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

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Bierbuikje
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PostPosted: Sat Jun 11, 2005 9:34 am    Post subject: Carb Icing Reply with quote

There was a lot of questions on another posting about carb icing so I felt the need to start a new post.

There is a lot of talk about Carb Heat & Carb Icing so just to confirm this is how the carb heat should be used on Robinson helicopters.

R22 Above 18" MAP (Hover, Cruise, Climb)
Apply enough carb heat to keep the CAT gauge reading +10?? to +15??C

R22 Below 18" MAP (Descent, Autorotation, Quickstop)
Apply FULL Carb Heat ATLEAST 15 - 20 seconds in before lowering the collective.

I know pilot's who have pulled the carb heat fully hot and lowered the collective straight away to descend and the engine has stoped due to carb icing, They should have APPLIED CARB HEAT ATLEAST 15 - 20 SECONDS IN ADVANCE and this would not have happened

On the R22 the sensor for the CAT gauge is located before the throttle butterfly.

So at low power setings with the throttle butterfly nearly closed you have a second venturi effect around the throttle butterfly.

This gives a colder area downstream of the throttle butterfly, ie Colder than where the CAT gauge sensor is located.

Result Below 18" MAP the location of the sensor can be above freezing temp but after the throttle butterfly (Because of the second venturi effect) can be below freezing temp.

This is what Robinson mean when they say below 18" MAP the CAT Gauge is unreliable, They just don't explain why.

R44
On the R44 Robinson placed the CAT sensor in the correct place, Down stream of the throttle butterfly, So you get the temp reading for the CAT gauge at the coldest place.

R44 (Cruise, Hover, Climb)
Apply enough carb heat to keep the CAT Gauge above the yellow ark.

R44 (Decent, Autorotation, Quickstop)
I still apply full carb heat at least 15 - 20 seconds in advance before descending.

On finals the Carb Heat should be re-set to the partial position for both R22 / R44 at about 200 ft AGL.

For those who fly Robinson's with CARB HEAT ASSIST don't forget this is NOT AUTOMATIC CARB HEAT, You still need to apply full carb heat at least 15 - 20 seconds in advance of lowering the collective to descend.

In realy realy low ambient temp's below -15??C applying carb heat could raise the temp in the carb into the icing range, So here it's best not to apply heat. This said how many of you fly in area's with such a low ambient temp.


Last edited by Bierbuikje on Mon Nov 06, 2006 4:57 pm; edited 1 time in total
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PostPosted: Sat Jun 11, 2005 3:13 pm    Post subject: Re: Carb Icing Reply with quote

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Last edited by Rdriver on Wed Nov 07, 2007 4:22 pm; edited 1 time in total
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PostPosted: Sat Jun 11, 2005 3:52 pm    Post subject: Reply with quote

One saving grace for the R22's is the new series 2 blades that are now coming out, and being updated on many machines.

If you've flown an R22 with the original blades, and then had them updated to the new blades, you'll notice that you get about an extra 1-1.5 inches of MAP, as they produce more lift than the old ones.

In theory, although this has not yet been discussed or given out as a Robinson directive, you could add about one extra inch to all your MAP limits, according to a few instructors I know.

Again, this is not officially recognised, and no instructor will 'officially' tell you this.

At the very least, on a machine that has been upgraded at least, you have the reassurance that you have a spare inch in there somewhere.

That said, of course, I am not advocating that anyone practice this in reality, as I would not want to be seen to be encouraging anyone to push the limits and get near to overspeeds or orverstressing the machine.

Just a throwaway comment really....but an interesting one.

If you've been flying a machine with the old blades, and then had new ones put on, you would notice that you can hover at about 19 inches or so, instead of 20-21 etc. I've certainly noticed the difference on one of the machines I fly.

