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Pendular action in a Chinook ?


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Sorry , I posted this in general but figured its better posted here.

 

Any help would be appreciated.

 

I am wondering if a Chinook gets pendular action & if there is any assistance in stability or is instability compounded by having the 2nd rotor system, either laterally or Longitudinally ?

 

Thanks again.

 

PRHTE

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Sorry , I posted this in general but figured its better posted here.

 

Any help would be appreciated.

 

I am wondering if a Chinook gets pendular action & if there is any assistance in stability or is instability compounded by having the 2nd rotor system, either laterally or Longitudinally ?

 

Thanks again.

 

PRHTE

 

Pendular action....nope. The aircraft, however, is inherently unstable and requires a computer package that assists flight. Actually, the aircraft prefers to fly sideways, that is with both rotor heads travelling parallel. It's fun to fly when you fly with the computer off, some say it is the equivalent of bull riding.

 

Now, the lateral or longutidal movement you are discussing can be caused by track/ balance issues. Not really what you are thinking about.

 

Very vague answer, but it's really much too complex to try and explain to most civilians.

 

-Steve

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Pendular action....well, your ideas are sound and you are thinking about all of the dynamic forces that the fore and aft rotors create.

 

The aircraft is assisted by a computer system that really flies the helicopter. As pilots, we only tell the computer what we want to do and the computer does the rest. Now, when we fly with the computer off, it gets fun. The tendency of the rotor systems are to both fly in clean air, which causes the aft end of the helicopter to try and swing forward. If you aren't thinking about flying the aircraft, then it'll bite you in the ass. Not really too hard to do, but is very challenging for some still.

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Sorry , I posted this in general but figured its better posted here.

 

Any help would be appreciated.

 

I am wondering if a Chinook gets pendular action & if there is any assistance in stability or is instability compounded by having the 2nd rotor system, either laterally or Longitudinally ?

 

Thanks again.

 

PRHTE

 

OK...the aircraft is inherently unstable because of the fore and aft rotor systems. Both of these rotor systems want clean air, so the aft rotor system tries to come around and fly parallel to the fore rotor system. To combat this, there is a computer system that acutally flies the aircraft by converting pilot input into flight control movement. When this computer system is selected off or malfunctions, you really notice it, especially those in the rear of the aircraft.

 

It really is more complex than that, but difficult to explain withough going on a while about all of the other influential factors which aid flight in a 47.

 

As far as lateral or longitudal instability, this is usually a track/ balance issue.

 

Hope the info helps.

 

-Steve

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yeah civillians are dumb :rolleyes:

 

 

I didn't say that civilians are dumb, but the aircraft is mechanically complex and there are really no civilian pilots that are familiar with the airframe.....I know about some private companies, but for the vast majority. I'd be happy to talk to you about it if you feel left out, it just tends to confuse people, that's all I meant.

 

I apologize if I came off condescending, didn't mean it that way.

 

-Steve

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  • 2 weeks later...
OK...the aircraft is inherently unstable because of the fore and aft rotor systems. Both of these rotor systems want clean air, so the aft rotor system tries to come around and fly parallel to the fore rotor system. To combat this, there is a computer system that acutally flies the aircraft by converting pilot input into flight control movement. When this computer system is selected off or malfunctions, you really notice it, especially those in the rear of the aircraft.

 

It really is more complex than that, but difficult to explain withough going on a while about all of the other influential factors which aid flight in a 47.

 

As far as lateral or longitudal instability, this is usually a track/ balance issue.

 

Hope the info helps.

 

-Steve

 

The instability (notably pitch and yaw) of tandem systems in forward flight has far more to do with the wetter area of the fuselage fore and aft of the center of gravity than it does with the rotors. The overall center of pressure is very close to the CG, and can be ahead of it with if the helicopter is aft loaded.

 

The PV-3 (XHRP-X), HUP-1/2/3, HRP-1/2, H-21, YH-16, Yak-24, McCulloch MC-4 (YH-30/HUM-1), and Filper Betas all flew without the benefit of computers, and some without hydraulics (the HUP-1/2/3 models have an ingenious control mechanism in this respect). All of these were outfitted with various forms of vertical and horizontal fins at the rear to aid in yaw and pitch stability in forward flight.

 

Most tandems have fin-like rear pylons noticeably larger than the front for the same reason. The addition of automatic stability control systems in later models (eg. the CH-46 and CH-47) removed much of the need for the fins, though they still retain the larger rear pylons.

 

Military vs. civilian has nothing to do with understanding stability. All of the above machines were designed by civilian engineers, and there are numerous civilian pilots who have and continue to fly them, including Boeing models 107 (civilian version of the CH-46) and 234 (civilian version of the CH-47).

