Lnewqban

Gliders have ballast tanks that can be filled with water. The addition of ballast increases weight, and this allows the glider to fly at faster air-speeds while maintaining the same glide ratio.

If all bodies free-fall at the same acceleration (g), why ballasted gliders fly faster?

AA5BY

They are not free falling.

otrcman

You have it right in your first sentence. And AA5BY has it right, too. Free-fall is truly valid only in a vacuum. Airplanes don't fly in a vacuum.

The flattest glide (best L/D) occurs at a specific angle of attack. For any given weight, the angle of attack for best L/D always results in the same airspeed. By increasing the weight, flying at best L/D results in a higher airspeed. Unfortunately, the sink rate also increases. So if an airplane with a 20:1 best L/D flies at 100 ft./sec, it's sink rate will be 5 ft./sec. Increasing the weight might increase the airspeed at best L/D to 120 ft./sec, but it also increases the sink rate to 6 ft./sec. On a day with good lift, the penalty of higher sink rate can be easily accommodated by better lift.

Dick

BFoote

Gliders work on rising air. If little rising air, then a heavier glider will fall out of the air sooner, though as mentioned above it ties into your L/D ratio at reynolds number etc. If in a very good rising air, the heavier glider will cover more distance, but won't climb as high. This can easily be observed in slope flyers. A gentle lady can fly on a beach forever with a VERY gentle breeze. A 3m glider wouldn't even be able to fly at all in same conditions. Conversely take same said models when there is more wind blowing and the 3m glider will destroy the gentle lady in performance.

No simple easy answer. Its a trade off like everything else in life.

speedracerntrixie

My Feedback: (29)

This is actually a quite difficult question as not all sailplanes are created equal. The current trend in unlimited sailplanes is 3M to 3.5M and in the mid 60 oz weight range. Back when I was flying sailplanes exclusively ( Late '90s ) the standard was 3M and up to 80 oz was pretty common. The lighter models are designed to fly slower, airfoils used have a better L/D at slower speeds while the models I flew were designed to be flown faster. Recently I picked up a used Synergy 91. Back in the day this was an all out F3B sailplane. After repairs it came out at a hefty 85 oz. Most guys did not realize it was that heavy because I was able to work lift with it well by just flying the airfoil in it's best speed range. Now if one would take a Gentle Lady and load it up to the same wing loading would it work the same? No way. I think the answer that may work for most sailplanes is a fairly generalized answer. Think as the additional ballast as potential energy and the sailplane is simply converting that potential into kinetic energy.

BMatthews

There's a tactical aspect to the use of ballast. As noted by the others it raises the airspeed at the same time it increases the sink rate. But the higher speed allows you to test a much bigger volume of air in the region for lift. And when around lift it's common to find equally as much areas of sink. And again the higher speed provided by the ballast allows you to get away from the sink with less loss of altitude.

Now you COULD suggest "why not just dive clear of the sink?". Or "why not just trim in some extra down trim to go faster?". But due to how the wing loading works with the airplane at the best L/D speed there is less of a sink rate penalty with ballasted speed gain with the glider still at or very near the best L/D point than there is with the same but lighter plane in a shallow dive to reach the same forward speed.

At least that applies as a general observation. Models or full size craft with high drag airframes will benefit little or be hurt badly thru ballasting due to the high amount of added drag from trying to load them down.

For example in the case of the Gentle Lady mentioned earlier. A fine glider for light air days and up to moderate winds. Not so good on windy days. So ballast it? Ain't gonna help. The airfoil and generally draggy airframe and tail surfaces will simply produce high amounts of drag and the model will sink like a stone.

Anyone that flew both the GL and the old Electra version equipped with the then state of the art can motor and big pack of sub C nicads will tell you that the Electra was a pig in the air compared to the light and pleasant Gentle Lady. Yet other airframes at the same or even higher weight did very well indeed even when ballasted up with the same sort of motor and battery weight.

In a case of mine the airfoil and airframe are clean enough that the 52 oz can motor/NiCd pack version worked FAR more nicely than the same wings and stabilizer on the 34oz pure glider fuselage. Oh sure, the sink rate of the heavier electic is slightly higher. But this is far more than made up for with the ability to cruise around and hunt for lift to greater distances and still make it back if I can't find any.

It also comes in very handy for returning from downwind after staying in lift for a longer time. A glider like a GL has to leave and dive for home far sooner than a heavier glider with the optimized airframe running a higher wing loading. In some of the higher performance examples this tolerable downwind distance can be 4 to 5 times more than what is doable with a GL from the same altitude on days of moderately high wind speeds.

Lnewqban

Thank you gentlemen for your detailed responses.

http://en.wikipedia.org/wiki/Free_fall

What force makes the heavier glider to move forward faster?

BFoote

Already explained up thread. Read and contemplate more.

