O ring-gasket bead - question

[spacesailor]

My air-cooled Half VW, motor gets heaps of cooling with it's Big fan in front.

" Cooling air flow control ! . "  should read : " Cooling air,  flow control ! ..

How about shrouds to  "WARM " your motor.

same difference.

Dense air gives Hummel's  "WINGS "

spacesailor

[RFguy]

maybe, but the pressure head available would be no different, just depends if the low airflow speed over 25 % of the fin area   (current) is better than a  lower airflow over 100% of the fin area

[skippydiesel]

Always loved the look of HummelBird's - just musing, what effect/good/bad/nil would  a full engine cowling have compared with the existing "pots" in the wind ??

[RFguy]

for  fine head finned engines like the jabiru, those fins only work if they have forced airflow through them.

 

blowing around in the wind wont be sufficient....

 

[Yenn]

You will not have forced airflow around the fins if you put something between them to stop the flow. I still cannot understand the question.

I have done some work on air cooling, but am not aware of what FR guy is trying to do. How anout a real explanation of what the problem is.

[spacesailor]

I believe that  the gasket at the end of the fins is to stop Air escaping, !

It will force that entrapped air to follow that bead of silicone, around the edge to wherever it,s needed to to go,  just as the same thing an aluminium cowling ( shroud ) does..

spacesailor

 

[RFguy]

yeah that's right spacesailor.  there are sections where I want to insert and remove easily, hence the bead. and also on the side between cylinders, there isnt room for an all encompassing shround. well there might be with 2 thou SS shim.

[Blueadventures]

On 27/01/2021 at 9:21 AM, RFguy said:

I have a quetsion about the use of circular or rectangular cross section gasket bead.

I want to push some silicone etc gasket bead in the fins on the top  couple of mm of the fins (so not to obstruct the available fin area more than necessary).

(why- because there is little.no clearance  for flat sheet on the outside of the fins in the region- so a pushed in bead an option if I cannot fit 2 thou SS sheet 

 

Is a circular bead goes to have better retention than a rectangular cross section cord ? it would be secured with a little suitable adhesive here and there.

suggestions. comments ? - intended use (orange bead below) and the small gap I have to work with (that reduces when it gets hot !) 

-glen

 

Looks to me you will get no gain with the bead. The first and second bend will reduce the flow of air getting through the first change of direction and the second will likely result in stopping or almost stopped the flow. Just my thoughts on it.

[RFguy]

yeah. I'm actually not too worried about getting around the corner clean - the pressure drop will take care of getting it out. 

should only be one change of air direction. The flow in though will be interesting- needs a inlet transformer. 

https://jabcamit.groups.io/g/main/message/13778

needs some refinement. concept is air must flow past the hot bits.

 

[kgwilson]

IMO anything that blocks the airflow will have a detrimental effect. The area directly behind the blocked area will not get any heat exchange quickly enough and the fins will be hotter. It is like this in an air conditioned room where an alcove remains warmer than the rest of the room due to air not circulating there and relying solely on thermal conductivity of air which is terrible.  Look at what happens when water is flowing and there is a blocked area. The water going past creates eddies & then the water in the blocked area just goes round and round. The pressure drop won't make any appreciable difference. Sucking as much air past all of the heat sinks as possible will always work best

[RFguy]

which is what the technique does, but forces 100% of the fins to be active, rather than abotu 30%

the air can only get out the bottom.

 

[spacesailor]

With the picture link.

How does Air get through those metal shrouds !.

Or are you trying to keep that cylinder warmer than the others. ( as in that HB picture )

I alwjays thought the Half VW motor was back to front, witn the exhaust,  faceing the wind, & the inlet in the hot. Low pressure at the rear..

but

IT was all for cooling the hottest parts first.

I still think, if turned around, you wouldn't need those fins at the inlet side & exhaust could hve a short stubby, large finned tuned pipe rearwards.

spacesailor

[RFguy]

you need to hold the air in all the fins. that's right- it will not let the air out. precisely!... until it exits underneath.

You might as well saw the fins off otherwise. 

