Interpreting "PERFORMANCE AIRSPEEDS FOR THE SOARING CHALLENGED" wrt ballasting

[rogerflies]

Whenever the subject of ballasting comes someone makes a reference to this very good article: http://home.att.net/~jdburch/polar.htm
Usually they refer to it to justify adding ballast on windy days so they can "penetrate with the best possible performance".

In the article, ballast is mentioned only in the very last paragraph, which reads:

"A different polar curve exists for the same glider at every gross weight. As the weight increases the polar moves down and to the right and becomes a little flatter, but retains approximately the same shape. Both airspeed and sink rate increase for equivalent points on the curves, but their ratio remains the same. Both heavy and light glider achieve the same best glide ratio, as shown here, but the heavy glider does so at a higher speed. This is the reason for carrying ballast to improve glide performance when the thermals are strong enough, even though the climb rate suffers."

Throughout the rest of the article, the gliders' airspeeds are varied only by changing the pitch, because the gliders are otherwise identical.

The important thing about the last paragraph is that the justification for using ballast is that the thermals are strong enough to make up for the lower climb rate that goes along with carrying extra weight. The higher speed means you spend less time between thermals, too.

It's been my experience (over the past twenty years of flying sailplanes) that the best thermals are found on days with light to moderate winds. That's why I don't see the point of adding ballast on windy days. And I don't think giving up the ability to climb well for the ability to zip around from thermal to thermal is a good bargain any day. Always having my best climb rate available makes up for whatever performance I lose by putting the nose down when I want to go fast.

Roger

[schrederman]

So... when the rest of the world, in full-scale soaring and R/C soaring, use the conventional wisdom of adding a bit of ballast, you're arguing against it? Not trying to be antagonistic, just not sure what your point is. I know from experience, both full-scale and from flying models, that ballast can be a really good thing, and the putting the nose down is just that... decreasing your glide ratio, sometimes drastically for the same effect. Besides, ballast doesn't really reduce the climb rate all that much...

In the lower Reynolds numbers experienced in R/C, that ballast can be a real blessing. When the wind's blowing aloft at the same speed your glider normally flys, adding ballast is a much better strategy for staying aloft than "putting the nose down."

Then again, if you're flying only moldies, they usually fly fast enough that I can understand why you might think that way.

Jack Womack

[Thermaler]

A few paragraphs up:

"To see how sinking air affects our glide let's compare the red and yellow gliders. If both enter an air mass that is sinking at 300 feet per minute (3 knots), their sink rates will increase accordingly. The yellow glider is now sinking at 4.9 kts (1.9 within the air mass plus 3 for the air mass sink rate). The red glider is now sinking at 6.2 kts (3.2 + 3.0). Both enter the air mass at the same altitude; our objective is to come out the other side with the least loss of altitude. Although the red glider is sinking faster, it is also moving horizontally much faster (62kts vs 42 kts). The result is that it gets out of the sinking air at a higher altitude than the yellow glider. The red glider has a better glide ratio, relative to the ground, than the yellow one even though the yellow glider has a better glide ratio relative to the air mass."

We usually see sink between climb in any wind conditions with or without ballast. :confused: I would rather be able to get out of sink faster and move on to the next lift patch than have to try recover the lost altitude with a better climb rate if the slower airspeed allowed me to make it to the next lift patch. :eek:
I may not climb as well but I have not lost as much altitude which might just make all things equal putting us back to pilot skill to keep flying. :D Then again I may have already used the next lift patch, gained more altitude and exiting the next sink patch into lift again by the time the better glide ratio relative to the air mass gets out of the drink, I mean sink :rolleyes:

Joe

[dharban]

That's why I don't see the point of adding ballast on windy days.
Roger

As long as you don't have to put the nose down to penetrate the wind you are flying in you probably don't gain much from ballasting. But the minute you have to put the nose down (change the angle of attack) to penetrate, you are better served to add ballast.

For the sake of simplicity, assume that you can control penetration only with elevator or ballast (no camber control). On a windy day you can penetrate (increase speed) by putting the nose down (elevator) or fly at a heavier weight (ballast). When you put the nose down (decrease angle of attack) you increase both the forward speed and downward speed (sink rate). If you work your way all the way through a polar analysis you will find that the increase in sink rate realized to achieve a particular forward speed by putting the nose down is substantially greater than the increase in sink rate realized by keeping the effective angle of attack up and adding weight to achieve your desired speed.

