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Author Topic: Older designs - engine cooling  (Read 2659 times)

Offline Mark Legg

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Older designs - engine cooling
« on: December 09, 2020, 01:28:11 AM »
Growing up in a world of British diesel powered designs, lots of provision is made around the cowl for keeping them cool.

I'm currently doing a van loo chipmunk, and I've noticed on a lot of other designs of the era, there doesn't appear to be much in the way of letting hot air out of the cowling, just the head poking out of the bottom and the exhaust slot.

What's the reason for that? Do glows naturally run cooler? Or should I add more means of heat escape? I mean it obviously worked for them. And if I can get a way with less cutting and carving, I will....

Offline Gary Dowler

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Re: Older designs - engine cooling
« Reply #1 on: December 09, 2020, 02:17:15 AM »
I have zero experience with diesel airplane engines, but the glow engines do well with what might appear to be minimal airflow.  They need it, yes, but if the design is a proven one, such as the Super Chipmunk (which I’m building as well) then the airflow you see in the design is adequate for the task. 

You can aid the cooling issue by running fuel with extra castor oil in it. Castor carries away heat in a way that synthetic oil simply cannot duplicate.   The high castor is essential in many older engines such as Fox or McCoy, etc.


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Offline John Park

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Re: Older designs - engine cooling
« Reply #2 on: December 09, 2020, 04:23:52 AM »
Diesels certainly do run a lot cooler than glows, and we never noticed any cooling issues with engines like AM35s, PAW .19s and ED 2.46 Racers in fully-cowled stunters such as the KeilKraft Spectre.  (The only problem with the Spectre was the kit tank, which was hopeless with any engine except the super-tolerant ED Racer!)  If I were building one of those old British designs today, for a glow motor, I might consider modifying it to allow a better flow of cooling air.  However, the Mercury Crusader, which had no more provision for cooling than the Spectre, suffered no overheating issues that I could detect with an Enya .29-III on 25% castor/5% nitromethane fuel.
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Offline Dennis Toth

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Re: Older designs - engine cooling
« Reply #3 on: December 09, 2020, 05:48:09 AM »
Mark,
If you are doing diesel and it is overheating add more ether to the fuel blend. For the glow engine if the head is exposed that takes care of the majority of the cooling needs although airflow through and around the case and upper fin section needs to be maintained, generally air exit should be 150% of the air inlet area. With glow you will know if you are having a slight over temperature issue if you hear the engine crackling as it gets loaded. One quick way to deal with this is to reduce the prop diameter in 1/4" increments. Also can go to a little cooler plug. Extreme overheating causes run-a-way or sagging. In extreme case remove the cowl and test fly to see if it is an airflow problem or a prop load. If prop load it will still overheat without the cowl in most designs. Also, as John stated make sure your fuel in the older cast iron motors has mostly castor and at least 27% ish total oil.

Best,   DennisT 

Offline Mark Legg

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Re: Older designs - engine cooling
« Reply #4 on: December 09, 2020, 07:37:35 AM »
Thanks for the replies.

Sorry, I should have made it clear what I was using. I'll be using an OS 35 Max-S in the chipmunk I'm building, and was just thinking about airflow as I'm now on with the cowling.

John, I've built a spectre, and it has a Fora Junior diesel for power. I had to add extra cooling slots at the rear of the cowl as it just kept getting hot and stopping - This is what got me thinking about Diesel Vs Glow.

If I can get away with just an inlet at the front, hole for the head and the exhaust slot, all the better! Makes for a much cleaner looking nose.
 

Offline Jim Svitko

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Re: Older designs - engine cooling
« Reply #5 on: December 09, 2020, 08:24:19 AM »
I make an air exit tunnel, or slot, in the bottom of the cowl.  I think it is necessary for air to flow around the engine and thru the engine compartment, thus carrying heat away.   With a tunnel on the bottom, it is pretty much out of sight.  And, with some careful work, it does not detract from the overall appearance.  I have done this on numerous planes with no indication of over heating.

Offline Robert Zambelli

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Re: Older designs - engine cooling
« Reply #6 on: December 09, 2020, 09:33:40 AM »
And here's another thing to consider.
If there is insufficient venting for exit air, you may wind up heating the tank compartment which will definitely change the engine performance.

Just my 2 cents.

Bob Z.

Offline John Park

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Re: Older designs - engine cooling
« Reply #7 on: December 10, 2020, 05:31:17 AM »
John, I've built a spectre, and it has a Fora Junior diesel for power. I had to add extra cooling slots at the rear of the cowl as it just kept getting hot and stopping - This is what got me thinking about Diesel Vs Glow.
Might need more running time.  PAWs can be a bit like that until they've had several hours' running - but of course, once they're well run-in they last for EVER! 
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Online Brett Buck

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Re: Older designs - engine cooling
« Reply #8 on: December 10, 2020, 09:52:17 AM »
Growing up in a world of British diesel powered designs, lots of provision is made around the cowl for keeping them cool.

I'm currently doing a van loo chipmunk, and I've noticed on a lot of other designs of the era, there doesn't appear to be much in the way of letting hot air out of the cowling, just the head poking out of the bottom and the exhaust slot.

What's the reason for that? Do glows naturally run cooler? Or should I add more means of heat escape? I mean it obviously worked for them. And if I can get a way with less cutting and carving, I will....

   I have nearly zero experience with diesels, but there's no particular reason to have limited or reduced cooling for glow engines. For conventional muffled engines (vintage, presumably) you don't want to scrimp on cooling outlet area. If you don't have enough cooling, it won't hold the setting, tending to "go lean" at some point in the flight, or taking a long time to recover the level flight speed/setting. It probably doesn't really "go lean", it just stays or takes a long time to recover in a 2-stroke due to thermal effects - hotter, so it will fire on every stroke despite an otherwise too-rich mixture.

