P-51 Civilian - Carburettor Icing/Temp at High altitudes

Arguably the finest fighter aircraft of World War II.
hughesj2
Airman
Posts: 27
Joined: 03 Nov 2011, 14:26

P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by hughesj2 »

Guys,
Quick question on cold weather/high altitude cruise and carb temperatures...
As an Accusim 17 pilot I'm careful about managing carb temps to prevent icing (and my chief bitchin!)..So my question is for the P-51 is there an optimal carb temp I need to be aiming for (gauge looks like 20-40C green arc?)

So from the manual "Ordinarily you will always use ram air, but, in the event of extreme icing or dust conditions, the carburetor air controls on the left cockpit pedestal allow you to select either un-rammed filtered or, in later airplanes, un-rammed hot air for operation"

Which I have been doing but I noticed my carb temp drifting down towards minus figures, where the OAT was around -40C?

Is that acceptable in clear air?

My natural instinct told me use un-rammed air (with no heat as I was over 12,000ft) which resulted in a carb temp around 12C, slight loss of power and a reduction in fuel consumption, otherwise no ill effects I noticed. Conversely I didn't notice any issues while the engine was running with Freezing carb temperature either?

Regards
James
Image

User avatar
n72.75
Senior Airman
Posts: 104
Joined: 25 Dec 2011, 18:54

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by n72.75 »

I use the air filter when under 10000 feet. Use carburetor heat if cruising or descending at low power below 12000 feet. Above 10k, the dew-point is so low that carb icing is very unlikely. The only time you would get icing at 35-40k would be in thunderstorm situations.

User avatar
CodyValkyrie
VIP Partner
Posts: 4560
Joined: 16 Feb 2007, 03:27
Contact:

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by CodyValkyrie »

This is a pretty interesting topic you have brought up, and one that has been heavily debated here. I wanted to comment on it immediately but hesitated. The P-51 carb intake does not have a great propensity for icing, and very little credible data is even available on the subject. In fact, some experts and pilots believe it is nearly impossible to ice the carburetor on a Mustang. To further complicate the situation, very few pilots today are willing to fly into known icing conditions to test it out. ;)

A couple of things to note however:
- While the Civilian Mustang is equipped with more modern IFR equipment, it only has pitot heat.
- Neither the military or civilian Mustang have de-icing boots or propeller heaters.
- Neither aircraft should be flown into known icing conditions.

The manuals are fairly vague when it comes to the conditions to utilizing the carb heater. What IS known is that some people who operate the P-51 on the ground utilize the filter/un-rammed air, and further that this should be done in dusty conditions. Some pilots today start and taxi with it, while others do not. I believe Dudley mentioned previously in other conversations that he never really touched it, and never started or taxied his aircraft with it. Some have argued that it is acceptable and likely wouldn't hurt to have the aircraft un-rammed when landing or taking off, although I personally wouldn't want to risk the penalty in power (albeit slight in most cases) unless the situation dictated such (perhaps operating in the desert or extreme weather due to poor planning). To achieve the most efficient power rammed air is necessary.

For the majority of your flying, you will not have a need to utilize the carburetor heat whatsoever. There are no cautionary placards or tales like in the B-17 or Cub for its use. I would suspect however that IF you did encounter severe icing conditions (you REALLY should be avoiding them at ALL COSTS), then my from the hip assumption would be to warm the carb above 32 degrees Fahrenheit or 0 Celsius (PREFERABLY HIGHER!) so as icing does not form, and that any icing that is already present can melt.

It's possible Dudley may chime in with some personal experience on the subject if he has time.
ImageImage
ImageImage

r4y30n
Technical Sergeant
Posts: 510
Joined: 31 Jan 2011, 01:47

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by r4y30n »

Just to expand on that, the early Merlin engines had coolant lines running through the carburetors to keep them warm without heating the intake air much, this pretty much made icing a non-issue with minimum penalty in power. The Packard Merlins have American-made pressure carbs, though, and I'm not sure if they're at all similar in that regard. Also, using un-rammed air in the Mustang above a certain altitude (not sure quite where) will screw up the carb's metering and skew your mixture into the dangerously lean zone, this is why many Mustangs today have the alternate air/filter system deleted. If you ever see a Mustang with the holes/louvers missing from the sides of the nose they have had that modification done.

-Dave
Image Image Image

User avatar
lonewulf47
A2A Chief Pilot
Posts: 6744
Joined: 03 Aug 2008, 10:41
Location: 1 NM east of LSZH

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by lonewulf47 »

n72.75 wrote:I use the air filter when under 10000 feet. Use carburetor heat if cruising or descending at low power below 12000 feet. Above 10k, the dew-point is so low that carb icing is very unlikely. The only time you would get icing at 35-40k would be in thunderstorm situations.
Just to make this clear and not generating any confusion :lol: : there is no selectable "air filter". You can only select rammed or un-rammed air. Un-rammed air is taken from within the engine cowling and furthermore allows use of carb heat. Rammed air is just straight-in from the ram-air scoop.
Oskar

ASUS MAXIMUS XI Hero, i9-9900K 8 Core OC 5.2 GHz, WIN10-64Bit, 32GB DDR5, Nvidia RTX 2080 Ti Strix, 2x 2 TB SSD M.2, 1x 2 TB SSD, 1 monitor 4k, AS4, EFB on remote PC

Image

hughesj2
Airman
Posts: 27
Joined: 03 Nov 2011, 14:26

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by hughesj2 »

Thanks for the replies guys...
So my take from this is I'm not doing anything horrendously wrong and due to the design of the intakes and carbs I don't need to worry about icing, as Cody mentioned I'd seen there was no additional ice protection on the airframe so I avoid dwelling in cloud.

