Alternator Blast Tube Intake Port

For those who have added an alternator blast tube, is there a preferred location on the right ramp baffle for the intake port? I was thinking of mirroring where the prop gov cable hole is on the left ramp, but just a little more inboard towards the centerline.

What about when it's raining?

Dave

Don't think water ingestion is an issue, but I'm open to suggestions as to a better location as I haven't cut a hole yet.

I put my blast tube intake on the bottom of the air intake ramp.

I brought air blast tubing from my right ramp and attached a plastic pipe cap with silicone tape. Tubing has slots which blow into diode cooling window on alternator. Cap is lower than slots and has 1/8" hole for water drainage. Not flying yet so take it for what it's worth...
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No blast tube on mine.

Shield from #1 exhaust pipe radiant heat on my to do list. So far no problems and I routinely see 55 A after startup charging 925/680 batteries and 22 A in cruise. 100+ degree OAT's on the ground last summer. TIFWIW.

Wayne Gillispie said:

Shield from #1 exhaust pipe radiant heat on my to do list. So far no problems and I routinely see 55 A after startup charging 925/680 batteries and 22 A in cruise. 100+ degree OAT's on the ground last summer. TIFWIW.

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I'm installing a blast tube because it's in my 70A alt's installation instructions. I'd rather err on the side of caution in this matter and follow the directions as written.

Source for small hose flange?

On a related note, does anybody have a source for spun (or even welded) aluminum flanges for 5/8" or 3/4" scat hose? ACS only offers them in sizes or 1" and up, and I'd rather not use hose that big for a blast tube. I know the Vans black corrugated plastic tube will do the job, but I was hoping to use someone more classy for this application.

thanks,
mcb

I went with no blast tube on my -4's alternator. After 110 hours that proved to be a mistake when one of the diodes went bad and started producing static over the radio. It still charged OK though. Replaced the diode pack and everything is good. Now I'm looking for a way to install a blast tube that won't ingest rain and is still far enough from the exhaust not to cause problems.

Alan
RV-4

Here is how I did mine

I made a small duct from scrap .025 thick alclad that I riveted to the cover on the aft side of the alternator where the air inlets for the diodes are located (painted it black to spiff it up a little). The duct will direct the air flow right onto the diodes. The corrugated tube snaps right in there with a little Proseal applied to prevent the metal from cutting into the tube over time.



The corrugated tube was routed so there is a low point between the air inlet at the baffle ramp and the alternator to catch water. I drilled a #40 inch hole in the tube at the lowest point for water to escape before it climbs back up into the alternator. It won't stop damp air from blowing into the alternator but it should prevent a jet of water from flowing in through the tube in heavy rain.



I plan to mount a radiation shield on the crossover exhaust pipe closes to the duct to reduce the heat load there. I could also wrap the duct with reflective thermo shield if necessary.

Appreciate all of the feedback and pics. My alternator came with a blast tube fixture which you can just see on the aft end of my alt in the photo below, just above and slightly left of the tag. (this is the only pic I have access here to at work and can post).

This is why my focus has been more on the location of the supply hole in the ramp baffle. (note: this is an old pic before I installed my prop and safety wired the tension arm bolts )

I know it's a bit off topic,

but what happens to rain water?

Does it get atomized by the prop and not pose a hazard to the electrical?

Water and electric are things I've always been told to avoid having in the same place.

copied some others

I found a light washer with a hole that fits the blast tube groove and then I RTV'd in place and closed the hole to .040 dia. with RTV for water drain. the blast tube is cut out to allow air flow to the back side of the alternator. not flying yet so I do not have any experience with this setup. I'm relying on other reports of similar config.

Blast tube

Plane power told me to direct the cooling tube on the decal in the aft center of the alternator. There is no opening there and that's where they want the cooling air. They also said that unless you are working the alternator at its maximum output there is no need for cooling and that the requirement for cooling air is obsolete.

OK, so I broke down and e-mailed PlanePower and the cliff notes answer is the blast tube is indeed optional and if you do install one, water isn't an issue. Here's the full response:

The blast tube is optional as far as we are concerned. To us water and blast tubs is a worry over nothing. All the way back to the introduction of the RV4 (or before) Vans has indicated a blast tube. Nothing wrong with it and it will cool your alternator by "a little, little bit". FYI, we have a blast tube that attaches to your AL12-EI70/B.

If you attach your tube to our blast tube it will dump air onto the center of the back cover. Under the back cover is the hottest part of the alternator. If water gets dumped there it will bounce off the rear cover. If you are simply terminating a tube at the alternator direct your tube at the center of the back cover. Do not dump air at the sides of the alternator for that is where hot air is exiting the alternator.

Run you tube downward so it is always draining water. Your alternator is below the baffle so you can easily run the tube so rain water simply flows down and drips out as will ram air. There are thousands of RVs flying with blast tubes and I have never heard of a water issue.

We appreciate your business.
Dick Carmichael
Tech Support

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So what I've decided to do is flip-flop and initially not install a blast tube. I will monitor buss loads during phase 1 and if it stays pretty low I'll probably leave the blast tube off permanently.

Auburntsts said:

So what I've decided to do is flip-flop and initially not install a blast tube. I will monitor buss loads during phase 1 and if it stays pretty low I'll probably leave the blast tube off permanently.[/FONT][/FONT]
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That is the same conclusion I came to. From what PP told me as well, unless the Alt is overworked at it's maximum output, then there is no need for cooling.

One more data point in the "blast tube on the alternator or not" question. I recently asked the same question of B&C about my alternator installation.

me: "I have an L-40 mounted to a Lycoming O-360 in an RV-6A. Thinking about long term reliability of the alternator diodes, I am trying to figure out if I need to install a blast tube to cool them. Do you recommend installing a blast tube? If yes, where should the blast tube be aimed on the alternator?"

answer from Greg at B&C : "A blast tube is not needed, however if you do use one you would want to focus on either/both ends. The air exits through the mid-section."

So, "install alternator blast tube" just got crossed off my to do list.

The enemy of all electonic components is excessive heat, keep them reasonably cool and they will outlast their hotter running counterparts.

Walt said:

The enemy of all electronic components is excessive heat, keep them reasonably cool and they will outlast their hotter running counterparts.

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As an electronics industry refugee, I understand completely. That's why after searching for info on VAF, I asked the alternator manufacturer. I figured his perspective from warranty return rate for diode failures would most likely determine his answer.

Also forgot to mention in my original post, my alternator is externally regulated, with the regulator mounted to the firewall. The only electronics inside the alternator, are the diodes.

Anyone running an internally regulated alternator needs to determine if external cooling is needed in that case. YMMV.

One other bit of interesting info. The alternators on both my cars (Honda Accord, Ford Ranger) have similar louvered areas on the back end, over the diode heat sink. I can't say whether they are internally or externally regulated. In both cases the alternators have a higher capacity (and most likely a higher normal operating load, therefor, higher diode heat generation) than my RV. Neither one has any type of external cooling added. Now admittedly, they both sit near the front of the engine compartment with some exposure to airflow past the radiator, as opposed to under the baffles of the Lycoming with airflow off a hot cylinder. And neither one has an exhaust pipe passing approximately 6 inches behind it as the RV does. Maybe the RV diodes will run hotter on the ground. On the other hand, the RV will be operating at much higher altitudes with lower outside ambient temperatures and lower loads, so may run cooler there. Short of measuring diode junction temperatures in a car vs. an RV under varying operating conditions, it is difficult to know for sure.

If the diodes outlive the alternator brushes wearing out, mission accomplished.