How will the main rotor torque be balanced if a helicopter doesn't have a tail rotor?
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2$\begingroup$ The vertical stabilizer usually provides enough torque balance at cruise speed - it will keep you straight if you lose the tail rotor in flight. Landing will be exciting, esp. if you have skids and not wheels. $\endgroup$– paulCommented Sep 21, 2014 at 5:35
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$\begingroup$ @paul What if i use two rotors one below the other, rotating in opposite directions and without any vertical stabilization and tail rotor. What then? will it fly?? $\endgroup$– seetharamanCommented Sep 21, 2014 at 16:04
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$\begingroup$ @paul: Well, landing will probably have to be carried out on autorotation in which there is no net torque. $\endgroup$– Jan HudecCommented Sep 21, 2014 at 16:36
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1$\begingroup$ @JanHudec autorotation is for landing after the engine quits. If your tail rotor and your engine fail at the same time you are probably going to have a messy ending. $\endgroup$– paulCommented Sep 22, 2014 at 0:11
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2$\begingroup$ @paul: Autorotation is also for landing if your tail rotor transmission fails because there is no torque. Otherwise the torque makes the helicopter spin out of control very quickly once speed is reduced. $\endgroup$– Jan HudecCommented Sep 24, 2014 at 9:33
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2 Answers
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The torque in a helicopter is generated by the engine driving the main rotor in one direction, which causes the fuselage to spin in the other direction. The tail rotor shaft is horizontally mounted and hence creates it own 'lift' to provide anti-torque. The pitch of the tail rotor gives directional control.
Common ways to design a helicopter without a tail rotor:
Tandem rotors : Two main rotors mounted one in front of the other. The rotor discs spin in opposite directions. All power from the engines is used for lift. Example: CH-47
Transverse rotors : Two main rotors mounted side-by-side. Example: Kamov Ka-22
Co-axial rotors : Two rotor discs are mounted one on top of the other. The engine units power both rotors. This structure requires a very complex swashplate mechanism. Example: Kamov Ka-25
Intermeshing rotors : In this main rotor assembly, the two set rotor masts are installed at a slight angle to each other, in a transversely symmetric manner, so that the rotor blades intermesh without colliding with each other. Example: Kaman K-MAX
Tip jet : This is an interesting solution to the no-tail-rotor problem. Instead of driving the rotors with a turboshaft engine, compressed air is sent through nozzles installed at the tips of rotor blades. The engines effectively push against the air rather than the helicopter fuselage. The rotor spins just like a Catherine wheel. Example: Hiller Hornet
NOTAR : NOTAR - NO TAil Rotor uses a fan inside the boom to build a high volume of low-pressure air, which exits through two slots and creates a boundary layer flow of air along the tailboom utilizing the Coandă effect. The boundary layer changes the direction of airflow around the tailboom, creating thrust opposite the motion imparted to the fuselage by the torque effect of the main rotor. Example: MD-900
Refer to Wikipedia for more!
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$\begingroup$ For unmanned vehicles, would it be practical to simply use vanes to limit the speed of rotation and then use guidance electronics which can cope with it? $\endgroup$– supercatCommented Sep 20, 2014 at 19:38
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$\begingroup$ Isn't the CH-47 Chinook also an example of intermeshing rotors? $\endgroup$ Commented Sep 20, 2014 at 20:27
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3$\begingroup$ @RedGrittyBrick : If you notice closely, the Chinook has its rotor discs installed at slightly different heights! Also, the intermeshing rotors concept has the rotor masts within the same housing. This is unlike in the Chinook, where the rotor shafts are separated by a very wide distance. $\endgroup$– RajCommented Sep 20, 2014 at 20:36
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1$\begingroup$ The point of the tipjet is that it's only the friction in the rotor bearing that would cause the body of the helicopter to counter-rotate? And that's presumably negligible compared wind buffetting and the air resistance of bodily rotating a helicopter so nothing much happens as long as you're not trying to hover for a long time in still air. $\endgroup$ Commented Sep 21, 2014 at 9:55
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2$\begingroup$ @Raj is entirely correct, these are all the current solutions to torque in helicopters. Each is something of a compromise. The tail rotor is very common, but uses some 15% of the engine power just to keep the helicopter straight. Tandem rotors like the CH-47 are great but give it a huge rotor area. Co-axial has a smaller foot-print, but the rotor mechanism is extremely complex and hence prone to failure. $\endgroup$ Commented Jan 9, 2015 at 4:47
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Another option to build a helicopter without a tail, is to use active flapping. Basically the idea is to not counteract the torque of the main rotor, but use a rotor concept that does not generate a torque.
At the Technical University Delft they applied this principle to build the Ornicopter (a mix of the words helicopter and ornithopter), a tailless helicopter. Flight tests have been performed using this prototype.
According to the university:
The Active Flapping
The active flapping of blades is the key for the Ornicopter concept. In this manner, the blade can generate a propulsive force to rotate itself, and hence the shaft torque is not needed. This results in a torque-less main rotor, i.e. the tail rotor can be eliminated.
Update
Concerning the question on scalability posed in one of the comments:
I found a PhD thesis on the topic (download) which did a feasibility study on a Bo-105 scale ornicopter. The thesis says:
"The thesis proves that the optimal design for the Ornicopter as compared to the Bo-105 benchmark is characterized by a lower blade loading, increased rotor tip velocity and larger vertical fin size. This optimal design results in an enlarged flight envelope due to the reduced rotor stall area and improved yaw stability in forward flight.
Nevertheless, despite these improvements in the Ornicopter's flight envelope, there is a slight increase in required power when compared with the Bo-105 specification (approximately 5% at 150 knots).
To compensate for the higher profile power needed for the Ornicopter's optimal design, a larger rotor radius is required in order to reduce the induced power and keep the increase in the total required power to a minimum. This thesis may be considered as a first step in rationalizing the expectations regarding the Ornicopter's tailless helicopter design.
The thesis proved that this new concept shows a slightly poorer performance than that of conventional helicopters regarding power consumption in forward flight and service ceiling. This is disappointing since one of the assumptions was that the elimination of the tail rotor would also eliminate the power consumption associated with a tail rotor. "
answered Jul 17, 2015 at 12:34
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$\begingroup$ Not that I'm aware of. The model is mainly a demonstrator. It was able to fly around freely, so perhaps it could already carry a small payload. No further effort was taken to scale it up to actually carry significant payload. $\endgroup$ Commented Jul 5, 2017 at 8:08
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$\begingroup$ @KorvinStarmast, I added a small part concerning a feasibility study for a bigger scale $\endgroup$ Commented Jul 5, 2017 at 8:18
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$\begingroup$ Ah, nice, improves the answer quite a bit. :) As ever, there is no free lunch. $\endgroup$ Commented Jul 5, 2017 at 23:13