February 7, 2010
Mathew Wich

All aircraft generate a certain amount of wake as they fly. The differences in pressure between the top and bottom of the wing cause air to wrap around the tips. This leads to vortices that generally swirl and descend for a few minutes after the wing flies by. Check out the pictures on Wikipedia and this short video on Youtube. Our PPGs generate a wake from our wings, and prop wash from our propellers. Unfortunately, the disturbances in the air are invisible.

Recently, Jerry Kerr was filming my take off at Springs East airport. I was attempting a reverse launch in 5-7mph winds. My back was to the wind (my takeoff path) and I was struggling to get my wing to inflate properly. What I didn't realize, was that another PPG had recently (about 10 seconds prior to launch) flown through my soon-to-be flight path.

Now, I've flown my PPG through my own wake several times before. More often this was during my "rookie" days of flying. I would mash on the throttle and pull a tight turn at 300 feet in the air. As I came 360 degrees through my turn, wham, I ran into my own wake. Remember, the vortices linger and descend. I presume that I wasn't doing myself any favors by staying on the throttle, which adds prop wash to the mix (pun) and helps maintain an altitude even with my wake. Anyway, after whaming my way through my own wake a few more times, I finally learned that 270 degree turns are better than 360's.

Surprisingly, the strongest encounter I ever had with wake turbulence, was when I flew through someone else's wake. At about 200ft I had seen the PPG flying slightly diagonal to my path about 1/4 mile in front of me. I didn't think much about it and soon forgot all about the other PPG. A minute later, big wham! I had clearly flown through the wake of the other PPG. Now "wham" and "big wham" are relative. In neither case did I see my wing collapse. In every case my wing probably had a small deflection then recovered before I even had a chance to look up. But to a guy who values his legs, I wasn't interested in repeating the experiences.

Back to my recent launch... With Jerry filming, I finally got the wing into the air, turned around and began a gradual climb into the air. My throttle was probably at 70-80% full. A few seconds of flight went by then, thud/wham! Well, that's what it felt like to me anyway. What the heck? I looked up to check my wing, lines, carabineers, etc. while maintaining an altitude of about 10 feet. I had no desire to climb after what I just flew through. While checking that everything was OK, I quickly had the Deja Vu recollection that this "event" felt a lot like the wake turbulence that I've encountered in the past.

Below is a clip of the "thud/wham" that I encountered...

I have to admit, the video didn't seem to do justice to what I felt in the air. I was expecting to see some sort of "wack" behavior on the wing. Something very abrupt to explain the downward motion I felt in the air. Instead, after watching the replay several dozen times, I think it's more appropriate to describe the motion of the wing as a "flutter". You can see from the video that the left side of the wing loses some pressure and the trailing edge comes up a few inches. My climb-out suddenly becomes level flight followed my swinging forward slightly as the wing regains its full pressure. In the end, it was pretty benign. My pants remained dry and I flew for about 10 more minutes before returning safely to the ground.

Sequence of Events

Here are the sequence of events that led to my encounter with the wake (see drawing below). As I was preparing to launch, with my back to the wind, pilot "X" flew upwind and parallel to my soon-to-be flight path at about 10-20ft AGL. I was focused on getting my wing ready for launch and failed to notice pilot "X" at all.  In fact, since I failed to scan the skies immediately before takeoff, I never knew he had been nearby at all. After passing the cameraman (Jerry), pilot "X" turned right and headed crosswind. After pilot "X" crossed my soon-to-be flight path, I inflated my wing and turned upwind for my launch. At that point, I still had no idea that a PPG had just flown by. Pilot "X" was ahead and to my right and I didn't see him in my peripheral vision. After pilot "X" crossed by flight path, the vortices from his wing (presumably) drifted downwind and toward me. I hit the wake a few seconds after lift-off.

Schematic

What Can we Learn From This?

The consequences of a collapse can be the most severe between 20-200 feet AGL because there is little time to recover from a collapse. However, we clearly need to fly through this region on takeoff and landing. Major airports will stagger the takeoff and landings of aircraft to avoid hitting wakes. Unfortunately, we rarely have the benefits of a control tower at our gatherings. As such we should keep the following under consideration when we fly...

  1. Be aware of the potential for wake turbulence during takeoff: Keep your eye on traffic and allow 2 minutes before taking off or landing through the (past) path of another aircraft/PPG. This can be more difficult to do during a reverse launch, so keep your eyes and ears open and stay vigilant.
  2. Avoid flying through the LZ when pilots are preparing for launch: This one has been on our "Safe Flying Practices" list for a while. But, as this incident shows, we now have another reason to stay clear of the LZ when pilots are setting up for launch.

Fly Safe, Mathew


risk&reward