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-   -   How much air is needed for proper engine cooling? (https://www.rcuniverse.com/forum/aerodynamics-76/11682678-how-much-air-needed-proper-engine-cooling.html)

meowy84 09-12-2020 04:27 PM

How much air is needed for proper engine cooling?
 
I did a search on this but couldn't find much and wasn't sure which forum to post this in so apologies in advance. Anyways, is there a rule of thumb to determine the volume of cooling air needed for proper cooling of a particular 4 stroke glow engine? I don't mean the rule where you need approximately 3 times the air exit area versus the intake area on cowled planes for instance. I mean how much cfm of air must flow past the cooling fins to satisfactorily cool a given 4 stroke? Or to put it another way (all things being equal) how do you determine the size of the opening in the cowl needed for sufficient cooling?

6a22a30b7c 11-09-2020 06:14 AM

There is a lot of air, and the average temperature of the air is suitable for long-term engine operation. It's simply a matter of exchanging the engine heat with enough air. Even liquid cooled engines do this, they use a fluid to move the heat to a radiator and from there into the air. An air cooled engine simply does it directly. It uses fans to circulate a lot of air over an engine structure with fins to provide enough surface area to exchange the heat.






Kodi nox

meowy84 11-09-2020 07:39 PM

Thank you for restating the obvious theory of heat exchange but it doesn't answer my original question: on average what volume of cooling air (in cfm for instance) is needed for a particular engine displacement when dealing with glow 4 strokes to keep it within the safe operating temperature? (I'm looking for actual quantifiable hard-and-fast rules not just theory.The stats would obviously be different for 2 and 4 strokes and also different for gas powered engines and different for glow engines because of the different heats generated by each fuel type.) Any actual mechanical engineers out there to shed some more light on this?

tedsander 11-09-2020 08:43 PM

I doubt you'll ever get an answer. Too many variables peculiar to each setup to give a meaningful guide. Particular engine design, prop, fuel, cowl shape, entry, exit, smoothness of the airflow, ambient temperatures, humidity...the list is almost endless. Best you can do is be as empirical as you can be - give it as much as the practical configuration will allow. If it gets hot, modify for more, play with baffles, run richer, or whatever it takes. If you have a way to measure temps, so much the better.....

init4fun 11-10-2020 03:29 AM


Originally Posted by meowy84 (Post 12631527)
how do you determine the size of the opening in the cowl needed for sufficient cooling?

I'll agree with Ted , I have never seen CFM specs for model engine cooling (Like your used to seeing in the full scale automotive world , where upgraded radiator fans ARE rated in how many CFMs they pull , for example) .



Outrider6 11-10-2020 07:13 AM


Originally Posted by tedsander (Post 12643699)
I doubt you'll ever get an answer. Too many variables peculiar to each setup to give a meaningful guide. Particular engine design, prop, fuel, cowl shape, entry, exit, smoothness of the airflow, ambient temperatures, humidity...the list is almost endless. Best you can do is be as empirical as you can be - give it as much as the practical configuration will allow. If it gets hot, modify for more, play with baffles, run richer, or whatever it takes. If you have a way to measure temps, so much the better.....

Excellent answer. Especially the part about too many variables. I am (or was) a mechanical engineer, and even though I have forgotten much of what I learned 35 years ago, I never would have been able to look at even my own plane and crunch any numbers to calculate anything. But I could look at my entire situation and "eyeball" the best scenario to cool it.

Do make certain that the exit area is larger than the intake area. If not, then increasing the intake area will accomplish essentially nothing. In other words, the controlling factor in cooling is the exit area, not the intake area. Make both as large as feasible, while concentrating more on the exit area.

jester_s1 11-10-2020 08:47 AM

I don't have a formula either, and I don't think one would even be useful given how different various cowl setups are. Little areas of stagnant air negate a lot of the benefits of the flow that you do have for example. From what I've seen though, it's more about directing the air through the cooling fins than it is about the total amount of airflow. Baffles that don't allow the air to go around the engine instead of through it do restrict airflow but improve cooling. So if you want to make it scientific, it's actually about the amount of air flowing through the cooling fins, not how much air is making its way through your cowling.

