Author Topic: frontal area questions  (Read 17921 times)

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Offline A2WindTunnel

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Re: frontal area questions
« Reply #15 on: October 13, 2010, 08:48:12 AM »
Think of frontal area as a full size cardboard cutout of your car.  If you were to test this cutout (flat plate) in a wind tunnel you would get a CD around 1.0 and when testing the real car the CD is now reduced because of the energy recovery from the body (CD around .330 of this style car pictured).  Frontal area is a measure of size and CD shouldn't change with scale.  If you build an exact 20% replica of your car and tested it, the frontal area is reduced but the CD will be the same as the full scale version.  When comparing car to car you really need to look at CDA (Coefficient of drag * Area).  A jumbo jet has a lower CD then a car but because of the massive size there would be more drag force on the jet because of the area the force is acting on vs a car.

An example:  Say you have 2 cars with different frontal areas but the same CD.

Car 1: Frontal Area = 22ft^2
Car 2: Frontal Area = 20ft^2
CD = .330 for both.

Car 1: CDA = 7.26
Car 2: CDA = 6.60

Car 2 would have less drag force and require less HP to go the same speed.

Click on the image to see it full size.  Its not just a car as it appears small.  It also illustrates frontal area.
« Last Edit: October 13, 2010, 09:08:02 AM by A2WindTunnel »
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Offline tortoise

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Re: frontal area questions
« Reply #16 on: October 13, 2010, 11:49:23 AM »
 Frontal area is a measure of size and CD shouldn't change with scale.  If you build an exact 20% replica of your car and tested it, the frontal area is reduced but the CD will be the same as the full scale version.


True if appropriate adjustments of fluid viscosity and/or speed are made. Tested at the same airspeed the smaller model will have a lower CD. The differences will be more apparent with lower CD forms.

Offline A2WindTunnel

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Re: frontal area questions
« Reply #17 on: October 13, 2010, 01:09:25 PM »
Quote
Frontal area is a measure of size and CD shouldn't change with scale.  If you build an exact 20% replica of your car and tested it, the frontal area is reduced but the CD will be the same as the full scale version.



True if appropriate adjustments of fluid viscosity and/or speed are made. Tested at the same airspeed the smaller model will have a lower CD. The differences will be more apparent with lower CD forms.

I should have stated the obvious of a scale model tested correctly will have the same CD.  Reynolds numbers, test speeds, and testing protocol is a whole other subject for another thread but thanks for keeping me honest.
« Last Edit: October 13, 2010, 01:11:01 PM by A2WindTunnel »
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Offline Chris Horoho

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Re: frontal area questions
« Reply #18 on: October 13, 2010, 03:18:10 PM »
these responses are what i am looking for
Thank you

now the tricky parts to many different types of fronts
and sorta off subject but which would be better for air inlet to feed turbo and vent cockpit
front ram as the ezhook
or a set of tunnels?
or would cfd testing be better for seeing that answer through?
yes i do plan on loads of research as i have been doing but seeing the many ways people do these liners gives me mixed emotions?? on which route to follow
"Pinky"

Offline 1212FBGS

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Re: frontal area questions
« Reply #19 on: October 13, 2010, 04:44:36 PM »
#1: Forever ban from your brain the concept of "frontal area".  

what an asinine statement!..... maybe not important if your building a 747! hey Blue you wanna take credit for designing the 747 also?

Chris, you want to do 3 things, 1st, make your "frontal area" as small as you can. 2nd, make your body as slippery as you can. 3rd never listen to Blue!....
Small and slippery is the most fundamental attention area for our vehicles
Kent

Offline joea

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Re: frontal area questions
« Reply #20 on: October 13, 2010, 06:38:52 PM »
.... :-D.....

Offline racergeo

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Re: frontal area questions
« Reply #21 on: October 13, 2010, 07:37:56 PM »
  It seemed like an over generalization to say frontal area was old news but I'm not on this sight to make waves, I let Kent do all my waving for me!!! It does seem like it would take less effort to poke a small hole in the air (water) then a large one.

Offline saltwheels262

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Re: frontal area questions
« Reply #22 on: October 13, 2010, 07:41:32 PM »
 

 hey Blue you wanna take credit for designing the 747 also?


[/quote]

lolol
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Offline wobblywalrus

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Re: frontal area questions
« Reply #23 on: October 14, 2010, 02:39:51 AM »
There is a stagnation point at the very front of torpedo shaped objects.  An excellent place for an air intake.  The EZ-Hook is using a good idea.

The frontal area and drag coefficient method is empirically based.  In other words, it is an observed relationship.  It is useful and our traditional way of looking at things.  It is a simplistic approach to a complex problem.  It does not give us much to go by when we want to look at a new idea.  By necessity, the aerodynamic relationships were examined further and methods were developed that give us the means to arrive at fairly good shapes based on theoretical relationships.  Testing is still very important, however.

The concepts Blue is describing are part of the more modern approach.  His posts should be read carefully and considered.  They have valuable information.

Offline ack

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Re: frontal area questions
« Reply #24 on: October 14, 2010, 10:16:46 AM »
  It seemed like an over generalization to say frontal area was old news but I'm not on this sight to make waves, I let Kent do all my waving for me!!! It does seem like it would take less effort to poke a small hole in the air (water) then a large one.

One of the first things my friend Ken Mort said when he began helping me with the aerodynamic design was don’t worry about frontal area the design is much more important.  Ken was one of the engineers (Stanford grad top of class) that ran 120 foot wind tunnel at NASA Ames and had much experience with full size aircraft and other aerodynamic body’s and has much practical experience. We have had some success with our design.

Offline JoshH

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Re: frontal area questions
« Reply #25 on: October 14, 2010, 10:20:58 AM »
We have had some success with our design.

I don't know what your talking about...  :-D

Offline tortoise

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Re: frontal area questions
« Reply #26 on: October 14, 2010, 12:33:24 PM »
Small and slippery is the most fundamental attention area for our vehicles

Reduced wetted surface, as Blue emphasizes, is also a kind of "small".  Are you suggesting that of  two forms, both with wholly attached flow, the one with less frontal area but greater surface area will have lower drag?

(Or are you just saying "Blue bad, Kent good"?)

Offline hotrod

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Re: frontal area questions
« Reply #27 on: October 14, 2010, 04:42:55 PM »
I think what is being missed in Blue's comments is that small frontal area is not a be all end all solution, the real number is the product of CD and frontal area. Sometimes by making a body larger in places that are friendly to the air flow your CD drops so much that you gain more than building a smaller shape that cannot be made as slick.

If you make the body too small you get forced into shapes that cannot maintain attached airflow and rapid transitions that increase drag and wake area.

Sometimes you have to give up a little frontal area to gain a lot of reduction in CD.

Larry

Offline racergeo

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Re: frontal area questions
« Reply #28 on: October 14, 2010, 05:06:29 PM »
  Don't get me wrong Larry I like what your saying, cause in kinda sounds logical. However you used the words frontal area 3 time and Blue admonished you to " forever ban from you brain" and I'm not going to use the rest of the quote because I'm starting NOW! I have already deleted that term from my program that helps me predict speed based on HP, CD, and that other term that I have banished from my brain. I think it may be time for Kent to jump in with some sooooothing words.

Offline maguromic

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Re: frontal area questions
« Reply #29 on: October 14, 2010, 05:20:16 PM »
I have herd Kent called a lot of things but sooooothing is a new one. :-D Come to think of it I can see him as a sooooothing balm.  :-o Tony
“If you haven’t seen the future, you are not going fast enough”