 

Bob

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  • 5 weeks later...

I would have to add that the Chinook will give you a Pendular TYPE action... I experienced that today when we turned the AFCS off and I picked it up to a hover. It's not the same as single rotor helicopters, but it gave pretty much the same feeling/effect. Loads of fun.

 

 

CHAD

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all i know is that they are complicated birds, especially to understand, they are rough as hell to ride in and my good friend broke his leg in one, when they were flying 100 ft off the deck and I guess the ride got rough some how, not contacting anything, and a large wooden pallet, bounced up, from inside the aircraft and came back down on his leg and snapped it..

 

in the mid 90's I got a ride in one , I think it was a 47 and it felt like I was riding in a wash machine. I got car sick from the flight with no windows and I don't remember it feeling smooth.

 

I also got the chance to try and fly an rc helicopter , model 47 and I could not fly it like I could my own regular helicotpers... and seeing the 4,000 dollar helicopter kit, and all of the belt drives and details inside the airframe was completely cool.... I don't know how they work and fly, especially in a smaller rc helicopter. but they sure are cool....

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The instability (notably pitch and yaw) of tandem systems in forward flight has far more to do with the wetter area of the fuselage fore and aft of the center of gravity than it does with the rotors. The overall center of pressure is very close to the CG, and can be ahead of it with if the helicopter is aft loaded.

 

<snip>

 

Most tandems have fin-like rear pylons noticeably larger than the front for the same reason. The addition of automatic stability control systems in later models (eg. the CH-46 and CH-47) removed much of the need for the fins, though they still retain the larger rear pylons.

 

<snip>

 

Bob

Two questions. How does the waterline affect pitch and yaw stability? The larger (taller) rear pylon always struck me as giving the rear rotor a shot at clean air in forward flight, how does a larger pylon in the rear provide greater pitch/yaw stability?
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Two questions. How does the waterline affect pitch and yaw stability? The larger (taller) rear pylon always struck me as giving the rear rotor a shot at clean air in forward flight, how does a larger pylon in the rear provide greater pitch/yaw stability?

The main reason the rear pylon is taller is so the rotor disks' plane is inclined relative to the fuselage, which allows the fuselage to be in a more level attitude in forward flight and more comfortable for the occupants. If the disks are parallel to the fuselage (like most of the early tandems), there is a pronouced forward tilt that doesn't ride well. Kinda like a 300 in fast forward flight, only more so.

 

For yaw stability the larger area of the rear pylon acts similar to fletching on an arrow, providing more area (and stabilizing forces) behind the cg than in front of it. Pitch [in]stability comes into play mostly during airspeed changes, and pilot technique comes into play. Automatic stabilization (ie computers) certainly helps, but isn't necessary or the early types would never have flown.

 

Bob

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I am sure that the larger aft pylon has something to do with the aft rotor system getting into cleaner air. However, you have to consider the size of the transmissions as well as the swash plates that it requires. All this to include that the 47's have LCT's (Long. Cyclic Trim) which in forward flight program and tilt the rotor systems forward giving the airframe a level attitude. As for stability, the aft pylon has a blunt shape (in the back) giving stability to the pilots during AFCS off flight.

 

 

CHAD

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I am sure that the larger aft pylon has something to do with the aft rotor system getting into cleaner air. ...

The only really "dirty" air in a tandem is in the area where rotors overlap, and it effects both rotors equally. The rear rotor does have slightly more induced flow compared to the front rotor due to transverse flow effect, but that is easily compensated by a slight increase in rear cyclic pitch.

 

... However, you have to consider the size of the transmissions as well as the swash plates that it requires. All this to include that the 47's have LCT's (Long. Cyclic Trim) which in forward flight program and tilt the rotor systems forward giving the airframe a level attitude. As for stability, the aft pylon has a blunt shape (in the back) giving stability to the pilots during AFCS off flight.

CHAD

LCT help maintain a level attitude over the wide cg range tandems have. The blunt shape in the rear is more likely a weight and overall fuselage length concession. It would be more yaw stable if it continued to a sharp trailing edge (ala CH-46/107), but only marginally so. Again, the early tandems didn't have LCTs, AFCSs, had varied amounts of disk overlap, both in-plane and parallel disks, with some tilted relative to the fuselage and some not. All flew reasonably well.

 

For those inclined, "Rotory-Wing Aerodynamics" by W.Z. Stepniewski and C.N. Keys, and "Helicopter Theory" by Wayne Johnson contain material regarding tandem aerodynamics. Lots of math, a number of drawings, and few pictures. Dry engineering stuff.

 

Bob

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