Landracing Forum Home
October 19, 2017, 04:16:51 AM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with username, password and session length
News:
BACK TO LANDRACING.COM HOMEPAGE
 
   Home   Help Search Calendar Login Register  


(Note: Donations are not tax deductible)







Live Audio Streaming and Archives of Past Events
Next Live Event: TBD
Pages: [1] 2 3 4   Go Down
  Print  
Author Topic: Drag coefficient fundamentals  (Read 5268 times)
0 Members and 1 Guest are viewing this topic.
Jack Gifford
Hero Member
*****
Offline Offline

Age: 77
Location: Phelps, NY
Posts: 1031





Ignore
« on: November 15, 2016, 01:42:49 AM »

Things I wonder about-

> Frontal area: the use of this term seems to confuse aero discussions. Some people look at a modified roadster with a tapered nose in place of a vertical grille and refer to its "small frontal area". Other people take the term to mean the area bounded by a projection of the car's outline onto a vertical flat plane perpendicular to the car's longitudinal axis (which would not be influenced by the nose piece mentioned above). Is there a bonafide definition of this term?

> Sixty-some years ago, competent motorsports engineers (Don Francisco, etc.) talked of drag coefficients in the ballpark of 0.9 for drop tank lakesters. No major drag-reducing changes have been introduced to this style car of today. So why do I read of a .27 drag coefficient in another thread? [Reply#90 of 'lakester aero help' thread in the 'Bonneville' forum]
« Last Edit: November 15, 2016, 01:48:34 AM by Jack Gifford » Logged

M/T Pontiac hemi guru
fordboy628
Hero Member
*****
Offline Offline

Location: Koozebane
Posts: 1833


GONE FISHIN' . . .




Ignore
« Reply #1 on: November 15, 2016, 05:20:57 AM »


> Sixty-some years ago, competent motorsports engineers (Don Francisco, etc.) talked of drag coefficients in the ballpark of 0.9 for drop tank lakesters. No major drag-reducing changes have been introduced to this style car of today. So why do I read of a .27 drag coefficient in another thread? [Reply#90 of 'lakester aero help' thread in the 'Bonneville' forum]


Based on what numbers I have been exposed to about aero, there is no way something with the wheels exposed fully to the air stream, could have a cd of .27

Maybe Woody will chime in with some authoritative numbers.

 cheers

 
Logged

I used to be a people person.  But people changed that relationship.

"There is nothing permanent except change."    Heraclitus

"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former."     Albert Einstein
jacksoni
Sr. Member
****
Offline Offline

Age: 73
Location: Annapolis, Maryland
Posts: 958




Ignore
« Reply #2 on: November 15, 2016, 08:48:39 AM »

You have to remember that the formula for total drag is D=CdA. The total drag is something that can be reasonably measured today and that the drag coefficient (Cd) is calculated IF you know an accurate frontal area. That area may be difficult to measure accurately or at least is very time consuming. Jack you are correct as I understand it that usually the A is the projection of the area in the frontal plane. I have tried to measure this on my car by a photo taken at a distance and using a drawing program to measure the area. Didn't work. Often for a usual road car the HxW numbers by an average 0.85 factor is used. Clearly a guess. Figuring this for a lakester would be very hard. A streamliner, easy. The Goldenrod was supposed to have a Cd of about .15 and frontal area 9.8ft^2. (per an article written by the designer). I recently observed a car in a wind tunnel. Published figures for Cd and frontal area were known. Initial runs in the tunnel suggested that the Cd was way off the published numbers. A number of changes were made that lowered the Cd by .12. One of those changes effectively decreased the frontal area (the car was lowered) so the total drag was reduced. If the new frontal area was used to calculate the Cd it would have been higher. I think if the final Cd that was found for this car was published without other comment, many people would call BS on it.

Interestingly Bonneville Pro has a Lakester as one of it's sample cars. It uses a frontal area of 16ft^2 and a Cd of 0.58. Perhaps realistic, who knows. (and 2000hp predicts 290mph) But the 0.9 you quoted is nearly a flat plate and I suspect these cars are better than that.

Bottom line of my thoughts are that in this situation, calculations are great as a baseline but actual data for these sorts of cars is harder to come by (those who know may not be talking) and the best way to find out is to measure, get as close as you can to the area and the magical low Cd falls where it may. The total drag is what counts. That sloping nose on the roadster does not have a lower frontal area, but may have a better Cd.

Woody published his calculations of body, axles and tires as an example and I don't doubt his thinking. But put the car or an accurate model in a tunnel and see what is measured.

