A question of ballast

[Steven N]

I have a Bird 3000 and where and how do you determine ballast on windy days? I guess I'd have to build something to put the weight in but then where do I place it and how much weight. The wings are 1000 sq. in.
Over all weight is 52 oz.

[SoCalGliderFlyr]

Ballast centered over the current balance point of the plane. Depending on your tow hook design you may have sufficient threads protruding into the fuse to slip lead bars over.

[Steven N]

I can do that and have several lead sticks available. What's normal or a rule of thumb for the ammount of weight needed? Like 1 to 2oz Or more?

[SoCalGliderFlyr]

Think four to six ounces at a time. Two pounds of ballast for a 3 meter sailplane is not unexpected. Some will biased the weight forward to shift the balance point for windy conditions.

[nuevo]

Try 6 to 8 oz increments in ballast. You won't be able to notice lesser changes in weight.

[invrtd]

Start with 10% of the weight of the model and work up from there depending on the wind. I usually balast my 3m - 6 oz on slightly windy days. Last weekend we had gusts up to 25 and I ended up with 13 oz to penetrate.

[solo6796]

Weak spar on that plane....be careful on windy days. I broke two of them.

AJ

[nuevo]

Uhhh..... do what AJ says. I was thinking of a much stronger plane I used to fly. Best wishes.

[Steven N]

I've already reinforced the spar. I had a feeling I needed more ballast. Thanks a lot.

[averen]

I remember reading somewhere that ballast has little effect unless you start approaching 150%+ of the original wing loading. I can't remember where I found this little tid bit of info though.

Jared

[Ollie]

The weight makes the air speed and sink speed as the squareroot of the weight.
Wt.% Speed %
100.0 100
110.0 104.88
125.0 111.18
150.0 124.74
200.0 141.42

[Steven N]

Ok Ollie, looking at your % ratio, what would be the % lets say at 10 to 15 mph wind? The weight of my bird is around 54 oz. I know this has to be too light for that type of wind.

[SoCalGliderFlyr]

Your really need to think wing loading per square foot. Some large planes are fairly light. The effect is refered to as apparent mass.

[D_Ryan]

Steven N; weight alone does not necessarily dictate whether a plane can penetrate, planform (especially airfoil) plays a part. Current foils like Mark Drela's AGxx series as used on the Supra, and the foils that Don Stackhouse engineered for the Spectre series are quite thin, and even though those aircraft have low weights, are still able to penetrate when the wind comes up. My 120" Spectre had an all-up weight of 47 oz. yet did not require ballast until the wind was approaching 15 mph.

[Ollie]

http://www.profili2.com/eng/default.htm
http://my.athenet.net/~atkron95/pcsoar.htm

[skybenchone]

Also, as you add ballast, the balance point can be moved a little further back. A flying technique for flying faster is put the balance point back a bit and add slight down trim (lessens wing incidence), adding ballast will enhance this set up. But, this only applies to modern airfoils, I'm sure this will not work as well on flat bottom airfoils. I add a 12oz lead slug to my 100" Big Bird when the winds exceed 15 mph or if it is a very dynamic, gusty type of day (spring time especially), the slug moves the balance point back an 1/8".

Ray Hayes http://www.skybench.com

[Steven N]

So far I have made a 4 1/2 oz and a 7 1/2 oz sticks with what I had laying around. After I finish re-covering the center section of the wing I'll find the true CG and test the weights for center as well.

[skybenchone]

Steven,

Just trying to be helpful, adding 4.5 oz to a thousand sq in wing is meaningless in winds over 5 mph, but I applaud you for considering what ballast can do for sailplane performance. Keep experimenting and do the math (oz per sq ft) and you will see what I mean.

Ray Hayes http://www.skybench.com

[fly1milehi]

[Also, as you add ballast, the balance point can be moved a little further back. A flying technique for flying faster is put the balance point back a bit and add slight down trim (lessens wing incidence), adding ballast will enhance this set up.]

Ray, Im not sure about this one. My experience with F3B sailplanes suggests this would be a great way to lose a good airplane. In the pursuit of world class speed runs I was pushing the CG limits on my RnR synergy. It had a build in ballast "box" in the fuselage, accessed via a hole in the bottom of the fuse. In checking the CG with ballast on the workbench it was obvious that the CG moved forward with the ballast filled. I measured and and found that .4oz added to the tail moved the CG to it original thermal location. HOWEVER.. and this is a biggie, it made the Synergy VERY happy to do snap rolls instead of fast 180 turns in the F3B speed run. It may be that the camber controlled trailing edges mixed with elevator etc on the F3B gliders makes the CG moving forward with ballast the way to go. (Yes a Synergy will survive a snap roll at 100+ mph, it looks very impressive too!)
Just my 2 cents on moving the CG back with ballast.
Greg