There’s a video floating around lately of a foreign pilot who tumbled their U2 145. If you haven’t seen it, here’s the video:
This video is a great conversation piece for a few principles we should all understand to stay safe out there. A quick rundown on what you see in the video: A pilot (consequently at 3/4 VG) pushes out to do a stall. When he recognizes the glider stalling, he pulls in very quickly/aggressively, initiating a very fast nose-down pitching motion. As the nose continues to drop well below horizontal, the pilot pushes out.
Going through the play-by-play, here’s what he did wrong… and why it’s bad:
-The Stall: Believe it or not, there is more than one kind of stall. In this video, there are actually 3 types! First is the typical stall we all know and love- angle of attack becomes too high and airflow separates over the wing. In this video we also see what is called an “accelerated stall”, which is a stall that occurs at a G loading higher than 1x. Because this pilot had some extra speed, and pushed out aggressively, the glider climbed before stalling, putting him at more than 1G wingloading- and raising the stall speed of the wing. Also, because he achieved such a nose-high attitude before actually stalling, we see what is called a “whip stall” where the glider climbs to a stop and very quickly rotates nose down with little-to-no airspeed. Whip stalls are especially dangerous in tail-less aircraft such as hang gliders, because of the very high pitch rates our wings are capable of. We also have a variable center of gravity, which leads me to…
-Pilot Input, part I: The pushout to stall is quick, pitching the glider up significantly. This pitch input is held until nearly 0 airspeed, at which point the pilot stuffs the bar quickly/aggressively. Pulling in is good if you’re stalling/stalled… but with low airspeed, and especially with such a nose-high attitude, pulling in THAT aggressively is bad. We can see in the video that stuffing the bar initiates a very, very fast nose-down rotation. The danger here is that the inertia of this rotation can be so strong that the glider continues to rotate well past equilibrium, as seen in this video.
-Pitch Stability: It is also important to understand how the systems in our gliders resist pitch-over tumbles. One factor is the shape of the wing- namely sweep/nose angle and aspect ratio… the less sweep a wing has, the faster it can rotate in pitch. Compare a Falcon to an ATOS (with no tail) and you could imagine how much faster the ATOS could pitch nose down than the Falcon. In general, the higher performance the wing the wider the nose angle and the higher the aspect ratio… and the faster it can rotate in pitch. Note that the shape of the wing DOES NOT prevent pitch-over situations… instead it is designed to provide enough resistance to slow the rotation so that there isn’t enough inertia to turn an “over the falls” pitch over into a tumble. If the rotation is dampened enough, as the nose rotates lower the other dive-recovery systems of the glider- luff lines, washout struts, SPROGS will create a nose-up pitching force. It is very important to note that these systems DO NOTHING if you have no airspeed. Since the force of air over the wing increases with the square of velocity, even a little more airflow over the wing results in MUCH more pitch-up restorative force. In this video, he had little-to-no airspeed, so his SPROGS were basically useless.
-Pilot Input, part II: As the glider continues pitching nose-down, you can see the pilot push out. This is something I have seen quite a bit of- many people instinctively want to push out to “push the nose up”. Unfortunately, pushing out doesn’t directly influence our pitch attitude… it merely moves our CG, which in effect changes the equilibrium angle-of-attack of the wing, raising or lowering the nose and altering our airspeed accordingly. When we have little or no airspeed, pushing out will not result in pitching the nose up (not until airspeed is increased). What moving our CG forward or back DOES do, is alter the pitch stability of the wing. The further forward our CG is, the more pitch-stable the wing becomes. And the more we push-out, the less pitch stable. This is exactly why pushing out while thermalling is very dangerous, and in this video it is a contributing factor in the glider rotating past 90 degrees nose down. The pilot in this video pulled in at the stall break, increasing the severity of the pitch-down rotation… but then pushed out, reducing the pitch stability of the wing. The two combined are the perfect way to initiate a tumble. If you’d like to read more about pitch stability and CG location, Wills Wing has a great article on their site: http://willswing.com/articles/Article.asp?reqArticleName=CenterOfMass
-VG’s affect on stall characteristics: First, many new gliders will trim at a faster airspeed with more VG pulled. This is because the VG tightens the sail, reducing how much twist or washout there is in the wing (reducing nose-up force). Pushing out, even from “trim”, at a high VG setting you can mean you have enough airspeed (energy)- as well as increased efficiency in energy retention, to get the glider significantly nose-high before actually stalling. Because of the reduced amount of washout at high VG settings, much more of the wing stalls at the same time, and the resulting pitch-down rotation can be quite dramatic. I know stalling my T2 at VG full is an eye opener! High VG settings also means a slightly wider nose angle, which means the wing can rotate a little quicker in pitch.
I DOUBT ANYONE ON THIS LIST WOULD DO WHAT IS SEEN IN THIS VIDEO, however I have seen several people doing one or two of these things in “active” air, and I want to make sure everyone really understands how our wings work, what not to do, and why. In even mildly active conditions, pushing out anytime other than landing is, in my opinion, a tumble waiting to happen. If you are climbing and get dumped out of it you now have very little airspeed, so your glider will be able to rotate nose-down much faster. You also have your CG aft, so the glider is far less pitch stable. If you pull in to get your CG forward (which you should do), you need to do it smoothly so as to not increase the rate of pitch-down rotation. Tricky stuff!
The more active the air is, I’d recommend first and foremost always flying with a little extra airspeed. If your glider has VG, be very careful of using higher VG settings. If you DO use higher VG settings… fly with LOTS of speed! VG full tight doesn’t make much difference at low speed anyway, so why risk it?! And, if the $h!t really hits the fan, smoothly pull in, and hold yourself there NO MATTER WHAT. Remember that the further forward your CG the more pitch-stable the glider becomes.