647jc

647jc

Well Known Member
I know this has been discussed in other threads but I don?t think a consensus was ever reached. Apparently it is felt that some alternators require additional cooling while others do not. I think most people (including myself) felt that adding a blast tube couldn?t hurt performance that much and better safe than sorry. However, I ran across another post in some other thread saying that installing an alternator blast tube in the plenum intake ramp as Van suggests would be like cooling your alternator with a garden hose, if you ever flew through rain. The alternator I have is the standard Vans 60 amp job (made in Japan label) and has an open frame design, probably for cooling purposes, and has a voltage regulator on the back also well ventilated. I really don?t think spraying water on it with a blast tube while it is running would be advisable and now I?m thinking no blast tube may be a better decision. So, as applicable to Vans 60 amp alternator with the open frame design and a voltage regulator on the back, what have other builders done and has anyone had any failures due to an over heated or a soggy alternator?
 
I did put the blast tube in per Van's in my 7A. I haven't had any failures in 150 hrs and I have flown in the rain. I have the 60 amp alt supplied by Van's.

Roberta
 
So you are Looking for Consensus?

Good luck with that. I have the 60 Amp Alternator with no blast tube 400 hours no problem. Blast tube holes do reduce performance and the faster you go the more they hurt it. If you are not looking for the maximum performance you wouldn't notice the difference. Roberta's experience should clear up the rain concern for you.

Bob Axsom
 
Roberta: You're using the internally regulated alternator from Van's, right? How did you provide overvoltage protection? Have been looking for a good method without success thus far. Glad the nosewheel mod went well. Happy New Year. Bill
 
I ran my first alternator with no blast tube, and when the VR started to wander I put in a rebuilt unit, and decided to try a tube. I haven't thought about it since, and it's still running. When I looked at the unit closely, it looked very well "potted" to me, so I decided not to worry about the rain. I also agree with bob that stealing air from the plenum can reduce performance and cooling - but I don't know if it is quantifiable.

No blast tubes on my Mags though....and they looked GREAT at the 600 hour (should have been 500) inspection of the inerds...

Paul

Paul
 
I don't think the automotive application for this alternator has any type of protection from rain, water, etc. Any time I have ever opened my hood after driving in heavy rain, the engine compartment is wet.
I install blast tubes on my alternators.
 
Its Your Dime

The original cheap, blast tube equipped 60 amp alternator purchased out of Van's catalog shot craps with relatively few hours on it. It failed both electrically and mechanically. See the link for pictures.

http://www.vansairforce.com/community/showthread.php?t=11768&page=2

I replaced it with a Plane Power unit and am so satisfied, I installed one exactly like it in the -8 project. The installation instructions that came with the new Plane Power alternator states the following and I quote: "Install alternator per included drawing. Supply outside ram air to rear of alternator by the use of a 1" scat tube or other suitable ducting."

As always, the builder is perfectly free to either comply with or dismiss the recommendation.
 
Hey Bill!! Happy New Year to you as well. Yes, I have the internally regulated unit from Van's. I did nothing regarding overvoltage protection or regulation. Haven't had a problem with voltage, battery or any of the components. Voltage seems to be just right as is.

Roberta
 
I have a 3/8" tube going through my cowling just ahead of the alternator. It will supply approximately 0.22 cf/sec of air at 200 mph. I don't think that this tube has a measurable effect on drag. The more of an electrical load you put on your system, the hotter the alternator will run. A lot of cowling designs will let air from the lower-half of the cowling go forward around the crankshaft and get pumped out around the back of the spinner. Since this is heated air, approximately 100F to 140F above ambient, it could add to the internally generated heat of the alternator as it flows past it. Plus, this air exiting the gap between the spinner and cowling is perpendicular to the flow from the spinner and causes a drag increase. Those in the know seal their crank opening to reduce drag.
 
That doesn't seem like that much airflow. If my math is right that's 13.2 cfm. All but the slowest 60mm (2 1/2") computer fans can do that. A 1" opening would be 4.5 cf/sec (271cfm), or more than double the airflow of a 120mm computer fan. (or about 6 60mm fans)

If everyone is using a 3/8" opening I'll guess there's just not enough extra air to make a difference. Come to think of it, those are probably outside diameters, so the airflow will be even less.
 
But it's cooler outside air, rather than the much warmer lower engine compartment air. OK, then how about a 3/4" tube to get about 53 CFM. Most of the installations I've seen no longer have the centrifugal fan on the front of the alternator, so they aren't getting any air pumping through the rotor and stator; the tube in front will blow air right through it.
 
Blast tube and OV protection

Hi Roberta: Sorry for hijacking the thread a little. Glad to hear your install has worked well with no problems -I know you fly a lot! Hope to fly in and visit you this summer. Bill
 
Stop in anytime, Bill.

The hose supplied by Van's is 7/8" ID, or better. The ID of the port on mine is better than 7/8". so it gets plenty of air. I doubt it affects my airspeed much. At 204 cruise, I'm not complaining.

Roberta
 
Is this going to be the year

Roberta in 2005 at Oshkosh you told me you really wanted to fly in the AirVenture Cup race. If that 204 mph cruise is accurate you would be in the money. The speeds from last year's race are inflated by an unusual tail wind so don't let the posted numbers mislead you. That is very impressive. Is this going to be the year?

I plugged all of the upstream cooling air ports except the oil cooler and found that the difference in speed was measurable but in the fraction of a knot range (less than 0.5kt as I recall). I agree that the difference from the one blast tube hole would be difficult to measure but we have purist that will tell you must have an indepentent hard top plenum with absolutely no leaks to fly fast. In the face of that accepting a 7/8" hole in the inlet ramp seems shocking.

Bob Axsom
 
I understand that air entering and exiting the cowl must have a minimal amount of differential pressure to achieve the best aerodynamics for speed. I just built the plane the way Van's told me to via the plans they provided. It seems to have worked and the numbers I get are comensurate with what was expected. I enjoy flying it as it performs as Van's said it should.

Thanks, Bob, but other interests preclude me from racing. But if that is your passion, go for it. Without the pioneers and inventors, in all areas, we would still be in the stone age. I applaude your inventiveness and passion.

Roberta
 
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