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HeliTorque :: View topic - Tandem Rotors in Autorotation Situations
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HeliTorque Forum Index » Flight Dynamics

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TripleDelta
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PostPosted: Tue Sep 13, 2005 7:25 pm    Post subject: Tandem Rotors in Autorotation Situations Reply with quote

How do tandem rotor helicopters (the Kamov line and the Chinook) behave in autorotation situations? I gather that they're a bit harder to steer, but is the sink rate then smaller, since you have two big rotors producing lift, as opposed to the "one big, one small" of a conventional helicopter?
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PostPosted: Thu Oct 06, 2005 10:42 pm    Post subject: Reply with quote

I've asked our training department at Columbia Helicopters for a response to your question, and the response comes from our Operations Administrator. He spent many years in a Chinook, and has a sense of humor.

His response: The aircraft is not harder to steer, but it does descend at a slower rate (depending on airspeed) because of the rotors and the wind hitting a bigger surface area on the bottom of the aircraft. On a Chinook with external tanks you have to jump up and down to get it to descend. Ha!
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PostPosted: Thu Oct 06, 2005 11:27 pm    Post subject: Reply with quote

Another response from one of the people in the Columbia Helicopters Training Department. This pilot has also had a lot of experience in a variety of aircraft.

"I can???t speak to the Kamov, however the Boeing tandem rotor design autorotates very much like other heavy helicopters (My experience is with Sikorsky S-61 and S-62 models). Steering is not an issue with the Boeing, but any heavy helicopter tends to build large sink rates which demand proper technique at the bottom. The tandem rotor design doesn???t seem to reduce the sink rate much."
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TripleDelta
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PostPosted: Fri Oct 07, 2005 9:49 am    Post subject: Reply with quote

Thanks for the info and big thanks to Columbia Helicopters. I've always wondered about this, since I thought that, in the case of the Chinook, tilting one rotor to the side to yaw around the heli's vertical axis would produce an assymetry of lift and a disruption of ballance, since there is no engine power anymore available to the pilot to overcome and correct this ballance situation.

My reasoning was that if you wished to yaw (the first thing I though about when considering tandem rotor helis) during autorotation for any reason (while level in the horizontal plane), in a single-main-rotor heli you'd vary the pitch angle of the tail rotor blades. The resultant increase in tail rotor thrust would in most cases be in the horizontal plane, producing a small or insignificant pitch moment.

On the Chinook however, as far as I get it, you'd tilt the rear rotor to the side, shifting the resultant lift as well. In this case, the vertical component of lift, counteracting the weight of the heli, would be reduced, creating an assymetry of lift between the front and rear rotors and causing the tail to drop. This could in turn be overcome by increasing the pitch angle of the tail rotor blades so that the resultant lift force is increased to the point where it's vertical component is ballanced with that of the front rotor. But increasing the pitch angle would increase drag as well, and with no engine powering the rotor anymore, it's RPM would drop, drawing lift in all components with it. This would then have to be counteracted by reducing the lift of the front rotor to regain ballance and sufficient speed for effective autorotation.

That would then again lead to some see-saw control manipulation, possibly increasing the pilot workload and making the heli harder to steer, despite it's lower sink rate. But then again, as I went on through this, and considering the input from Columbia Helicopters, I'd guess the simplest thing when yawing would be to reduce the pitch of the front rotor when tilting the rear one, thus keeping sufficent speed for an effective autorotation at the cost of a little altitude... Someone please correct me if I'm wrong, since my knowledge of flight mechanics is based largely on fixed-wings.

As for the Kamov system, having both rotors on the same line in the vertical plane (i.e. having no torque arm between the rotors like the Chinook) would not produce such a ballance change, but then again I presume the roll and yaw capabilities would be reduced for the same reason.

EDIT: I meant the above for Kamovs in power-out situations. I've read that with power on, few helicopters can match the yaw rate of a Kamov combat heli.
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PostPosted: Fri Oct 07, 2005 10:30 am    Post subject: Reply with quote

Or am I missing a rotor cyclic issue here?
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PostPosted: Fri Oct 07, 2005 3:29 pm    Post subject: Reply with quote

From our Training Department representative (his was the second response):

When you displace the yaw pedals in the tandem rotor design you???re tilting the rotors in the opposite directions by equal amounts. The loss of vertical lift is nearly insignificant as far as the rate of descent goes, however it is equal unless you???re cross controlling. In the cross control situation the pilots conditioned response would be to correct for the asymmetry of lift with cyclic inputs to maintain the desired pitch attitude. The bottom line is that with a couple exceptions a tandem rotor flys the same as a single rotor in terms of pilot inputs and aircraft responses.

