The Air Affair : Wingnut Chronicles

California Cold and the Cheap Bastard Engine Pre-heater

January 2013

My Grob hates cold weather. Having spent it's whole life in coastal California, it has never even seen snow! But even in California, the temperatures do drop some, and when that happens, the Grob becomes very reluctant to fire up.

This is a shame because winter days offer some of the best flying. The temperatures are not sweat-generating, the air is clear and smooth, and the hills are green (and even rarely, white!)

So when I arrive at the airport at 8am, the temperature in the low 50s(F), after a night in the low 40s or even high 30s, the engine and fuel are cold soaked, and I know I'm in for a challenge.

Over the years, I've honed my cold start technique, which involves:

Sometimes this procedure works.  Other times, the battery runs out.  A 1pm departure on a sunny day improves the odds considerably.

For a long time, I've pondered how to get some heat onto the engine.  If you have the luxury of a hangar, various electric warming devices are possible.  But at my airport, a box hangar costs almost $900 per month, so the Grob lives outside.

There are commercially available engine preheaters, but they are expensive, and require batteries and propane cylinders. That's a lot of equipment to be hauling out to the ramp to use only a few times per year.

While driving home from work one cold night, I cranked up the car's heater fan full blast, and had a flash of inspiration. The car itself could be the heat source, but what would be the best transfer method?

I stopped at the hardware store and bought a length of dryer exhaust hose and an infrared thermometer.

The following Saturday, I was out at the airport connecting my car's heater to the Grob's cylinder cooling intakes. The infrared thermometer reported the surface temperature of the cylinders as 45 F. The air from the car heater was 135F.

I placed the end of the hose over the vent on the driver's side, using duct tape around the side mirror to hold the hose in place.  To direct as much airflow as possible to this vent, I closed the outboard vent on the passenger side, and placed a large beach towel over the vents in the center of the dashboard.  The weight of the towel was enough to block the air, and the towel was not affected by the hot air.

I started out using the full 20-foot length of dryer hose, but a lot of heat was lost, both by the surface area of the long hose, and by allowing some of it to rest on the cold pavement.

The duct tape didn't take well to the heat of the hose even several inches from the heater vent.  I guess it's not designed for ducts after all!

After applying 20 minutes of hot air to each side of the cylinder bank, the infrared reading rose to 72 degrees.

And it was a success! The engine started within three blades using the standard starting procedure.

On my second attempt at this procedure, I trimmed the dryer hose to about 4 feet, and tried to keep it from touching the ground. Much more of the heat reached the cylinders, shortening the run time to 15 minutes per side.

Of course this method applies little, if any heat to the oil, so it would probably not do as well for below freezing temperatures. But for high 30s to low 50s temperatures, especially if no sunlight warming is available, it could certainly help out.

A future improvement that I have not tried yet: attaching the hose to the heater vent using pipe cleaners. This should hold the hose in place better, while avoiding any damage or tape residue on the dash.

Someone asked me if I planned to try using the car's engine exhaust instead of heater air. While the exaust would be at a much higher temperature, it also has a lot of water vapor, which would condense on the cold aircraft engine, imparing the heat transfer and making a mess. Also I would be concerned about what effect other combustion products would have on the aircraft engine and airframe. So I don't plan to try this method.

Another more exotic heat transfer method involves the water-cooled heat sinks used by hardcore computer gamers to cool their computer processors. One (or more) of these could be attached to the oil pan of the engine, and hot coolant could be routed to them from the car. This, however, would involve more than temporary modifications to both the car and the aircraft. Still I would like to see someone try it!