Adam.
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PostPosted: Sat Jun 11, 2005 3:55 pm    Post subject: Reply with quote

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Last edited by weekend warrior on Tue Jun 06, 2006 2:30 pm; edited 1 time in total
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PostPosted: Sat Jun 11, 2005 4:54 pm    Post subject: Reply with quote

Adam, I don't see how different blades will allow you to pull more power from the engine - the blades produce more lift, so you require less power to do what you were doing, but that doesn't mean more power is available. Or am I missing something.

RDriver: just an interesting point regarding applied carb heat, that I found out on the flight safety course (and detailed in the thread about it here.) The R22 engine is derated to 124/131bhp (M.C.P./Max 5 minute) - the MAP chart shows you the MAP pressure required for that output. The limit shown on the MAP chart is based on density altitude - that changes with temperature. Obviously, when you apply carb heat to the air inlet, the temperature goes up! So what happens? Your MAP limit increases. Handy, huh?

There is an approximate 1% reduction in power per 4??C. Full carb heat on a R22 raises the temperature by 40??C. So power output is reduced by 10%.


    R22 Beta
    For the R22 Beta, the Lycoming power chart shows that 1" of Manifold pressure = 7 bhp.
    Therefore, with full carb heat MCP 124bhp will occur at approximately 12??7 = 1.7" manifold pressure above chart limit.
    And for Max take-off 131bhp will occur at approximately 13??7 = 1.8" manifold pressure above chart limit.

    R22 Beta II
    For the R22 Beta II, the Lycoming power chart shows that 1" of Manifold pressure = 8 bhp.
    Therefore, with full carb heat MCP 124bhp will occur at approximately 12??8 = 1.5" manifold pressure above chart limit.
    And for Max take-off 131bhp will occur at approximately 13??8 = 1.6" manifold pressure above chart limit.


So you're not really being robbed of 2 inches at all!

Oh, and one other thing as well, while I'm here. I'm sure Bierbuikje is aware, but I just wanted to make sure it was clear. The distinction between low and high power settings for the derated helicopter engines in terms of throttle butterfly aperture is not very much at all to look at! At 18" MAP, the butterfly has an angle of 35??. At 23", it's 50?? - 24"=53??, 25??=55?? approximately - IMPORTANTLY, the throttle IS NOT WIDE OPEN! Because we use derated engines in our R22s and R44s, we're almost always using what Lycoming call "low power settings".

More details on these concepts can be found in Richard Mornington-Sanford's leaflet "No Ice, Thank You" which can be ordered from his website here for ??3.70 - a leaflet well worth having!
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PostPosted: Sat Jun 11, 2005 5:01 pm    Post subject: Reply with quote

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Last edited by weekend warrior on Tue Jun 06, 2006 2:30 pm; edited 1 time in total
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PostPosted: Sat Jun 11, 2005 5:03 pm    Post subject: Reply with quote

Err, no. Just stay within limits, I guess.

I certainly didn't know it before the safety course. Rolling Eyes
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Bierbuikje
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PostPosted: Sat Jun 11, 2005 5:08 pm    Post subject: Reply with quote

Hi

RDriver I do work in the real world, Which Ive been doing for about the past 10 years (No offence taken or intended) How ever I do not operate in an area where the ambient temp is as low as -15??C (Where are you bassed BTW?)

How do you operate the carb heat when the the ambient temp is to low to raise the carb heat above the yellow ark on the CAT gauge?

Have I misunderstood you when you talk about hovering & moving slowly you need all the power you can get, If you can't hover in MAP limits with the correct ammont of carb heat applied then you should not be trying to hover under these circumstances.

What type of work do you do which would require such a high power setting?

Is there not another solution other than hovering? Surely if the ambient temp is so low this will increase peformance from the helicopter.

My point is if you have to overboost the MAP to hover this is not good (R22 recent blade failure's) (Im not sugesting or saying you do this)

However if you have to operate with to little carb heat this is also not good (Chance of carb icing and engine stopage although at a higher power setting this is less remote but still possible)

Maybe I missunderstood what you said there?

