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Sealing baffes vs blast tubes?

bret

bret

Well Known Member
Been doing a lot of reading about the importance of sealing every crack and escape path for the cooling air coming into the cowl, or plenum, seams like having all those blast tubes for mags, alternator, ect, is contrary to sealing everything up with RTV? I might have one for the EFII fuel pumps and filters filters doghouse but that's it, how many do you have, and do you have cooling problems?
 
Bret - I have what is probably considered typical, if there is such a thing;
Air for Heat (per plans)
Blast tubes for Alternator, Engine Driven Fuel Pump, and Mags.

I cool well under the Lycoming limits but need some care when the engine is already hot from flying, long taxi's, etc... to keep under 400 (my personal, time to do something, limit). Lower the nose a tad and all is fine.
I0-360, per plans baffle system.

So, I don't consider this to be a cooling "problem" but some would probably not consider that ideal. Some may tell you that if you can't climb full out on a 100 degree day and stay below 400, something is wrong..... Nobody I know and fly with meets than standard, but there are those that post they routinely do....
 
pressure and flow, where you need it!

I think the smart engineer types would agree, if you seal the 'leaks', then you'll have the excess pressure and airflow to direct it to useful places, and still achieve adequate cooling.

Interesting thought, when I pull carb heat on, I am 'stealing' whatever flows thru that scat tube from the air going in the left cowl inlet.
obviously not a cooling issue, but has an effect nonetheless.

Do you tape over your oil cooler in winter? If it's mounted or feeds from the back of the baffles, you now have a whole bunch more pressure pushing cool air thru your cylinder fins, right when you don't really need it. :rolleyes:

P.S. Gardnerville looks reel Purdy! how do you stand all that scenery???
 
flyboy1963 said:
I think the smart engineer types would agree, if you seal the 'leaks', then you'll have the excess pressure and airflow to direct it to useful places, and still achieve adequate cooling.

Interesting thought, when I pull carb heat on, I am 'stealing' whatever flows thru that scat tube from the air going in the left cowl inlet.
obviously not a cooling issue, but has an effect nonetheless.

Do you tape over your oil cooler in winter? If it's mounted or feeds from the back of the baffles, you now have a whole bunch more pressure pushing cool air thru your cylinder fins, right when you don't really need it. :rolleyes:

P.S. Gardnerville looks reel Purdy! how do you stand all that scenery???
Click to expand...

on the net somewhere there is a pic above KMEV up high, 18K or so, looking west, mountain range and then Lake Tahoe. On my list of things to do is get special permission to do a celling test in phase one and take one of these awesome photos!
 
flyboy1963 said:
Do you tape over your oil cooler in winter? If it's mounted or feeds from the back of the baffles, you now have a whole bunch more pressure pushing cool air thru your cylinder fins, right when you don't really need it. :rolleyes:

P.S. Gardnerville looks reel Purdy! how do you stand all that scenery???
Click to expand...
I do have a shutter over my oil cooler and love it as I can regulate my oil temp very closely. But to my surprise, I have found it has very little effect to my CHT if it is closed or wide open. Another surprise was my scat tubing for my heat muff which is not from the back of my baffle but at the right hand cowl inlet. I had expected if I close that in summer time, there is no air to the bottom cowl which will reduce the pressure and less competing air to the exist ramp. Closing it completely by covering the inlet had zero or no measurable effect on my CHT. My CHT numbers are more like Jon, typically hit 400 on longer climbs or if the engine is already rather hot.
 
Bavafa said:
I do have a shutter over my oil cooler and love it as I can regulate my oil temp very closely. But to my surprise, I have found it has very little effect to my CHT if it is closed or wide open. Another surprise was my scat tubing for my heat muff which is not from the back of my baffle but at the right hand cowl inlet. I had expected if I close that in summer time, there is no air to the bottom cowl which will reduce the pressure and less competing air to the exist ramp. Closing it completely by covering the inlet had zero or no measurable effect on my CHT. My CHT numbers are more like Jon, typically hit 400 on longer climbs or if the engine is already rather hot.
Click to expand...

