Helicopter CG

[Journeyman 0]

3) "If the object is unconfined and the force is applied at some distance from the center of gravity, the object both translates and rotates about the center of gravity."

http://exploration.grc.nasa.gov/education/rocket/torque.html

This still does not take into account the gyroscopic effect of the spinning rotors. Suma is right in that most choppers rotate at a point above their C of G due to this effect. It would be somewhere between the C of G and the rotor pivot depending on the chopper/how much power/speed etc. But I agree the camera needs to come away from the rotors and placed nearer the C of G to give more realism to the view.

If you have ever played around with a gyroscope you would understand this effect much better! Which reminds me I haven't seen any of those little gyroscopes for ages, I had two of them when I was a kid!  

[4 IN 1 0]

3) "If the object is unconfined and the force is applied at some distance from the center of gravity, the object both translates and rotates about the center of gravity."

http://exploration.grc.nasa.gov/education/rocket/torque.html

This still does not take into account the gyroscopic effect of the spinning rotors. Suma is right in that most choppers rotate at a point above their C of G due to this effect. It would be somewhere between the C of G and the rotor pivot depending on the chopper/how much power/speed etc. But I agree the camera needs to come away from the rotors and placed nearer the C of G to give more realism to the view.

If you have ever played around with a gyroscope you would understand this effect much better! Which reminds me I haven't seen any of those little gyroscopes for ages, I had two of them when I was a kid!  

no, we should tie him up to a real camel, eject right after we gain agl and leave him to fly that damn thing so that he can really feel how the effect affect his flight path

[frederf 0]

3) "If the object is unconfined and the force is applied at some distance from the center of gravity, the object both translates and rotates about the center of gravity."

http://exploration.grc.nasa.gov/education/rocket/torque.html

This still does not take into account the gyroscopic effect of the spinning rotors. Suma is right in that most choppers rotate at a point above their C of G due to this effect. It would be somewhere between the C of G and the rotor pivot depending on the chopper/how much power/speed etc. But I agree the camera needs to come away from the rotors and placed nearer the C of G to give more realism to the view.

If you have ever played around with a gyroscope you would understand this effect much better! Which reminds me I haven't seen any of those little gyroscopes for ages, I had two of them when I was a kid!

"If the object is unconfined" also assumes there is no air. Most helicopters are surrounded by it on at least 2 sides

[max power 21]

Apparently, gyroscopic precession has been disproven as a primary factor in helicopter flight, and the weirdness of helicopter flight has much to do with the fact that the rotors are semi-free to flap around on hinges. What was previously thought as precession forces are now thought to be just a product of the blades accellerating and flapping around.

[LeftSkidLow 1]

Apparently, gyroscopic precession has been disproven as a primary factor in helicopter flight, and the weirdness of helicopter flight has much to do with the fact that the rotors are semi-free to flap around on hinges. What was previously thought as precession forces are now thought to be just a product of the blades accellerating and flapping around.

Precession isn't really what Suma was talking about I don't think, for the most part precession is countered by off setting the pitch horns on the rotor head by 90 degrees so that cyclic inputs aren't 90 degrees off. There are areas where precession does affect the cyclic inputs by the pilot but they aren't simulated in ArmA. I think he is mostly referring to the gyroscopic property of rigidity in space, the gyro tends to resist any deflecting force. Its the same reason why attitude indicators (artificial horizon) tend to stay level with the horizon.

[max power 21]

I was thinking that what he's describing sounds like just a product of complex aerodynamic forces. Fixed wing aircraft appear to rotate around an axis outside of the aircraft when you apply elevator to a roll. I'm no expert, though.

[kestrel7e7 0]

Precession isn't really what Suma was talking about I don't think, for the most part precession is countered by off setting the pitch horns on the rotor head by 90 degrees so that cyclic inputs aren't 90 degrees off.

The 90 degree phase lag is the more significant manifestation of the gyroscopic precession phenomena but there are other manifestations.

"If the object is unconfined and the force is applied at some distance from the center of gravity, the object both translates and rotates about the center of gravity."

http://exploration.grc.nasa.gov/education/rocket/torque.html

This holds for all objects, even gyroscopic ones.  An unconstrained preceeding gyroscope translates and rotates with respect to its center of gravity but seems to rotate about a different point in much the same way as a circling aircraft appears to rotate about a point in the air in the middle of its circle. But you don't use that observation as the basis for coding the physics of an aeroplane in a flight sim. :P

The proportion of mass in a helicopter's spinning rotors is small compared with the amount of mass in its airframe so making analogies with gyroscopes in which all of the mass is spinning leads to huge over-estimations of gyroscopic effects.

Ultimately these effects on the center of rotation of a helicopter are too negligable to be included in a non-flightsim game like ArmA, yet alone to exaggerate them to such an unrealistic level... I've played many sim and non-sim games with helicopters in the past but I don't think I've encountered anything like this.

[frederf 0]

Gyroscopic forces apply even in a vacuum.

[LeftSkidLow 1]

Quote[/b] ]The proportion of mass in a helicopter's spinning rotors is small compared with the amount of mass in its airframe so making analogies with gyroscopes in which all of the mass is spinning leads to huge over-estimations of gyroscopic effects.

Yeah I think I agree, the most in depth training I've had about gyroscopes when flying helicopters has just been about precession, which would never come into play with ArmA. I'm guessing the problem here is in a hover the fuselage acts more like a pendulum and with significant forward airspeed the fuselage rotates around a different point. I don't even notice it when I'm flying I just do what needs to be done with the cyclic, this whole argument gets a little overboard at times but I'd agree something is a little strange.

[kestrel7e7 0]

Gyroscopic forces apply even in a vacuum.

Of course, for example the Earth exhibits gyroscopic precession with a period of 25,800 years.

[Journeyman 0]

The proportion of mass in a helicopter's spinning rotors is small compared with the amount of mass in its airframe so making analogies with gyroscopes in which all of the mass is spinning leads to huge over-estimations of gyroscopic effects.

That is precisely why it is possible for the helicopter to turn at all! If the helicopter was to behave like a conventional 'toy' gyroscope where more than 90% of the mass is in the gyro, then I don't think that we would just be looking at a slightly different rotation point!  

For those that don't understand this 'resistance to turn' of a gyroscope have a look at THIS short clip of how it affects a bicycle wheel. Alternatively go play with one of THESE!

It is perfectly acceptable to expect that even though the mass of the spinning rotors is a mere fraction of the total mass of the helicopter that the gyro effect of the spinning rotors will still imply a resistance to turn sufficiently to cause a slightly different rotation point in the fuselage of the aircraft.

I don't believe however that this would be at the point of the rotors and like I said would be somewhere between the C of G and the rotor pivot. But I think that Suma said that he was correcting this.  

[kestrel7e7 0]

The non-rigid, flapping nature of helicopter rotors is why helicopters don't exhibit quantifiable gyroscopic precession, making analogies with rigid gyroscopes is impossible.

"The rotor of a helicopter has very little inertia for its size. In fact, most pilots would like it to have more inertia during autorotation. I.e. The rotational speed of a helicopter rotor is very slow compared to that of a gyroscope. The mass at the circumference of a helicopter rotor is very small compared to that of a gyroscope."

"Lu said

"If you have an infinitely rigid rotor you in effect have a gyroscope."

You have to forget gyroscopic precession & the helicopter rotor. For gyroscopic precession to take place, the rotating device must have considerable [Angular Momentum] A helicopter's rotor does not have enough."

http://www.pprune.org/forums/showthread.php?s=&threadid=51070