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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 9:07 pm    Post subject: Reply with quote

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


My goodness. Your all giving my nogin' a workout today.

A manifold pressure gauge measures the absolute pressure of the fuel/air mixture inside the intake manifold and is more correctly a measure of manifold absolute pressure (MAP).

At a constant rpm and altitude, the amount of power produced is directly related to the fuel/air flow being delivered to the combustion chamber. As you increase the throttle setting, more fuel and air is flowing to the engine; therefore MAP increases.

When the engine is not running, the manifold pressure gauge indicates ambient pressure 1013.2 mb/29.92 in. hg.
When the engine is started, the manifold pressure indication will decrease to a value less than ambient pressure (ie: idle).

Correspondingly, engine failure or power loss is indicated on the manifold pressure gauge as an increase in manifold pressure to a value corresponding to the ambient temperature at the altitude where the failure occurred.

Now, doesn't that clear things up?

Enough time on this. clock

I need a rest. Laughing Laughing

Ref: FAA PHAK manual
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PostPosted: Sun Feb 15, 2009 9:07 pm    Post subject: Reply with quote

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


My goodness. Your all giving my nogin' a workout today.

A manifold pressure gauge measures the absolute pressure of the fuel/air mixture inside the intake manifold and is more correctly a measure of manifold absolute pressure (MAP).

At a constant rpm and altitude, the amount of power produced is directly related to the fuel/air flow being delivered to the combustion chamber. As you increase the throttle setting, more fuel and air is flowing to the engine; therefore MAP increases.

When the engine is not running, the manifold pressure gauge indicates ambient pressure 1013.2 mb/29.92 in. hg.
When the engine is started, the manifold pressure indication will decrease to a value less than ambient pressure (ie: idle).

Correspondingly, engine failure or power loss is indicated on the manifold pressure gauge as an increase in manifold pressure to a value corresponding to the ambient temperature at the altitude where the failure occurred.

Now, doesn't that clear things up?

Enough time on this. clock

I need a rest. Laughing Laughing

Ref: FAA PHAK manual
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PostPosted: Sun Feb 15, 2009 9:19 pm    Post subject: Reply with quote

I'll add a caveat to my last Don't dick around with Carb Heat in the Hover
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PostPosted: Sun Feb 15, 2009 9:22 pm    Post subject: Reply with quote

I think you've failed to grasp the argument again AB. No-one is refuting your quotes.

If we are putting warmer ie less dense air into the engine, then quite simply we need more of it. Hence open the throttle a little, hence MAP rising.

Not sure I can simplify it any more......
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PostPosted: Sun Feb 15, 2009 9:23 pm    Post subject: Reply with quote

Good caveat Veeany.
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PostPosted: Sun Feb 15, 2009 9:23 pm    Post subject: Reply with quote

veeany wrote:
I'll add a caveat to my last Don't dick around with Carb Heat in the Hover


You can, but only if you put the collective friction on first... Laughing Shocked
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PostPosted: Sun Feb 15, 2009 9:27 pm    Post subject: Reply with quote

Good evening,

Dick Sanford has published a leaflet about Carb Icing - his background of course is Robinson helicopters. You can download part of it here or purchase it for a small fee. (I'm sure that was a freebie download for the whole thing when I last looked!)

Anyway, one of the topics discussed here is how carb heat affects power. To quote from the leaflet (I'm just going to give the example of R22 Beta II):

Quote:
The application of full carb heat will cause a power loss of approximately 13%. Aircraft engines horsepower output is checked at a corrected standard temperatue of 15C. There is a simple rule of thumb for the effect of heat on power: for every 4C of heat above the standard 15C there is a 1% power loss. Since the average heat source for the carb provides at least 40C of heat above the 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. In the following charts I have rounded it down to 10%

R22 Beta II with Lycoming LYC 0-360-J2A engine.

MCP 124 BHP
Max 5 minutes rating: 131 BHP
Air temp rise @ full carb heat is approx 40C
There is a 1% reduction in power per 4C temp increase - therefore 40C increase sees power reduce by 10%.

10% of 124BHP = 12 BHP

Per power chart in Lycoming operators manual, 1 inch of manifold pressure = 8BHP.

Therefore, with full carb heat MCP 124 BHP will occur at approximately 12/8=1.5inches MAP above chart limit.

Therefore with full carb heat, max take-off 131 bhp will occur at approximately 13/8=1.6 inches above chart limit.




(And, it's been a couple of years, but we've had at least one discussion about this before: here
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PostPosted: Sun Feb 15, 2009 9:34 pm    Post subject: Reply with quote

Cheers for that James. Like the way it ended in Handbags at Dawn.
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PostPosted: Sun Feb 15, 2009 9:39 pm    Post subject: Reply with quote

Heliwhore wrote:
I think you've failed to grasp the argument again AB. No-one is refuting your quotes.

If we are putting warmer ie less dense air into the engine, then quite simply we need more of it. Hence open the throttle a little, hence MAP rising.

Not sure I can simplify it any more......


Probably. I do that at times. Sorry. Crying or Very sad
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PostPosted: Sun Feb 15, 2009 9:40 pm    Post subject: Reply with quote

No offence intended Wink
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PostPosted: Sun Feb 15, 2009 9:49 pm    Post subject: Reply with quote

Heliwhore wrote:
No offence intended Wink


None taken. At times these threads take on a life of their own, and after awhile, I forget the original question. Laughing Laughing

As has been said.....
We can agree to disagree.

That's how we learn. Is it not?

Very Happy

ADDED:
James,
Very interesting post both yours and the included link.
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PostPosted: Sun Feb 15, 2009 10:12 pm    Post subject: Reply with quote

fendersim wrote:
veeany wrote:
I'll add a caveat to my last Don't dick around with Carb Heat in the Hover


You can, but only if you put the collective friction on first... Laughing Shocked


I seriously hope you are joking Rolling Eyes Rolling Eyes

Otherwise I'll have to slap you!

Sarah
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PostPosted: Sun Feb 15, 2009 10:19 pm    Post subject: Reply with quote

No no, it's true. You can even take your hand off the cyclic if you have enough friction on it as well. At times like that, I like to put a brick on the pedals. Rolling Eyes
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PostPosted: Sun Feb 15, 2009 10:21 pm    Post subject: Reply with quote

Ok, what the heck was the original question?

Are we into physiology now?

Laughing
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PostPosted: Sun Feb 15, 2009 10:24 pm    Post subject: Reply with quote

Oh, I forgot. Dick mentioned there might actually be forthcoming revision to the MAP limit chart that will put this whole thing into perspective. He has recommended that RHC redesign their MAP limit chart based taking into account the carb temperature as opposed to simply calculating it on OAT. Nothing is confirmed but he said he's hoping they will make the revision at some point.

Sarah
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