When working with an internal combustion engine, we always have to deal with the heat that is created by this process. In many cases, the opening in the cowl will allow enough air to flow over the engine to maintain a cool temperature. But when performing extended 3D maneuvers, we have only the air produced by our prop to keep the engine cool, and sometimes this may not be enough. That is when we want to direct the air to flow over the main engine component that is creating the most heat-the engine head. Extra airflow over the engine can be accomplished by manufacturing ducting inside the cowl to direct the air where you want it to go. Here are three very important facts about air:
- Air will always flow in the path of least resistance.
- Air pressure will form a wall that will prevent any airflow from coming into the cowl if it is allowed to build up. That is why the exit hole is always recommended to be three times larger than the entry hole.
- By funneling air, it will increase in speed.
If we make a ducting system at the opening of our cowl, it leads only to the engine head(s). It will become the path of least resistance that will force the cooling air to travel over cylinder head(s). I used materials common to most model airplane enthusiasts. The cowl ducting can be made from a variety of materials including fiberglass, tin, plywood and balsa wood. Let’s take a look at what we need for the project.
1 The materials I used include (left to right) 3/32 balsa wood for the ducting or baffles, hobby blade, 5-minute epoxy (or 30-minute epoxy), microballoons, pattern transfer gauge, felt-tipped pen and a Dremel tool with drum sander bit.
2 My first step is to increase the airflow coming into the cowl by enlarging this front opening. By increasing the opening size in a downward direction, I also center my cowl entry hole to the engine’s cylinder head.
3 The cowl on this TOC Katana comes in two parts and allows me to work on the lower part while it is still attached to the aircraft. This makes my job of fitting the duct work much easier. I begin by enlarging the entry hole using the sanding drum on my Dremel tool. My Shop-Vac sucks up any dust created from the sanding drum and keeps the area clean.
4 I start by using a pattern duplicating tool to make a rough outline of my engine head. I then transfer this outline to the 3/32 balsa wood. This pattern does not have to be exact and can also be created from cardboard or any other material you want to use.
5 With the outline transferred to my balsa wood, I begin cutting out the major portions with my hobby blade. Then, I trim up the edges and do any final modifications with my Dremel tool. By using balsa wood, this is a quick and relatively easy process. Consider your first piece a pattern piece that may need extra work, or you may need to make an entirely new piece to get it just right.
6 Here is my first piece with some scribe lines that show additional material that needs to be removed. Again, I use the Dremel tool for all the detailed removal. I cut out two pieces, one from the bottom of the opening and the other for the top; in most cases, they will be very close to the same size.
7 I now take both pieces and tack-glue them into place using BSI thick CA glue and placing two to three drops around the edge where the ducting contacts the cowl. Then I hit it with a quick spray of CA accelerator to hold my piece firmly in place. I repeat this process on the upper duct, or baffle, so it is also tacked in securely.
8 I now work on my side baffling. This does not have to be cut with precision; I am only concerned with directing the airflow to the top and bottom of the cylinder head. After cutting my pieces to the correct length and angle, I again tack them in with two drops of thick CA and accelerator. After repeated fittings with my upper cowl and using my Dremel tool with the sanding drum, I finally get a perfect fit. Now, when my upper cowl is attached, there is a 1/16-inch gap between the side baffles and the upper baffle. I want to make sure that everything fits correctly before I final-glue the baffles. To this end, I bolt on the upper and lower cowl, along with the side screws, to ensure I have a proper fit.
9 All that is left to do is to mix up some epoxy and microballoons and apply it to all of the corners of my ducting/baffles. I added some triangle balsa to the bottom of the ducting where it attached to the cowl for added support. Make sure you work with fresh epoxy; if it starts to cure, mix a new batch. Fresh epoxy will flow into the wood fibers and make for a stronger bond. I only mixed up enough epoxy for each corner; I ended up mixing about 12 small portions of epoxy for this side alone.
10 As you can see from this view, I now have a larger opening for air to flow in and help cool the motor. All air that now flows in through the cowl opening has to go over the engine cylinder head on its path through the engine compartment. That makes efficient use of all cooling air, resulting in a much lower overall engine temperature. This is a simple addition to any engine compartment that will always improve your engine’s performance and efficiency. Try it and enjoy!
Nice work on the air ducting. one point you left out is putting a return lip on the bottom of the cowl. this creates a low preasure zone behind the lip and encourages the higher preasure in the cowl to chase the low preasure and in effect creates vacuum and pulls air through the cowl and over the heads. works a treat.
Ross Bathie, can you explain what and where the lip is with more detail?
I’m disappointed that MAN would perpetuate the myth that the exit area needs to be three times larger than the inlet. This is complete garbage and a long-standing modeling legend. As Ross alludes in his comment above, pressure differential is what drives the system. If the air is not given a reason to exit, the size of the exit hole will make no difference.
Air must be directed through the cooling fins. Creating a low pressure area at the exit will ‘power’ the cooling system. This can be done with the general cowl shape, ramps, louvers, Gurney flaps etc.
one question.. What is micro balloons? I never see them advertised in my tower hobbies site or any other. I have used balsa rite for filler but have never seen micro balloons. Been building for 20 years and get confused every time I see this…
Your method is great for the elegant and may be better than my way. I am too lazy to do all that work and just put a full width sheet of foam board with a cut out for the head that is just slightly smaller than the fins and hole for the prop shaft and glue it to the inside of the cowl in front of the engine. Fast, cheap, simple, and most importantly effective. The entering air goes through the cooling fins and does the job. The lip on the exit of the cowl is also done by heating the cowl and bending a lip out or using a 1/4 inch triangle stock glued to the cowl and painted to match.
But now what have you done to increase the exit air. You stated correctly that 3x the exit opening is best but you did not show what you did to accomplish this.
Can poor ventilation create issues other than those created by heat?
My DLE120 takes a while to idle back when in forward flight. On the ground it seems fine.
Is there a pressure increase that could cause this?
After reading some posts I feel I need more ventilation
Brandon, you may need to solder a brass barb on your carb above the fuel pump diaphragm (over the small air relief hole) and place a piece of gas line on that barb to run to the inside of the fuselage. This modification will cancel the pressure variations in the cowl which affect the performance of the carb. The carb will get air to the pump diaphragm from the dead or steady air inside the fuselage. If I could post a photo it would make the explanation much better. This is a fairly common modification on gas engines. LaneC
I feel that exit area is the most overlooked aspect of cowled engine cooling. And the reason is because the solution is “not scale” in that an exit area sometimes requires an additional hole be cut in the cowl that isn’t on the prototype aircraft. A modeler needs to drop the idea or mental image that his “Corsair” might have actually fought in WWII and thus cannot have anything on it that is not to scale. An obsession with “looking cool in the pitts” is another way of stating this problem. Of course, if you plan on competing in Top Gun, then you may have a problem with static judging with engine cooling modifications in the cowl. An acceptable way to open the cowl for exit area and not affect static judging is to cut the cowl flaps and glue them in an open position. Also, less scale, is the use of a pressure lip to create a low pressure area behind the lip to pull hot air out of the cowl. The pressure lip is placed in front of what I call a “cheater hole” to the aft of the engine on the bottom of the cowl. Not as cool in the pitts but very important if you plan on making more than one pass down the runway on a hot day! And, I tell others that the cheater hole and pressure lip won’t be noticed as I zip by at 90 MPH!
Comments are closed.