Minimum sink and optimum thermaling speed

[Phil Stang]

There is something that does not make sense to me re: the optimum thermaling speed in relation to the minimum sink airspeed. fwiw I can thermal pretty well and I think understand the basics.

For point of reference, it is conventical wisdom to fly at the 45 degree bank angle and at the slowest speed where one still has reasonable control.
In most gliders I find this to be about 5 knots
than bug speed.

For example, in an ASK 21 this is about 45 knots. In a Libelle 201 it can be as low as 40 to 42 knots.

However when I look at it from a polar/spec perspective something does not seem right.

My assumption is that the minimum sink speed is in level flight. Using the ASK 21 the min sink is 36 knots. At 45 degrees of bank the loading is 1.4 so this would make the minimum sink speed 50 knots. This is much faster than the 45 knots I would actually be using.

Is my understanding of min sink incorrect? Is flying at the "optimum" flying speed (slowest with still good control) actually sinking faster than optimal? A few more experienced pilots do seem to thermal a few knots faster than I do.
Is there another explanation for the apparent contradiction?


- Phil S

[Michael Bamberg]

Phil,

If you have access to the SSA magazine archive you can find several articles, published in the last 3-5 years, that explains all of that.

Flying at the minimum control speed lets you make a smaller radius circle, useful in a small radius thermal. You may find that the faster speed you calculated at 1.4x the min sink may result in a faster climb, if the thermal is large enough.

Several good ones: January 2019, May 2019 and August 2019.

MB

[Eric Greenwell]

The lift generated by the wing increases with the square the airspeed, so the factor is
not 1.4, but the square root of 1.4, or 1.18. So, 36kt x 1.18 = 42.5 kt, close to the 45
kt you see on the ASI when you are thermalling.

--
Eric Greenwell - USA
- "A Guide to Self-launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications

[Richard Livingston]

I fly at minimum sink while thermaling, but there are a number of factors at play you might want to consider:
-Flying a little faster gives you more maneuverability, depending on the glider you are flying. Better maneuverability might help you stay in the core better.
-The minimum sink speed is an optimum. Flying 5 kts faster or slower has virtually no impact impact on your sink speed.
-Different gliders have different low speed handling characteristics. If yours is right on the edge of breaking into a stall/spin you might want to increase the speed a bit above "optimum" sink.
-Thermals do rise faster in the core, so a smaller radius (i.e. lower airspeed at a given bank angle) theoretically will perform better, but I have rarely encountered a theoretical thermal. Your ability to make constant adjustments to stay near the core is, I think, more important than the exact speed you fly.

My two cents,

Rich L.

[Phil Stang]

Eric: Thanks for solving the mystery and clearing up a misunderstanding. Looking at some tables I now see the difference between the g-load increase and the stall speed (and therefore also sink speed) increase.

Rich and Michael: I agree with your comments. I am coming to the conclusion that flying at the "theoretical perfect optimum" (45 degrees, slowest speed but still adequate control) has only a very slight advantage in only the best circumstances. The actual climb rate will be less if many small corrections or small mistakes are made. In addition to safety margin, flying at 40 degrees (vs 45) or 45 degrees with a few knots more, is a bit easier thereby reducing the workload resulting in more mental resources are available for looking at the next cloud, etc. Reduced workload also also for longer flights. You also have a mental reserve for when you really need such as concentrating very hard to thermal "perfectly" when climbing to avoid an off field landing.

When I start flying in real life again I am going to thermal a few knots faster or at 40 degrees (2 sides of the same coin) in most conditions.

Cheers,
Phil S

[Hank Nixon]

As an old advisor once said to a hard question- it depends.
If the thermals are really small, turn tight(rarely over 45 degrees) and as slow as is good enough for control.
Some gliders don't climb well if pulled too hard due to airfoil characteristics. The '21 is not one of those.
Don't fly tighter than you can fly accurately.
You probably will not be as steep as you think you are.
Big circles make it easier to lose the thermal.
Once you think you have it, hold your bank and speed steady so as not to wander out.
Slow gives a smaller circle due to centrifugal force. If the choice is slow or steep- fly slow if the glider allows.
Add a couple knots below 1500 feet or your personal margin.
On most days I try tightening until my climb gets best and use that as a reference going forward.
FWIW
UH

[Eric Bick (DY)]