tedsander 11-10-2020 10:21 AM

And don't run the engine lean! That oil we all hate to clean up is a major factor in carrying heat away from the engine!

meowy84 11-13-2020 03:37 PM

Thank you to all the esteemed gentlemen who answered. I was afraid there wouldn't be a simple answer (heh heh...there seldom is for anything). I do of course realize the broad scope of what I was asking and all the variables as some of you have pointed out. However, I do see a consensus among you on some basics like the exit area and the baffling and such. As for richening or leaning the engine as a method of temp control, I'm a little hesitant to rely on that method as a primary one because I think that if you have to resort to controlling the temperature to a large extent with the mixture then there are fundamental deficiencies in your cooling system to begin with. I typically try to run my engines a bit on the rich side anyways just for the sake of engine health and longevity. Well, i guess I'll have to fiddle with the baffling and such and make some temp measurements and adjust accordingly. As always, I tip my hat to all who commented. Good info guys. Cheers!

invertmast 11-13-2020 08:10 PM

It depends.

1:1 is the general old school rule of thumb but with proper baffeling, as little as .3:1 (exit to inlet) is sufficient.

jester_s1 11-13-2020 10:03 PM

I'd actually explain it backwards from what Invertmast said. 3/1 exit to entry will still give you enough flow with a marginal design, whereas good baffling and having the only air intake at the cooling fins lets you get away with less exit.

speedracerntrixie 11-14-2020 07:04 AM

As has said already, there are many variables. One that hasn't been brought up yet is the shape of the exit. If at all possible try to create a low pressure area at the exit. For example, on some of the big twin engines some guys used to add a lip at the front of the exit hole. A current project of mine has the bottom edge of the cowl dropping down 1.5" lower then the fuselage. It's much more efficient to pull the air through the cowl then it is trying to push it. As mentioned, you need to get flow through the fins and not just around them. A good example of this is some of the F-1 racing airplanes.

https://cimg3.ibsrv.net/gimg/www.rcu...a1888f814b.jpg
F-1 race plane. Fairly small intake due to efficient baffling.
https://cimg5.ibsrv.net/gimg/www.rcu...e8363fa097.jpg
A current project of mine, the original owner went a bit overboard with the intakes. I've also closed up the bottom.
https://cimg6.ibsrv.net/gimg/www.rcu...a1bd49cc64.jpg
Gas engines produce more heat then equal size glow engines. My YS 140 doesn't require anything more then having the cylinder head peeking out the bottom of the cowl.

astrohog 11-14-2020 01:04 PM


Originally Posted by speedracerntrixie (Post 12644560)
If at all possible try to create a low pressure area at the exit.

That is the basis of the 3:1 theory..........

Astro

speedracerntrixie 11-14-2020 01:46 PM


Originally Posted by astrohog (Post 12644646)
That is the basis of the 3:1 theory..........

Astro


I see your point however with a little thought put into it you can reduce the size of the exit by designing in a low pressure area as opposed to just cutting a huge hole in the bottom of your cowl.
https://cimg9.ibsrv.net/gimg/www.rcu...fa623a8863.jpg
Low pressure area at the exit of the cowl created by extending the cowl below the fuselage bottom.
https://cimg0.ibsrv.net/gimg/www.rcu...716520cfba.jpg
This shows the exit size. This exit along with the holes for the muffler stacks and carb are very close to the intake area.

astrohog 11-14-2020 06:02 PM


Originally Posted by speedracerntrixie (Post 12644652)
I see your point however with a little thought put into it you can reduce the size of the exit by designing in a low pressure area as opposed to just cutting a huge hole in the bottom of your cowl.

Low pressure area at the exit of the cowl created by extending the cowl below the fuselage bottom.

This shows the exit size. This exit along with the holes for the muffler stacks and carb are very close to the intake area.