BMatthews

Sometimes a different way of saying or showing the same thing makes an explanation more clear for some.

A glider flies forward against the retarding effect of the drag due to the airfoil's lift being at right angles to the direction of flight (DoF). And because the DoF is slightly down there's a component of thrust involved. It's the same effect that keeps an autogyro's rotor auto-rotating. They are like glider wings going around in a circle.

But the pull of gravity is directly down. So if you make the effect of gravity stronger by increasing the weight of the plane then the wing has to lift more. And when the wing lifts more the portion of the lift vector that is pulling the wing forward increases as well.

In this sketch you can see what I'm saying more clearly. Lift is perpendicular to the wing itself. Gravity just pulls directly down. Only the part of the lift that is directly up is fighting the gravity force. The remainder is used to keep the model flying ahead against the retarding force of the drag.

So there's two ways to fly faster. One is to increase the downward DoF angle to force an increase in the thrust component. This will increase the size of the forward thrust component of the lift. And as the angle of the descent increases beyond 10 to 15 degrees gravity plays a larger part as well. But for angles of under 10 degrees it's relatively minor. The other is to increase the mass so gravity pulls harder. In response the model has to fly faster to generate more lift. And for a given DoF angle this means that the available thrust increases as well. So the model trims out at a faster flying speed for the same DoF to the oncoming air.

Of the two the increase in weight has the advantage of producing more ground coverage per foot of lost altitude. So that is why it is the preferred method for glider guiders.

I've got a nice little sketch to show this but RCU image uploading isn't working at the moment.

Lnewqban

Thank you very much, BMatthews; much clearer now!

I look forward to seeing that schematic posted.

BMatthews

Here it is now that RCU is running right.

EDIT- Well maybe not. It took the file but isn't displaying it. I'll check back in a while.

Note that the forward component of the lift vectors that provides the thrust is balanced by the overall drag of the aircraft. I left that part out rather than muddy up the sketch for THIS discussion.

Lnewqban

Thanks, BMatthews!

It seems that we will have to wait until the pictures' issues get resolved. [] [] []

In the meanwhile, let's talk about the trim process.

Does adding weight creates the need for re-trimming the model (more applicable for free flight), or it falls naturally to the best L/D ratio without the need for that?

As I understand it, two models (or the same glider with different weights in this case) with the same best L/D will follow identical slope during their natural descend.............is that correct?

BMatthews

IF the weight can be added equally around or right on the CG then no trim changes are needed. And if they were then adding the weight is being done wrong. If the added weight moves the CG then the model will no longer fly at the otherwise optimized CG location and won't be as efficient or consistent in handling. So no, all it SHOULD do is make the model fly faster at all the otherwise "usual" trim settings.

I'm reminded of my day at the US Nats back in 1991 (Tri Cities area of Washington State) I believe it was where we had near hurricane conditions on the day for Standard and Unlimited class soaring. Finding and following a thermal was simply out of the question. And to make matters worse the best useable winch line direction on that field was still mostly cross wind. The ONLY hope of making the target time was to run up the line with a near 40 degree yaw angle, come off the top and dive/penetrate to the line of windbreak trees that were only about 150 yards upwind and slope soar off the tree line. Lighter models tried to dive toward the tree line only to end up under the tree line and barely make it back for landing points after about 40 second flights.

The winner was Troy Lawicki flying his own design "Rubber Ducky". In talking with him after the event I mentioned that he had seemed to have less issue with fighting upwind to the tree line. I asked what his secret was. He looked me straight in the eye and with a slight grin said "..... 6 lbs of ballast." I kid you not. He ADDED 6 lbs to his 2.5'ish meter span model just to claw that 150 yards directly upwind to get to where he stood the only practical chance of making his flight time.

Lnewqban

Amazing story !!!

.........6 lbs must have felt like one ton to that model.

BMatthews

As I recall the model's empty weight was around 70 to 80 oz. So call it 4.5 to 5 lbs. So he slightly more than doubled the wing loading.

It did the trick though. He was one of only a very few that were able to battle the atrocious conditions and get to that tree line. I saw LOTS of them which couldn't get there and were back on the ground in around 1 minute.

Lownverted

My Feedback: (4)

Yeah that correct, same slope but different speeds. All else being equal, both a light plane and heavy plane (same plane) would fly the same slope, say 500" to the ground, & cover the same distance, with the only variable being time. Obviously the heavier one being much faster. So when the name of the game is staying up as long as you can, lightweight is the way to go. When the need for speed hits, load 'er up! That's the reason the FAI limits surface loading on competition. Otherwise you could load up as much as the conditions would allow. Man on Man slope racers in the 2.5 to 3 meter range can be 11+lbs in the right conditions. It's amazingly fun!!

http://www.youtube.com/watch?v=ECi18gzyM2U