REFLECTOR: AIR INTAKE DUCT

Lou Stedman stedmanlou at adelphia.net
Thu Apr 17 08:37:22 CDT 2008


Thanks Al. I appreciate this information. Not being an engineer, I am not sure about the calculations. How do I determine the cfm of the 3" hose. Is it 8230 cfm?I have a K&N filter #33-2060 which is mounted right in the belly scoop. The length of hose from there to the fuel servo is 26". I am using skeet tube so the inside of the hose is fairly smooth. Do you think the 2 1/2" is large enough or do you think I need to go to the 3". 
Lou Stedman
Velocity N7044Q
Olean, NY
  ----- Original Message ----- 
  From: Al Gietzen 
  To: 'Velocity Aircraft Owners and Builders list' 
  Sent: Thursday, April 17, 2008 12:52 AM
  Subject: Re: REFLECTOR: AIR INTAKE DUCT


  I have a tech. question.  I have a LYC. IO-360. I am replacing the Skeet duct which runs from the belly scoop, through my K&N filter, through my alternate air T, and into the throat of my fuel servo. The hose that is on it now is 2 1/2". My A&P tells me it should be a minimum of 3" diameter, yet the throat opening of the fuel servo is only 2 3/8". How can I determine what is the right size to supply needed air flow?

  Lou Stedman



  Lou;



  It's all about pressure drop.  Pressure drop in the induction path directly reduces the amount of air/fuel charge entering the cylinder. The throat of the fuel servo is very short, so there is not a lot of frictional loss there, and you want high velocity at that point for fuel dispersion, etc. It sounds like you have a fairly torturous path for the induction air to travel before getting to the intake manifold.



  What is the right size? Well, that's a bit complex.  You could measure the pressure at the entrance to the fuel servo, and if it is more then 1" of HG less than the pressure at the entrance to the scoop, the loss is probably higher than you'd like.  Or you could do some analysis based on the engine displacement, rpm, air flow velocity, and compute the pressure drop.  Not very easy.  



  But just a back of the envelope analysis - your engine would like to take in 180 cubic inches of air each revolution.  At 2700 rpm that's 280 cubic ft per min (cfm). You 2 ½" duct has an area of only 0.034 sq ft; so the velocity in there would be 8230 ft/min, or about 95 mph. Depending on the length of tube, that could be considerable friction and turbulence loss, especially if the inner surface is not real smooth.  And does it also have to make a turn? More loss.  How big a filter?  Pressure loss there as well.



  A 3" diameter would slow the velocity by 30%; a big help.  3 ½" would be even better.  Pressure losses generally go as the square of the velocity.



  FWIW;



  Al



------------------------------------------------------------------------------


  _______________________________________________
  To change your email address, visit http://www.tvbf.org/mailman/listinfo/reflector

  Visit the gallery!  www.tvbf.org/gallery
  user:pw = tvbf:jamaicangoose
  Check new archives: www.tvbf.org/pipermail
  Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.tvbf.org/mailman/private/reflector/attachments/20080417/9930f48d/attachment.htm 


More information about the Reflector mailing list