# Helicopters with two rotors

Was watching Monster Machines on TLC, and they did a piece on one of them giant helicopters with two rotors that can lift 10 tons. They mentioned that the two rotors spin in opposite directions, and that this is important, but they didn’t explain why. So, why is it important that the rotors spin in opposite directions?

Torque. Without having two opposite forces on the fuselage, the helicopter will spin out of control. The upright tail rotor on other 'copters performs the same purpose, exerting a force in the direction opposite the torque rotation.

I’m sure someone else will do a better job explaining this.

Its a very simple concept to be honest, each of the rotor sections spin in opposite directions. The blades in each of the rotor sections have a specific angle of attack in relation to relative wind. each generating a downward air flow, hence forth pushing the aircraft up! Woo how easy was that… Ok picture it this way, take one of those ocillating fans… it pushed air in one direction if you where to aim it at the ground it would generate some lift, not take another fan and reverse the angle of the blades, and reverse the direction of rotation… bang there ya go… next time ask a better question, like how a helicopter makes banking turns… then we will get into something called gyroscope percession…

I have to know these types of things for my job, so dont be thinking i am some kind of weirdo or whatever…

-Mikey

Ah. That makes a lot of sense. I always thought the tail rotors were just for steering.

There are two basic reasons. The first, as Flymaster pointed out, is to balance the torque output from the rotors. Ever notice how when a helo loses its tail rotor assembly it spins almost uncontrollably? That’s because the function of the tail rotor on a conventional helo is to basically push in the opposite direction that the rotation of the main blades try to spin the body. When you have twin blades and spin them in opposing directions at the same rate, the torque balances, and the aircraft is stable.

The second reason that the blades spin in opposite directions is that on some models of helos, the radii of the blades overlap somewhat, and it is FAR easier to synchronize the blades so that they do not impact each other when you spin them opposite.

it seems that my message has been mis-understood… I was speaking of the main rotors… the tail rotor does prevent the helicopter from spinning out of control, it also control the heading of the aircraft… if you have seen the H-46 that is a helicopter that lacks a tail rotor… it has two main rotors… one at the front of the aircraft and one at the back… perhaps this is the aircraft that we are speaking of? Or are we speaking about the russian helicopter that has two rotor blades directly intop of each other?

Yeah, Mikey, that’s a good point. I was talking about those US military jobbers that the boats drive right into and stuff, the Sea Knight and the Chinook. The Russian one (the entire Kamov line, from what I gather from my handy dandy Jane’s Aircraft Recognition Guide, for those of you scoring at home) uses two rotors for a reason that I’m not sure about. I’d guess that Mike hit the right answer above, as he seems to be some sort of aeronautical engineer. I’ll defer to him on that, although the Kamovs do not have a tail rotor, as it is not necessary to counter the torque. The dual rotor assembly does that quite well by itself.

Flymaster,

What do you do? I am curious as to you affiliation with aeronautics…

Yeah, I was talking about the ones with one rotor on each end, not one right above the other.

well, to make a long story short with the helicopter that you are speaking about (the H-46, which i see on a day to day basis) the two rotor blades spin in opposite directions to oppose eachother torque… otherwise it would flop through the air like a butterfly with 1 wing cut off… and the H-46 is FAR from giant… if you want to talk about giant helicopters you must look up the H-53… I was just flying in one not to long ago… those are HUGE… there is a helicopter bigger by only 3 inches… a Russian aircraft… actually the H-53 is made by a Russian company (Sikorsky) so i guess you could call them both russian… whatever… I hope your question has been answered… Feel free to fire away with some more avation related questions…

-Mikey

Man is certainly stark mad: he cannot make a worm, yet he will make gods by the dozen. – Michael de Montaigne

Nothing in the field. I just read a lot of books. Currently, I’m a first semester junior working towards a BS in CompSci. Maybe something’s rubbed off from the engineering school. BTW, is my understanding of the issue correct, or am I just talking out of my misinformed ass?

You are correct…there is so much physics to the flight of helicopters you put it in the perfect simple explanation

Mikey, Igor Sikorsky was originally from Russia, true, but the Sikorsky Aircraft Company is in Connecticut. Ol’ Igor did have his own company in Russia during WW1, but he left during the Revolution.

The Russian Mil helicopters with 2 counterrotating coaxial rotors also have that design to eliminate net torque on the fuselage right at the source. The extra complexity does lead to extra weight, though.

It’s also true that the tail rotor on a “conventional” layout does control the direction the helicopter is pointed. Their blade pitch can be changed to provide either less or more force, allowing the fuselage to swing under greater force, or be swung by rotor torque. The actual direction the helicopter is flying, not just pointing, is controlled by making the pitch of the main rotor blades higher on one side than the other. Some of that is necessary just to keep the helicopter flying straight - since the main rotor blades on the advancing side are traveling faster relative to the air, their pitch must be lower to keep them from producing more lift than the trailing blades and flipping the machine over.

hrmm i guess i should say thanks for the second lesson on helicopter flight…
but i wont…
-Mikey

Let me toss in one more explaination. It doesn’t seem that the point has been missed, but I haven’t seen it explained like this, which to me seems the most accurate.

Remember that there is a motor spinning an axle, obviously. On one end of this axle is the rotorblades, and on the other is the helicopter. That axle applies an angular force on both ends, in equal and opposite magnitudes. When the aircraft is in the air, there is nothing to cause either end to resist the force except air resistance. Air resistance is, in a very round about way, what provides lift. That lift makes the lighter, but more resistant to rotation, blades harder to spin than the massive fuselage. The fuselage then begins to rotate instead of the blades. So, depending on the pitch of the rotors, the body of the helicoper is going to have to resist a varying angular force in order to remain stable.

