flight path angle?"

was correct. Including the table of G-loading for various bank angles and L/D ratios. The decrease in G-loading (or lift force) due to the flight path through the airmass being descending, not level, is very small for typical sailplane glide ratios, but it is still very real and fundamental to understanding the theory of gliding. And yes you are absolutely right of course, on my last post I meant to type "glide angle" not "glide ratio". Glide ratio (in still air) would be simply L/D. If you are defining lift correctly. If you are using lift to mean the total VERTICAL aerodynamic force, than you can no longer say that the still-air glide ratio equals the L/D ratio. The L/D ratio and the W/D ratio are very close to each other, for flat glide angles, but they are not exactly the same.

OK, enough on that. Really!

S

G-load has absolutely nothing to do with L/D or (stationary) speed of airmass.

Bert TW