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Carb heat

Old 10th Dec 2005, 15:00
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Carb heat

My aircraft has a Lycoming O-235 which, legend has it, is prone to carb icing. I therefore give it a blast of hot air about every 10 minutes in this weather. Typically today Stansted were giving 6 degrees with a dew-point of 4 degrees. So cruising revs of 2450 set, blast of carb heat for 30 seconds, revs drop to 2350, no coughing and banging, push in carb heat knob and the revs only return to 2410 or thereabouts. It always happens....why?
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Old 10th Dec 2005, 15:35
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Carb icing

I have the wreck of a once beautiful Ryan PT22 in my garage.

Cause of the accident? Carb icing.

Unless you have a serious build up there won't be any significant coughs or bangs. What you are doing is bang on the money and almost certainly removing the initial ice deposits.

Dont do what you are doing and the build up is quite rapid thereafter.

I'd fly with you any time.

HP
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Old 10th Dec 2005, 16:08
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So cruising revs of 2450 set, blast of carb heat for 30 seconds, revs drop to 2350, no coughing and banging, push in carb heat knob and the revs only return to 2410 or thereabouts. It always happens....why?
Hot air less dense than cold, therefore less power, therefore RPM drops.

RPM drop is a the check for carb heat working properly, so that's good. Try keeping it hot for a couple of mins and see if it recovers to 2450 - if so, you had a little ice and the carb heat needed more tim eto totally clear it, if not, its just a quirk of your aircraft .... and they all have theor own little ways .

Just keep setting it to ''on'' and checking, it's good practice
 
Old 10th Dec 2005, 16:38
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I can understand how the carb ice forms, with the venturi effect in the carb intake dropping the temp of the air dramatically etc, and having read about carb icing incidents on the AAIB site and in the CAA GASILS (all thankfully available online), I for one will certainly be getting into the habit of a good 15sec+ burst of carb heat, at least every 10 mins, and 'always on' on low power settings eg descents (apart from low power on the ground, due to the unfiltered air).

One thing I wonder about though. I'd never heard of carb icing before taking up some flying training. In the 'good ole' 70's, when only the expensive cars had fuel injection, how come it didn't happen in normal auto carburettors - since they have venturis also? Suppose they must be diferent design to the aero carbs?
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Old 10th Dec 2005, 16:55
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auto carbs

Hi There,

Car engines have been known to suffer from icing. In my days as a student in the early 90s I was running a 1967 Triumph that would 'loose power' on motorway driving, to the point where I couldn't maintain speed. After a few minutes standing, whilst I scratched my head, I could restart it, drive off and everything would be fine - for about 1/2 hour, when it would loose power again. Only when I started flying did I work out what used to cause this problem. Some seventies cars used to have a vacuum operated device that would draw hot air into the filer from a shroud round the exhaust manifold on any power setting except full throttle.

The seventies cars lead to an interesting thought - why don't light aircraft draw hot air to the filter, instead of using unfiltered air when we pull carb heat on?? I was getting icing whilst taxying and holding at Coventry this morning, so using carb heat on the ground was essential, despite the fact that this can draw dust and debris into the engine. . .

Cheers,

David
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Old 10th Dec 2005, 16:57
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It did happen to some cars. My wife, before were married, had a long time ago a Mini Clubman. On cold moist days it would sometimes stutter to a complete stop. After a short wait (during which the ice melted from residual heat in the engine), it would start and behave fine - for a while.

SSD
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Old 10th Dec 2005, 17:01
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Unless I'm mistaking, what Ozplane is asking, is why does the engine not return to 2450 rpm, once he removes the carb heat? ie. why has it lost 40 rpm, by only returning to 2410.

I don't know the answer. The only guess I can make, it that the aircraft has slowed a little while the carb heat was applied, because of the reduced power. Then, if it's a fixed pitch prop, it won't quite develop the same rpm until the speed of the aircraft through the air has caught back up to where it was.

Having said that, I'd be skeptical if that would make the difference of 40rpm.

dp
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Old 10th Dec 2005, 17:23
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Presumably it must return to normal or else the throttle would move further and further towards the dash as CH is applied.
 
Old 10th Dec 2005, 18:17
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Dublinpilot

I use a rough rule of thumb that says -50rpm = 100fpm descent on a fixed prop.

So your theory may be spot on - the pilot would unconsciously hold back pressure (or retrim) to prevent a 200fpm RODwith the carby heat on and the A/C would slow.

On returning the heat to normal, the AC would need to accelerate back to the trimmed speed or be retrimmed and a drop of 40rpm doesn't sounds daft at all.

Anyone else like to comment?
 
