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alternate carb heat



 
 
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  #1  
Old October 17th 04, 06:03 PM
Ray Toews
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Default alternate carb heat

Has anyone ever tried using cowling air under the engine as heated air
for carb heat.
Because of tight cowling space I am thinking of building a door in the
lower cowl for fresh ram air and then closing it and drawing air from
under the engine, cooling air coming off the cylinders which seems to
run about 120 degrees as my heated air in case of carb ice.

I have measured the under engine air on two airplanes now and it seems
to run around 120 deg. so this must be fairly typical and would seem
to be more than adequite to get rid of carb ice. I have never had the
opportunity to measure heated air coming off exhaust manifold.

I would build it so fresh ram air is unfiltered and alternate, heated,
air would be filtered. in this way I would always taxi using warm
filtered air but in normal flight use cold ram air.

Makes sense to me, but then so does capitalism.

Ray
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  #2  
Old October 18th 04, 01:02 AM
Dave Hyde
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Default

Ray Toews wrote...
Has anyone ever tried using cowling air under the engine as heated air
for carb heat.


Serveral RV-builders including me have tried something similar.
Van's sells a mini heat muff that doesn't do mucgh but serve as
a mount for scat tube, so in essence we're drawing undercowl
air into the airbox when carb heat is selected. Sam Buchanan
has a picture on his website:

http://home.hiwaay.net/~sbuc/journal/engine4.html

about 1/2 way down the page. I did the same thing on an
RV-4 and I get a barely perceptible RPM rise when I select
carb heat, which tells me it's not helping much.

Your call.

Dave 'hot air specialist' Hyde



  #3  
Old October 18th 04, 01:14 AM
Kyle Boatright
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Default


"Dave Hyde" wrote in message
...
Ray Toews wrote...
Has anyone ever tried using cowling air under the engine as heated air
for carb heat.


Serveral RV-builders including me have tried something similar.
Van's sells a mini heat muff that doesn't do mucgh but serve as
a mount for scat tube, so in essence we're drawing undercowl
air into the airbox when carb heat is selected. Sam Buchanan
has a picture on his website:

http://home.hiwaay.net/~sbuc/journal/engine4.html

about 1/2 way down the page. I did the same thing on an
RV-4 and I get a barely perceptible RPM rise when I select
carb heat, which tells me it's not helping much.

Your call.

Dave 'hot air specialist' Hyde


You get an RPM increase??? Sumpthin' is way, way wrong with that... ;-)

With the exact same setup, I get a barely perceptible RPM decrease...

KB



  #4  
Old October 18th 04, 01:57 AM
Dave Hyde
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Posts: n/a
Default

Kyle Boatright wrote...

You get an RPM increase??? Sumpthin' is way, way wrong with that... ;-)


Argh. Something's wrong alright, but not
with the airplane.

Dave 'fatfinger' Hyde



  #5  
Old October 18th 04, 02:48 AM
UltraJohn
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Default

Sorry Dave
In this case I don't think it's the fingers that are 'fat'. ;-)
John


Dave Hyde wrote:

Kyle Boatright wrote...

You get an RPM increase??? Sumpthin' is way, way wrong with that... ;-)


Argh. Something's wrong alright, but not
with the airplane.

Dave 'fatfinger' Hyde


  #6  
Old October 18th 04, 02:55 AM
Jerry J. Wass
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Default

I know this is a homebuilt group, but for naturally aspirated engines-CFR
23.1093
says---" each engine shall have a preater to raise the inlet temp. 90 deg
F, from an
ambient of 30deg F w/ no visible moisture present.---AND-at only 75%
power. This is not remotely possible with under cowl air temps. The
pucker factor gets pretty high when your altitude is 600 ft over a forest,
or lake, and the engine starts choking down.
..Jerry

Ray, Toews wrote:

Has anyone ever tried using cowling air under the engine as heated air
for carb heat.
Because of tight cowling space I am thinking of building a door in the
lower cowl for fresh ram air and then closing it and drawing air from
under the engine, cooling air coming off the cylinders which seems to
run about 120 degrees as my heated air in case of carb ice.

I have measured the under engine air on two airplanes now and it seems
to run around 120 deg. so this must be fairly typical and would seem
to be more than adequite to get rid of carb ice. I have never had the
opportunity to measure heated air coming off exhaust manifold.

I would build it so fresh ram air is unfiltered and alternate, heated,
air would be filtered. in this way I would always taxi using warm
filtered air but in normal flight use cold ram air.

Makes sense to me, but then so does capitalism.

Ray


  #7  
Old October 18th 04, 11:34 AM
smjmitchell
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Default

I would do nothing less than ensure the system complies with the FAR requirements.

If want to proceed then I would instrument the carby by placing a thermocouple inside inthe induction system as close to the carby thoat as you can get it. I would also place a second thermocouple outside the aircraft to measure OAT (you could also use an existing thermometer for this if installed). Climb up until the OAT is 30 deg F set 75% power and then turn on the carby heat (i.e. switch to the alternative inlet inside the cowl) and let the system stabilize. If you get the required 90 deg F rise in temp then good .. if not get rid of it an install a conventional system. I have done this in a number of airplanes and it is no big deal to set up and run the tests. Note that in most cases you will get an initial higher temperature and then the intake air will stabilize at a lower temperature. You must stabilize the system at a delta of 90 deg F.