Same here with the shutter.
I suspect the plenum is being more than adequately charged and losing some air through things like scat tubes for cooling, heat, etc... isn't depleting that pressure too much. I also suspect that work could be done to improve the differential between the plenum and lower cowl on the stock design. If you open the shutter, that air then goes into lower cowl. You would think it would be double bad, but it has no affect, at least on mine.
Dan H. did a lot of testing, real testing. Without going to that degree you really can't fine tuning inlets, exhaust outlet area, etc....
So...., many of us simply built it per plans and find the cooling more than adequate for the type of flying and conditions we operate in.
Could it be better? Lot's of examples show it can be without too much trade off in cooling drag.
Much of this was really not messed with "back in the day" as the performance of the stock plans build airplanes was so good, why mess with it. Took me long enough to finish the airplane as it was.
 
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Interesting on your test closing off the heat muff, with no change? one thing I forgot to mention is that I am using the SJ long cowl, so I don't know if that makes a difference on how critical it is of the gap sealing and cooling tube leakage compared to the Vans cowl, a lot bigger intakes. excess air to waste?
 
cooling

Bret don't worry about cooling cylinders they will be way cool its the oil that's the problem with our set up get the best cooler you can I'm running a 13 row an wish I had room for the 17 row.
Bob
 
dang! really? I am going to start out with what the Mooney had on it, an 8 row, guess ill find out on that first flight....Hey Bob, did you try an exit shroud on the cooler with scat down near the exit?
 
It depends. Same as others, I have blast tubes for the (p)mags, alternator, etc. as well as two heat muffs and of course the oil cooler gets a shot at the air from the upper plenum as well.

My cruise CHTs are low 300s in the summer. I have no problems with extended climb outs at > 110kts, either.

Where it gets tough is extended periods at full power and high alpha. The tandems don't seem to have these problems, but I have yet to hassle with a side-by-side that doesn't. If this isn't your regime, you should be fine.
 
bret said:
Been doing a lot of reading about the importance of sealing every crack and escape path for the cooling air coming into the cowl, or plenum, seams like having all those blast tubes for mags, alternator, ect, is contrary to sealing everything up with RTV? I might have one for the EFII fuel pumps and filters filters doghouse but that's it, how many do you have, and do you have cooling problems?
Click to expand...

It's about mass flow...the quantity of air, in pounds, flowing through the system.

We can flow a lot of mass, and heat it just a little. Or we can flow a little bit of mass, and heat it a lot. Either will carry away the same quantity of heat.

A system with poorly sealed baffle tin and many blast tubes will need to flow a lot of mass, as heat transfer (from hot parts to the air) will be poor. No surprise; much of the air never gets in close proximity to really hot things. However, if it can flow the necessary mass, it will cool just fine.

On the flip side, cooling drag is mass x loss of velocity, so changing the velocity of a lot of mass results in a slower airplane.

If acceptable cruise cooling with high reliability is the primary goal, then the above is fine. However, it does seem nonsensical to buy "speed and efficiency" toys, then negate them with crappy baffling and blast tubes.

Break.

Do blast tubes really improve reliability, or could they be eliminated with better design and/or installation? You're offering an excellent example when you talk of blast tubes to a pump and filter dogbox. Why locate your pumps and filters in a hot place? Why not simply locate them someplace cool?
 
