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HeliTorque :: View topic - Carb Icing
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HeliTorque Forum Index » Flight Dynamics

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afterburner
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PostPosted: Sun Feb 15, 2009 7:44 pm    Post subject: Reply with quote

HaggisHunter wrote:
And yes, if you add carb heat it may raise the temperature in the carb and induce icing if the correct conditions are present.
HH


I have to disagree with you here. UNLESS the conditions are present for carb ice, using carb heat should not "induce" such a condition. It will however, change the fuel air mixture and create an engine roughness problem and if left to persist, can result in an engine shutdown.

Yes, the colder air is less likely to form ice in the venturi, but pumping hot air into the venturi cannot create icing conditions that did not exist prior.
At least from my understanding of carb icing, which has never been challenged in the more than 30 years of flying and teaching.

But then again....I'm always willing to learn. Laughing
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PostPosted: Sun Feb 15, 2009 7:48 pm    Post subject: Reply with quote

HaggisHunter wrote:
Dick is a man of incredible knowledge. Did you do the safety course with him?


I did indeed, and we have another one running at Coventry in early July if anyone is interested. I thoroughly recommend every Robinson pilot does this course.

Another thing he talked about related to this subject was pulling a lot of carb heat in the hover will often put you close to, at or even over your MAP limit. I struggle to find a good way to explain this to students. In my opinion we should never encourage anyone to exceed a limitation, but Dick's take on it was that if the limit on a particular flight is say 24"MAP, and with carb heat applied in the hover you end up seeing 24.2" on the gauge, then you're not actually pulling 24.2" in equivalent horsepower, i.e. if you are hovering with no carb heat and you get your passenger to pull the carb heat full hot, the MAP gauge indication will show around 1" more but technically you are not actually pulling any more horsepower from the engine.

I'm not really sure how to explain this to students without them getting the wrong idea and thinking it's ok to bust a limitation. Have any other FIs come across this issue either from the safety course or otherwise?

Sarah
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PostPosted: Sun Feb 15, 2009 7:55 pm    Post subject: Reply with quote

As an afterthought....

It is not cold air that causes or prevents carb ice, but the relative humidity of the air.

You can have carb ice on a warm summer day when the temp is 38C with relative humidity as low as 50 percent!

It normally occurs at temps below 21c and RH at or above 80%. The likelihood of icing increases as temperatue decreases down to 0c, and as relative humidity increases..
Below freezing, the possibility of carb icing decreases with decreasing temperatures.

I know that you are all aware of this , but I'm bored, so I'm a bit more chatty than usual. Laughing Laughing

All I do is go "up and down" ...."up and down" Heli Boy
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PostPosted: Sun Feb 15, 2009 7:57 pm    Post subject: Reply with quote

AB, you are right. Brain fart on my part, possibly my hangover that caused it.

HH
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PostPosted: Sun Feb 15, 2009 8:13 pm    Post subject: Reply with quote

Sarah, hows this: if the aircraft is hovering with 10 deg on the carb guage, it is using x amount of horse power and putting y amount of torque through the gearbox, and we are at 24" MAP.

If we now pull 20 deg on carb guage the MAP might go to 24.5" for example and break our limit. We are still doing the same hover, we therefore must be using the same x horsepower and still putting the same y torque through the gearbox.

At least thats the way I explain it to my students....
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PostPosted: Sun Feb 15, 2009 8:16 pm    Post subject: Reply with quote

WhirlyGirl wrote:
Another thing he talked about related to this subject was pulling a lot of carb heat in the hover will often put you close to, at or even over your MAP limit.

Why would you be in a hover at max MAP? How do you expect to safely climb out? You can always toss out a passenger, but this tends to be bad for public relations. If you are at max MAP in a hover, then maybe you shouldn't be in a hover at all.

Quote:
Dick's take on it was that if the limit on a particular flight is say 24"MAP, and with carb heat applied in the hover you end up seeing 24.2" on the gauge, then you're not actually pulling 24.2" in equivalent horsepower.....

Is that what Dick said? Strange. I would think that since we are changing the volume of air entering the carb, then the engine output must change to compensate for the load we are placing upon it, therefore the MAP rises to indicate this. I am not an engine guru, but how can the gauge read an MAP if the engine is not supplying the corresponding horsepower?

Well, right now I'm thoroughly confused by Dick's statement.

Any A and Ps out there with some insight?