I have reviewed FAR 23.1091 and 1093 quickly ..... I would suggest that some detailed study is in order. These requirements were revised many times since this original amdt until Amdt 51 which set in place the rules that are effective today. The rules at Amdt 51 are very different from those earlier requirements and have now split the requirement between FAR 23.1091 and 1093.

What I think you should do is study the amendment history and understand the way that the FAR's have developed over the years. This may give some insight the basis of the requirements and the reasons for the changes - usually a valuable history lesson. In particular I would study the NPRM's and other preambles may also provide some useful background on the reasons for change.

The earlier amendments (up to Amdt 43 in 1993) had different requirements and specifically state that the induction air from the alternative intake must have a temperature at least equal to the temperature of the cooling downstream of the engine with a carburetter that tends to prevent icing (presumably a fuel injected engine) BUT you must meet the requirement for a 90 deg F increase compared to 30 deg F ambient with a convention venturi carburetor. Don't get confused by this. For reason explained in the NPRM's they ammended the first requirement to require to state that engines with a fuel metering device (fuel injection) tending to prevent icing must demonstrate a 60 deg F temperature rise at 75% power. The 90 deg F requirements remains for convention carby's.

The FAR also specifically state that you must have an alternative air intake (I don't think a door on the front of the cowl qualifies but that may be open to some interpretation. I think they mean physically separate systems with a value in the induction system.

Also FAR 23.1091 provides guidance on where this intake can be located. I would review this.


"Jerry J. Wass" wrote in message ...
I know this is a homebuilt group, but for naturally aspirated engines-CFR 23.1093
says---" each engine shall have a preater to raise the inlet temp. 90 deg F, from an
ambient of 30deg F w/ no visible moisture present.---AND-at only 75% power. This is not remotely possible with under cowl air temps. The pucker factor gets pretty high when your altitude is 600 ft over a forest, or lake, and the engine starts choking down.
.Jerry
Ray, Toews wrote:

Has anyone ever tried using cowling air under the engine as heated air
for carb heat.
Because of tight cowling space I am thinking of building a door in the
lower cowl for fresh ram air and then closing it and drawing air from
under the engine, cooling air coming off the cylinders which seems to
run about 120 degrees as my heated air in case of carb ice.
I have measured the under engine air on two airplanes now and it seems
to run around 120 deg. so this must be fairly typical and would seem
to be more than adequite to get rid of carb ice. I have never had the
opportunity to measure heated air coming off exhaust manifold.

I would build it so fresh ram air is unfiltered and alternate, heated,
air would be filtered. in this way I would always taxi using warm
filtered air but in normal flight use cold ram air.

Makes sense to me, but then so does capitalism.

Ray


  #8  
Old October 18th 04, 03:24 PM
Matt Whiting
external usenet poster
 
Posts: n/a
Default

Kyle Boatright wrote:

"Dave Hyde" wrote in message
...

Ray Toews wrote...

Has anyone ever tried using cowling air under the engine as heated air
for carb heat.


Serveral RV-builders including me have tried something similar.
Van's sells a mini heat muff that doesn't do mucgh but serve as
a mount for scat tube, so in essence we're drawing undercowl
air into the airbox when carb heat is selected. Sam Buchanan
has a picture on his website:

http://home.hiwaay.net/~sbuc/journal/engine4.html

about 1/2 way down the page. I did the same thing on an
RV-4 and I get a barely perceptible RPM rise when I select
carb heat, which tells me it's not helping much.

Your call.

Dave 'hot air specialist' Hyde



You get an RPM increase??? Sumpthin' is way, way wrong with that... ;-)

With the exact same setup, I get a barely perceptible RPM decrease...


I thought the same at first, but he may be referring to turning on the
carb heat after he's already accumulated enough carb ice to noticeably
decrease his RPM. In that case, applying the carb heat should increase
the RPM as the ice is removed.

Matt

  #9  
Old October 18th 04, 04:06 PM
Cy Galley
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Default

When you measured the under cowl temperature, I'll bet it was on the ground.
In this situation there is little air moving and poor cooling leaving higher
temps than one would find when flying.

When carb ice occurs, the already low temp is rapidly reduced due to lack of
power. If you do not apply carb heat quickly enough, you become a glider.

The trick is to get carb heat applied before the engine quits so you have
heat to melt the ice. You need a lot of heat and quickly,

Ray Toews wrote in message ...
Has anyone ever tried using cowling air under the engine as heated air
for carb heat.
Because of tight cowling space I am thinking of building a door in the
lower cowl for fresh ram air and then closing it and drawing air from
under the engine, cooling air coming off the cylinders which seems to
run about 120 degrees as my heated air in case of carb ice.

I have measured the under engine air on two airplanes now and it seems
to run around 120 deg. so this must be fairly typical and would seem
to be more than adequite to get rid of carb ice. I have never had the
opportunity to measure heated air coming off exhaust manifold.

I would build it so fresh ram air is unfiltered and alternate, heated,
air would be filtered. in this way I would always taxi using warm
filtered air but in normal flight use cold ram air.

Makes sense to me, but then so does capitalism.

Ray



  #10  
Old October 18th 04, 10:21 PM
Steelgtr62
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Posts: n/a
Default

Some British engines used an oil passage through the carb venturi, a far more
elegant solution. However using fuel injection or mounting the carburetor on
top of the engine instead of underneath would largely obviate the problem.
 




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