FreeFlight Advice: Wingloading and “Ground Effect”
Originally Posted on FreeFlight Advice: February 12, 2016
Q: Does weight affect how long you stay in ground effect?
My buddy and I fly the same model and size glider, however, he is about 30lbs less than me and it appears he stays in GE longer than I do and needs a less aggressive flair. It appears my flair window is shorter and I need to flair harder if I want a no-stepper.
Please note: He is a little on the light side of his glider and I’m more in the middle to upper end of the weight range.
A: This question follows nicely with the last question we got, which also had to do with wingloading… but this one involves a very specific phase of every flight- ground skim or “ground effect”.
Before getting into the effects of wingloading, I should really talk briefly about “ground effect”. Let’s start with what it is in very general terms: An increase in aerodynamic efficiency (improved glide) that occurs when an aircraft flies near the ground. Understanding where it comes from will also be important in a minute, so let’s delve into that for a second…
There are two models as to how a wing makes lift; one says it creates a difference in pressure between the top and bottom of the wing, and this difference in pressure naturally wants to equalize, and so a lifting force is applied to the wing. The other model is that the wing works to deflect air downward, and because every action has an equal and opposite reaction, in pushing the air down the wing is also pushed upward some (lift). It doesn’t really matter which model you like, they’re both kind of the same here (in truth, they are probably two perspectives on the same thing, and therefore both accurate). Anyways- a wing makes lift, right? But where the wing ends, out at the tips, some of that lift “spills” around the tip, and swirling tip vortices occur as a result. A flying wing is pulling the air in these vortices, and that’s called DRAG. Glide performance is lift divided by drag… so even minor reductions in drag can noticeably improve glide. The idea of “ground effect” is that, when the wing is close to the ground, those swirling vortices hit the ground and get broken up, and therefore the mass of air the wing is pulling is less… in other words, less drag. Less drag = more glide.
The reason I wanted to delve into what ground effect is and where it comes from, is because hang gliders are a bit different than airplanes (no duh, right?). A hang glider doesn’t have a tail (again, no duh, right?)… or does it?! To accomplish the same thing a tail would, the tips on a hang glider- which are the farthest aft part of the wing because of sweep- are twisted to be at a lower angle of attack than the root (center). Lift increases with angle of attack, and so does induced drag (including the tip vortices). The opposite is also true- the lower the angle of attack, the less lift, and the less drag, right? So… if you think about the angle a hang glider’s tips are moving through the air… there’s probably not a ton of lift there, and so there’s also probably not huge vortices, either. In addition to that, the pilot and control frame both exist below the wing… which limits how close the wing can get to the ground.
Am I saying “ground effect” doesn’t occur in hang gliders?!?! No, that’s not what I said! A good friend and one of the brightest people in hang gliding, whom I have massive respect for, has said just that… and I have an awful hard time disagreeing with someone I know to be a whole lot more knowledgeable and experienced than I… and at the same time, I have a hard time accepting there is NO “ground effect” increased efficiency in hang gliders. Perhaps there’s a middle ground- it does exist, but it’s not nearly as much as we think.
If we replace the phrase “ground effect” with ground SKIM… that provides an entirely fresh perspective. Perhaps most (if not all?) of what we attribute to increases in aerodynamic efficiency, is much more so just the conversion of kinetic energy… airspeed… into potential energy… altitude. But in the ground skim, we aren’t gaining altitude, right? Relative to the ground, no. But if a hang glider typically sinks through the air at say 200 ft/min, and during the ground skim the glider is no longer sinking, that increased lift has to be coming from somewhere… and the answer is, it’s being fed from built-up airspeed.
Let’s explore that- airspeed as stored altitude. An object in motion will remain in motion until acted on by an equal or greater force… says the dude that got hit on the head by an apple. In this case, the object in motion is the pilot/glider combo… and the force slowing it down is drag.
And finally we are ready to examine wingloading’s role here (sorry that took so long)! If we discount the probably minimal effects of wingloading on a flexible wing- increased twist/washout- increased drag… we could conclude the force drag exerts on the glider and pilot during the ground skim is about the same. BUT- a heavier loaded wing will have more momentum than a lighter one traveling the same speed. So maybe we’d suspect the heavier wing would be able to travel farther before coming to rest? Maybe… but a heavier loaded wing also has a higher stall speed. So if both pilots flew until they couldn’t fly any more, the heavier loaded wing would crash before the lighter loaded wing. Who would go farther? Hmmm…. We don’t really know, because HOPEFULLY these pilots aren’t crashing! They’re flaring, and landing on their feet. Without getting into the aerodynamics of flaring… let’s remember that heavier loaded wing has more momentum at a given airspeed… and it stalls at a faster airspeed… and that adds up to mean that heavier loaded wing is going to need to flare much earlier. The lighter loaded wing can keep flying a little longer (speaking time, not distance), and to a slower airspeed, before needing to flare… because it doesn’t take as much to stop that slower moving wing with quite a bit less momentum.
The heavier loaded wing should have the same glide performance, L/D, but it occurs at a faster speed and slightly higher angle of attack. Higher angle of attack might mean larger tip vortices, so maybe again we’d think the heavier wing would go farther in “ground effect”, because that pilot would see a bigger reduction in drag? But if ground effect is greatly overstated in hang gliders…
Earlier I said something that we need to come back to before wrapping this up. I said:
If we discount the probably minimal effects of wingloading on a flexible wing- increased twist/washout- increased drag… we could conclude the force drag exerts on the glider and pilot during the ground skim is about the same.
What if we DON’T discount wingloading’s effect on the flexible wing? The twist and washout in a hang glider is largely controlled through lateral sail tension- specifically along the trailing edge. Opening the wings wider (pulling VG) tightens the sail and flattens the wing by reducing twist. The tighter the sail, the less wingloading is going to add more twist or washout. In a high performance wing, flying at a higher wingloading will still increase stall speed, trim speed, sink rate, etc… but probably has very little effect on performance. On a much looser sailed glider, though… flying at a higher wing loading might have a greater effect. In this message, the pilot says that he and his buddy are flying the same model and size glider, just at different wingloadings, and the buddy flying at a much higher wingloading goes farther in ground skim… which seemed a bit counter-intuitive. Perhaps the specific glider model is a factor here?
Also with noting- hang gliders are more or less hand made. The largest manufacturer is able to afford a bit more tooling and machinery to ensure accuracy and consistency, but even then the sails are stuck together and stitched by humans. And the other, much smaller manufacturers? I’m not at all putting them down, or saying they don’t do a great job… but without the same resources, they just aren’t able to be quite as accurate when drilling holes and cutting sails. My point- I have observed that, with some manufacturers more than others- there can be noticeable variation in how well a glider “goes”… regardless of pilot skill.
And then there’s PRONE FORM- which pilot is able to be the cleanest through the air, even if you’re both flying the exact same harness? And remember it’s not just body position, but also that any weight shift inputs are reducing efficiency… so who is able to be the cleanest and smoothest through the ground skim phase? And… who has smoother and more confident transitions to upright? Being able to hold that clean prone form even just a little longer can make a difference in 10’s of ft covered. And if there are any pitch bobbles during that transition, that is airspeed (energy, distance) lost…
So, after all of that… why is your friend flying the same wing, but at a lighter wingloading, able to ground skim longer (distance or time?), and why does he seem to need a less aggressive flare? The answer to both is probably wingloading… and hopefully if you’ve made it this far you have a deeper understand of WHY that is. And also a better understanding of what “ground effect” is, and some of the variables that can have an impact it.
Thanks for writing in, and if I missed anything or this just leads to new questions, please let me know!