Optimal TWR with Stock vs. FAR Aerodynamics

[numerobis]

You seem to be more of a mathematician than an engineer i.e. you seem to know more theory than me

I studied computational geometry and scientific computing -- not engineering, indeed!

You could design an item that had Cd fall faster than speed^2. I think an object with wings on springs would do, but if not, just put in a sensor that retracts the wings past a certain speed. That object would have two terminal velocities.

By the way, given your excitement about very-high-TWR spacecraft, you might be interested in the 10-minute challenge.

Edited December 16, 2014 by numerobis

[arkie87]

I studied computational geometry and scientific computing -- not engineering, indeed!

Yeah, i really like scientific computing and numerical methods. 'Stuffs fascinating to me

You could design an item that had Cd fall faster than speed^2. I think an object with wings on springs would do, but if not, just put in a sensor that retracts the wings past a certain speed. That object would have two terminal velocities.

Interesting. While retracting the wings might decrease drag coefficient and definitely decreases area, I would seriously doubt if it changed the overall drag coefficient/drag force of the craft that significantly, since most of the drag is experienced by the fuselage, no--wings are designed to be low drag, after all. But this is beside the point...

Let's pretend the entire craft is a wing: a sensor that retracts the wings will essentially be changing the drag force vs. velocity curve. Thus, before it retracts its wing, it has one terminal velocity; after it retracts, it has another. It does not simultaneously have two roots i.e. two terminal velocities. Retracting the wings in this manner is like attaching wings to decouplers.

A better example is the wings-on-springs, since the deformation would be continuous (i assume by wings-on-springs, you mean the wings deform like a spring proportional to the drag force) and inherent to the system, rather than a sudden change. Even so, I dont think there would be two roots based on the reasoning in this example:

Consider a craft going at terminal velocity, Vt, with drag force = weight i.e. D = W and "wings on springs" (which sounds awesome by the way). If we try to push the craft negligibly faster, the drag force will increase slightly and the wings will recede slightly. If the wings receding decreases the total drag force to below the terminal velocity drag force i.e. our initial value (which is what is required for their to be two roots), the wings will bounce back to their initial condition/displacement at terminal velocity.

The point being, its not possible for the drag to decrease with increasing velocity due to the wrings receding, since if that were to occur, the wings would "un-recede" since receding only occurs with increasing drag force...

If by wings on springs you just mean some arbitrary controller that recedes the wings at will, then the spacecraft will have two (or more) roots, but the control system will be basically changing the spacecraft. A command pod has two terminal velocities-- one with its parachute stowed and one with it deployed... but that clearly never was the case we were discussing

By the way, given your excitement about very-high-TWR spacecraft, you might be interested in the 10-minute challenge.

I will check it out