fin/wing tanks freezing

Had a Nimbus 3 in the shop some years ago with massive structural wing
damage due to freezing tanks. Took the upper wing shells clean off the
spars and/or cracked the shell right through just in front of the spar (the
width of the tanks), and the resulting 'new' aerofoil made flying it very
interesting according to the pilot. Interesting repair as well.

Damage can be massive, control problems and weight/balance problems would
be an issue too with freezing water. Most flight manuals specifically
state: water to be dumped before reaching freezing level. Even when not
using common sense these factors should be enough to make up your mind, I
would say.

On Wednesday, June 5, 2013 9:51:04 AM UTC-7, Croft Brown wrote:
....... I am amazed that this question has even been posted......

cb



You could focus on coaching instead of deriding a fella for asking a genuine question.





At 16:23 05 June 2013, Matt Herron Jr. wrote:

I fly a Ventus C with integral tanks (no Bags) any issues with antifreeze

and carbon/fiberglass tanks? What would be the recommended amount?

Sounds

like an environmental hazard to dump, and I certainly dont want to land

wet

unless I have to.



I have had issues with the Ventus fully loaded with water where the flaps

seem to stick a bit. Not sure if it was a freezing leak or what, but was

a

little scary. Frozen rudder must not have been too fun.



Matt

Of course your right. We must focus on coaching. Read the flight manual
section on water ballast.
cb




At 13:32 06 June 2013, TravisBrown73 wrote:
On Wednesday, June 5, 2013 9:51:04 AM UTC-7, Croft Brown wrote:
....... I am amazed that this question has even been posted......

cb



You could focus on coaching instead of deriding a fella for asking a
genuine question.





At 16:23 05 June 2013, Matt Herron Jr. wrote:

I fly a Ventus C with integral tanks (no Bags) any issues with
antifreeze

and carbon/fiberglass tanks? What would be the recommended amount?

Sounds

like an environmental hazard to dump, and I certainly dont want to
land

wet

unless I have to.



I have had issues with the Ventus fully loaded with water where the
flaps

seem to stick a bit. Not sure if it was a freezing leak or what, but
was

a

little scary. Frozen rudder must not have been too fun.



Matt

What I was wondering is whether there are any gliders out there with
thermometers in the tanks. My hunch is that, together with non-leaking
valves, that would allow a great deal of added flexibility. It would also
reduce the amount of speculation in that thread

Best
--Gerhard

Can anyone share some wisdom on using water at high elevations for long durations? How do you know your fin or wing tanks will not freeze? If I am at 18K for 6 hrs in the Sierras, I really don't want my vertical stab splitting open in flight. Has this ever happened? Any guidance would be appreciated.

I have had 3 different malfunctions as a result of prolonged flights above the freezing level when carrying water ballast.

1. In an ASW 20 with a aftermarket brass valve actuated tail ballast system.. The valve froze and I was unable to dump the tail ballast. I wasn't aware of the malfunction until after landing and the valve thawed and water came running out. It was fashionable at the time to fly the ASW 20 right at or slightly behind the aft limit so dumping the wings and not the tail resulted in a dangerously tail heavy condition. I was blissfully unaware of any change in the behavior of the aircraft and landed uneventfully. The fix was to put a pint of RV antifreeze in the tail tank. RV antifreeze is propylene glycol and is safe even if you drink a little.

2. In a Nimbus 3 when flying with full wing ballast water would bubble out of the vent in the filler plugs on the wing top surface and run back over the flap aileron junction on the trailing edge and freeze, resulting in frozen controls. It was quite alarming but not too hard to dislodge with a vigorous thrashing of the controls. I learned to cycle that gap frequently to break it off before it became thick enough to be a problem.