I modeled a typical RES ship and found that there was little advantage to adding ballast up to about 12 mph. But there was more than a 30% improvement in sink rate utilizing ballast over trim to fly at 20 mph -- and the calculated optimum amount of ballast was nearly equal to the plane weight!

I know guys take their AVA's and Bubble Dancers out and say they successfully flew them with little or no ballast in moderately high winds. No doubt. But a full analysis of the polars will show that they were not flying at the minimum sink rates possible. Often in windy conditions, especially in RES the game is to launch high and hang or surf bubbles that come through. In these conditions, minimizing sink rate (for the speed you need to fly) is the name of the game.

[OVSS Boss]

Roger, I rub the button here for a bottom line. Yes, you can use pitch to run with, but since I do not have loose as much as you do to cover the ground, I am ahead there. Second, depending on the day, there is huge to light air in wind, but the difference in climb rate in our two ships is not that much. My attitude for ballast is that I do not want to go fast, just move at as close to my normal L/D as possible. Now at this same moment, your normal L/D is going in the crapper to make the change in trim you have to make to cover the ground you want to come home or go to lift.

I realize that if you found the boomer and went downwind with it, you feel that you have no problems. But what if you were Arend Borst at the WSM, in the finals he flew a 62ish ounce Supra in a solid 15+ mph wind and went way downwind. Now, he had in 20 ounces from what I heard, he came back very impressively and the lift was not great, just a bit better than sustaining kind of air on that flight. With no lead he would not have made it home, he would have landed out and not made the money crowd, just that simple.

You can decide not to fly with ballast, but as Jack said, the research and numbers are very well known, and guys trust their butts in the seats with them. Choose to use the numbers to your advantage or not, but this is solid stuff here.

Marc

[rogerflies]

But the red glider in the section dharban quoted above got the increased airspeed by putting the nose down, not by adding ballast. If it's OK to put the nose down to increase the speed in that situation, it's OK to put the nose down when you're flying into the wind.

Now, the situation MAY change if you're flying cross-country into a headwind. The performance advantage of getting your speed from ballast isn't going to be offset by the disadvantage of carrying the extra weight when you're going in some other direction or circling. But it's ONLY going to work if the thermals are strong enough to offset your reduced rate of climb.

On a typical day, the sinking airspeed in the areas between thermals isn't going to be as high as the rising airspeed in the thermals, since the downward flow takes place over a much broader area. Cutting your losses in sink doesn't make up for being able to maximize your gains in lift. I think you'd be better off learning how to recognize where NOT to fly.

I think people tend to spend too much time playing with ballast when they really need to learn to read their planes, judge the air they're flying in and the terrain they're flying over, and plan their flights better.

It may not be much of a factor with the moldies, but adding ballast reduces the margin for error in judging the airspeed of the built-up sailplanes. The added weight reduces the airspeed where flutter sets in as it increases the "normal" airspeed, so you've got a narrower window of safe airspeed to work with. Seeing a wing disintegrate is a real bummer.

Roger

[schrederman]

If you're worried about flutter, you're apparently not flying a Houston Hawk...

[Batmanwpg]

<It may not be much of a factor with the moldies, but adding ballast reduces the margin for error in judging the airspeed of the built-up sailplanes. The added weight reduces the airspeed where flutter sets in as it increases the "normal" airspeed, so you've got a narrower window of safe airspeed to work with. Seeing a wing disintegrate is a real bummer.>

Only practice will overcome your "error in judging" the airspeed. Adding ballast is like flying a whole new airplane. It doesn't turn as tight, flies faster etc, etc. Practicing with ballast will only make your non-ballast days seem like a cake walk. As said before by adding ballast the plane will fly faster and with this the airfoil actually gains by boosting the Re and having the polar shift left on an L/D graph. The looses by adding ballast are actually small, it's more the psychological factor we have to overcome.
If your wing flutters apart, well you need a better design!

[Thermaler]

I believe I did the quoting and I do not see were it says the red glider used down elevtor. The red one uses it's mass to gain speed.

Joe

[Miami Mike]

I'm enjoying this thread very much and don't have much to contribute, except possibly this:(From http://home.att.net/~jdburch/polar.htm)

Suppose we have four Schweizer SGS 1-26 gliders, identical except for color. For our test, we will line them up and fly them at the speeds and sink rates in the following table:My read is that since they're identical, they weigh the same. The only difference is the way that they are flown.

