FreeFlight Advice: Wing Loading and Ballast
Posted on FreeFlightAdvice.com: September 11, 2015
Q: Recently I was trying to figure out if it would be useful to put a few extra kilos in my harness to get a better glide and to become faster overall. Is it still common practice for top comp pilots to use it, now that gliders get better and better, also the smaller ones? I’m on the low side of the ideal pilot weight for my WW T2C 154, so maybe you can write a blog on this matter and give me a word of advice on this.
A: This is an excellent topic and question- because there’s a simple answer, which is mostly correct, and then there’s the “it depends” answer when you really get into understanding wing loading.
The short answer, if you’re in a hurry and want simple yes/no, is that NO, adding some ballast in your harness will NOT improve your glide. Glide- as in Lift/Drag- remains pretty much constant regardless of wing loading. But YES, increasing your wing loading would make you faster. Where you go faster is unknown; do you cover more ground in a day, ending up with a longer distance flight… or do you just end up in the same place sooner… or do you end up landing sooner, and going less far?!
Let’s get into understanding the variables in this, and then hopefully you can make an educated choice for what suits your needs. As with anything, it’s a series of trade-offs and compromises…
First, let’s understand exactly what changes in wing loading does. And to understand that, I want to briefly explain what a glider’s polar curve is. A polar curve is the graphical representation of a wing’s performance across it’s speed range. The X axis (horizontal) is airspeed, and the Y axis (vertical) is sink rate. Hopefully everyone knows that “best glide” occurs at an airspeed FASTER than minimum sink- and we see this in the curve as we follow it from left-to-right. And as airspeed increases beyond best glide, we see sink rate begins to rapidly increase. Drag increases with the square of airspeed, right? 2x as fast, 4x more drag, 3x as fast, 9x more drag, etc
Here are some general polar curves provided as a rough guideline from Wills Wing
Getting back to the topic of wing loading again- what changes in wing loading actually do is shift this polar curve left (lighter loading) or right (higher loading). The curve remains the same, but stall speed, minimum sink, best glide speed, all shift faster/slower.
So, if the glide performance remains constant, why is it sometimes considered an advantage to fly with ballast? There are two reasons- the first is RACING. Many competitions are races, so if your “best glide” speed occurs at a higher airspeed than your opposition, you have a racing advantage. The down side is that your stall and min sink speeds have also increased, so landing will likely be more critical (and risky). Maneuvering your wing to stay in lift will also be more challenging and require more skill, since flying faster makes it harder to turn as tightly. It may actually mean climbing slower, because of the need to bank more steeply to accomplish the same diameter 360 as a more lightly loaded wing. On the topic of racing- even flights that aren’t technically races, like cross country or open distance comps, are still essentially a race against the sun. When the sun goes down the thermals stop… so the faster we are able to cover ground the farther we can go (all else being equal).
The other reason has to do with the Speeds to Fly concepts. Our wings have one very definitive “best glide” airspeed, where they achieve their maximum efficiency gliding through the air. However, since we’re usually not so concerned with how much AIR we flew through, but how much ground we covered… we must account for wind. If our best glide speed occurred at 20mph, and we were flying into a 20mph headwind… we wouldn’t cover any ground, despite flying very efficiently through the air. But, in that same example, if the pilot flying next to you had higher wing loading, and their best glide occurred at 23 mph… they’re going to have an advantage covering ground. Flying upwind, we must always fly faster than best glide- and having a higher wing loading becomes an advantage.
Downwind, however, we will most efficiently cover distance if we fly slower than our best glide airspeed… and so having lighter wing loading may prove advantageous. There are still the racing competition or sun/thermal aspects, so maybe in the big picture slowing down a lot isn’t REALLY an advantage? it depends…
Since we fly weight shift aircraft, our wing loading also effects our control over the wing. A heavy pilot on a small glider will have much more authoritative control than a light pilot on a big glider.
And then there are some even finer details- like higher wing loading on a flex wing hang glider might increase the flex of the airframe, the billow twist in the sail, decreasing gliding efficiency, and actually (slightly) altering the SHAPE of the polar curve, and not just shifting it as I stated earlier. How much this happens or if performance is actually measurably effected is a topic of much debate; who knows for sure?! Hang gliding is just too small for the scientific testing and measurement required to quantify how wing loading deforms the wing.
What you might be starting to realize is that wing loading isn’t a simple idea, and on different days or for different kinds of flying, it’s probably not one-size-fits all. Hopefully you can now also see what the trade-offs are, and that a happy balance will always be best.
Before wrapping this up, I would like to draw attention to two safety concerns: First, if you want to try flying with higher wing loading, do small increments. And do it in conditions that give you a great safety margin (smooth wind to launch and land in being the primary thing to look for). The second concern, is be careful what you use as ballast, and where you put it. If you were to happen to have a hard landing, is the ballast in a spot where it could hurt you?
Also- can you use ballast that you can release before landing? Water is a great one, as it’s pretty dense (heavy for it’s volume), and dumping it won’t hurt anyone. Water also tends to be abundant enough that filling up before each flight should be feasible. Other materials, like sand, can work well… but if you dump all your sand, you might not be able to find more before the next flight.
Thank you for the EXCELLENT question and article topic, and if you have any more questions (about this or anything else), please don’t be shy about asking!