AUSTIN VERT COMMENTS ON REAR SPOILERS.

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1973 Mustang Convertible
Hi to all,

First up apologies to all, as this topic has been somewhat done to death over the years. But i did want to revisit it, as i wanted to bring home a couple of points that i thought needed clarifying to some extent.
It basically concerns the spoiler mounting angles of our '71-'73 Mustang hardtops and convertibles.

Before i start, i am aware that some folk like the look of spoilers on our 1,2,3's and some don't. Fair enough, but i'm not going down that road of discussion today. So the rear deck spoilers have two basic things going for them.
They have a certain look that they bring to our cars (LIKE THEM OR NOT), and secondly, they are supposed to improve the handling performance out on the road. I did a little online research before posting, to brush up on the technical side of rear deck spoilers mainly to back up my own personal opinions. I have included that stuff in my links below, and makes for good viewing i think.

So, i have always been of the personal opinion that when mounting a rear deck spoiler, the blade angle should end up being tilted forward, so that the front leading edge is lower than the rear back edge. The main idea of the rear spoiler is to try and create downward pressure on the rear end of the car out on the road. This can be done if the spoiler angle is tilting downward at the front leading edge. The more the spoiler blade tilts downwards the more greater the downward force is applied to the rear of the car. However, if the angle of the spoiler blade is parallel to the body, or tilting slightly backwards, so the front leading edge is higher than the rear edge, then that creates little to no downward pressure on the rear end of the car. The spoiler then becomes a visual thing only, not a functional one as well. Tilting the spoiler blade forward or down on the front edge will produce some drag, but that is the trade off of going that way. Over the years, i have noticed that most Mustang owners like to mount their blades horizontal to the body line, or front up slightly with back edge down. Looks cool, but doesn't do much for improved handling.(See my pics below ). I have included a couple of pics as well showing more rarer examples of owners with a slight tilt forward. You could even tilt those blades down at the front more for greater downward force to the rear end.

Also, i think that the higher the spoiler sits above the body line, the more it will capture the wind air currents, thus having or creating a better downward force on the rear end. However,i have never really liked the look of high mounted rear deck spoilers on our Mustangs. The lower to the body panel the better they look in my opinion. But that i guess is the tradeoff on cool looks as adversed to better handling performance.

Some folk believe that for a spoiler to work or assist handling, the car has to be going at high speeds. I think the jury is out on that as there is evidence claiming that a kick in handling effect can take place at lower speeds as well.
In any case, i have always thought that to have a spoiler installed, and mounted in the correct blade angle ( down at the front edge), does improve the handling of the car to some extent, but probably not in a dramatic way.

LINKS ....................................................................













Greg.:)
 

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When talking about our 71-2-3 Mustangs, there are a number of things to realize. First, any appreciable difference in handling is likely only going to occur at speeds beyond current legal speed limits. Second, what we have is an upside-down wing. On an airplane, the wing shaped cross-section creates low pressure underneath, creating lift, on our cars, the wing is upside-down, which puts the low pressure on the top surface and creating a down pressure, NOT the angle of attack. If, our cars came with deck devices that had a constant cross-section, the angle of attack would be more important. That's "air deflection", and also creates drag. Talking about angle of attack, a high angle of attack is displayed in the bottom video picture, and will cause an airplane wing to "stall" and lose lift. Too much angle nose down on our cars does the same, stalls lift ( downforce on our Upside -down wings). But by placing the wing into the angle of airflow off the back of the car, negates any drag, turbulent air, forming at the rear of our cars. Remember, we have WINGS, NOT SPOILERS. The same thinking applies to the chin spoiler, or air dam, on the lower valence in the front. It's purpose is to move the air from going under the car, to around the car. Air going under the car will tend to lift the car, so air dam devices are there to push the air from going under and help plant the car to the road. Granted, again, that effect is only going to happen at only higher speeds, but you can help the front air dam by having the nose as close to the ground as your driving landscape will allow. We live in a world with speedbumps, inclined driveways, and such, so perfect control of frontal air is a compromise at best. All that having been said.......since the aero devices really only have effect at higher speeds, and we realize the factory put them on because it sells the car as a performance car, put your wing wherever you like the looks of it . It won't make any difference taking your Mustang to the Burger Joint on Sunday.
 
