Chapter 23 - Engine - Cooling System

Overall Approach

During the four years I've been building this plane I've read a lot of interesting theory about cooling. BTUs and Delta Ts. Inlet to outlet area ratios and thermal transfer rates. It's all been interesting, and of some value, but when it came down to the practical implementation I must admit to something of a "suck-it-and-see" attitude.

I want the fastest airplane I can build, so nasty big scoops are a last rather than a first resort. PL says the NACA won't work, so there's an incentive right there to use the NACA as the sole source of inlet air for all five (count 'em) heat exchangers at the back of the airplane. (Note: There are another two at the front in the form of the AC evaporator and the cabin heater.) The five at the back are one big-assed radiator, two 3rd gen stock oil coolers, a stock 3rd gen intercooler and the AC condensor from a '92 Honda Civic. To help with air movement during taxi I added a Ferrari fan and an augmented exhaust system.

Positioning the rads

Radiator and twin oil coolers roughly in place Radiator and twin oil coolers roughly in place Once I had the cowl in place I spent some time trying to figure out where the radiator and oil coolers will go. Greg Richter gave me a great deal on an all aluminum Howe Racing radiator (23 * 19 * 2) which he had bought, but couldnt fit in his Cozy III cowl. I got a 2nd gen (long) oil cooler, but couldn't find a good place for it. I experimented with the twin stock coolers that came with my engine. (Actually one is the stock oil cooler, the other is the stock tranmission cooler. ) They fit nicely either side of the water radiator in the NACA duct. The total cooling area of the two oil coolers is exactly equal to that of the 2nd gen cooler. It'll take one extra pipe, but I think they'll cool well down in the airsteam from the NACA. I made temporary brackets out of 1/16 aluminum. Once I have the pieces all in place, I'll make (or have made) permanent brackets.

George Graham does an EAA tech councilor inspection A little later, once the cowl was finished, George Graham flew over for a visit in his 13B powerwed EZ. He examined my project carefully and pronounced it good. He had one complaint - The engine wasnt properly protected from the elements. He's right, of course. The tape I had blocking all the holes had come off and needed to be replaced. The engine will have to be very carefully flushed and checked before it's run.

Looking closely at his installation I realized that brackets for the radiator and oil coolers would be redundant. George built ducts into his cowl which both direct the air and serve as mounts for the radiators. I have to build ducts anyway - I decided to follow George's example and make a duct. I started with the radiator in wedged in place wih blue foam, then cut a template for the gap between the cowl and the rad. Next I did a 3 foot * 3 foot 3 BID layup on the bench top. I cut the shape of the planned duct in flat fiberglass panels and 2 BID taped it together while adding a reinforcement lip which will bolt to a flange around the radiator.

Building the duct Building the duct I was pretty pleased with how this design came together. If the "me" of 3 years ago had watched the "me" of last night making this duct it would have looked like a guy at childs party making animals with ballons.

Building the duct Once the duct was made, I cut holes in each side for the oil coolers and glassed aluminum mountings brackets to the edges of the holes. AT this point my third Dremel tool for the project died. I rushed out to Home Depot and bought a Black & Decker RTX. Much better than a dremel. Better design and more power. So much power that my cutting wheel got stuck and the RTX broke the retaining screw rather than stall. The cutting tool was rotating at 30,000 rpm when it came off. My abrasive wheel disappeared off down the yard at 60 MPH, never to be seen again.

My cowl flap As I was fitting the finished duct to the cowl and preparing to glass it in place I had a brainwave. Well, at least I think it was a brainwave, I'll let you know later. The idea of cowl flaps has been around for many years. You need lots of cooling when taxiing and during climb out, but once you get to height, or when the weather's cold, you dont need as much cooling. Cooling equals drag, so cowl flaps divert some of the air away from you're draggy cooling set-up whenever you dont need it. Well, the back angled portion of my cowl duct didnt fit very well, so I bent it downwards. This was when the idea hit. If I put this puppy on a hinge I'd be able to let the air out of the back of the duct, straight into my "boat-tail" and out the back to the prop. The air would be coming into the NACA at 200 mph, and exiting the back of the cowl at the same speed. Hmmm. Maybe, once at I'm at altitude I can lift my cowl flap a bit, cut back on the excess cooling, reduce the drag and dump more air into the prop all at the same time. Sounds like a plan to me. I put the back flap on a hinge. Now all I have to is figure a way to open and close it. I know - KISS, but when I spotted this opportunity, I just couldnt resist it. It only cost me an hour or two to cut off the flap and add the hinge. It will be interesting to test this item one day in the future. I'll be really pleased if this gives me a few extra knots.