[tonyestep]

Speaking of ballast, most people agree that the coolest ballast system is the Icon's. However, for a more conventional slug/spacer arrangement that is very cool, check out this link for the Stratos 6 ballast system:
http://www.soaringusa.com/cart/cart.php?target=image&action=detailed_image&id=1025
Having that little picture would make it much easier at the field when you are down on your knees groveling inside the fuse, trying to figure out what you're supposed to be loading.

[rogerflies]

That's correct, Mike. Down near the bottom of the article, just before the paragraph I quoted, he says:
"The performance data used here is based on a SGS 1-26 at 575 pounds gross weight, as shown in Appendix A, The Joy of Soaring."

I'm not totally anti-ballast. It's a useful tool to have in your arsenal, but I dispute the idea that says:

"It's a windy day, so I've got to put in some ballast."

There are other factors to consider, and you make some compromises when you put in the ballast.

Heck, I've even used ballast myself. When I had my Cessna 172, I'd fill the tanks and throw some extra stuff in the plane if I was going to fly on a "bumpy" day. The extra weight made the ride smoother. Of course, it wouldn't climb as well or go as fast, and it burned more fuel, but the smooth ride was more important. I wouldn't add extra weight on a cross-country flight with a headwind, and I wouldn't do it at night when the air was smooth, either.

That points up one of the trade-offs. "Ballast = smooth ride" means the plane won't signal lift quite as well. Hmmm....

Roger

[dharban]

The many points made concerning the importance of how you fly your plane being important in successful flying are well taken. There is no substitute for flying skill and familiarity with your plane.

That being said, if all other things are held equal (skill, etc.) the simple fact is that above some particular speed (for RES ships, for example probably 12-14 mph) the sink rate realized by adding ballast will be less than the sink rate achieved by putting the nose down -- period. Again, all other things being equal, there is an optimum weight to achieve minimum sink for each speed that the glider flies. For a RES ship (without camber control) that weight increases rapidly as airpeed increases.

For any glider, there is an advantage to adding ballast whenever wind strength becomes a factor in staying over the field -- not just for maintaining the speed that is needed, but for achieving minimum sink, which on many windy days is at least as important as finding lift that may be moving through at speeds which exceed our ability to keep up with.

Don

[rdwoebke]

It's been my experience (over the past twenty years of flying sailplanes) that the best thermals are found on days with light to moderate winds. That's why I don't see the point of adding ballast on windy days. And I don't think giving up the ability to climb well for the ability to zip around from thermal to thermal is a good bargain any day. Always having my best climb rate available makes up for whatever performance I lose by putting the nose down when I want to go fast.


I have not been flying sailplanes for 20 years, but I have been flying contests for over 10 now. I might agree with your logic if a person was just working thermals that were known to be kicked off from the area you are launching from then using the altitude you gained in the thermal to work yourself back upwind towards the launch area.

BUT, thermals often tend to be kicked off from little hills and treelines. It seems the area we launch from is often the flattest/cleanest area of the surrounding geography. I have been to many contests where there is a little ridge or treeline upwind say 1500 feet or so. If you have to fly outside your best glide ratio in order to penetrate upwind to have enough altitude to work/surf that ridge, you are better off giving up a little bit of min sink rate with ballast.

Say for example your glider glides at 12.5 mph (18 FPS) and sinks at .8 foot per second (probably a bit optimistic). This means from a 600 foot launch in a 10 mph (15 FPS) head wind, you are now making 3 FPS ground speed, towards that ridge 1500 feet away. It takes 500 seconds to get to the ridge and you are at 200 feet. Lets say you ballast up to twice your original weight, and your glider now has a sink rate of 1 feet per second. I have read that if you double your weight you increase your glide speed at best L/D by about 50%, so now lest assume we glide at 18.75 mph (27 FPS). This means we can arrive at the ridge in 125 seconds and at 475 feet. From my experience it would be much easier to try to work lift 1500 feet away at 475 feet rather than 200 feet.

Somebody correct me if I have messed up the math anywhere in this example...

Ryan

[POF]

Another reason to add ballast on windy days is that landings will be safer. On windy days you will probably have more turbulence and with ballast the glider is less sensitive to turbulence when flying fast on your finale with a bit more mass.

