To see this working, head to your live site.

Birds possible?

Would this work to make a bird? You have Flying animals - Birds listed on your Applications section, but watching the various videos I only saw a dragon statically flapping its wings, but not moving. I understand how the wing model could work if a bird wing isnt flapping, like normal gliding flight as that is just a basic wing, but birds also generate lift forces from flapping to gain altitude and take off from a stand still and it looks like your asset needs a moving wind field to work on the wing body. As a bird can flap and take off with no wind since its pushing against the air itself at that point, would this sort of force work?

Just wondering if youve got any models or demos of this kind of dual lifting force that happen for birds.

Thanks

8 answers0 replies

Comments (8)

Zalz Torus

Nov 03, 2023

Wow thats a lot to digest, frankly Im not great at algebra so this might be beyond my pay grade, but thank you for all the info Ill do my best. I was thinking more along the lines that somewhere in your AeroObject code the drag would be calculated as you say, then applied to the rigidbody via "Rigibody.AddForce()" and was thinking I could just modify or limit that vector3 right before its applied.

I think I do understand however needing to reduce the fixedDeltaTime. Mine is set to 0.01111111 which is 90 per second, which is the minimum recommended for VR. I've tested at 0.002 and I am not longer getting the run away forces so I suppose this is the solution. I can try running with a faster deltaTime although I dont know if this will become a performance issue. Ill have to test that.

Looks like drag velocity on the cube was fine, I was just trying to figure out what was going on. My cube example actually wasnt having the issue, my wings were during their downward stroke, which is when I was increasing the AO local dimensions, the cube was just a test I ran to remove variables, I should have been more clear about the two.

Edited

Conor

Oct 16, 2023

Hi Zalz,

Birds are absolutely possible with these tools. Here's an example of a bee flying by flapping its wings and also affecting the fluid around it with a custom wake model: https://youtu.be/mSj5ib-0S2E?si=LkCWm0kLeZZznmTF

The rotor blades of a helicopter work in a similar way to a bird wing flapping motion too, though they spin in a circle instead of moving back and forth.

A good starting point would be to add aero objects to your bird's wings and mark them as kinematic - this will mean they get their motion from the transform component and do not need to reference a rigid body for their velocity and angular velocity. Then you can move the wings according to sin waves for the flapping and twisting motions.

Hope that helps!

Conor

Oct 18, 2023

Replying to

I wouldn't recommend using a group for this application as the panels will be moving separately to each other. The grouping is intended for a rigid wing structure like a typical aircraft.

By not using a group you may notice the ellipsoid bodies will slightly underestimate the platform area of the wing. You can make them larger to account for this or see how they perform as is. Generally I haven't found an issue with the slight underestimate!

Zalz Torus

Nov 01, 2023

Replying to

Is there a way to limit the amount of force acting on the rigidbody? Im using the AeroObject Local Dimensions to vary how the wings behave during movement, or just to get the "correct" amount of lift flapping wings should produce, but I'm finding that increasing the the dimensions or high speed movement can suddenly fling my game object out of bounds.

Not sure if I did something wrong with my project but I made a simple unmodified 1x1x1 cube, with just "has Drag" on as a test and I find it falls very slowly, like 6 velocity. Almost like I have too much air reistance or something. I found Global Fluid properties, maybe this is set too high?

Conor

Nov 01, 2023

Replying to

Hi Zalz,

Your fluid properties look fine!

If you're using the latest verison of Unity you can look at https://docs.unity3d.com/ScriptReference/Rigidbody.GetAccumulatedForce.html to find the total force acting on a body - you can then compensate for large forces by adding forces in the opposite direction of the accumulated force. Note you would need a script running in a larger execution order to ensure all the forces have been added to the rigid body before saturating them. Check out this for more on execution orders: https://docs.unity3d.com/Manual/class-MonoManager.html

For your test, you can check the expected terminal velocity of an object using the following equation:

v = sqrt( 2mg / pACd )

Where v is the terminal velocity, m is mass, g is acceleration due to gravity, rho (p) is the density of the fluid (1.23 by default), A is the area (0.785 m^2 for your cube - note we use ellipsoids to approximate bodies) and Cd is the drag coefficient (0.5 is a good approximation for a cube). I get 6.37 m/s for a 1x1x1 m cube at 1 kg. Our drag model is a bit more involved than that equation which will explain the discrepancy, the cube actually reaches 6.33 m/s in my tests.

During development we decided it was best to use Unity's physics solver so that our tools are fully compatible with Rigidbodies. Unfortunately, this means that drag can become unstable when the drag force and the weight of an object are close in magnitude.

A rough estimate for instability is to compute the value of:

4m / (p S Cd v)

(m is mass of an object, rho (p) is the density of the fluid (1.23), S is the largest area on the object, Cd is drag coefficient (in this case use the max of 1.2), v is an expected velocity for the object). If that value is less than the fixed delta time in your simulation, then you will likely have instability and objects will shoot off. The cube experiment for example gives a value of 0.545 so a simulation of that cube falling at 6.33 m/s should be stable with a time step of around 0.5 seconds - though I would advise using a much smaller one!

You can ensure stability by either: reducing the fixed delta time, increasing the mass of your objects, or reducing their areas. We have found a good value for fixed delta time to be 0.002 seconds.

Hope that helps!

Edited