RC Groups

RC Groups
    Thermal
        Discussion Ballast Use: How much do you use in what conditions and why?

#1 R2R Dec 29, 2012 08:31 PM

Ballast Use: How much do you use in what conditions and why?
 
2 Attachment(s)
I got the green light to attend a few nearby contests this winter and spring, so I spent some time over the last couple of days installing a ballast tube in my Sapphire and cutting some ballast rods. They are 5/8" diameter steel.

3 1/2" x 5 oz
5 7/8" x 8 oz
9 7/8" x 14
15 1/8" x 22 oz

The tube is rolled fiberglass with a blind nut epoxied to it and the aft-end sealed off. (Thanks to the personal stash of a great vendor/fellow RCG'er for that one and another one to be installed in my RES!! ) In the tail boom the tube is epoxied to the tail floor with a piece of foam wedged above and epoxied to tie everything in. At the forward end of the tube, it is glued to the hardwood dowel cross-brace on top and to a 1" diameter dowel that is glued to the fuse floor. The front of the dowel is also epoxied to my servo tray. A #6 screw will lock the ballast into place, screwing down into the blind nut into the dowel. The forecast is for 9MPH winds tomorrow, so I should get the chance to try out the lighter weights.

It was very timely receiving my Model Aviation this month. The excerpts on how the Masters use ballast was super interesting, especially JW's comments on Max LD to wind speed in marginal lift conditions and D.Hobby's comments. What I got out of it was that, both tune ballast using thermal activity as the overriding factor, while still keeping in mind wind speed. Seemed a little more concrete/systematic than what Dave Thornburg has captured in Old Buzzards.

I have never used ballast, so I can't offer any personal experience; how much do you all use for certain situations and why?

Allan

#2 satinet Dec 30, 2012 07:20 AM

What's your glider like?

Ballast is realitive to the weight and characteristics of the model imho.

#3 R2R Dec 30, 2012 07:37 AM

satinet,

It is 64.4 oz @ about 9.5 oz/sq-ft loading without ballast. SD7037 airfoil. 120" wingspan, fullhouse. I'm not sure what other characteristics ...

#4 satinet Dec 30, 2012 08:30 AM

sounds like a pretty floaty model.
Hard to say isn't it. try and bit and see how you go. A few ounces won't drastically effect the sink rate.

#5 Curtis Suter Dec 30, 2012 08:35 AM

I have never flown that airfoil that light. I used to fly it at 12 oz sq/ft and it was great! Camber changing is real important with the SD7037 too. I can't wait for your test results.

Would you happen to have a small GPS receiver that can export NMEA data?
If you do and would like to accomplish some trailing edge L/D testing send me a PM.
The weather isn't conducive for me to accomplish that for a few months or more!

Curtis

#6 R2R Dec 30, 2012 09:29 AM

satinet, I will give it a try, hopefully this afternoon, when it warms up a bit outside! Thanks.

Curtis, I don't have a GPS receiver, but thanks for the offer! Didn't know people were flying the SD7037 at 12oz/sqft.

#7 williamson Dec 30, 2012 10:44 AM

Quote:

Originally Posted by satinet (Post 23658738)
sounds like a pretty floaty model.
Hard to say isn't it. try and bit and see how you go. A few ounces won't drastically effect the sink rate.

If the ballast is centered on the CG, there should be no need to change the elevator trim. Under those conditions, the plane will glide along essentially the same glide path independent of the weight of the plane. Increased ballast will result in increased speed down that glide slope. The L/D (lift/drag) will remain nearly the same.

Under the conditions described above, the speed of the plane will be proportional to the square root of the weight. Doubling the weight of a plane will increase the speed by 41% (sqrt(2)=1.414...). Adding 6 oz. of ballast to a 60-oz. plane will increase the speed by 5% (sqrt(1.1)=1.05...). Increased speed means both a higher horizontal speed and a higher vertical speed (rate of descent). If thermals are strong, the increased rate of descent isn't a problem. The increased horizontal speed means that it easier to find new thermals and also easier to come upwind to land.

Increased weight also means the plane will be tossed around less by wind gusts.

#8 glidermang Dec 30, 2012 12:26 PM

R2R:

I fly DLGs, and ballast is a big factor in those. How I use it is this way: when the turbulence makes the airplane difficult to control, I add ballast. It's not a function of wind speed, primarily.

