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Venturi-icing?

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Venturi-icing?

Post  roddie on Fri Dec 16, 2016 6:23 pm

This thread has obviously gotten your attention.. but it may be a stupid topic. For that; I take full responsibility. Laughing It was really cold here today.. (11F when I left for work this morning). I'm always (daily) thinking about flying a model airplane. A site-search here for a thread on "Winter/Winter-Flying" doesn't bring-up much. There however have been threads here.. where modelers have mentioned flying in freezing/near-freezing temperatures.. and advice on what they've done to overcome some obstacles.

I'd done a little reading on the effects of carburetor-icing on full-scale aircraft, and today it made me wonder if this could become an issue with our small-displacement engines.. under certain atmospheric-conditions/temperatures...? How would you know? You'd need to actually see the engines' venturi in flight.. wouldn't you? Once you land the plane, the engine-heat would probably sink into the case and melt-away any evidence of icing. If this was actually happening.. what could you do about it? If you've ever experienced your engine's fuel-mixture turning "rich" into a cold-weather flight.. could this be the reason? Could the phenomenon be magnified with small-displacement engines?

Warming the fuel... and wrapping a Cox glow-heads' fins with a few turns of sewing-yarn have been discussed to get the engine to start and run in cold weather.. but what about when the models' in the air tearing-up the sky? The wind-chill will surely have an effect on how the engine is breathing. Could the intake fuel-air charge be warmed somehow.. to offset the effect?

I leave this for you to ponder...  Eyebrows
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Re: Venturi-icing?

Post  balogh on Sat Dec 17, 2016 3:20 am

Roddie

A good point. I experienced venturi frost with old Soviets-era pickups my company had 30+ years ago where the air intake nozzle on the filter box had to be manually turned in winter towards the exhaust manifold so as to suck in warm air. Once I forgot to do so and when checked the erratically running engine I found a nice ice cone built on the carb intake. It was minus 10 Celsius ambient.

With the small model engines I have never seen venturi icing when flying in subzero temperatures probably because the venturi is close to the hot crankcase and engine heat reaching the venturi by conductance  prevents it from freezing. This is especially so with metal backplate and metal carb body COX engines but even plastic components will allow some heat conductance to keep the venturi above freezing temperatures.

By the way because of the adiabatic expansion that causes temperature drop by itself the ambient temperature need not be freezing to expect venturi frost. A few degrees above freezing is already a zone from where the expanding air may end up freezing it's humidity in the venturi.

You see a similar phenomenon when seated over the wing of a passenger airplane and streaks of frosty air are seen on the top of the wing (air undergoes here adiabatic expansion) during landing.
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Re: Venturi-icing?

Post  Oldenginerod on Sat Dec 17, 2016 4:33 am

balogh wrote:Roddie

..... adiabatic expansion that causes temperature drop by itself the ambient temperature need not be freezing to expect venturi frost.

I'm not at all familiar with the term used, but in a previous life I used to install & service car air conditioners. Sounds like the same principal as the compressed gas expanding once forced through a restriction, causing it to absorb surrounding heat. Opposite effect to air temperature rising under compression (as in a compression ignition engine.)
Just having an air conditioner installed in my house today, so I've been thinking about this stuff.
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Re: Venturi-icing?

Post  balogh on Sat Dec 17, 2016 4:38 am

Yes Rod these all flow from the laws of gases like the Boyle-Marriotte and others.
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Re: Venturi-icing?

Post  KariFS on Sat Dec 17, 2016 6:18 am

I have had freezing in full-scale carburettors a few times. Usually the temperatures have been just above the freeze and the conditions somewhat humid.

Once in my '58 Saab two stroke, it has a snorkel thing that should be installed between the air intake and a shroud on the exhaust manifold, but some parts had been changed and it would not fit. So, I started to lose power and limped to a packing lot of a roadside cafe. After maybe 10 minutes enough heat had transferred from the engine to warm up the carb, and it was fine. This Saab has a bit different design, the engine is way up front and the radiator behind it, so the engine bay runs a bit cooler than usual.

Another time, my lawnmower (Briggs&Stratton) had the same symptoms, and the remedy was the same too. Left it for a few minutes, had some coffee and it was fine.

The freezing is caused partly because of the way the relative humidity and dewpoint change when the air pressure and speed changes in the carburettor, but another significant reason is the heat that the fuel absorbs as it vaporizes. If you get fuel (gas, methanol, ethanol, kerosene, ether, whatever) on your hands you will notice it feels cold, that's the same phenomenom. When the temperatures are far below freeze, the air is so dry that there is no risk for frost buildup.

