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Engineering, Maintenance, Certification Bits & Pieces
This is the second in a series of articles on aircraft combustion heaters. I guess it is back to work
after the 4th of July celebrations - I hope everyone had a chance to grill a few hot dogs and hamburgers
and sip a few of your favorite beverages, and maybe get a few flight hours in too! This months article
will address the ventilation air blower. Yup, pretty similar to the combustion air blower.
By Jim Davidson, President & FAA DER
Davidson Engineering Resources, Inc.
The ventilation air blower is pretty similar to the combustion air blower - an electric motor that rotates
some form of fan that moves air through the combustion heater. This month the focus is on the
ventilation air blower motor/fan assembly.
Ventilation Air Blower
The ventilation air blower motor is also 12 or 24-volt motor with a propeller type fan assembly attached
to one end of the motor shaft. The metal exterior covering of the ventilation air blower is designed to
directly connect to combustion heater tube. The propeller type fan assembly is designed to pull in fresh
air which passes through the combustion heater. When you compare the design of the combustion air blower
with the ventilation air blower you'll see differences and similarities.
Similarities & Differences
They both have motors, either 12 or 24 VDC. That means they both have brushes and bearings that will
wear. As I said last month, I would rather have the ability to replace brushes than to just throw a motor
away. Brush wear can tell you a lot about how the motor is operating in your aircraft. Also the
environment in which you operate your aircraft has a telling effect on the rate of brush wear.
I know of instances where the fine powdery Iraq sand has caused severe generator brush wear on a certain
aircraft and the result was a dual generator failure. So if you operate in a dusty, dry, sandy
environment you need to think about your heater motor brush maintenance program (and your alternator too -
but that is for another article).
Both assemblies have fans, but they differ in design. The combustion air blower uses a squirrel-cage air
mover whereas the ventilation air blower uses a fan or propeller-type air mover. The differences in
air-mover type, way back when the engineers were designing their combustion heater systems, boiled down to
cubic space available, combustion heater overall design, and maybe parts availability. The ventilation
blower has to pull air linearly through the heater. A small, compact motor/fan assembly is a good choice
for this application. The fan accelerates the air linearly through the combustion heater, no curves, and
no bends - just a straight shot. The combustion air blower seems to have been designed with installation
space as a prime design concern - remember it is not installed on the heater but elsewhere close to the
heater. The combustion air blower assembly squirrel-cage pulls air in through a central opening and then
accelerates the air radially away from the spinning cage. The design and form of the combustion air
blower cover funnels the air into the combustion blower air outlet. Same function, that is to accelerate
air, but totally different methods.
Heat transfer is the prime function of the combustion heater - combustion heat is transferred to the
ventilation air. The combustion air blower provides the air (oxygen) to sustain the combustion heat
source, while the ventilation air blower provides the air that absorbs heat from the heat source. The
combustion air blower must provide a constant supply of air for the combustion process - too little and
the combustion process becomes rich; too much air and the combustion process leans out. You'll know the
problems associated with both situations, spark plug damage or fouling, excessive fuel consumption,
excessive wear on the heater components, etc... So the combustion air blower assembly is designed to
provide the proper amount of air for the combustion process. The same holds for the ventilation air
blower assembly. The amount and speed at which the ventilation air passes through the combustion heater
determines just how warm the ventilation air is at the combustion heater output, and how warm you are in
Since each motor/fan assembly is designed to provide airflow at specific rates, don't tweek the impellers
or fan blades to try and "improve performance"!
You can end up with a fan that is out of balance (bearings wear faster), actually reduce the airflow
(messed up combustion or poor heat transfer), vibration
and failure of the impeller or fan (broke heater right smack dab in the middle of that long winter flight
you are on)...These units are certified, and just
like placing a non-FAA approved part on them, "tweeking" beyond approved maintenance procedures can be
considered taking the heater out of its certified configuration. Both assemblies use electrical motors, so
the discussion last month about wiring applies to the ventilation blower motor assembly too. As does the
section on attachments. The ventilation air blower is attached to the combustion air heater, so all
fasteners need to be checked to assure that the ventilation air blower assembly is tight and
Ram Air and On-Ground Operations
As discussed above - the ventilation air blower assembly pulls air through the combustion heater which
heats up as a result of the transfer of heat
from the combustion chamber. Aircraft designs differ on how the outside air is ducted to the inlet of the
ventilation air blower assembly, and the bottom
line is that the passage or tube or duct needs to be checked on a regular basis for connection security,
cracks, obstructions, and loose fittings. Your aircraft may be designed to use ram air for ventilation
while in flight. In this case, the ram air has enough energy to go through the combustion heater and
provide adequate warm cabin temperatures. If you aircraft uses ram air in flight for ventilation air you
may have a ventilation blower "ON/OFF" switch. Follow you aircraft owners manual or operation
instructions for operating the ventilation blower both for on-ground and in-flight operations.
If your aircraft doesn't have a ventilation blower "ON/OFF" switch the heater system installation may not
be designed to use ram air in flight. If this is the case with your aircraft, and you want to install a
ventilation blower "ON/OFF" switch - STOP!! and think about what you are going to do. Can your aircraft
provide adequate ram air for proper heating? Remember "tweeking" with airplane systems can take your
aircraft out of its' type certified configuration. If you want to control your ventilation blower motor,
ask the aircraft manufacturer about the ram air characteristics first. Does the ram air have enough
energy to be used as the ventilation air source? What is the pressure drop in the ducting from the
exterior air "scoop" to the ventilation blower face? Will there be enough airflow through the heater to
provide the necessary cabin heat? Maybe there is an STC out there to install a ventilation blower control
switch for your aircraft. You can go to the FAA web page and search for owners of STC's. Contact your
local ACO or a DER about the proposed design change, it may be possible.
The control of the ventilation blower motor, at the proper time, can provide some additional power to the
rest of your aircraft systems. Think for a moment, you are sitting a taxiway in your Piper Comanche
PA-24-250, powered by an O-540-A1A, it has a 12V, 70A ALX-8521. You have the combustion heater on,
landing lights on, radios and Narco 136 Audio Panel, MX COM, Bendix 2000 ADF, Narco 150 Transponder Mode
C, Digital Clock, Narco COM IIB NAV, Dual Narco 114 NAV, Narco 890 DME, HSI, and Garmin 55 GPS all
operating, and wonder if you'll have enough power for everything? Take a look at the amount of current
the ventilation blower motor consumes! A few amps shed from the ventilation motor for a few minutes (ok -
so you'll get a bit cool) can be just enough to let the rest of the electrical systems operate normally.
The design of a ventilation blower control circuit is pretty straight forward, but it has to be certified
as being in compliance with applicable FARS - you may need an FAA DER for that.
The bottom line - ventilation air is what ends up in the cabin after being pulled through the combustion
heater by the ventilation blower assembly and warmed by the transfer of heat from the combustion process.
A ventilation blower that is in good operating order, with all other associated systems in top-shape will
keep you warm next winter. Keep an eye on brush wear if you live in a dusty, sandy location like Yuma,
Arizona. And don't tweek any combustion heater component to try and make it run better - the designers of
the old Janitrol and Southwind heaters knew what they were doing way back when. If you find that the
cabin warmth is not what you think it should be - call a certified repair station.
Let the experts take care of the heater so you can enjoy the winter scenery passing below while you are
nice and toasty.
For Information on Aircraft Electrical Design Certification:
Contact: Jim Davidson, President and FAA DER
Davidson Engineering Resources, Inc.
Phone (520) 977-9824
Fax (520) 232-3660