AS355 TwinStar
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.
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Cheers
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!
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Devil49 has the answer....don't hover it in, just do a smooth no-hover run-on landing ( we're talking just a few inches of dragging the springs)
Smooth as silk every time.
Smooth as silk every time.
Coanda effect - a cylinder can generate lift in an airflow by rotating on its axis, or by inducing the air to flow faster on one side.
Coriander effect - big night on the beer and curry, can cause urgent visits to the bog.
Throttles on the collective seem to rotate in the same direction, throttles on the floor (Squirrel) or roof (twins) have the same orientation of forward for full throttle.
Bell 430 is the odd one out, with throttles looking like motorbike setup, but (I believe) rotating the opposite way.
Coriander effect - big night on the beer and curry, can cause urgent visits to the bog.
Throttles on the collective seem to rotate in the same direction, throttles on the floor (Squirrel) or roof (twins) have the same orientation of forward for full throttle.
Bell 430 is the odd one out, with throttles looking like motorbike setup, but (I believe) rotating the opposite way.
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Coanda effect
As stated by Ascend Charlie - in an aviation sense.
The coanda effect goes back to even more basics. Any liquid or gas if flowing along a surface will tend to follow that surface even if the surface changes direction.
If its air - add in the changes in air pressure that occur with air flow changing direction and you get lift in funny directions
As stated by Ascend Charlie - in an aviation sense.
The coanda effect goes back to even more basics. Any liquid or gas if flowing along a surface will tend to follow that surface even if the surface changes direction.
If its air - add in the changes in air pressure that occur with air flow changing direction and you get lift in funny directions
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.
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.
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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
RB
"still bouncing on the concrete with the best of em "
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
RB
"still bouncing on the concrete with the best of em "
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Thanks Guys !
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$e French people' again ? (No offence to short or French people but the ergonomics don't seem quite right for a relatively spacious cabin.)
We also have a Squirrel, that I haven't flown yet, that does have the twist grip throttle - believe it's a mod or you can request it from the factory. Not for the twin though I don't think...?
As for 222, yes, the throttles are the opposite way to a motorcycle.
Now I've got a straightfoward explanation - shape of the belly and recirculation plus large tail boom - and a scientific name for the whole thing - Coanda effect - that I can tell students to go research so it all seems terribly aerodynamically complicated and they'll stop asking me questions ! Perfect.
It's a high skid B that I 'm flying and I must admit to being a bit nervous at a zero speeder on the hard stuff, especially when light. Will give it a few goes when no-one's looking.
As for sitting nearer the windscreen, I'm fairly tall and as it is I can't easily see the red marks on the torque gauge. Any nearer and I'd have to crouch. Seat won't go back as far as I'd like. Is this more 'design for short ar$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.
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.
The French
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Sorry to whinge at the previous participant but:
The rotor does not turn the "wrong way", merely the other way ( much more ergonomic if you ask me)
The space in the cab is excellent, you can shove all sorts of bits and pieces left and right and most of it doesn't fall out of the ddor when you open it (ie 206)
The throttle is fine where it is, never had a problem with it at all!
The tank, well at least it's big enough to get you somewhere without having to re-fuel every two hours.
NO I am not French, but US helicopters are certainly NOT gods gift to us pilots.
PS anyone else had Nr, Ng and TOT needles all wobble(fluctuate) at the same time in a 206? Bloody strange if you ask me
The rotor does not turn the "wrong way", merely the other way ( much more ergonomic if you ask me)
The space in the cab is excellent, you can shove all sorts of bits and pieces left and right and most of it doesn't fall out of the ddor when you open it (ie 206)
The throttle is fine where it is, never had a problem with it at all!
The tank, well at least it's big enough to get you somewhere without having to re-fuel every two hours.
NO I am not French, but US helicopters are certainly NOT gods gift to us pilots.
PS anyone else had Nr, Ng and TOT needles all wobble(fluctuate) at the same time in a 206? Bloody strange if you ask me
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All interesting theory but...........
