Go Back   EcoModder Forum > EcoModding > Aerodynamics
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 04-15-2010, 06:57 PM   #1 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 14,496
Thanks: 22,498
Thanked 6,700 Times in 4,240 Posts
BOAT TAIL Drag Reduction Estimates

This is just a quicky for those contemplating the addition of a tail and what it might do.I can't do a graphic today,will do numbers.
Data is from Hoerner's AERODYNAMIC DRAG,Fig.3.23,p.35,1951,which is referenced to Kosin & Lehman,'Aerodynamic Characteristics of Fuselage Bodies,'Deutsche Luftfahrtforschung,1942,p.I 241.

What I will do,is have you imagine your car already' with' a full boat tail based on the Aerodynamic Streamlining Template,which with proper ground clearance extends to about 80% when it 'strikes the 'line'.

Note: The aft-body of the example fuselage is responsible for a 567% drag reduction when added to the point of max camber.

From zero-to-20% chopped-off,the drag rises 3.95%
30%-chop = 12.5% drag increase
40%-chop = 29.6% drag increase
50%- chop = 59.6% drag increase
60%- chop = 99.85% drag increase
70%- chop = 257% drag increase
80%- chop = 330% drag increase
90%- chop = 426% drag increase


These values are for 'profile-drag' only( covering form and skin-friction drag ) ,so your car will still have components of cooling system drag,interference drag,induced drag.
You can see the significance the rear of the car plays.
With wings,the aft-body is responsible for a 1,000% drag reduction.
Most modern cars will achieve Cd0.13 with a full tail.With under-car wheel fairings the Cd can go even lower.

If you can do a breakdown of your existing drag components,you can use the table to estimate the profile drag as a function of boat tail length, holding all the other parameters as a constant to arrive at the theoretical Cd for your mods.
I've already done a parametric comparison to Walter E.Lays wind tunnel investigations and there is very close agreement between his work and the fuselage data.
Jaray,Kamm,and Lay,all did pumpkin seeds with forms close, if not in perfect agreement( Lay) with the Aerodynamic Streamlining Template.
The pressure recovery profile appears to be universal for streamline bodies of revolution regardless of thickness ratio.
It's my opinion that the 'phantom tail' concept is losing air.When you make a very close investigation of the research you find context which must be taken into account when making general claims about the performance of a tail.
Gotta go,clocks about run out!

  Reply With Quote
The Following 3 Users Say Thank You to aerohead For This Useful Post:
BamZipPow (04-15-2010), jime57 (03-07-2012), Piwoslaw (04-16-2010)
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 04-15-2010, 09:07 PM   #2 (permalink)
Master EcoModder
 
Join Date: Oct 2008
Location: Mid-Atlantic
Posts: 491

OurInsight - '06 Honda Insight
Thanks: 170
Thanked 69 Times in 44 Posts
Quote:
Originally Posted by aerohead View Post

Note: The aft-body of the example fuselage is responsible for a 567% drag reduction when added to the point of max camber.

From zero-to-20% chopped-off,the drag rises 3.95%
30%-chop = 12.5% drag increase
40%-chop = 29.6% drag increase
50%- chop = 59.6% drag increase
60%- chop = 99.85% drag increase
70%- chop = 257% drag increase
80%- chop = 330% drag increase
90%- chop = 426% drag increase
So,if I interpret this correctly, the best one can hope far in about 3.95% worse than the ideal template, with a boattail that is probably impossible long and low. RobertSmall's proposal looks to be about a 30-40% chop, if one degrades it a bit for the upward slop of the bottom, so he is looking at something like 20 % drag increase from the perfect template. The stock Insight looks to be something like a 70-80% drag increase from the template. Am I in the ball park?
  Reply With Quote
Old 04-15-2010, 10:40 PM   #3 (permalink)
Master EcoModder
 
3-Wheeler's Avatar
 
Join Date: Dec 2008
Location: Southern WI
Posts: 827

AlienMobile - '00 Honda Insight
Team Honda
90 day: 80.05 mpg (US)
Thanks: 101
Thanked 558 Times in 191 Posts
I'm just hoping for a decrease in drag similar to Metro's.

