VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics


Go Back   VAF Forums > Model Specific > RV-14
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 10-19-2018, 05:35 AM
MED MED is offline
 
Join Date: Jul 2015
Location: Aiken, SC
Posts: 306
Default Alternator Blast Tube?

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?
__________________
MED
140236
N435MD reserved
Reply With Quote
  #2  
Old 10-19-2018, 06:25 AM
BigJohn BigJohn is offline
 
Join Date: Nov 2008
Location: Gloversville, NY
Posts: 1,544
Default 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.
__________________
John Peck, CFII, A&P, EAA Tech Counselor, Flight Advisor.
“Master Pilot” Award, UFO Member.
RV-12 N37JP 120176 Flying since 2012.
One Week Wonder Build Team, OSH 2018.
VAF paid through 10/2018.
Reply With Quote
  #3  
Old 10-19-2018, 06:58 AM
Nova RV Nova RV is offline
 
Join Date: Aug 2015
Location: Leesburg, VA
Posts: 244
Default

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.
__________________
Chris Moon

Leesburg, VA
RV-14A kit # 140243 (flying as of 11/18)
VAF 2017 and 2018 paid
www.mykitlog.com/chrismoon/
Reply With Quote
  #4  
Old 10-19-2018, 09:14 AM
Hartstoc's Avatar
Hartstoc Hartstoc is offline
 
Join Date: Aug 2017
Location: Sebastopol,CA
Posts: 139
Default 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.
__________________
Otis Holt-
RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2018 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"
Reply With Quote
  #5  
Old 10-19-2018, 10:06 AM
Bavafa Bavafa is offline
 
Join Date: Oct 2007
Location: Sacramento, CA
Posts: 2,461
Default

Quote:
Originally Posted by Hartstoc View Post
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.
__________________
Mehrdad
RV7A - IO360M1B - SOLD
Dues paid
Reply With Quote
  #6  
Old 10-19-2018, 10:09 AM
Ralph Inkster Ralph Inkster is offline
 
Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 420
Default

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!
__________________
Ralph
Maintain lots, upgraded & repaired some, modified more, rebuild a few, & built 4 of 'em
Reply With Quote
  #7  
Old 10-19-2018, 11:51 AM
Hartstoc's Avatar
Hartstoc Hartstoc is offline
 
Join Date: Aug 2017
Location: Sebastopol,CA
Posts: 139
Default

Quote:
Originally Posted by Bavafa View Post
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.
__________________
Otis Holt-
RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2018 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"

Last edited by Hartstoc : 10-19-2018 at 12:00 PM.
Reply With Quote
  #8  
Old 10-19-2018, 04:15 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,801
Default

Quote:
Originally Posted by BigJohn View Post

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?
__________________
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
Reply With Quote
  #9  
Old 10-19-2018, 04:18 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,801
Default

Quote:
Originally Posted by Hartstoc View Post
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.
__________________
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
Reply With Quote
  #10  
Old 10-19-2018, 07:27 PM
MED MED is offline
 
Join Date: Jul 2015
Location: Aiken, SC
Posts: 306
Default

Quote:
Originally Posted by rvbuilder2002 View Post
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!
__________________
MED
140236
N435MD reserved
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 04:41 PM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.