Carosel43

That's correct, as the airflow passes the cowl flaps it will create a suction and draw the hot air through the cowling. The wider the flaps are open (on full size) the greater the suction and thus cooling. Below is an airflow diagram for a lycoming flat 4 or 6 as you would find in almost every Cessna and Piper on the planet as well as all the Extras and pitts aircraft etc. it shows the same principal as we are using here its just the layout of the engine that has changed.


http://www.avweb.com/newspics/188945...ow_baffles.jpg

BJ64

I read what you're all saying - but I must say that I'm a bit out to lunch on some of that theory.

While I can understand how a lot of 'stalled air' might not be able to go anywhere and thus potentially reducing the cooling efficiency... does it make that much difference in an RC plane?

Are there any quantitative figures to work from? i.e. actual contolled-experiment data?

I can kinda grasp the idea if there was a huge difference in inlet and outlet size, but once the 400 is enclosed inside a cowl - e.g. the ESM Corsair - there's a pretty big hole for it come back out through the glassed pretend cowl-flap opening at the back. Not to mention the venturi effect created by the angle of the fake flaps.

Is there perhaps a 'formula' to work from? Like... with EDF's, where you work towards an intended reversed effect - compression of the moving air - of around 90% of FSA?

When I look at the cowl on the F4U, and take into consideration the rolled lip on the nose and the front-on area already taken up by the motor, there doesn't seem like there's going to be all that much air going "in" that needs to get back out through the cowl-flap gap.

I'd like to learn more on this and understand it a bit better...

BJ

Carosel43

If you work out the area of the inlet on the corsair it will likely be very large. If you then measure the area of the outlets they will actually be quite small.

As I don't have the cowling dimensions I cant work it out but I can make up some numbers to show the point.

If we assume the cowl has a front opening of 210mm, the rear diameter of the cowl around the flaps is 240mm and the fuselage diameter where it joins the cowl is 220mm we can work out what we have.

Incoming area is 34636 mm2

rear opening is 45238 mm2

But the fuz creates a blockage of 38013mm2

leaving an opening of 7725mm2

So, from this example we can see that the incoming area is almost 4.5 times larger than the outlet and even with a suction from the cowl flaps that's just not going to work. Even if we added the space taken up by the engine its still not going to add up

Now if the front is baffled off, and 40x80mm openings are cut in front of each cylinder than gives an inlet area of 16000mm2. As the physical mass of the cylinder will block off most of that area we can assume there is perhaps 10mmx80mm of actual space for the air to flow through for each cylinder. this gives us 4000mm2 of actual air opening and 7725mm2 of air escape. that's going to work for sure as the air will flow nicely and (because of the baffle) will flow where we want it to.

I appreciate that I have just made up all of these figures and so they are not going to be quite accurate, but with luck they will show the principal. Unless I have dropped a clanger on my maths!

BJ64

To chuck a spanner in the works of all this 'math' (assuming that it's ballpark)....

How much of the air intended to pass between the cylinder gaps in the baffle actually makes it between the gap/s and enhances the cooling effect?

I undersand the theory - I'd just like to see some empirical evidence before I go manufacturing something that may not enhance anything - let alone produce an adverse deleterious effect....

BJ

PS - some basic measurements - some actual, some optical as best as I could estimate (cowl and motor in-situ)

Cowl opening to front of aircraft - 165mm diam.

Nut/Boss - 50mm diam (centre).

Prop blade width 40mm x 2 (mean average).

Cowl exit - 260mm diam with clear 'flap opening' step of -10mm to fuze diam (~240 mm fuze diam at that point).

Mean cowl diam ~ 235-240mm (behind cowl "nose-over"/lip - the average i/d of the cowl after the fold of the nose as viewed from the front of the airframe)

Visual motor/exhaust/counterwieght (DLE 55 CDI) area as seen from front of cowl opening - at least 50%, possibly up to 60%.

blw

My Feedback: (3)

Here is a good explanation that is an easy read and makes sense. Air pressures are explained quickly and easily:

http://www.troybuiltmodels.com/ns/le...g-easy-how-to/

Carosel43

in an ideal world the baffle would touch the engine so that no air can escape but as that is not practical a gap of up to 5mm is fine.

