Streamliner body design

[blackslax]

Good topic Blackslax. Question for all: If the air inlet is right at the nose, doesn't that reduce drag? My thinking is that there is less mass of air to deflect as the air column that would have been deflected around the nose is now being pushed/pulled into the engine. And if the engine is drawing in air faster than the relative airspeed of the vehicle, wouldn't there be a "pulling" effect?
Thanks
Don

In theory I think there would be a pulling effect, but it would be negligabe as each intake stoke is being used to "pull the bike", so I think the engine perfomance would suffer much more than the pulll would help.  Take an air hose with a jet tip and feel how much it pushes when you press the valve. that is 100+ PSI.  A NA inginge will only produce about 2 psi of pressure in the plenum at 200+ mph. there is some math that can easily be used to size it so that the correct amount is pulled in.

But, Does anyone know why the nose inlet was refered to as the achilles heel of the E-Z?

[hotrod]

I think this link might help you answer that question.

http://www.motorcycle-usa.com/499/1499/Motorcycle-Article/2006-Bonneville-Streamliner-Battle.aspx

It might be due to the air duct limiting the size of the front wheel from how I read that article.


Design of internal air flow is always a challenge. You can never achieve theoretical ram air pressure in real systems. You should figure on only getting about 50%-75% of the theoretical ram boost. NACA type ducts work best where there is a very thin boundary layer and high speed attached flow --- in other words they work a lot better on the front of the car than they do near the rear of the car, as the boundary layer gets thicker as the flow moves down the body.

As mentioned above you need to decelerate the airflow from the ram scoop smoothly with minimal turbulence to get maximum pressure recovery. This means rapid section changes in the ducting can waste a lot of your airflow energy you are trying to convert to pressure as turbulent mixing.

This was one of the problems with head light cool air packages. The length of the ducting negated a lot of the theoretical gain from an ideal pickup point. The short path hood scoop generally out performed the longer ducting due to its shorter path and reduced friction losses and heating.

http://www.melmoth2.com/texts/NACA%20inlet%20sizing.htm

In short the intake inlet must be larger than needed to feed the engines air demand. If it is too large you increase drag excessively but the air flow slow substantially and you get a large fraction of your theoretical ram boost (assuming the ducting is not causing energy robbing turbulence). As with the NACA duct you want it just a bit too large so the air slows to around 3/4 -1/2 its free air speed at the cars top speed. How ever that sizing will strangle the engine at low speeds and high throttle settings so you need to provide an alternate path for additional air that will allow the engine to "suck open" a flapper valve or some similar air path. You see this sort or arrangement on some jet fighters where they have additional low speed air inlets that allow adequate air flow for max power takeoffs but close as air speed increases and ram pressure builds up in the intake duct.

See page 24 of this doc.

http://www.scribd.com/doc/24418180/Study-of-Air-Intake-in-aircraft-report

You also need to manage flow friction and turbulence in the duct, pay as much attention to internal drag as you do to external drag and your inlet will work for you, not against you.


Larry

[blackslax]

Good topic Blackslax. Question for all: If the air inlet is right at the nose, doesn't that reduce drag? My thinking is that there is less mass of air to deflect as the air column that would have been deflected around the nose is now being pushed/pulled into the engine. And if the engine is drawing in air faster than the relative airspeed of the vehicle, wouldn't there be a "pulling" effect?
Thanks
Don

that is exactly what i was thinking. Placing the inlet at the highest pressure point would greatly reduce drag.


Larry, you make some great points.  So can I ask, If I have an inlet size requirement of x, what would be acceptable inlet runner size to minimize turbulence and keep pre sure as high as possible? And how would length effect it?  Would a sizing of 4x or 6x be acceptable?

[hotrod]

It does not need to be that much bigger. Look at the air horn on the old style air cleaners. The inlet was small enough to cover with your hand but the tube that went back to the air filter box gradually increased in diameter. This allowed the air flow to slow down smoothly and fill the air cleaner can with air at a higher pressure.


If the change in size is too abrupt the air flow will tumble and separate from the walls of the ducting, greatly reducing the effective size of your duct.
You want to increase the size enough that the airflow slows down significantly, this both increases flow pressure due to recovery of kinetic energy of the moving air flow, but the slower moving air has less drag (friction) with the walls of the duct so pressure losses due to flow friction go down.

Ever notice how the blower fan on your cars heater makes almost no noise at the low speed setting but gets quite noisy at setting 3 or 4?
That is due to turbulence generated as the airflow moves faster. Same thing applies to intake ducting. You can move all the air most engines  can handle with a 4 inch to a 6 inch duct.

A 6 inch duct has a cross section of 0.196 sq ft. If you are moving 1000 cfm through that duct it has to move 85 ft/second or just a tad slower than 60 mph.
As you probably know friction drag due to air flow on a car below 60 mph is so insignificant it can be largely ignored but increases rapidly as speeds go above 80-100 mph.

The same thing happens in an air duct, flow friction increases at the square of the flow velocity, so if you keep air flow velocities down around 80-90 ft per second in the duct, you will minimize friction flow (obviously a smooth duct helps too), and you get as much pressure recovery as is likely in a real world setup.

The ducting does not need to be huge, just smoothly formed and with gentle bends, that are kind to the air flow, and drops the air speed by about 1/2 of its free air velocity as it enters the intake system. That will cut flow drag by 4x, and give you 50% of the theoretical pressure recovery possible at a given speed.

Good topic Blackslax. Question for all: If the air inlet is right at the nose, doesn't that reduce drag?

internal drag is just as important as external drag. The air does not know if it is inside a tube or flowing over the outside of a body. It will still exert drag against the walls of the tube. That is one reason cars go faster if you can block off the radiator flow. By forcing the flow out over the smooth body, internal drag is reduced.