In autorotation the rotors are not being driven by the engines, but keep in mind that they are still being driven, by the aerodynamic forces created during autorotation.
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PostPosted: Fri Oct 07, 2005 9:08 pm    Post subject: Reply with quote

In forward flight, yaw controls are only used to stay in balance (skid ball) and the movements are small - you wouldn't detect any loss of lift from tilting the rotors.

If you made a yaw input so big that it affected lift, you wouldn't see it anyway, because your eyeballs would be stuck to the inside of the window from the spinning. Wink
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PostPosted: Sat Oct 08, 2005 4:28 am    Post subject: Reply with quote

Quote:
On the Chinook however, as far as I get it, you'd tilt the rear rotor to the side, shifting the resultant lift as well.


I have no Chinook time, but I would think that you shouldn't get chasing the idea of "shifting the resultant lift" to far, as my understanding in an autorotation, you don't have any lift. Hence the rapid falling from the sky!

Think of the disk as more of a plate, and then think about how that plate would react as it falls through the air. If it is tilted to the right, it will slide off to the right, (or any other direction it is tilted) and usually the helicopter tends to follow.

T/R as required to keep it straight with NO concern of how much or how little, RPM is, or will be affected. If the AutoRevs are set correctly, (and everyone should know what they shoud be for the helicopter they are flying) you will have plenty of RPM to conduct the auto, regardless of T/R requirements.

Autrotations are usually a matter of speed control, while having the collective full down, (or as required) to control rotor RPM control, (depending on the AUW of the AC) and learning how to control the speed of the helicopter (cyclic) to get to your spot, using the proper control of that speed.


Quote:
the resultant lift as well


This will come back in the final touchdown of the auto as collective is used to cushion and stop the rate of decent, which has kept your blades spinning for this specific reason, and space in time...
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PostPosted: Sat Oct 08, 2005 7:17 am    Post subject: Reply with quote

Skids Up says:
" as my understanding in an autorotation, you don't have any lift."

Sorry, Skids, but in a steady auto, your lift is exactly equal to your weight. Same as in a steady climb, same as in level flight. Lift equals weight, forward thrust equals drag.

If lift is less, rate of descent builds up, angles of attack increase, and lift increases to again equal the weight, but with a higher steady rate of descent.

And the disc isn't anything like a plate sliding on the airflow. It is like a paper toy helicopter blade, which you hold up and release. It is extracting energy from the airflow as it descends, spinning down slowly. If there was no lift, it would fall like a brick. It doesn't. Tilt it to one side, and the lift vector will make it move that way until some other force exerts its influence to stop it.

Regarding the coaxial Kamov types, yaw control comes from differential collective pitch - one set of blades receives more torque and yaws the machine in the opposite direction. But that only works in powered flight. In auto, the differential lift between the systems is almost the same. As the collective is lowered, a special linkage operates the collective controls, and it is then differential transmission drag that yaws the aircraft.
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PostPosted: Sat Oct 08, 2005 4:12 pm    Post subject: Reply with quote

DanSweet wrote:
When you displace the yaw pedals in the tandem rotor design you???re tilting the rotors in the opposite directions by equal amounts.


That makes more sense. I originally thought that only one rotor would tilt, while the other one would remain untilted. Then if both would tilt to the opposite sides, any assymetry would be cancelled out, plus you'd get a stronger yaw moment around the CG than with just a single rotor, requiring less rotor tilt and lower concentrated stress on the airframe...

Ascend_Charlie wrote:
In forward flight, yaw controls are only used to stay in balance (skid ball) and the movements are small - you wouldn't detect any loss of lift from tilting the rotors.


Thanks for the info. I thought that the deflection and movement of the controls would be greater, producing a correspondingly greater and more significant change in force ballance.

Concerning that, what is the max sideways tilt of the rotor on a Chinook, and does it vary from the front to rear rotors?

SkidsUp wrote:
to far, as my understanding in an autorotation, you don't have any lift


A rotor in an autorotation situation behaves much like a windmill (hence also the "windmilling" effect on propeller aircraft). The relative wind flows over the rotor, rotating it much like wind rotating a windmill rotor. The resultant lift, in autorotation, is less than the weight of the aircraft, but is far from zero and still counteracts the weight of the craft, producing a sink rate as Ascend_Charlie said.
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PostPosted: Sun Oct 09, 2005 12:49 am    Post subject: Reply with quote

Quote:
Sorry, Skids, but in a steady auto, your lift is exactly equal to your weight. Same as in a steady climb, same as in level flight. Lift equals weight, forward thrust equals drag.


I can stand corrected, but then how come when I'm at 100% N2 flat pitch on the ground, lift does not equal the weight of the helicopter? If it did, you couldn't sit on the ground at 100% 'cause a gust of wind could then blow you away... which it doesn't.

