Design and application of NOTAR as replacement for classical tail rotor.
Key words: NOTAR, Coanda effect, Tail Rotor
Due to the market requirements to increase the safety of helicopters, and by reducing the helicopter generated noise, the designers have come up with the application of different design solutions in replacement of the classical tail rotor. One of the solutions is also the NOTAR (NO TAil Rotor).
The NOTAR system is based on using the Coanda effect and vectored thrust to create the force necessary to counteract the torque effect and control the helicopter around the vertical axis. It is, namely, that the airflow, which after entering at intake is driven by the variable pitch fan through the helicopter tailboom, exits partly through the slots on the side of the boom, and the residual airflow is used to produce vectored thrust.
The part of the airflow which exits through the slots on the side of the tailboom, due to the way in which it streams around it (consequence of Coanda effect), causes a force which, after adding the force generated as consequence of vectored thrust, is sufficient to counteract the torque of the main rotor.
The result is of using NOTAR system is that the tailboom becomes a "wing", flying in the downwash of the rotor system, producing up to 60 percent of the anti-torque required in hover. The balance of the directional control is accomplished by a rotating direct jet thruster.
The anti-torque system of a helicopter has a major impact on the weight, performance, agility, reliability, flight and ground crew safety, and vehicle survivability.
MDHC has been working on the No-Tail Rotor (NOTAR) concept for the past 20 years. This antitorque system is in production and exists on current MD 500 series and Explorer vehicles.
[FIGURE 1 OMITTED]
The Coanda Effect has been discovered in1930 by the Romanian aerodynamicist Henri-Marie Coanda (1885-1972). He has observed that a steam of air (or a other fluid) emerging from a nozzle tends to follow a nearby curved surface, if the curvature of the surface or angle the surface makes with the stream is not too sharp.
The reason why airflow follows a certain surface lies in its viscosity. The air viscosity is very low; however, it is sufficient to make the air molecules adhere to the surface. On the very surface, the relative speed between the surface and the air molecules closest to it equals zero. Following the relative airflow speeds in the molecule layers further from the surface, it may be noted that these increase. As consequence of the difference in relative airflow speeds the forces are generated which cause twisting of air flow along the surface.
[FIGURE 2 OMITTED]
If pressures along the tailboom sides are considered, it may be noted that on the side with the slots through which the air exits from the tail the pressure is lower, whereas it is higher on the opposite side. The reason for this difference in pressures is the different airflow along the sides of the helicopter tail. A part of the airflow which bends around the tail prevents normal airflow on the side opposite of the one with the slots through which the air from the tail interior exits. Thus, the airflow speed on that side is reduced, increasing the pressure. The pressure difference results in a force which pushes the helicopter tail to the side with the air slots.
The NOTAR anti-torque system eliminates accidents and mishaps caused by the exposed tail rotor striking objects in flight, and significantly reduces ground incidents with people or equipment. Potential damage caused by foreign objects is also eliminated.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Inherent design capabilities of such helicopters reduces the susceptibility to loss of tail rotor effectiveness and the problems associated with quartering tail winds. The helicopter's power and stability in various wind conditions increase operational safety.
The disadvantages of implementing the NOTAR system can be seen in higher necessary power of the powerplant, thus increasing the fuel consumption and reducing the helicopter flight range, and in somewhat more complex design compared to the one of the classical tail rotor, may have problems operating in high cross winds. Furthermore, the cost, complexity, and less mature technology make the NOTAR less desirable than the conventional or fan-in-fin configurations.
The NOTAR anti-torque system eliminates all of the mechanical disadvantages of a tail rotor, including long drive shafts, hanger bearings, intermediate gearboxes and ninety-degree gearboxes.
In spite of this, due to a number of advantages that result from the implementation of this system in the helicopter design we may expect in the future an increase in the number of helicopters being equipped with such a system. The number of accidents dropped from 30 per 100,000 flight hours in 1970 to slightly over 9 in 2002.
To keep the helicopter light and achieve the maximum useful load, components are made as light as possible while maintaining structural integrity. On the other hand, rotor blades, transmissions and other components whose functions are essential to safe flight are designed to be stronger than necessary by a wide margin.
Application of Notar system eliminates the use of ordinary tail rotor, resulting with significant helicopter weight decrease. Considering the oil price growth trend on the world's market, application of Notar system shows its obvious advantage, among all the others, related to the lower levels of fuel consumption. Therefore, such helicopters have significant comparative and economic advantages resulting with higher level of competitive abilities on the market.
W. F. Powers: Adwanced Materials and Processes, May 2000.
Poston, Ken: Advanced Manufacturing and assembly technologies for future aircraft fuselage structures, Bombardiers Aerospace, Aerospace--A 2020 Vision, Royal Society, 2002.
Riddle, D. W.; and Eppel, J. C.: "A Potential Flight Evaluation of an Upper-Surface-Blowing/Circulation-Control-Wing Concept," Proceedings of the Circulation-Control Workshop 1986, NASA Ames Research Center, Moffett Field, CA, NASA CP 2432, February 19-21, 1986.
Kozachuk, A. D.: "Experimental Studies of Air Flow in the Channel of a Circulation-Control Rotor Blade," Problems inthe design of helicopter rotors (A93-32173 12-05), Izdatel'stvo Moskovskogo Aviatsionnogo Instituta, Russia.
Keys C., Sheffer M., Weiner S., Heminway R., "LH Wind Tunnel Testing: Key to Advanced Aerodynamic Design", AHS 47th Annual Forum, Phoenix, Arizona, May 1991. Costes, M., Collercandy, R., Kroll, N., von Geyr, H. F., Renzoni, P., Amato, P., Kokkalis, A., Rocchetto, A., Serr, C., Larrey, E., Filippone, A., Wehr, D., "Navier-Stokes Calculations of Helicopter Fuselage Flowfield and Loads", AHS 54th Annual Forum, Washington, DC, May 1998.
Fig. 5. Shematic presentation of design and operation principles of the NOTAR system All Accidents 6% Avoidable Accidents Saved 14% With NOTAR System Accidents 80% Other Than Tail Rotor Note: Table made from pie chart.
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|Title Annotation:||no tail rotor|
|Author:||Frankovic, I.; Rados, B.; Rados, J.|
|Publication:||Annals of DAAAM & Proceedings|
|Article Type:||Technical report|
|Date:||Jan 1, 2005|
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