helicopter plus turbine generator

[IAHM-COL]

Another question.... I have a HOTAS

I envisage using the yoke\joystick for the cyclic control.....

The collective however I see being controlled using the throttle....

How are existing helicopters in FG set up ?

IIRC, that's exactly how FG sets up helicopters. Only one note, the collective is with throttle, inversed. All up is low, all down is high.

[bomber]

Update of where I am.....

People may or may not know that JSBsim calculates a vehicles inertia around it's CoG. This works fine for a fixed wing plane but as a helicopter in flight is suspended from its rotor, and rotates around this point it's not fine for helicopters.

I had a suspicion at the start that this could be an issue and the last two weeks have been an investigation into JSBsims use of empty weight, inertia values, point masses and fuel tanks.

An interesting thing has been revealed. In the past I've used a point mass to represent the weight of the undercarriage and moved this point mass as the undercarriage is lowered and raised. This has the required effect of moving the CoG but what it doesn't do is adjust the inertia values.... Inertia values via point mass are not adjusted on the fly. Inertia values however are adjusted on the fly via content change of a fuel tank.

Work still to do on this, but getting there.

Regards

Simon.

[mue]

People may or may not know that JSBsim calculates a vehicles inertia around it's CoG. This works fine for a fixed wing plane but as a helicopter in flight is suspended from its rotor, and rotates around this point it's not fine for helicopters.

There is nothing wrong with calculating the moment of inertia around the COG. You know, rigid objects rotate around it's COG if no external forces are applied. Now you only have to add the external forces and moments and JSBSim's EOM solver does the rest. There is no fundamental difference between fixed wing planes and helicopters. The only challange is to find a good rotor model that gives you reasonable forces and moments from the rotor.

An interesting thing has been revealed. In the past I've used a point mass to represent the weight of the undercarriage and moved this point mass as the undercarriage is lowered and raised. This has the required effect of moving the CoG but what it doesn't do is adjust the inertia values.... Inertia values via point mass are not adjusted on the fly..

That's incorrect. Moving point masses does change the moment of inertia tensor of the aircraft (I tested it in Flightgear 2018.3.2).

[IAHM-COL]

There is nothing wrong with calculating the moment of inertia around the COG.

aha! Now this thread starts to get really interesting.

I was reading (like several times) Bombers post about inertia and COG, and I still cannot fully understand what is the main point.
Clearly, my ignorance is the main problem on my understanding.

From a simple point of view inertia and mass are supposed to be completely correlated. Isn't COG supposed to be a point where interactions over the mass are supposed to be able to be simplified? Is this is so, how is it that inertia cannot be understood to be applied on the COG, and how this is an acceptable simplification for fixed wing but not rotor wing aircrafts?

Me... still thinking....

[bomber]

Ok look at this video

https://youtu.be/G73CSDKFN-g

The questions are...

1) Where's the CoG of the object ?
2) Where's the rotation point ?

They're not the same point.... JSBsim expects them to be the same point, as in a fixed wing plane they are.... but not in a helicopter

Simon

[mue]

Ok look at this video

https://youtu.be/G73CSDKFN-g

The questions are...

1) Where's the CoG of the object ?

I would say inside the sphere.

2) Where's the rotation point ?

It depends on where you set your rotation reference point.
Here is what I and JSBSim "sees": The rotation point is at the CoG, and the CoG moves on an arc trajectory.

They're not the same point.... JSBsim expects them to be the same point, as in a fixed wing plane they are.... but not in a helicopter

JSBSim calculates the (translational and rotational) motion about the CoG. Because in this case the rotational dynamics can be separated from the translational dynamics. Simple rigid body physics. It doesn't matter if the rigid body is an airplane or a helicopter. As I wrote, the difficulty in helicopter simulation is to get what forces and moments of the rotor are applied to the aircraft.

[bomber]

However the point I was making was that in the video the object doesn't rotate around it's CoG...

If JSBsims inertia calcs are based on rotating around the CoG then they must be the wrong values for rotating somewhere else....such as at the rotors.

[mue]

However the point I was making was that in the video the object doesn't rotate around it's CoG...

There is no such thing as THE rotation point. As I wrote, you can choose any point on your rigid body as rotation point and then describe the rigid body motion as the combination of translation of the rotation point and the rotation about the rotation point.
JSBSim uses the CoG as rotation point. That means the motion of the pendulum consists of the translation of the CoG along the arc trajectory and the rotation about the CoG.
You choose the pivot point as rotation point. In this case the pendulum motion doesn't contain a translation (because the pivot point is fixed) but only the rotation about the pivot point.

[bomber]

Ok I've reread it half a dozen times and I agree with what you're saying....

The problem lies in the practicalities of JSBsim.
When it initialises it uses the empty CoG location and it's inertia values to calculate using any additional pointweights and fuel the new CoG and inertias around the CoG

So far so good....

A real life helicopter does not rotate around it's CoG. .... It swings from its rotor.

But in the JSBsim world the CoG is exactly where it attempts to rotate your model/FDM..... So these inertia values calculated at startup are incorrect.

[IAHM-COL]

As I wrote, the difficulty in helicopter simulation is to get what forces and moments of the rotor are applied to the aircraft.

Isn't expected that any and all momentums generated from the rotor wings to be applied to an helicopter?
How can a rotor generate a momentum that happens to not become applied to the helicopter body that is thethered to it? and if this is not possible, then why would one need to establish in JSBsim which momemtums to apply and which ones to disregard?