B

bjustus

Well Known Member
<Title should say "Potential Icing in IFR" edit feature won't let me change it.>
(Done; S. Buchanan)
Talked to a rep at Van's yesterday about cutting off the forward bend in my fuel vents. I'm upgrading the panel for IFR flight and worry, (as several others have mentioned here in past threads related to pitot heat) about fuel vent icing potential.

He said not to cut them off with a 45 angle to the rear as I was thinking about doing as it may cause a negative pressure. "You don't want to get into any kind of icing conditions in an RV anyway," he said. I agree, but the only way you can absolutely guarantee that is never go into a cloud. Forecasts can be very, very wrong, and icing can come up when you least expect. Cherokees, Grummans, etc. fly IFR all the time with flush or angled vents that do the job and don't ice up -- what am I missing?

Any suggestions/solutions out there?

Thanks,

- Brett
 
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More info

I have not hear of these mods and they are pretty timely for me. Can someone who has had good luck doing something different post some links of pics? Thanks
 
Yes, a pic would be great if possible. My plane is already flying so I'm hoping for a simple solution that doesn't require tank removal. I don't want to vent inside the structure.



RickWoodall said:
I have not hear of these mods and they are pretty timely for me. Can someone who has had good luck doing something different post some links of pics? Thanks
Click to expand...
 
I mounted my fuel vents on the bottom of the wing. The air input is through a parallel-wall, submerged duct 1/4" wide by 1/4" deep at the aft end, where it goes into a 1/4" tube that had been widened out into a square. The PWSD is one of the family of submerged ducts; its most well known brother is the curved, divergent SD or NACA duct. Keep the ramp angle at the recommended 7 deg. which means the duct will be 2" long. Since it is flush, there is almost no possibility of rain entering it, since there is almost no flow in a vent, and the rain wouldn't want to make that turn at 200 mph or thereabouts. A 1/4" tube vent should provide more than enough flow unless your FF is really incredible. My 1/4" vent line starts at the highest point outboard in the tank, goes all the way to the inner tank bulkhead, turns 180 deg and goes back out to the far end of the tank where it connects to the vent duct. This keeps fuel from spilling when yaw or an uncoordinated turn is present.
 
elippse said:
I mounted my fuel vents on the bottom of the wing. The air input is through a parallel-wall, submerged duct 1/4" wide by 1/4" deep at the aft end, where it goes into a 1/4" tube that had been widened out into a square. The PWSD is one of the family of submerged ducts; its most well known brother is the curved, divergent SD or NACA duct. Keep the ramp angle at the recommended 7 deg. which means the duct will be 2" long. Since it is flush, there is almost no possibility of rain entering it, since there is almost no flow in a vent, and the rain wouldn't want to make that turn at 200 mph or thereabouts. A 1/4" tube vent should provide more than enough flow unless your FF is really incredible. My 1/4" vent line starts at the highest point outboard in the tank, goes all the way to the inner tank bulkhead, turns 180 deg and goes back out to the far end of the tank where it connects to the vent duct. This keeps fuel from spilling when yaw or an uncoordinated turn is present.
Click to expand...

Lemme see... how do I say this in your language?

Here goes, "HUH"?

:confused:

Can this ADHD sufferer get a pic please?

I was doing an Angel Flight last week in a Cherokee 6 and upon landing I post-flighted the ship and discovered ice clinging to the tip tank vents. I chipped it away and launched back home.

Now, this got me thinking about the RV vents too!

I too, am very interested in a way of employing an ice RESISTANT method of venting the tanks.

Just to beat a dead horse... NOT FOR KNOWN ICING!

I know... I know.

:cool: CJ
 
Some folks drill a small hole in the back side of the vertical part of the external vent tube. This is supposed to provide an alternate way for the air to get in if the vent ices over. If the vent isn't iced over, the fact that this hole is quite a bit smaller than the forward facing hole means the vent will still be pressurized by ram air.

I'm planning on drilling this small hole, but I haven't yet ran the sums to figure out how big it needs to be to be sure that it can pass about 15 GPH of air.
 
Very interesting. That is definitely a simple solution easily adaptable to an already flying aircraft if it works.

Kevin Horton said:
Some folks drill a small hole in the back side of the vertical part of the external vent tube. This is supposed to provide an alternate way for the air to get in if the vent ices over. If the vent isn't iced over, the fact that this hole is quite a bit smaller than the forward facing hole means the vent will still be pressurized by ram air.

I'm planning on drilling this small hole, but I haven't yet ran the sums to figure out how big it needs to be to be sure that it can pass about 15 GPH of air.
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LifeofReiley said:
Do not put the prescribed vents in. Use an aluminum coil vent inboard the fuel tanks. Works well...
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This may be a good vent (Others, do a search, there are pictures on here.) only one problem, I don't see how this will solve the icing issue.

Up front, where Van's recommends might just be the best place as there heat from the cabin heater, thus you may never actually get any ice in that location.
 
