banking/turning

[rancherman]

I noticed on a couple of POV videos, that during a right turn/bank, the stick is held mostly right of center during the duration of the turn.
During a left turn, the stick is momentarily moved to the left, but to maintain the banking angle for rest of the turn, the stick seems to be a little right of center.
Is this due to the leading/retreating effect of the rotor.. or cross wind effect for that particular turn.. or a little of both?

 

[WaspAir]

Crosswind should affect ground track only, not the control input necessary to establish or maintain a bank. I have never detected any roll direction preference attributable to the rotor in my rotary wing aircraft. Are you seeing a torque roll response due to the prop on a gyro? In long span fixed wing aircraft ( e.g. 18 meter sailplane), there is an overbanking tendency that requires top aileron, but it is symmetrical.

 

[rancherman]

Geez, I forgot the torque roll effect too!

This was a Nano J2.
Might of been the angle of the camera, although it seemed the pilot favored right of center on a left bank

 

[Doug Riley]

Rancher, you're right that, as each blade passes through a position on the OUTSIDE of the turn, that blade sees a different net airspeed, compared to level flight at the same aircraft airspeed. By the same token, each blade also sees a different airspeed when passing through the INSIDE position that it would in level flight. The odd part is that both the "inside" and the "outside" blade experience (a) more airspeed in a left turn and (b) less airspeed in a right turn.

Obviously, this is a different result than a pair of fixed wings in the same situation (one wing has an increase in A/S, while the other has a decrease).

Why the difference between aircraft types? Because, in a rotorcraft, the retreating blade's airspeed is its airspeed from rotation MINUS the aircraft's airspeed. The advancing blade's airspeed, OTOH, is its rotational airspeed PLUS the aircraft's airspeed.

Bottom line: the airspeed differences between advancing and retreating blades in a turn don't result in any rolling tendencies that differ between left and right turns. A CCW rotor will make slightly more lift in a left turn than in a right turn, due to the higher blade airspeeds during a portion of each rotation. Therefore, the pilot may need a bit less back stick in a left turn than in a right one.

 

[okikuma]

Geez, I forgot the torque roll effect too!

This was a Nano J2.
Might of been the angle of the camera, although it seemed the pilot favored right of center on a left bank

Robert,

Perhaps during normal level flight one has to hold slight left cyclic to fly level to compensate for the torque. Because of the "fish eye" lens of the camera, the slight left cyclic position appears vertical / centered, and the actual vertical position of the cyclic appears to be right of center.

Wayne

 

[rancherman]

thanks! Yeah, just putting this info in my 'tool bag' and didn't want to have the wrong idea of what's what!

 

[rancherman]

Speaking of torque roll, are we talking about the way the prop wash strikes one side of the vertical surfaces... or actual engine countering forces?

 

[okikuma]

Torque from the prop turing counter-clockwise as viewed from the rear of the gyroplane.

The corkscrew airflow from the counter-clockwise prop rotation would hit the left side of the vertical stabilizer / all flying rudder, and thus swinging the nose to the left. Right rudder input or an offset vertical stabilizer would be needed to counter this and center the yaw string. With clockwise prop rotation from Rotax 503. 532. 582 powered gyroplanes, left rudder input is needed.

Wayne

Right rudder inputs



Left rudder inputs from Rotax 503. 532. 582 powered gyroplanes.

 

[rancherman]

wow~ seems like a lot of correction!..
still, wouldn't striking a vertical surface below the CG cause a simultaneous roll too?

 

[Doug Riley]

Well, the spinning propwash shoves the vertical fin one way or other -- which way of course depends on the direction of the prop's rotation. The fact that the fin wants to move sideways means that the frame experiences both a yawing moment and a rolling moment. However, both of these effects result from the same force -- the sideways push on the fin. Once we eliminate this force by deflecting the rudder, we neutralize both tendencies.

