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Jul 18, 2019, 01:45 AM
Right on the Edge!
Thread OP
Help!

Engine baffling and air flow. Will this work?


Hi there,

I hope this is the right section.

I just modified the cowl to baffle my engine, a 60 CC twin, and was wondering if there's anything I can do to improve my job or if it is already OK. My concern is not aesthetics (as you can see! ) but functionality. If it serves a cooling aid, that's all I care about.

So basically what I did was to add plywood baffles to the front, close any other openings (as much as possible) in the front of the cowl, then open four outlets holes in the back, as you can see. This is actually the part I am most doubtful about. Will that be enough to let hot air out?

The inside of the fuselage is set up to accomodate a canister, which I don't use, so I would guess that this area also functions as an outlet, since it has two wood mesh from which the air can flow out (check pics).

It is the first time I ever "baffle" an engine so I am not sure I am doing things right!...

Thanks guys
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Jul 18, 2019, 09:19 AM
Registered User
Yes what you have done will work just fine. The vents on the bottom of the cowl could even be considered overkill. You could easily reduce their height by half and be just fine. The cowl inlets are perfect.
Jul 18, 2019, 09:42 AM
Registered User
richard hanson's Avatar
You can improve what you have if it isstill too warm
Add a sheet attached to top of each inlet baffle which extends past fins then curves downward
The bottom outlet ducts are necessary to create a low pressure aft the engine
The result is the air infront is now forced to flow thru fins and is pulled downward thu cowl bottom exits
You cant push air into the cowl
It flows from higher to lower pressure along whatever route is provided
Jul 18, 2019, 11:43 AM
Right on the Edge!
Thread OP
Thanks guys.
I am glad I did fine.

For now I think I won't add the sheet you mention -- but that's the next modification I'll keep in mind, was I to still measure high temps.

Rather, my main concern was that the outlets on the bottom of the cowl have an "angle of attack", so to speak, too high, and that air wouldn't stick to them but rather "stall", as would a too high angle of attack of a wing -- ultimately not pulling air out from the inside of the cowl. Is this a real concern? I'm not sure I was able to explain what I mean exactly, English is not my native language but I hope it was clear enough.
Jul 18, 2019, 12:20 PM
Registered User
richard hanson's Avatar
Bottom cowl outlets are good
The idea is to create low pressure behind them it is not required to have smooth flow
You could remove more material behind them but ok for first tests
Look at cowl flaps on old military planes
They are just sheetmetal which can be opened as needed and were usually open on the ground to create lots of low pressure behind the cylinders
If Ir can not flow out, it cannot flow in that is the basic concept
Jul 18, 2019, 12:53 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by Hlaalu11
Thanks guys.
I am glad I did fine.

For now I think I won't add the sheet you mention -- but that's the next modification I'll keep in mind, was I to still measure high temps.

Rather, my main concern was that the outlets on the bottom of the cowl have an "angle of attack", so to speak, too high, and that air wouldn't stick to them but rather "stall", as would a too high angle of attack of a wing -- ultimately not pulling air out from the inside of the cowl. Is this a real concern? I'm not sure I was able to explain what I mean exactly, English is not my native language but I hope it was clear enough.

The angles shown won't "stall" like you're suggesting. The angles will create a suction at the openings which is what you want.

What I'm thinking though is that the first duct is located right under the cylinders. That may not be a good spot. And even the second one might be too far forward. If you look at full size cowls of this sort the main air exit is generally a big open scoop at the lower rear of the cowl. Often with a lip at the openings leading edge to aid with creating a suction zone behind the lip just like you are doing with the angled openings.

The baffling you added is better than no baffling for sure. But as mentioned above there's nothing to keep the air flowing through the fins once it gets past your opening baffles. The good news is that you could easily "fix" this by removing the ones you have now and making them longer and extending back in a longer "conical box" that ducts the air in such a way that it's forced to pass through the fins to get into the back area of the cowl. Same with a longer angled baffle for the cylinder head to again maintain control of the air so it is forced to pass through the fins to get to the rear open area of the cowl.

With such longer baffles that extend back to the rear of the cylinders the exit louver locations become far less important. The air coming out the back of the baffles can be allowed to move ahead and go out the louvers you made.

