Learn me some about rear spoilers

[Piotrsko]

Only way I know to reduce drag is to add better laminar airflow. Where and how you get there is the real argument.

[Vman455]

Originally Posted by j-c-c

Doesn't seem to me that a typical rear spoiler often reduces drag, leading to better mpg.

This is a good example of the non-intuitive nature of aerodynamics, because appropriately-sized spoilers typically reduce lift and drag (cf. Aerodynamics of Road Vehicles pp. 191-195). You can see examples of this on production cars such as the Civic hybrid, where a small spoiler was fitted to the trunk edge:

2005 Civic sedan, non-hybrid:


2005 Civic sedan, hybrid:


2008 Civic sedan, non-hybrid:


2008 Civic sedan, hybrid:

[j-c-c]

Maybe, but with only a quick glance, I don't see any real "spoiling" of air flow taking place in the pics, related to my earlier intuitive definition of what a spoiler does.

I do see a "lip", which as has been discussed here elsewhere, helps the rear turbulent air behind the vehicle air reattach better to the rear airflow off the trunk lid.

I am not trying to open a semantics discussion, but believe clarity is important here.

On a historical note, I saw my first real spoiler on the winning 250 TR Ferrari at the 1962 Sebring 12 hrs. It was not intuitive as to its purpose or method at the time.

[Vman455]

Originally Posted by j-c-c

I do see a "lip", which as has been discussed here elsewhere, helps the rear turbulent air behind the vehicle air reattach better to the rear airflow off the trunk lid.

I am not trying to open a semantics discussion, but believe clarity is important here.

A "lip" is a small spoiler. There's no governing body defining "spoiler" with a minimum dimension, and the literature uses the term very generally. For example, Hucho defines rear spoilers as "attached plastic parts made of a soft foam in order to reduce risk of injury, or parts drawn from the body panel" (Hucho, W.H. "Aerodynamic Drag of Passenger Cars." In Aerodynamics of Road Vehicles, 4th ed., W.H. Hucho, ed. [Warrendale: SAE International, 1998], 191). He includes figures that show testing of spoilers from 0-100mm tall, with drag minimum around 20mm (less than an inch)--right around where those Civic spoilers appear to be. He also argues that the purpose of rear spoilers is three-fold:

1. Reduce drag.
2. Reduce rear-axle lift.
3. Reduce dirt on the rear surface.

(Ibid.)

Further, the spoiler does not have to be large to have a substantial effect. Hucho provides data for the VW Corrado, one of the first production cars with an active rear spoiler. The difference between these:





...is a reduction in rear-axle lift of 33% at 120 km/h.

I don't see much point in arbitrarily restricting one's definition here, especially in a thread that very generally asked about rear spoilers.

[Grant-53]

Historically I would say a spoiler has been a panel that is near vertical to the rear deck to increase down force. A deflector lip would be used to direct flow. Depending on the angle of the rear window a deflector at the back of the roof may be helpful.

[freebeard]

IMO, a rear spoiler is supposed to "spoil" air flow, and therefore increasing the local air pressure upon a surface up stream, to intentionally increase DF.

Maybe, but with only a quick glance, I don't see any real "spoiling" of air flow taking place in the pics, related to my earlier intuitive definition of what a spoiler does.

Gurney Flap/wickerbill: https://en.wikipedia.org/wiki/Gurney_flap

Think of it as an inarticulate trim tab.

[Piotrsko]

Originally Posted by freebeard

Think of it as an inarticulate trim tab.

Not moving or DOESN'T talk much?

[aerohead]

Originally Posted by j-c-c

IMO, a rear spoiler is supposed to "spoil" air flow, and therefore increasing the local air pressure upon a surface up stream, to intentionally increase DF. Increased drag is thought to be an acceptable trade off in most cases.

A flat added extension (trunk?) should, again IMO, maybe better be called a "smoother (or divider?)", which might reduce drag. If a "smoother", just by forming a hard boundary between a higher pressure area and a lower pressure area in space, will also increase potential DF, with little drag increase, if any.

Doesn't seem to me that a typical rear spoiler often reduces drag, leading to better mpg.

The spoiler is there to spoil lift.The lift is a function of fast,low-pressure turbulence,created by separated flow over a moment arm created by position over the body.Kamei created the first commercial airdam and spoiler for the BMW 2000,in 1982.The rear spoiler created a flow reattachment at the tip of the 23-degree slope-to-spoiler top,thus capturing a vortex,preventing the separation line pressure from telegraphing all the way to the end of the boot.Without the rear spoiler,low pressure would make it behind the car,lowering the base pressure,and increasing pressure drag.At this 'sweet-spot',the spoiler both kills rear lift,while also reducing drag.

[aerohead]

Originally Posted by Piotrsko

Only way I know to reduce drag is to add better laminar airflow. Where and how you get there is the real argument.

Hucho writes that all aerodynamic streamlining has to do with minimizing or eliminating flow separation in the aft-body.This can only be achieved with a very gradual Bernoulli Equation-related deceleration/pressure recovery which will not compromise the turbulent boundary layer,the key to attached,laminar flow,as you mention.Hucho cites Buchheim et al.,and their 23-degree maximum angle streamline contour which agrees favorably with W.A.Mair's 22-degree maximum boat tail contour,which also happens to correlate with the NACA airship body contour research,of which the aerodynamic streamlining template originates.
Automakers don't mind using it to achieve world records,but otherwise appear to hate it.Until Elon Musk reared his beautiful head.

[aerohead]

Originally Posted by j-c-c

Maybe, but with only a quick glance, I don't see any real "spoiling" of air flow taking place in the pics, related to my earlier intuitive definition of what a spoiler does.

I do see a "lip", which as has been discussed here elsewhere, helps the rear turbulent air behind the vehicle air reattach better to the rear airflow off the trunk lid.

I am not trying to open a semantics discussion, but believe clarity is important here.

On a historical note, I saw my first real spoiler on the winning 250 TR Ferrari at the 1962 Sebring 12 hrs. It was not intuitive as to its purpose or method at the time.

I believe that the Ferrari incorporated Richtie Ginther's spoiler,which he co-opted from his experience around flaps,slats,and spoilers in the US Air Force.If you have an side image of the Ferrari,look at the slant from the roof to the spoiler tip.As long as some of the downwash strikes the spoiler,you're on your way to eliminating rear lift.