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Mag blast tubes

vgb

Well Known Member
In Vans plans it calls for blast tube for the mags. Seems like to me the blast tubes would be like a leaking cowl seal. So whats everyone doing and are they necessary.
 
Cool it...

In Vans plans it calls for blast tube for the mags. Seems like to me the blast tubes would be like a leaking cowl seal. So whats everyone doing and are they necessary.

Hey Vern,
Mag blast tubes are necessary as the max operating temp recommended by Slick is 190F with Bendix at 200F. Blast tubes rob very little airflow from your plenum and are effective, without them the un-cooled under-cowl temps can reach 300F.
The coil in a mag is the weak link as the majority of failures are the coil. Keeping it cool extend it's life considerably. My RV's and Rocket have all had blast tubes pointed at the Mags and P-Mags and only one Mag failed (Slick) before 500 Hours. My Sonerai 2 had a single Slick 4316 on it's 1835CC VW so I definitely kept it cool...

http://www.greatplainsas.com/imagtime.html

FYI...One of the many mods I made on my RV4 over the years was Electronic Ignition. Setup 1 was a single Slick mag paired with Jeff Roses first Electroair Electronic Ignition. This after the #1 Slick Mag failed twice. The performance difference with the EI was noticeable and the remaining Mag became the weak link. I now run dual electronic with a P-Mag replacing the Mag with improved performance, lower fuel flow, cooler temps, automotive spark plugs and smoother running....:)

https://emagair.com/

V/R
Smokey
 
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Thanks for the info. When I build something its nice to know why its built this way.
 
Nice explanation Smokey Ray. Thank you, I learned something there. I put the blast tubes on my mags when building per Vans instructions and for the 800+ hours on the engine, I've never had an issue with the mags (overhauled at 600 hrs). The engine runs plenty cool so it doesn't appear the blast tube holes negatively impact the baffling.

VGB: Don't forget to put a blast tube on the alternator as well. From everything I've read, it makes a big difference on longevity of the alternator.

Chris
 
Curious

So I can see how the blast tubes cool things while flying.

Seems they aren?t going to do much when the plane is sitting after shutdown and everything is getting heat soaked from residual engine heat.

Thoughts?
 
So I can see how the blast tubes cool things while flying.

Seems they aren’t going to do much when the plane is sitting after shutdown and everything is getting heat soaked from residual engine heat.

Thoughts?

The post shutdown heat soaking starts with the mags at a lower temp due to the blast tubes thereby minimizing the risk of getting to those damaging temps.
 
Two blast tubes don't rob a lot of pressure, although they are definitely a leak, with a total area of roughly 1.5 sq in. Bobby Looper told us an ECI R&D project established a deltaP loss of about 1" H2O for each 1 sq in of leak area. Yes, lots of engines flying around with worse seal leaks, but it's not a positive thing.

Slick's operating limitation of 190F is unlikely to be exceeded in flight. It can be exceeded after shutdown, when the blast tubes don't do zip, but not by a lot. Lots of P-mag discussion regarding those points.

Nothing in the upper accessory case area is anywhere near 300F. I have a thermistor up there right now.

Heat soak starts from a lower temperature? Yes, but it would make little difference in the time to reach ambient temperature after shutdown.
 
Two blast tubes don't rob a lot of pressure, although they are definitely a leak, with a total area of roughly 1.5 sq in. Bobby Looper told us an ECI R&D project established a deltaP loss of about 1" H2O for each 1 sq in of leak area. Yes, lots of engines flying around with worse seal leaks, but it's not a positive thing.

Slick's operating limitation of 190F is unlikely to be exceeded in flight. It can be exceeded after shutdown, when the blast tubes don't do zip, but not by a lot. Lots of P-mag discussion regarding those points.

Nothing in the upper accessory case area is anywhere near 300F. I have a thermistor up there right now.

Heat soak starts from a lower temperature? Yes, but it would make little difference in the time to reach ambient temperature after shutdown.

Dan, is there a way to correlate (loosely) 1" H2O delta plenum pressure to delta CHT in flight? Seems I've seen some info on that before but can't remember where. Would one assume correctly that lower plenum pressure remains the same if one covers (2) mag blast tubes, but upper plenum pressure increases by roughly 1.5" H2O? (I realize I could buy the equipment, build piccolo tubes, and test the configurations, but these are lazy man questions I ask :D)

Also, this post seems to be saying a Slick mag will not necessarily benefit from a blast tube. Is this a correct interpretation?
 
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Dan, is there a way to correlate (loosely) 1" H2O delta plenum pressure to delta CHT in flight? Seems I've seen some info on that before but can't remember where.

A standard Lycoming cooling chart offers some insight. Here I've plotted the effect of one inch less deltaP, 5 vs 6 at 5000 feet. At 75% power and 60F OAT, pretty common at 5000 feet in the summer, one inch deltaP is the difference between bumping max CHT and running somewhere below it. The numbers are appropriate for a cruise climb around 135 KTAS.

COOL%20O-360%20One%20Inch.jpg


Do remember that the cooling charts are empirical, and assume "standard baffles", an undefined factor. Our goal as builders is to better this performance.

Would one assume correctly that lower plenum pressure remains the same if one covers (2) mag blast tubes, but upper plenum pressure increases by roughly 1.5" H2O?

Reducing a leak means the lower plenum pressure would become slightly less, the degree depending on the area of the exit, and the ratio (leak flow/cyl-OC flow).

Upper plenum pressure would increase, roughly by 1.5" if Looper's number can be applied literally. Thinking about it this AM, I'll recant slightly and venture an opinion that blast tube removal would not equate to 1.5", because (1) corrugated plastic tube is a very draggy duct, and (2) the inside diameter is smaller than 1", i.e. an area of 0.785. A quick look at the net says ID is around 13/16, so area would be about 0.5 sq inches per tube, and corrugation would make it flow less than a simple hole of 0.5" area.

I'll venture 1/2" H2O loss, an opinion. Measurement trumps opinion, so someone with blast tubes should do a with and without. I'm not cutting holes in my rear baffle and installing tubes just to quantify a leak I think is useless anyway.

Also, this post seems to be saying a Slick mag will not necessarily benefit from a blast tube. Is this a correct interpretation?

My opinion, yes, but if you want 'em, go for it. I'm sayin' it ain't no free lunch.
 
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Dan,

One thing to note, Vans corrugated tube flows very poorly. I wouldn't be surprized if the flow were less than half of the same ID smooth bore tube. I don't have any hard numbers to back that up, but I tried to use some for an avionics cooling fan duct; it whistled and the flow through about 12" was almost non existant. The smooth tube I endes up using flows quite a lot.

The point being, the equivalent area of pressure loss may be much less.
 
Dan, One thing to note, Vans corrugated tube flows very poorly. I wouldn't be surprized if the flow were less than half of the same ID smooth bore tube. I don't have any hard numbers to back that up, but I tried to use some for an avionics cooling fan duct; it whistled and the flow through about 12" was almost non existant. The smooth tube I endes up using flows quite a lot.

The point being, the equivalent area of pressure loss may be much less.

Thanks Colin. Good note.

If it doesn't have much flow, it can't be doing much cooling.
 
We just installed dual PMags on a friends skybolt. The folks at Emag were very helpful on where the blast tube location needed to be and why. The heat from the engine oil is the primary source of heat that travels back to the electronics area of the mag. Placing the blast tube on the round area of the Pmag helps cool that area thus avoiding damage to the electronics in the mag.
 
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