Jim.
  Reply With Quote
Old 04-15-2010, 11:56 PM   #4 (permalink)
Left Lane Ecodriver
 
RobertSmalls's Avatar
 
Join Date: Aug 2008
Location: Buffalo, NY, USA
Posts: 2,257

Prius C - '12 Toyota Prius C
Thanks: 79
Thanked 286 Times in 199 Posts
Quote:
Originally Posted by jimepting View Post
RobertSmall's proposal looks to be about a 30-40% chop, if one degrades it a bit for the upward slop of the bottom, so he is looking at something like 20 % drag increase from the perfect template. The stock Insight looks to be something like a 70-80% drag increase from the template.
I can't study this thread tonight, but I had to at least crunch these numbers.

If the stock Insight's drag is 75% greater than that of a full length streamlined car, then the latter has a Cd of 0.14 or so. Apply a 20% penalty to this, and you have 0.17. These numbers use a baseline Cd of 0.25, which is a stock Insight with mirrors, grille openings, and stock underbody coverage.
  Reply With Quote
Old 04-17-2010, 01:15 PM   #5 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 14,496
Thanks: 22,498
Thanked 6,700 Times in 4,240 Posts
best

Quote:
Originally Posted by jimepting View Post
So,if I interpret this correctly, the best one can hope far in about 3.95% worse than the ideal template, with a boattail that is probably impossible long and low. RobertSmall's proposal looks to be about a 30-40% chop, if one degrades it a bit for the upward slop of the bottom, so he is looking at something like 20 % drag increase from the perfect template. The stock Insight looks to be something like a 70-80% drag increase from the template. Am I in the ball park?
I ran the profile out with some minimum constants for cooling system drag ( 0.011 ),'features drag'( gaps,cut lines,door handles,antenna,wheel openings,etc.) ( 0.007 ),and then subtracted out 0.015 for a complete wheel fairing set using data from the Sunraycer.
It appears that if we do everything 'right',we could get a car down to a conservative Cd 0.126,at 80 % of the template length.
The last bit of drag would be insignificant,residing within the 'phantom' tail.
Running the tail out to 100 % would only get the Cd down to 0.123.
  Reply With Quote
The Following User Says Thank You to aerohead For This Useful Post:
jime57 (04-17-2010)
Old 04-17-2010, 01:26 PM   #6 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 14,496
Thanks: 22,498
Thanked 6,700 Times in 4,240 Posts
Insight

Quote:
Originally Posted by RobertSmalls View Post
I can't study this thread tonight, but I had to at least crunch these numbers.

If the stock Insight's drag is 75% greater than that of a full length streamlined car, then the latter has a Cd of 0.14 or so. Apply a 20% penalty to this, and you have 0.17. These numbers use a baseline Cd of 0.25, which is a stock Insight with mirrors, grille openings, and stock underbody coverage.
I'm seeing the stock Insight at 30 % aft-body.
If I use Cd 0.25 and add the Sunraycer's original wheel fairings,I'm seeing Cd 0.241 at stock length.
If I run the tail out to 50 %,and with cooling system mods( ductwork) we might be looking at Cd 0.163.
With a one-wheel range-extender trailer with gap-fillers taking it out to 80 % template,Cd 0.119.
  Reply With Quote
Old 04-17-2010, 10:05 PM   #7 (permalink)
Left Lane Ecodriver
 
RobertSmalls's Avatar
 
Join Date: Aug 2008
Location: Buffalo, NY, USA
Posts: 2,257

Prius C - '12 Toyota Prius C
Thanks: 79
Thanked 286 Times in 199 Posts
What do you make of my grille block, in terms of its contribution to Cd: Insight grille block

I will probably also block most of the are to the driver's side of the license plate, leaving <10in˛ of cooling air inlet low in the bumper, and ducted to the front wheel wells.

I see in the other thread you translated the above percentages into Cd figures:
Code:
zero------ Cd 0.488
10 %----- Cd 0.375
20 %----- Cd 0.299
30 %----- Cd 0.240
40 %----- Cd 0.194
50 %----- Cd 0.162
60 %----- Cd 0.135
70 %----- Cd 0.125
80 %----- Cd 0.118
90 %----- Cd 0.118
100 %---- Cd 0.115
It is interesting to compare your table to this one:

Specifically the half-sphere (approximating a 0% chop) and streamlined half-body (which has Cd=0.09 with zero ground clearance and no features).

Like the EV1 and the UFE-III, the Insight's rear glass doesn't exactly follow the template. I believe a 35-40% chop is a better approximation of the Insight's shape. That would place it around a Cd of 0.21, i.e. a 0.04 Cd reduction from aeromods other than the tail, and the potential for 0.10 Cd reduction from a complete boat-tail.

