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TripleDelta
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Twin Main Rotors In Autorotation Situations

Sun Jul 18, 2004 12:24 am

How do the twin main rotors (like on Kamov helicopters) behave in autorotation situations? Do they help to slow the descent rate since two rotors are better than one? Are there any other advantages / disadvantages of this system in autorotation?
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SlamClick
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RE: Twin Main Rotors In Autorotation Situations

Sun Jul 18, 2004 11:58 am

Never flew twin or tandem rotor helicopters, but I expect that they would perform similar to single rotor. The issue is blade loading and the engineers tend to design the rotor system to match the loads intended to be carried. Don't think two rotors are actually better than one - just different.

One factor that has really improved autorotation performance in the last couple of decades is weighted blades. Early helos, say, 1950s vintage had very light blades, balsa wood with maybe bronze or aluminum spars and leading edge caps. They would bleed energy (RPM) very quickly when loaded up in autorotation, as in pulling a little pitch to carry the helicopter a bit farther in the descent. Blades with internal weights carry a lot more inertia.

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Airtractor
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RE: Twin Main Rotors In Autorotation Situations

Sun Jul 18, 2004 2:27 pm

This is a very good question.

A Kamov helicopter with contra-rotating blades behave very well in auto rotation as there is no tendency for the aircraft to yaw out of control on flare before touch down, like all other "conventional" helicopters do without torque control ( Tail rotor )

As for tandem rotor aircraft such as the Chinook I have no idea how they would handle an auto-rotation.
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TripleDelta
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RE: Twin Main Rotors In Autorotation Situations

Sun Jul 18, 2004 4:16 pm

Hmm...I never thought about the yaw issue. Interesting. Two blades spinning at same speed but in different directions would pretty much cancel each other's effects. I'd guess that with the Chinook-style ones, the effect is the same, though maybe the stress on the fuselage is greater, on account of the torque "arm", i.e. the distance of the rotor from the CG. Possibly then an imbalance between the two rotors would cause additional adverse effects along with yaw, maybe something like a sliding motion.
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Starlionblue
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RE: Twin Main Rotors In Autorotation Situations

Mon Jul 19, 2004 2:19 am

Don't think two rotors are actually better than one - just different.

IIRC the biggest advantage is smaller rotor diameter, leading to slower tip speeds and also ease of handling on/near the ground.

Of course, the solution is trickier to engineer with all that gearing in the same place.
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UPS763
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RE: Twin Main Rotors In Autorotation Situations

Mon Jul 19, 2004 11:17 am

I fly single rotor helicopters and perform several touchdown autorotations a week and am interested is what you mean when you say the aircraft yaws out of control?

The only time the aircraft will have a high yaw rate is during the initial power failure or throttle roll off, which will require right pedal in most american built helicopters, during the actual touchdown when you pull collective pitch to cushion the landing, there is no tourque being generated by the engine hence no yaw or need for a pedal input.
 
N766UA
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RE: Twin Main Rotors In Autorotation Situations

Mon Jul 19, 2004 11:19 am

I asked a Chinook pilot about this once when I was visiting an Army Guard hangar. He basically said that if they lose either 1 or both of the rotors, they're screwed. Thus, they pay extra close attention to keeping them in top shape. Their chinook was down because they found metal in the oil and the engine had to be replaced.
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PaveLowDriver
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RE: Twin Main Rotors In Autorotation Situations

Tue Jul 20, 2004 8:32 am

Gents,

Regarding the original question posed:

"Are there any other advantages / disadvantages of this system in autorotation?"

I don't have any stick time in a contra-rotating aircraft, but there are a couple of notable differences in the coaxial (Kamov) type system during autorotation that I'm aware of. Most notably in the how the aircraft behaves about the yaw axis.