What we're left with is the rolling tendency caused by the mechanical torque reaction between the prop and the frame. This reaction is quite powerful, especially in gyros with reduction drives (=torque multipliers). It's a hundred foot-pounds or more, even on a little gyro. It's enough to flip the gyro quite rapidly in the air (it's happened). In fact, it's almost enough to flip a light, high-powered gyro on the GROUND if no one's in the seat (it's nearly happened).

We tilt the rotor thrust slightly to one side of vertical to counteract this rolling tendency. People often make their control pushrods different lengths, so that the stick is centered when the rotor spindle is tipped the correct amount. So you may see the stick continuously off-center, or not --depending on how the control pushrods are adjusted.

The problem with using this tilted-rotor thrustline as roll-torque compensation is that rotor thrust varies in flight. If it is momentarily low enough while power is up, the gyro can roll over in the air -- again, it's happened. Tail surfaces that counteract torque roll aren't subject to this difficulty. Either a long-span H-stab or a long-span vertical stab, in either case immersed in the propwash near the center of the wash, will both provide roll-torque compensation and prevent the yaw issue.

 

[Jean Claude]

The odd part is that both the "inside" and the "outside" blade experience (a) more airspeed in a left turn and (b) less airspeed in a right turn.

A CCW rotor will make slightly more lift in a left turn than in a right turn, due to the higher blade airspeeds during a portion of each rotation. Therefore, the pilot may need a bit less back stick in a left turn than in a right one.

I not agree, Doug.
Since the gyrocopter's airframe is not rotationally linked to the rotor, the rate of a turn does not lead to any change in Rrpm/air, which is due solely to the required lift.

Only the rotor tachometer, indicating the rrpm relative to the structure, is influenced, not the lift.

 

[Aerofoam]

I not agree, Doug.
Since the gyrocopter's airframe is not rotationally linked to the rotor, the rate of a turn does not lead to any change in Rrpm/air, which is due solely to the required lift.

Only the rotor tachometer, indicating the rrpm relative to the structure, is influenced, not the lift.

I was thinking about this and "Modeling in my head" and suspected the same thing unless you were within about 20% of the speed where you would start feeling the presence retreating blade stall area.
Would it be possible to essentially have a "Tip Stall" if you were approaching the top end of the speed range and turned into the receding blade direction?

 

[Jean Claude]

Would it be possible to essentially have a "Tip Stall" if you were approaching the top end of the speed range and turned into the receding blade direction?

In a steep turn, the additional forces to be generated to change the plane of rotation (i.e. gyroscopic) must raise the nose of the disc (think of a 90-degree banked turn).
To raise the nose of the disc, you need to reduce the angle of attack on the retreating side.

 

[Aerofoam]

Let me re-phrase,:
If you were at a speed where the receding stall area was starting to develop and you banked into the receding side in a turn,
could it induce what would essentially be a tip stall of the disk?
I know it's not really a tip stall, but would it act like one?

 

[Jean Claude]

Putting the stick to the side to obtain a roll rate modifies the angles of attack of the blades passing in front and behind. Therefore no change of the stall area on the retreating blade .

 

[bryancobb]

Putting the stick to the side to obtain a roll rate modifies the angles of attack of the blades passing in front and behind. Therefore no change of the stall area on the retreating blade .

Jean Claude,
Once the desired bank angle is reached, in a steady-state steep turn toward the retreating half of the rotor disk, wouldn't the turn radius of the advancing half be greater than the turn radius of the retreating half? And if so, and if advancing half has speed of rotation PLUS speed of flight, and retreating half has speed of rotation MINUS speed of flight, would the shape and area of the stalled portion change and increase?

 

[Jean Claude]

Jean Claude,
... and if advancing half has speed of rotation PLUS speed of flight, and retreating half has speed of rotation MINUS speed of flight, would the shape and area of the stalled portion change and increase?

Sorry for the delay in my reply.

Unlike a wing attached to the fuselage, the airspeed velocitys composition you suggest applies only to an immaterial zone and not to airflow on a real element of the blade. So, you cannot conclude from this that it generates a physical dissymmetry of lift.