Tail rotor negative pitch question
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Tail rotor negative pitch question
I have an aerodynamic question relating to the negative pitch angle available on the tail rotor.
I have always assumed that the tail rotor would only ever require positive pitch to counteract the torque provided by the main rotor for pedal turns to the left (counter-clockwise rotating main rotor) and that turns to the right would only ever require less positive pitch or even neutral pitch - but not negative pitch.
I cannot understand how the helicopter would ever be a situation that would require power to be supplied the tail rotor for a right pedal turn - shouldn't the torque of the main rotor always provide enough yaw for this?
The only reason for negative pitch would be in an auto, when the torque of the main rotor is no longer an issue.
The following is a quote from the Rotocraft Flying Handbook:
Now for the question. Are my assumptions correct or is there a situation in powered flight/hover that would require a negative pitch angle on the tail rotor?
Thanks for your input.
Jim.
I have always assumed that the tail rotor would only ever require positive pitch to counteract the torque provided by the main rotor for pedal turns to the left (counter-clockwise rotating main rotor) and that turns to the right would only ever require less positive pitch or even neutral pitch - but not negative pitch.
I cannot understand how the helicopter would ever be a situation that would require power to be supplied the tail rotor for a right pedal turn - shouldn't the torque of the main rotor always provide enough yaw for this?
The only reason for negative pitch would be in an auto, when the torque of the main rotor is no longer an issue.
The following is a quote from the Rotocraft Flying Handbook:
The maximum positive pitch angle of the tail rotor is generally somewhat greater than the maximum negative pitch angle available. This is because the primary purpose of the tail rotor is to counteract the torque of the main rotor. The capability for tail rotors to produce thrust to the left (negative pitch angle) is necessary, because during autorotation the drag of the transmission tends to yaw the nose to the left, or in the same direction the main rotor is turning.
Thanks for your input.
Jim.
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Lets say that you are using the right pedal to keep the aircraft straight in the hover.
As you transition to forward flight and increase power you will need more right pedal.
As you gather speed and the vertical fin starts to take over you will need less right pedal until above a certain speed the tail rotor is at zero pitch.
If you reduce power for a powered descent you will need left pedal.
In auto you will need a lot of left pedal.
Another situation where a lot of left pedal is needed is where the engine fails in the hover. You will go from lots of right pedal to lots of left to keep straight.
Substitute left for right dependant on which way the main rotor turns.
As you transition to forward flight and increase power you will need more right pedal.
As you gather speed and the vertical fin starts to take over you will need less right pedal until above a certain speed the tail rotor is at zero pitch.
If you reduce power for a powered descent you will need left pedal.
In auto you will need a lot of left pedal.
Another situation where a lot of left pedal is needed is where the engine fails in the hover. You will go from lots of right pedal to lots of left to keep straight.
Substitute left for right dependant on which way the main rotor turns.
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Depends on the aircraft design (i.e. how much "weather cocking" tendency it has) but in your example of an anti-clockwise main rotor, hovering in a crosswind from the left side will increase the requirement for right yaw pedal / tend towards a negative tail rotor pitch input.
In autorotation, the tail rotor needs enough negative pitch, in addition to that required to overcome transmission drag, to allow the pilot to balance the aircraft in a turn to the right.
In autorotation, the tail rotor needs enough negative pitch, in addition to that required to overcome transmission drag, to allow the pilot to balance the aircraft in a turn to the right.
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Autorotation is one.
Strong Crosswinds at hover is another.
Another aspect is when rt rdr (say +ve angle) changes to lt rdr, the angle does not necessarily change to -ve. The rudders neutral are generally not the zero angle. So when from rt rdr you are now applying a little left rdr (say after transition) you are not applying -ve thrust, just reduced +ve angle (though rdr is left of neutral position!)
Strong Crosswinds at hover is another.
Another aspect is when rt rdr (say +ve angle) changes to lt rdr, the angle does not necessarily change to -ve. The rudders neutral are generally not the zero angle. So when from rt rdr you are now applying a little left rdr (say after transition) you are not applying -ve thrust, just reduced +ve angle (though rdr is left of neutral position!)
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if one was to fly the aircraft at say twenty knots along a straight line at say five foot height and forced by use of the pedals the aircraft to rotate smoothly and fluently at a constant rate, say one revolution per 10 secs around the axis of the mast in either direction, it wouldn't matter which way the M/R blades rotated, you would in each instance be using more and less left and right pedal up to full deflection either way as you encountered more and less crosswind effect (as ShyT mentioned) and as the answer to your question.
that is for flight powered by the engine which causes torque,
in flight powered by gravity which produces anti-torque (transmission drag) one could experience the same pedal effects during aircraft rotations about the mast axis, but of course one might need to start those experiments well above the firma terrors as gravity being the driving force, begorrah at the point that it runs out things get kinda solid.
that is for flight powered by the engine which causes torque,
in flight powered by gravity which produces anti-torque (transmission drag) one could experience the same pedal effects during aircraft rotations about the mast axis, but of course one might need to start those experiments well above the firma terrors as gravity being the driving force, begorrah at the point that it runs out things get kinda solid.
