FreeFlight Advice: Pitch Stability
March 4, 2015
Normally this site runs on a Question and Answer format… but there’s topic that comes up time and again and I’d like to talk a bit about it for those interested in understanding it better (there’s a lot of myth and mis-information out there).
This topic has come up twice for me just recently- first in a discussion on our local pilot chat list about the inefficiencies in flying wings, and now during the FAI Hang Gliding World Championships, where a pilot experienced a tumble and had to deploy his reserve.
So let’s talk about pitch stability, and I’ll try to keep it light and simple. In aerodynamics, stability is as a wing’s resistance to changes in attitude, or how strongly it tries to return to steady state. Hang gliders are generally very pitch stable- they want to find ‘trim’ and stay there. Pulling in or pushing out to change the angle of attack we experience bar pressure, which is a force created by the aerodynamics of the wing trying to return to center.
Where this force comes from is one big question we should explore. In airplanes, it comes from the tail. But hang gliders have no tail. Weelllllll….. visually, that is true- hang gliders have no tail. But aerodynamically, we have the same needs as airplanes and just meet them in a different way. In hang gliders, the outboard part of our wings, and especially the trailing edge of the wing out near the tips, is our “tail”. By giving a hang glider swept wings, the tips are moved aft of the center of the wing. And by twisting the tips to a lower angle of attack, this portion of the wing is essentially our tail. It’s a little abstract to think about the “tail” on a tail-less aircraft, but hopefully the simplicity of it makes it understandable.
In terms of efficiency, this is where we run into issues. First, swept wings aren’t terribly efficient. Some of the air we want flowing OVER the wings becomes span-wise flow, along the angled leading edges… resulting in a loss of potential lift, as well as increased drag. Another efficiency issue is that a hang glider’s “tail” isn’t very far aft of the rest of the wing, so in order to create a nose-up pitching force we need a decent amount of twist in the wing- which means less of the wing is creating lift, and again more drag. If you think about a simple lever and how mechanical advantage works… and you compare how far aft a hang glider’s “tail” is with an airplane’s… you might realize that an airplane’s tail can create the same pitch forces as our tips, but without being angled as far off the direction of airflow (IE making way less drag).
These inefficiencies are what they are… they are the price we pay for portability. If you look at all the hot sailplanes, they have straight wings and tails… and their performance blows ours away. But they only pack into a trailer, and you certainly can’t foot launch or land them like hang gliders. Our glider’s need to stay tail-less to keep them what they are, and there’s nothing wrong with that…
Regarding the recent tumble at the worlds, an interesting comment was made:
“What’s really strange is that he’s flying with a tail, so it’s hard to imagine how that could have happened…”
Let’s think for a moment about what we just talked about above- where the pitch stability of a hang glider comes from. And then let’s think about how long the keel on a hang glider is, and how effective mounting a horizontal tail on it might be. Hopefully what you might be realizing is that the keel doesn’t really stick out any farther than the tips- in fact, most manufacturers determine the length of a keel as just long enough to keep the tips off the ground during setup. Unless the keel is extended dramatically, mounting a horizontal tail there in hopes of creating pitch stability will not be any more useful than twisting the tips. It WILL allow less twist in the tips, and therefore a greater area of wing lifting… which would detrimentally impact the handling of the glider, because the effective wingspan would now be much wider, and we’re still trying to steer it by mere weight shift. The twist in the tips also creates some nice effects, like gentile and progressive stall characteristics… so reducing that twist to make use of a tail would make the wing much less forgiving on launch and landing.
So maybe the horizontal tail is creating a pitch-up force IN ADDITION to the tips… increasing stability? Not likely, because remember that creating pitching force means creating drag… and we’re seeing these horizontal tails on high performance ships only, where every bit of drag matters.
But there is ONE real advantage of these tails- and that is in creating a damping force during a pitch rotation. That’s really a fancy way of saying it makes it harder to rotate along the pitch axis. This benefit is experienced even if the tail is at a neutral angle of attack and not creating any pitch forces to speak of. Pitch damping is an important factor in hang gliders, because we don’t have a conventional tail, and because our bodies move the CG of the wing below the wing itself (we hook in heavier than the weight of our glider). If you experience turbulence and say the nose pitches over 20 degrees very quickly… your body, which is below the CG of the wing, will have more momentum than the wing after that 20 degrees of rotation… which can actually “push” the wing into pitching over farther! Many tuck/tumbles appear to have some element of this, where the glider was pitched down dramatically, and it was actually the “rotational inertia” that completed the tumble. Pitch damping, in simple terms, is designed to resist this pitching rotation- NOT TO PREVENT IT- but to slow it (hence “damping”) so that when the pitch over stops there’s less rotational momentum built up. Make sense?
So, getting back to the comment made about the pilot with the tail that tumbled at the worlds… why shouldn’t we be surprised?! Tumbles don’t happen very often these days, and even in this very article I’ve said that hang gliders are generally very pitch stable, and that even horizontal tails that create no pitch force are still helpful in deterring a tumble? The answer lies in the last, but most important, factor in hang glider pitch stability. The relationship between CG and pitch stability.
It’s a simple relationship- the farther forward the CG is (the more we pull in), the more pitch-stable our glider becomes. Pushing out, on the other hand, moves our CG aft, decreasing the pitch stability. Because hang gliders have a pretty short distance between the lifting part of our wings and the part acting as a tail… and because our aircraft is lighter than our bodies… we have a pretty massive amount of variance in fore/aft CG. Translation- we have a whole lot of pitch control! And with great power, comes great responsibility…
I’m not at the Worlds. I didn’t see the tumble. But knowing it’s a worlds and everyone’s likely to be flying certified gliders with pitch stability systems (sprogs) that are within the designer’s certified specs… and this pilot was even flying with a horizontal tail… I think it’s a safe bet to say he probably pushed out at exactly the wrong time. Maybe he was climbing in a gnarly thermal, and got a little too greedy in trying to fly slower and climb faster- pushing out- and he destabilized his wing enough that turbulence was able to overcome all the pitch-safety systems. Or maybe he was flying normally and got pitched down, and responded by pushing out to “push the nose back up”… a common instinct people have when they don’t fully understand how wrong that is. But the point is, should we be surprised that a “safe” hang glider can tumble? NO. Being surprised shows a lack of understanding, and respect, for how it all works. And if we can understand how it works, and respect what the wing needs to keep from tumbling, we can feel “safe” and confident while flying.
For more reading about pilot position and pitch stability, check out this great article with graphs and test-vehicle data published by Wills Wing.
And to see EXACTLY how to tumble a hang glider, even flying a certified and pitch-stable wing in perfectly smooth air, watch this dude who uses a stall to initiate a pitch-rotation, and then pushes out to destabilize the glider in pitch. He did it “just right”… and in doing so, shows us exactly what NOT to do (or if we think about doing the opposite, we can see what TO do in the event of a pitch over).