View Single Post
  #28  
Old June 9th 13, 04:50 AM posted to rec.aviation.soaring
[email protected]
external usenet poster
 
Posts: 220
Default fin/wing tanks freezing

On Saturday, June 8, 2013 8:23:00 PM UTC-7, jfitch wrote:
On Saturday, June 8, 2013 8:04:15 PM UTC-7, Craig R. wrote:

Not having studied thermal dynamics, etc, I have two rookie questions. How does the container material (thin plastic "milk jug" in freezer vs layered fiberglass for tail tank) and the shape of the container (minimal surface area of a milk jug vs long thin tail) effect these calculations? ... also, head space in jug is zip and in tail tank is large....




Having not studied thermodynamics you have saved yourself from a lifetime of boredom (even it if was only an hour a week). To a good approximation, the shape is accounted for in the surface to volume estimate, and the head space is not consequential to heat exchange.


I looked up the effect of various materials on heat transfer and it would appear that 1/4" of foam-composite sandwich roughly halves the heat transmission, extending the time to freeze by roughly double.

The starting temperature of the water doesn't matter that much in the end result (if the end result you are looking for is a solid block of ice). The enthalpy of fusion (turning 32-degree water into 32-degree ice) represents 75-90 percent of the heat transfer versus 10-25 percent to take the water from its starting temperature to 32 degrees.

Again, you would have to be on one of those 16-hour, 25,000 foot Sierra wave flights to worry about frozen-solid ballast tanks - even then I remain a bit skeptical that you'd have a problem. The main risk on a typical summer thermal flight in the 16-18,000' range is from leaking water freezing up on a valve or hinge that you care about. People who have experienced this seem to report pretty consistently that you can break free most of the time. I probably would avoid dumping ballast in freezing conditions if possible.

9B