TedMo

Opening a can of worms? F rom all I have read in past engines/motors require cooling to operate properly. This is of particular importance to those that are cowled and especially for electric powered aircraft; understandable. This is what has me perplexed. I have read that it is necassary to have an air inlet and outlet with outlet about 3 times larger then the inlet. Well I presently own and fly 25 models of varying sizes and types, glow and electric. None of them meet the requirements as stated. We will concern ourselves strictly with cowled aircraft since those open to the air are not likely to be a problem. Ok, I have several that have no air outlets at all; these include an At-6, Mustang both of which are 40 size electrics, others have very minimal outlets. They all fly well and I have not had overheating problems of any kind. I'm very pleased but also very perplexed. Anyone with any comments?

DavidAgar

My Feedback: (108)

I have one word that seems like it might fit. Lucky.....  I am thinking that you would be the rare one. I know what happens to my engines if I do not have an air outlet... Good Luck, Dave

LesUyeda

I have found that with electrics, the vast majoriity of the heat is in the batteries, not the motor.  If you are not allowing cooling air around your batteries, you are doing them a disservice, and they will not last as long as they should.  Unless you are really pushing the motors, they will generate very little heat, and are much more toleerant.

For fuel type airplanes, the only cooling the engines get is air going past the cylinder head fins, and it is critical that they be allowed to cool properly, or they will get too hot, pre ignite, and seize.

Larger outlet than there is inlet, because you want the air to flow, not just to pack the engine area.  By having a larger outlet than inlet, you create a negative pressure inside the engine compartment, so that air is sucked into the compartment.  P-51 oil cooler is a prime example.  Originally with the inlet open the oil got hotter and hotter, until they opened up the outlet area=no more problem.

Les

TedMo

I agree with all comments. Maybe lucky is the explanation. I fly very aggresively so would expect a problem. These aircraft are ARF's that are built in this manner, not something I have done to inhibit air flow. This is why I'm perplexed since I would think proper air cooling would have been taken into consideration in the design. With my electrics I generally fly using 3 or 4 cell Li Po's of 2200 Ma 25 C and time my flights for about 7-8 min. fter a flight batteries are not hot just warm and generally have about 25-35% of charge remaining. I'm not attempting to cause any argument only comments and understanding. Very pleased that I'm lucky to be doing this well. I've been building and flying for many many years and enjoy reading these forums since still learning.

Lnewqban

For glow engines, a great deal of heat is released by the exhaust gases and the unburned oil, more for rich settings.
Castor oil also helps keeping parts lubricated at high temperatures.
If the muffler is exposed to the air stream, much heat transfer from the cylinder to the cooler muffler.

However, your case is remarkable, and I believe your engines may be working too close to seizure.

DRC1

My Feedback: (10)

Do you have any pics of your planes with the cowls on? Although you may not have intentional exhaust vents it is possible that any holes or openings to route the exhaust or just gaps between the fuse and cowl are enough to keep the engine cool?

Very curoius to pics of the cowls on you fleet...

LLD

da Rock

Here's the deal............

Yes, you need cooling air.  But it's not cooling air unless it's required to pass close to the hot object.  Allow lots of space around the object, and the air will NOT choose to pass close to the hot air around the hot object.  That's because hot air is harder to move than cool, and the air coming into the area will simply move around the hard to move hot air that's around the hot object.  The cool air coming in will simply follow the cool air that's leaving.   So whatever you want cooled often needs baffling around it.

As for having read that you need 3X the exhaust.......  that's not really true and in fact is not even practical most times.   1.5X works well.  No matter what ratio, the exit needs to be in an area of negative pressure or neutral pressure.  It won't work no matter what size, if it's got airflow into it (positive pressure).    

da Rock

You wonder how your models that have what looks like no hot air exits can be working?

The things that might need cooling might not be getting hot enough from your usage to need cooling.   Batteries often have their life shortened by lack of cooling.  They don't always melt and drip out of the plane.   But they do go bad "too soon".    Or don't deliver power they might otherwise produce.  How would anyone know they should be getting a 20 minute flight instead of 10 minutes if that airplane has never had good cooling? 

