One should be careful on this matter. Please think again. I seem to
recollect a case in America with an FRP sailplane years ago where the
vents on the wing surface to the water tanks were taped over and a
flight in wave conditions was undertaken. Also the tank outlets were
closed and so the tanks were thus sealed . This did not present any
problems on ascent to some serious altitude (something like 15 to 20,000
feet). On ascent the excess higher pressure in the tanks caused the
tape at the vents to lift sufficient to bleed the air out of the tanks
via the vents progressively. The problem came on long descent from high
altitude where the air pressure differential at the vents pushed the
tape hard to the surface and prevented partly or wholly the air flowing
back into the tank. The result was that towards the end of the descent
both wing tanks, leading edge to spar, collapsed due to the large and
crushing pressure differential.
Many sailplanes have wing undersurface tank outlets where a tapered plug
sits into a tapered seat built into the wing shell. Higher interior air
pressure as compared to exterior pressure, such as in the long descent
situation above, will push the plug into the taper improving the seal.
There is a good argument for duplicated vents to each tank with the vent
outlets in different locations, given that a vent may become blocked say
due to mud wasps.
The vents are there for a reason, because venting unwanted pressure
differential is good.
Regards, Roger Druce
On 23/04/2016 8:15 PM, Justin Couch wrote:
On 23/04/2016 2:30 AM, Jim Staniforth wrote:
Can a pilot tape over the dump holes to reduce noise, or is that a
breach of contract?
I'm pretty sure some tape over the holes would be fine - plenty of
gliders with underwing dump valves have a little mylar or rubber flap
over them for fairing purposes - our club DG1000 does. I've seen tape
plenty of times over the Jantar dump outlet too for the same reason.
I can't say for 100% coverage, but the flight manuals of gliders that
I've read state that the tanks should be left open if flying without
water. Obvious reason for this is to prevent the pressure change
splitting the wing open at altitude. A bit of tape won't hold much
pressure behind it so the manufacturers instructions are still being
obeyed anyway.
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