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question on s-turns-vs- attitude


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Hello all,

 

I am about 5 hrs into my flight training, and i have noticed while doing S-turns, that the attitude of the helicopter is different from a left turn to a right turn.

 

I am flying a 300cb and it seems that at about 60 knots, while turning left the helicopter wants to go into a nose down attitude, while in a right turn the helicopter seems to hold altitude or even climb slightly. Is this possible? I realize that i am a newb, and mabee it just seems that way to me, but something feels quite different between a left turn and a right turn.

 

I am hoping some of you experienced pilots here can explain this to me in simple terms, or just tell me i am crazy, which ever the case mabee.

 

Thanks in advance :)

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There are a couple of things that could be going on in your descending left and ascending right turns...first I'd probably guess that when you apply left cyclic you're also pushing forward because of the mechanics of the arm, and vice versa for the right turns: try to concentrate on applying only lateral cyclic, even look down while practicing it to be sure.

Secondly, the schwiezer usually sits in a left skid low attitude, due mostly to CoG issues with the fuel tank, if you dont have the heli trimmed properly in a turn the nose can dip or rise due to this yaw. Keep an eye on the trim string.

And third; wind has an effect on performance in a turn. Steeper into the wind and shallower away from the wind.

 

As to whether you're crazy or not....you are trying to become a heli pilot arent you?

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...and fourth, and most important, the rotor system experiences different aerodynamics in a left vs right turn. When you turn left, the advancing blade is getting a higher relative airspeed (it is on the outside of the circle), so it wants to flap up, requiring forward cyclic to counter the tendancy. When turning right, the effect is reversed.

 

BTW, the left-skid-low attitude in a Schweizer (and many light helicopters) is mostly due to translating tendancy - the tail rotor is pushing right, so the main rotor has to be tilted left (by the pilot) to keep the helicopter stationary, this causes the left tilt of the airframe.

 

The best way to keep the helo level when banking into turns is to maintain a consistant disk-to-horizon gap during the bank. Note the point on the disk that the disk seems to pivot about when you bank, and keep that spot steady re: the horizon.

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...and fourth, and most important, the rotor system experiences different aerodynamics in a left vs right turn. When you turn left, the advancing blade is getting a higher relative airspeed (it is on the outside of the circle), so it wants to flap up, requiring forward cyclic to counter the tendancy. When turning right, the effect is reversed.

 

 

This seems to be on the right track. Only it is acting opposite, the left turn caused a slight nose down, while a right turn causes a slight nose up. :blink:

 

Could it be more of a retreating blade issue in a turn, rather than an advancing blade issue?

 

In otherwords in a left turn the retreating blade is to the inside of the turn which causes even less lift (or less velocity) than normal and that causes the helo to take a nose down attitude.

 

In a right turn the retreating blade is to the outside of the rotor disc which has a slight increase to velocity which adds lift and causes the helo to rise in altitude.

 

I have looked in all of my books, but none of them mention this phenomenon, or atleast i havent found it yet <_<

 

The above is only my theory, based on the small amount of aerodynamic knolwedge that i currently have, so if i am off base please let me know.

 

 

Patrick.

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Fling- what you're describing about adv. vs. ret. blades is true but if you read his post he's getting nose low in left turns and the higher rel. vel. of the advancing blade in a left turn will precess into a nose high att. In my experience this is almost always caused by students pushing fwd cyclic as they reach across their body to turn left.

Also, the left skid low schweizer condition is not because of translating tendency. Translating tendancy only accounts for a very small portion of the left skid low. Fly a 300c with dual tanks and two pax of similar weight and the left skid low is almost imperceptable. Its also why on pickup its an aft right instead of an aft left motion. Ask the factory if you dont believe me.

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The pilots position also plays a factor in the climbing/descending in a turn

 

Sitting in the right seat, when the aircraft banks left the pilot feels like they're climbing, (above the horizon) and in the right turn, they feel like they are descending (below the horizon)

 

This is probably more so in airplanes than helicopters, but I've still seen it with new (and not so new) pilots.

 

Fly Safe

Clark B)

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Ask the factory if you dont believe me.
Don't have to - the only CCW-turning helicopter I know of that hovers level is the Enstrom, and that's because the rotor mast is rigged with a tilt to the left. R22? left-low. B206? Left-low. B47? Same. EC120? Right-low.

 

If you run a W&B on a single-tank 300CBi, you will find that the lateral CG is pretty close to zero (it's +0.02") with 25 gallons in the tank and a 155-lb pilot, yet it still hovers left-skid low. I've watched my 290-lb student land solo after a long XC with around 8 gal in the tank, and then (with a +2.33" CG) it lands almost level.

 

Don't discount translating tendancy - the tail rotor on a Schweizer is putting out 100 lbs of thrust or better at the hover. That's 100 lbs of force pushing the helicopter sideways which must be countered by tilting the main rotor the opposite way. To put it another way, to the main rotor, it's the equivelent of an added 10~12-kt crosswind.

 

As far as the attitude change in turns - yep, I missed the nose-down-left thing which Mr. New described. I'd say you were right, it may be unconscious control inputs. The aerodynamic/gyroscopic attitude change in turns is well documented - have a look at Ray Prouty, Shawn Coyle, and W.J. Wagtendonk.

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newbie,

 

Keep your trim in the turn, or rather retrim in the turn.

 

 

So judging by the responses, I assume there is nothing different aerodynamicly that should cause any difference in helo attitude between a left turn and a right turn? I am concious of my cyclic inputes while turning, and i dont think i am adding forward cyclic while making a left turn, but apperently i must be.

 

thanks for the info

 

Fair winds,

Patrick

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The gyroscopic precession (or phase lag) in forward flight for left and right turns causes the nose to be low in left turns and high in right turns. In a left turn, the advancing blade upflap begins over the tail and retreating blade downflap begins over the nose, both inducing a nose forward momentum which results in the nose low attitude in the left turn. For the right turn, the upflap of the retreating blade is induced at the nose and the downward flap of the advancing blade is induced at the tail resulting in an upward motion to the nose which causes a nose high attitude in the turn. Both of these conditions result in the aircraft being out of trim in the turn, hence my comment.

 

Normal, in-trim, left turns will feel a bit like the tail is inside of the turn (off to your left) and the right turn will feel almost level. It also depends on the angle of bank or the onset of the roll into and out of the turn, as to how severe the effect will be. Like I said, check your trim in the turn, chances are that you are not in trim.

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  • 3 weeks later...
Hello all,

 

I am about 5 hrs into my flight training, and i have noticed while doing S-turns, that the attitude of the helicopter is different from a left turn to a right turn.

 

I am flying a 300cb and it seems that at about 60 knots, while turning left the helicopter wants to go into a nose down attitude, while in a right turn the helicopter seems to hold altitude or even climb slightly. Is this possible? I realize that i am a newb, and mabee it just seems that way to me, but something feels quite different between a left turn and a right turn.

 

I am hoping some of you experienced pilots here can explain this to me in simple terms, or just tell me i am crazy, which ever the case mabee.

 

Thanks in advance :)

 

I agree with the response about the fact that since you're sitting on the right side of the helicopter and not the center, your perception of the helicopter's attitude is different in a left turn and a right turn. The mechanics of the arm moving the cyclic has, in my experience and reasoning, to cause a climb in a left turn and a descent in a right turn. My advice to you is to remain aware of the problem you're having and make a conscious effort to correct for it. Good luck.

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