It also sounds like I don't need to get too uptight over the carb temperature staying in an optimal zone? So for example it makes no odds in clear air whether my carb temp is -5c or +20c?

Cheers
James
Image

User avatar
CodyValkyrie
VIP Partner
Posts: 4560
Joined: 16 Feb 2007, 03:27
Contact:

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by CodyValkyrie »

I wouldn't worry about it too much. Just keep an eye on things mate. There's just no way to keep it optimal at high altitude.
ImageImage
ImageImage

User avatar
CodyValkyrie
VIP Partner
Posts: 4560
Joined: 16 Feb 2007, 03:27
Contact:

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by CodyValkyrie »

A quick note, the carburetor temperature control will NOT work unless the carburetor air control is in the un-rammed position (down).
ImageImage
ImageImage

hughesj2
Airman
Posts: 27
Joined: 03 Nov 2011, 14:26

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by hughesj2 »

Cheers Cody, no worries with the operation of rammed/un-rammed air control or the fact it needs to be un-rammed for the heater control to work...Was more whether I needed to pay much attention to trying to hit an optimal carb temp range..And from the various replies (including avoid icing conditions) I don't think I do. Will report back if I break anything through ignoring the controls :P
James
Image

ft
Staff Sergeant
Posts: 358
Joined: 01 Feb 2005, 08:13

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by ft »

r4y30n wrote:Also, using un-rammed air in the Mustang above a certain altitude (not sure quite where) will screw up the carb's metering and skew your mixture into the dangerously lean zone, this is why many Mustangs today have the alternate air/filter system deleted. If you ever see a Mustang with the holes/louvers missing from the sides of the nose they have had that modification done.
Using hot (not unrammed) air above 12,000 will risk causing a too lean mixture. Source: POH.

The blank doors are for winterization, forcing the engine to pull all its air through the hot air door when unrammed. Extensively used in my air force back in the days, when we operated Mustangs in the subarctics (and no, I wasn't around back then... unfortunately!).

Cheers,
/Fred
Be warned: Aero engineer, real life pilot, sim programmer. Nothing good can come out of that.

Nachtflieger
Airman
Posts: 34
Joined: 13 Jul 2018, 23:06

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by Nachtflieger »

lonewulf47 wrote:
n72.75 wrote:I use the air filter when under 10000 feet. Use carburetor heat if cruising or descending at low power below 12000 feet. Above 10k, the dew-point is so low that carb icing is very unlikely. The only time you would get icing at 35-40k would be in thunderstorm situations.
Just to make this clear and not generating any confusion :lol: : there is no selectable "air filter". You can only select rammed or un-rammed air. Un-rammed air is taken from within the engine cowling and furthermore allows use of carb heat. Rammed air is just straight-in from the ram-air scoop.


I was just checking into this, wondering if the air restriction and minimal power loss were modeled in any way by use of the filtered air (non-rammed) position. It's really irrelevant at lower altitudes as the supercharger can easily produce takeoff power either way.

But yes, there is a selectable air filter. Unrammed air is filtered. Page 5, Section 1 of the P-51D manual clearly states this. Operation in dusty or dirty conditions would have you wanting to use this position.

User avatar
Jacques
Senior Master Sergeant
Posts: 2376
Joined: 26 Jun 2011, 17:54
Location: West Coast, USA

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by Jacques »

A careful reading reveals that is precisely what Oskar was saying: The filter itself is not something that is selectable- Unrammed air is, by default, filtered.


Sent from my iPad using Tapatalk

Tomas Linnet
Senior Master Sergeant
Posts: 2285
Joined: 05 Nov 2013, 10:48
Location: Oksboel, Denmark

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by Tomas Linnet »

Nachtflieger wrote:I was just checking into this, wondering if the air restriction and minimal power loss were modeled in any way by use of the filtered air (non-rammed) position. It's really irrelevant at lower altitudes as the supercharger can easily produce takeoff power either way.

But yes, there is a selectable air filter. Unrammed air is filtered. Page 5, Section 1 of the P-51D manual clearly states this. Operation in dusty or dirty conditions would have you wanting to use this position.
Note you have answered a 6 year old thread :D
Kind Regards
Tomas

Sim: FSX SE
Accu-Sim aircraft in my hangar:
C172, C182, P51 Civ, P51 Mil, B17, Spitfire, P47, B377 COTS,
J3 Cub, T6, Connie, P-40, V35B
A2A Accu-Sim Avro Lancaster Loading:............0.000003% complete, please wait.

Nachtflieger
Airman
Posts: 34
Joined: 13 Jul 2018, 23:06

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by Nachtflieger »

I can read carefully enough, and I noted the date. Does the age of the thread somehow invalidate any response? Or did I miss something in the forum terms and conditions?

As for careful reading, I leave it to you:

"The air induction system supplies either ram air or unrammed, filtered air to the carburetor."

It's a fifteen-word sentence. It shouldn't take a law degree to understand, should it? You can either get the straight shot with no filter, or divert the air to a filter using the control. The earlier post was in error and I simply looked it up and corrected it because I was curious myself.

Nachtflieger
Airman
Posts: 34
Joined: 13 Jul 2018, 23:06

Re: P-51 Civilian - Carburettor Icing/Temp at High altitudes

Post by Nachtflieger »

Jacques wrote:A careful reading reveals that is precisely what Oskar was saying: The filter itself is not something that is selectable- Unrammed air is, by default, filtered.


Sent from my iPad using Tapatalk
No.

The filter is absolutely selectable.

If the pilot want unfiltered, rammed air, he uses that position.

If he/she wants the air to pass through the filter to protect the engine and induction system, he/she selects that position.

new reply

Return to “P-51 Mustang (Military & Civilian)”

Who is online

Users browsing this forum: Google [Bot] and 10 guests