Terminology

[Jack Gifford]

Unfortunately, my engineering education did not include a fluid dynamics course (or courses). I'm hoping some of you can can provide a fundamental definition of "frontal area". Is the area-of-concern the total "shadow" cast on a plane perpendicular to the direction of motion? I've always assumed this is the case, but the presence of the word "front" in the term makes me wonder if it's defined otherwise?

[fastman614]

Jack, your understanding of the term sounds like mine.... you look at a vehicle from the front and see (on an open wheeled car) the square inches of the tires, the axle tubes, the width & height of the body at its widest and tallest points....

If an expert can give a "better" definition, I would like to have it....

[hotrod]

Your perception is correct, it is the total area of the vehicle a seen from the front.
One way to visualize it is the area of the smallest hole the vehicle could fit through, or the shadow the vehicle would cast on a wall if illuminated by a point light source a long ways in front of the car.

The real effective frontal area, is actually a bit smaller than that projection of the forward face of the vehicle, I have seen some references that assume a value of 85% of the total projected frontal area is the "effective" frontal area.

For practical purposes the number does not need to be all that exact,(usual precision is to the nearest tenth of a sq ft) it is a reference area for comparison and calculation. In the real world you typically find out the aero drag for the vehicle at some given speed using a coast down test or power needed to achieve a given speed, then work backwards from that to compute the Coefficient of drag based on that total aero drag and your best estimate of the frontal area.

As long as the frontal area you use and the calculated CD you come up with give the actual air drag results in calculations, precision is good enough. It does not matter if the CD or frontal area is a bit off as long as the product of those two numbers (which is what really matters) works in the aerodrag calculations to give you the right total air drag.

Larry

[fastman614]

Your perception is correct, it is the total area of the vehicle a seen from the front.
One way to visualize it is the area of the smallest hole the vehicle could fit through, or the shadow the vehicle would cast on a wall if illuminated by a point light source a long ways in front of the car.

The real effective frontal area, is actually a bit smaller than that projection of the forward face of the vehicle, I have seen some references that assume a value of 85% of the total projected frontal area is the "effective" frontal area.

For practical purposes the number does not need to be all that exact,(usual precision is to the nearest tenth of a sq ft) it is a reference area for comparison and calculation. In the real world you typically find out the aero drag for the vehicle at some given speed using a coast down test or power needed to achieve a given speed, then work backwards from that to compute the Coefficient of drag based on that total aero drag and your best estimate of the frontal area.

As long as the frontal area you use and the calculated CD you come up with give the actual air drag results in calculations, precision is good enough. It does not matter if the CD or frontal area is a bit off as long as the product of those two numbers (which is what really matters) works in the aerodrag calculations to give you the right total air drag.

Larry

Larry,
Thanks for that.... this "little" point about 85% being a possible factor in determining the "real" number is interesting. I am going to get our "theory man" with his desktop dyno program to try a bit of fudging to get his outcomes a bit closer to our real life results.... (his computer constantly comes up with "we can't go as fast as we just did" type of results.... but then, maybe the salt flats tilt - but only for us!)

[McRat]

Hijack/sidebar:

If I have a vehicle on a 100 foot rope, being towed at 60 mph.

I put a pull scale on it, and it reads 100 lbs.

How do I translate that into HP required?  ie - 100 lbs of drag at 60 mph is ? HP.

 

[tortoise]

this "little" point about 85% being a possible factor in determining the "real" number is interesting.

Multiplying height times width times .85 gives a fairly good estimate of frontal area for typical road cars, if it isn't worth the trouble to measure it accurately. That's where the "85%" comes from.

[tortoise]

100 lbs of drag at 60 mph is ? HP.

 

60 mph = 88fps
88fps x 100 lb = 8800 lb.ft/sec
1 hp = 550 lb ft/sec
8800/550 = 16 hp

[McRat]

100 lbs of drag at 60 mph is ? HP.

 

60 mph = 88fps
88fps x 100 lb = 8800 lb.ft/sec
1 hp = 550 lb ft/sec
8800/550 = 16 hp

THANKS!

Do I divide the weight by gravity?  

[Jon]

Hijack/sidebar:

If I have a vehicle on a 100 foot rope, being towed at 60 mph.

I put a pull scale on it, and it reads 100 lbs.

How do I translate that into HP required?  ie - 100 lbs of drag at 60 mph is ? HP.

 

Too slow typing if a tortise can beat me   will leave it here anyway

1hp = 33,000 foot-pounds per minute.
60mph = 5,280 feet per minute
Multiply that by your 100 lbs measured pull = 528,000 foot-pounds per minute.
Divide that by 33,000 = 16hp.

If you want to get a reasonable measurement of your frontal area cut out a square the size of the units you want to work in i.e. a square foot if imperial or 10cm square if metric.
Stand it next to your vehicle and take a front on picture from a reasonable distance with a long focal length lens.
Rule up the picture with a grid the size of your square cutout and count the full and partial squares and that's your frontal area.

Cheers
Jon

[tortoise]

Do I divide the weight by gravity?  

Sure, why not? Post a picture.

[hotrod]

(his computer constantly comes up with "we can't go as fast as we just did" type of results.... but then, maybe the salt flats tilt - but only for us!)

When models disagree with reality, reality is always right!  

Since you are not designing an airplane all you need is an ability to repeat measurements to determine if a change helped or hurt. As long as your satisfied no really silly mistake is being made, tweak the frontal area and CD until the computer model results match the real world and use those numbers in the future.


Even the big boys use low tech analysis to solve problems.

http://www.degreeinfo.com/off-topic-discussions/8194-burt-rutan-rolls-out-his-new-spaceship.html

Stability problems with the spacecraft tail were detected in unpowered drop tests over the Mojave desert, making some redesign work necessary. (That's why they have test pilots.) So the schedule slipped.

Rutan doesn't use wind tunnels. He uses a sophisticated computational fluid dynamics computer model to virtually design aircraft. But the tail problem necessitated some empirical experiments. Here's their "wind tunnel" to test new tail ideas: A Ford pickup truck with model tails mounted on it:

http://www.scaled.com/projects/tierone/New_Index/photos/images/800/wind_tunnel_800.jpg  <--- link is broken may be on the web site but probably re-named

Another location has the image ---- http://stargazer2006.online.fr/others/trucktunnel.htm

Not exactly high technology! (But it worked.) Can you imagine NASA doing that?? A pickup truck and a few weeks? NASA is so afraid of the political repurcussions of failure that they would have spent 24 billion dollars and 14 years studying the problem.

The tail is all fixed now.

Here is a test rig used by NASA to test chute deployment:

http://www.dfrc.nasa.gov/gallery/photo/X-38/HTML/EC96-43844-4.html

You don't have to be fancy to get basic aero info.

Larry

[tortoise]

Too slow typing if a tortise can beat me  

I'm not the first tortoise to win a race.

[McRat]

Do I divide the weight by gravity?  

Sure, why not? Post a picture.

Still working on it. 

[WOODY@DDLLC]

Too slow typing if a tortise can beat me  

I'm not the first tortoise to win a race.

All modeling is an approximation! How accurate does an approximation have to be to become useful?  Regarding models read my byline below! 

All the equations and some formulas have "k" factors! 

[fastman614]

Hotrod- As true as it is, I got a laugh out of it!.... So, to quote you - What I can't wait to tell him is this:

When models disagree with reality, reality is always right!