NACA design specs
HPV Cooling - NACA Duct Rational and Construction

Article by Warren Beauchamp, 5/1/2000

A major issue with well sealed, fully faired HPVs is that of overheating. Racing during a typical 85 degree and 90 percent humidity Midwestern afternoon can generate in-fairing temperatures in excess of 110 degrees fairly rapidly. At that point it's only a matter of a short period of time before a competitor experiences overheating and has to stop racing. This is not a good thing. In the past, streamliner builders have employed a few techniques to allow them to make it through a race without expiring:
  • Train for the heat - It has been found that if you train in high heat conditions, your body becomes acclimatized.
  • Ice in the water bottle/bladder - drinking cold water helps a lot
  • Spray bottle - some racers mount a spray bottle inside their fairing and spritz themselves during the race.
  • Air ducts - some racers put holes in the fairing or canopy to allow some air to cool the fairing.
  • Matt Weaver's Virtual Edge used a human powered fan, which sucks air in from rear area of fairing and shoots in out the back .
  • Fancy cooling jackets - Umm... Right....

I have reservations about training for heat, it doesn't sound like fun, and fun is why I race. The spray bottle works great in low humidity conditions, but in  high humidity the effect is greatly reduced. Fans sound too complicated and can rob power from the rider. Cooling jackets have to radiate large amounts of heat away from your body. I have heard that a human body at high exertion can melt a cubic foot of ice in an hour. I don't know about you, but I don't want to carry that much ice around in my fairing...

The Barracuda version1 fairing had an air scoop which blew air on my face. This seemed to help a lot, so the search was on for a low drag way to duct some air in to blow on my face. Some fairings utilize a hole in the nose of the fairing, theorizing that it will disturb the air the least there, but the plumbing involved with getting the air to the rider is extensive. I decided to try a NACA duct, which is designed to be low drag, and to place it just in front of the canopy, where a high pressure zone is likely to build.

NACA duct design specs:
After wading through a bunch of documents and drawings on the NACA web site, I determined that an optimum NACA submerged inlet duct design should employ curved diverging ramp walls with a width to depth ratio between 3 and 5, and a ramp angle of between 5 and 7 degrees. The entrance lip at the rear of the duct should be a blunt airfoil leading edge shape; however, the edge formed by the duct and the surface elsewhere should remain crisp. The thicker the surface boundary layer in the area of the duct is, the less efficiently the duct will work. This means that if the surface boundary layer begins to detach from the fairing surface before it gets to the duct, you won't get as good ventilation as you would if the aerodynamics of your fairing were better in front of the duct.

The drawing below shows ramp profiles (side view) for an 11.5, 9, 7 and 5 ramp angle, as well as the plan form (top view) for the various angles.

The picture below shows the curved ramp floor. A curved ramp floor gives slightly higher duct pressure than a straight one.

I determined that a duct 5" wide was about the right size for the Barracuda and then based on the width to depth ratio above, I decided on a 1.5" duct depth. Based on that depth, and a 7 degree ramp angle, I made the duct 9.5 inches long. I drew the outline of the duct on the fairing just forward of the canopy, as that would enable me to use a very short duct to redirect the air toward my face, and I figured there would be a high pressure zone in that area. nacaduct_actual1.jpg (4628 bytes)
nacaduct_actual2.jpg (10155 bytes) I cut the hole in the fairing (yikes!), made the duct itself from some scrap Coroplast, then pop-riveted the duct to the fairing. 

 

Now came the time to test it, and what better time than at a race! At the 2000 Northbrook, Illinois and Kenosha, Wisconsin HPV races I took the new streamliner out for the first time with the canopy on.  The duct worked great! At speeds above 20MPH I started to feel a breeze from the duct. At cruising speeds of 35+ MPH, I got a great blast of air on my face. I am now confident that I will be able to cruise at high speed for an extended length of time without overheating.

10/21/04 Update

I have been racing the Barracuda streamliner now for 4 years, at numerous diverse events each year. I have raced at venues from Toronto Canada to Battle Mountain, Nevada. Though I have no numerical data to prove the duct's cooling ability, or it's low drag, I can provide the anecdotal information below:

  • There is no noticeable difference in speed with or without the duct open, but I'm sure it adds some minute amount of drag. Because of this I tape the vent shut or partially shut during high speed sprint races or races on cooler days when the vent is not needed. 

  • The vent works great at speeds over 20MPH, and blows enough air on my face and chest to keep me from overheating. At 40MPH the volume of air is wonderful. This solution works well even on long (1 hour) races on extremely hot and humid days. I am consistently fresher and less sweat soaked after the races than my competitors.

  • I think it's important that the vent be placed in front of the windshield as there is a high pressure zone there caused by the windshield bubble. The vent bleeds off some of this pressure. It also makes it easy to have the air blow on your face, as that's where the most blood vessels are to cool you off.

  • NACA ducts in other locations will be less effective.

You should be search on "NACA duct drag" and find lots of articles if you are looking for numerical data about NACA ducts.

For my latest Cuda-W streamliner, I will either build a more permanent fiberglass version of the duct, or buy one of the commercially available NACA ducts designed for planes and race cars.

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