Saturday, October 1, 2011
Around the World: Avoiding LTE
Loss of tail rotor effectiveness (LTE) has been around for a long time and the majority of helicopter pilots are familiar with this aerodynamic phenomenon. They will have been briefed on it during basic training and have probably discussed it when fraternizing with their rotary wing colleagues.
Very few will actually have had it demonstrated to them during training and Heaven forbid, experienced it firsthand during their day-to-day operations.
Whenever a helicopter is maneuvered in close proximity to the surface, either in the hover or low speed forward flight, there exists the potential for development of this condition. The following is a brief description of LTE.
• Weathercock instability downwind, i.e., the pilot has a high pedal workload with varying tail rotor thrust in order to prevent the helicopter weathercocking into wind—occurrence mainly in the hover.
• Left crosswind with anti-clockwise rotor system and vice versa, which could lead to development of a vortex ring state on the tail rotor with subsequent loss of thrust—occurrence either in the hover or in forward flight.
• Left quartering crosswind which entrains the main rotor disc vortex straight into the tail rotor with subsequent disruption of airflow on the tail rotor and loss of thrust—occurrence either in the hover or in forward flight.
Most Causative Factors:
Left quartering crosswind while terminating transition, particularly during steep, constant angle approaches. Left crosswind in either IGE/OGE hover.
Sufficient IAS should be maintained when operating close to surface and not G/S—any uncommanded right yaw should be corrected immediately.
An increase in forward speed with a possible minor reduction of collective—the CP not to be lowered completely since this will create accelerated ROD and a rapid increase of CP to correct the ensuing increase in sink rate will drive the M/R straight back into vortex.
What is not mentioned in most publications is the fact that allowing rotation to develop will exacerbate the vortex ring situation on the tail rotor (I stand corrected here since initially the tail rotor will go from a vortex ring situation to a windmill brake state and thereafter a completely stalled condition if the rate of rotation is extreme enough).
This will rob the main rotor of rpm, resulting in a large loss of thrust and the aircraft settling, the pilot applying CP to correct this situation and the main rotor developing vortex ring, all of which will result in an accident—irrecoverable if the pilot is not aware of what is happening.
Caution should be exercised during operations close to the surface, i.e. hovering IGE/OGE, confined area inspections/approaches/game catch/cull operations/crop spraying/vehicle tracking, to mention a few.
If this occurs below 50 feet AGL it will result in a rapid, uncontrolled rotation with a hard landing at the very least; if it occurs between 250 and 150 feet AGL on approach it will result in a catastrophic accident.
I discovered these secondary effects the hard way. I was asked by a client to actually demonstrate the LTE condition and subsequently did so. The results as described above were frightening and all the helicopters on which I have subsequently demonstrated LTE have experienced a rapid development of vortex ring state on the main rotor in less than one rotation.
Therefore it must be understood that the onset of both vortex ring and LTE can be violent and rapid if conditions are right, giving the pilot very little warning and leaving extremely little chance of recovery, particularly if the pilot is not aware of what is happening. Keep in mind that this condition can occur on any helicopter with a conventional anti-torque system.
I have seen a training video at Bell that shows a Eurocopter Dauphin (Fenestron-type anti-torque system) suffering from this phenomenon and crashing in an uncontrollable fashion from low altitude. The client I mentioned suffered from this in that he was orbiting a confined area carrying out an inspection prior to landing—he maintained a constant G/S as opposed to IAS in a very strong wind during the orbit, found himself settling on D/W at low speed, rapidly applied CP to correct this, rotated out of control and developed vortex on the main rotor resulting in a serious accident from an altitude of 500 feet AGL.
Note: S.P. “Buzz” Bezuidenhout is a senior helicopter flight instructor and designated examiner based in Johannesburg, South Africa.