Water leak from underside

My 6A is pretty tight up top, even with a tip-up. I posted previously about a 3 piece fiberglass tray which prevents any leakage from the front canopy seal from making its way into the instruments, although the canopy seal rarely lets any water in. Flying some practice IMC approaches the other day in heavy clouds with 700' ceilings resulted in water dripping out of the drain holes in the bottom of the fuselage, specifically the forward most drain hole which is centerline, just aft of the seat back bulkhead. I'd say that approximately a pint of water leaked out after I parked the plane in the hanger, following about 1.5 hours in the clouds (no rain just lots of moist humid clouds).


I found this extremely disturbing and set right away to identifying the source of the leak, while everything was still wet and I could hope to track it. Off came the seat backs, floor panels and the baggage bulkhead panels. I figured it must be coming from the wing area, even though I had sealed the outer floor ribs to the skins at construction which had been recommended. I found that virtually the only evidence of water was along the center of the aircraft, between the two ribs which create the tunnel for the elevator pushrod. The drain hole is in the baggage compartment portion, but there was also water in the portion of the tunnel forward of the compartment in the seat area and even the aft part of the insulation in the center tunnel of the floor area was very wet. No evidence of it coming from outboard and working its way to the center. Aft of the baggage bulkhead was virtually dry, with only a few drops of water visible.

I had previously tape sealed the vertical/horizontal stabilizer fairing thinking it may have leaked there and run forward along the bottom of the fuselage. That tape was still intact and the drain hole aft of the baggage compartment was not leaking. The firewall is well sealed and there is no evidence of it running aft as only the aft portion of the floor insulation was wetted.

My only conclusion is that the source of the water leak is actually the drain hole!!! I'm guessing that the shape of the fuselage and location of the drain hole just aft of the seat back bulkhead must create a low pressure area there and actually force water into the drain hole, which is about 5/32" diameter. My plan is to put a screw with a rubber washer into the drain holes to seal these and next time I fly in the clouds to remove them and then see if any water comes out.

Just wondering if anyone may have had a similar experience or have any other advice. I was really stumped when I couldn't find evidence of water other than in the center channels of the fuselage bottom.

trib said:

Flying some practice IMC approaches the other day in heavy clouds with 700' ceilings resulted in water dripping out of the drain holes in the bottom of the fuselage,

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They are called drain holes for a reason....
They are to provide an exit path for water that will get in if the conditions are right.
Think about the openings at the aft end of the fuselage.... elevator torque tube exit holes, small gaps between the emp fairing and fuselage, etc.
With heavy rain flowing aft on the exterior of the fuselage in flight combined with the dynamics of high speed, and it is easy to see how water would get inside.
That is why we install drain holes. It is good that you have them.... many RV's don't.

You may be surprised..

After I put a smoke system on my -4, I found oil residue in the back of the belly behind the baggage compartment. I thought my tank was leaking, but couldn't find anything obvious with it. After much head scratching, I realized there is a negative pressure at the rear bulkhead/aft spar of the vertical fin.The oil residue was going down the belly and being sucked in the small openings at the fingers of the rear bulkhead attachment, and it was working its way back forward inside the plane. I would give this some thought, as you may need to seal up some areas that would not seem like a contributing factor.

fixnflyguy said:

After I put a smoke system on my -4, I found oil residue in the back of the belly behind the baggage compartment. I thought my tank was leaking, but couldn't find anything obvious with it. After much head scratching, I realized there is a negative pressure at the rear bulkhead/aft spar of the vertical fin. The oil residue was going down the belly and being sucked in the small openings at the fingers of the rear bulkhead attachment, and it was working its way back forward inside the plane. I would give this some thought, as you may need to seal up some areas that would not seem like a contributing factor.

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I'm trying to picture this. Did you mean vertical fin?

I thought the technique for drain holes was to insert a rod into the hole and pull the rod backwards, which shapes the hole with a lip at the front and a depression at the rear.

Is this incorrect?

Vertical Fin clarrification

Vertical Fin, or Stabilizer, either or..point of aft most belly skin and rear bulkhead where vertical fin attaches and tailwheel fitting (On TD models). Seems to be a low pressure area there, and fluids are drawn back into the tailcone. I wouldn't have figured this unless I had excessive oil from smoke system turned up too high and oil residue down the belly came back in that way. I can just assume water may do the same.

Got the picture now, thanks!

I had initially thought that the water was coming from aft, primarily the empanage fairing, which is why I sealed it with vinyl tape. The tape was intact and no water could have found its way in there. The water was also not coming from the aft drain hole, which is just aft of the baggage compartment rear bulkhead, only from the hole just aft of the seat back bulkhead. I opened the baggage bulkhead and confirmed it was essentially dry. That's why my current guess is that the water actually came in the same hole it was draing from.

Interesting technique on creating a lip at the leading edge of the drain hole. Might give that a try.