Justin's Raven

[AndyCAP]

Hi Justin

Sorry to hear there is even more damage than you initially thought . Glad to hear you got a hangar... much better location than you mentioned to me many moons ago

I am not an engine expert so am speaking purely from a mechanical engineering background . I am not 100% sure if I understand the concept of the tight fitting fibreglass baffles? I think it would be better to leave a slight gap beteen the tops (outer edges) of the cooling fins and the baffle? if you havae fibreglass touching the outer edges of the fins (i.e. a flat plate lying across the tops of the fins), it might act as isolation at the points of contact and be counter productive. On the contrary, Aluminium touching the fins will act like a heat sink - for radiant and conducted heat. By all means, seal the baffles against sections of the engine which does not have cooling fins (as well as the inside of the cowl), but I would guestimate that one should leave about a 5 - 10mm gap between the baffles and the cooling fins (where they run close around the cooling fins). It might also be better to have a slightly larger mass of air flowing around the cylinders - even if its not all going to be directly in contact with the coolingfin surfaces. I also think that one should try and streamline the route to reduce airflow resistance though the ducting. You are not going to be flying at the speeds that the racers are and would be much more interested in longevity of your engine than they might be.

going out on a limb a bit... It would be interesting to hear the experts' opinion of my 'gut feel engineering'

[Boet]

The oldest VW powered BB still flying is ex Louis Chanu`s BB, CNL. I designed his baffling system as wwell. This engine had other faults, but it definately never overheated. It was used for some serious adventure flying. Here are some pics. One of Louie trying his hand at a "deadstick" spot landing competition. Once he drummed up enough courage to switch the engine off, and gotten used to the feeling, he just could not get enough, and he kept on practicing untill he got it right. Some experience that was invaluable later when the main power supply cable vibrated off, and he had to make a REAL dead stick landing in the dry riverbed of the Hamrivier in the south of Namibia.

[German]

Seker baie dors met daai carbs!

[justin.schoeman]

The oldest VW powered BB still flying is ex Louis Chanu`s BB, CNL. I designed his baffling system as wwell. This engine had other faults, but it definately never overheated. It was used for some serious adventure flying. Here are some pics. One of Louie trying his hand at a "deadstick" spot landing competition. Once he drummed up enough courage to switch the engine off, and gotten used to the feeling, he just could not get enough, and he kept on practicing untill he got it right. Some experience that was invaluable later when the main power supply cable vibrated off, and he had to make a REAL dead stick landing in the dry riverbed of the Hamrivier in the south of Namibia.

As a matter of interest, about how much would it cost me for a paint job, cowl and baffles, if I towed this plane down to your place for a week, or two?

[justin.schoeman]

This is one of the few research articles freely available:

http://ntrs.nasa.gov/search.jsp?R=69952 ... D201%2B286

It does come from the 40's, and it does refer to radial engines, but those were the most powerful piston engines ever produced...

They have a lot of good points about the ideal % of the circumference of the cylinder to seal, and such. But they also say the baffles must seal against the fins.

This is more contemporary, and covers a lot of the points you mentioned. It is also more VW specific:

http://home.hiwaay.net/~langford/faq/ch ... cool1.html

3. Shell baffles

Shell baffles ensure that the cooling air passes between the cooling fins on the cylinder walls and the cylinder heads. They cover the fourth side of small cooling passages, of roughly rectangular cross section. Shell baffles thus prevent 'dead water' areas where the local air flow between the cooling fins is either reduced or stagnant. Extensive deadwater areas may occur if the shells do not cover enough of the circumference of the cylinders. But if the shells cover too much of the circumference then unrealistically high pressure differentials would be required to create adequate rates of cooling airflow through the passages. Some engines benefit from an inter cylinder shell baffle which further reduces dead water areas.

It is extremely important that all the shell baffles actually make contact, throughout their length, with the edges of the cooling fins. Only then will the fins cool effectively to their full depth. Otherwise, the cooling air takes the easy way out along the unobstructed gaps.

There are many different designs of cylinder and cylinder head baffles - shell baffles stamped from a single sheet are possibly one of the best - but these can be incredibly expensive to buy and their manufacture requires facilities beyond the reach of most home-builders. It will pay to inspect as many other installations as you can in order to get good ideas.

This designer is fairly adamant that the baffles contact the fins. Although he is using aluminium, and not glass, the tiny relative proportion of fin area covered should make no difference.

OK - now these are a bunch of articles describing how, and why it should be done, BUT, just about every baffle I have seen has left a gap between the edge of the fin and the baffle... This worries me - what do they know that I don't know. I will be bugging one of my friends with a hydrodynamic simulator to run the problem, and see what the ideal solution is.

[Boet]

Seis, nee nogal nie. Hy was baie lig op petrol, so 9-12 lpu. Leefbaar. Die errietjie het "awesome" lekker gevlieg, amper soos n lekker C172. Met die 1ste toetsvlug het die VW geloop soos n klok, en met die 2 de vlug, toe ek vir ou Louie vir n spin vat in sy errie, toe tjorts die amper in sy broek, en die volla loop en hik en snik so n hele slapperige circuit vol, tot terug op die grond. Ek wil nie SO n enjin he nie. OF die donner loop reg, of hy vrek en kry klaar.........

