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Alternator Blast Tube?

MED

Well Known Member
Should I install a blast tube for the belt-driven alternator? I have installed one on the stand-by alternator, but there are no provisions in the plans to install one for the primary. It would be easy to install a tube in the right, forward baffle, but I was concerned it might direct rain right into the alternator. Thoughts? :confused:
 
To blast or not to blast...

I recently had occasion to work on a client?s RV-7, to replace the original failed alternator. The original had no nameplate or identification of any kind so we were unable to do a direct replacement, but we did find that the Plane Power unit that Van?s sells was able to be adapted, but that did require a different length belt and prop removal to install it, but I digress.

The original alternator had no blast tube and had lasted about ten years and 500 hours, but Plane Power recommended one. However, the installation of a blast tube would be quite onerous in that particular airplane. The owner does not fly in IMC or at night, therefore rarely uses pitot heat or lights or other high draw items which would put a heavy load on the alternator (read that produce a lot of heat), so we decided to forego the blast tube.

The point I am trying to make in this ramble, is that in making your decision you need to cosider the type of equipment you will be powering and the type of flying you plan on doing.
 
Med, one comment from Vic on my inspection was to add a blast tube to the stock PP alternator (pointed to the rear electronics) so that's exactly what I'm going to do. It's not in the plans so not required but he did strongly suggest adding one.
 
Not all blast tubes are created equal-

Just be aware that most blast tubes for belt driven alternators out there are of questionable value, and may actually be counter productive. Most are installed just inside the lip of cooling inlets at or near 90? to the high-velocity inlet stream. In many instances this can result in no flow or even reverse flow through the blast tube itself. The only way to know for sure is through a careful study with thoughtfully mounted manometers that can compare pressures at various points in flight in a variety of flight conditions.

If you want a blast tube That is certain to work properly, consider a longer one that is fed from the rear baffle wall, where plennum pressure recovery is at its highest value and velocity is essentially zero. Also be sure to aim and secure the business end of the tube where it will do the most good.
 
Just be aware that most blast tubes for belt driven alternators out there are of questionable value, and may actually be counter productive.
Just curious if you have done any test or study for this? The inlet air for the heater is located at this location with the same orientation and most if not all get a very nice flow of air thru that.
 
In addition, study which end the alternator sucks the cooling air through it. Some alternators draw the air in from the back, others have the air flow through from the front, depending on how the cooling fan is set up. No use directing blast tube air flow at the wrong end!
 
Just curious if you have done any test or study for this? The inlet air for the heater is located at this location with the same orientation and most if not all get a very nice flow of air thru that.

Back in the days of CAFE research we did look at this on a couple of aircraft, but it is a very installation specific question.

The same issue should be addressed with many of the Superior forward facing induction system inlets that are also usually located very near the cooling inlet. Not having the air scoop seems like an attractive idea, but you do get a real ram-effect benefit if they are well designed. Again, this is a very installation specific issue, worthy of some study in each case to make sure that poor induction flow is not robbing potential power. There can be very wierd flow dynamics in the region of the inlets because of the high velocity, and these can vary considerably with angle of attack. Flow separation anywhere in the inlet will reduce net effective inlet area.

I’m not a big fan of round cooling inlets with sharp edges at the entry point for this reason. At high angles of attack, the sharp break can cause massive flow separation around the lower lip. A generous radius at the lip and a properly designed decelleration ramp can greatly improve performance of round inlets.
 
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The original alternator had no blast tube and had lasted about ten years and 500 hours, but Plane Power recommended one.

During the development of the RV-14 FWF kit I had a personal phone conversation with the head engineer at Plane Power.
He said they had done testing of numerous airplanes to measure the max. under cowl temp experienced after shut down and that it never exceeded the max. temp they would consider acceptable, so the did not require a blast tube (which doesn't do anything for you anyway when they consider the most critical time to be parked after shutdown). He said in flight temp data showed that to not be any concern.
He said he didn't see any harm in having one unless it could induce water during flight in rain, etc., and possibly damage the bearings (depending on how it was designed).

Can you point me to the info that states they now recommend one?
 
The same issue should be addressed with many of the Superior forward facing induction system inlets that are also usually located very near the cooling inlet. Not having the air scoop seems like an attractive idea, but you do get a real ram-effect benefit if they are well designed. Again, this is a very installation specific issue, worthy of some study in each case to make sure that poor induction flow is not robbing potential power. There can be very wierd flow dynamics in the region of the inlets because of the high velocity, and these can vary considerably with angle of attack. Flow separation anywhere in the inlet will reduce net effective inlet area.

Many years ago when the induction "snorkle" was developed for use of the fwd induction engines on RV's, a lot of testing was done to assure the best pressure recovery performance possible, and the final design was found to be about equal to the standard (scoop style) induction system.
 
During the development of the RV-14 FWF kit I had a personal phone conversation with the head engineer at Plane Power.
He said they had done testing of numerous airplanes to measure the max. under cowl temp experienced after shut down and that it never exceeded the max. temp they would consider acceptable, so the did not require a blast tube (which doesn't do anything for you anyway when they consider the most critical time to be parked after shutdown). He said in flight temp data showed that to not be any concern.
He said he didn't see any harm in having one unless it could induce water during flight in rain, etc., and possibly damage the bearings (depending on how it was designed).

Can you point me to the info that states they now recommend one?

Thank you, Scott. This answers my question! :)
 
Can you point me to the info that states they now recommend one?

Hi Scott. It was on the installation instruction sheet. I didn?t keep a copy but I will double check next time I see the owner.

Hey, I see EAA flew off the 40 hours on ?our? airplane!
 
The Plane Power installation instructions do say to "supply outside ram air to rear of alternator by the use of 1" scat tube or other suitable ducting." Nevertheless, if no one is doing this, the plans don't call for it, and there have not been a rash of failures, maybe it is not needed. I know Van's planes are not used for IFR, but I will - at least passing through clouds going up or down, so I am worried about directing water into the alternator. It might be better for me to just plan on more frequent alternator replacements, rather than precipitating one - no pun intended. :rolleyes:
 
There is a poll here for alternator reliability and it is worth looking it up and check it out if you have not. It does not specifically indicate if blast tube has been used or not but it has different brands/time and people have provided info on their experience and type of failures they have had.
 
There is a poll here for alternator reliability and it is worth looking it up and check it out if you have not. It does not specifically indicate if blast tube has been used or not but it has different brands/time and people have provided info on their experience and type of failures they have had.

Thanks. I Have not searched for overall reliability. :eek:
 
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