So, each half of the system (blades and fuselage) has a moment of inertia (resistance to spinning). The moment of inertia is heavily dependant on the mass of the object, and you can extrapolate that the blades moment is much much lower than the body. In addition each half has a magnitude of air resistance, the proportions of which fluctuate wildy based on the flight dynamics at the time.

Knowing this, you can see that while the force is equal on both ends of the axle, the resistance to that force of each part is different at any moment.

There are two ways to combat the propensity of the fuselage to rotate, the most common is by simply adding a second blade perpendicular to the first at the tail. This blade also must have a variable pitch to adjust its lift (horizontal) to match the difference between the force and the resistance to that force I described earlier.

The second way, as you noticed, is by adding a second vetically lifting rotor. The control dynamics of this system are much more complex, but it has the benefit of doubling the applied lift. This rotor (assuming it moves at the same speed and pitch as the first) must be placed an equal distance from the center of rotation (related to the center of gravity) so that the torque forces are aplied equally to the fuselage. When the helicopter like this wants to rotate on its axis, ie change its heading, it slows or speeds one of the blades and uses the torque its been trying to combat to its advantage in a controlled fashion. In other words it offsets the stability the two blades created and that difference results in a spin.

After typing this, I’m not sure if it clarifies, or simplifies anything, but its accurate for the most part (nelgecting some of the more complex mechanics and flight dynamics). Its really pretty damn cool.

Additionally, as mentioned, there is a design in which the two opposing rotors are stacked directly ontop of each other. This still adds the double lifting force (for the most part) and removes the need for careful positioning of the two relative to one another. The mechanics of the drive train are very complicated, so its not as efficient or reliable to use this design.

…but without the benefit of caffeine at the moment, I don’t have a lot to add.

There are four types of twin-rotor designs. The most familiar is that on the Boeing CH-46 and CH-47. As mentioned, the reason they rotate in opposite directions is to counteract torque. I don’t really buy the secondary reason that they counterrotate is to lessen the chances of them crashing into each other when they overlap. It’s to counteract torque.

Another type is primaryly (solely?) used by the Russians. Again, they counterrotate to counteract torque. The advantage of this design is that it’s easy to fly and that you’re not going to have a tail rotor strike if you don’t have a tail rotor. It is subject to vibration though, which increases fatigue both of the rotor system and of the pilot.
Then there is the meshing rotor design. In this system, the rotors each have their own mast which is offset from vertical. The blades cross in the middle (sort of like an egg beater). The USAF used this design in the H-46 Husky. Without the anti-torque rotor in the back, it was very easy to fly. So easy, I’ve read, that it was deemed unsuitable for training. There is also the advantage of not having to worry about tail rotor strikes, and it was safer to load troops or casualties from the rear. Currently, Kaman (I believe it is pronounced “ka-MAN”) is building a meshing-rotor design for special lifting work.

The fourth twin-rotor design uses rotors that are extended laterally from the fuselage. Hanna Reich flew the Focke-Wulf FW-61 in the 1930s, which was of this design. The design has fallen out of favour since the single main rotor is more efficient, but it lives on in the V-22 Osprey.

Other points:

Sikorsky is an American company. Igor Sikorsky was a successful aircraft designer in Russia, but left after the Revolution. His first successful helicopter design was built in the U.S. It was Sikorsky who developed and proved the single main rotor design that is so successful today.

The anti-torque rotor (tail rotor) in a single rotor design is there to counteract torque. It is also used for yaw control. This is much more pronounced near the ground where you may want to make a hovering turn. In flight, the a/t rotor is not used as much as the rudder in an airplane. Once you’ve set your pedals for the torque you are producing, you steer with the cyclic. Sort of like a giant video game. (Except when it’s “game over”, it’s OVER! ) The a/t rotor is also used to trim the aircraft for crosswinds or during turns.

Okay, I need more coffee.

ok everyone and anyone else who intends to read/reply to this thread must understand this… The orignial question was from someone who was inept as to how the helicopter in question worked. So a simple and plain answer was in order… so please lets end it here ok? How many times do we want to hear about the same thing of the same type of smartass over and over… Now if you people want to go head to head in the area of helicopter knowledge we will start with the AFCS (automatic flight control system) for the Sikorsky H-53… Any takers?

-Mikey

To complete the field of anti-torque methods, we need to mention:
NOTAR - a Boeing (nee McDonnel Douglas) technique that blows air down the tailboom, which is slotted for circulation control, and vented out a duct at the tail. There are some ships certified and being sold with this system. There is no tail rotor.
Tip Drive- rotor propulsion is provided by jets or ducts at the tips of the rotor blades. Therefore, no torque is applied to the fuselage, and no anti-torque is required. AFAIK, this has been done only on experimental aircraft. It works, but isn’t practical.

mikey, your post serves no purpose. Also, your first post to the OP was rather rude. And as to your last post, “inept” is not how we usually describe a person who is asking a question.

friedo asked a question and Flymaster answered it. Your answer came off as very condescending, and by your own statement it was unnecessary. Frankly, you’re the one coming off as a “smartass”.

I let it slide before because this is not The Pit, and this thread does not deserve to go there. I see that you are a new user and perhaps some latitude can be granted. But please learn some etiquette.

Otherwise the mods will close this thread.

[Moderator watch ON]

Just stepping in here to confirm what Johnny L.A. said: If you can’t say somethin’ nice, don’t say nuthin’ at all. Also, just because the OP has been answered is usually not a good reason to close a thread: No single post will ever contain the entire body of knowledge on a given subject, and subsequent posts are likely to add related information which many will consider interesting.