pe reivers

I hope this post can provide some guidance on how to set up a cooling for your cowled air cooled gas engine.

For good cooling, only two things are realy needed:

1] The air should pass throug the engine cooling fins and not take the easy route around the engine with less resistance. In order to achieve this, one has to use:
1.1) engine baffles
1.2) jackets to guide and force the air through the fins which must have adequate dimensions.

2] To achieve an air flow, there must be a pressure difference.
To set up a pressure difference a high pressure is needed in front of the baffle, and a low pressure after the baffle.
2.1) High pressure: The centre of the prop is a very lousy fan design, and all it does is stir up the air. This turbulent air is however good for cooling the front of the engine. If the plane has a decent forward speed, the incoming air will provide a pressure high enough. At low speeds and in long verticals, this speed is very low, and thus the high pressure will be absent.
2.2) Low pressure. This can be achieved by carefully looking at the flow around the cowl and making use of the high speed prop blast that hugs the fuselage.

Some principle images of the above:














For radial cowls it looks like this:
If the cowl rear edge flares to a slightly larger diameter than the fuselage, it will greatly enhance the cooling. Black is the basic baffle. The added jacket (in purple) is the ultimate in cooling.
A good example can be seen here: http://www.mini-iac.se/forum/topic.asp?TOPIC_ID=277


Today, I finished a personal test case, using my old trusted and power tuned MVVS58 tow plane engine in different cooling configurations.
To pull the 13.5 kilo tow plane vertically out of sight, 7.0 HP at low rpm is required, which the engine delivers using a TD75 canister muffler. For tuning, an extra boost port with sub-piston flow is added, and a larger reed block is used for better mixture flow.

Observed temperatures after one minute full power (except run 3):
1) Standard engine, Static, full power, In excess of 200°C, (no provisions for extra cooling, not hopped up yet with power ~6.0 hp).
2) Engine as in 1), Power 7.0hp, Static, full power, cylinder jacket added, 158°C
3) engine as in 2), Static, 60% power, cowled in, In excess of 200°C.
4) Engine as in 2), Static, full power, cowled in, air extractor added, 128°C (mixture little rich)
During tests, outside temperatures were about 12°C

Engine 2) with cylinder jacket added for cooling air guiding through the fins. (158°C). The sensor is tied to the back of the cylinder wall.


Engine 2) in plane with cowl fitted, air extraction at cowl bottom only and good flow over the engine head.
This is considered good cooling by most people.
200++°C. 200° degrees was obtained at slightly over half throttle.
WOT lifted temperatures even higher into the danger zone, but was not
sustained. Engine did not show any distress!


Engine 2) special air extractor lower duct wall in place to extract air over the lower cooling fins using the cylinder jacket air guiding. (no temps taken)


Upper duct wall added to guide upper cylinder air flow away from, and provide low pressure exit for lower cylinder airflow. 128°C

_________________

CK1

My Feedback: (60)

Pe ,
Very helpful .
I have done similar cooling fabrications using G10 and glass layups , I have alway been concerned about the use of alluminum or sheet metal because of possible RFI issues wih the metal etching and chattering . Have you noticed any concerns and if so how would you resolve or prevent possible interference problems.
Thanks ,
Chris

pe reivers

The ducting is glued with contact cement (AKA goop?) The cylinder jacket is tied with wire, and kept under tension using silicone tubing inserts between duct and fins.
So far, no RFI, but hard to tell for sure because I use 2.4GHz. which blankets a lot of RFI that is present.

It is however very easy to make the ducts of thin cardboard or mylar, or anything non-metal.

MTK

I mold 1/32" wet balsa around the engine, let it dry and come back with 0.5 glass cloth, epoxy painted in place. The wet balsa goes around compound curves easily. I leave 1/4" clearance (soft mounted engine). It is easy and lightweight

What Pe sez is on the money...you have got to baffle these beasties if you want best cooling in large cowls

pe reivers

What I really wanted to show is how the 3 times larger exit just does not work out. A real small low pressure exit works a lot better, even though the air entry is not even aimed at the cylinder yet. All provided that the air must pass through the engine cooling fins.

aerobear

My Feedback: (4)

Nice diagrams.

Another method to accomplish the baffling on fully enclosed engines is to glue foam rubber to the inside the cowling. It is easily shaped as needed so that when the cowl is in place the foam almost touches the cooling fins, forcing air through the fins. The foam is light and handles vibration well. I used white foam from the fabric department of the local Mega Mart. A quick blast of black spray paint and it disappears inside the cowling.

pe reivers

Good solution.
@Everybody: any pictures of your solutions and results please?

