“----But I was wondering if the underlying problem in this crash (and possibly Terry's) might be that hang gliders are insufficiently stable under those conditions. What struck me was that the glider started moving to the right (from camera perspective) and the pilot corrected but ended up to the left where
he crashed. Is this a case of PIO?”
BC’s Response,
I believe that most hang gliders are sufficiently stable for towing. Gliders like the Airwave K-2 with their alleged designed anhedral for ease of roll control are in my opinion insufficiently stable for low airtime pilots.
Any glider that will exhibit adverse oscillating yaw at any speed within its VNE is insufficiently stable for free flight ( forget towing.) in my opinion.
A vertical stabilizer installed on the old K-2 glider will cure the oscillating adverse yaw problem. (I’ve done it so I know that it works.)
If you return to the Bob Buxton Youtube video and run it in HD you will see the tow line sliding over the base tube against Bob Buxton’s left handed Bar Mitt and hanging up there. This is the beginning of the lock-out. The towline at this point is pulling the control bar base tube toward the tow vehicle. That banks the glider away from the tow vehicle and to Bobs right. (Put another way, to the left side of the camera.)
http://www.hanggliding.org/viewtopic.ph%20...%20highlight=
Releasing upon discovery of the towline being routed improperly is necessary.
Failing to notice the improperly routed (over the base tube) towline the next clue would be the glider not responding to control input. This also makes releasing necessary.
Quote from Bobk,
“-----What struck me was that the glider started moving to the right (from camera perspective) and the pilot corrected but ended up to the left where he crashed. Is this a case of PIO?”
Bill C. response,
It doesn’t look like PIO to me. More likely the texture of the air or a thermal induced turn.
It could even be Bob Buxton’s right hand applying more forward force than his left hand. Remember he is being pulled by his harness. On lift off if Bob was pushing out to climb away from the truck and if he was slightly right of center on the control bar with his hips, he could be yawing the glider to its left. (The right hand pushing forward more so than his left hand.)
Another possibility is that upon lift off the tow line over the base tube was slightly left of center pulling down on that side of the base tube causing the turn to the left.
Whatever the cause for the initial left turn it did not look serious. Bob may have put too much control input to the right and over shot his desired position behind the truck that lead to the certain lock-out due to the towline being in contact with the base tube.
When routing the towline over the base tube-----
If a pilot is towing from the shoulder straps only (which I don’t like to do) the pilot can gain more altitude without having the towline come in contact with the base tube than if the pilot connects the towline near his/her hips.
I heard talk on the above video about giving Bob 35 miles per hour airspeed for lift off. If towing in Arizona I would be asking for around 30 or 31 miles per hour airspeed for PL towing. Towing here in New Mexico at 4,550’ msl I prefer 32 miles per hour airspeed for PL take off. In Minnesota at 1,300’ msl I would ask for 29 mph for PL lift off. I didn’t arrive at these speeds off of a DA chart but just through trial and error. (poke and pray).
Higher airspeeds than needed can put a double surface glider closer to an oscillation yaw situation. I’m not saying that 35 was too fast for Bob Buxton to be launching. I just prefer being closer to the middle of the envelope.
We all know that our flight envelope is not too fast and not too slow with good reasons for both.
While free flying I want to be somewhere between stall and VNE but for different reasons, like close to the ground, I like to fudge a little higher then near stall speed. In a thermal I fudge closer to stall, but not always.
When I have used the acronym, “DIO,” I was referring to Design Induced Oscillation. Not Driver Induced Oscillation.
However I do believe the driver is doing a lot of flying of the glider from the tow vehicle. If a pilot is out of position left or right behind the vehicle and holding that position thing can still go along just fine but if the driver increases tension on the towline the sweet spot narrows and it can narrow to the point of the driver creating the lock-out. Low tension means a wide sweet spot. High tension means a narrow/small sweet spot to be flying in.
I’m real sure a vertical stabilizer could have prevented a lot of towing accidents.
I’m real sure a vertical stabilizer could have prevented a lot of PIO during a lock-out on the way to the impact zone. (It will not prevent a lock-out!)
Sam, Here is a trick question, (using only a two word answer) What is YOUR favorite sport?
. It surprised me.