TedMo

I wish to thank all that replied to my can of worms. All interesting comments and in agreement with my own thoughts. Many comments pretty much were in respect to cooling for glow but my main concern was the lack of cooling for electrics; which appears to be insufficient judging by what has been stated is needed. I'm not complaining since I've been lucky I guess. Having been involved in RC for over 50 years I still don't know all the answers so am still looking for them. No sense argueing about the fact that whatever source of power; it needs cooling. My point is to try to understand since this is so why are so many aircraft, particularly electrics, designed without sufficient air inlets and outlets? Or have we been mislead to believe what is necassary? My most drastic aircraft that lacks cooling is my E-Flight T-6 which has no exit at all; with next in line is my Hyperion Mustang which has one exit hole about 1"x1.5" total area. They have not given me any problem but just arouse my curiosity. Thanks again for you replies

JohnBuckner

My Feedback: (1)

Only having a few electrics to dable with since my primary interest has always been and will remain glow. I know virtually nothing when it comes to electrics. But one can speculate that when a discussion of the effects of undercooling with an electric system and a glow system are very different and the effects on the glow system will be quite apparent with possible engine failure in flight but it seems that battery damage is the more likely early effects of an undercooled electric system and that damage is likely cumlative with the first result as someone already mentioned in this thread is just a shortened battery life. That of course would not be so readily apparent or obvious.

John

jooNorway

Would be a big difference wheather you run a nice rated setup far from max load, or if you run parts of the setup at the max limits.

da Rock

I've helped a guy who is physically challenged build and fly a bunch of airplanes for some years now.  He loves the stuff and we're always working on a new one.  He also likes electrics if only because he can't start or adjust gas/glow.  We've found that any glow arf that's converted needs battery/esc cooling.  How did we learn that?  We ruined a few.  We also had a few that "ran out of battery" way too soon after takeoff.  And discovered they did better if the cooling we added was actually cooling, not just routing air through the fuselage.  It's not really a surpise with a model that wasn't designed for electric. 

We also discovered that a lot of the electric ARFs don't have adequate cooling.  Some don't have any, and a couple even box the battery completely.  Why do they do that?   Lots of ARFs are mfg'd by toy factories.  If you have watched what's for sale over the years, you will have noticed the fairly quick demise of a number of electric ARFs.  A bunch of them were battery/ESC killers.  And their mfg's simply sold 'em, ate the complaints and moved on.  Lots of those complaints went to the battery retailers.

Gray Beard

My Feedback: (-1)

There is another forum here for electrics and if you posted there most of your answers would have nothing at all mentionedabout wet engines. That 3 times outflow is for people living in a perfect world but you don't see it often but you do see air flow venting a lot. Ihave found it in some Epowered ARFs I have assembled for people. Just small vents someplace on the fire wall with a closed in area with an outlet for the air so it flows over the battery. MOst the builders I know make up there own vent system in there planes.
Iwas given a very nice complete giant scale Decathlon because the owner couldn't keep the engine running. Ijust opened up the imbossed areas in the cowl for the air vents and opened up the bottom of the cowl for the outflow, they didn't bother with venting. The engine runs just fine now, it really is important for both wet and electric, just a bit different.

jetmech05

Let's look at our full scale brethren.....I have crewed and worked on round motored round cowl airplanes.....there is no way that cowl flaps on lets say a Convair 440 gives even 1.5 times the exit that the entrance has......true some of the airflow exits through the bellmouth....But those titty bitty cowl flaps work very well....
with no exit cowl flaps closed, CHT rises quickly, like right now quickly...a good exit is required for cooling

da Rock

Radial engines in nacelles like on a Convair 440 are in quite a different world than our models.  The inflow area of those mothers actually has an initial step down.  They call the things "pressure cowls" because there is baffling around the cylinders that reduces the wide open intake area down to an area directly adjacent to the fins on the cylinders.   Then the outlet areas behind the cowl flaps is in what you'd most emphatically call "negative pressure".  So we'd be comparing apples to .....  hmmm.... apples to watermellons.