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I asked Robert where he would mount the pumps when I was installing the EFII system, he mentioned he likes all the High pressure fuel stuff on the engine side, I like that idea, so that's where they are, as you know, the system circulates a high volume of fuel, so I believe the flow exchange rate will negate any specific amount of fuel and return to tank before picking up any substantial amount of heat, the pumps come with a heat shield but I am making a new one, longer, .063, and shrouding pumps for scat attached air flow. bonus, I have lots of leg room inside;-) Hey, Dan, on a slightly different note I was reading one of your many comments from back in 2009ish about #3 and #2 cooling fin airflow and lack of, what is todays recommendation for the front of # 2 as far as getting the flow around that one? Thanks.
 
bret said:
dang! really? I am going to start out with what the Mooney had on it, an 8 row, guess ill find out on that first flight....Hey Bob, did you try an exit shroud on the cooler with scat down near the exit?
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Many of us have the opposite problem with the stock cooler... Oil is too cool, even in high ambient temps. I have never figured out why some have oil cooling issues on the hot side while others the cool side, even with identical set ups.
 
JonJay said:
Many of us have the opposite problem with the stock cooler... Oil is too cool, even in high ambient temps. I have never figured out why some have oil cooling issues on the hot side while others the cool side, even with identical set ups.
Click to expand...

I suspect the vernatherm has (some contributing factor) to do with oil too cold or hot....
 
bret said:
...he likes all the High pressure fuel stuff on the engine side, I like that idea...
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Tell me why you like it. "High pressure" here is about 45 psi, vs 25 to 30 psi for all the other fuel injected airplanes with cabin mounted boost pumps.

Anyway, cabin mounting of the dual pump module only adds two pressurized connections; at the pump outlet and at the firewall bulkhead fitting. And with the circulating return, you already bought into an entire series of pumped return line fittings and valve connections.

A boost pump (or here, the primary pump) located in a cool place is not subject to vapor formation.

Call the pump manufacturer and ask about an operating temperature limit. Like anything else which relies on a blast tube for survival, it's going to 180~220F after shutdown.
 
I don't believe the pumps "require' external cooling, they look to be internally cooled by the fuel flow, I have two very large fuel coolers that can handle any heat that may be absorbed by the engine side mounted fuel system :D so, it may help to have fresh cool air blowing past them or not, I don't know. I am thinking the fuel flow rate will keep the fuel cool enough during the race around the engine and back to the tank, I guess I could temporarily install temp gauges at the fuel valve during flight to confirm. that may be a while.......
 
JonJay said:
Many of us have the opposite problem with the stock cooler... Oil is too cool, even in high ambient temps. I have never figured out why some have oil cooling issues on the hot side while others the cool side, even with identical set ups.
Click to expand...

bret said:
I suspect the vernatherm has (some contributing factor) to do with oil too cold or hot....
Click to expand...

Even though Lycomings are air cooled engines a lot of cooling is done by the oil.

A lot of the variation in oil temps is a result of how good the ring/cyl break- in was. This influences the amount of blow by. The high temp blow-by is moving into the lower engine case area where all of the oil is stored, thus heating the oil.
So, if you have an engine that has piston cooling squirt nozzles, and a poor ring break-in (which will have little influence on the actual performance of the engine so a lot of people aren't aware), you can have significantly higher oil temps than the next guy.
 
EFII fuel pumps

The EFII fuel pump is set to 35 psi in our installations.
The pump module can be mounted in the cabin or on the firewall - either solution is valid and works fine. The pump module includes a heat shield that can be used when mounting on the hot side of the firewall. The pumps self cool from the fuel passing through them. The heat shield is all that is necessary to keep them happy on the forward side of the firewall. No fuel coolers required.

Robert Paisley
 
rcpaisley said:
The EFII fuel pump is set to 35 psi in our installations.
The pump module can be mounted in the cabin or on the firewall - either solution is valid and works fine. The pump module includes a heat shield that can be used when mounting on the hot side of the firewall. The pumps self cool from the fuel passing through them. The heat shield is all that is necessary to keep them happy on the forward side of the firewall. No fuel coolers required.

Robert Paisley
Click to expand...

Hey Robert, I was just being funny, the fuel coolers are the wing fuel tanks with lots of air flow, ;-)
 
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