Anyway, what you did should still be a good improvement. Try it and see if the engine suffers or if it seems happy. But if it proves to be not quite enough of an improvement there's room for more with just a little more effort.
Jul 18, 2019, 10:24 PM
Local Cropduster
I think yours will work better than a stock setup. A fully baffled engine/cowl setup would be far more work that the results would be worth. In the full scale experimental aircraft world, there are many rules of thumb on this....lots of variables....but, for your application, I think the most important thing to consider is cowl inlet vs exit ratio. Make sure the area of the exit is larger than the inlet, like 20% or so. The air exiting is heated and benefits from larger exit area, and larger exit will help create that low pressure area you need to get the air out of the cowl. It is a little drag, but cooling is what you are after.
PS- BMatthews is spot on.
Jul 19, 2019, 02:08 AM
Right on the Edge!
Thread OP
Thank you guys for all the detailed answers.

I had sort of feared that the outlets could be too afterwards, but I was dealing with an already cut cowl and tried to make the best out of it. I actually took inspiration from this:
Baffling your gas RC engine for better cooling (3 min 56 sec)
. My hope is that the canister housing will also function as an additional outlet.

Adding longer baffles, or upper sheets, as Richard Hanson suggests, could actually be not as easy as it sounds, because this being a one piece cowl, I already found adding the present baffles quite a challenge. I was even tempted to split the cowl in half to facilitate the work. The problem is that anything curved, be it a sheet or the baffles themselves, to force air pass over and the down the engine, presents a problem for sliding in and out the cowl off the firewall... any solid structure would make it impossible.

I can see all of that happening with a split cowl that opens like a lid, like the one in the video, then I would be willing to tinker more with it.

But as it is now, perhaps the only way to do it would be working on the engine itself, as opposed to the cowl, something like this: http://www.rcuniverse.com/forum/gas-...#&gid=1&pid=12

But honestly it looks like a hell lot of work, I'm not sure is worth it for my case. For me, knowing that I haven't do anything so wrong that it is actually worse than before, is already very comforting.

As for increased drag, that could in fact even being a plus, being this a 3D airframe flown as such (which by the way is the reason I wanted to make sure it had better cooling, because it won't be flown full speed all the time really...). Anyway I am pretty sure however increased the drag may be, it won't be much noticeable in these kinds of airplanes which are already so overpowered.

So... to piece together what's come out from your replies, I think I will first of all test it out before making any other modifications. As I said, at least I am happy to know that present setup is "kind of" fine.

Thanks again folks and happy flying weekend.
Jul 19, 2019, 08:42 AM
Registered User
I think most in this thread are overthinking this. On my twin powered airplanes all I ever did was put in a plate that kept the incoming air from going below the cylinders as pictured on my 3 meter span Extra. Thousands of flights on a half dozen different airplanes and I have never had overheating issues.
Jul 19, 2019, 10:44 AM
Registered User
richard hanson's Avatar
That setup also works well on many setups
As long as the setup directs air thru the fins and has a low pre.ssure outlet at lower rear of cowl, it usually worked I used infrared meters to see how well the cylinders balanced
using a good oil /gas mix and getting needles correct ( not too lean) , was part of the difference in meter readings
The baffling used on full size four cyl craft works too
The usual problem came from users who thought you could push air into the cowl an also tried to get max rpms in level flight and used very low oil mixes
they just burned up the engines
Jul 20, 2019, 01:40 AM
B for Bruce
BMatthews's Avatar
I've also seen solutions over the years where folks added the baffles to the actual cylinders using things like aluminium roof flashing and wiring them in place around the cylinder. Then the cowl fits over the baffles and the shape of the baffles in close proximity to the inside of the cowl holds them from shifting out of place.

So doing self standing baffles would be an option too. And such wired on separate baffling would work for a one piece cowl that fits on from the front.
Jul 20, 2019, 12:41 PM
Registered User
IMO this just like the wing flutter thread below simply requires the application of fairly simple tried and true modeling techniques. It's fun to theorize and point to examples of what full scale designers do to fix issues but those avenues are usually overly complicated and have the potential to confuse the guy who is asking for help.
Jul 20, 2019, 02:52 PM
Registered User
richard hanson's Avatar
True
The basic truth is that air only flows from high to low pressures , Solves the basics here
Jul 21, 2019, 12:45 PM
Registered User
Of course some planes as well as some people are easily baffled
Jul 22, 2019, 12:27 PM
Registered User
Quote:
Originally Posted by xlcrlee
Of course some planes as well as some people are easily baffled
Perhaps then you could help people at their level rather then use these threads as a tool to demonstrate how educated you are. Your end goal is to help your fellow modeler be successful right?


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