AB....\
be back. Tea pot is whistling. Going inside for a spot. Laughing
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PostPosted: Sun Feb 15, 2009 8:21 pm    Post subject: Reply with quote

Just found SN-31 on the Robinson site. It says:

Quote:
GOVERNOR CAN MASK CARB ICE

With throttle governor on, carb ice will not become apparent as a loss of either RPM or manifold pressure. The governor will automatically adjust throttle to maintain constant RPM which will also result in constant manifold pressure. When in doubt, apply carb heat as requiredto keep CAT out of yellow arc during hover, climb, or cruise, and apply full carb heat when manifold pressure is below 18 inches. Also remember, if carb heat assist is used it will reduce carb heat whenyou lift off to a hover and the control may require readjustment in flight


Doesn't mention the very cold temperatures thing though.

Sarah
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PostPosted: Sun Feb 15, 2009 8:28 pm    Post subject: Reply with quote

I think it's mentioned in the Systems Description section Sarah, unfortunately I don't have a flight manual here to check.

HH
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PostPosted: Sun Feb 15, 2009 8:32 pm    Post subject: Reply with quote

The MAP is only a pressure guage indicating how much the throttle has opened to maintain the engine output at a certain horsepower. It is only a guide to engine power, a pretty good one if the temp is just out of the yellow, but at any other temp it is telling us the wrong thing.

As far as hovering near max MAP, when are we not, whilst instructing in summer with some of the chubbies we teach.
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PostPosted: Sun Feb 15, 2009 8:34 pm    Post subject: Reply with quote

.....and whilst I'm ranting, its also possible that you are over-torquing if the gauge is reading very cold. Just a thought.....
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PostPosted: Sun Feb 15, 2009 8:37 pm    Post subject: Reply with quote

afterburner wrote:


Any A and Ps out there with some insight?

AB....\
be back. Tea pot is whistling. Going inside for a spot. Laughing



I could be wrong, but my understanding is that the MAP limit is derived from the current 'actual' OAT and pressure altitude.

If you then artificially heat that air before it enters the engine (with carb heat) then you're effectively moving up the y axis (OAT) on your MAP limit chart, meaning your MAP limit will increase.
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PostPosted: Sun Feb 15, 2009 8:46 pm    Post subject: Reply with quote

HaggisHunter wrote:
AB, you are right. Brain fart on my part, possibly my hangover that caused it.

HH


At least you have a hangover! You young pups are all alike Laughing Laughing I remember when..... Those were the days. If you had a bad one, who could remember. Laughing Laughing

No problem. We all get a bit "fogged" at times. Laughing Laughing Laughing

Cheers!
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PostPosted: Sun Feb 15, 2009 8:50 pm    Post subject: Reply with quote

WhirlyGirl wrote:
Just found SN-31 on the Robinson site. It says:.............

Doesn't mention the very cold temperatures thing though.Sarah


Yes precisely. Cold temp thing really is not a factor, but both R-22 and 300 use carb heat almost under the identical conditions for hover, and in flight.

Good post. Thanks.

AB
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PostPosted: Sun Feb 15, 2009 8:52 pm    Post subject: Reply with quote

Heliwhore wrote:
As far as hovering near max MAP, when are we not, whilst instructing in summer with some of the chubbies we teach.

Yes, that is why I recommended tossing one out on takeoff. Laughing Laughing Laughing

Why do we do it??????

Because we love it!!!

Laughing Laughing Laughing
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PostPosted: Sun Feb 15, 2009 8:56 pm    Post subject: Reply with quote

Heliwhore

I believe you have a pretty good take on it, MAP is not an absolute measure of horsepower but its the closest thing we've got in most pistons so its what we use.

Power as we all know (at least the iinstructors will) is RPM * Torque. If you leave the lever where it is (leave the torque demand alone ) and pull carb heat in the hover the RPM wants to go down, manually opening the throttle to maintain RPM or allowing the governor to do so will put the engine power output back where it was (without changing the ERPM). The MAP has gone up because the throttle has been opened, but the air density has gone down so the engine power output has decreased for the corresponding throttle setting because the air is warmer.

As to the issue of busting limits, AB is right why do you need to sit in the hover on the MAP limit in the first place (can't say I've never done it particularly at the start of a series of Trial lessons on abusy Saturday or over the hedge in a fully loaded A109) just put less fuel in, or would that demonstrate that perhaps the helicopters we use for training aren't quite as flexible as we would like to believe). Some even have CAA supplements that point out the fact that the engine doesn't deliver rated output and the aircraft won't meet the rate of climb performance the RFM specifies (we do all know whats in our flight manuals don't we ?). If you are stuck in that situation I would have to agree with Dick but its not exactly a good practice to be doing without any support from the RFM ( I don't remember there being any alleviation in the POH but I might be wrong), but then there is nowhere in the POH/RFM that allows you to operate with the RPM down at 80 odd percent as demo of low RPM recovery on the safety courses but that goes on as well. Don't read the last bit as a criticism of Dick or his courses, he is a good man with an intimate knowledge of his subject, the job he does is a fantastic one.

GS
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