Aerodynamics

[Alain - The Red Scorpion]

I agree with the turbulence theory over the trunk lid, here is a still pic from a video I took at 120kph of short wool threads taped to the lid and the edge of the engine cover of my track X to test a small boot lip spoiler. The threads moved around a bit, but quite often pointed forward rather than backward. I then did similar tests with a higher spoiler with string on higher posts. It looked like it may be a bit more effective, but still of doubtful benefit. I did go ahead andView attachment 53590View attachment 53591 fit a spoiler mainly just for looks, as expected, it has made no difference to track times.

Thank you for the input. Confirms my "theory" that a ducktail/spoiler is useless, except for looks. I should have mentioned that I have entered my Vintage prepared X (still street legal) in its first event. With AVO coilovers and wider wheels/tires, it has done well, considering its still stock engine and transmission.

 

[Steve Hoelscher]

The thing you need to understand about automotive aerodynamics from a downforce/lift perspective is that they don't do much under 100 mph... Not much use on a street driven car... I'm sure there are gains to be had as far as cooling/airflow goes, but most factory add-on aero just causes drag....
A ducktail is somewhat effective on a coupe/fastback shape. On a car like an X/19, with a sharp roof/ rear window transition, that air is nowhere near that ducktail. Most effective wings are large and high enough to be in the 'clean' air, over any turbulence from the edge of the roof...

I am going to disagree with you on this. Speed is relative to the device employed. In the case of a ducktail spoiler on the X, the airflow over the spoiler itself, and the rear decklid, might not have much change but even a small ducktail can help reduce tumble in the airflow coming off the roof.

My MR2 autocross car is a perfect example. Wings are not permitted but a spoiler (max 10" tall) is. Mounting the spoiler as far back on the rear deck and tuning the angle produced noticeably more rear grip and required the addition of a rather large splitter and hood vent to balance the rear downforce (or more likely; reduction of rear lift) and return the car's handling balance. Testing with and without the spoiler/splitter aero demonstrates the car is typically more than a full second faster on a 60 second autocross course with the aero. Note that typical autocross speeds (as determined by data logging) range between 35 and 60 mph with the one or two points in excess of 65 mph.


The fastest autocross cars have been employing aerodynamics to produce downforce for about a decade now. The data is there that demonstrates that aero can work at most any speed if the device used is specific to the application. Most production cars use aero to reduce drag and any negative lift produced is the result in the reduction in positive lift or turbulence.

I will also note that while I am not an aerodynamicist I work with them daily.

My MR2:

 

[Dr.Jeff]

I will disagree with another statement that was made in a earlier post regarding the movement of air through the engine bay.

A lot of turbulence over the entire rear area does not necessarily mean the air is moving upward through/across the engine cover. Turbulence moves in all directions. That's why the strings dance about randomly. It creates mostly dead air over the lid; it is as likely to create as much positive pressure (moving air downward through the engine lid) as negative pressure (pulling air upward). There may be some particular spots where it is positive while at the same time other spots where it is negative. And those places will continually move about, changing in intensity, location, and direction. Lots of variables will affect it, for example speed is one. So while the movement of air is random over the engine lid, it may be a bit more upward at times, a bit more downward at times, or neutral at times.

However that is only respective to the turbulence above the engine bay. Another factor that will have a big influence on the direction of the air movement (up or down) within the bay is what's happening on the other side, below the engine cover. The pressure differential between the air above the lid and the air below the lid will determine the direction of movement across the lid (and therefore through the engine bay). Again, the pressure below the lid will be affected by several variables. And that will also change depending on the conditions.

While I agree there may be a tendency for some upward air movement, for the most part I suspect the air inside the engine bay will tend to be fairly stagnant overall due to various factors cancelling one another out. But this could be altered by modifying several aspects of the bodywork (from stock) if additional cooling is the goal. Those mods can cause either more upward or more downward movement depending on exactly what is done.

I am not a aerodynamic engineer so this opinion is based solely on my general knowledge of scientific principles and not on any specific testing. Hopefully Steve H has some additional knowledge from the aerodynamicists he has worked with, or someone else is a aero engineer that can shed more light on this aspect (direction of air movement through the engine bay).

 

[johnph]

Check out the aero mods on my old H production SCCA road racer...

