Hood Scoop Opinions....

[dwarner]

Tom,

The reason for the expansion of rule 4.R is an attempt to control the hood scoops we see on coupes. It appears that your roadster got got in Miller's rewrite. May I suggest that you fill out a rule change request and request some sort of allowance for street roadsters.

The form can be found at www.scta-bni.org.

Dan

[Rex Schimmer]

Rick Byrnes,
Please take what ever I say with a "grain of salt" you need to remember that I have NOT built a car for Bonneville and you have, as of right now you are miles ahead of me! I am presently working on a car design but I am at least 3-4 years away. You have been there with racing iron. You are now converting to a streamliner which will probably be much different than your present lakester and going with a NACA duct may be a good option for your air inlet, it may not but what ever you pick I am sure it will be based upon your best thinking and that is what makes Bonneville so great! My back ground is engineering and some good "hands on" race experience, aerodynamics is my hobby and passion not my occupation and I realize that there are lots of ways to "skin a cat" aerodynamically so I am looking forward to seeing your new streamliner shape. My basic thinking is that nothing is "mystic" when it comes to aero and mechanical design (not sure about electronics!) everything has a cause and effect, we just need to be able to fugure out what they are.

Really looking forward to seeing your new shape.

Rex

[836dstr]

Dan,

Thanks for the clarification!

I looked at the rulebook definition of "Air Intake" being a non-tapered "hood scoop" and may proceed in that direction an see what works best. I hope to get some testing in at Bonneville this year.

Thanks again for you help!!

Tom

[rebelce]

Has anyone done work on the airflow AFTER a submerged inlet?

[Rick Byrnes]

Rex, not to worry
I have thought about NACA ducts since beginning the original lakester body and did use smaller ones to provide a little airflow thru the engine bay.

Traditionally many old time lakester guys insist that NACA ducts don't work because of turbulance off the canopy, or the rather thick boundary layer where the inlet is located.
Every AERO engineer I have talked to insist the opposite and can show me the math.  Not much data but they can be very convincing.
From engine dyno testing, I know there is about a small hp loss going away from the straight short trumpets but there is a larger drag loss by having the mail box sticking out in the wind.  We don't know the number, but I don't think I need to be an aero engineer nor do I need specific data to know that its more than just a few hp.

Before we actually make the duct, I'm hoping to have the body CFD work done and a lot better idea of how the boundry layer acts in that area.  Design of the duct is going on now and it is pretty large.
The other part of course is the air box and how the air transition is managed. (rebelce question)  In my world there is no data.  F1 and Indy guys have done lots of work, and to emulate their designs is probably where I'm going. 
The process though is, I sketch, one of the two counselers look at it and throw up or smile.   We make compromises and decide what to build and test.  Since I don't have a ton of money the testing is real world on the salt.  The CFD guy is not committed to do these parts.
I'm happy with having just the body done for now.

As things progress I'll keep you guys informed.  I'm trying to spend more time in the shop than on the computer so a build diary is out of the question for now.
At the this time, I'm finishing the rear suspension.  Starting the front suspension and steering.  real real slow steering.
removable tail extension and removable nose section.  The car will look pretty conventional as I'm not prepared to do a complete front clip and lower the cage.  I need to finish this thing and go racing.  Many of us love to concept and build, but I need to get to a more financially stable part of the LSR game.   Building has made me broke.
(historically for me, actual racing is cheaper than building the damn thing)

[Stainless1]

Has anyone done work on the airflow AFTER a submerged inlet?

We were always worried about turbulence, so we put a flat dry K&N in the airbox, figure it straightens out the air a little and eliminates the chunks that would otherwise make the motor run poorly as time goes on...  8-)  We tap the salt out between runs, if you use an oiled one, the salt will stick.

Couldn't do it that way with the turbo, had to use a cone filter in the box to make everything fit.

[Tman]

So, since this thread was started some of you have HAD to learn something to add to the conversation?

[hotrod]

Traditionally many old time lakester guys insist that NACA ducts don't work because of turbulance off the canopy, or the rather thick boundary layer where the inlet is located.

It depends on the specific situation. NACA ducts are more effective/effecient in the front part of a body or an area that is upward sloping into the airflow. In both cases you have relatively thin boundary layer and attached laminar flow with little if any turbulence in the boundry layer.

The NACA duct basically strips off the boundary layer and recovers the pressure gain from slowing the high speed flow down.
It is not quite as efficient as a well designed forward facing scoop positioned above the boundary layer, but as noted above pays back some of that loss of efficiency with reduced over all drag.

The real trick to pressure recovery in a scoop or air inlet is to manage to smoothly slow down the high speed airflow with a minimum or losses to turbulence inside the scoop and duct. The NACA duct with a proper shape, ramp slope and inlet nose shape have very little drag or induced turbulence in the air flow.

Scoops placed above the car body, can have a host of problems getting the air slowed down, the flow straightened out without having buffeting inside the duct that causes large random pressure changes that upset intake fuel air mixture management. Well designed they will give better pressure recovery than a NACA duct but not by a great margin and if poorly designed can do more harm than good.

NACA ducts on the rear third of the body are often in poor locations to work well as the body is usually beginning to narrow down toward the tail of the vehicle and this is frequently the area where flow separation begins which largely kills the function of a NACA duct.

The value of the duct depends a lot on where specifically it is located and the condition and direction of the local airflow.
If you note the NACA ducts used in the rear quarter windows of NASCAR sprint cup cars they often are not pointed directly toward the front of the car. Instead they are placed at an angle that aligns the axis of the NACA duct with the local airflow along that part of the body.

Tuft tests are your friend to help figure out where to put the ducts but it takes some time and testing to sort out the best place considering the shape of the body, the airflow over the body and the packaging of the ducting under the body to get the air where it needs to go.

Larry

[836dstr]

Tman,

Interesting that you brought this topic up again. My reply has nothing to do with NACA ducts. In 2010 I had a one core non-functional radiator made with the widest fin spacing I could get. I had the back of the radiator blanked off with a 8" diameter hole at the top. I built a bellmouth and expanding ducting back to the plenum. With a panel over the old hoodscoop hole in the hood I figured I had the hot setup.

The first pass we were off 35 MPH and my Brother-in-law said the car would not pull. We blanked off both ends of the duct and after a quick trip to Mike's Carquest put on a 9" x 2" Aircleaner sticking out in the breeze and regained the lost speed and ran 170.

So much for a better idea!

Tom

[javajoe79]

We ran this scoop on the sonoma http://www.eharwood.com/catalog/product.php?productID=347

Way bigger inlet then needed but we ran out of time and I didn't get to make a reducer for it. We went 219mph with that scoop. We have turbos in the works now but we would like some day to run without the scoop to see if we pick up speed.

[Tman]

Well, I brought this back up since air inlets were last night project. I have the plug made for a small cockpit NACA like Rick has on the flower of Scotland. And I am going to follow Stainless' lead and try a NACA for the engine as well. I did find some neat scoop designs for offshore racing boats.