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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.
Tucson, Arizona
July 2004

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 the cockpit.

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 secure.

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. Bottom Line

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.
jimdavidson@davidson-der.com
www.Davidson-DER.com
Phone (520) 977-9824
Fax (520) 232-3660