     From some of the responses I can see you are talking about a OS-35S - those are pretty well-known for being "meltdown specials" due to the (very unfortunate) inclusion of two venturis. What seems to happen over and over is that people see the two, figure they can Get More Power using the big venturi, and then have burn-down after burn-down in their stunt plane. The real problem is the excess venturi, not cooling, but by the time people realize it, they have lost the small one. We used the smaller venturi from the OS25S. Randy knows what the dimensions of the small OS-35 venturi are, I don't, I just ended up putting my OS20S and 25S "small" venturis in various other people's 35Ses to get them to run - and never getting them back!

    Brett

Offline Steve Helmick

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Re: Older designs - engine cooling
« Reply #9 on: December 10, 2020, 01:16:39 PM »
Perhaps the plans don't clearly show the cooling air exhaust...probably a slot in the bottom of the fuselage nose. IIRC, the rule of thumb is to have the outlet half again bigger than the cooling inlet, because heating the air causes it to expand.

As nice as an OS .35S is...they're generally considered a bit weak for a kit-built Chipmunk. The .46LA would be better, especially if you need a muffler. A ST G.21 .46 ABC* would also be really good.  y1 Steve

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Offline Jim Svitko

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Re: Older designs - engine cooling
« Reply #10 on: December 10, 2020, 01:38:01 PM »
I would also recommend the 46LA for the Chipmunk.  An Aero Tiger 36, if you can find one, or Magnum 36, would also work pretty well.

You would have to build the Chipmunk very light for the OS 35S to have a chance.   You will enjoy the plane much more if you give it more power.

Offline Air Ministry .

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Re: Older designs - engine cooling
« Reply #11 on: December 12, 2020, 06:31:27 PM »
If initially you can leave the cowl off , you get a cross check as to if its working . the flow through . Whole Head OUTTside maybe ?? you need 1/2 the cooling ??

The top vent set up is somewhat so it wont perculate on the ground after landing . But a C' Case cooling outlet / inlet can help stop runnaway , seems if the heat up they can switch & stay locked lean .

Offline Mark Legg

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Re: Older designs - engine cooling
« Reply #12 on: December 14, 2020, 10:14:02 AM »
Thanks for the replies everyone. The OS 35 I have is in extremely good condition,  I'm not sure it's even broken in as it still has light honing Marks on the liner.

I will be running it with a tongue muffler only to keep weight down, and on the small venturi (I've made plugs to reduce it if that is too big)

This isn't a kit chipmunk, it's a scratch built jim van loo one, using the very best quality wood I can find, so we're looking at coming out under 40oz at the moment if I'm careful with the paint. I deliberately chose the OS max as I'm familiar with them and they're light.

Will see how we go

Cheers

Mark

Online Brett Buck

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Re: Older designs - engine cooling
« Reply #13 on: December 14, 2020, 10:38:00 AM »
Thanks for the replies everyone. The OS 35 I have is in extremely good condition,  I'm not sure it's even broken in as it still has light honing Marks on the liner.

I will be running it with a tongue muffler only to keep weight down, and on the small venturi (I've made plugs to reduce it if that is too big)

This isn't a kit chipmunk, it's a scratch built jim van loo one, using the very best quality wood I can find, so we're looking at coming out under 40oz at the moment if I'm careful with the paint. I deliberately chose the OS max as I'm familiar with them and they're light.

   If you know it runs OK, then by all means, go ahead. But to the original question, put in what you think is sufficient cooling air exit, and if it show inadequate cooling (presumably causing it to take several laps after, say, the square 8, to settle down to the steady-state setting), make it bigger.

     I would also point out that you need to make sure that the air actually goes around the cylinder instead of bypassing it. So the "choke point" needs to be near the engine inside the cowl.
 
      Brett

Offline Mark Legg

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Re: Older designs - engine cooling
« Reply #14 on: December 14, 2020, 10:58:51 AM »
Thanks Brett

I'll have a mess about. I'm not a competition flyer or anything, yet..... we don't do much in the way of classic stunt over here in the UK. It's more for down the field fun when I'm not flying combat comps.


Offline Mark Legg

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Re: Older designs - engine cooling
« Reply #15 on: December 16, 2020, 11:50:25 AM »
The OS has a 7mm venturi with a 4mm spray bar, does that sound about right?

Offline Air Ministry .

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Re: Older designs - engine cooling
« Reply #16 on: December 17, 2020, 08:52:11 PM »
" The OS has a 7mm venturi with a 4mm spray bar, does that sound about right? "

Yea.

Maybe try a 11 x 4 prop , if its light, & you wanna go slow . Theyre ( OS 35 ) good on the 11 x 4 3 blade Tornado, anyway . Up to a 11 x 5 wood, maybe thinned .

Id try it ( on the bench ) with & without the toungue muffler , to see if it chokes or overheats things . Some'd breath easier than others .

Offline Chris Wilson

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Re: Older designs - engine cooling
« Reply #17 on: March 10, 2021, 08:26:26 PM »
A lot of the older designs used no muffler with the engine and so its not hard to imagine high speed exhaust gases exiting a cowl dragging the cowls contents out with it through the slotted clearance around the manifold.
Especially so with side exhaust engines.
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Offline Howard Rush

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Re: Older designs - engine cooling
« Reply #18 on: March 11, 2021, 05:42:23 PM »
Castor carries away heat in a way that synthetic oil simply cannot duplicate.

IIRC, the rule of thumb is to have the outlet half again bigger than the cooling inlet, because heating the air causes it to expand.

Doesn't speak well for Washington science education, does it? 
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