How to Build an Aircraft Engine Preheater
By Sam McNair, EAA 770296, for Experimenter
I read the article on preheaters in the November 2011 issue of Sport Aviation, and it caused me to recall a preheater that I built about 20 years ago based partially on another design that I saw in Sport Aviation.
It answered my need for a powerful preheater that didn't require electricity (unavailable out on an exposed ramp) or propane (which doesn't vaporize well at subzero temperatures and is hazardous to store) and was still robust and secure enough to leave at my tiedown. It was simple and easy and inexpensive to build, easy to use, and one doesn't have to transport fuel, a battery, or drain the aircraft battery by powering a puny blower. This design achieved all these objectives and I've used it for years.
The unit gets its huge forced airflow and heat from a modified 5-hp walk behind a lawn mower. It cost about $50 to build (unless you buy a new mower). It is built as follows:
- Find an inexpensive, used 3.5- to 5-hp gasoline push mower. Remove the handle but retain the throttle controls and the safety trip cable. Leave the wheels on. Add an eyebolt to the deck to give you a place to anchor it to your tie-down ring.
- Remove the blade and heat the tips with a torch. Twist the outer 2 to 3 inches down to 90 degrees from the flat of the blade to make a simple fan blade. Exact twist isn't critical; just don't twist too much of it or your new "fan" blade will pull too much horsepower and overload the engine. Note: Once you've assembled the unit, you may need to go back and add more or less twisted area to the blade so that at full throttle the engine is in fact operating at near full load. It's sort of like trial-and-error prop pitching. Don't cut the blade unless you have the ability to balance it afterwards. Heating and twisting works quite well.
- Remove the engine from the deck and space it up about 1 inch using washers and longer engine-mounting bolts. Ensure that the blade adequately clears the deck. This gap is the "fan" air inlet.
- Use plywood or sheet metal to enclose the bottom of the deck to make it airtight. I used 1/2-inch plywood and angle clips since they were available.
- Buy a metal garbage can. You can use a plastic one, but you'll have to install an inner ring of aluminum flashing as a heat shield if you do. Cut the can so that the now-open bottom sits on the mower deck around the engine and the top extends about 8 inches above the top of the engine. A little under 24 inches is tall enough. If the lid won't stay on in high wind by itself, equip it with a bungee or hasps.
- Trim and flair the bottom edges of the can for a tight fit to the deck, and secure it with short screws with locknuts so that it's removable. Seal with RTV. With the lid on, the engine is now totally enclosed. If the lid isn't fairly airtight, you may have to fashion a gasket with weather stripping.
- Using dryer hose duct fittings, create an adaptor for 4-inch flexible dryer hose to the discharge chute of the mower, attach, and seal it. This is your hot air supply connection.
- The safety trip cable (dead-man's switch) that many mowers come with can be rigged through a small hole drilled vertically in the discharge chute and out the bottom of the enclosure for the bottom of the mower deck. String a good quality nylon tie-wrap to the end of the cable, pull the cable taut (in the run position), and secure it to a pin (a short nail or screw works fine) on the bottom of the deck enclosure. This is your over-temp cutout. If a fire or over-temp occurs, the place that will always get hottest first is the discharge. The tie-wrap will melt, the cable releases, and the unit will shut down. The idea is not to have a gasoline-fueled and powered blowtorch if you have an engine fire.
- Mount a simple cooking thermometer dial into the discharge adaptor to monitor the overall air temperature.
- Into the side of the garbage can engine enclosure, on the same side as the discharge, mount a 4-foot dryer hose duct fitting with rivets and seal with RTV. This is your cold air return inlet.
- Fabricate an extended exhaust pipe that loops around the engine base as near to 360 degrees as possible, given your clearance between the engine and the enclosure, and route it out through the engine enclosure wall, then reattach the muffler. I made my extension out of metal electrical conduit using a bender, but you can also use small-diameter spiral wound flex exhaust pipe. Position it so that the air flowing into the gap between the engine and deck (that was created in step three) flows over and around it as much as possible. Be sure to support the extension so that it doesn't vibrate and break off. Seal it where it exits the enclosure with a split flanged ring as if it were a firewall penetration.
- Route the engine throttle control to the outside and mount it conveniently to the deck.
