## JSBSim dynamically change empty mass and inertia in-flight

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

### Re: JSBSim dynamically change empty mass and inertia in-flig

You're not wrong.... even a stopped clock tells the correct time twice a day.

And so wiith a force that offers zero moment when in two perpendicular directions.

But the other 358 degrees it wrong.

I was kinda hoping I could get some agreement on this before moving onto the erroneous damping moments calculated by JSBsim from it's mass rotatng around the force location (canopy).
"Stay away from negative people.They have a problem for every solution." - Albert Einstein
S&J

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### Re: JSBSim dynamically change empty mass and inertia in-flig

polly wrote in Mon Sep 18, 2023 9:30 pm: I can't see the distiction being made between a paraglider and a high-winged monoplane.

Me neither, assuming the paraglider can be modelled as a rigid body.
amue

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### Re: JSBSim dynamically change empty mass and inertia in-flig

That assumes that a high winged aircraft is correct.

That's too flippant a reply, let me try again.

A plane rotates around it's CoG and JSBsim internal moment calcs are set up for this. And the perpendicular distance armature between the wings pressure and CoG isn't that large and only really effects SIDE force and as such roll and with a high wing with little dihedral it's negligible.

But with a paraglider this distance is massive and is not negligible. And a paraglider doesn't rotate around it CoG.

So if we can only agree on one thing then can it be that a paraglider doesn't rotate around it's CoG but instead rotates around it's canopy.

https://youtu.be/LBANhMOSIr0?si=lORLgmBvxABldtaZ
"Stay away from negative people.They have a problem for every solution." - Albert Einstein
S&J

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### Re: JSBSim dynamically change empty mass and inertia in-flig

S&J wrote in Tue Sep 19, 2023 9:28 am:So if we can only agree on one thing then can it be that a paraglider doesn't rotate around it's CoG but instead rotates around it's canopy.

Or the paraglider does rotate around its CoG while the CoG simultaneously moves along an arc trajectory with the arc center at the canopy...
amue

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### Re: JSBSim dynamically change empty mass and inertia in-flig

I can run with that as an idea.

So the next question that arises from this is if the above is correct what forces are involved that once the paraglider returns to a level straight flight aligns the CoG and canopy.

Does the canopy rotate into alignment or does the CoG rotate into alignment ?

I imagine holding a pocket watch by it's chain end.
"Stay away from negative people.They have a problem for every solution." - Albert Einstein
S&J

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Joined: Wed Aug 26, 2020 7:31 pm

### Re: JSBSim dynamically change empty mass and inertia in-flig

If you have any changes to improve the paraglider (and parachutist) I would be happy to add them but for now I have little time to work on it or look up how I implemented it.

Erik
Current: Parachutist, Paraglider, Pterosaur, Pilatus PC-9M and variants, ERCO Ercoupe, Fokker Dr.1, Fokker 50, Fokker 100
Less active: Cessna T-37, T-38, Santa Claus. Previous: General Dynamics F-16. Worked on: Wright Flyer
erik

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### Re: JSBSim dynamically change empty mass and inertia in-flig

S&J wrote in Wed Sep 20, 2023 6:54 pm:So the next question that arises from this is if the above is correct what forces are involved that once the paraglider returns to a level straight flight aligns the CoG and canopy.

Does the canopy rotate into alignment or does the CoG rotate into alignment ?

I'm not sure if I understand the question correctly. But aerodynamic forces (mostly from the canopy) rotate the paraglider (or more exactly change the angular velocities) around the CoG.

Imagine the paraglider from your death spiral video. If the aerodynamic forces would be abruptly removed (imagine replacing the air environment with vacuum) while in the death spiral, you would see that the CoG of the paraglider travels at a ballistic trajectory (because of gravitation) and that he uniformly rotates (e.g. no change in angular velocity) around the CoG (because of inertia).
amue

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Joined: Tue Apr 03, 2018 10:13 am

### Re: JSBSim dynamically change empty mass and inertia in-flig

Hi Erik, just talking flight modelling in general here.
"Stay away from negative people.They have a problem for every solution." - Albert Einstein
S&J

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Joined: Wed Aug 26, 2020 7:31 pm

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