Of course, sometimes water is accumulated in the fuel tank, that may cause problems too. Nowadays we have ethanol in the fuel, the only thing good about it is that it should absorb water and carry it into the engine and away from the tank. Back in the old days when gas smelled like gas and there was lead in it, no ethanol, engines ran better, but on the other hand, we had to add some isopropanol to the fuel to avoid water accumulation.

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Re: Venturi-icing?

Post  KariFS on Sat Dec 17, 2016 6:28 am

Oh, Andras, when you mentioned the pickup truck, did you by chance mean this beauty:



That is of course the utility vehicle "IZH", they also made motorcycles like the 1951 model 350, it was based on the German pre-war DKW:



A lot of both were imported to Finland too. IZH is nowadays a member of the Kalashnikov group... Besides the utility vehicles and motorcycles they also made rifles.

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Re: Venturi-icing?

Post  balogh on Sat Dec 17, 2016 7:14 am

Yes, Kari, exactly the IZH, but with the boxed cargo area... I think apart from the DKW replicas IZH made, the 4-stroke boxer engined IZH motorcycles - also seen in the past here in Hungary - were the replicas of the BMW-s that the Soviet Union made after the Germans had paid world war II indemnification to the Soviets by shipping plants like a fully fledged BMW motocycle production line...

I am not a weapons expert but the IZH vehicles now made by the Kalashnikov group definitely "hang fire" more often than the AK semiautomatic assault rifles.
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Re: Venturi-icing?

Post  KariFS on Sat Dec 17, 2016 8:14 am

Those 4-stroke boxer twins are sort of fascinating bikes. Heavy and utilitarian, shaft-driven beasts, simple bullet-proof technology (provided that it is first sorted out by the owner), some of the sidecar-equipped ones even had reverse gear. In Finland they were called "Ural" and "K-750". One had side valves and the other was OHV.

I am too young to remember if I have ever seen any of these, or the numerous Jawas, MZ's or Pannonias (these were from Hungary, right?) that were imported in the '50s and '60s in daily use, but nowadays many of them have quite a following here, especially Jawas. Same kind of nostalgia, I guess, as it is with our beloved Cox engines Smile

But I do remember the Soviet cars, as well as the old-school Skodas with rear engines, the 2-stroke Wartburgs from the DDR as well as Polski Fiats (FSO). Can't really say I miss any of them Smile When the iron curtain fell, a lot of Russians came to Finland as tourists, smuggled vodka and some military surplus (funny hats, submarine captain watches, badges and such), sold them here and bought practically all the VAZ "Ladas" and took them back to Motherland. Fair trade, I'd say lol!
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Re: Venturi-icing?

Post  rsv1cox on Sat Dec 17, 2016 9:55 am

Loved Kari's picture of the bike and truck, especially the truck.  Reminds me of the early '70's Mazda Wankel pickups.  Always wanted one.

Bird feeder icing.  This morning West Virginia.



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Re: Venturi-icing?

Post  roddie on Sat Dec 17, 2016 11:20 am

Yup.. that's bird-feeder icing for sure. Maybe gutters and a down-spout would help prevent that.. Huh...

I feel really bad about my lack of bird-feeding lately. I used to enjoy it. The grey squirrels are gluttons. I had a terrific squirrel-baffle that worked. It was a clear-acrylic dome.. and after a couple of years, started to craze/crack until it finally couldn't be repaired with tape anymore. I bought a "tin" conical baffle on-clearance last Spring. It was missing the pole/pivot-mount. The baffle needs to tip about its' axis to be effective. I made a pivot/sleeve out of foam pipe-insulation and used hose-clamps to secure it. The squirrels ate the foam.

I currently have 15-20lbs. of custom-blended seed.. and a half dozen suet-cakes. The squirrels can and will eat 5lbs. of seed in two days. That's the amount of seed it takes to fill my two feeders. One feeder is a tube-type for "perching" birds.. and the other is a trough-style with an over-hanging roof. Between and below the seed-feeders are two suet-baskets that are more squirrel-resistant. I wrap tie-wire around the basket-handles and around screw-head posts in the pole.



The Cardinals are shy and illusive. Thistle is one of their favorite seeds. It's expensive.. and if the squirrels get to it.. they'll empty a trough-feeder in a day.

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Re: Venturi-icing?

