Helicopters with two rotors

[friedo]

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?

[Flymaster]

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.

[mikey07005]

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

[friedo]

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

[handandahalf]

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.

[mikey07005]

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?

[Flymaster]

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.

[mikey07005]

Flymaster,

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

[friedo]

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

[mikey07005]

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

[Flymaster]

Originally posted by mikey07005
What do you do? I am curious as to you affiliation with aeronautics....

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?

[mikey07005]

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

[ElvisL1ves]

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.

[mikey07005]

hrmm i guess i should say thanks for the second lesson on helicopter flight.....


but i wont.....


-Mikey

[Omniscient]

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.

[Johnny L.A.]

...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 (http://home.earthlink.net/~gawebster/fockeachgiles/fa61folder/fa61.html) 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.

If you really look at what a rotor blade is doing, you'll probably think twice about flying in a helicopter. First, it's going round and round. Then it's turning on the feathering hinge (for pitch). Of course, if you change the pitch, the blade is going to want to climb or descend; so you have a flapping hinge. But when the blade flaps up, the ends of the blades move closer to the centre of rotation causing that blade to move faster due to the Coriolis effect. The classic example is of an ice skater whose arms are spread when she's spinning. When she moves her arms closer to her body, the speed of the rotation increases. Since the other blades are at different points in the rotation, they don't want to speed up. So you need a lead-lag hinge. (Of course in a two-bladed system, the blades lead and lag and flap as a unit. I'm talking about a 3-or-more blade system.) So each of your blades is twisting along its longitudinal axis, flapping up and down, and moving forward and backward in the plane of rotation all at the same time.

Okay, I need more coffee.

[mikey07005]

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

[Race Bannon]

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.

[Johnny L.A.]

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.
So a simple and plain answer was in order... so please lets end it here ok?
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.

[Chronos]

[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.

[friedo]

Don't worry about it mikey, I have enjoyed the extra info. Never can learn too much.

[Omniscient]

Chronos, can I borrow your Moderator Hat to smack this punk upside the head with please?

[Fletch]

A Russian built anti-submarine helicopter Nato designation (Hormone) has counter-rotating rotors. Some of the advantages have already been posted but another is that the helicopter can hover "outside the wind". It is very difficult for single rotor helicopters hover unless they are heading into the wind.

[ElvisL1ves]

One more advantage of a dual-rotor system is that all engine power, less transmission losses, goes into producing lift or what amounts to thrust. With a tail rotor system, some of the power fights itself - the tail rotor works against the main rotor. That effect is offset in smaller machines by the single-rotor system's lower weight and cost.

The Kaman system (Johnny, you're right about the pronunciation) has the 2 masts so close together that most of the rotor circles overlap; but conversely, they're actually too close for the blades to hit each other. On the Boeing CH-46 and -47, there is very little overlap, just enough to make blade strikes or mast strikes a worry; but the larger total rotor circle area allows for greater lift capacity. There's a Russian Mil design that has the 2 rotors located side by side on long truss booms, but that would seem to weigh more unnecessarily.

Mikey, let us show off a little, will ya?

[Johnny L.A.]

I forgot to mention that the fore-and-aft twin rotor system, as on the CH-47, allows for a wider centre of gravity range than a single main rotor system.

ElvisL1ves: now that you mention it, I do remember seeing a picture or some footage of the Russian "outrigger" helicopter. I think it was on The History Channel, The Learning Channel or A&E, but I can't recall which. Do you know what it's called?

And on the subject of such an arrangement being kept alive in the V-22 Osprey: The Focke-Wulf used the rotors for lift and thrust (the propellor in front was there to cool the engine, not for propulsion). Having rotors on the wingtips allows the Osprey to vector its thrust and fly as a conventional twin-engine aircraft, albeit with oversized props.

[CurtC]

Another digression...

Twin-engine airplanes' propellers rotate the same direction. The ailerons can compensate for the torque, and it's less complex to have interchangeable engine parts, so counter-rotating wouldn't be worth it.

And I've heard it said that, at least when hovering, helicopters don't as much fly as they beat the air into submission.

Johnny, thanks for the note about the wider CG range for dual-rotor. Makes sense, but I had never thought of it.

[bernse]

Originally posted by CurtC
Another digression...

Twin-engine airplanes' propellers rotate the same direction. The ailerons can compensate for the torque, and it's less complex to have interchangeable engine parts, so counter-rotating wouldn't be worth it.