The advantage of ballast is the higher speed at which the airplane will fly, for a given L/D (a lot of folks will climb all over this, but that's my point of view). The price of ballast is increased sink rate, which is sometimes a heavy price to pay in windy conditions when thermals may not be as strong.

I had a pair of Taboos a couple of years ago, and experimented with ballast quite a bit. They both weighed 8.7 ounces or so. I could easily load as much as five ounces of ballast, but not only did they fly faster, they also broke in less-than-perfect landings. I realized that three ounces was the practical limit - less might not make the airplane more manageable, more increased the probability of damage. I routinely flew those Taboos in winds approaching 20mph with two or three ounces of ballast. I fly a pair of Blaster 3s now, and have a 2-ounce ballast stick. They weigh right at 10 ounces, and simply don't need more.

It's a personal preference, but I install ballast so that the CG is unchanged. Makes it a lot easier to put in and take out - no requirement to re-trim for ballast.

Good luck, and have fun at the contest. That's the best place to learn.

Yours, Greg

#9 R2R Dec 30, 2012 01:36 PM

williamson,

That looks like a nice formula for estimating speed gained by adding ballast. My ballast system is set up so that the CG doesn't change regardless of the amount added. Hopefully no trim adjustments will be needed.

#10 R2R Dec 30, 2012 01:48 PM

Greg,
Interesting methods of deciding when and how much ballast. I can see the advantages of being more resistant to turbulence, in addition to better speed. Like you, my ballast system is set up so that the CG doesn't change regardless of the amount added.

#11 aeajr Jan 19, 2013 11:29 PM

Supra - 8 to 14 ounces
Ava 9 to 12 oz
Radian - 4-6 ounces
Easy Glider 4-9 ounces
Spirit 4-12 ounces

#12 R2R Jan 20, 2013 06:41 PM

Quote:

Originally Posted by aeajr (Post 23867836)
Supra - 8 to 14 ounces
Ava 9 to 12 oz
Radian - 4-6 ounces
Easy Glider 4-9 ounces
Spirit 4-12 ounces

Thanks for posting, Ed. From the looks of your weights, I am probably fine with my first 3 sets of ballast (5-14 oz) and probably won't ever need the 22-oz setup.

I flew the 5-oz set last weekend. The wind was only about 5-9 mph and the lift was weak, but I wanted to get a feel for flying with the ballast. It was interesting to see the difference in speed in thermal turns and the momentum while landing. I would deploy flaps on final, and the plane's glide path/pitch was the same as without ballast, but the speed was higher and it just wanted to maintain the trajectory. I found that I have to flare earlier than usual -- took about three times to figure it out (yeah, I'm a little slow that way with piloting), one time flaring much too late and unintentionally dorking at about 30 degrees, tail stuck in the air, nose buried. :o

#13 aeajr Jan 20, 2013 06:53 PM

Dependng on the plane I would typically not put ballast in till we are around 10 mph for the light ballast and build up from there.

Ava, Radian maybe a little sooner.

#14 R.M. Gellart Jan 20, 2013 08:22 PM

From my Sapphire days, I had a solid steel wingrod I used for ballast, it added 16oz, and that was my start point. I would probably wait till 12-14mph of wind. Remember, that when ballasting, you need to make a significant difference in loading on an airfoil like the 7037. Your 5 and 8oz bars, I am guessing you will not even notice the difference, but the 14 and 22oz bars will be useful. Your install is very nice.

The new airfoils, and current very light airframes, ballasting has changed a bit since those days and when we flew 7037's alot.

Marc

#15 aeajr Jan 20, 2013 09:03 PM

I have always looked at ballasting as a % of of the unballasted weight.

At 15% I can tell the difference.

For a Radian at 28 oz that is about 4 oz
For a 60 ounce Supra that is about 9 oz

I have rarely every felt the need to ballast more than 25%

Again for the Radian that would be 7 oz
for that 60 oz Supra that would be 15 oz

If I have to ballast more than 25% I better be flying for a world championship because those conditions must be wicked!

I ballasted my Supra at 60% once, just for testing. She flew just fine into a 25 mph wind. Clearly the sink rate was higher but I was able to push right through the wind to find lift that I probably would not have reached at 25%. But the sink rate was certainly much higher. And it landed fast.

Naturally your smileage will vary.


All times are GMT -5. The time now is 09:53 AM.