Steve Platt has written at length in Soaring magazine “Glider Aerodynamics,” using the “Standard British Thermal” for thermaling analysis: speeds to fly, angles of bank, masses of glider. These are great guidelines - in theory. Soaring out of Moriarty, NM, I find the theory to only be a guideline. When we start in the morning, the thermals can be relatively soft and wide, single core (4 -6 kt), so constant bank angles of 40-45 degrees work, and flying near minimum sink. As the day develops, the thermals can start having irregular cores, and often multiple cores, and I find it necessary to constantly adjust the bank angle, including up to 60 degrees, but generally 45 - 50 degrees with 6 - 12 kt up, and often 15 kt or more in the core. This often leads to flying through multiple cores, slowing in lift, accelerating in sink, adjusting bank angle as circling. I find I seldom thermal in these booming conditions near minimum sink speed. Somewhere around 60 kt will often be comfortable, sometimes up to 65 kt when it’s really boiling, especially helpful when an invisible hand reaches out, grabs your glider and tries to invert it - quite a helpless feeling, but speed is your friend when this happens and you work to regain control. (Imagine you have one wing tip in 10 kt up and one in 10 kt down and a gust hits you from behind - don’t know if that is what is actually happening, but sure feels like it.) And, these strong cores tend to push you out of the thermal quickly unless you bank steeply. I’d say about 50 degrees is sometimes common to maintain position, judging from the instrument screw positions on my panel relative to the horizon.

As a side note, I’ve flown almost exclusively in the U.S. southwest desert and mountains (which entails some other considerations as to speed in thermals and position relative to the terrain). I’ve flown in Florida twice (Seminole in June), wondering where the thermals were - in the two flights I had there, I didn’t adjust to the idea that 2 - 3 kt could be a good thermal, and 2500 - 3000 ft AGL was good soaring height.

[R]

Good info.
I tend to ignore speed, but match pitch to the conditions of the thermal.
Smooth thermals are easy.
Choppy requires a compromise.
My goal is to have as little change in pitch throughout the circle. The best way to perfect this is learn, for your specific glider, where the horizon cuts thru the nose or canopy. This 'cut' changes with thermal stability, or as you climb.
A rising and falling nose chasing airspeed (pitch change) reduces the efficiency of the climb.That in turn requires more time in the thermal.
Try to ignore looking at airspeed and 'pitch' your way into the groove. We all know that bank requires pitch, but make it easy on yourself. Keep it at 45* and find the balance. Learn that and all the rest will fall in place.
Good luck!
R

[Eric Greenwell]

Where is an Angle Of Attack meter when you need/want one? Maybe it's in Hawk?

[2G]

Trying to do these calculations and g-loadings is a fool's errand. In practice you feel what is optimal and safe; if you get too slow you start feeling stall burbles and speed up. Spending effort trying to fly a specific airspeed or bank angle just detracts from more important duties.

Tom

[Hank Nixon]

I could not agree more.
UH

[Richard Livingston]

On Thursday, March 31, 2022 at 6:25:37 PM UTC-5, Hank Nixon wrote:
> On Thursday, March 31, 2022 at 12:48:59 AM UTC-4, 2G wrote:

...

> > Trying to do these calculations and g-loadings is a fool's errand. In practice you feel what is optimal and safe; if you get too slow you start feeling stall burbles and speed up. Spending effort trying to fly a specific airspeed or bank angle just detracts from more important duties.
> >
> > Tom
> I could not agree more.
> UH

I did these calculations once for my glider and my flying weight. I calculate the minimum sink speeds for level flight, 30, 45 and 60 degree bank angles. The nice thing about optimums is that +/- 5 kts variation in speed has little impact on the sink rate, so you really don't have to sweat the exact speed, but given the angle of bank and the conditions it is easy to memorize these four speeds and adjust a little bit up or down as appropriate.

Rich L.

[R]

But, those learning the art of Soaring, as all GREAT instructors know, must have a starting point to build upon.
Telling a newcomer to forget speed or bank angle ( UH, ? hello...you there?) ala caveate creates a foundation of peat moss.
The ops question, I believe, is seeking a starting point based on his question. As experience is gained, the pilot will connect the dots in a drawn out eureka enlightenment, stand on the podium and tell us all...how he did it.
Then he can do more "... important duties..." like fiddle with his LX9000 or play Condor on his mega-phone while racing.
Maybe even enjoy the view.
Baby steps.

R