Above is my understanding and thoughts about this stuff. FWIW
Logged

Jack Iliff
 G/BGS-250.235 1987
 G/GC- 169.741  2009
 G/GMS-178.835 2010
Stainless1
Administrator
Hero Member
***
Offline Offline

Age: 66
Location: Wichita KS
Posts: 6421


Robert W. P. "Stainless" Steele Wichita, Kansas



« Reply #3 on: November 15, 2016, 09:56:36 AM »

I think it was our old (RIP) buddy Jack Dolan that said

"Theoretical records are set by theoretical vehicles"   cheers Jack, we still think of you  cheers

What should be and what is appears to generally be different.  There are 2 affordable wind tunnels and a Subaru-load of software out there that can measure and predict.  Results can be affected by actual things like traction and the actual power you make on the race course.
Logged

Stainless
Red Hat 228.039, 2001, 65ci, MSA Bockscar Lakester with a little N20 
MSA Bockscar Lakester #1000 my fastest mile 245 and change, 84 ci turbobusa motor... but Corey's 233 MPH H/BFL record is still 3MPH faster than mine.
Bob Wanner
Sr. Member
****
Offline Offline

Posts: 368




Ignore
« Reply #4 on: November 15, 2016, 10:23:37 AM »

Jack,
Was that Francisco # .9 or .09  ? I still feel more lurker than racer on this list in the talks about anything Aero. I do think a wind tunnel session the best investment, info  gained second only to actually racing on a long straight. And, as for frontal area POVs, a F104 and a full grown pick-up  truck might have similar frontal areas, but which is more slippery  ?
Logged
Jack Gifford
Hero Member
*****
Offline Offline

Age: 77
Location: Phelps, NY
Posts: 1031





Ignore
« Reply #5 on: November 16, 2016, 01:27:45 AM »

Thanks- more stuff to ponder.

jacksoni- I have yet to do it, but I'm hoping to fairly accurately measure the cross sectional area of my lakester. I'll take straight-on photos as you did (at each side and at top and bottom), but I'll only use them to spot which parts of the car comprise its projected outline. Then I can individually measure those parts and get a total area number.
When you mentioned area being reduced by lowering a car, I assume not an open-wheel car? I don't see that lowering a lakester could change its area.

Bob- Don Francisco published a table of representative drag coefficients of various shapes, including at the low end the Auto-Union long-time record holding streamliner @ 0.08, and at the high end a flat plate @1.1x. A figure of 0.9 was shown for a typical drop tank lakester of 1953 (I don't know whether that would have been with wheel discs or with old Kelsey-Hayes wire wheels).

1953 was not the stone-age of aero knowledge. Especially in southern California, where wind tunnel data was not all that difficult to obtain. And engineers of the time were just as well grounded in Newtonian physics (including fluid dynamics) as any are today.

« Last Edit: November 16, 2016, 01:31:49 AM by Jack Gifford » Logged

M/T Pontiac hemi guru
jacksoni
Sr. Member
****
Offline Offline

Age: 73
Location: Annapolis, Maryland
Posts: 958




Ignore
« Reply #6 on: November 16, 2016, 08:58:27 AM »

Jack- yes, it was not an open wheel car. More or less stock production body. From a wind tunnel standpoint the space under the car is included in the frontal area, at least as I understand. A very low front air dam nearly kissing the ground certainly makes it so.   

There are several online programs that allow tracing around an irregular object and from that it calculates the area for you. I tried this with two different programs and got same problem with both. I had an accurate reference length in the photo and both the car with some known measurements and in this case the garage door, in which the car was sitting. The programs would not consistently show the known measurements as accurate when scaled to the reference length so the area calculations were bogus. I never figured what I was doing wrong and so we were left with the HxWx.85 deal. Bottom line the drag of the car in the wind tunnel using the calculated area produced a Cd number. Changing the car improved ( or not) that number as the drag changed. That is what we wanted. Don't care if the area or the Cd is "accurate", the drag improved and that was the goal.

Your plan to accurately measure the components of the car should work well to give a baseline. But to really come up with a Cd so you can guess at performance potential after you get your engine dyno'd is going to be harder. I am sure there are folks out there who have had their lakesters in a wind tunnel. Have not seen any of them pipe up with numbers.  sad  So, we will continue to follow your build and look forward to the Great White Dyno giving the real answers.  grin cheers
Logged

Jack Iliff
 G/BGS-250.235 1987
 G/GC- 169.741  2009
 G/GMS-178.835 2010
Sumner
Global Moderator
Hero Member
***
Offline Offline

Age: 73
Location: Blanding, Utah
Posts: 3961


Blanding, Ut..a small dot in the middle of nowhere


WWW
« Reply #7 on: November 16, 2016, 10:28:11 AM »

The following is from a page on my site where you can download various spreadsheets.  The spreadsheet it refrences is one where you can enter one set of data for frontal area/CD for the body and another set for the wheels/tires.......