My tail extension will be cantilevered, not towed, and its underside will depart from that of the template to allow good clearance angles. This departure occupies 17% of the tail extension's area, in rear projection, which could not result in a drag penalty of more than 0.017 Cd.

It looks like I'm on track to build a car with a Cd of 0.13-0.15! I will be impressed if I achieve a figure in that range.
  Reply With Quote
Old 04-17-2010, 10:21 PM   #8 (permalink)
Master EcoModder
 
Join Date: Oct 2008
Location: Mid-Atlantic
Posts: 491

OurInsight - '06 Honda Insight
Thanks: 170
Thanked 69 Times in 44 Posts
Got my Hucho!

I found interesting his comment that there had been no agreement between CFD simulations and the results achieved by real cars ;-)

Last edited by jime57; 04-17-2010 at 10:29 PM..
  Reply With Quote
Old 04-20-2010, 05:25 PM   #9 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 14,496
Thanks: 22,498
Thanked 6,700 Times in 4,240 Posts
grille block

Quote:
Originally Posted by RobertSmalls View Post
What do you make of my grille block, in terms of its contribution to Cd: Insight grille block

I will probably also block most of the are to the driver's side of the license plate, leaving <10in˛ of cooling air inlet low in the bumper, and ducted to the front wheel wells.

I see in the other thread you translated the above percentages into Cd figures:
Code:
zero------ Cd 0.488
10 %----- Cd 0.375
20 %----- Cd 0.299
30 %----- Cd 0.240
40 %----- Cd 0.194
50 %----- Cd 0.162
60 %----- Cd 0.135
70 %----- Cd 0.125
80 %----- Cd 0.118
90 %----- Cd 0.118
100 %---- Cd 0.115
It is interesting to compare your table to this one:

Specifically the half-sphere (approximating a 0% chop) and streamlined half-body (which has Cd=0.09 with zero ground clearance and no features).

Like the EV1 and the UFE-III, the Insight's rear glass doesn't exactly follow the template. I believe a 35-40% chop is a better approximation of the Insight's shape. That would place it around a Cd of 0.21, i.e. a 0.04 Cd reduction from aeromods other than the tail, and the potential for 0.10 Cd reduction from a complete boat-tail.

My tail extension will be cantilevered, not towed, and its underside will depart from that of the template to allow good clearance angles. This departure occupies 17% of the tail extension's area, in rear projection, which could not result in a drag penalty of more than 0.017 Cd.

It looks like I'm on track to build a car with a Cd of 0.13-0.15! I will be impressed if I achieve a figure in that range.
Robert,if the grille block isn't cooking anything under the hood then I guess you're good to go.
Since the hybrids have expensive MOSFET and IGFET power transistors requiring cooling,I'm at a loss for recommendations with these advanced powertrains.
With respect to the drag reduction contribution from the block, crude logic might suggest that if you've cut airflow by 50%,then maybe that load has been halved.Just a SWAG.
Don Sherman got 0.4 mpg @ 70 mph with complete block to Pinto.2.0 mpg when airdam was added,then after cutting some block away for minimum cooling,gain came in at 1.4 mpg @ 70 mph.
With respect to the tail,I see no reason why you couldn't achieve your 0.13-0.15 numbers.
  Reply With Quote
Old 04-21-2010, 12:24 AM   #10 (permalink)
Pokémoderator
 
cfg83's Avatar
 
Join Date: Dec 2007
Location: Southern California
Posts: 5,864

1999 Saturn SW2 - '99 Saturn SW2 Wagon
Team Saturn
90 day: 40.49 mpg (US)
Thanks: 439
Thanked 529 Times in 356 Posts
aerohead -

I started with this image :



Are the red vertical lines the correct chop-off points? :



Orrrrr, the original black vertical hatched lines?

CarloSW2

__________________

What's your EPA MPG? Go Here and find out!
American Solar Energy Society
  Reply With Quote
Reply  Post New Thread


Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
CdA list Daox Aerodynamics 173 03-13-2022 11:40 AM
Vibe Kammback / Boat tail trikkonceptz Aerodynamics 9 01-19-2014 09:59 AM
Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph MetroMPG Aerodynamics 500 05-10-2012 10:30 PM
Sources of Aerodynamic Drag in Automobiles and Possible Solutions SVOboy Aerodynamics 12 02-17-2010 02:09 PM
Subaru Outback Prototype Boat Tail Test Results brucey Aerodynamics 10 10-02-2009 05:03 PM



Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2022, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com