 

[Steve Hoelscher]

I am not a aerodynamic engineer so this opinion is based solely on my general knowledge of scientific principles and not on any specific testing. Hopefully Steve H has some additional knowledge from the aerodynamicists he has worked with, or someone else is a aero engineer that can shed more light on this aspect (direction of air movement through the engine bay).

I have some knowledge from looking at airflow through the MR2 engine bay and what I have assumed based on the X1/9's design.

Looking at the X1/9's design and how air would flow over/around the bodywork one can determine basic airflow. Air flowing over the roof creates a low pressure area above the engine decklid and that draws air through the side scoops and from under the car. Anyone that ever had a water hose break or had the car boil over could see the steam follow that path. Further, with the OE splash shields in place in the engine bay the primary source of airflow would be the side scoops. And a basic understanding of aerodynamics would lead one to determine that the low pressure area above the engine decklid would draw air from the engine compartment and in through the side scoops.

That doesn't mean that the air doesn't tumble as it comes off the roof. It certainly does. The tumbling action and the blending of airflow coming in diagonally from off the sides of the targa bar, creates a lot of turbulence behind the rear window and over the entire rear deck.

From experience looking at flow (and CFD) off the rear end of various cars, the spoiler (ducktail) helps reduce the amount of turbulence behind the car and that you will not see in the yarn taped to the rear deck. Reducing that turbulence and smoothing out the re-attachment of the airflow off the roof and over the rear spoiler, does a lot more for reducing drag and lift than actually creating downforce.

On my MR2 I was able to measure the deflection of the rear spoiler at various speeds by mounting a reference next to the spoiler and a go-pro looking at the reference. At 35 MPH there was less than 3mm of deflection but at 65 there was more than 10.

 

[gene cooley]

And if a little bit of wing/ spoiler is good then wretched excess is better....
(except in our case the front wing probably disturbs the air on the "main" wing.

 

[Dr.Jeff]

I have some knowledge from looking at airflow through the MR2 engine bay and what I have assumed based on the X1/9's design.

Looking at the X1/9's design and how air would flow over/around the bodywork one can determine basic airflow. Air flowing over the roof creates a low pressure area above the engine decklid and that draws air through the side scoops and from under the car. Anyone that ever had a water hose break or had the car boil over could see the steam follow that path. Further, with the OE splash shields in place in the engine bay the primary source of airflow would be the side scoops. And a basic understanding of aerodynamics would lead one to determine that the low pressure area above the engine decklid would draw air from the engine compartment and in through the side scoops.

That doesn't mean that the air doesn't tumble as it comes off the roof. It certainly does. The tumbling action and the blending of airflow coming in diagonally from off the sides of the targa bar, creates a lot of turbulence behind the rear window and over the entire rear deck.

From experience looking at flow (and CFD) off the rear end of various cars, the spoiler (ducktail) helps reduce the amount of turbulence behind the car and that you will not see in the yarn taped to the rear deck. Reducing that turbulence and smoothing out the re-attachment of the airflow off the roof and over the rear spoiler, does a lot more for reducing drag and lift than actually creating downforce.

On my MR2 I was able to measure the deflection of the rear spoiler at various speeds by mounting a reference next to the spoiler and a go-pro looking at the reference. At 35 MPH there was less than 3mm of deflection but at 65 there was more than 10.

Would smoothing out the airflow above the engine lid have any relative impact on the amount of upward airflow through the bay? In other words does less turbulence above the lid create more or less upward flow through it.

On my current X project I have dramatically opened up the side scoops, removed the rear bulkhead between the engine and trunk, completely opened the rear taillight panel, and closed off the floor under the engine. The goal is to move the air through the bay and out the tail.

 

[Lowtechprime]

My MR2 autocross car is a perfect example. Wings are not permitted but a spoiler (max 10" tall) is. Mounting the spoiler as far back on the rear deck and tuning the angle produced noticeably more rear grip and required the addition of a rather large splitter and hood vent to balance the rear downforce (or more likely; reduction of rear lift) and return the car's handling balance. Testing with and without the spoiler/splitter aero demonstrates the car is typically more than a full second faster on a 60 second autocross course with the aero. Note that typical autocross speeds (as determined by data logging) range between 35 and 60 mph with the one or two points in excess of 65 mph.View attachment 53607

That's interesting.. I can't imagine that at 35 mph there's that much down force generated, but if that's the results that you are seeing, who can argue? Although I will say, C'mon Steve, that's hardly a ducktail spoiler, more of a 'goose' tail.. That spoiler is more than half the height of the rear pillars. Not the same as a little 1 or 2 inch lip on most cars...