Larry

[blackslax]

internal drag is just as important as external drag. The air does not know if it is inside a tube or flowing over the outside of a body. It will still exert drag against the walls of the tube. That is one reason cars go faster if you can block off the radiator flow. By forcing the flow out over the smooth body, internal drag is reduced.
Larry

SO, the $64,000 will using the stagnation point reduce drag as it is using quantity of it to feed int the engine? 

[landracing]

Hello everyone,

 I read somewhere online that the open front was the "Achilles heel" of the E-Z Hook.
)

Thank you for any help you can give.

Umm  hate to tell you that the EZHook Achilles Heel was the front wheel not the opening in front. It probably is still the most aero vehicle on the salt....  You neeed to do some more research on the why the opening is in front of the liner.

JonAmo

[blackslax]

I think that what larry was saying was that the air inlet in the front may have led to design limitations which led to the use of the problematic front wheel.

[stwheeler]

It was the front wheel    AERODYNAMICS     FRONTAL AREA, 3.62 SQ. FT., DRAG COEFFICIENT .1007

[blackslax]

Sam,
Thank you for the clarification and the phone chat today.
your bike is a true testament to running smarter instead of harder.
once again, thank you.  You are a true gentleman.

[stwheeler]

Thanks for the kind words, but that wasn't Sam
, it was his brother. The info was from the web site.  

[blackslax]

I did talk to your brother today.  So, thank both of you.

[Hans Blom]

I've often thought about Bernoulli when seeing the carb hats for the blow through superchargers like procharger or any front mount blower or turbo. Seeing that the pressurinzed air coming from either one of these devices is no different than ram air other than the amount of differential pressure, its seems odd that all the carb or throttle body hats you see are just a small as possible to cover carb inlet with a 4 or 5 inch outlet for your hose....not only from a PxV standpoint but also thinking about the air trying to turn into the carb at almost a 90deg angle. Obviously there are packaging concerns  when talking mass production and the majority of these supercahrger kits are designed for people bolting them on a production vehicle...and as your volume of you hat gets larger your pressure increases(good) but this makes fabrication and strength of said product more of a struggle. I guess I answered my own question, or i'm totally off...one or the other.

And I guess you intake design pays a large role in this as well as when you are wide open basically you have one large volume of pressurized air , some with fuel, from the outlet of your pressure device to the head of your intake valve...or the piston if it is on the intake stroke.....

[hotrod]

You hit on some issues. In the turbo-supercharged community there have been a number of situations where intake manifold plenums that were too large blew the top off at high boost. This includes plenums that were fabricated out of 1/4 aluminum. If you are running 35 psi boost and the roof of the plenum is only one square foot the force trying to blow the top of the plenum off is 5040 pounds. Even if built strong enough to handle the load initially constant flexing of the plenum will crack the corner welds and lead to failure. The usual solution is to build the plenum as small as practical, build it several times stronger than seems necessary, and include internal struts tying the roof and floor of the plenum together to minimize flexing, and stress on the corner welds. Also a round tube is inherently more stable under high internal pressure than a box like structure.

In theory you are correct that smooth large radius bends and a diverging section into a large plenum would help but packaging, cost and mechanical strength issues usually force the design toward a very strong small tube.

When comparing to ram air however there is a big difference as the pressure in the boosted intake duct is static pressure and does not depend on the air velocity, the ram pressure developed in a ram scoop is entirely dependent on initial air velocity, and efficiently slowing it to recover as much of that kinetic energy as pressure gain as possible. By comparison the boosted intake has an unlimited supply of high pressure air so it can throw away small gains in efficiency in exchange for other advantages like mechanical strength, and ease of manufacture.

The ram air system under the best of circumstances is only good for very small pressure gains so you cannot afford to throw away any bit of it if you can avoid it.

John Baumann developed the hood scoop used by the Chrysler drag racing cars in the late 1960's and 1970's. It did not use a smooth flair to slow the air flow down but a scoop of relatively small entry dumping into a small plenum, provided the rapid deceleration of the air required to develop ram boost. It was found that the distance between the floor of the entry and the top of the carburetor was somewhat critical. The carburetors wanted to be just an inch or two below that scoop horn. Too high and rapid airflow and turbulence really screwed with the carburetor air flow, and too much depth created too much air drag for the scoop and too big of a plenum.

It would have been more effective with a small flare in the horn and turning vanes in the top of the plenum. This design also put the intake horn up above the boundary layer on the hood so the air entering the scoop horn was moving at the maximum speed possible.

http://www.allpar.com/racing/missile/motown.html

Larry

[Hans Blom]

So, it would sem that a larger plenum at the intake of your supercharger would gain you some benefit as well since it will give a rise in pressure and therefor give a very slight decrease in pressure differential inbetween you ram boost and supercharger boost, making your HP needed to drive said super charger ever so lightly less...?

[blackslax]

So, it would sem that a larger plenum at the intake of your supercharger would gain you some benefit as well since it will give a rise in pressure and therefor give a very slight decrease in pressure differential inbetween you ram boost and supercharger boost, making your HP needed to drive said super charger ever so lightly less...?

The size of the plenum does not raise the pressure within it.  The increased size leads to increased surface area wich leads to an increase in the force exerted on the inner surface and the joints.  When the large flat surface decides to "oil can" it tears the edges open.  I deal with boilers, and most are designed with round pressure vessels or flat ones have rods from side to side to eliminate the problem.

The advantage of a large plenum is slow down the charge, deturbulate it, and to lessen any pressure drop as the cylinder draws in its intake charge.