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#21
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New Vent!
In article ,
Don Johnstone wrote: Do you blokes suffer from major flatulence problems? Seems a lot of effort to remove air from the cockpit but I could understand if the air was contaminated in some way :-) Around here (northern Virginia) there's a month or two of summer where I spend pretty much the entire flight with my hand stuck outside to get as much air in as possible to try to stay cool. On one miserably hot day this past July I had to cut a flight short as I began to seriously overheat despite all my best efforts. I have one of Paul's extractors on order and I'm hoping it'll make next summer much more pleasant. -- Mike Ash Radio Free Earth Broadcasting from our climate-controlled studios deep inside the Moon |
#22
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New Vent!
from Paul (sisu1a):
..... *But as I understand it, the pressure at the tail is kinda on a ship by ship basis. Some ships even suck water up the tailpipe when blowing ballast, which is a pretty good indicator of a poor choice for a 'low pressure' location. ... Paul (ALL) I have this problem with my Genesis 2 (both water and "other" fluids). Does this come from a "venturi" effect of air leaking from Horz/Vert Stab Junction or poor Rudder Sealing? It amazes me how much gets sucked back into fuselage. (Genesis has VERY short fuselage) Another Genesis owner (DK) has experimented with an exit duct out the fuse hatch over wings. ... comments Don??? Curt - 95 |
#23
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New Vent!
On Dec 8, 12:46*pm, CLewis95 wrote:
from Paul (sisu1a): ..... *But as I understand it, the pressure at the tail is kinda on a ship by ship basis. Some ships even suck water up the tailpipe when blowing ballast, which is a pretty good indicator of a poor choice for a 'low pressure' location. ... Paul (ALL) I have this problem with my Genesis 2 (both water and "other" fluids). Does this come from a "venturi" effect of air leaking from Horz/Vert Stab Junction or poor Rudder Sealing? *It amazes me how much gets sucked back into fuselage. *(Genesis has VERY short fuselage) Another Genesis owner (DK) has experimented with an exit duct out the fuse hatch over wings. ... comments Don??? Curt - 95 Likely due to pressure recoveing over the very short length of fuselage and large reduction in cross section over that length. High location closer to centerline would probably better in order to avoid effects on root fillet area. Just speculating- It's cold here. UH |
#24
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New Vent!
I have this problem with my Genesis 2 (both water and "other" fluids). Does this come from a "venturi" effect of air leaking from Horz/Vert Stab Junction or poor Rudder Sealing? *It amazes me how much gets sucked back into fuselage. *(Genesis has VERY short fuselage) Another Genesis owner (DK) has experimented with an exit duct out the fuse hatch over wings. ... comments Don??? Curt - 95 I have had an air outlet on my Genesis2 for several years. It was modeled after the outlet on DB's ASW-22. It seems to work very well. Curt is correct that on a Genesis 2, dumped water ballast will enter the fuselage thru the fairings over the rudder horns. These fairings are suppose to be the air outlets. I have taken photos of that area with tufts taped around the fairings. And indeed the tufts turn and are sucked into the "outlets". Don (DK) |
#25
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New Vent!
On Dec 7, 12:47*pm, Don Johnstone wrote:
Do you blokes suffer from major flatulence problems? Seems a lot of effort to remove air from the cockpit but I could understand if the air was contaminated in some way :-) At issue is that ventilation air tends to pressurize the cockpit, and then leak out around the canopy perimeter. Anywhere that there is air escaping through the canopy frame gap, that leak will trip the boundary layer and increase drag. If you can keep the cockpit at lower than ambient pressure, you run a good chance of maintaining laminar flow across the gap between the fuselage and the canopy, which can result in several more square feet of laminar flow than you had previously. I happen to think that many original designers got it right; that the most effective vent is back at the base of the rudder, where it is convenient to exhaust air around the rudder cable horns. Unfortunately, something often got lost in translation, and most production gliders allow too little exhaust area through the vertical fin spar, causing inadequate ventilation flow and too much cockpit pressure. They also offer many restrictions on the path from the cockpit to the tailboom, which reduces the flow rate. So I think that these trendy exhaust vents, while perhaps not the best possible solution, are still a lot better than you can get without removing the rudder and doing some relatively major surgery on the fin spar. Thanks, Bob K. http://www.hpaircraft.com |
#26
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New Vent!
