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Old 06-28-2010, 08:46 AM
ninerhawk ninerhawk is offline
Join Date: Aug 2005
Posts: 27
Default Blast tube considerations

Are there any recommendations for initial cooling blast tube installations on a 320 engine for an RV-9? Seems natural to add provisions for these up front, or should one wait to monitor initial temps? I'm planning to start with factory mags and may upgrade to EI later on. Also, I have seen pictures of blast tubes pointed at either front or rear of a Plane Power alternator - which end needs cooling the most?

Do most add temp stickers to monitor on intial flights or how do you diagnose temp related problems before it is too late? Anybody using simple temp data acquisition on initial flight(s)?
Howard Kaney
West Bend, WI
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Old 06-28-2010, 09:55 AM
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kentb kentb is offline
Join Date: May 2005
Location: Canby, Oregon
Posts: 1,785
Default I installed two blast tubes on my 9A

A one inch tube on the left inlet ramp to direct air at the back side of my alternator. It needs to be directed at the voltage regulator to keep the diodes cooler.
The other one is mounted on the right back baffle to cool my engine mounted fuel pump. I don't have anything pointing at the single mag. The other side is powered by EI.

I don't know if this is optimal or not. I had to replace the voltage reg after only 90 hours.

Kent Byerley
RV9A N94KJ - IO320, CS, tipup
AFS 3500, TT AP, FLYING....
Canby, Or
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Old 06-28-2010, 10:20 AM
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f1rocket f1rocket is offline
Join Date: Jan 2005
Location: Martinsville, IN
Posts: 2,336

I think the most important is the alternator, just like Kent suggested pointed at the back.

After that, I always put tubes on my magneto and mechanical fuel pump but to be honest, I don't know how critical it is to do them.
Randy Pflanzer
Greenwood, IN
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Old 06-28-2010, 11:02 AM
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Ironflight Ironflight is offline
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Join Date: Jan 2005
Location: Dayton, NV
Posts: 12,612

I definitely agree with Randy on this one! I cooled the alternator diode package with a carefully aimed and secured tube after having my first ND alternator start with a wandering voltage after a few hundred hours. the second (withe the blast tube), is going strong at 800 hours or so. I have never been a big believer in cooling the big mass of the Slick mags with a little stream of air, but I have been wrong before....

Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
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Old 06-28-2010, 11:49 AM
Bavafa Bavafa is offline
Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,540

Like others, I am cooling the back side of my alternator (B&C) with a 1" tube but I also have blast tubes to my mags.

I wonder if the engine driven fuel pump needs it as I have seen very few with the blast tube directed at it. I like to get more input on this before I start drilling another hole in the baffle.
N825SM RV7A - IO360M1B - SOLD
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Old 06-28-2010, 03:03 PM
elippse elippse is offline
Join Date: Dec 2006
Location: Arroyo Grande, CA
Posts: 938
Default Blast tubes

I have a 3/8" diameter aluminum tube installed in the cowling immediately ahead of the alternator. I flared the input so as to allow flow from the various directions that may be present behind the prop. I also have a 1" W X 1/2 H inlet on the top of the cowling at the very front. It feeds an expanding duct on the inside-top of the cowling. There are four 1/4", 90 degree tubes branching from it with their apertures pointed toward the ignition coils on top of the engine. At the back of the cowling this duct empties into a small plenum on the firewall from which I take off five plastic tubes - one 3/8" tube to each of the two electronic ignitions mounted on the engine-side of the firewall, one 3/8" tube to the gascolator, and one 3/8" tube to the engine-driven fuel pump. A fifth tube, 1/4" plastic, goes into the crankcase breather separator assy. to condense the oil vapors. I have yet to experience any vapor lock at high, hot airports though I did on three occasions in my friend's L235/320! A 3/8" tube pointed into the airstream at 190 mph can flow as much as 0.21 cfs, 12.8cfm.
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Old 06-28-2010, 03:24 PM
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DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 9,957

Does anyone have any actual temperature measurements to share?

Called Plane Power.....nice folks! These temperature numbers are more or less applicable for the the AL12-EI60 and similar. I think the data actually came from measurements for certification of similar models.

Max stator 223C (433F)
Max rectifier 150C (302F)
Max bearings 120C (248F)

The unit has two internal fans, one at each end. Air enters at both ends and exits around the case.

Their very general rule; 40 amps load or less usually doesn't need a blast tube.

Now a personal thought, mine, not Plane Power's. The undercowl area contains a huge radiant heater, aka the exhaust system tubing. One leg is near the alternator and another is under the fuel pump.
Dan Horton
Barrett IO-390

Last edited by DanH : 06-28-2010 at 04:46 PM.
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Old 06-29-2010, 02:56 PM
elippse elippse is offline
Join Date: Dec 2006
Location: Arroyo Grande, CA
Posts: 938

Originally Posted by DanH View Post
The undercowl area contains a huge radiant heater, aka the exhaust system tubing. One leg is near the alternator and another is under the fuel pump.
I'll second that, Dan! On a car belonging to a friend, his ignition wires used to dry out and crack because of the exhaust manifold nearby. It is so important to put a thin metal shield between an exhaust pipe and any nearby object that you might not want to get heated. It's good to look at those pix of old which showed a night takeoff of a plane with flames coming out of the exhaust and the pipe glowing red!
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