My comments in the auto was that you are in this position in an auto, and lift does not equal the weight of the AC or you wouldn't desend.

At flat pitch (0 lift) the disk is just like a sheet of plywood in the wind and it can only be moved so fast through the air, but produces no lift. (Parachute?)

Rate of desent in the auto is a function of weight, and speed. "At Flat Pitch" in the auto, the more weight you have, the faster the rate of desent, and the more speed you have, the faster the rate of desent.

Quote:
A rotor in an autorotation situation behaves much like a windmill (hence also the "windmilling" effect on propeller aircraft). The relative wind flows over the rotor, rotating it much like wind rotating a windmill rotor. The resultant lift, in autorotation, is less than the weight of the aircraft, but is far from zero and still counteracts the weight of the craft, producing a sink rate as Ascend_Charlie said.


I'm just not convinced that the air flowing "up" through the blades in the auto, causes a great deal of lift, but functions to keep the blades spinning, (storing energy) until needed to produce the lift needed at the end.

I seem to remember from early FW days that AC such as the Twin Otter, when the props went into what was called the "Beta" range, which was a netural pitch, neither pulling or pushing, but effectively creating a huge "disk" that was difficult to push through the air, thus slowing you down, and allowing a much steeper angle of desent without and increase in speed.

Always related the "flat pitch" on a helicopter to that, and thought that it worked the same way.

I'm not a techy on this and there are probably a lot of vector calulactions that would prove it all wrong.
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PostPosted: Sun Oct 09, 2005 1:39 am    Post subject: Reply with quote

Skids says:
"I can stand corrected, but then how come when I'm at 100% N2 flat pitch on the ground, lift does not equal the weight of the helicopter?"

Because you are not trying to produce lift - you are at flat pitch. The ground produces a force equal to the weight of the machine, so you stay put. If weight was more than ground force, you would sink into the earth at an increasing rate.

As you increase pitch, the lift increases, and more weight is taken up by the rotor system. When lift equals weight, there is no force on the ground. When lift exceeds weight, you accelerate upwards until other forces come into play to balance the forces, and you are either in a steady climb or a hover.

The forces are in balance. Unbalanced force equals acceleration.

When you first enter auto by lowering the collective, the weight is no longer balanced by the lift, and you start to accelerate downwards. Rate of descent increases, and the airflow changes from sucking air from the top and pushing it downwards, to being airflow from the bottom coming upwards through the disc. Angles of attack are changed, resultant vectors are a mixture of pointing forwards (to pull the blades around) and pointing backwards (trying to slow the blades.) and lift increases as the acceleration increases. When lift equals weight, the acceleration downwards stops, rate of descent is stable, and Bob's your uncle.

You say "I am just not convinced..."
Well, we don't have to convince you that it works, it just DOES.

Get a book on heli aerodynamics, it is easier than trying to write it all here for you.
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PostPosted: Sun Oct 09, 2005 2:50 am    Post subject: Reply with quote

Quote:
Because you are not trying to produce lift - you are at flat pitch. The ground produces a force equal to the weight of the machine, so you stay put. If weight was more than ground force, you would sink into the earth at an increasing rate.


Quote:
Sorry, Skids, but in a steady auto, your lift is exactly equal to your weight


So why am I producing "lift" in an auto when I'm at "flat pitch", but not on the ground when I'm at "flat pitch" Question
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PostPosted: Sun Oct 09, 2005 6:16 am    Post subject: Reply with quote

Quote:
So why am I producing "lift" in an auto when I'm at "flat pitch", but not on the ground when I'm at "flat pitch"


Because you're missing the point that you have a relative airflow through the disc that creates an positive Angle of attack, so you have lift to turn the blades and keep you from falling to the ground like a stone.

On ground you are at flat picht and flat AoA what in a symmetrical airfoil equals to no lift.

It's better to see the pictures in a book.

Regards.
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PostPosted: Sun Oct 09, 2005 6:19 am    Post subject: Reply with quote

Because in an auto, you are falling at 1800' per minute, with lots of air rushing up under your blades, and you have probably 60 kt of airspeed as well. If you are in the hover when you commence the auto, the ROD will be much higher until the airflow is sufficient, or you gain forward airspeed as well.

Plenty of air to provide the energy to drive the rotor. On the ground, you don't have the upward rush of air. Sometimes a wind gust across the disc can cause it to flap a lot and hit the tail boom.

I have been in a Huey, which we landed on a hilltop in strong winds. After shutting off the engine, the wind rushing up the hill from below kept the blades spinning for 20 minutes, with me flying the disc and trying to prevent serious flapping.
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