N941WR said:
Up front, where Van's recommends might just be the best place as there heat from the cabin heater, thus you may never actually get any ice in that location.
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That was my thought. Heat transfer through the bottom skin from within the cockpit; and perhaps additional heat from the exaust pipes to the skin and surrounding air.

L.Adamson
 
Another solution is to "tee" a oneway check valve into the vent line inside the cockpit right at the top along the longerons. Have the cehck valve facing down so that the weight of the actual check valve keeps it closed. It takes very little suction to open it, and solves the icing problem as well as the mud dauber problem. You can put it on one or both tanks. I have it on one, and just remember to change it if icing conditions are encountered. Hopefully, the plan is to get out of icing conditions ASAP, so just the fuel in one tank should be needed.
I've not posted this before as I don't want to seem to encourage equipping our RV's for flight into known icing. But, as someone mentioned, the forecasts can be wrong. I only needed it once in 30 years, but once is enough. :)

Vic
 
Bill, Good point! I never thought about the possibility of coincidental inadvertent heating from the engine exhaust and the cabin floor!

I do also like the check valve idea, Vic!

:) CJ
 
L.Adamson said:
That was my thought. Heat transfer through the bottom skin from within the cockpit; and perhaps additional heat from the exaust pipes to the skin and surrounding air.

L.Adamson
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Not enough, at least under fairly extreme conditions.

There was an article several years ago in Sport Aviation about Bruce Bohannon's tiger. He described a deadstick landing from one of his record attempts.

At the extremely high altitudes he was climbing to, the oil cooler required a spray bar because of the thin air. The mist coming out of the cowl froze and blocked the fuel vents, resulting in a power loss.

The resulting negative pressure actually damaged the fuel tank skins.

I've wondered, at least on the RV-8, about relocating the fuel vent behind the gear leg fairing, like Cessna does with the fuel vents on 100 series airplanes. This would make a poor man's "inertial separator."
 
I installed an AN T bulkhead fitting with a cap inside the cockpit on the vent line. I have been told that if the vent line is blocked that the tank will start to collapse. I do not plan to fly this airplane into known icing. In the event inadvertent Icing I could remove the cap and vent the tank into the cabin (in theory). Not that I think that venting the tank into the cabin is a good idea. Some venting is better than no venting. I might put a valve on the tee rather than a cap. Kinda like alternate static venting.
007dy4.jpg

More pictures
008qn6.jpg

009ll0.jpg

Aaron
 
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AMURRAY said:
I installed an AN T bulkhead fitting with a cap inside the cockpit on the vent line. I have been told that if the vent line is blocked that the tank will start to collapse. I do not plan to fly this airplane into known icing. In the event inadvertent Icing I could remove the cap and vent the tank into the cabin (in theory). Not that I think that venting the tank into the cabin is a good idea. Some venting is better than no venting. I might put a valve on the tee rather than a cap. Kinda like alternate static venting.
007dy4.jpg

Aaron
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deleted --- A tee I see, or just don't actually see.... :)
 
L.Adamson said:
Problem is, that without venting, the fuel will eventually not flow. It's like all those old gas cans, that have a vent to unplug on the opposite end of the spout. You need air pressure, and a bit of ram pressure, as with Van's method, is even better

L.Adamson
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It is teed into the vent line it is not blocking the vent line. I still have the vents on the bottom of the plane.
Aaron
 
AMURRAY said:
It is teed into the vent line it is not blocking the vent line. I still have the vents on the bottom of the plane.
Aaron
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Yes, I figured that out, after a quick re-read of the AN "T" fitting & deleted; but you beat me to the punch. :).

L.Adamson
 
I have the standard 45 degree cut of bulkhead fitting per the plans (at least the 10 year ago -6 plans!). But, I drilled a small hole in the aft side just below the bulkhead nut to act as a "backup" in case the main forward facing opening gets iced.
 
I placed a bypass valve in my fuel vent. A simple on/off crank valve that is located near the pilots knee. Vents to the cockpit (guaranteed to never freeze) which is not ideal, but only for emergencies.

Be careful about the hole drilled in the line. It may leak during high temp environments (summer) and may not be large enough to feed the fuel at the correct rate (gph).

This icing over actually happened to the Mattiuck Exxon Flying Tiger. Engine quit and never restarted. Landed wfi. Of course he was at over 20k when it happened. During the decent the fuel tank collapsed.

Steve
RV7A
 
bjustus said:
<Title should say "Potential Icing in IFR" edit feature won't let me change it.>
(Done; S. Buchanan)
Talked to a rep at Van's yesterday about cutting off the forward bend in my fuel vents. I'm upgrading the panel for IFR flight and worry, (as several others have mentioned here in past threads related to pitot heat) about fuel vent icing potential.

