Don’t worry about it mikey, I have enjoyed the extra info. Never can learn too much.
Chronos, can I borrow your Moderator Hat to smack this punk upside the head with please?
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.
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?
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.
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.
Thats not always true. Several are counter-rotating.
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!
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.
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The two engines on the Cessna Skymaster counterrotate, but only because one is mounted backwards at the aft end of the fuselage.
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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.”
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.
/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/