In regard to airframe Icing Ive operated in an ambient temp of about -5 quite often (Typical Belgium winter November - December) but I avoid (Airframe) icing conditions by not flying in precipitation when the ambient temp is below 0??C. (Fly around the precipitation or fly lower to a warmer temp, If this does not work land) All the helocopter's I fly or have flown are not authorized for flight in icing conditions.

My point being if your only talking about an ambient temp below 0??C this is not icing condition's so would not cause a problem (In fact it would increase your performance) add in some precipitation now you have a problem (Airframe) icing conditions.


Last edited by Bierbuikje on Sat Jun 11, 2005 5:55 pm; edited 3 times in total
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PostPosted: Sat Jun 11, 2005 5:38 pm    Post subject: Reply with quote

Hi Adam Baily

I think what your trying to say is the machine or machines where you fly have new blades, As a result of this you need less MAP to hover, Which give you more MAP in hand.

There is nothing wrong with that your not overboosting the engine or useing more MAP than allowed. You still have the same MAP Limit according to Temp / Altitude.

The new blades have 5 spindels instead of 3 (This is why there is less chance of damage through an overspeed)

But as far as I know the external surface of the blades are just the same, So there is no differance in external blade design to make a differance in lift.

I think because they are new blades (ie they are not pitted or worn) They are producing better lift than the old blades. Give them a few year's and the see how much MAP you need to hover.
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PostPosted: Sat Jun 11, 2005 7:05 pm    Post subject: Reply with quote

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PostPosted: Sat Jun 11, 2005 7:45 pm    Post subject: Reply with quote

Rdriver wrote:
Heh, couple of words and suddenly there are specialists to advice me of what i do and know wrong.

James T: I'm aware of what you explained and have heard that explanation before. But: 22 (Beta) POH doesn't agree with you and others in that, cause in that case it should be aloud to pull over red line (when it's warm). Obviously you would not need carbheat in that case but it's not prohibited to use it in any temp. I don't remember what B2:s Poh says about it. Funny if the factory theaches people to operate against POH, which they have written themselves. So very handy indeed, but i rather wait a confirmation from the factory (written in a handbook, please).


I don't know about specialist, but I made a point of paying attention in the safety course back in February, and I'm just relaying the information from it. Seriously, check Richard Sanford's leaflet. The numbers I quoted before are taken from it.

As for confirmation, I think it probably IS written in the handbook, isn't it? Or at least, implied. I don't have one to hand, but surely there's a qualification on the MAP chart about what it's showing. If the air intake is suddenly 40??C higher, then just read the chart accordingly.

Let me quote from said leaflet:

Richard Mornington-Sanford wrote:
The application of full carb heat will cause a power loss of approximately 13%. Aircraft engines horsepower output is checked at a correct standard temperature of 15??C. There is a simple rule of thumb for the effect of heat on power: for every 4??C of heat above the standard 15??C there is a 1% power loss. Since the average heat source for the carb provides at least 40??C of heat above standard, this heat condition causes an average power loss of 10%. Add to this the fact that warm air is less dense, so the mixture becomes richer, we get up to the 13% power loss caused by the application of heat. [Which is rounded down to 10% for the examples, as given above.]


As for pulling power over the red line, does it state in the POH not to do this?
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PostPosted: Sat Jun 11, 2005 8:32 pm    Post subject: Reply with quote

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Last edited by Rdriver on Wed Nov 07, 2007 4:24 pm; edited 1 time in total
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PostPosted: Sun Jun 12, 2005 12:17 am    Post subject: Re: Carb Icing Reply with quote

Rdriver wrote:
Bierbuikje wrote:


This said how many of you fly in area's with such a low ambient temp.


I do....and i agree with you, partly. Those recommendations work very well if you're just flying around, unfortunately in the real world it does not always work like that. For example some jobs require very low speed/ hovering and if you're near max gross weight (R22) you just can't use carb heat because it reduces max available power (it needs almost 2 inches more to hover with full carbheat applied than without it). And believe me, in some cases those 2 inches are plenty... Wink

I've always found ice to be a bigger problem in other parts of a helicopter, like blades. It can happen very fast and is usually more lethal than carb icing.


