Is it possible to make gravity as function of time on a mechanism simulation?

Question

Is it possible to make gravity as function of time on a mechanism simulation?

Aerodyns (Aeronautics)

(OP)

1 May 24 18:50

Hello all,

Althought it could sound weird, I am interested in introducing the gravity as a function of time in an ADAMS mechanism simulation. I am not looking for a complex expresion, just an step function which passess from 0 to 9.8 at certain moment.

I would expect it to be possible by introducing something like IF(time-tref:0,9.8, 9.8), or through a STEP, but ADAMS does not let me introduce any function on the gravity expression... Do you know by chance how could this be done?

I have a begginer level, but I ve spent a quite amount of time trying to fix it, without sucess.

Thank you very much in advance for any answer!

Best regards,
Aerodyns

GregLocock (Automotive) · Answer 1 · 2024-05-01 20:46:17

I think you'd have to stop the model in the acf, change the value in ACCGRAV, and restart it. I have low confidence in this.

ADAMS works best when you simulate something the way physics actually works, gravity does not work like that, so what are you really trying to simulate?

Cheers

Greg Locock

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Aerodyns (Aeronautics) · Answer 2 · 2024-05-02 07:46:52

First of all, thanks for your answer - I will try it. By the way, what do you call "acf" on your message?

I am working on simulating a glider (non-powered airplane) landing and its post-landing loads while taxiing across the runway.

During the Landing, the Aerodynamic Lift is compensanting its weight, which provides a smooth touch-down (constant vertical speed). I normally analyze this part as a zero-G situation in ADAMS (physically is not exactly the same, but differences are negligible).

The problem is, if gravity is zero, after the touch down the glider will rebound and won't follow the runway. In real life, once the glider touches down, its aerodynamic configuration change (flaps/spoilers, angle of attack, airspeed), making the lift smoothly dissapears. Unfortunately this fluid dynamics problem cannot be easily implemented on ADAMS. This is the point where I want to "activate gravity", so it sticks to the ground once it touches and I can analyze the loads during taxi.

Maybe I am doing the things more complicated than it could be done

Best regards,
Aerodyns

GregLocock (Automotive) · Answer 3 · 2024-05-02 23:34:16

acf is the adams control file.

I think your approach is unwise, ADAMS is quite happy to simulate aero forces.

Cheers

Greg Locock

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GregLocock (Automotive) · Answer 4 · 2024-05-03 02:20:11

if you can't be bothered with aero (it isn't hard but ok), then how about a vertical force at the cg say an SFORCE single acting force that you smoothly vary as the thing lands and slows down? I can talk you through how aero is implemented in ADAMS/Car where we use it for crosswind simulation.

Cheers

Greg Locock

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ichbinpulkit (Automotive) · Answer 5 · 2024-05-15 08:19:53

Something like:
IF(Glider_Speed<10mps);
G=9.8;

Might work giving out a function on value of Gravity.

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Is it possible to make gravity as function of time on a mechanism simulation?