2 things are happening -
1/ The cyclic induced PIO. Friction up the cyclic as per instructions in the Llama RFM to the point where it is quite difficult to move. You'll get used to it. The controls are obviously fixed to the frame and if the mast/transmission start to oscillate it subsequently moves the controls more. The better the balance of the MR the better the hover as well. Don't stir the pot. With friction on you'll soon tire of trying to.
I have had a 355 hover for 45 seconds or more without touching the cyclic. This is with .02 IPS.
2/ The wake from the MR OGE misses the tail rotor due to contraction and when IGE moves across the tail rotor and causes translation lift in the TR but not consistently. This causes a yaw roll couple and gets worse the closer you are to the ground. This can be proved by hovering in a tail wind. Keep it straight with your feet. The wake around the cabin is minimal.
The strake on the B2 was to create a Coanda effect to help the tail rotor by destroying lift on the right side. I doubt there is much Coanda effect without it as per B, D. Interesting the B3 doesn't have it.
The tank sloshing is limited to earlier unbaffled tanks and will NOT be noticed in the hover unless your a stick stirrer, only in the cruise at about 60-70%.
Try it. Works for me, steady as a rock.
1/ The cyclic induced PIO. Friction up the cyclic as per instructions in the Llama RFM to the point where it is quite difficult to move. You'll get used to it. The controls are obviously fixed to the frame and if the mast/transmission start to oscillate it subsequently moves the controls more. The better the balance of the MR the better the hover as well. Don't stir the pot. With friction on you'll soon tire of trying to.
I have had a 355 hover for 45 seconds or more without touching the cyclic. This is with .02 IPS.
2/ The wake from the MR OGE misses the tail rotor due to contraction and when IGE moves across the tail rotor and causes translation lift in the TR but not consistently. This causes a yaw roll couple and gets worse the closer you are to the ground. This can be proved by hovering in a tail wind. Keep it straight with your feet. The wake around the cabin is minimal.
The strake on the B2 was to create a Coanda effect to help the tail rotor by destroying lift on the right side. I doubt there is much Coanda effect without it as per B, D. Interesting the B3 doesn't have it.
The tank sloshing is limited to earlier unbaffled tanks and will NOT be noticed in the hover unless your a stick stirrer, only in the cruise at about 60-70%.
Try it. Works for me, steady as a rock.
Ok, two cents from me as well
I once had the pleasure to talk to one of the designers of the AS 350, and one of my questions was of cource why it is so unstable. Should not have said unstable
I got an instant reply "It is not unstable, it is precise". I guess it could be argued. Anyway he said they (Eurocopter) where not really sure why exactly is was a bit twitchy (he agreed on that), a lot of contributing factors he said, no magic item.
He also said when they installed the long tailpipe on the exhaust on the B3 it became much better, and this was a surprice to them.
I have had the pleasure to fly many diffrent versions (all but the D) and in my opinion there are some diffrences.
Low skids are worce then high( I know it was argued the other way, but this is my opinion).
Short T/R output drive worce then long (standard now).
Twinstar blade with trailing edge tab much better than without.
B3 exhaust also much better.
Best contributing factor - lots of houres on the machine and to have it recent. Look on a high time 350 pilot not flying anything else - steady as a rock, but do not put that guy in a steady dampend platform, such as the S76, he will make it look bad by overcompensating
I once had the pleasure to talk to one of the designers of the AS 350, and one of my questions was of cource why it is so unstable. Should not have said unstable
I got an instant reply "It is not unstable, it is precise". I guess it could be argued. Anyway he said they (Eurocopter) where not really sure why exactly is was a bit twitchy (he agreed on that), a lot of contributing factors he said, no magic item.He also said when they installed the long tailpipe on the exhaust on the B3 it became much better, and this was a surprice to them.
I have had the pleasure to fly many diffrent versions (all but the D) and in my opinion there are some diffrences.
Low skids are worce then high( I know it was argued the other way, but this is my opinion).