[O.L. Adcock]

Ryan, that sounds about right. What happens if you take the unballasted ship and lowered the nose to fly that speed?? I'd suspect you'd even be lower when you got there.

Few things are constant in this evaluation but some things are for sure...For any given altitude, the heavier plane has the most PE available to it. A 50% increase in velocity will result in a 200% increase in drag (assuming no camber change) so your going to need more kE to counter that. The only way you can get that extra energy needed in the unbalasted ship is by trading altitude for it.....O.L.

[OVSS Boss]

Ryan, you are right on in your evaluation. Also, I read something that Roger wrote and if it the way he views flying ballast, it is the root of his problem. Roger made a comment of "zipping" around in a ballasted state. Pleas read what I said again in my earlier post, when a pilot that knows his stuff ballast up in a given condition of wind, he only wants to establish the best L/D for the ship in those winds, that is not zipping around! That is an L/D that allows for unfettered travel, but he is not going fast[I]. There is a big difference and the properly ballasted ship feels nearly identical in wind to the unballasted ship in light conditions.

Now, Roger, if you are only flying stick built models, then yes, you will approach Vne at a lighter weight than a moldie. But you can still ballast and still utilize the potential of ballast there. I can carry about 2 pounds in my Grand Esprit and do not heitate to use when necessary.

Marc

[rdwoebke]

I'm really not an expert sailplane pilot (actually a fairly crummy one), so take my thoughts with some salt. The numerical example I posted earlier was not completely made up. I extrapolated (to the best of my ability, it might not be perfect) the numbers from Dr. Drela's Bubble Dancer plans:

http://www.charlesriverrc.org/articles/bubbledancer/PDFs/bd_V3.pdf

Anyhow, what really helped me the most with ballast was flying electric sailplanes. See, I have 2 gliders that I can configure as an electric sailplane and as a pure glider (Paragon and Allegro Lite).

The Allegro's electric power plant does not add a lot of weight, 4-6 ounces (a bit over 1 ounce per square foot wing loading increase) depending on battery type on a plane that starts out at 19 ounces. But the Paragon, I am using a geared Astro 035 and 7 or 8 cell sub Cs. This configuration adds about a pound (increasing the wingloading by a bit over 2 ounces per square foot). Since I built a new fuselage for the electric Paragon, I tried to set the CG on the e glider to see that I was not messing that up, since I had really felt I had the glider Paragon dialed in. Once I flew it, I was amazed at how well the elctric Paragon flies. I did a lot of "late evening" testing (mostly to get a baseline for what kind of dead air time I had on a "A saillplane LMR motor run") and really don't think the sink rate suffers that much. The plane does cover a lot more ground in this configuration, giving me the ability to search more area for lift. Since flying this configuration I have since found that I needed to add a bit of weight to the tail of the glider Paragon when carrying ballast slugs (I thought the slugs were placed over the CG, apparently not). I still like the light Paragon for flying in calm conditions or fun flying, but now I'm certianly not afraid to ballast it up (before when I was inadvertently changing the CG I really felt my circling peformance suffered) if it gets much above say 8 mph winds.

Ryan

[rogerflies]

When I was first flying sailplanes, I built copies of several of the planes the other members of the club were flying. These were Oly IIs, Sagittas, Cumics, and then Lovesongs. I was a meticulous builder, and I was fanatic about weight. Their planes were built well, too, but they didn't get nutso about the weight. My planes usually came out 15-25% lighter than theirs, so it was easy to make comparisons. There was hardly ever a case where the heavier plane outflew the lighter plane, and the lighter planes were always easier and more fun to fly.

I also had the opportunity to fly a well-built Spirit 100 and the electrified version of the same plane. The glider was decent to fly, while the electric was junk with the motor running and worse with it off.

My experience with power planes has only reinforced the idea that lighter is better. My 42% Spacewalker with a 17oz/sf wing loading was a joy to fly with the motor running or not. I added 8 pounds of ballast one day, bringing the weight up to 33 pounds. It flew faster most of the time, but I don't think it would go as fast on the straight and level as it would without the ballast. It wasn't nearly as forgiving either, and it sure didn't glide as well with the engine off.

"Lighter is better" held for all the other power planes I built compared to their heavier cousins, too. They always had a wider speed range and were easier/more fun to fly.

Maybe I'll try to talk myself into putting some ballast in the Lovesong, but it's going to be a hard sell.

