Debunking Glider Spoiler Turns Causing Spin Thinking

On Thursday, June 4, 2015 at 10:41:59 AM UTC-5, jfitch wrote:
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Are you saying that the wing loading goes down in a turn because the glide ratio deteriorates?

Yes, I am. The worse the glide ratio (horizontal distance covered per vertical distance covered), the less lift the wing must generate, for any given bank angle. As L/D approaches 1:1 and glide ratio approaches something really terrible (worse than 1:1, if the aircraft is banked), the lift vector becomes less than the weight vector. Because much of the aircraft weight is being supported by the drag vector. See my table from previous post. As we start at any given bank angle, and hold that bank angle constant, and degrade the L/D ratio by increasing the drag coefficient, the flight path will end up aiming more and more steeply downward into a descending helix, with the ultimate end point being a vertical rolling dive at 0:1 L/D and 100% of the aircraft weight supported by the drag vector. (The drag coefficient must be infinite, and the airspeed must be zero, to achieve this so we never get there in real life.)

See my table from previous post.

That is incorrect. Wing loading and glide ratio, as you are using them here, are unrelated.

In steady state, unaccelerated flight, Lift = Mass and the wing loading is constant.

No-- see my table from previous post. Lift does not equal weight in unbanked gliding flight, unless L/D is infinite. Do you know how to draw the lift -drag- weight vector triangle for unbanked gliding flight? It appears in most sailplane instructional materials-- I should hope!

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.

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 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 10:41:59 AM UTC-5, jfitch wrote:

Are you saying that the wing loading goes down in a turn because the glide ratio deteriorates?

Well-- of course I'm not saying that holding L/D constant and increasing the bank angle will decrease the wing loading! That would be very silly.

I am saying that holding bank angle constant and decreasing L/D will decrease the wing loading due to the deterioration in glide angle.

See previous posts and especially the table of wing loading for various L/D ratios and bank angles. The table says it better than I can say it in words.

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

On Friday, June 5, 2015 at 7:01:27 AM UTC-7, wrote:
On Thursday, June 4, 2015 at 10:41:59 AM UTC-5, jfitch wrote:

Are you saying that the wing loading goes down in a turn because the glide ratio deteriorates?

Well-- of course I'm not saying that holding L/D constant and increasing the bank angle will decrease the wing loading! That would be very silly.

I am saying that holding bank angle constant and decreasing L/D will decrease the wing loading due to the deterioration in glide angle.

See previous posts and especially the table of wing loading for various L/D ratios and bank angles. The table says it better than I can say it in words.

S

This is a unique use of the term "wing loading". By that definition, an HP with 90 degree flaps, pointing straight at the ground in steady state, has zero wing loading, and parachutes always have zero canopy loading.

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

I confess this one stumps me. I'll have to think about it a while.

Disregarding drag (maybe I shouldn't), if lift does not equal weight the result is unbalanced forces, which result in acceleration (in a vector sense). The glider will either accelerate or fly an increasingly steep glide; maybe both.

Maybe it's like an optical illusion.

This is a unique use of the term "wing loading".

In my table I referenced g-loading, not wing loading, but they are not unrelated. Assuming you define g-loading as lift / weight. Another reasonable way to define g-loading is net aerodynamic force / weight-- that's a useful definition for orbital re-entry dynamics and such, but that's not what you'd be reading on a conventional panel-mounted aircraft g-meter.

By that definition, an HP with 90 degree flaps, pointing straight at the ground in steady state, has zero wing loading

If the flight path is directed straight down, the wings must be at the zero-lift angle-of-attack. The g-meter on the panel will read zero, and there is no lift generated by the wings.

and parachutes always have zero canopy loading.*

I'm not very interested in parachutes, but if I were, I don't think I would define my terms as you seem to be suggesting.

Once again-- you don know how to draw the lift-drag-weight vector triangle for unbanked gliding flight, right?

S

Without drawing any pictures -

At constant speed the glider is descending, therefore the lift vector
(perpendicular to the wing) is canted forward. Adding in the drag
vector (opposite the glider's longitudinal axis) yields a vector which
is directly opposite to gravity. It is this resultant vector (the sum
of the lift and drag vectors) which offsets the weight vector.

Did I say that well enough?

On 6/5/2015 9:33 AM, Jim Lewis wrote:
"Lift does not equal weight in unbanked gliding flight, unless L/D is infinite."

I confess this one stumps me. I'll have to think about it a while.

Disregarding drag (maybe I shouldn't), if lift does not equal weight the result is unbalanced forces, which result in acceleration (in a vector sense). The glider will either accelerate or fly an increasingly steep glide; maybe both.

Maybe it's like an optical illusion.

--
Dan Marotta

Can someone help educate these folks by posting a link to a nice illustration of the lift-drag-weight vector triangle for straight-line gliding flight?

S