 

[Steve Hoelscher]

Would smoothing out the airflow above the engine lid have any relative impact on the amount of upward airflow through the bay? In other words does less turbulence above the lid create more or less upward flow through it.

On my current X project I have dramatically opened up the side scoops, removed the rear bulkhead between the engine and trunk, completely opened the rear taillight panel, and closed off the floor under the engine. The goal is to move the air through the bay and out the tail.

Assumption warning!!!

The big low pressure area is created by the void between the engine decklid and the end of the roof. Smoothing out the airflow likely reduces the void and thus the low pressure generated. That is unless you used a device to reattach the airflow behind the engine decklid and maintained the void. I don't know how you would accomplish that but its a possibility.

I have looked at adding a wickerbill/gurney flap (tiny lip spoiler) to the end of the roof panel for the purpose of reducing the tumble effect, (which it _should_) and thus improve the flow to the rear spoiler. It would also increase the low pressure area above the decklid slightly.

 

[Steve Hoelscher]

That's interesting.. I can't imagine that at 35 mph there's that much down force generated, but if that's the results that you are seeing, who can argue? Although I will say, C'mon Steve, that's hardly a ducktail spoiler, more of a 'goose' tail.. That spoiler is more than half the height of the rear pillars. Not the same as a little 1 or 2 inch lip on most cars...

An airfoil's effect is relative to the speed of the air flowing over it. At lower air speeds you need more area to generate the same lift. This is why jets have retractable wing surfaces on both the leading and trailing edges of the wing. The car that won A-Mod at this years Solo Nationals had ground effects venturis that were enormous, paired with massive wings. The car wasn't that quick down the straights but it hardly slowed for the corners. It was obvious the 600cc motorcycle engine didn't have the horsepower to push the aero any faster.

The spoiler on the MR2 is 10" in total length. Its at about 60 degrees from horizontal and spans the entire width of the car. It does catch a lot of airflow off the sides of the greenhouse so it sees more than just airflow over the roof. It would be a problem at 100 mph.

 

[NEG]

Given the amount of road muck that ends up in the engine bay and how the engine lid flys like a magic carpet at low speeds if you forget to latch it there’s quite a bit of upward air movement through the engine bay!

 

[carl]

The engine lid lifting up is probably caused by low air pressure on top, just like your front trunk or hood lifting up if not latched.

I'm pretty sure the usual duckbill spoiler is a fashion statement, just like a FAZA snorkel.

 

[Dr.Jeff]

Assumption warning!!!

The big low pressure area is created by the void between the engine decklid and the end of the roof. Smoothing out the airflow likely reduces the void and thus the low pressure generated. That is unless you used a device to reattach the airflow behind the engine decklid and maintained the void. I don't know how you would accomplish that but its a possibility.

I have looked at adding a wickerbill/gurney flap (tiny lip spoiler) to the end of the roof panel for the purpose of reducing the tumble effect, (which it _should_) and thus improve the flow to the rear spoiler. It would also increase the low pressure area above the decklid slightly.

Regarding the movement of air through the engine bay and the low pressure area above it. I agree there would be a difference depending on how the airflow was "smoothed out". For example if the air coming off the roof could somehow be directed more downward, and made to flow straight across the engine cover. That laminar flow (relatively) should significantly reduce the turbulence above the cover, which would decrease the low pressure as you say. Unless it somehow creates a venturi effect in the engine bay (but I don't think it could), that decreased low pressure above the engine cover will change the relative pressure difference above and below it and decrease the upward movement of air.

That same airflow flat across the engine cover should also benefit the rear spoiler. Therefore rather than adding a wickerbill, would it work to go the other direction and add a "scoop" across the rear edge of the roof to direct the air downward? This was what @Alain referred to in post #37, "Would a roof deflector à-la-Stratos be useful in "bending" the airflow downwards over the rear end of the car?" BMW also did this on the 3.0 CSL ("batmobile"):

Grandma's station wagon also had one, to keep the rear window clean :

 

[dllubin]

I have been running my X for several months without the engine cover rain tray. I began to notice dust patterns on top of the engine cover that were never there before. They appear in the area right above where the rain tray was, and not where the two openings are on each side of the engine cover are that don't get exposed with the rain tray removal. I also noticed a bunch of dust and dirt in my previously clean engine compartment. All the factory engine compartment sheet metal is installed.

