Reading SGS 2-32 Polar Curve

[runner4065]

I'm struggling how to read the polar curve for an SGS 2-32. It can be found on page 19 of the flight manual found here:

http://www.airsailing.org/downloads/2-32%20Flight%20Manual.pdf

What are the two different groups of lines? I'm only familiar with one (the top set).

Any significant in a band vs. a line?

Typically the origin (0,0) is in the top left with sink rate increasing as you move down on the Y-axis. This chart is different.

What is the L/D-S=7.44 or Vs-S=7.44?

My experience is minimum sink is a vertical line at the apex. So I believe at 1020 lbs, this would be about 53 mph.

Best L/D is typically a line from the top left (0,0) origin, or about 61 mph. But again, there is no 0,0 origin and I'm not sure what the L/D-S vs the Vs-S lines are.

Any help is appreciated.

Thanks

[Michael Opitz]

The bands are probably the result of data point scatter when they did
flight tests. For L/D and Min Sink speeds see page 9 of the manual -
depending on gross weight....

[Papa3[_2_]]

There are 4 curves in total: L/D solo (wingloading at 5.67 lbs/sqft) and dual (7.44 lbs/sqft). Those are read against the left Y axis. Sink rate solo and dual. Those are read against the right Y axis. As far as the bands, I suspect it's "truth in advertising" by Schweizer; i.e. "it's our calculated L/D so don't hold us to it" :-)

[Steve Leonard[_2_]]

On Tuesday, March 15, 2016 at 3:52:27 PM UTC-5, Papa3 wrote:
> There are 4 curves in total: L/D solo (wingloading at 5.67 lbs/sqft) and dual (7.44 lbs/sqft). Those are read against the left Y axis. Sink rate solo and dual. Those are read against the right Y axis. As far as the bands, I suspect it's "truth in advertising" by Schweizer; i.e. "it's our calculated L/D so don't hold us to it" :-)

And the upper curves are the L/D curves. Well, upper at the left hand side of the chart. The lower curves are the sink rate curves.

Don't you just love the optimism of showing best speed to fly for next expected thermal of up to 1600 feet per minute!

Steve Leonard

[Steve Leonard[_2_]]

To read the L/D curve, solo, the plane will have a best L/D of about 35 at a speed somewhere between 52.5 and about 56 MPH. The curve is fairly flat up there. L/D read from the left side scale, airspeed read on the horizontal axis.

To read the VS (vertical Speed) curve, for this same speed range, the VS will be roughly 2.1 to 2.4 feet per second (read from the Right side vertical scale). As you indicated, hard to tell from the VS curves exactly where best L/D occurs since they truncate the vertical and horizontal axis of the plot. Must be why they also show the L/D curves on the same chart.

I believe you would use the VS curve to draw the polar as you are use to seeing it. Complete with airspeed going all the way to zero (so you can more quickly do graphic analysis) and sink rate going all the way to zero. Lopping off the unobtainable portions of this graph allows a manufacturer to show a "flatter" polar, making you think the plane will not increase its sink rate rapidly as you increase the airspeed.

Hope this is more helpful than my previous reply.

Steve Leoanrd

[runner4065]

Helpful. Am I correct in assuming this L/D curve is interpreted differently than a polar curve? And therefore the apex of the curve is the best L/D, not a line touching the curve beginning at the origin?

How do I compensate for wind? If I have a 15 mph headwind, what becomes my best L/D?

[Steve Leonard[_2_]]

On Tuesday, March 15, 2016 at 5:04:14 PM UTC-5, runner4065 wrote:
> Helpful. Am I correct in assuming this L/D curve is interpreted differently than a polar curve? And therefore the apex of the curve is the best L/D, not a line touching the curve beginning at the origin?
>
> How do I compensate for wind? If I have a 15 mph headwind, what becomes my best L/D?

Correct. On the L/D curve, max L/D is the apex of the curve. On the "polar curve" as you describe (horizontal and vertical speeds), best L/D is a line drawn through the origin, tangent to the polar curve.

To see what affect wind has, you can take the existing chart, extend the airspeed to the left to find the zero airspeed, and the vertical speed down to zero vertical speed, and use this origin. It will be "upside down" from what you are use to, but you should be able to work with it, drawing tangents to find speeds to fly for various winds and anticipated climb rates.