Quote
Horse Power Needed For A Lakester or a Car to Run a Certain MPH -- Need to know how much HP you need to run a certain speed? This spreadsheet uses the formula from the spreadsheet above this one to figure total HP needed based on the frontal area/Cd for the body independent of the tires used and another frontal area/Cd for the tires/wheels. It combines the HP needed to drive both to the given speed.

I've found Cd's for tires that have the width/diameter ratio similar to Goodyear Land Speed tires with Cd's of between .18 to .58. So take your pick. I'd use numbers between .45 and .60 myself as that way you will be erring on the conservative side.

The Spreadsheet will also give you figures for the weight needed on the drive wheels for different salt conditions in order to get the HP/Thrust to the ground since you don't have the same traction as pavement.

"One point-- the "weight" required on the rear wheels doesn't have to be just ballast weight, rather it is the total of the weight plus the aerodynamic downforce. This can be really significant at high speed." Regards, Neil

NOTE: For a car or streamliner don't fill in the tire inputs.
( HP Needed For A Lakester- Excel Spreadsheet Format ) Version: 10-13-13

Here is a link to the page where you can download the spreadsheet if you so desire.....

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html#HP needed for A Lakester

Sumner
Logged

jacksoni
Sr. Member
****
Offline Offline

Age: 73
Location: Annapolis, Maryland
Posts: 958




Ignore
« Reply #8 on: November 16, 2016, 01:27:41 PM »

Sumner- thanks for putting this up, I am sure it will be very helpful. The area of all the various pieces surely can be measured pretty well. Do you think that the wide variation of Cd of the tire samples is due to testing variations/difficulties or in fact different (though the dimensions seemed similar to the LSR tires as you mentioned) tires with similar sizes?  Also, what are your thoughts about the various components affecting the drag of adjacent parts? As in the "whole is greater (or less than as the case may be) than the sum of its parts?
Logged

Jack Iliff
 G/BGS-250.235 1987
 G/GC- 169.741  2009
 G/GMS-178.835 2010
jacksoni
Sr. Member
****
Offline Offline

Age: 73
Location: Annapolis, Maryland
Posts: 958




Ignore
« Reply #9 on: November 16, 2016, 01:48:11 PM »

 I was digging around in an old text on fluid dynamics that has a section on drag of motor vehicles. There is a bit on the 1934 Auto Union race car (google it for info) photo attached. This is at least similar to a lakester with respect to general lay out/shape and exposed wheels and wheel and tire size. There is more suspension in the breeze than a current lakester and it is bigger with a frontal area of about 12.4ft^2. A significant difference is radiator air flow which we know has a big effect. Anyway a wind tunnel developed Cd was .65 with all this. With wheel and axle fairings, Cd was .50. I am sure it would be much less with radiator blocked.

Just for fun comparison.


* autouniontypea_1934_ii-4.jpg (264.71 KB, 1255x741 - viewed 95 times.)
Logged

Jack Iliff
 G/BGS-250.235 1987
 G/GC- 169.741  2009
 G/GMS-178.835 2010
WOODY@DDLLC
Hero Member
*****
Offline Offline

Age: 72
Location: Cincinnati
Posts: 1121


It's GONE ......... the Ohio Mile! :-(


WWW

Ignore
« Reply #10 on: November 16, 2016, 02:38:54 PM »

I debated whether to add the Cd to that chart but since I did and you "caught" me I'll elaborate a bit.

One of the issues [not really a problem] with computer models is they are perfect!

None of the lumps, bumps, rivets, waviness, seams, warts, wrinkles or variations of real world parts. So that model says if those parts were exactly those shapes and they were placed exactly the same in a perfect wind tunnel this could be the potential Cd. Cd is a dimensionless number! WTF does that mean? We can measure density, frontal area and velocity but there is no number to measure shape - hence a dimensionless number is derived that enables us to compare different shapes. If the same methods and measurements are used to compare shapes then the "value" of that number has no meaning other than one shape is better than another shape. As has been stated - don't get too hung up on that dimensionless number!

Think of a pressure gage with no increments - if the needle goes CCW that's good but if it goes CW that's bad or vercey-visey! It still gets me the improvements I want just not quantified to any standard.