My guess is that removing the rain tray increased the air flow out of the engine cover by quite a bit. I think there may be quite a bit of air coming in from under the car in addition to the side scoops based on all the dust.

 

[Steve Hoelscher]

I have been running my X for several months without the engine cover rain tray. I began to notice dust patterns on top of the engine cover that were never there before. They appear in the area right above where the rain tray was, and not where the two openings are on each side of the engine cover are that don't get exposed with the rain tray removal. I also noticed a bunch of dust and dirt in my previously clean engine compartment. All the factory engine compartment sheet metal is installed.

My guess is that removing the rain tray increased the air flow out of the engine cover by quite a bit. I think there may be quite a bit of air coming in from under the car in addition to the side scoops based on all the dust.

I have run a few X's without the rain tray, including one of my racecars. Yes, the rain tray is the primary restriction on flow. If you want to increase the flow through the engine bay, remove the tray.

Jeff, the big low pressure area behind the rear window is what draws the air through the engine bay. Reducing the low pressure void will reduce the volume of flow. And.... directing more airflow from the side scoops and under the car through the engine bay and out the deck lid will reduce the low pressure void and thus lift over the rear axle.

 

[NEG]

The engine lid lifting up is probably caused by low air pressure on top, just like your front trunk or hood lifting up if not latched.

I'm pretty sure the usual duckbill spoiler is a fashion statement, just like a FAZA snorkel.

If it’s a low pressure lifting it up then there must be a positive pressure under it or it wouldn’t lift up! Think aeroplane wing…the high pressure is always trying to find a way to the low pressure and creates a force on anything in its way.

 

[Dr.Jeff]

I have run a few X's without the rain tray, including one of my racecars. Yes, the rain tray is the primary restriction on flow. If you want to increase the flow through the engine bay, remove the tray.

Jeff, the big low pressure area behind the rear window is what draws the air through the engine bay. Reducing the low pressure void will reduce the volume of flow. And.... directing more airflow from the side scoops and under the car through the engine bay and out the deck lid will reduce the low pressure void and thus lift over the rear axle.

Steve, I neglected to say in my case I'd like to reduce the upward movement of air through the engine bay. Look back at my prior comment: "On my current X project I have dramatically opened up the side scoops, removed the rear bulkhead between the engine and trunk, completely opened the rear taillight panel, and closed off the floor under the engine. The goal is to move the air through the bay and out the tail." I guess what I didn't specify is the air will then move downward through the engine cover. In other words I'm creating the low pressure at the back of the car, and higher (relatively) pressure areas at the side scoops and engine lid. This would be different from the typical X where you'd want as much air to flow upward as possible.

In another thread about my turbo build I explained how the intercooler lays flat across the top of the engine bay (to the driver's side above the transmission). And it has a large Spal fan to move air across it. Initially I thought I'd mount the fan to push the air upward. But with further development it will be better to pull the air downward (for lots of other reasons I won't get into here). So now my goal is to direct as much air downward as possible.

 

[Dr.Jeff]

If it’s a low pressure lifting it up then there must be a positive pressure under it or it wouldn’t lift up! Think aeroplane wing…the high pressure is always trying to find a way to the low pressure and creates a force on anything in its way.

It doesn't need positive pressure under it, just less negative pressure than above it. It's really not about positive or negative, but gradients. In other words the relative pressure differential makes it move up or down. Even no pressure is less than any negative pressure and will make things move.

 

[NEG]

No, sorry that’s not correct. If the pressure is the same both sides it won’t move, you need a differential to create the force required to move it. If you create a negative pressure above it as you say then automatically the pressure below it must be greater.

Edit: re reading your post Inthink we are saying the same thing, it’s a pressure differential.

 

[Alain - The Red Scorpion]

Just saw the post by Dr. Jeff, illustrated with the BMW 3.0 CSL. Thefore same set up as the Stratos. Remains to see if it is any use on the X at higher track speeds. More testing needed. Any volunteers?