- I had a C-182. So to introduce the hot air, I made a pair of foam cowl inlet plugs with 3-inch holes in the center of each one. Into that I inserted 3-inch-diameter, 90-degree elbows (made from aluminum) into each one and oriented them to face down. I then interconnected them with a tee that had two 3-inch connectors and one 4-inch inlet. Each of the two elbows were then connected to the tee, pairing them down to one 4-inch inlet, using 3-inch flexible dryer hose. Then a section of 4-inch hose was used to connect the tee to the hot-air outlet on the converted blower. I now had the capability of creating heat (from the engine) and blowing it into the engine's upper plenum, through the cowl inlets.
- To return the cool air to the heater engine enclosure, I made one simple rectangular-to-4–inch-round adaptor to fit the open cowl flap, by using a residential forced-air heater floor or ceiling register fitting (standard hardware store stuff). I bent a slight lip at the top edge to "latch" into the top of the cowl flap opening, and slotted the extended bottom edge to clear the cowl flap actuator rod. It would just snap into place with hand pressure. Put it in the pilot's side away from the gascolator drain and crankcase vent line.
- Using cowl plug foam, I made a plug to tightly fit the other cowl flap opening. On this one, be sure to install a "remove before flight" streamer.
- Scraps of cowl flap foam were cut (and safety "streamered" together) to fit the three "gill" cooling air outlets on each side of the cowl. You may have to use your imagination for some other aircraft types, but the objective is to get the hot air well distributed throughout the engine compartment, and to return most of it to the heater, as the heater won't provide heat very quickly in a "once through" operating mode. When not in use, store the adaptors and hoses in the open space above the lawn mower engine inside the enclosure. Keep this storage space in mind when you decide how tall to make the enclosure.
- Paint the whole thing some eye-catching, highly visible color(s) so that when it is anchored to your tie-down ring, no one taxies into it. (Use a bicycle flag and pole to mark it if you expect deep snow.)
- You have a ready supply of fuel on hand by draining all you need from the plane's gascolator or sump. A caution here: Make sure the fuel that you drain doesn't have any water in it before you use it to fuel the heater engine. (Experience speaking here!)
- Use a good grade of synthetic multiviscosity oil in the heater (lawnmower) engine. It's going to have to start very cold and will run pretty hot.
- If you have bitterly cold conditions, keep a spraycan of ether (starting fluid) on hand to get the heater engine going. Caution: Do not have the hot air discharge line or return air lines connected when starting or using ether, and be careful. Ether is explosively flammable if misused. You don't want to suck ether into the heater and blow it into your engine compartment. Exploding your cowl will run your whole day. You may also need an engine blanket for your aircraft during preheat if the heat loss is too excessive.
- After the preheater engine is started and will idle, connect the hot air supply and return hoses. Put the lid on the engine enclosure.
- Bring the pre-engine up to full throttle and monitor the hot air discharge as it rises. If the temperature gets above about 160°F (and not to exceed 180°F — ever), then you need to remove some of the plugs in the cooling air "gills" on each side of the cowl to let a little bit of cold air in to dilute the hot air. Or you can make adjustments by slowing down or speeding up the preheater engine.
In this system nearly the entire 3.5 or 5 hp of the engine is turned into heat either by direct conduction, convection, or friction. It generates a lot of heat (20,000 to 28,000 Btu per hour depending on engine size, compared to around 5,000 Btu per hour for a 1500-watt electric heater), and it flows a lot of air to ensure rapid heat transfer and contact with all parts of the engine. In fact this unit will put out as much heat as you can safely introduce to the engine compartment. How long it takes to preheat is dependent upon the ambient temperature, size of the engine, and the volume of the cowling.
The goal is to get the cases and cylinders warm to the "core" and the oil just warm enough to flow. At near zero temperatures the case and bearings "shrink" around the crankshaft, closing down bearing clearances to nearly zero. And cold cylinders with suddenly hot pistons in them can severely scuff since the clearance at the top of the cylinder at the point of maximum choke can also approach zero.
With this or any preheater, don't get into a rush when warming your engine. It takes time to do the job right. If you do rush the job and crank a cold engine, you can take hundreds of hours off of your engine's lifespan in one cold start.
As a final tip, on my Cessna 182 there was a carburetor air-temperature gauge. When it reaches 50°F to 60°F you know that you have good heat penetration into the engine. Another way to determine if the engine is warm enough is to pull the dipstick and feel the oil to see if it's no longer cold to the touch to determine when you have sufficient preheat.
With either method, it's important to be sure the engine is sufficiently warm for a safe start, or else you've simply wasted time and money, something I'm hoping you can avoid through this article.