So the new chart show what happens if when we don't have perfect parts. So if we fudge some of the drag values to guesstimate what might happen if the tank is not as slippery or the tires are bigger or a different shape or you have a roll cage hanging out in the breeze you can see that the Cd goes south in a hurry. In all cases the tires and axles still have the most drag but the tank is getting pretty bad at the end. Wonder what some F1 tires would do? Double or triple the tire values and you will have an idea. If you can't change the shape you have to make it smaller.

Me thinks that just confirms many of the real world comments already stated in this and other threads.

As my byline says: All models are wrong but SOME are useful! That's my story and I'm sticking to it!  angry


* Lakester Drag Forces 2.jpg (104.28 KB, 278x781 - viewed 151 times.)
Logged

All models are wrong, but some are useful! G.E. Box (1967) www.designdreams.biz
jacksoni
Sr. Member
****
Offline Offline

Age: 73
Location: Annapolis, Maryland
Posts: 958




Ignore
« Reply #11 on: November 16, 2016, 03:25:13 PM »

In at least one of my earlier posts I said that Drag equaled area times the Cd. This is incorrect. It is proportional to the A x Cd. The velocity of the fluid (air in this case) squared and the density of same also are involved. My apologies.

Woody: Thanks for the reality check. I love computer horsepower. So much easier than building an engine. If only we could just send the results to the proper sanctioning body and clear a space on the trophy shelf.....
Logged

Jack Iliff
 G/BGS-250.235 1987
 G/GC- 169.741  2009
 G/GMS-178.835 2010
Sumner
Global Moderator
Hero Member
***
Offline Offline

Age: 73
Location: Blanding, Utah
Posts: 3961


Blanding, Ut..a small dot in the middle of nowhere


WWW
« Reply #12 on: November 16, 2016, 04:39:00 PM »

....Do you think that the wide variation of Cd of the tire samples is due to testing variations/difficulties or in fact different (though the dimensions seemed similar to the LSR tires as you mentioned) tires with similar sizes?

Good question and one I don't have an answer for as the CD's are all over the place.  Maybe it has something to do with how they hold the tire in the tunnel.  How about 10 of us put in $100 each and get some real data from a wind tunnel on various tires/wheels, with and without moon disks?  We could share the data between us.

....... Also, what are your thoughts about the various components affecting the drag of adjacent parts? As in the "whole is greater (or less than as the case may be) than the sum of its parts?

A good question for Woody.  I would recommend getting THE LEADING EDGE ....

https://www.amazon.com/Leading-Edge-Engineering-Performance/dp/0837608600

... not cheap but if one gained one little improvement it would be then considered a cheap source.  Most of the book is over my head cry (Rex understands it  smiley  ), but they summarize and that proves useful.  Especially parts about where things attach to the body.  In my case I have side pods, similar to Hammon's old car, and similar to how wings attach to a fuselage and they have some good info on that,

Sumner
Logged

jacksoni
Sr. Member
****
Offline Offline

Age: 73
Location: Annapolis, Maryland
Posts: 958




Ignore
« Reply #13 on: November 16, 2016, 09:20:03 PM »

Well, the guy at A2 told me they had been testing garbage cans so I guess a series with wheels only would be cool. cool The 10x100 will just cover the two hour minimum.. Best have ducks in a row to make changes. Hardest being how to hold the tire in place without the hold down interfering. They do use ratchet straps over the front wheels. Maybe a really thin wire strap deal could do it.
Logged

Jack Iliff
 G/BGS-250.235 1987
 G/GC- 169.741  2009
 G/GMS-178.835 2010
WOODY@DDLLC
Hero Member
*****
Offline Offline

Age: 72
Location: Cincinnati
Posts: 1121


It's GONE ......... the Ohio Mile! :-(


WWW

Ignore
« Reply #14 on: November 16, 2016, 10:01:24 PM »

A2 uses small, compact straps on the front tires with a deflector to "simulate" the tire rolling. When you change tire sizes or wheel discs you find out how much HP you gain or lose real quick!  cheers
Regarding various components - it's a 3-dimensional interactive problem - leading to the two most common wind tunnel comments: "Wow, look at that!"  shocked and "I have no idea!"  huh
TEST - DON'T GUESS!  cheers


* A2 straps.JPG (232.54 KB, 768x1024 - viewed 168 times.)
Logged

All models are wrong, but some are useful! G.E. Box (1967) www.designdreams.biz
Pages: [1] 2 3 4   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Simple Audio Video Embedder
Valid XHTML 1.0! Valid CSS!


Google visited last this page October 17, 2017, 01:11:09 AM