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There are a number of conditions which require negative pitch. One of the more obvious is weathervaning. The Bell 206, for instance, weathervanes strongly, and in a 20 kt wind will stay into the wind without any pedal input at all, in an OGE hover. I've done it offshore, going from flat pitch up to an OGE hover and back to the deck with my feet on the floor, not touching the pedals. If you need to turn to the right, you have to put in a lot of right pedal, and it simply won't turn without it. Most helicopters will weathervane to some extent, and with a crosswind it can take a lot of negative pitch in the tail rotor to make the nose point where the pilot wants it.
I've done it offshore, going from flat pitch up to an OGE hover and back to the deck with my feet on the floor, not touching the pedals.
I've never flown the JR, and don't have access to one to try the above. In that situation, will the pedals retain their position when you remove your feet, or will they seek neutral due to aerodynamic loads, etc.?
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fly sideways
Flying sideways to the left in a counter clock wise rotating rotor machine normally faster than the point where translational lift is achieved. Do it in a 44 fast enough and you will run out of right pedal!
Its perhaps worth pointing out that the tail rotor pitch angle depends (in nearly all helicopters) on two things, the position of the yaw pedals and the position of the collective. Many helicopters will fly at Vh with the pedals roughly in the middle, and also fly in autorotation with pedals roughly in the middle.
Therefore with normal flight collective position, full left (european) or full right (american) pedal may not give a negative pitch angle on the TR. It certainly will do in autorotation, partly because some negative pitch is needed to overcome the transmission drag (as TET says below), and also because a helicopter that does not require a large TR pedal input when going from powered to autorotational flight is a well designed one. In other words, in auto there is lots of negative pitch available, but its unlikely to ever be needed (esp if you do you EOL into wind!), and this is just a consequence of making the neutral-ish pedal position correspond to a slightly negative TR pitch with the collective down.
In some types, the amount of pedal travel to the left (EU) or right (USA) reduces as the lever is lowered - in other words if you have full pedal with the collective up, then lower the collective, the pedals are forced back towards the middle (best to try this shut down on the ground!). In some types this is not the case, perhaps to reduce the probability of a pedal jam restricting collective travel, with the inconsequential side effect that there is more negative TR pitch available with collective down than one could reasonably need.
HC
Therefore with normal flight collective position, full left (european) or full right (american) pedal may not give a negative pitch angle on the TR. It certainly will do in autorotation, partly because some negative pitch is needed to overcome the transmission drag (as TET says below), and also because a helicopter that does not require a large TR pedal input when going from powered to autorotational flight is a well designed one. In other words, in auto there is lots of negative pitch available, but its unlikely to ever be needed (esp if you do you EOL into wind!), and this is just a consequence of making the neutral-ish pedal position correspond to a slightly negative TR pitch with the collective down.
In some types, the amount of pedal travel to the left (EU) or right (USA) reduces as the lever is lowered - in other words if you have full pedal with the collective up, then lower the collective, the pedals are forced back towards the middle (best to try this shut down on the ground!). In some types this is not the case, perhaps to reduce the probability of a pedal jam restricting collective travel, with the inconsequential side effect that there is more negative TR pitch available with collective down than one could reasonably need.
HC
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Negative TR trust
Jim,
1. Conform the quote from the handbook, a fully symmetric helicopter would require a small amount of negative trust in autorotation because in autorotation a small amount of power (5-10 %) is drawn from the rotor to drive a number of systems: this requires an opposite force to the usual torque canceling force in powered flight.
2. For efficiency reasons, many helicopters are not symmetric, and an aerodynamic torque canceling force will be provided by (for instance) the tail fin in order to reduce the amount of power that the tail rotor has to provide in forward flight. In autorotation this aerodynamic configuration needs to be neutralised increasing the need for negative trust.
3. As stated by Steekvlieg, Fast lateral flight (or compensation of strong lateral wind) creates a situation where negative trust is needed in powered flight.
4. Dynamic conditions can arise where negative trust is needed : for example in a hover power failure, delay in control inputs will most likely make the helicopter rotate. To arrest that motion negative trust is needed.
5. Possibly (fast) backward flight or hover with strong tail wind, could create a condition where negative trust is required to control the helicopter.
d3
edited point 5 added
1. Conform the quote from the handbook, a fully symmetric helicopter would require a small amount of negative trust in autorotation because in autorotation a small amount of power (5-10 %) is drawn from the rotor to drive a number of systems: this requires an opposite force to the usual torque canceling force in powered flight.
2. For efficiency reasons, many helicopters are not symmetric, and an aerodynamic torque canceling force will be provided by (for instance) the tail fin in order to reduce the amount of power that the tail rotor has to provide in forward flight. In autorotation this aerodynamic configuration needs to be neutralised increasing the need for negative trust.
3. As stated by Steekvlieg, Fast lateral flight (or compensation of strong lateral wind) creates a situation where negative trust is needed in powered flight.
4. Dynamic conditions can arise where negative trust is needed : for example in a hover power failure, delay in control inputs will most likely make the helicopter rotate. To arrest that motion negative trust is needed.
5. Possibly (fast) backward flight or hover with strong tail wind, could create a condition where negative trust is required to control the helicopter.
d3
edited point 5 added
