Debunking Glider Spoiler Turns Causing Spin Thinking

Thank you for your interest in educating us.

Yes, thank you.

At an L/D of 45:1 I suppose the drag vector would be significant?

On 6/5/2015 9:01 AM, jfitch wrote:
Snip

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...

Maybe some of the confusion stems from terminology? It's been a long time
since my college days when I learned the definition of 'wing loading' (and I
haven't bothered to look it up now!), but I still think of it as simply a
mathematical construct: aircraft weight/wing area (including that projected
through the fuselage).

As such, it's a simple, static concept; the definition says nothing about
aircraft attitude, dynamic loading, load distribution, etc. Complexity enters
the picture when dynamic/structural 'stuff' needs to be considered, e.g.:
non-steady-state flight conditions (turns, spirals, changing flight
trajectories, ...), wing-borne loads, etc.

Considering the flight condition above, the wing's structural loading (even in
the lift-plane perpendicular to the wing chord) of a glider under that
condition will be quite different from the wing's definitional wing loading.
Imagine a glider descending at a zero-lift AOA (which the above example is not)...

HTH,
Bob W.

How about this? Lift weight drag of a glider
https://www.google.com/search?q=lift+drag+weight+vector+diagram&espv=2&bi w=1280&bih=675&tbm=isch&imgil=P0lMoDrEgSfJCM%253A% 253BhJTVL_XvdI7R2M%253Bhttp%25253A%25252F%25252Fww w.kitestrings.org%25252Ftopic10-250.html&source=iu&pf=m&fir=P0lMoDrEgSfJCM%253A%25 2ChJTVL_XvdI7R2M%252C_&usg=__qfFN38kSGKUAQBuynNOaD mlnsAE%3D&ved=0CDMQyjc&ei=NdlxVdafA4WlsAXkh4PQAw#t bm=isch&q=lift+drag+weight+vector+diagram+glider&i mgdii=htd7GlKMM3-dPM%3A%3Bhtd7GlKMM3-dPM%3A%3BGHSbBYUdPbo61M%3A&imgrc=htd7GlKMM3-dPM%253A%3BlbPHPYZSArLXbM%3Bhttps%253A%252F%252Fww w.grc.nasa.gov%252FWWW%252Fk-12%252Fairplane%252FImages%252Fglidvec.gif%3Bhttps %253A%252F%252Fwww.grc.nasa.gov%252FWWW%252Fk-12%252Fairplane%252Fglidvec.html%3B709%3B531

On 6/5/2015 10:56 AM, wrote:
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

--
Dan Marotta

Re NASA diagram linked above in recent post--

That doesn't cut the mustard!

A) they didn't draw L, D, and W in the form of a closed vector triangle, so it's unclear that the net force equals zero

B) If you look closely at the three force vectors, you see that they COULD NOT FIT into a closed vector triangle of the appropriate shape (right angle between L and D).

Don't feel bad guys, NASA evidently doesn't get it either...

Don't the SSA instructional materials include a good diagram showing L, D, and W as a closed vector triangle of the appropriate shape?

S

"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.

All the math is fine......
But, if your head is up your arse (Sorta British spelling), it does not matter if you leave a "dent in the ground".

I learned by/taught, "What does it sound like, what does it feel like, what does it look like?".
Period.

Either you were not taught well, or you, "Missed part of the discussion."

Can we please now close this thread? The horse has been whipped so many times, there is nothing left to whip............
Sigh..........

PS, off-field landing a ASW-20C with a full load of water (unknown to me until too late) I would have NOT made a decent landing. But the "feel" was wrong so I added speed.......

"For more information go to Knauff or Piggott. They've got a hell of a lot more experience and knowledge than almost anybody.*"

Yes and no... for example I heard Tom Knauff explain to a seminar that the wing reaches the stall angle-of-attack with the stick further aft in a turn than in wings-level flight, but he was unable to explain why. He offered an explanation that it had to do with the increased wing-loading-- which would suggest that we ought to see the same effect when we load up with water ballast.

This is a sincere question that I'll explore myself the next time I fly-- in wings-level flight, and it shallow bank angles where we can stall the wing before running out of aft stick travel, do most ships reach the stall angle-of-attack with the stick further aft with the airbrakes open than with the airbrakes closed?

S

I'm coming in at the end of a long ...er discussion, so excuse me if I"m missing something.
But I believe the diagram is correct. Regardless of how it's "drawn", the vector math for forces is simple.

L cos(a) + D sin(a) - W = 0
L sin(a) - D cos(a) = 0

Are necessary & sufficient for gliding without net force (acceleration) in the horizontal and vertical directions.



On Friday, June 5, 2015 at 12:29:01 PM UTC-5, wrote:
Re NASA diagram linked above in recent post--

That doesn't cut the mustard!

A) they didn't draw L, D, and W in the form of a closed vector triangle, so it's unclear that the net force equals zero

B) If you look closely at the three force vectors, you see that they COULD NOT FIT into a closed vector triangle of the appropriate shape (right angle between L and D).

Don't feel bad guys, NASA evidently doesn't get it either...

Don't the SSA instructional materials include a good diagram showing L, D, and W as a closed vector triangle of the appropriate shape?

S

On Friday, June 5, 2015 at 5:35:29 PM UTC-5, Sarah wrote:

... But I believe the diagram is correct. ...

Hi Sarah. First of all I erred in referring to a link to a google search as if it were a link to a single diagram. But these educational diagrams from NASA came up near the top of the search:

http://www.grc.nasa.gov/WWW/k-12/airplane/glidvec.html
http://www.grc.nasa.gov/www/K-12/airplane/ldrat.html

I have no quarrel with the math on those pages-- I haven't looked at it closely.

To me, a vector diagram conveys a concept much more intuitively and concisely than an equation, and the vector diagrams in these drawings immediately leap out to the viewer as being hopelessly screwed up. There is no way you could re-arrange those three vectors into a closed triangle. In relation to L, W is much too small and D is much too large. The glider cannot be in a steady-state-- it must be accelerating upwards and aftwards. So those diagrams are an embarrassment to NASA, and an obstacle rather than an aid to the children they are hoping to educate.

What I really don't understand, is why they chose to draw the diagram with the tail end of each arrow at the CG of the glider, rather than drawing the diagram with the arrows arranged head-to-tail into a closed triangle. A closed triangle of three force vectors elegantly and instantly conveys the idea that the forces on the body are perfectly balanced, never mind about sines and cosines.

It seems bizarre that the first one hundred images on a google image search for "Lift weight drag of a glider" didn't include a single diagram drawn as I have described above-- with the force vectors arranged head-to-tail into a closed right triangle. It seems to me that such a diagram ought to be near the beginning of any description of how a glider works.

Here's a diagram that's correct, but less elegant than a simple closed triangle of three vectors drawn head-to-tail:

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

S