MTK

Yes I know....The exit hole should be allowed to breathe in the right direction. Simply opening a discharge hole does not mean it will discharge. Ram air from the outside may actually enter the exit hole and spoil flow through the engine. It is similar to how sunroof works on a car. These should have a small deflector that pops up at the front, creating a slight negative and vent of the cab

TimBle

Here's one of my solutions using the pipe tunnel to good effect.
Whats missing from the pics but fitted is the air flow splitter just ahead of the pipe tunnel exit.
Also the whole afflr construction is light balse 1.5mm thick and it needs a coat of glass and some thin EPE foam around the engine to close the gap a little more and add sound damping to some extent.
Its all feed by the lower opening in the cowl. I will add some ducting to the inlet next to the spinner if Irequire it

3136

Iused the little aluminium vents you can buy at the hardware store.
I have one in the chin block and one in the back of the scoop.
It could do with a bit of baffling, but if you aren't doing 3d this should be fine shouldn't it?

pe reivers

Those louvers are extremely effective in creating a low pressure zone.

ggraham500

My Feedback: (4)

Where do you find such small vents? I've never seen them that small before.

pe reivers

They mostly are in two rows on a square and you have to snip them free. Walmart or similar stores. Not all have them, so you must be on a constant lookout.

clivemc

pe
nice work - pics and explanation are great - thanks for sharing!
i am surprised at the effectiveness of adding the top cover to the lower fin area. i wouldnt have thought a 'louvre' to create a negative pressure would have worked so well right behind the cylinder head. but clearly it does.

av8tor1977

My Feedback: (6)

You really did a nice job with the louvers. How did you attach them?

Thanks,
AV8TOR

3136

Thanks mate!
I tried a few things which all rattled loose, eventually I used roof and gutter (neutral ph) silicon.
It holds the parts together well and absorbs the vibrations, after 20 or so flights they are held in just as strong.

pe reivers

That happened to be in combination with a richer mixture.
Last part of the test: Leaning the mixture for best power and to get a white insulator tip again (still a black mixture ring down under) then drove the temperature up to 152° static at sustained WOT (previously 200°++)

The cooling jacket exit is a quite narrow chute; it's cross section reduces towards the end thus accelerating the cooling air flow before it merges into the main flow. This has a suction effect on the main flow, and reduces drag. The effective exit area is quite a bit less than a 1:1 area rule, let alone a 3:1 rule (sic!). Working on reducing air entry surface and directing all entry air onto the cylinder could further drive peak temperatures down whilst further reducing plane drag for quicker aerotows or more effective prop thrust at all speeds.

@ 3136
Using contact cement for these bondings works just as well. As the cement hardens (rubber polymerisation) it shrinks some but still remains flexible. After a few days you will have difficulty separating the parts again.

av8tor1977

My Feedback: (6)

I've had excellent luck gluing various oddball things together with "Shoe Goo" from Walmart. It has held dissimilar things together under stress that I would have thought impossible. Think it would work? Or what about just pure silicone glue?

AV8TOR

TimBle

Try Silpoxy  by Smooth On. Its flexible and really tenacious

pe reivers

latest update:
In flight temperatures during normal flight with towering verticals up to sight limits were a peak of 131°C at ambient 15°C

pe reivers

I just received an PM asking:
can a engine get cooled too much that it results in less performance? At what point would this be considered?

That hardly is possible because there must be quite a temperature difference between air and cylinder before any decent amount of cooling sets in.

If the engine at full power would not reach 80°C in the cylinder head that would be of some concern. Never seen that happen though.

hobby_man

My Feedback: (8)

what should we be shooting for with reguards to cylinder head tempature? 250 F 350 F ???

pe reivers

150°C (300F)or less is a safe temperature for air cooled engines. Absolute peak temperatures of 200°C (400F) for very short durations (ROTAX manual). Rich mixtures can manage as low as 110°C (230F) peak. If you measure less, your setup probably is not OK.
Measurement should be done by a sensor under the spark plug. Failing that, measure at the cylinder top in the cooling air leeway. The sensor should not be cooled by the air flow. Sample and hold devices like Venom sells are nice. Datasampling like Eagle tree is very nice.
Infrared gun gun pointing after landing is pointless (pun intended). The IR gun can however be used with the plane static and engine full blast. Point the gun just behind the spark plug. A little black soot rubbed on the surface you point the gun at makes for more reliable readings. Do not measure through the prop disk. The lower prop temperature will influence your reading big time.
Keep in mind, that in a tethered plane the engine runs hotter. The prop center section is not a very good cooling fan.

3136

Hi Pe, I haven't maidened this yet, do you think this will cool ok?
I'm inclined to open up the slots on the sides a bit more.

3136

actually, I think I'll use the louvers again where the red area is marked.
Painted green that should look alright.