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  #1  
Old 12-19-2018, 01:06 PM
Dunkelmann Dunkelmann is offline
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Curious effect of visual model affecting physics model

I have a prototype Slowpoke electric model where I have entered the physical measurements of the Durafly version. I've taken a guess at the motor but at least I know its kV.

The model flies but I've no way of knowing how this compares to real life.

That apart, the problem is that the nose tips over on the ground meaning it has to have a hand launch.

The CG is roughly right, about 80-85mm back from the main wing leading edge. That is in front of the physics model undercarriage so it shouldn't tip forward.

Now the visual model's undercarriage is behind the CG so my suspicion is that this is being used as a pivot point instead of the physical model's.

As a small experiment I greatly increased the length (vertical) of the undercarriage expecting the model to appear visually higher on the field, appearing to be floating.

Can anyone shed some light on this phenomenon? I have attached the rfx.
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File Type: rfx Durafly SlowPoke Proto_EA.RFX (364.1 KB, 3 views)
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  #2  
Old 12-19-2018, 02:02 PM
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legoman legoman is offline
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Quote:
Originally Posted by Dunkelmann View Post
I have a prototype Slowpoke electric model where I have entered the physical measurements of the Durafly version. I've taken a guess at the motor but at least I know its kV.

The model flies but I've no way of knowing how this compares to real life.

That apart, the problem is that the nose tips over on the ground meaning it has to have a hand launch.

The CG is roughly right, about 80-85mm back from the main wing leading edge. That is in front of the physics model undercarriage so it shouldn't tip forward.

Now the visual model's undercarriage is behind the CG so my suspicion is that this is being used as a pivot point instead of the physical model's.

As a small experiment I greatly increased the length (vertical) of the undercarriage expecting the model to appear visually higher on the field, appearing to be floating.

Can anyone shed some light on this phenomenon? I have attached the rfx.
your problem is the cg is not between the three wheels. just changing the physics does not change the graphics which determines how the plane sits on the ground.

If you can get the cg cross hairs surrounded by the wheels


Picture one is your picture two is a mod by myself. 80mm back from the wing is too close to the gear my picture is closer to where it should be about 100mm back
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File Type: jpg 2018-12-19 12_35_17-Window.jpg (67.4 KB, 16 views)
File Type: jpg 2018-12-19 12_59_00-Window.jpg (27.4 KB, 16 views)
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  #3  
Old 12-22-2018, 01:59 PM
Dunkelmann Dunkelmann is offline
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Quote:
Originally Posted by legoman View Post
your problem is the cg is not between the three wheels. just changing the physics does not change the graphics which determines how the plane sits on the ground.

If you can get the cg cross hairs surrounded by the wheels


Picture one is your picture two is a mod by myself. 80mm back from the wing is too close to the gear my picture is closer to where it should be about 100mm back
That confirms my thoughts: "Now the visual model's undercarriage is behind the CG so my suspicion is that this is being used as a pivot point instead of the physical model's."

That is weird since I would have expected the physics model to extend to ground behaviour too. I regard a visual model as an overlay on a simple axial physics model (I mean let the physics model and the visual model have coordinate systems where the axes and the 0,0,0 point coincide).

OK then it looks like my next steps are to:
  1. Decouple the visual model so I can edit it
  2. Edit the visual model so that the location and properties of the undercarriage components match my physical model
  3. Join the updated visual model to the physical model

I therefore need to know:
  1. How to decouple
  2. What tool(s) I need (I think 3ds and Gimp - I have no idea what they are but I'll find out)
  3. What to edit
  4. How to join it all up

This being the holiday period I can start reading up. Without your help I am jiggered. I will try to find the overview which explains the process at a high level.

Note: If your model needs CG at 100mm then it doesn't correspond to the real model so I still have some work to do on weights.

I appreciate your time.
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  #4  
Old 12-22-2018, 04:09 PM
asj5547 asj5547 is online now
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Physics Tutorial, this might help.
http://www.knifeedge.com/forums/show...ysics+tutorial
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  #5  
Old 12-22-2018, 07:03 PM
Dunkelmann Dunkelmann is offline
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Asj, many thanks. I realised that there are design threads for other versions that I need to trawl through.

It looks as though I need a 3D visual model first, then I create a physics model. So is my talk of decoupling/reverse engineering the 3D visual model impossible?

I have registered for an educational copy of 3ds. Perhaps I can find a 3ds file for the Slowpoke that I can import.
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