Old 10th Dec 2005, 19:13
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Thanks aerobat and Shaggy for the replies.

Although in danger of wondering off-topic for the thread, it's good to see that the physics 'hold true', and that auto engines did indeed suffer.

May well be the reason that my first car (Mk 1 Escort) had trouble some mornings - and I always put it down to flooding ?

Like the Triumph ref aerobat. I learned to drive in a Dolomite, and my brother had, at the time, a Triumph 2.5 PI (fuel injection) with a switch on the gearstick for 'overdrive' (5th gear to the younger ones).

Ahh - the nostalgia

cue Mr Clarkson......
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Old 10th Dec 2005, 19:33
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Speke2me

1 - 15 seconds is probably too little, I leave carb heat on for 1-2 mins, depending on the conditions

2 - you MAY sometimes need it on the ground, I have experienced carb ice iduring taxi, in a PA28, normally not known as an airframe/engine combination that is particularly susceptible, which just goes to show that you cannot be complacent
 
Old 10th Dec 2005, 22:11
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GASCo (General Aviation Safety Council) want to see accidents through carb ice become a thing of the past and would like to see changes.

The use of unfiltered hot air is probably due to the difficulty of ducting filtered air to the heater muff. Pressure at the carb intake has to be lower than atmospheric, otherwise air wouldn't flow through the carb. Most hot air muffs leak all around, not a problem for heat because the air being drawn in passes over the exhausts and warms up. If however you tried to draw it via the filter the air would find it much easier to flow through the leaks than through the filter.

One solution would be to fit the filter between the hot air valve and the carb but there's no room in the vast majority of installations.

In practice, while people are worried about unfiltered air, icing requires humidity, which in turn usually means that conditions are not dusty.

I like the idea of hot air automatically being selected when the throttle is closed. It wouldn't be difficult to devise a linkage that would also slightly lean the mixture. You could add a ground/air control enabling you to prevent unfiltered air being selected while on the ground.

The application of modern automotive techniques such as fuel injection and variable ignition timing would greatly help economy. Maybe the rise in fuel prices might help stimulate something in this direction.

Apropos of cars, I well remember having a car with "Winter" and "Summer" settings for the air filter. It clamped on to the top of the carb and could be swivelled so that its intake was nearer the exhaust manifold in winter.
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Old 11th Dec 2005, 03:44
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The classic carb heat scenario is repeatedly increasing throttle to make up for increasing amounts of ice. This is a form of painting yourself into a corner

I never just add throttle if I seem to be losing rpms, airspeed and/or altitude without first checking for carb ice.

Cruising close to a ceiling puts you into a situation where the temperature and dewpoint spread is close and carb ice is more likely. The first symptom can be a small loss of altitude from a loss of a couple dozen revs on a fixed pitch prob -- or MP on c/s.
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Old 11th Dec 2005, 14:34
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Thanks for your answers chaps. I was hoping to try out a few of the theories today but the horizontal viz never quite got to my self-imposed minimum 0f 6km. Mind you that didn't seem to worry an Extra which I just saw heading North at about 200 feet. Rule 5 anybody? Anyway I tend to dublinpilot's theory as the Airtourer needs to be flown "on the step" and the slight reduction in airspeed when I apply carb heat is probably enough for it to come slightly out of trim. I'll report back after the next trip.
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Old 11th Dec 2005, 20:56
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Everybody should just vote with their wallet and never buy anything that flies unless it is fuel injected.
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Old 12th Dec 2005, 10:58
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Or water cooled !!
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Old 12th Dec 2005, 11:17
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Everybody should just vote with their wallet and never buy anything that flies unless it is fuel injected.
I completely agree. Hopefully then I'll be able to pick up some really interesting aircraft for a bargain
 
Old 12th Dec 2005, 13:44
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My old RS2000 used to get carb icing and at times, I would need to floor the throttle to open the second choke on the Webber Carb to keep the engine running. Replacing the hot air ducting under the air filter sorted the problem out.

I'm amazed that in the 21st century, carb heat is still a manual operation when the consequences of getting it wrong can be fatal. The Luftwaffe sorted out fuel injection almost 70 years ago.

Can anyone explain why icing is more prevalent at low power compared to full throttle ?
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Old 12th Dec 2005, 13:50
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Can anyone explain why icing is more prevalent at low power compared to full throttle ?
I would guess that it is because the air is moving more slowly so the ice has more time to form within the carb. In addition, carb heat is less effective at low RPM due to lower exhaust manifold temperature.
 
Old 13th Dec 2005, 11:24
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Can anyone explain why icing is more prevalent at low power compared to full throttle ?
and it's because the pressure differential across the butterfly is greater.
 

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