That exit hole would be more effective with a small lip extending downward As seen in the picture below.

https://cimg3.ibsrv.net/gimg/www.rcu...231a06b063.jpg


speedracerntrixie 11-14-2020 06:29 PM

Quite possibly but what I have will work just fine. I bought the airplane second hand and what I have done is an improvement to what it was originally. I flew it for two years with the old setup and had no cooling issues.

astrohog 11-14-2020 06:52 PM


Originally Posted by speedracerntrixie (Post 12644710)
Quite possibly but what I have will work just fine. I bought the airplane second hand and what I have done is an improvement to what it was originally. I flew it for two years with the old setup and had no cooling issues.

How do you measure an improvement if there were no cooling issues before? aesthetics?

Astro

speedracerntrixie 11-14-2020 07:00 PM

Nope, the prior setup had oversized intakes and no baffles. The new setup will have baffles. Not my first rodeo with large twins.

https://cimg1.ibsrv.net/gimg/www.rcu...61a131b63c.jpg
https://cimg4.ibsrv.net/gimg/www.rcu...0426e9e454.jpg
https://cimg6.ibsrv.net/gimg/www.rcu...eab2198c60.jpg
https://cimg8.ibsrv.net/gimg/www.rcu...f1daae019a.jpg
https://cimg2.ibsrv.net/gimg/www.rcu...8f5dd16dd7.jpg
https://cimg3.ibsrv.net/gimg/www.rcu...09bfcead80.jpg

astrohog 11-14-2020 08:04 PM


Originally Posted by speedracerntrixie (Post 12644717)
Nope, the prior setup had oversized intakes and no baffles. The new setup will have baffles. Not my first rodeo with large twins.

Hmmm. I didn't mention anything about twins or baffles or how many rodeos you have attended. I asked how you measure an improvement over a previous setup that had no issues?

Astro

speedracerntrixie 11-14-2020 08:17 PM

Common sense and decades of experience. A full cowl with baffles is without a doubt going to cool the engine better then a hack job on the cowl. Don't you have anything better to do then follow me around trying to bust my chops? Oh wait, almost forgot one of my favorites. BTW, none of these experienced any overheating issues.

https://cimg0.ibsrv.net/gimg/www.rcu...fac1517b49.jpg

franklin_m 11-15-2020 06:02 AM


Originally Posted by astrohog (Post 12644704)
That exit hole would be more effective with a small lip extending downward As seen in the picture below.

(CORRECTED)
Astro is correct here. Mechanical things like these lips function to redirect airflow in order to expand the low pressure area downstream ... which enhances the airflow through the cowl.

These and other air exits are sometimes placed on the bottom of the aircraft for the simple reason that air flowing through those fins often pick up dirt and minor bits of oil and other fluids, and most owners would rather have that dirt on the bottom of the plane than on the sides. However, that is not universally true either.

Below is an example of the other low pressure areas on that same cowl, which sure looks like it's a super cub.

https://cimg7.ibsrv.net/gimg/www.rcu...633c2234f6.jpg

franklin_m 11-15-2020 07:07 AM

An actual expert in this, particularly for model aircraft, is Andy Lennon. He wrote a lot for MAN and other hobby magazines. He also holds a degree in aeronautical engineering. On page 78 or his book, he says: "The cooling air exit(s)' rule of thumb is the total exit area be 140 percent of the entry area."


jester_s1 11-15-2020 12:17 PM

Ok lovely people. While I recognized and fully affirm the life and death importance of keeping engines on toy airplanes at optimal operating temperature and the need to fight to the death with anyone who even slightly disagrees with you, I think it is possible for everyone who wants to discuss this vital topic to do so in a civilized manner.

I've deleted the bickering. It's a shame too, because there was some good info in there.

Here's what I tell my 12 year old students:

1. If you know you can't talk with someone without getting into an argument, then don't talk to them.
2. If your only reason for saying something is to bait or create conflict, then don't say it.
3. If you see yourself getting angry, offended, frustrated, or otherwise despising someone, remove yourself from the situation and ask for help if you need to.

Most 12 year olds can do all of this. I think men in their 40's and 50's can too. How about giving it a try and seeing how it goes?

astrohog 11-15-2020 12:31 PM

I don't believe I broke any forum rules with any of my posts. If I did, I would certainly love clarification so it doesn't happen again.

Astro

jester_s1 11-15-2020 08:01 PM

Pm'ing, you Hog.


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