You said "how I operate the carb heat works well if your just flying around but say they don't work well in the real world", PLEASE EXPLAIN THIS.

When I conduct a training flight, pipeline flight, photo flight or any other type of flight which I may need to conduct is this not the real world?

You also said "some jobs require very low speed / hovering",
I did ask what type of work you do which requires very low speed / hovering? You replied "there is no alternative to hovering"

From what you said I presumed you did some sort of observation work with the R22 which require's low speed / hovering OGE, Is this correct? This is why I asked, Is there another alternative to hovering?

or are you purely talking about hovering IGE at the airport etc?

Are you operating at Sea Level or a much higher Altitude?

If you can't hover the helicopter, With the correct amount of carb heat applied and not fly with in your allowed MAP limits then there is somthing wrong either with your helicopter or you need to lighten the load on your helicopter.

Through out the year we operate our helicopter's between -10?? to +35?? C and we can hover at all times with the correct amount of carb heat applied and still in MAP limits but we have to carry less weight in some of the R22's here to stay in MAP limits. (Some R22 are more under powered than others)

A colder ambient temp will result in less MAP available but better helicopter performance, So you won't have to use so much engine power in a colder enviroment.

At ambient temperature's of about - 25??C applying carb heat could raise the the temp in the carburettor air tempereture guage (CAT) into the icing zone so I understand if you do not use carb heat at such a low ambient temperture.

However from about -15??C to about + 30??C you have a serious chance of carb icing and should be useing enough carb heat as required. (See my previous posting) If this means carrying less wait to keep the MAP in limits then carry less wait.

I have to admit I'm a bit confused by some of the thing's you said you start by talking about "specalist's given you advice" but latter you say "Thanks for the advice I realy needed it"

If you decide to take the advice thats upto you, I won't be offended if you don't.

Do you know Richard Sandford is the Europeon Robinson Accident Investigator (That was the guy who James was talking about) or he was the last time I spoke to him which was about 2 years ago, So I wouldn't doubt to much what Richard say's,

I read what James said and he did not tell you you could pull over the red line on the MAP gauge.

You know the definition of Airframe Icing but you misunderstood what I said about Airframe Icing.

So I will clarify my self, If you ONLY HAVE A AMBIENT TEMPERETURE BELOW ZERO (This include's -0??C to -25??C or even lower) with NO PRECEPITATION then you will NOT have a problem with icing.

However If you have a AMBIENT TEMPERETURE BELOW 0??C with PRECEPITATION now you have a icing condition's.

I presume you do not fly in (airframe) icing condition's as the R22 is not allowed to fly in (airframe) icing condition's.

I am curious to know in which country you fly and what type of work you do, I think one of the Scandinavian countries but Im not sure.

One final thing don't think that people are atacking you here I got the impresion thats what you thought, Every thing Ive said here is meant in a friendly way.

All the best Bierbuikje.
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PostPosted: Sun Jun 12, 2005 12:50 am    Post subject: Reply with quote

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Last edited by Rdriver on Wed Nov 07, 2007 4:29 pm; edited 1 time in total
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PostPosted: Sun Jun 12, 2005 11:30 am    Post subject: Reply with quote

Hi Rdriver

I did point out and try to make it clear that I was NOT attacking you, You seem to have a problem exepting this. Maybe you should read my post a bit more carefully.

I also asked a few simple questions regarding where you operate and what type of work you do, To get an idea of the situation your talking about but no reply.

I would say what is more dangerous, Depends on where you operating and the condition's at the time of the flight. (At no point did I say Carb Ice is more dangerous than Airframe Ice)

If you can't handle people asking a few question's when you reply to a post, Then why are you here?

All the best Bierbuikje.
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