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aerohead · 04-15-2010, 07:57 PM

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!

04-15-2010, 07:57 PM	#1 (permalink)
aerohead Master EcoModder Join Date: Jan 2008 Location: Sanger,Texas,U.S.A. Posts: 16,314 Thanks: 24,441 Thanked 7,386 Times in 4,783 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!