I do not want to go THAT tecnical. It just buggers up the fun. Keep baffeling simple, keep the baffles tight around the cylinders, seal all potential air leaks, and use ALL the trapped air for cooling, and Bob`s your uncle.

The secret is not ONLY how much cool air you get in at the top, but of how much of the HOT ait you get out underneath. For that purpose the outlet underneath should have a "ventyri" shape, to help suck the hot air out by creating a lower pressure area underneath the engine. See pic.

[Oupa-G]

Boet understands the airflow and with the baffles on the Bosvark our two heads were one. The reason the plates should touch the fins is that the air must run between the fins and not over the top. Hence having springs that pull the baffles tight is so important. Look at a Cessna 210 baffling or the Baron baffles or the baffles of a Twin Comanche or the Baffles on my Vansin they are all tight the air that flows through the cowling must pick up heat and because the warm air expands it must have a nice sucking hole the leave the cowling. All these principals were also applied to the Bosvark.

Cheers Oupa-G

[justin.schoeman]

Thr BMW is a lekker engine to baffle. For all practical purposes it is a 2 cylinder radial, so you can take this article:

http://ntrs.nasa.gov/search.jsp?R=69952 ... D201%2B286

And apply all their tricks.

I had a close look at your pictures of the BMW baffles when you posted them, and I did see that they seemed to follow just about every recommendation in the book.

For my VW, I think I will do nice, tight cowls pressed up against the fins. I have no idea why so many other people leave a gap around the fins. I hope it is not for a good reason!

[AndyCAP]

Clearly my gut feel was wrong - although I did mention that I think aluminium baffling should touch the fins since it will also have a bit of a heat sink effect .

My concern was mostly about the use of composite baffling and the insulating effect that the composite could have on the engine, but in reality the contact area beteen the baffling and the fins is probably very small so it probably wont have a negative effect.

Thanks for the interesting info.

[justin.schoeman]

Clearly my gut feel was wrong - although I did mention that I think aluminium baffling should touch the fins since it will also have a bit of a heat sink effect .

My concern was mostly about the use of composite baffling and the insulating effect that the composite could have on the engine, but in reality the contact area beteen the baffling and the fins is probably very small so it probably wont have a negative effect.

Thanks for the interesting info.

Aluminium would definitely be better. But unless you are Boet, making nice, tight fitting aluminium shell baffles is fairly difficult .

For the simpler pieces, like the side plates, I want to use aluminium, and bolt it directly to the outboard fin at a number of places - effectively extending the last fin. For the more complicated bits, I will have to go composite...

PS - My initial gut feel was also that getting air to flow PAST the fins was important, but every scrap of reading I have done says THROUGH the fins. Makes a hell of a lot of sense when you look at it (a second time).

[Boet]

Justin, ek wens ek kan die hoop aly sien wat ek "gescrap" het in my pogings om die perfecte verkoeling sisteem aanmekaar te timmer. Sorg dat jy GENOEG aly byderhand het, en maak oor, as jy nie met poging no 1 tevrede is nie. Ek het nog nooit eens oorweeg om n glasvesel baffeling systeem te maak nie, want veselglas vervorm as dit te warm word. EN het jy al gesien hoe lekker BRAND dit????

[justin.schoeman]

Justin, ek wens ek kan die hoop aly sien wat ek "gescrap" het in my pogings om die perfecte verkoeling sisteem aanmekaar te timmer. Sorg dat jy GENOEG aly byderhand het, en maak oor, as jy nie met poging no 1 tevrede is nie. Ek het nog nooit eens oorweeg om n glasvesel baffeling systeem te maak nie, want veselglas vervorm as dit te warm word. EN het jy al gesien hoe lekker BRAND dit????

If Boet battled like that, I don't know how I am going to manage... Maybe I should buy shares in Hulet!

As to fibreglass, I have a resin that will do the job. Can't get it to light in a gas flame, and it will only distort long after the heads are cooked... Used for making molds for plastics. Should be safe enough.

Anyway, about to go back into the workshop and strip the bottom end. Hope things will look good down there. Wish me luck!

[justin.schoeman]

Anyway, about to go back into the workshop and strip the bottom end. Hope things will look good down there. Wish me luck!

Wishing doesn't help. All the journals are scored. #3 and #4 main can twist sideways on the crank, so they are toast. That much free play on the nose has probably fatigued the crank. Will get the engineering shop's opinion, but it must probably be replaced.

I don't know what happened to that engine. All the oil that came out was clean, but there were obviously particles of some sort rattling around in there...



Justin

[AndyCAP]

Ag Nooo! my heart bleads for you Justin

there is WAY too much damage for the previous owner not to have known

I think you need to speak to him and if he does not cooperate, send him a lawyers letter. (Yes I know it will probably not be worth the cost of litigating - let alone the time and schlepp). This sounds like a clear case of misepresentation to me.

If all else fails consider name and shame

Good luck

[Boet]

Justin, you hyave been shafted. Gaan sien die ou wat hom aan jou verkoop het. Vat my saam asb.