Hi everyone. The following is a reply I received from the previous owner of GKB regarding the wing flapping at 90 knots. Ron is a very experienced and skilled CFI. In hind sight it all makes sense to me so I am heading out to practice. I included the bit about the offset tow hook as well as that may be of assistance too.
Cheers Kevin
"As for anything strange happening in flight up to VNE i have to say none that i didnt cause myself, I flew KB in all types of W.X and at V.N.E on a regular basis with no problems even in Aerobatic manovers such as loops, stall turns,steep turns, fully developed spins and low level ridge soaring at VNE. How ever the problem that you experienced can happen- more so in Aircraft with all flying tail planes and more so at high speeds known as Pilot Induced Oscillations i did have this experience in the past but knew darn well it was me at the time.- mine occured while flying for some time on the Kaimai range in a long high speed flight and getting a bit to comfortable with the speed,i made a small unintentional movement on pitch this causes a small violent dip/rise in horizon which suddenly wakes u up as your natural instint is to correct it. This starts the wing flap you described and then from here on in it becomes a problem of cat and mouse as the hand eye co ordination try to match what is occuring to the glider. More so if you are not a regular flyer at high speeds, as this is something that can occur in these speed ranges. You naturally bought it under controll by slowing down or gently pitching up. I did this as well on the first time it occured with me and later after i was Aerobatic Trained and well practiced ,i automatically just locked the controlls hard and held steady and it almost immediatly stops. Number 1 rule is to keep a firm and positive hand at speeds above what you are used to flying at. Practice fly at V.N.E when the W.X suits, i.e rembering max wrough air conditions it cannot hurt you or the ship and builds confidence. I am sure there is a bit in the Flight Manual about it.
Now you also mentioned some recent work on the controlls. If every thing is assembeled correctly, tail plane fitted to the ship the correct way up etc, then i would only look at this after going up and practicing V.N.E flight at a safe height, all so put some S turns into your V.N.E flight path and some climbing turns as well after getting the feel for it, this gets some positive controll input and the brain hand eye co ordination going.
Glad to here you are mastering the offset tow hook on take off. this is a challenging time for any pilot and anyone who says they are a piece of cake is kidding you. You can only make it easier for your self by choosing a reliable wingrunner and reminding wingrunners that it is a bit of a hand full if they let you go half heartedly, i will even start the gliders with offset underbelly tow hooks slightly weather cocked to one side of the runway if there is a slight crosswind and then run like hell with the wing and i can say i have never had an aborted take off thru a wing drop when i have had a good wing runner. I had to have a word with one of my teenage students when he went thru the stage of wearing his trousers hanging down showing off his boxer shorts and was trying to run wings and hold his trou up at the same time, wasnt a good look!"