Obviously, in a conventional helicopter, yaw is controlled by the application of pitch to the antitorque rotor - in the coaxial system, however, yaw is controlled by the application of differential torque to each rotor system. This allows helicopters with coaxial rotor systems to dispense with the long tailboom common to almost all other configurations and results in all of the Kamovs looking sort of like a fat goldfish. This config reduces the aerodynamic moment of the tail - i.e. the fuselage does not "weathervane" into the relative wind as easily as a conventional helicopter. What this necessitates on most coaxials is a large set of vertical tailplanes on the horizontal stabilizer, usually equipped with a set of rudders....

In short, since yaw control in a coaxial system is based on engine torque being applied to the system, when that torque is removed during a freewheeling autorotation, the only yaw axis control remaining is that provided by the aerodynamic surfaces, resulting in the aircraft being sort of squirrelly, heading-wise, on the way down through the auto.

To quote from Shawn Coyle, "Maintaining directional control [in a coaxial system] is difficult, as the amount of differential lift between the two rotors is not high, and may in fact, reverse. The rigging that produces a left pedal turn in powered flight may produce a different effect in autorotation."

On a different note, I've heard nothing but rave reviews from my friends who used to fly HH-43s and some dudes who are flying K-MAXs now. Apparently with the intermeshing (eggbeater) config, autorotational performance is pretty impressive. You've still got the yaw control problems requiring extensive stabilizer set-ups, but apparently they are some of the most forgiving rotor systems out there. I've heard the old HH-43 guys talk about autos in which, after rolling the throttle off, you could take the aircraft all the way to the ground without raising the collective from the floor. Some of 'em have even said you could do an auto without even lowering the collective....pretty amazing.

I'm all ears if anybody has any contradicting info or thinks I'm off-base....

Hope this helps......

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Starlionblue
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RE: Twin Main Rotors In Autorotation Situations

Tue Jul 20, 2004 10:03 pm

PaveLowDriver, welcome to the A.nut forums and thanks for your info.

Pave Lows are HUGE! I once saw two passing my window in London when Bush was there. Man was that impressive!
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HaveBlue
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RE: Twin Main Rotors In Autorotation Situations

Tue Jul 20, 2004 11:36 pm

Yes, welcome to the forums PaveLowDriver. I'm a big fan of the whole 53 series.

You seem kinda young to have friends that flew the Husky  Smile . I've never had the pleasure of seeing one fly, but I'm guessing they were pretty much out of service by the end of the Nam war, late 70's at latest? Pretty interesting that it is so docile in the auto though, I would have never guessed. I have seen the KMax fly, interesting bird there.
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PaveLowDriver
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RE: Twin Main Rotors In Autorotation Situations

Wed Jul 21, 2004 11:02 am

Thanks for the hearty welcome, gents.....

Starlion....Yep.....Paves are one of the biggest [western] helos you're going to see flying around....Although the Marine E-model is significantly larger, with our FLIR/radar setups in the nose, the Pave Low definitely looks more aggressive....but of course I'm biased.

HaveBlue....Yeah, the guys I know that flew the Husky have got a couple years on me, and as far as I know, they were pretty much out of service by the late mid-70's, with the exception of some local air base rescue units.....If you're interested, check out...

http://www.pedroafrescue.org/

The K-MAX is definitely a different kind of bird.....A great example of "form follows function..." I've heard from the pilots that the ride is a little bumpy, but other than that, a great bird.

Cheers!

PAVE LOW LEADS!
 
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Starlionblue
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RE: Twin Main Rotors In Autorotation Situations

Thu Jul 22, 2004 7:15 am

PaveLowDriver, I became a Pave Low fan after reading Clancy's "Clear and Present Danger". Still one of my favorite books. I stopped wondering if you could actually land a Pave Low on a CG cutter (but I'm sure you can enlighten us) in a storm after my boss told me this story:
He was in the Swedish Navy and a Boeing Vertol driver was showing off by holding the nose gear steady on the deck of a pitching ship (and there aren't any big ones in the Swedish Navy) while unloading pax in a mild storm. He was "flying" sideways compared to the ship with only the nose gear on the deck. Just following the movements of the ship.

Maybe OT, but we can throw autorotation in there  Big grin: How would you compare Western helos with, for example, the Hind, Hip or Halo when it comes to flight characteristics, behavior and safety features?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

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