3 ms·

> All the dynamics are different and easier at smaller scales. Are you sure about this part? (not rhetoric). Non-linear scaling of surface and volume are simpl

by tomxor 7y ago

> All the dynamics are different and easier at smaller scales. Are you sure about this part? (not rhetoric). Non-linear scaling of surface and volume are simple to understand, but dynamics doesn't look so straight forward to me... in my short lived experience trying to fly very small model helicopters, it was clear that the smaller they are the more unstable they were. I wasn't sure how much of this was due to limitations in human reaction time and how much was inherent aerodynamic instability at smaller scales. In these rocket models the human limitation is clearly removed, the remaining dynamics look faster at least which may or may not run up against higher frequency sensor data and processing requirements... are there other dynamics i'm missing? i guess materials don't bend much at this scale?

TeMPOraL 7y ago
Different? Yes. Easier? Depends. One thing you gain with size is intertia, which grows with N³, and depending on the design, can be helpful for stability.
- tomxor 7y ago
  Yes, thanks, this is what I meant but the relationship was not clear to me before, it seems obvious in retrospect: n^3 * density = mass scales cubed, which as far as control is concerned is both good (lower impulse requirements), and bad (lower relative mass requires much finer control)... happy to be told how to express the later formally :)
  - TeMPOraL 7y ago
    I think in terms of "less mass = bad", the formal answer will be found in control theory.
  - lutorm 7y ago
    The think that makes this scaling "bad" is that moment of inertia is going down faster than everything else and this means the required control frequency goes up. A Falcon-sized rocket may only need to correct its course 50 times a second while a 3-foot scale model would need to do it 10x faster. This means your sensors need to be faster, your guidance computer has to run faster, and the actuators have to be able to respond faster. The hard things on the other side tends to be power requirements, for exactly the same reason. Mass scales up as the cube of size, including the mass of the thnigs you have to move. Although you don't have to move them as fast, the net effect is still that your power requirements become very large for large-scale vehicles.
illys 7y ago
You are certainly right on helicopters' dynamics... I was more thinking of planes and rockets when writing on dynamics: they are shaped to break through and be tunneled by a non-moving air for stability, and their weight decreased faster than their wing surfaces, making lift easier at smaller scales. Helicopter are a different realm: they need to survive in the middle of the wind (and turbulence) they create to lift, and they are not tunneled (their body is not aligned on the vertical flow). Thus, as Temporal mentions it, inertia is important for helicopter stability and it is reduced with weight at smaller scales.
- tomxor 7y ago
  > I was more thinking of planes and rockets when writing on dynamics: they are shaped to break through and be tunneled by a non-moving air for stability, and their weight decreased faster than their wing surfaces, making lift easier at smaller scales. I suppose there is also relative viscosity to take into consideration? so even if smaller scales are going to be more fidgety and "unstable" in terms of inertia (as TeMPOraL more clearly expressed)... taking aerodynamics into consideration (depending on the design) may provide more significant benefits to dynamic stability at small scales anyway. Without being very scientific, it feels like the small scale dynamics are not merely easier, but significantly different. It's intuitive to see how insignificant aerodynamics are at take off in full scale rockets are compared to models, and how models are going to be more sensitive to aerodynamics than inertia... i suspect the proportions to the problem of dynamic stability might even be flipped.
  - heavenlyblue 7y ago
    You also have to understand that small-scale models need the time dimension to be slowed down by X too in order to get the same dynamics.
deleted 7y ago
[deleted]

by tomxor 7y ago

TeMPOraL 7y ago

Different? Yes. Easier? Depends. One thing you gain with size is intertia, which grows with N³, and depending on the design, can be helpful for stability.

tomxor 7y ago
Yes, thanks, this is what I meant but the relationship was not clear to me before, it seems obvious in retrospect: n^3 * density = mass scales cubed, which as far as control is concerned is both good (lower impulse requirements), and bad (lower relative mass requires much finer control)... happy to be told how to express the later formally :)
- TeMPOraL 7y ago
  I think in terms of "less mass = bad", the formal answer will be found in control theory.
- lutorm 7y ago
  The think that makes this scaling "bad" is that moment of inertia is going down faster than everything else and this means the required control frequency goes up. A Falcon-sized rocket may only need to correct its course 50 times a second while a 3-foot scale model would need to do it 10x faster. This means your sensors need to be faster, your guidance computer has to run faster, and the actuators have to be able to respond faster. The hard things on the other side tends to be power requirements, for exactly the same reason. Mass scales up as the cube of size, including the mass of the thnigs you have to move. Although you don't have to move them as fast, the net effect is still that your power requirements become very large for large-scale vehicles.

illys 7y ago

You are certainly right on helicopters' dynamics... I was more thinking of planes and rockets when writing on dynamics: they are shaped to break through and be tunneled by a non-moving air for stability, and their weight decreased faster than their wing surfaces, making lift easier at smaller scales. Helicopter are a different realm: they need to survive in the middle of the wind (and turbulence) they create to lift, and they are not tunneled (their body is not aligned on the vertical flow). Thus, as Temporal mentions it, inertia is important for helicopter stability and it is reduced with weight at smaller scales.

tomxor 7y ago
> I was more thinking of planes and rockets when writing on dynamics: they are shaped to break through and be tunneled by a non-moving air for stability, and their weight decreased faster than their wing surfaces, making lift easier at smaller scales. I suppose there is also relative viscosity to take into consideration? so even if smaller scales are going to be more fidgety and "unstable" in terms of inertia (as TeMPOraL more clearly expressed)... taking aerodynamics into consideration (depending on the design) may provide more significant benefits to dynamic stability at small scales anyway. Without being very scientific, it feels like the small scale dynamics are not merely easier, but significantly different. It's intuitive to see how insignificant aerodynamics are at take off in full scale rockets are compared to models, and how models are going to be more sensitive to aerodynamics than inertia... i suspect the proportions to the problem of dynamic stability might even be flipped.
- heavenlyblue 7y ago
  You also have to understand that small-scale models need the time dimension to be slowed down by X too in order to get the same dynamics.

deleted 7y ago

[deleted]