He said not to cut them off with a 45 angle to the rear as I was thinking about doing as it may cause a negative pressure. "You don't want to get into any kind of icing conditions in an RV anyway," he said. I agree, but the only way you can absolutely guarantee that is never go into a cloud. Forecasts can be very, very wrong, and icing can come up when you least expect. Cherokees, Grummans, etc. fly IFR all the time with flush or angled vents that do the job and don't ice up -- what am I missing?

Any suggestions/solutions out there?

Thanks,

- Brett
Click to expand...

Brett,

Just to keep things in perspective, in a previous life I had job flying equipment that was certified for flight in icing conditions. Millions of dollars were spent to achieve that certification and even after that, the operation was restricted. If the weather report (or forecast) included moderate or heavy freezing rain the operation was shut down.

Granted, freezing rain is a most severe type of icing one can encounter but the point is - no matter what you do to provide for icing conditions with the RV, it will not be good enough. Any air frame ice will degrade performance, there is no way to prevent ice formation and no way to remove it. Power available is limited as is, with ice on the prop, wings, engine intake, windshield, and control surfaces, the airplane will be descending much sooner than later. The only anti ice system available, pitot heat, will not save the day.

The RV series of airplanes (by design) are basic VFR flying machines. Yes, they can be equipped for IFR flight and many are, but a safety envelop for such flight can not include icing conditions of any type. By definition, icing conditions are visible moisture with the OAT at 0 degrees C or less.

I say again, you can not make the airplane safe for flight in icing conditions. It it is not prudent to even think about it.
 
I can't agree more that an RV should not/can not be made, "safe for icing conditions." That was never my intention and I would never fly one in icing conditions, or even close. My point is that even when you think those conditions don't exist, conditions can quickly change, forecasts can be wrong. Been there, done that. I don't care how careful you are, the only way you can absolutely guarantee no ice ever is to never penetrate a cloud. If you are going to go in the clouds it's nice to know that if you get some of that unforcast, unexpected icing, (or for that matter, unforcast/unexpected clouds that happen to produce ice) it's nice to know that your engine will keep running and your wings won't crumple up like a Coke can.

- Brett


David-aviator said:
Brett,


I say again, you can not make the airplane safe for flight in icing conditions. It it is not prudent to even think about it.
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I had the opportunity to experience a fuel tank vent ice up this past Sunday that resulted in an engine failure in my RV-8 while IMC in heavy snow. No fuel tank collapse was noticed from the cockpit.
conditions were:
Decending out of 10k'(clear on top) to 6k' IMC heavy snow. Surface was 400', 1mile vis. 28degF blowing snow.

25deg OAT. 6k', level for ~10 min on approach to home base. Engine began leaning. Over a 5 minute period leaning continued unitl Engine quit with fuel pump on. Injected motor.
Switching tanks solved the problem.
No airframe icing experienced. No ice build up on the vent pickup. Freeze was up the line somewhere. Vent lines are at plans position, straight down 1" from fuse, cut 45deg into the wind.

Just another data point.
 
Kahuna said:
I had the opportunity to experience a fuel tank vent ice up this past Sunday that resulted in an engine failure in my RV-8 while IMC in heavy snow.
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First of all, glad you're OK. :)

How do you know this was vent related? How do you know where the blockage occurred? Couldn't it have been the vent, but the blockage melted before you were able to get eyeballs on it? Or was it still blocked on the ground? How long did it take to clear? Did it only clear after heating (i.e. putting in heated hangar)?

I for one would be very interested in any route cause diagnosis and recommendations (design or operational) you might come up with as a result of this incident.
 
I worried quite a bit about a blocked vent. In the end, I moved the vents more towards the centerline on my RV8 and covered them with a screen to keep the insects out. The inboard position is just outside the exit ramp from the engine.

My reasoning (not verified by any means) is that the warm engine exit air will keep the vent ice free, but I don't plan on testing it or ever approaching any possible icing conditions.

Another backup approach would be a 1/8 inch hole somewhere up in the vent line (probably in the gear tower). This was fairly commonly done on BD4's and worked very well. On the BD, the vent line came from the top of the wing tank and ran to the rear of the wing and exited with a bend forward, much like an RV. The hole was drilled on the top of the vent line inboard in the wing. No problems with fuel feeds were reported, but of course the BD's fuel system was quite different from an RV's (and quite a bit more troublesome in other aspects). So, I'm not recommending this to anyone, just bringing more info about the subject to the discussion.
 
I think the risk of iced up fuel vents is overblown. I installed the fuel vents per the plans with insect screens glued on. On a couple of occasions, I've had the pitot tube (Vans plain stainless steel tube) freeze-up with the indicated airspeed winding all the way down to zero. The engine (O-360 carb with fixed-pitch prop) just purred along, without a hiccup.

The pitot tube cleared, on cue, as I descended into the warmer air below. The engine didn't know the difference.

The fuel caps seal pretty well, much better after I adjusted them and installed new o-rings, but they still let in a little water when it sits out on the ramp in the rain. I the water gets in around the cap, air can get in. Sounds to me that you'll have your backup fuel vent there.
 
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