Steve Leonard

[Andy Blackburn[_3_]]

On Tuesday, March 15, 2016 at 1:25:54 PM UTC-7, runner4065 wrote:
> I'm struggling how to read the polar curve for an SGS 2-32. It can be found on page 19 of the flight manual found here:
>
> http://www.airsailing.org/downloads/2-32%20Flight%20Manual.pdf
>

I love the cover of the manual - a series of nouns that together describe the entire experience of owning your own glider.

A.

[Andy Blackburn[_3_]]

On Tuesday, March 15, 2016 at 1:25:54 PM UTC-7, runner4065 wrote:
> I'm struggling how to read the polar curve for an SGS 2-32. It can be found on page 19 of the flight manual found here:
>
> http://www.airsailing.org/downloads/2-32%20Flight%20Manual.pdf
>

I love the cover of the manual - 3 nouns that together describe the entire experience of owning your own glider.

9B

[kirk.stant]

The whole flight manual is so quaint!

I had forgotten that Vne is higher with the dive brakes open (158) than closed (150), and is called "Max Glide Speed". Are you still technically gliding when going straight down standing on the rudder pedals? (BTDT)

Aerobatics get two short sentences, basically, "no restrictions, but be careful". Meanwhile, flying in cold weather gets a couple of pages, including a paragraph on Wheel Freeing that ends with "The consequence for landing with a locked wheel is not severe at most, a blown tire."

Anyone who has been around a 2-32 with a flat or blown tire knows how un-fun that can be!

Still my all time favorite Schweizer!

Kirk
66

[Charlie M. (UH & 002 owner/pilot)]

On Wednesday, March 16, 2016 at 12:48:49 PM UTC-4, kirk.stant wrote:
> The whole flight manual is so quaint!
>
> I had forgotten that Vne is higher with the dive brakes open (158) than closed (150), and is called "Max Glide Speed". Are you still technically gliding when going straight down standing on the rudder pedals? (BTDT)
>
> Aerobatics get two short sentences, basically, "no restrictions, but be careful". Meanwhile, flying in cold weather gets a couple of pages, including a paragraph on Wheel Freeing that ends with "The consequence for landing with a locked wheel is not severe at most, a blown tire."
>
> Anyone who has been around a 2-32 with a flat or blown tire knows how un-fun that can be!
>
> Still my all time favorite Schweizer!
>
> Kirk
> 66

In the 1980's (?) I believe Schweizer came out with an "update" to all their flight manuals stating "Aerobatics are not approved", mostly to cover themselves "in case someone pulled the wings off, etc.".
Having an old manual does NOT mean you can "do what you want" since the FAA would look at the current MFR manual.
;-)

[[email protected]]

On Tuesday, March 15, 2016 at 1:25:54 PM UTC-7, runner4065 wrote:
> I'm struggling how to read the polar curve for an SGS 2-32

Others have taken care of the important points, so I will address the semantics: The L/D curves you are looking at are not polars. They are plots that show the same data as a glide polar, but in a different way.

However, I think that the sink rate curves shown lower on the graph are indeed polars in the traditional sense. They are shown upside-down, and the origin is beyond the scope of the data shown, but if you extrapolated the horizontal speed out to 0 and converted the vertical speed to something less crazy (feet per second? Really?) you can use it just like any other polar.

I could be wrong, but I think that at the time that the 2-32 manual was published, what we now think of as a standard glide polar was not standard here in the US. Or it might be that the FAA folks who reviewed the manual were not familiar with the polar format. So maybe Schweizer formatted the data in a way that they thought their US-centric regulators and customer base would be more comfortable with.

Thanks, Bob K.

[runner4065]

> In the 1980's (?) I believe Schweizer came out with an "update" to all their flight manuals stating "Aerobatics are not approved", mostly to cover themselves "in case someone pulled the wings off, etc.".
> Having an old manual does NOT mean you can "do what you want" since the FAA would look at the current MFR manual.
> ;-)

Aerobatics not authorized per a SB in 1987, covering most (if not all) of their sailplanes.