3. I have always used the wax from wax toilet mounting rings to seal the dump valves on Schempp Hirth gliders. After a long wave flight in NZ in my Nimbus 3 I landed at Omarama and promptly did a wild ground loop. Omarama is a wide grass field and the glider escaped unscathed. On most airport runways I probably would have done heavy damage. The wax had become so stiff that one valve stayed stuck shut, and there was enough slack in the dump linkage to allow the cockpit dump handle to stow in the dump position, so I had no way of knowing that one wing was not dumping. Interestingly, although I had been flying for some time with one wing empty and the other full, I was not aware of the asymmetry until the heavy wing went down on the landing. The solution was to switch to chap stick which stays fairly soft when cold. It doesn't seal as well but is a wiser choice.
I guess one could conclude that when flying for extended times, in below freezing temperatures, with water ballast it is advantageous to have been born lucky.

DLB

On Jun 6, 3:37*pm, wrote:
What I was wondering is whether there are any gliders out there with
thermometers in the tanks.

I think some, if not all, LS gliders, have tail tank thermometers and
that it was a requirement of the type certificate. Never understood
why it should be required for one make/model but not for gliders from
other manufacturers.

When I had my ASW-19b I was concerned out wing ballast temperature.
Since the 19 uses bags it was easy to fit a thermistor probe under one
of the ballast bags and connect to a cockpit thermometer. (Radio Shack
indoor/outdoor thermometer with a connector break in the outdoor probe
lead). I was surprised at how warm the ballast stayed despite long
cold soaks at altitude.

The risk of tail ballast failing to dump is taken seriously by some
manufacturers but not by others. For the ASW-28 the aft cg limit
moves forward as mass is increased. If I operate within limits I
will not exceed the aft CG limit if my tail tank fails to dump.

On the other hand, the Duo Discus handbook recommends an increase in
minimum front seat mass if a tail tank is fitted. It then goes on to
say it's only a recommendation and you can ignore that safety
protection if you want to.

Failure to dump the tail ballast is not just a freeze risk. The valve
can jam or the cable can break when it's nice and warm outside. I've
even seen a well meaning crew tape over ballast vent holes.

Andy

I think some, if not all, LS gliders, have tail tank thermometers and
that it was a requirement of the type certificate. Never understood

Not the brandnew LS10 I was flying in Chile 2 1/2 yrs ago (manufactured
by DG)...

lead). I was surprised at how warm the ballast stayed despite long
cold soaks at altitude.

Water does has a high heat capacity... And if bags are used, they could
potentially made from some insulating material (are they?).

Best
--Gerhard

On Wednesday, June 5, 2013 8:41:20 AM UTC-7, Matt Herron Jr. wrote:
Can anyone share some wisdom on using water at high elevations for long durations? How do you know your fin or wing tanks will not freeze? If I am at 18K for 6 hrs in the Sierras, I really don't want my vertical stab splitting open in flight. Has this ever happened? Any guidance would be appreciated.



Matt


What you're basically looking for is a solution to a reasonably straight-forward heat transfer problem. We can make a set of assumptions to make this a solvable problem.

1) The temperature of the water is basically spatially uniform.
2) The temperature of the surrounding atmosphere doesn't change.
3) The fluid properties (density and specific heat) are constant.
4) The heat transfer is convective only (from cold air flowing around the water container). No conduction or radiation.
5) There's no forced convection (i.e. no fans or forced flow into the ballast tank)
6) The free convection heat transfer is fairly efficient or the cold air motion outside of the water container is substantial (i.e. Gr and Pr are large)

If you do this, you end up with an equation that looks like this:

-h * A * (T - To) = rho * V * (Cp * dT + hfs) / dt

h = the convective heat transfer coefficient, which basically measures how efficiently heat is being transferred. With our assumptions, we can roughly approximate h = 1.52 * (T - To)^(1/3).
A = the surface area of the water container
T = the temperature of the water at time t, We'll use 32 °F here since most of the heat transfer will be occuring while the water is undergoing the phase change
To = the outside temperature (this needs to be below 32 °F)
rho = the density of the water
V = the volume of the water
Cp = the specific heat of the water (assume around 4.2 kJ/ kg K)
dT = the difference between the initial and final water temperatures
dt = the time it takes to freeze
hfs = the enthalpy of fusion (about 334 kJ/kg)

Rearrange the equation and we set the energy required to freeze the water (hA(T-To)) is equal to the energy required to cool it (rhoVCpDT/dt) plus the energy required to turn it into a solid (rhoVhfs/dt).
Suppose you have a typical water bottle as a simple example.