Regarding specific evidence I have none to present, other than 20 years of experience telling me that if I don't do it my engine over heats and stops. Also we are following full size practice in doing it. And lets not forget that even with baffles full size radial powered aircraft like the FW190 and B29 suffered major overheating problems, as did many of the Russian designs like the La7.

The problem is two fold, getting the air where we want it for cooling, and getting the hot air out and away from the engine. There is only so much air we can get out of a given hole and so that is our limiting factor. Also as the air heats up it expands and so needs even more space to exhaust. The other problem is that air is both sticky (it has viscosity) and lazy. as such it will try to escape through the gaps between the cylinders as this is easier and airflow close to the cylinders will then stagnate and just sit there getting hotter and hotter.So by baffling it we get greater flow velocity and less stagnation which gives us a happy engine

I know that Rev's 400 is unbaffled and he is having no issues, but the area of air outlet on his model is massive. Roger in the states has had two engine failures with his unbaffled (I believe, correct me if im wrong) engine that could be down to overheating. these failures left him with a broken model. I understand the issue regarding the extra work required to make one but I personally think its time well spent as the cost and effort of making a simple baffle plate is nothing against the cost and time wasted on rebuilding a broken model and/or engine because it overheated and stopped.

Reverend

My Feedback: (3)

Just to back Jon on this: I did my maths and the outlet area of my Cowl (including the air leaving through the Fuse) is approx twice that of the incoming air. I am still thinking about baffling the engine but I flew four flights on a 32°C Day and had no heat issues.

BJ64

Ok - I think I understand it a bit better now.

Thx for all the info & links etc

BJ

Carosel43

No worries. As it happens I am having this exact conversation with a friend about his Laser 200v and he refuses to believe me!

Carosel43

This evening we had one of our club late night flying evenings and I took the 400 out to play. I flew it twice as the rest of the time was spent eating sausages, helping a friend with his model, drinking a cold beverage (after flying was done) and because the crowd of people around the radial was such that I almost had to beat a path to get in there and start it up! Anyway the engine ran very well and (as usual) attracted many look and comments of admiration. But im just happy its running well and seems, for the moment, to be in good shape

Kmot

My Feedback: (24)

It is just responding well to all the love.

Carosel43

love? more like open heart surgery and multiple organ transplants!

Reverend

My Feedback: (3)

sounds great caro! My 400 is running so well I am taking the chance to fly it in front of an audience on the 06. and 07. july in Oberhausen. THAT is what I call a real classic meeting :-D

check last years footage out (cold beer and a bag of crisps (us:chips) required)
[youtube]http://www.youtube.com/watch?v=pRSNNpr6YbY[/youtube]

Rev

BJ64

Das ist aber das Beste ...

Nur die Österreicher konnten eine Show wie der an setzen

BJ

Reverend

My Feedback: (3)

Almost, its in Bavaria but with strong austrian participence.

Really looking forward to it.

BJ64

Oh well.... they speak like 'us'.... LoL

BJ

wherewaldo

Hey guys its been a while since I posted ...

Kmot

My Feedback: (24)

Sorry to hear about your mishap.

I can only speculate. I think a 20x10 3-blade is a very heavy load for this engine.

Losing a cylinder could happen for many reasons, such as a 'iffy' glow plug; valves out of adjustment; glow driver wiring to the affected cylinder have a mechanical problem; etc.

It seems I read about owners of Sonic Tronics radial glow drivers having more troubles than successes. That could be a part of the problem. 5 amp hungry glow plugs will drain a battery quickly too. If you ran the engine on the ground for several minutes doing pre-flight checks, that could have negatively affected your in-flight glow power time.

wherewaldo

Thanks for the reply ..I will try a 20 *10 two blade 

Carosel43

Hi Waldo,

I agree with Kmot on this one. Lighten the load on the engine as I found that I would loose cylinders when the rpm was below 7000 or so. Also there has been some discussion regarding cooling and baffles etc to prevent overheating, you may want to look into this as well. A peak at your valve clearances wouldn't hurt either

Kmot

My Feedback: (24)

http://www.rcuniverse.com/forum/m_11..._1/key_/tm.htm

Reverend

My Feedback: (3)

RIP Pe, always listening, always trying to help.

Big loss for our community. My first flight in oberhausen will be dedicated to you.

BJ64

Likewise - RIP.

Sincere condolences to family, friends, and associates.

BJ

tim750r

Here, here...

Carosel43

Always sad to loose a pair of oily hands