I finished off the duct and set it inside the cowling. I decided I'd better do a final fit check before glassing it in place. With the radiator in place I remounted the engine and installed the top cowl. There was a half inch gap between the cowlings. When I lifted the lower cowl up to meet the upper cowl I heard a clunk as the rad hit the sump. Damn. I took the duct off again and cut it into 3 pieces. I guess this is what's called "iterative engineering". I rebuilt the duct around the rad, allowing the rad to go an inch lower.

When I came to fit the oil coolers I was looking for a good way to screw them to the duct. I made an aluminum base plate on each side with two nutplates for the bottom attachments. For the top, I decided that the best way would be to drill and tap the aluminum side pieces Greg Richter had made for the rad and screw up into them from below. When I drilled the hole I found that Greg had already anticipated exactly where the oil cooler mounts would go, and had provided threaded holes. I could have kissed him. Well, maybe not, but it's nice when something works out like that.

The bottom cowl is now finished, except that I haven't yet figured out a good way to move the cowl flap.

Cooling Fan

I'd heard on the ACRE site that Lancair's are using cooling fans, so at Sun & Fun I looked for, and found a firewall forward Lancair automotive engine provider who had a cooling fan mounted on the radiator. This makes a lot of sense to me provided that the fan doesnt overspeed, come apart and go through my prop.

My Saab 9000 Turbo will often be running its fan as I get out and walk away from the car. Sometimes, in the Florida summer the fan will run for 10 minutes or so. At first glance this seems like an obvious solution to the problem with cooling at idle, during taxi and after landing. Rather than design the cooling system to handle these low airspeed situations, why not make it much less draggy and design it to handle cruise, then add a cooling fan to make up the difference?

The fan they were using on the Lancair package was a Spal off a Ferrari.

It's operating inside a plenum and wont be trying to cope with 200mph air.
It has a permanent magnet motor so it will resist rotation when switched off.
(I think there's also a wiring trick to make this work even better)
It's designed for a Ferrari that can cruise almost as fast as my plane.
It's been proven on 300mph Lancairs

It costs almost $400.

I'd mentioned the problem to a collegue at work who drives a Ferrari asking where I could go for discounted Ferrari parts. He smiled, and said that when you drive a Ferrari you dont worry about discounts.

A couple of weeks later I just happened to be getting Married (Self reminder - Anniversary is April 27th). You'll never guess what turned up as a wedding gift from my work buddy! A brand new shrink-wrapped Ferrari fan. Wow. The really funny part was that he and his wife were concerned that Charmaine might be upset that they sent a fan as a wedding gift, so they also sent beautiful crystal toasting glasses. They don't know my Charmaine very well. You guessed it - Char glanced breifly at the glasses, then grabbed the fan and spent an hour chatting excitedly to anyone who'd listen about how this would solve all our cooling issues.

The fan is Ferrari Electrofan part number 173030. It weighs 5.5 lb., is 16.5 inches in outside diameter and 4 inches deep at the motor. It has straight sides allowing it to fit in a 16 inch space and it comes with a custom rubber seal. My radiator housing is 16.5 inches so the fan is a perfect fit.


Someone emailed asking for a picture of my NACA scoop. It's fairly standard except that I tried to make it a nice airfoil. I know that the theorists say the NACA won't be sufficient. I plan to try it anyway. I'd like to get away with minimum drag, even if I have to back off on the climb. I can add armpit scoops later if I need to.


My totally unsubstantiated theory is that I can run ducts off the plenum to cool the AC Condenser and intercooler. I cut holes and used PVC pipes to make the ducts. On the left side I added a 3 inch duct for the intercooler. Later I added a 2 inch duct to run cool air to the coils. On the other side The AC got a 3 inch and the turbo intake got a 2 inch. When all the ducts were in I painted the inside of the lower cowl with gold paint. Why? It looks a lot better than bare glass and weighs a couple of oz.