Short T/R output drive worce then long (standard now).
Twinstar blade with trailing edge tab much better than without.
B3 exhaust also much better.
Best contributing factor - lots of houres on the machine and to have it recent. Look on a high time 350 pilot not flying anything else - steady as a rock, but do not put that guy in a steady dampend platform, such as the S76, he will make it look bad by overcompensating
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Cyclicrick,
I totally disagree with you that the throttle is OK where it is. I guess you havent had too many emergency's that require throttle manipultion.
High side gov failure in an Astar is a little differnt, you must immeaidtly lower collecitve and take the FCL out of the govering range.
Stuck pedals, kinda hard to adjust throttle to help control yaw.
There are several other things i wouldnt mind having it on the collective for.
They shouldnt have waited for the B3 to do it , but put it on the B2 years ago.
RB
I totally disagree with you that the throttle is OK where it is. I guess you havent had too many emergency's that require throttle manipultion.
High side gov failure in an Astar is a little differnt, you must immeaidtly lower collecitve and take the FCL out of the govering range.
Stuck pedals, kinda hard to adjust throttle to help control yaw.
There are several other things i wouldnt mind having it on the collective for.
They shouldnt have waited for the B3 to do it , but put it on the B2 years ago.
RB
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THE REASON THE SQUIRELL WOBBLES WHEN IT LANDS IS THE RESULTING FORCE CAUSED BY THE MAIN ROTOR HEAD AND THE POSITION OF THE TAIL ROTOR BEING AT DIFFERENT HEIGHTS
THIS RESULTANT FORCE CAUSES THE HELICOPTER TO "HANG" RIGHT SKID LOW .
THIS COUPLED WITH THE "STINGER" {FITTED TO ALLEVIATE GROUND RESONANCE} CAUSES THE HELICOPTER TO INITIALLY LAND ON THE RIGHT REAR STINGER TIP WHICH IN TURN CAUSES A PIVOTING/ BOUNCING MOTION-USUALLY OVERCORRECTED BY MOST PILOTS I AGREE THAT THE BEST WAY TO LAND IS A GENTLE RUN ON LANDING
THIS RESULTANT FORCE CAUSES THE HELICOPTER TO "HANG" RIGHT SKID LOW .
THIS COUPLED WITH THE "STINGER" {FITTED TO ALLEVIATE GROUND RESONANCE} CAUSES THE HELICOPTER TO INITIALLY LAND ON THE RIGHT REAR STINGER TIP WHICH IN TURN CAUSES A PIVOTING/ BOUNCING MOTION-USUALLY OVERCORRECTED BY MOST PILOTS I AGREE THAT THE BEST WAY TO LAND IS A GENTLE RUN ON LANDING
Helicopter Pilots Get It Up Quicker
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as355f1 you sound more qualified to comment than me as I ve never flown the squirrel and guess I m not likely to in the near future 
but...
Are there any helicopters where the main rotor head is at the same level as the tail rotor?
In my limited experience all helicopters tend to hover with one skid low - or is that just my flying?
I always thought the stringer wa the part which stopped me dinging the tail rotor - what is it on the squirrel?
And surely one of the bonuses of helicopters is you don't need to 'run on' during 'normal ops'?
PW
PS - got a headache so please don't shout!
but...Are there any helicopters where the main rotor head is at the same level as the tail rotor?
In my limited experience all helicopters tend to hover with one skid low - or is that just my flying?
I always thought the stringer wa the part which stopped me dinging the tail rotor - what is it on the squirrel?
And surely one of the bonuses of helicopters is you don't need to 'run on' during 'normal ops'?
PW
PS - got a headache so please don't shout!
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pilotwolf, i believe the stingers that 355f1 refers to are actually the small "beavertails" installed at the aft end of each skid tube, those are for ground resonance attenuation. remove those and things can get interesting. also there are tolerances for the tails and they need to be replaced if the angle / clearance is out of limits
dr
dr