When talking about our 71-2-3 Mustangs, there are a number of things to realize. First, any appreciable difference in handling is likely only going to occur at speeds beyond current legal speed limits. Second, what we have is an upside-down wing. On an airplane, the wing shaped cross-section creates low pressure underneath, creating lift, on our cars, the wing is upside-down, which puts the low pressure on the top surface and creating a down pressure, NOT the angle of attack. If, our cars came with deck devices that had a constant cross-section, the angle of attack would be more important. That's "air deflection", and also creates drag. Talking about angle of attack, a high angle of attack is displayed in the bottom video picture, and will cause an airplane wing to "stall" and lose lift. Too much angle nose down on our cars does the same, stalls lift ( downforce on our Upside -down wings). But by placing the wing into the angle of airflow off the back of the car, negates any drag, turbulent air, forming at the rear of our cars. Remember, we have WINGS, NOT SPOILERS. The same thinking applies to the chin spoiler, or air dam, on the lower valence in the front. It's purpose is to move the air from going under the car, to around the car. Air going under the car will tend to lift the car, so air dam devices are there to push the air from going under and help plant the car to the road. Granted, again, that effect is only going to happen at only higher speeds, but you can help the front air dam by having the nose as close to the ground as your driving landscape will allow. We live in a world with speedbumps, inclined driveways, and such, so perfect control of frontal air is a compromise at best. All that having been said.......since the aero devices really only have effect at higher speeds, and we realize the factory put them on because it sells the car as a performance car, put your wing wherever you like the looks of it . It won't make any difference taking your Mustang to the Burger Joint on Sunday.
Hi Spike,

Thanks for your reply and feedback.

I don't want to get into some kind of long winded debate on this topic per say, but reading through your reply, leaves me somewhat confused as to your explanations of upside down wings/ airplane winged shaped cross sections/wings not spoilers/ blade angle high and low pressure outcomes etc. I got lost and confused in your explanations as such, and really don't know what you are talking about sorry. Also, speed of vehicle and kick in spoiler effects needs to be more tested and calibrated to produce more factual information and evidence on what real impact a spoiler has on handling performance at varying speeds. Has this been done already? - i don't know.

I myself am no expert in the car aerodynamics field, and so like most people, i go to the online videos for some kind of more knowledgeable feedback on this subject. The videos i have presented in my post, clearly describe and show in good detail, the downward air pressure forces on a rear deck spoiler, and hence, the rear end of the car, when the mounting tilt angle changes. I have put my faith in what i have seen in these videos as to all be correct and true. Maybe more research could be needed on my part to learn more about car aerodynamics as such, i don't know.

In any case, for now, i will stick with the technical feedback that has been presented in the videos as true and factual, and go along with that.

Greg.
 
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Here is an example of a wing, upside -down, on the front of this dragster. Open the picture and look closely at the airfoil shape, flat on the top, and curved on the bottom, exactly the reverse of a wing as used on aircraft, but exactly like ours. The shape creates lift on aircraft,and downforce when upside-down, as is used here, and with our cars. In this case, applying downforce at speed to keep the front wheels in contact with the dragstrip at speed. Now, there is some nose-down angle of attack in how it's mounted here, but the relative wind is going to hit it straight on, as opposed to flowing on a downward direction as it does when flowing off of our rooflines, so the angle of attack is not as radical as it is if you nose-down the deck wing on your Mustang. Another thing to note here, is, that this wing is hinged, so that as the cars speed increases and pushes the chassis down, the slight angle here decreases, hitting the wind more straight-on. This is effective at 200 miles per hour, may not be perceivable at all on our car speeds, save for long tracks where our Mustangs could see benefits. Wind tunnel tests showing downforce and C/D of a Mustang Sportsroof would tell all.
 

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