Apr 23, 2012, 09:09 PM
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United States, CA, Mountain View
Joined Apr 2001
692 Posts
Quote:
 Originally Posted by Tuomo My knowledge about full scale soaring is limited, but basically I think you hit the point I was trying to make. To simplify my thinking: in windy conditions going up is rarely the problem, getting back home is the tough part of the flight. So you better ballast (mostly) for the homebound stretch... (it is always a compromise). Please explain better Francesco, I am running out of words
I'll try...
For this example, I will use the speed polar for a Supra, calculated with XFLR5. A 1500g Supra (green curve) may reach a maximum L/D ratio of about 27, this means a 27:1 glide ratio relative to the air mass it flies in; graphically, the operating condition is summarized by the point in which the polar touches the tangent drawn from the origin.

Flying in perfectly calm air, the glide ratio relative to the ground reference frame is simply equal to the glide ratio relative to the air mass.
If there's a headwind, things change a lot. If we don't change the glider trim, it will keep its trimmed airspeed, and so its speed relative to the ground will decrease, whereas the sink rate remains (hopefully) the same. This means that the glide ratio to the ground will also decrease. Of course, the glider doesn't care about anything relative to the ground, so ideally its flying will not be affected in any way by the headwind, but if we need to make it back to the landing spot, or if we want to cover ground in order to get to a possible trigger point or simply to "explore" a wider area of possible triggers, then the glide ratio relative to the ground is very important for the pilot!
In the attached chart, I supposed that the model is flying against a 6 m/s (12 kts) headwind: if we don't change the trim, the glide ratio to the ground drops to just 5:1! This means only 500 ft of ground covered while spending 100 ft of height!
Indeed, the best glide trim for no-wind conditions is NOT the same as the trim for the best glide (relative to the ground) for headwind conditions. The best glide trim for this 6 m/s headwind can be found by re-drawing the tangent line, starting not at the origin but at the 6 m/s point on the airspeed axis. This means FASTER airspeed: in my example, this condition requires flying at about 12 m/s (24 kts), and yet it would only yield a glide ratio of 10:1.
The only way to further increase the ground-relative glide ratio for windy conditions is to carry ballast. Ballasting a glider shifts its polar towards higher airspeed. In my example I supposed (orange curve) I could carry 1500 grams of ballast in my Supra (probably impossible unless with depleted uranium, but that's for fiction). The new best glide ratio to the ground would be achieved at about 14 m/s, and it would amount to about 16:1. What is probably more important and less obvious, the ballasted glider flying at ITS OWN best ground glide ratio will sink LESS than the unballasted glider flying at ITS OWN best ground glide.
Is that the point that you wanted to make, Tuomo? it's a very simplified view of the speed-to-fly theory.

# Images

Last edited by Francesco; Apr 23, 2012 at 11:31 PM. Reason: Clarified which curve is which loading
Apr 23, 2012, 11:27 PM
Registered User
Joined Aug 2003
2,660 Posts
Quote:
 Originally Posted by Francesco What is probably more important and less obvious, the ballasted glider flying at ITS OWN best ground glide ratio will sink LESS than the unballasted glider flying at ITS OWN best ground glide. Is that the point that you wanted to make, Tuomo? it's a very simplified view of the speed-to-fly theory.
That is just what I am thinking.

In practice, I think we all use mostly much less ballast than what would theoretically ideal. I do not know if more ballast would do the trick, but it is sure something to think about.
Apr 24, 2012, 12:37 AM
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United States, CA, Mountain View
Joined Apr 2001
692 Posts
Quote:
 Originally Posted by Tuomo That is just what I am thinking. In practice, I think we all use mostly much less ballast than what would theoretically ideal. I do not know if more ballast would do the trick, but it is sure something to think about.
Maybe I should further clarify what I meant by "will sink less": not only the ballasted glider, when flown at the best ground glide ratio, covers more ground than the unballasted one trimmed for its best glide, but it also stays aloft for more time in those conditions. The unballasted glider can still be trimmed for minimum sink rate, in this case its sink rate will be less than the ballasted glider, but the price to pay in terms of ground glide ratio will be huge.
Last edited by Francesco; Apr 24, 2012 at 12:50 AM.
 Apr 24, 2012, 09:44 AM Win=span\massXpractice+lu ck Joined Jul 2007 5,460 Posts I should have a couple new Xplorer MKII in my hot little hands by the end of the week! My build area is all set. I might even have one ready for a weekend contest..if I am lucky. I appreciate the technical explanations on theoretical best approaches. I am really looking forward to seeing how the MKII covers ground and ill have an older 3.8 to compare it to as well as a 3.5 MKII and 3.5 standard Xplorer...should be an interesting but less technical exposure.
 Apr 24, 2012, 10:00 AM Jody Miller Orlando, Fl Joined Jun 2009 643 Posts David, Glad to hear your models are on the way. Make sure to take a few minutes to post some pics!!! Thanks Jody
Apr 24, 2012, 10:01 AM
Registered User
United States, CA, Mountain View
Joined Apr 2001
692 Posts
Quote:
 Originally Posted by webbsolution I am really looking forward to seeing how the MKII covers ground and ill have an older 3.8 to compare it to as well as a 3.5 MKII and 3.5 standard Xplorer...should be an interesting but less technical exposure.
From what I've been told by friends who already have it, the new airfoils are more slippery, that should be good for covering ground.
 Apr 24, 2012, 10:02 AM Registered User Joined Oct 2003 2,120 Posts I will be at the Bulgaria Cup deep in NAN country on the weekend... It will be interesting to see if they will show the other new design they have been playing with..... If I see or hear anything new I will post it here..CA You can follow it here http://www.f3jbulgaria.com/2012info.html
Apr 24, 2012, 10:28 AM
Win=span\massXpractice+lu ck
Joined Jul 2007
5,460 Posts
Quote:
 Originally Posted by Larry Jolly I will be at the Bulgaria Cup deep in NAN country on the weekend... It will be interesting to see if they will show the other new design they have been playing with..... If I see or hear anything new I will post it here..CA You can follow it here http://www.f3jbulgaria.com/2012info.html
That will indeed be interesting I was told that it wont be available until after the WC but im sure the elite pilots will get a preview for SA
Apr 24, 2012, 04:30 PM
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Southern California
Joined Feb 2003
2,010 Posts
X2s

Howdy,
X2s are in the house...

Only a couple left!

B

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Latest blog entry: 2015
 Apr 25, 2012, 10:22 AM Launch high. Fly low. United States, CA, Lake Elsinore Joined Aug 2003 4,433 Posts Droooool... Hey, Bob! That huge SUSA Plane quiver you gave me (THANKS!!!) has plenty of room for another tenant in there! My 3.8X could use the company of an unpainted X2 sibling! LOL! Soar! Jun
Apr 25, 2012, 11:22 PM
Registered User
USA, CA, Moreno Valley
Joined Nov 2003
165 Posts
Just picked up my 3.5Xplorer II. Looks like it will come in lighter than the std 3.5 Xplorer