Go Back  RCU Forums > RC Airplanes > Aerodynamics
Reload this Page >

How much air is needed for proper engine cooling?

Notices
Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

How much air is needed for proper engine cooling?

Old 09-12-2020, 04:27 PM
  #1  
meowy84
Thread Starter
 
Join Date: Oct 2004
Location: , IN
Posts: 1,063
Received 2 Likes on 2 Posts
Default 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?
Old 11-09-2020, 06:14 AM
  #2  
6a22a30b7c
Junior Member
 
Join Date: Mar 2020
Posts: 25
Likes: 0
Received 0 Likes on 0 Posts
Default

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
Old 11-09-2020, 07:39 PM
  #3  
meowy84
Thread Starter
 
Join Date: Oct 2004
Location: , IN
Posts: 1,063
Received 2 Likes on 2 Posts
Default

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?

Last edited by meowy84; 11-09-2020 at 07:43 PM.
Old 11-09-2020, 08:43 PM
  #4  
tedsander
 
Join Date: Jul 2004
Location: White Bear lake, MN
Posts: 397
Likes: 0
Received 25 Likes on 25 Posts
Default

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.....
The following users liked this post:
daveopam (11-10-2020)
Old 11-10-2020, 03:29 AM
  #5  
init4fun
 
init4fun's Avatar
 
Join Date: May 2009
Location: Massachusetts
Posts: 3,690
Received 15 Likes on 12 Posts
Default

Originally Posted by meowy84 View Post
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) .


Old 11-10-2020, 07:13 AM
  #6  
Outrider6
Senior Member
My Feedback: (2)
 
Join Date: Apr 2020
Location: Metro Atlanta GA, USA
Posts: 101
Likes: 0
Received 7 Likes on 7 Posts
Default

Originally Posted by tedsander View Post
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.
Old 11-10-2020, 08:47 AM
  #7  
jester_s1
Moderator
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,095
Likes: 0
Received 20 Likes on 17 Posts
Default

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.
Old 11-10-2020, 10:21 AM
  #8  
tedsander
 
Join Date: Jul 2004
Location: White Bear lake, MN
Posts: 397
Likes: 0
Received 25 Likes on 25 Posts
Default

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!
Old 11-13-2020, 03:37 PM
  #9  
meowy84
Thread Starter
 
Join Date: Oct 2004
Location: , IN
Posts: 1,063
Received 2 Likes on 2 Posts
Default

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!
Old 11-13-2020, 08:10 PM
  #10  
invertmast
My Feedback: (23)
 
invertmast's Avatar
 
Join Date: Dec 2003
Location: North Port, Fl
Posts: 7,909
Received 13 Likes on 11 Posts
Default

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.
Old 11-13-2020, 10:03 PM
  #11  
jester_s1
Moderator
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,095
Likes: 0
Received 20 Likes on 17 Posts
Default

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.
Old 11-14-2020, 07:04 AM
  #12  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 7,944
Received 56 Likes on 53 Posts
Default

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.


F-1 race plane. Fairly small intake due to efficient baffling.

A current project of mine, the original owner went a bit overboard with the intakes. I've also closed up the bottom.

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.
Old 11-14-2020, 01:04 PM
  #13  
astrohog
My Feedback: (1)
 
astrohog's Avatar
 
Join Date: Dec 2001
Location: Bellingham, WA
Posts: 2,351
Likes: 0
Received 2 Likes on 1 Post
Default

Originally Posted by speedracerntrixie View Post
If at all possible try to create a low pressure area at the exit.
That is the basis of the 3:1 theory..........

Astro
Old 11-14-2020, 01:46 PM
  #14  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 7,944
Received 56 Likes on 53 Posts
Default

Originally Posted by astrohog View Post
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.

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.
Old 11-14-2020, 06:02 PM
  #15  
astrohog
My Feedback: (1)
 
astrohog's Avatar
 
Join Date: Dec 2001
Location: Bellingham, WA
Posts: 2,351
Likes: 0
Received 2 Likes on 1 Post
Default

Originally Posted by speedracerntrixie View Post
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.



Old 11-14-2020, 06:29 PM
  #16  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 7,944
Received 56 Likes on 53 Posts
Default

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.
Old 11-14-2020, 06:52 PM
  #17  
astrohog
My Feedback: (1)
 
astrohog's Avatar
 
Join Date: Dec 2001
Location: Bellingham, WA
Posts: 2,351
Likes: 0
Received 2 Likes on 1 Post
Default

Originally Posted by speedracerntrixie View Post
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
Old 11-14-2020, 07:00 PM
  #18  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 7,944
Received 56 Likes on 53 Posts
Default

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







Old 11-14-2020, 08:04 PM
  #19  
astrohog
My Feedback: (1)
 
astrohog's Avatar
 
Join Date: Dec 2001
Location: Bellingham, WA
Posts: 2,351
Likes: 0
Received 2 Likes on 1 Post
Default

Originally Posted by speedracerntrixie View Post
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
Old 11-14-2020, 08:17 PM
  #20  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 7,944
Received 56 Likes on 53 Posts
Default

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.


Old 11-15-2020, 06:02 AM
  #21  
franklin_m
 
franklin_m's Avatar
 
Join Date: Nov 2005
Location: State College, PA
Posts: 3,688
Likes: 0
Received 1 Like on 1 Post
Default

Originally Posted by astrohog View Post
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.


Last edited by franklin_m; 11-15-2020 at 09:03 AM. Reason: Corrected - brain f*rt
Old 11-15-2020, 07:07 AM
  #22  
franklin_m
 
franklin_m's Avatar
 
Join Date: Nov 2005
Location: State College, PA
Posts: 3,688
Likes: 0
Received 1 Like on 1 Post
Default

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."

Amazon Amazon
Old 11-15-2020, 12:17 PM
  #23  
jester_s1
Moderator
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,095
Likes: 0
Received 20 Likes on 17 Posts
Default

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?
The following users liked this post:
allanflowers (11-16-2020)
Old 11-15-2020, 12:31 PM
  #24  
astrohog
My Feedback: (1)
 
astrohog's Avatar
 
Join Date: Dec 2001
Location: Bellingham, WA
Posts: 2,351
Likes: 0
Received 2 Likes on 1 Post
Default

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
Old 11-15-2020, 08:01 PM
  #25  
jester_s1
Moderator
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,095
Likes: 0
Received 20 Likes on 17 Posts
Default

Pm'ing, you Hog.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.