[Wing] Fw: Fw: Carb Heat

[George McNutt]

Does anyone have any temperature rise information on the 2" tube type carb
heat (see below)
Is this type of reply typical of Ken Scott?
The implication being the manufacturer supply the specs, not the builders.

----------------snip

There is a requirement for Amateur Built aircraft in Canada to show
that the carb heat system is capable of providing a temperature rise of 90
degrees F. shown on AMA:549.13/2  15 April 1987,  My concern is, is this
system capable of providing that much heat, I really doubt it.

My reason for asking about this carb heat system is because I am an
(MD-RA)inspector for amateur built aircraft in BC and I want to be sure
that the system works properly. Last weekend I inspected an RV4
built by ***** *******, it was the first time I had seen this type
of carb heat assembly.

 Terry Elgood

> > From: "Ken Scott" <kens at vansaircraft.com>
> > To: "T&M Elgood" <elgood at aebc.com>
> > Sent: Friday, April 04, 2003 9:27 AM
> > Subject: Re: Carb Heat
> >
> >
> > > It seems to work under most circumstances, but we have never
> > > done formal tests on it.

----------------------snip


> > > Subject:        Carb Heat
> > > Date sent:      Thu, 3 Apr 2003 19:54:13 -0800
> > >
> > > In your catalogue a carb heat muff is listed, it is a simple
> > > assembly with what appears to be a 2" aluminum tube with a cut out
> > > to fit over the exhaust pipe. Considering the small surface area
> > > against the exhaust does this assembly provide enough heat, do you
> > > have any test information showing the temperature rise above
> > > ambient.
> > >
> > > Terry Elgood

---------------------------------------------------

HI Terry & RV Builders

I will chime in with my opinions because I think the carb heat issue is very
important to those of us flying here in the Pacific Northwest. In the past I
have posted questions on this subject on the RV-list with very little
response. (I am a old guy so temperatures below are farenheit!)

(1) Regulations

The AMA Terry quotes provides guidance for acceptable means, but not the
only means, of showing compliance with the requirements of preventing ice
build-up in carburettors of amateur-built aircraft. The AMA is based on
regulations for certified aircraft such as, FAR 23.1093 (1983) paragraph (1)
states that a carb preheater should provide a heat rise of 90 deg F with the
engine at 75 % power and OAT 30 deg. (for conventional venturi carburetors).
Also 23.1105 calls for a 100 degree rise where the heated air passes through
a filter.

Note these regulations calls for a heat muff rise and this temperature would
be read at the outlet of the heat muff or in the carb air box, not in the
carb venturi where the temperature has dropped again due to the latent heat
of vaporization of fuel, etc. Therefore a CAT gauge normally cannot tell you
if you meet the regulation, only that you have a safe temp in the carb
venturi. I would assume that a safe venturi temperature at 75% power on a 30
deg day meets the heat muff requirements.


(2) Underdog's Carb Heat.

I did not like the looks of the small Van's heat muff and returned it for
credit. I had a larger heat muff made up, looks much like the cabin heater
muffs but slightly shorter, 9" long and encasing the forward exhaust
crossover pipe. Because I fly IFR I am still not satisfied with the
indicated carb heat rise and plan on making a larger heat muff that wraps
around both crossover pipes.

On runup @ 1700 RPM I show a 15 degree rise on the CAT gauge and a 30 RPM
drop. In cruise @ 2200 - 2400 RPM the temp rise is 15 - 20 deg and a 50 RPM
drop. On approach at 1400 RPM (less airflow through muff) I get a 40+ degree
CAT rise.

I have flown my RV6A in cloud at near freezing temperatures (carb heat on)
for a 30 minute period with no sign of carb ice, so I assume I am getting
enough heat but would like more as a comfort factor. The only actual carb
ice I have experienced was when taxiing out in rain.

Fly safely.

George McNutt