Does anyone have expertise in Damping Ratio vs Static Margin?

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jahall4

Well-Known Member
Im finding the some designs (e.g. the V-2) when the Static Margin is the typical 1 caliper will have a Damping Ratio less than what I read is recommended (0.05). But, if you tighten the Margin, say 0.4, the Damping Ratio can be raised to approach 0.1.

So what could happen when you try to fly with a Damping Ratio of say 0.04?

mpitfield

Moderator
Staff member
Global Mod
Im finding the some designs (e.g. the V-2) when the Static Margin is the typical 1 caliper will have a Damping Ratio less than what I read is recommended (0.05). But, if you tighten the Margin, say 0.4, the Damping Ratio can be raised to approach 0.1.

So what could happen when you try to fly with a Damping Ratio of say 0.04?
Okay for the non-aeronautical engineers in the crowd can you explain what a "dampening ratio" is in the context of stability margin?

jahall4

Well-Known Member
Okay for the non-aeronautical engineers in the crowd can you explain what a "dampening ratio" is in the context of stability margin?
Ever see the tail of a rocket oscillate back and forth for no obvious reason (e.g. a damaged fin)?

markkoelsch

Well-Known Member
I am not an aerospace engineer. I do have a background in electrical though. These are my thoughts, and I could be wrong.

In general, damping deals with the oscillations of a body or system, and how they are controlled. In a rocket, the oscillation is likely something that disturbs the flight path- say a wind gust. This damping ratio is a number that describes how quickly the oscillation are in essence cancelled out.

In general terms, a damping ration of under 1 is under or undamped. A ration=1 is what is called critically damped.

I would think that a damping ration in the case mentioned is low. Likely to indicate that the design is likely to take some time to stop oscillating once an oscillation is induced. This makes sense in the case a V2- it was designed to carry a payload of a ton, and to be actively guided hence dampening the oscillation.

jahall4

Well-Known Member
In a rocket, the oscillation is likely something that disturbs the flight path- say a wind gust. This damping ratio is a number that describes how quickly the oscillation are in essence cancelled out.

In general terms, a damping ration of under 1 is under or undamped. A ration=1 is what is called critically damped.

I would think that a damping ration in the case mentioned is low.
... So when nothing less than 0.05 is recommended what could happen when you try to fly with a Damping Ratio of say 0.04?

Xrain

Well-Known Member
... So when nothing less than 0.05 is recommended what could happen when you try to fly with a Damping Ratio of say 0.04?
You generally compensate with an active system. Sidewinder and other high dynamic missiles are massively unstable. Stability means it requires more energy to affect a change in the vehicle's trajectory. Something you don't want if you want the thing to do loop-de-loops, since a lower amount of energy required means you can get larger movements with a smaller control input. The lower your damping ratio and static margin the more responsive the vehicle becomes to control inputs.

For non actively controlled systems less damping ratio would mean a higher frequency of oscillation, it would also not return to a zero angle of attack and constantly oscillate back an forth. It is also possible to get pitch-roll coupling in this area which can cause problems. Too much damping makes it weathercock worse. Too little makes it unstable, or just increases drag from it never being at zero angle of attack.

UhClem

Well-Known Member
Topics In Advanced Model Rocketry covers this and much more. That recommends the coupled damping ratio be no less than 0.05 and the decoupled damping ratio be no greater than 0.30. (Coupled meaning that the equations of motion include roll.)

jahall4

Well-Known Member
It is also possible to get pitch-roll coupling in this area which can cause problems.
I guess that is really what I'm asking about... "roll coupling". Apparently it is NOT a given that it will occur since there are scale models Like the V2 that are flown with a Damping Ratio of less that .05. Its also clear from the simulations I have run, that in at least some rockets, narrowing the static margin increases the Damping Ratio.