On Dec 8, 11:20*am, Bob Kuykendall wrote:
On Dec 7, 12:47*pm, Don Johnstone wrote: Do you blokes suffer from major flatulence problems? Seems a lot of effort to remove air from the cockpit but I could understand if the air was contaminated in some way :-) At issue is that ventilation air tends to pressurize the cockpit, and then leak out around the canopy perimeter. Anywhere that there is air escaping through the canopy frame gap, that leak will trip the boundary layer and increase drag. If you can keep the cockpit at lower than ambient pressure, you run a good chance of maintaining laminar flow across the gap between the fuselage and the canopy, which can result in several more square feet of laminar flow than you had previously. I happen to think that many original designers got it right; that the most effective vent is back at the base of the rudder, where it is convenient to exhaust air around the rudder cable horns. Unfortunately, something often got lost in translation, and most production gliders allow too little exhaust area through the vertical fin spar, causing inadequate ventilation flow and too much cockpit pressure. They also offer many restrictions on the path from the cockpit to the tailboom, which reduces the flow rate. So I think that these trendy exhaust vents, while perhaps not the best possible solution, are still a lot better than you can get without removing the rudder and doing some relatively major surgery on the fin spar. Thanks, Bob K.http://www.hpaircraft.com .... and as JS pointed out, many of us fly where it is REALLY hot, with ambient temperatures of 43c, 110f, and closed cockpit temperature over 55c, 130f. It doesn't take long at that temperature to ruin an otherwise great flying day. |
#27
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New Vent!
What keeps rain, wasps, mice, etc. from entering the vent hole when on
the ground? |
#28
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New Vent!
On Dec 9, 3:11*am, "Matt Herron Jr." wrote:
What keeps rain, wasps, mice, etc. from entering the vent hole when on the ground? We don't have mice running around on our launch grid and rarely grid in the rain. Guess I just didn't think about those problems. UH |
#29
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New Vent!
At 08:11 09 December 2010, Matt Herron Jr. wrote:
What keeps rain, wasps, mice, etc. from entering the vent hole when on the ground? Canopy cover? |
#30
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New Vent!
My two cents:
- complementing the post by Bob Kuykendal, and as an example, the air passage though the tailfin spar of an LS8 is comprised of three small holes with a combined area of barely 3 square inches. Added to the other constrictions along the way, this means the ventilation pressure drop occurs mostly after the cockpit. Thus, of course the cockpit will stay significantly above ambient pressure in an unmodified LS fuselage. - regarding the reingestion of ballast water (or pee...) at the end of the tailboom, perhaps it is linked to lower pressures at the top end of the rudder hinge? The location of the horizontal tailplane on the Genesis would suggest suction occurs there. - finally, and after applauding the designers of all these fine new outlets, perhaps the next step is to locate the inlet in a neutral or even a low pressure area? Why, you may ask? Because there is no reason in principle to pursue the highest possible ventilation pressure drop. With a nose inlet and a turtleneck exit, the total ventilation pressure drop approaches twice the dynamic pressure of the outside free flow (i.e. the pressure coefficients may approach +1 at the nose and -1 at the turtleneck). The power lost to the ventilation flow is the product of this pressure drop by the flow rate, e.g. at 100 kts a flow rate of 20 litres/second costs 30 Watts. This power is subtracted from the performance of the glider. If the inlet is located instead in a neutral pressure area (and the cross-sections are suitably sized), the same cooling flow will cost only 15 Watts - and the cockpit will achieve an even lower pressure than before, which is doubly good for performance! Going further: an inlet may even be located in a moderately negative pressure area (I envision exchanging the pop-out window for a small naca-entry connected to a small eyeball vent). The Cp at this location is about -0.7; with partial pressure recovery, perhaps we get -0.3 in the cockpit. As the pressure at the turtleneck exit remains even lower, it is still possible to create an effective airflow. Result: the most energy-efficient ventilation possible. Sounds counterintuitive, but should work and be easy to implement in a new design (existing designs may be constrained by the impossibility of increasing the cross-section of inlets). |
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