Starting with h, if you begin at room temperature, h = 1.52 * (25-0)^(1/3) = 4.4 (W/m^2 K)

Assume a cylindrical container meauring about 6" x 2.5". So PI * D * L = 0.03 (m^2)

The outside air is at 0 °F (-18 °C) and the water starts at about room temperature (20 °C). So T - To = -40 (°C)

The volume is about 500 mL, the density is about 1000 kg/m3, so the mass is about 0.5 kg.

The specific heat we said above was 4.2 (kJ/kg K) or 4200 (J/kg)

The temperature change will be from room temperature to freezing, so 20 °C.

The enthalpy of fusion from above is 334,000 J/kg.

Plugging all this in and solving for dt gives a time of 31,667 s or about 8 hr. 48 mins. For any volume of room temperature water going into a freezing environment, the only thing that will change will be the surface area (A), and the volume (V). The rest you can keep the same as a basic approximation so you have:

t = 1,900,000 V/A

where V and A are in m^3 and m^2, respectively.

The V/A for tail tanks and wing tanks vary only slightly. We should probably adjust the heat transfer coefficient downward a bit from the above example since the tanks in most gliders are insulated with the composite/foam sandwich of the structure. This will make the time longer. If it's warmer than 0 °F it will take longer still, so for normal thermal soaring you would expect never to get to frozen solid. Maybe on a really long wave flight you should worry.

Lastly, as has been pointed out, depending on your venting, valving and CG considerations, you may have localized water management issues from small-scale freezing, but I wouldn't worry about a giant block of ice exploding from my wing or tail.

9B

After reading this post I am just glad my elementary school math teacher was wrong when she said I needed to learn math because I wouldn't always be walking around with a calculator.

I don't recall a thermometer in the tail tank as a requirement in either of
my LS-6s. Maybe because they were early models...

My LAK-17a Pilot's Manual requires an OAT gauge if water ballast is carried
and states further:

Warning:
Flight with water ballast must be conducted at an OAT greater than 1° C
(34°F). If there is a risk of freezing temperatures, all water must be
dumped before freezing temperatures are reached.

Being somewhat of a "test pilot", I often fly with water at below freezing
temperatures (it's cold at 18,000' MSL regardless of seasons), but I
generally don't spend more than a couple of hours at those altitudes and
temperatures. I also spend plenty of time on the descent to allow things to
warm up at a reasonable rate.

Having said that, I've had a stuck wing dump valve more than once. It's not
noticeable until the glider comes to rest.

"Andy" wrote in message
...
On Jun 6, 3:37 pm, wrote:
What I was wondering is whether there are any gliders out there with
thermometers in the tanks.

I think some, if not all, LS gliders, have tail tank thermometers and
that it was a requirement of the type certificate. Never understood
why it should be required for one make/model but not for gliders from
other manufacturers.

When I had my ASW-19b I was concerned out wing ballast temperature.
Since the 19 uses bags it was easy to fit a thermistor probe under one
of the ballast bags and connect to a cockpit thermometer. (Radio Shack
indoor/outdoor thermometer with a connector break in the outdoor probe
lead). I was surprised at how warm the ballast stayed despite long
cold soaks at altitude.

The risk of tail ballast failing to dump is taken seriously by some
manufacturers but not by others. For the ASW-28 the aft cg limit
moves forward as mass is increased. If I operate within limits I
will not exceed the aft CG limit if my tail tank fails to dump.

On the other hand, the Duo Discus handbook recommends an increase in
minimum front seat mass if a tail tank is fitted. It then goes on to
say it's only a recommendation and you can ignore that safety
protection if you want to.

Failure to dump the tail ballast is not just a freeze risk. The valve
can jam or the cable can break when it's nice and warm outside. I've
even seen a well meaning crew tape over ballast vent holes.

Andy