Cowl Flap

Obviously, if you can cool the engine during take-off & climb, you can overcool it during cruise. Ideally there's a need to loose some cooling (and its related drag) during cruise. Earlier on during construction of the plenum I'd figured a way to dump excess air out of the back of the cowling without making it pass through the radiator. The cowl flap is a hinged fiberglass flap which lifts up against the radiator, thus opening the bottom of the cowl.

Eventually I figured out a way to actuate it. I got Charlie to weld three fingers onto a steel rod. On the end he welded an actuator arm. I drilled oversize holes for the fingers in the flap, filled the holes with flox, inserted the fingers, then BID taped around the rod onto the top and bottom of the flap. I'm concerned about this flap because it'll be seeing some serious pressure and will go straight into the prop if it breaks. I think the result is strong enough for the task.

My throttle cable (from was too long at 120 inces, so I used it to actuate the cowl flap and ordered another shorter one for the throttle. The 120 inches was a bit long, even for the cowl flap, but I sent the cable a circuitous route to make it fit. I anchored the outer cable on a piece of aluminum angle bolted to the radiator flange.

cowl flap closed cowl flap open

Expansion Bottles

I asked around about overflow bottles and expansion chambers and got different opinions. Some say you can get away with just an overflow bottle. What convinced me to go with an expansion chamber was that a couple of people mentioned that they'd tried this first, but were always loosing coolant. I'd bought a 16lb cap, but was persuaded to increase this to 21lb. These aren't my bottles. They're designed by Al Gietzen on the flyrotary list for his plane. I asked Al for an explanation of how they work. This is the info he came back with:
The two small tubes (3/16 Al) are air bleed lines from my two radiators.  Both my rads are horizontal flow – tanks on the ends.  Air can be trapped in the upper corners when filling; but also, since the flow velocity in the tanks is very low, it is the likely place that any entrained air will eventually separate.  Because that bottle is at the lowest pressure point in the system (the bottom of that bottle connects to the line near the pump entrance) any air will be driven to that bottle where it is eventually bled from the system through the overflow bottle.

That is a low-level switch - magnetic reed float type, right angle.  You can get it from Mcmaster-Carr.  It is the nylon one which is good to about 300F – Part #47465K83. 

Both bottles would be filled on initial fillup.  As the coolant expands it would be ejected overboard via the overflow bottle until the max temp point was reached.  Air is drawn back into the overflow bottle upon cooling.  Subsequently the air is compressed and expanded in the overflow bottle as the coolant temp level changes.  If the level gets below the float switch it would indicate loss of coolant while the actual cooling system was still full.

The pressure sender at the pump outlet side will see fairly high pressure at full power (temp) as it is reading cap pressure plus pump pressure head, minus the drop through the engine block.  You can play mind games with how the pressure will fluctuate as power level varies, but there will always be positive pressure (above ambient external) at the pump inlet, and somewhat higher pressure in the block, giving max protection against boiling or pump cavitation.

At least that’s the theory; and probably more info than you ever wanted.  Oh, yeah; I haven’t flown this yet, so actual operating data will be interesting. 
Al made his expansion tank from aluminum tubing, and his overflow tank from a fire extinguisher, but the filler necks are more than half the cost of a prebuilt tank, so I ordered a rectangular CAN80-200 tank and a STANT 22-24 PSI RADIATOR CAP W/ LEVER from pitstopusa

When the expansion chamber arrived I mounted it high on the firewall. OK, so now I need a hose from the bottom of the tank to the radiator. I removed the lower cowl, again. I didn't want to remove the radiator and have to replace all the RTV sealant, so I took the whole cowl over to Charlie so he could weld a 1/4 NPT aluminum flange onto the tank. WHen he was done I noticed that the flange was in place, but there was no hole. What? I pay all this money, and I don't get a hole? Charlie didnt want to lug the cowl over to the drill press, so I drilled the hole with a hand drill.

I'm still short of an air bleed from the top of the engine, and an overflow tank, but as of 12/27/03 the cooling system is almost there.

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