I asked a pal of mine the exact same question, he is a CPL and used a twin Squirrel every day, he passed it over by saying he had never really tried to work out why, but he went on to say, it did make you look a little under-experienced.
But yet I watched an RAF FI and student land a single Squirrel and never saw the same wiggle, could it be big engine covers create dead spots when trying to come down slowly?
My best but completely unscientific guess is that the low tail rotor placement causes it's rotor wash to interact with the ground, varys thrust efficiency. Add a horizontal stab that's at the margin of the main rotor down wash and you've got a lot of changes happening in the control trim position.
Good news, forget the tendency and fly the aircraft all the way down. Concentrate on no translation as you slowly descend and above all- don't try to feel for the ground- and it lands like any helicopter.
Interference between the rotor downwash bouncing back upwards and the frame. Put it in hover at 4 or 5 m and it's steady as a still life. The engine cowlings have little influence, yet I find the twin wiggles more than the single engine.
Low skid/high skid? From what I can recall, the high skid Squirrel puts the airframe spot on the top of the ground cushion, just before the skids touch (depending on weight, power, etc), whereas the low skid is through the downwash and stable before the skids touch. The Squirrel dance is basically trying to get on the ground whilst the ground cushion has other ideas: that was my excuse for the erratic landings, Guv!
Milt - Henri Marie Coanda was a Rumanian engineer/ scientist/ aircraft designer born in 1886 and he designed what could be considered the first turbofan/bypass engine in 1910, exploiting an effect he had observed where engine exhaust gases from a piston engined aircraft tended to follow the curved shape of the fuselage much further than expected. He took the propeller off the piston engine and used the drive to power a compressor that forced air into a ring shaped combustion chamber and produced sufficient thrust to get the aircraft airborne (albeit briefly). As ascend charlie has said, we see coanda effect on tail-booms as the downwash creates lift on one side of it (the angle the air hits the boom forces it around one side faster than the other). The effect can be enhanced by blowing air through a slot along the length of the tail boom (NOTAR does this) which encourages and enhances the boundary layer. The Sea King had a long strake added on the port side of the tail boom to break up this powerful lift because, with a wind from the right, the aircraft was trying to yaw right and full left pedal was frequently insufficient to prevent it. I have seen pictures of a rotor system built in the 60s with circular section blades that had a blown slot long the top surface - it apparently worked but the engineering problems (weight and power required) associated with getting high pressure air up the rotor mast and along the blades meant it didn't see full production.
Location: 1 deg south, avoiding Malaria P Falciparium
Well, some of you are close. there are 2 main things that make it a little squirely in a hover. 1) the shape of the belly .. has a tendancy with the rotorwash to recirculate. 2) The 146 gallon unbaffled fuel tank under the M/R transmission... its called fuel slosh.. very noticeable when your really light and it has under 40% fuel... and once it starts.... if you try to correct it... youll make it worse. My theory, is it came out of a Pugeot... its got the same gas cap 3) its french and they make everything difficult like getting in and out of the thing.. like the blades turning the wrong way
I love flying the thing, but the fuel tank a couple of other things makes me wonder about the people who designed it. Like whay cant they put the damm throttle on the collective and why cant the seat be 4-6 inches closer to the window!
Oh lets talk about my HYD failure last week and the whimpy green belt where the gule came apart....hmmm can you say weak link.
You know they have a saying in france " that may work in practice, but it does not work in theory, there fore we can not do it.
you tend to see guys in a really high hover, or a really low hover... that be why..
OK I am done rambling
"still bouncing on the concrete with the best of em "
Now I've got a straightfoward explanation - shape of the belly and recirculation plus large tail boom - and a scientific name for the whole thing - Coanda effect - that I can tell students to go research so it all seems terribly aerodynamically complicated and they'll stop asking me questions ! Perfect.
It's a high skid B that I 'm flying and I must admit to being a bit nervous at a zero speeder on the hard stuff, especially when light. Will give it a few goes when no-one's looking.
As for sitting nearer the windscreen, I'm fairly tall and as it is I can't easily see the red marks on the torque gauge. Any nearer and I'd have to crouch. Seat won't go back as far as I'd like. Is this more 'design for short ar$e French people' again ? (No offence to short or French people but the ergonomics don't seem quite right for a relatively spacious cabin.)
We also have a Squirrel, that I haven't flown yet, that does have the twist grip throttle - believe it's a mod or you can request it from the factory. Not for the twin though I don't think...?
As for 222, yes, the throttles are the opposite way to a motorcycle.
Many things conjoin and conspire to make the Squirrel squirrelly right near the ground. If it were "simply" the low tail rotor, 206's on low skids and certainly 222's would be similarly afflicted. Unless there's one on the left side also, the strake on the right side of the tailboom would seem to be Eurocopter's attempt to take advantage of the Coanda effect to lessen the right-pedal demands at a hover.
Rotorboy correctly brings up the fuel sloshing in the unbaffled plastic tank (although at 146 gallons it must have come out of one really long-range Peugeot). This can be uncomfortable, not to mention the source of PIO, and one that is certainly not needed when other factors are coming into play.
People have mentioned the rounded belly and how it affects the hover airflow. I suppose that's as good an excuse for my squirrelly hovering as any. And it probably does play a part.
I've never figured out Eurocopter's philosophy on control harmony. The D-models I flew had notchy collectives even with the friction all the way off, the pedals were strangely stiff considering they were boosted, and the cyclic was your typical "wet noodle." The cyclic, collective and pedals all ought to move with the same perceivable effort and effect, and this did not seem to be the case with the Astar. Ease in some pedal and nothing happens at first. A little more and...whoa, that's too much! Eh- maybe it was that limber, elastomeric tail rotor.
Adding to this was the puzzling cyclic linkage. The French do like a "sporty" feel to their helos, but the gearing of the Astar cyclic linkage and bellcranks seemed to me to be needlessly "quick" considering how responsive the rotor system is. Perhaps in designing the AS-350, the Aerospatiale engineers used the same basic control linkage ratios that they used with the (non-starflex) Gazelle and wet-head Dauphin, which were both sporty without being twitchy.
Although some pilots seem to adapt to the Astar flying qualities quickly, it takes awhile for many others to develop a feel for the aircraft. Fly an Astar at night, when the landing light illuminates the very front arc of the rotor disk. Even the tiniest movement of the cyclic causes that arc to jiggle up and down. To my mind, that's a control system that's not got the right bellcrank gearing.
High-skid or low, the Astars I flew all seemed to get weird right near the ground. It's aggravating because placing a skid exactly where I want it is neither as easy nor predictably accurate as it is in a Bell (or virtually any other helicopter for that matter). Flying shouldn't be that much work.