I've been considering a mod for my RV-7 that I haven't seen mentioned elsewhere. I've read some stories about RVs suffering power loss at high/cold altitudes where the culprit was assumed to be icing of the tank vents. I say "assumed" because there's usually no evidence after the plane lands since the ice has presumably melted. I believe that Bruce Bohannon experienced this on one of his record attempts.
Of course, I don't know any better than anyone else whether fuel vent icing was really the culprit. However, looking at the RV fuel vent design, I think I see a vulnerability. The vent line is run from the high point of the tank near the fuel filler, down to the wing root, into the cabin, up to its highest point near the canopy, then back down to through the cabin floor to exit underneath the plane. It took a few minutes of staring at the plans to figure it all out. I guess it's done this way to prevent fuel spillage when doing aerobatics. As far as I know this is the standard design for all RVs. The problem I see is that it creates a low point in the vent line in which water an possibly collect. Either by flying through rain, condensation, or (as in Mr. Bohannon's case) runoff of a water-spray cooling system, it's possible for water to make it's way into the vent tube and collect at the low point. The relatively low air-flow rate into the tube probably isn't enough to clear out the moisture; especially in the gently sloping section from the low point to the fuel filler. When the plane climbs to high altitude, the moisture starts freezing, and eventually blocks airflow into the tank.
I don't really know how likely that scenario is. However, it seems clear that it's possible. There's also a simple fix: simplify it. Instead of running the vent into the cabin and up, why not just bring the vent out the bottom of the wing fairing at the root? This would eliminate the low point and reduce the chances of ice accumulating in the vent. It would also make the plane a bit lighter. I can't see any drawbacks. It seems like it would even work for inverted flight with full tanks.
So what do you all think? Is this a good idea or am I missing something?
Thanks,
Kev
Of course, I don't know any better than anyone else whether fuel vent icing was really the culprit. However, looking at the RV fuel vent design, I think I see a vulnerability. The vent line is run from the high point of the tank near the fuel filler, down to the wing root, into the cabin, up to its highest point near the canopy, then back down to through the cabin floor to exit underneath the plane. It took a few minutes of staring at the plans to figure it all out. I guess it's done this way to prevent fuel spillage when doing aerobatics. As far as I know this is the standard design for all RVs. The problem I see is that it creates a low point in the vent line in which water an possibly collect. Either by flying through rain, condensation, or (as in Mr. Bohannon's case) runoff of a water-spray cooling system, it's possible for water to make it's way into the vent tube and collect at the low point. The relatively low air-flow rate into the tube probably isn't enough to clear out the moisture; especially in the gently sloping section from the low point to the fuel filler. When the plane climbs to high altitude, the moisture starts freezing, and eventually blocks airflow into the tank.
I don't really know how likely that scenario is. However, it seems clear that it's possible. There's also a simple fix: simplify it. Instead of running the vent into the cabin and up, why not just bring the vent out the bottom of the wing fairing at the root? This would eliminate the low point and reduce the chances of ice accumulating in the vent. It would also make the plane a bit lighter. I can't see any drawbacks. It seems like it would even work for inverted flight with full tanks.
So what do you all think? Is this a good idea or am I missing something?
Thanks,
Kev
