frontal area questions

[Chris Horoho]

on determining frontal area I understand it enough to be slightly confused but yet not

First if I come to a mild point of sorts (not quite but close) on a motorcycle streamliner would the shape of the vehicle profile from the front become the frontal area am I correct in thinking this?

Second say I go to a ball shape up front and slowly go to my profile shape would this then (the ball up front) become my frontal area instead of the profile?

sorry if these are rookie ?s I'm just doing a lot of research and this to me seemed a bit vague

Thank you

[Jonny Hotnuts]

frontal area is the amount of area as viewed from the front. Doesnt matter if the object has things forward or aft, if it can be seen from the front, its frontal area.

Another thing you should concern yourself with is wetted area.

Good luck and god bless google.

~JH

[Chris Horoho]

that's kinda what lost me a bit before as someone told me if I went to a round front it would decrease my frontal area
This was said to not be as big an issue with subsonic vehicles (the example was a 747 isn't pointed (and yes i see this as well as the saying is its an airplane not a land vehicle but as I said I am just tryin to do research for a possible future build))
I understand the deal with drag and the goal is the get it as low as possible

again sorry I am prone to ramble

[Jonny Hotnuts]

that's kinda what lost me a bit before as someone told me if I went to a round front it would decrease my frontal area

They were misinformed.
It does change wetted area.

Points are good for supersonic.

Dont apologizes for rambling.

~JH

[Chris Horoho]

that's kinda what lost me a bit before as someone told me if I went to a round front it would decrease my frontal area

They were misinformed.
It does change wetted area.

Points are good for supersonic.

Dont apologizes for rambling.

~JH

ok that i understand
and that have been what they meant and were just responding on short notice??
Thank you
now i have to figure out what style and design to stick with and model it up


so jonny you still have that MP or did you change it out

[hotrod]

It is basically a measure of how much air you have to move out of the way as the car passes, so the entire cross section of the vehicle as viewed from the "direction of the airflow" is your frontal area.

If there is absolutely no cross wind frontal area is the front view of the car.
If there is a slight cross wind the actual frontal area is the cross sectional area of the car seen from the direction of the wind. That is one of the reasons small cross winds can drastically slow a land speed vehicle down. The other is as the wind strikes at an oblique angle the air flow can change substantially and areas of the car on the down wind side of the car will change from smooth attached flow to turbulent detached flow which significantly increases drag.

One way to think of frontal area, is it is the smallest possible hole the car could pass through.

An easy way to get a good approximation of the frontal area, is to get well back from the car with a camera and take a picture of it at the same level as the axis of the vehicle.

Everything you see of the vehicle is frontal area.

Larry

[interested bystander]

Chris and Hotnuts have it pegged.

Putting your object against a wall longitudinally and then shining a light on the shadow from the front is another handy visualization tool.

[Chris Horoho]

ok lets add to this discussion a bit
lets compare the easyrider vs sam wheelers on Cd vs frontal area and wetted area and the calculations of required HP to achieve desired speeds





now this looks to be the where i get a little lost
the ez hook has a Cd at around .103
the easyrider is around .2
look to have similar frontal areas ??
this is why i get lost as the wetted area looks to be more on the ez hook then the easy rider??
and yes i understand these terms and designs but this just makes me not know if the design in my head for a liner is going to be any good (and no im not any good at sketching to get a drawing up i am more of a cad type of person lol sorry)

[JoshH]

the ez hook has a Cd at around .103
the easyrider is around .2

I would question these numbers. If one of these is an "estimate" it might be misleading.

Many times these numbers are arrived at by calculating drag based on actual runs, there are many factors that can skew this.

[WOODY@DDLLC]

As already explained, frontal area is simply the area of the hole you punch in the air. Cd is a measure of how slippery a shape is. With Cd you can compare different shapes to see which is more slippery. As a shape gets more streamlined the pressure drag will go down and the skin friction from the wetted area will then become the major source of drag.

CdA is the Cd times the frontal Area and tells you how well you are punching a given size hole for a given shape. You can have a larger area that is a more slippery shape than a smaller one. An egg and a sugar cube obviously do not have the same Cd but they have the about the same CdA. If you had to power these there would be more room for engine and driver in the egg but if you have a small enough motor and driver then a smaller egg-shaped sugar cube might be the better way to go.


Finding the best working balance of all the factors is the tricky, fun part!

[JoshH]

Woody is right. After posting I was thinking about the size difference of the two liners in question, size and shape is likely a major factor.

[horsewidower]

Please explain "wetted area."

Thanks
Bob

[Chris Horoho]

ok thank you
i do understand what you guys are referring to

the design i am pondering is going to have a frontal area of 4.8
i would only hope to have a Cd of .15 or better but would only hope
as that would put my goal and available HP about where they would need to be with a little HP in the bank as to keep it reliable

[Chris Horoho]

Please explain "wetted area."

Thanks
Bob

wetted area is the area basically hitting the air or rubbing the air could be another way to explain it

[Blue]

#1: Forever ban from your brain the concept of "frontal area".  Until you do, you will learn very little about aerodynamics that will improve your vehicle over anyone else who still uses a term we stopped using in aerodynamics about 60 years ago.

#2: http://www.landracing.com/forum/index.php/topic,3900.msg51288.html#msg51288

#3: For a subsonic vehicle, drag is the wetted area times the skin friction coefficient plus all of that nasty stagnation and separation drag associated with blunt front and back ends.