Roger

[tonyestep]

[QUOTE=O.L. Adcock]...what happens if you take the unballasted ship and lowered the nose to fly that speed?? I'd suspect you'd even be lower when you got there.
---------
Yep. In fact, there's no need to suspect, it's a physical fact. Dharban summed it up: "the simple fact is that above some particular speed (for RES ships, for example probably 12-14 mph) the sink rate realized by adding ballast will be less than the sink rate achieved by putting the nose down -- period."

Sinking speed = forward speed / (L/D). In other words, for a given forward speed, your sinking speed, as well as your rate of climb in a thermal, is determined by the L/D. As the "Soaring challenged" site so cogently points out, putting the nose down kills L/D, so the lighter plane sinks faster when speed up. Ballasting up does not reduce L/D, and may in fact increase it, especially for an old-timey airfoil like that on the Lovesong.

Of course, when you are flying a plane that can flutter, your performance envelope is so restricted that it's hard to really figure out how best to approach difficult conditions -- which makes learning that much harder.

[dharban]

For all of my advocacy of ballasting up, I still have to admit that adding ballast (especially heavy duty ballast) truly tests my pucker factor -- it just doesn't feel right. When I get weak knees I just recall the Sailaire I flew in the late "70's. I flew it once at Visalia when they flew a speed task (yea -- for real dude!). I ballasted up to 11 pounds and blasted through the gates -- the old gas bag was very competitive.

Having overcome my ballastphobia, I flew the same plane later in Tulsa ballasted again to the max in 20+ mph (launching at that weight was no problem even with the old winches in the strong wind -- just kite it up). This time the event was duration and the gas pig had no trouble penetrating and surfing on whatever lift waves which passed through. Any less weight and the bird would have been long gone downwind. With ballast, it flew well and combined with the great launches that the high wind offered did not seem handicapped with a particularly high sink rate.

As much as I hate ballast, I will remain convinced that heavy air competition calls for a plane strong enough to get the highest launch and enough time with the plane in heavy air to be confident that the ballast will work.

[tonyestep]

Yeah, I've seen a Sailaire at 11 lbs -- an awesome sight!!

Speaking of RES speed events, I flew in the 1976 FAI team selection finals (now F3B) and we all were loading in chunks of plumber's solder and launching off hi-starts, which was the official launch mechanism in that day. It called for a finely calibrated guess, since the weight you added inevitably took away from your launch height in the light breezes we experienced. On the other hand, in the semi-finals, the wind was whistling at 20 kts and I remember stretching my hi-start rubber straight up on the kite launch, then diving my plane (7 lbs) like a banshee for the upwind mark.

But ya know, Dharban, sometimes you need a little extra zip even in light breezes if your plane is light. For example, I have a 41 oz. Bubble Dancer, 124" span. It stays up longer when loaded with about 6-8 oz. in just about any kind of the air we get around here, as did my Ava. Right now, I'm in the last stages of finishing a bagged Supra which will weigh less than 50 oz. It's gonna need a lot of ballast on many days, so I made provision for 24 oz. of tungsten slugs.

Marc Gellart flies a huge and slow-moving Insanity; he can comment about proper ballast setup for that plane, with which he had a fabulous contest season last year.

[OVSS Boss]

Roger, on the given correct conditions, probably 10-12mph, I would not even hesitate to put in a half pound in the WS and from there go to a pound easy, finding somewhere is another question. I have Sags with a pound as a starting point for ballast. Also, you are correct that in still air, my old Sag 900 at 50+ ounce can not stay with a 40 ouncer. But that day is the abnormal day, not the norm, and I even can ballast that ship.

I would not own a ship I could not ballast now, the onlyone I have is my HL.

Marc

[schrederman]

... we've all typed until there are no prints left on our finger tips... will we ever convince Roger that ballast has it's place in soaring? :D Maybe, and maybe not... Oh well... to each his own... As windy as it is where I live, it didn't take me long to figure it out.

JW

[dharban]

As windy as it is where I live JW

Jack,

For some of us it is windy wherever we live :D

We're helpless and hopeless.

Don

[davidleitch]

An alternative to adding ballast might be to make the wing thinner and just use camber to achieve still air hang time.

[Robglover]

An alternative to adding ballast might be to make the wing thinner and just use camber to achieve still air hang time.


Yep, that's a great alternative. We call that plane a Supra right now. Mine will hold up to 40 ounces of ballast. :D