The 1:1.5 ratio was evolved for models at model speeds.  It seems to be more than appropriate for the electric conversions.  But would rely on the outlet being in neutral pressure worst case.

jetmech05

Rock remember that a Convair 440 was powered by a P&W  R2800-52W. twin row....there is no baffleing on the cowling.....or the engine as this would restrict airflow to the aft row of jugs......Unlike most round cowled engines the Convairs had a bellmouth in the back of the motor...but had 4 only 4 very small cowl flaps at 2, 4 , 8, and 10 oclock positions......I have a big scar on my left thumb from the one at the 10 oclock....anyway what I was getting at...was with the cowl flaps closed CHT went up right now....no exit......

da Rock

Yeah, I got my references mixed up. Was going to include a picture of the baffling in the pressure cowl in a T-6.But my error and it's correction illustrates even more the issues we face. Different engines (single row vs. twin row) and different operating speeds and..... on and on and on............

Goodpoint about the result with flaps closed. Of course, when you've got two rows of cylinders pumping outheat instead of one..........

(You know, the 440 was a graceful looking little airplane. They were in use byEAL when I hired on.)

I figured I might as well attach a picture that shows something worth seeing.Notice the direction of the arrows all around this cowling. Looks like only one area of negative pressure available for theexit. (the green area of arrows)

TedMo

Well appears my can of worms got lots of response, thanks to all. my conclusion is since all of us realize cooling is a necassity best thing to do is provide it.

jbdismukes

My Feedback: (2)

Rock,

On another thread, I saw a pic of a metal "wind tunnel" which had been fabricated to attach to the edges of the baffle opening and extended back the approximate depth of the cylinder with minimal clearance all around. Assuming there was adequate air exhaust at the rear of the cowl, it would appear that all air drawn through the baffle was directed through the cooling fins of the cylinder. Seems a good idea to me, but is it overkill as compared to just a baffle at the fron edge of the cylinder?

Jack

jetmech05

Rock.....Yes it was, I was on them at Andrews Air Force, Navy side,  Marine Aircraft Support Detachment from early 77 until late 79...Remember the oil tank was in the top of the main wheel wells...well I had this Gunny that used to preach over and over again about not servicing oil with the prop feathered...cause there's 5 gallons of oil in the prop dome in feather.......
Sittin at my desk one day and in walked the Gunny,  black from head to toe...there wasn't anywhere on him that wasn't coverd in thick(60 Weight) oil....couldn't help myself I started rollin...I said "there was this Gunny I knew that used to preach about servicing oil with the prop feathered...ever meet him?".....he questioned my parents hertiage..and I got to help him pull the tank......he bought the beer after we were done
I still chuckle

da Rock

Tight baffling the airflowis one of those decisions that's appropriate sometimes but not most times. I don't fly any of my R/C models for speed, so never considered going to all the trouble. However, I usually prop almost everything with as much diameter as possible on the prop.Therefore, I carefully select props AND almost always do a little bit of work on the airflow through the cowl. About all that's usually necessary is to either block or baffle theoffside intake when the cowling has two, or blockmost of the openingin a cowl like you'd see on a Convair 440 or a Corsair. The idea is to stopany airflow that'd naturally bypassthe cylinder. Cool air coming into the openings will simplyfollow any air that's exiting. Cool air is easier to move than hot. Air coming in that encounters hot air has difficulty moving it and won't bother if there is coolto move. So whatever exit is working will havethe easily moved cool filling it on the way out.

My P47's engine run wasn't what I wanted the first few outings. There really wasn't much exit area, but a bunch of intake. I hadn't planned to use the dummy enginefor a couple of reasons,so the entire opening wasfilling the cowl with air. And what went out the exit area wasn't taking much heat with it. And the planemaidened in the hottest time ofthe year here. So I took about a half-hour and put in a baffle thatblocked off the worthless intake area. The engine flew great the next time out with the new baffle and that day was appreciably hotter than the previous ones.


......BEFORE ................... AFTER