Post  RknRusty on Sat Dec 17, 2016 8:26 pm

My best friend back in high school days was given a dilapidated Zundap motorcycle that we wanted to fix up. But it was too far gone and beyond our technical and financial abilities. Any of y'all aver heard of that? German, I believe. We ended up resurrecting a 6 cyl. 54 Belair with 3 on the tree that got us around town for several years. We made a front bumper from a pine 4x4. Two tone blue and white. That was a good ol' car.

This is more or less what the Zundap looked like:



Off topic, I suppose, since we don't have carb icing on wheeled vehicles in SC.
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Re: Venturi-icing?

Post  Oldenginerod on Sat Dec 17, 2016 9:48 pm

I don't believe model engines stand much chance of icing, as mentioned, due to the close proximity of the venturi to the engine. Vehicle carburettors are generally insulated from the manifold by a fibre block, to save vapourization. With the venturi as part of the engine block, I can't see that on a running engine it could possibly get cool enough to ice.

I had a 1976 Corolla that I fitted early Sprinter twin carbs to. Never had the original air cleaner, so just a couple of chrome Lynx foam sports air cleaners. I had to drive an hour to work at the time, mostly at highway speed at 6.30 in the morning, and those carbs used to ice up as quick as anything. I learnt where every slight down-hill undulation was in that road and I'd throw the clutch and coast for a few seconds. The engine would generally stall but it only took a few seconds for the ice to melt, I'd drop the clutch and we were off again. If it got real bad I'd pull over and again, it only took a few seconds for the ice to melt once the heat started to rise from the manifold.
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Re: Venturi-icing?

Post  fit90 on Sat Dec 17, 2016 10:58 pm

The few times I had experienced carb icing in full size planes were all in visible moisture. You would have to be flying in an exceptionally humid environment to experience venture icing without actually being in the condensation (clouds). Although, it is a possibility I wouldn't bet on it.
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Re: Venturi-icing?

Post  Oldenginerod on Sun Dec 18, 2016 1:39 am

fit90 wrote:The few times I had experienced carb icing in full size planes were all in visible moisture. You would have to be flying in an exceptionally humid environment to experience venture icing without actually being in the condensation (clouds). Although, it is a possibility I wouldn't bet on it.

I recall my late brother's Luton Minor with a VW engine fitted icing the carb while sitting running on the ground. It was a "cool" day, but certainly not cold or particularly damp like many of you experience way up north from us.
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Re: Venturi-icing?

Post  roddie on Sun Dec 18, 2016 9:12 am

Thanks for all the comments guys. I've enjoyed reading them.. on-topic or otherwise. Better to have a thread evoke thought and conversation.. than to have it languish in disinterest.

It seems to be the general consensus that venturi-icing is unlikely on our model engines. I guess I can understand it being less likely in a front/rotary-valve induction because of the additional heat generated in the front of the case by the crankshaft. A rear rotary-valve or reed-valve engines' venturi is more isolated. I think too much sometimes about such things.. but I have to wonder. You couldn't actually "see" it happening with a reed-valve engine, because of its' radial mount.. but you might see icing with a rear rotary-induction engine running on a stand outdoors.. under the right conditions.

As I mentioned, the weather here in Rhode Island was chilly last week.. with the coldest being on Friday morning (11F). Yesterday we got 6" of snow and temps held at 28F through most of the storm. By the time it ended.. it was too dark to drag the snow-blower out of the shed, and I was involved all day doing some trim-work in a room downstairs. Temps were forecasted to rise.. so I gambled. This morning it's overcast in the mid-50's.. and the snow is almost gone. Crazy weather. Foiled my plans to go to the post-office yesterday to mail a few packages.
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Re: Venturi-icing?

Post  fit90 on Sun Dec 18, 2016 11:06 am

Oldenginerod wrote:
fit90 wrote:The few times I had experienced carb icing in full size planes were all in visible moisture. You would have to be flying in an exceptionally humid environment to experience venture icing without actually being in the condensation (clouds). Although, it is a possibility I wouldn't bet on it.

I recall my late brother's Luton Minor with a VW engine fitted icing the carb while sitting running on the ground.  It was a "cool" day, but certainly not cold or particularly damp like many of you experience way up north from us.

I wonder if there was a big bend in the induction system before the carb. The pressure will decrease as the air is going around the bend and allows for condensation which can lead to icing. That sounds like it was a cool and fun plane. I hope you two had the opportunity to enjoy it together.
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