I pulled a FastEddie....

[CharlieTango]

A yaw string on my CT worked fine.

[zaitcev]

A couple of months ago, a local instructor at my home field took a potential student for an "introductory ride" ... totaled a DA20, probably one of the easiest planes there is, to fly. He developed a habit of landing too flat and after his 2nd bump of the nose wheel, induced so much PIO, the nose gear ended up coming up through the floor of the airplane.

Those strikes are actually very common on DA20. The one at Bode Avia at KAEG was ruined like that. There is one sitting outside of the school in KAKR right now, too. For that reason I do not consider DA20 an easy plane to fly. A seasoned pilot might find them easy, but that's a different reference.

[FastEddieB]

Those strikes are actually very common on DA20.

And Grummans and Cirrus' and Mooney's. Really any plane with minimal nose gear damping.

I think there's a YouTube video of one in a CT, if I'm not mistaken.

Key of course is to keep that stick coming back and to try to have it all the way back or nearly so on touch down. There's some debate on that, of course, but without extra speed on touch down one cannot porpoise.

[drseti]

without extra speed on touch down one cannot porpoise.

Actually, one can - but it has to be done on porpoise.

All jest aside, the best cure for this is to get some taildragger training. Those used to flying conventional landing gear seldom bounce on the nose wheel. Of course, we sometimes scrape the tail tiedown ring...

[zaitcev]

Those strikes are actually very common on DA20.

And Grummans and Cirrus' and Mooney's. Really any plane with minimal nose gear damping.

IMHO nose gear damping has nothing to do with it, because porpoising has little to do with gear's characteristics. It occurs because of the sudden increase of the angle of attack due to the contact with the runway. The cure, in airplane with conventional gear, is to touch down on the main gear and stick it. If you do that, it does not matter if and how the front is sprung.

The real problem here is that all these aiplanes have tails that make it necessary to make a finely controlled flare to avoid a tail strike. For an experienced Cirrus driver like you it may sounds like a non-problem, but anyone who cannot keep the Vref exactly on the dot is in the world of hurt, because the attitude of the airplane changes with the AoA.

[FastEddieB]

IMHO nose gear damping has nothing to do with it, because porpoising has little to do with gear's characteristics.

It always seemed to to me.

Land a Cherokee or a 172 a bit flat and fast and there's a decent chance the oleo will absorb some of the energy and the plane will stay on the ground*.

In the aforementioned planes with no effective damping in the nose gear, the stage is set for that extra energy to efficiently push the nose back up, increasing the AOA and beginning the porpoising.

Just my impression over the years, is all.


*Of course this goldie oldie show that oleo struts certainly don't make a plane immune!

http://youtu.be/NMmHYWjEmkY

[zaitcev]

IMHO nose gear damping has nothing to do with it, because porpoising has little to do with gear's characteristics.

It always seemed to to me.

Land a Cherokee or a 172 a bit flat and fast and there's a decent chance the oleo will absorb some of the energy and the plane will stay on the ground*.

The logical lapse here is that while oleo can save a botched touchdown, it does not have to.

BTW, here's an extreme case of the wrong that you mentioned:
https://www.youtube.com/watch?v=cclH8ZSDVOM

[Jack Tyler]

Actually, there is a shock absorber in the Grumman 4-place nose gear. It's a torsion bar affair, in-between the fixed steel strut and its fixed attachment points on both sides of the firewall. Not the equivalent of bungee cords or oleo struts but with a fair bit of give, nonetheless. This is one reason, I believe, why we hear far less about Grumman nose gear failures than e.g. we do about RV nose gear failures, where the attachment to the firewall is both fixed and not attached across the full width of the firewall. Much beefier gear on the Grumman, as well.

"...anyone who cannot keep the Vref exactly on the dot is in the world of hurt, because the attitude of the airplane changes with the AoA."
I haven't found this to be the case, and I've certainly made many landings at higher than appropriate speeds. In fact, planes are often landed at higher speeds than appropriate because the pilot uses the same approach & flare speeds that are intended for MTOW when the single pilot is flying a relatively light a/c and the appropriate speed is ~5 knots lower. Simply letting the plane's speed dissipate in the 'float' while easing back the stick/yoke results in a good if long landing. PIO is often the result of the pilot choosing to force the stick/yoke forward a bit, thinking that will 'plant' the a/c on the runway.

[FastEddieB]

Actually, there is a shock absorber in the Grumman 4-place nose gear. It's a torsion bar affair, in-between the fixed steel strut and its fixed attachment points on both sides of the firewall. Not the equivalent of bungee cords or oleo struts but with a fair bit of give, nonetheless.

I see this as conflating two seperately functions.

Analogy: in a car you generally need both springs and shock absorbers. The springs allow the wheel to move and then return to its original position. The shock absorber damps out that movement to keep it from being cyclical.

I think all nose wheels will have a "spring" function - a torsion bar can do that, or rubber pucks, or the air in an oleo strut. All allow the wheel to move up and down. But most of these, save the oleo, lack any appreciable damping - the energy that goes into them comes right back out - which I still hold makes porpoising more likely in a flat landing.

But don't get me wrong - they are all perfectly serviceable. Four of the planes I've owned have lacked damping of the nosewheel - 2 Grummans, a Cirrus and now my Sky Arrow. In none of them has that been an issue for me*, but I make slow, nose high landings a habit.

Anyway, Cirrus airplanes have long been prone to porpoising, part of which I think was caused by some initial training leading to fast landings. In more recent models, damping was added - my assumption is because the engineers thought it would help by making the gear more forgiving.

Finally, I am not totally immune. A few years back I was landing the Mooney I've recently posted videos and photos of. The amount of pull required on landing was so different from the Sky Arrow I'm used to that it touched down fast and bounced. And then another. And then...saw things were going downhill and went around - it happens!

*I did have one renter catch the prop on my Traveler after an apparent porpoising incident. He had checked out fine, but again - it happens!

[FastEddieB]

The logical lapse here is that while oleo can save a botched touchdown, it does not have to.

Logical lapse? Moi???

I can see that criticism if my position was that an oleo prevents botched touchdowns. From the video I posted that's clearly an indefensible position. Once begun, even the most finely crafted nose gear damping will not prevent pilot induced oscillations from overpowering it.

My position is that nose gear damping can reduce the likelihood of porpoising. And for now I'll stick to that position.

[FastEddieB]

BTW, here's an extreme case of the wrong that you mentioned:
https://www.youtube.com/watch?v=cclH8ZSDVOM

Cool!

I think that's a great example of nose gear damping working as advertised. Note how each "cycle" is smaller than the last, and by about 4 "cycles" the oscillations have been effectively damped out.

[Paul Z]

I do “Fast Eddy” Landings all of the time. My Runway is 5999 Ft X 100 Ft, I aim for the Left or right side depending on the winds, and cut as much Cross Wind out as Possible. I had an instructor in Idaho teach me to do that, and I’d much admit it is better than doing a Ground loop in a Tail Dragger.

[MrMorden]

BTW, here's an extreme case of the wrong that you mentioned:
https://www.youtube.com/watch?v=cclH8ZSDVOM

Cool!

I think that's a great example of nose gear damping working as advertised. Note how each "cycle" is smaller than the last, and by about 4 "cycles" the oscillations have been effectively damped out.

Though it's hard to know how much of the recovery in that video is damping from the gear and how much is pilot correction. If the pilot sensed the problem after the first bounce and immediately pulled the yoke full back, that could have aided the recovery as well.

[MrMorden]

I’d much admit it is better than doing a Ground loop in a Tail Dragger.

That's the problem with modern pilots. No sense of adventure.