Thats not always true. Several are counter-rotating.

[Venkman]

Originally posted by CurtC
Another digression...

Twin-engine airplanes' propellers rotate the same direction. The ailerons can compensate for the torque, and it's less complex to have interchangeable engine parts, so counter-rotating wouldn't be worth it.



Just to pick a nit and show off some of my (limited) knowledge on the subject, there are some exceptions to this. IIRC on the P-38 is one of the exceptions - the engines rotate in opposite directions. This helps eliminate the "critical engine" problem in the case of an engine failure, but as CurtC mentions above it causes a whole 'nother set of problems with maintenance since you need different parts for the left and right engines

[Vestal Blue]

Originally posted by Venkman
Originally posted by CurtC
Another digression...

Twin-engine airplanes' propellers rotate the same direction. The ailerons can compensate for the torque, and it's less complex to have interchangeable engine parts, so counter-rotating wouldn't be worth it.



Just to pick a nit and show off some of my (limited) knowledge on the subject, there are some exceptions to this. IIRC on the P-38 is one of the exceptions - the engines rotate in opposite directions. This helps eliminate the "critical engine" problem in the case of an engine failure, but as CurtC mentions above it causes a whole 'nother set of problems with maintenance since you need different parts for the left and right engines

Boy howdy! and with the limited combat lifespan of the Allison "time bomb", the maintenance troops were kept busy. I've always wondered what a P-38 with Merlins could've done.......IIRC, this also applied to the Dehavilland Mosquito, though I'm a little fuzzy. Great info on helicopter flight dynamics, BTW. Thanks!

[ElvisL1ves]

Originally posted by Johnny L.A.
ElvisL1ves: now that you mention it, I do remember seeing a picture or some footage of the Russian "outrigger" helicopter. I think it was on The History Channel, The Learning Channel or A&E, but I can't recall which. Do you know what it's called?[/B]

I saw the same show you did, apparently. Looking in my trusty Aviation Week index, that must be the Mil Mi-26. I recall from the same show that it was made by using 2 entire rotor/drivetrain systems from what must be the Mi-10, in a quick and cheap way to get double the lift capacity and have a counterpart to the CH-64.

[ElvisL1ves]

The two engines on the Cessna Skymaster counterrotate, but only because one is mounted backwards at the aft end of the fuselage.

[Flymaster]

Originally posted by ElvisL1ves
I saw the same show you did, apparently. Looking in my trusty Aviation Week index, that must be the Mil Mi-26. I recall from the same show that it was made by using 2 entire rotor/drivetrain systems from what must be the Mi-10, in a quick and cheap way to get double the lift capacity and have a counterpart to the CH-64.

My Jane's book shows the Mi-26 as being a fairly standard looking design, albeit with 8 rotor blades. Perhaps the picture is just bad, and the two rotors are mounted VERY close together, but the only outrigger I can find is the Osprey, plus the Kaman with the 2 very close together engines.

Maybe the one you saw was just an experimental model based on the Mi-26? The model I'm looking at has a NATO designation "Halo."

It is, however listed as a "Heavy lift helicopter," that was built with the goal of doubling the capacity of the Mi-6, and it was, as of the 1999 publication of this book "the largest helicopter in production."

[Gunslinger]

Continuing the hijack AND lending to the original discussion...

Both the Mil-26 helo and the "Bear" heavy bomber have pairs of counter-rotating props on the same shaft (er...you know what I mean) In the Bear's case, there are 4 pairs; on the helo, 1 pair.

[Venkman]

Originally posted by Vestal Blue

Boy howdy! and with the limited combat lifespan of the Allison "time bomb", the maintenance troops were kept busy. I've always wondered what a P-38 with Merlins could've done.......IIRC, this also applied to the Dehavilland Mosquito, though I'm a little fuzzy. Great info on helicopter flight dynamics, BTW. Thanks!

/hijack in progress..../

Were the Allisons that bad? I always thought that the Merlins put out more power but were less reliable than the Allisons, but that's completely biased by reading about the unlimited air races. It's a wonder the Merlin in a typical modified P-51 can make it through a single lap, let alone complete a whole race.

Reliability seems to be the big advantage radial engines have in air racing - you can blow a cylinder clear through the cowling and the engine will probably keep running long enough to get the plane safely back to the runway, plus it will be relatively cheap and easy to fix the engine and have it running again. Do the same thing with a Merlin and not only are you landing NOW you're replacing the whole engine afterward.

/hijack/