 

Because the fins represent a narrow long duct, If the air has ANY CHANCE to go around the fins, it will....

 

The default setup is probably less than 25% fin utilization. Maybe less.

 

 

 

 

 

 

 

[jackc]

IF I had Jabiru cooling problem,  too old and not enough time to gin around.  Just suck it up, spend the money....

https://www.rotecaerosport.com/lch

and fly away happy!  

[onetrack]

I seem to recall someone on here previously mentioned the Rotec water-cooled heads conversion for the Jab engine - and the general opinion was, the gains were minimal, for a big increase in complexity.

 

Look at that huge pile of coolant hoses! Every one of them, a potential engine failure in the making.

 

EDIT: - I found the thread ...

 

https://www.recreationalflying.com/topic/36284-watercooled-jabiru-2200/

 

Edited January 30, 2021 by onetrack

[spacesailor]

BUT,

Those Jabs grounded whenever the air reaches 41,d,C.

Doesn't,t the water cooLed heads allow a bit more fly time ?. ( at the Oaks foxbat flying, Jabs grounded ).

spacesailor

 

[RFguy]

41 is fine if you have the engine (air) cooling sorted....

 

just got the ex valve out #5 . slight ream required in the guide. minimal. but certainly required. now passes 7.04 mm . lots of gunk up the ex vavle stem, possibly stopping valve returning all the way.  seats and face look good.  stems in no way necked, hard to see any cracking etc under the buildup. will x-ray tomorrow or next week. 
 

Edited January 30, 2021 by RFguy

[spacesailor]

NN,!.

All the Jabiru training planes were grounded due to high air temps,

Lucky me in the opposition,s, Foxbat A22.

spacesailor

[Yenn]

I would have thought that if anything stopped the exhaust valve from closing there would have been visible damage, either to the seat or the valve.

[RFguy]

well there is alot of gunk on the stem, and where the stem hits the guide. 

 

oven cleaner for the valve.... bit of MEK for the head.

 

inlet looks good.  nice fit of the 7.04mm go-no go.

 

Are the stems suposed to be harder than the guides?

or the guides supposed to be harder than the stems ?

 

 

 

[facthunter]

 That gunk that reams easily will hold a valve open without difficulty  Cast iron or bronze for the guides. Very little clearance on the inlets. Need about .003" on the exhausts.  Valve material on stems? Best is nitrided or industrial chrome.  Replace rather than reface the nitrided ones. No thickness of nitride on seats.   Inlets aren't that fussy  Stainless can mean many things but it tends to GALL (pick up bits from the guides) easily and then it wears rapidly. (acts like a file).  Nev

[RFguy]

thanks Nev-  . yes exhausts reamed fairly easily and now everything is a nice fit. no need to touch the inlets-

so in general  :

Are the stems suposed to be harder than the guides?

or the guides supposed to be harder than the stems ?

IE what is supposed to wear ?

[onetrack]

Glen - Stems are harder than guides, and guides are usually where the wear is. Valve guides are made from a wide range of materials - with cast iron and bronze being in the majority.

Cast iron is "hard" and bronze is "soft" - but you're not looking for hardness in valve guides, you're looking for wear resistance. Most importantly, you have to be looking at surface finish of a guide, so that it allows just the right amount of oil through, between stem and guide, to allow lubrication - without allowing excessive amounts of oil through, that causes oil burning and combustion chamber coking up.

The desirable surface finish for a guide is between 30Ra and 80Ra. Ra is Roughness average. Also known as the arithmetic mean, this measurement represents the average of all peaks and valleys. Lower numbers indicate a smoother finish.

[RFguy]

Moving back to the hot head thread....

https://www.recreationalflying.com/topic/37195-hot-rear-cylinder-example-jabiru-3300a-2575-and-plenums/?do=findComment&comment=501610&_rid=11148

 

 

 

Edited January 31, 2021 by RFguy

[facthunter]

With a supercharger the oil will be forced back up the guide anyhow and most of those have a very effective seal fitted at the top, so lubrication is a delicate thing. Proper ( wear and temperature compatible ) metals play a big part in internal combustion engine poppet valves lasting any time at all.  Nev