j1b3h0
Line Up and Wait
Just about anyone who has trained in or taught in tailwheel airplanes knows of the dreaded porpoising that can happen, especially when practicing wheel landings with a bit of extra speed. You know the drill, your student (or you) don't quite arrest the sink rate enough before the mains touch down and then, Boing, you bounce back in the air and then, if left unchecked by some back stick, the nose will point down more sharply until you bounce again - this and each time harder, untill either you correct for it, or the airplane does a face-plant on the runway and flips upside down. Many airplanes have been crashed in this fashion. What causes the porpoising phenomena?
I'll give you a hint: When you open the door of a Citabria and unfasten the seatbelt, what happens to the stick?
Took me quite some time to understand why the airplane porpoises well enough to explain it to my students. Enlightenment came to me while attempting to teach wheel landing to a student who (ironically) had taken some of my previous advice to heart: He flew with a very light touch. "Three fingers and a thumb," I would tell him, explaining that it would help him always keep the airplane in trim and be easier for him to tell if he was holding the proper amout of rudder pressure in the climb, because the wing tends to drop, etc. And because I didn't want him to be a hack. After a couple times around the pattern, it seemed to me he was excaserbating the porpoising tendencies of the airplane by actually pushing the stick forward after a bounce. "Don't push the stick forward after you bounce," I would caution him. "I'm NOT!" he told me...which I found hard to believe because I just watched the stick physically move forward about six inches after the last bounce.
Then came the epiphany: The airplane bounces, the nose pitches up (because the mains are forward of the CG), and a split second later, the effect of having hit the ground deflects the elevator downward. It was the G loading of the bounce that made the stick go forward. My student was right, he never pushed the stick forward - the bounce did.
On airplanes with long fuselages - and hence a long distance from the main tires to the elevator, the porpoise happens relatively slowly. Short coupled airplanes, like a Pitts or even a RV can have a viscious porpoise. Stiff landing gear make it worse, but I never witnessed it happen in a Great Lakes with its mushy oleos. Wouldn't happen at all in an airplane with a statically balanced elevator.
I look forward to comments, questions. D
I'll give you a hint: When you open the door of a Citabria and unfasten the seatbelt, what happens to the stick?
Took me quite some time to understand why the airplane porpoises well enough to explain it to my students. Enlightenment came to me while attempting to teach wheel landing to a student who (ironically) had taken some of my previous advice to heart: He flew with a very light touch. "Three fingers and a thumb," I would tell him, explaining that it would help him always keep the airplane in trim and be easier for him to tell if he was holding the proper amout of rudder pressure in the climb, because the wing tends to drop, etc. And because I didn't want him to be a hack. After a couple times around the pattern, it seemed to me he was excaserbating the porpoising tendencies of the airplane by actually pushing the stick forward after a bounce. "Don't push the stick forward after you bounce," I would caution him. "I'm NOT!" he told me...which I found hard to believe because I just watched the stick physically move forward about six inches after the last bounce.
Then came the epiphany: The airplane bounces, the nose pitches up (because the mains are forward of the CG), and a split second later, the effect of having hit the ground deflects the elevator downward. It was the G loading of the bounce that made the stick go forward. My student was right, he never pushed the stick forward - the bounce did.
On airplanes with long fuselages - and hence a long distance from the main tires to the elevator, the porpoise happens relatively slowly. Short coupled airplanes, like a Pitts or even a RV can have a viscious porpoise. Stiff landing gear make it worse, but I never witnessed it happen in a Great Lakes with its mushy oleos. Wouldn't happen at all in an airplane with a statically balanced elevator.
I look forward to comments, questions. D