Which spoiler works best?

[JMortensen]

As soon as the smooth flowing air separates from the upper surface of an airplane wing.. it stalls... it looses lift...

I think "stall" is the wrong word here. How else would you explain that you can improve take off and landing and slow speed manueverability with the VGs? Unless you think that the wing on just about every aircraft is stalled just as the airspeed is high enough to take off. I think by making the air stick to the top of the wing it make the flaps more efficient and allows the flaps to work better, and that's where your increased lift comes from, but what the VGs really do is reduce drag. It reduces the vacuum created by wings and that makes it faster and easier to fly and makes the controls more responsive because the flaps aren't sitting in a turbulent vacuum, and that vacuum isn't trying to pull the plane backwards.

 

Getting back to the automotive world, did you not agree that the Mitsu paper says that the downforce is increased by adding the VGs? How else do you explain this statement: "Application of the VGs of the optimum shape determined through the aforementioned analyses to the Mitsubishi LANCER EVOLUTION showed a 0.006 in both the drag and lift coefficient" and even better, this: "As a result of the verifications, it is confirmed that VGs create streamwise vortices, the vortices mix higher and lower layers of boundary layer and the mixture causes the flow separation point to shift downstream, consequently separation region is narrowed. From this, we could predict that VGs cause the pressure of the vehicle's entire rear surface to increase therefore decreasing drag, also the velocity around the rear spoiler to increase, and the lift to decrease."

 

"...VGs cause the pressure of the vehicle's entire rear surface to increase therefore decreasing drag" sounds just like how I imagine the plane wing to work. The plane wing doesn't get more lift from the VG's, it actually gets more pressure on the top of the wing when compared to the boundary layer it would have with no VGs. This airflow and pressure can then be used to greater effect by the control surfaces, and THAT can be used to generate more lift.

[JMortensen]

Here is another example. Look at the picture and answer the following question: Where is the pressure? Is there more pressure in the blue box where the "low pressure" air is not separated from the wing and the yarn lies totally flat against the wing, or is there more pressure where the air has separated from the wing in the red box and the yarn is floating around and moving in all directions?

 

[bjhines]

I owuld assume there is more lift on the area in the blue box

[JMortensen]

duplicate

[JMortensen]

At least we've isolated where you are wrong. I think maybe you're confused because you equate fast moving air and low pressure. That's true, but you don't realize that there is no air density in the red area, so it is not slow moving high pressure air. It is a void, or a lack of air, a vacuum. So it's pressure is lower than the low pressure air in the blue box.

 

Take a look at figure 10 in the Mitsu paper. It deals with air VELOCITY. You can see the reduction of the blue boundary separation layer, which is a vacuum, behind the rear window with the VGs installed. So there is NO velocity at the base of the window. The air doesn't go around the sides and top of the vehicle then just suddenly stop in that area. There is no velocity there because there is no air there, because it is in the boundary separation layer.

 

Then check figure 11 in the Mitsu paper, this one shows PRESSURE. You can see side by side that the area at the base of the rear window has higher pressure with the VGs than without. This is because of the smaller boundary separation layer. If you could get the boudary separation layer totally eliminated and get that air to flow directly across the body, the pressure would be higher still.

 

I don't know how else to prove the point, so I'll let someone else take over the argument if they care to.

[Guest Mike]

JON... Judging by your airplane wing example, it seems that the wing(s) should extended several feet to either side of the car to be effective and a centrally located wing aft the fuselage (car body) is ineffective. How does the wing differ as mounted on an airplane (outwardly and to the side) as opposed to where one is mounted on a car (centrally at the rear). In other words, how might the fuselage (car body) affect the wing(s)?

[Guest Mike]

I think we need an aeronautical engineer here:confused:

[Guest Mike]

I believe correct aerodanics should be built into the design of the car body rather than adding wings. Didn't they try that in the 50's?

 

Yeah... I know that formula cars have them but they travel 200MPH.

[JMortensen]

The wing in the picture is stalled. Normally wings don't have airflow like that, and the flow should be relatively equal across the surface of the wing. That is an extreme example, but I used it to point out that the part with the yarn pressed against the wing has some pressure on it, and the part where the yarn isn't pressed against the wing has no pressure. I think you could get much more effective downforce if the wings extended off the side of the car like a plane, but that might make it difficult to drive though, especially at an autox...

 

As far as reasonable modifications go, the body has the effect of putting "dirty air" from the boundary separation layer onto a low mounted wing. That's why we're discussing the VGs, because if they can clean up the airflow and reduce the boundary separation layer then we can make the wing or spoiler more effective. That's what the thread is really about, we've just gotten a bit sidetracked.

[Guest Mike]

"Diry air"... sounds like burrito fuel to me:biggrin: I still think, for street-driven cars anyway, the car body should do the work:wink:

[JMortensen]

Here's something else John. I think your understanding of how a wing works (and mine, prior to some internet reading this afternoon) is fundamentally flawed. Bernoulli's Principle and the idea of the air on top of the wing going faster than the air on bottom due to the shape of the wing does not explain lift in an airplane wing. If it did, how could a plane fly upside down? The long side is on the bottom, so the air on the bottom would flow faster than the air on the top, and the plane would slam into the ground, right???

 

Check these out if you have the time or the interest:

http://www.av8n.com/how/htm/airfoils.html

http://www.aa.washington.edu/faculty/eberhardt/lift.htm

 

After reading this and a bunch of other crap I think the angle of attack of a wing is much more important than anything else as far as producing lift. So again, it could be said that the VG's on an airplane wing make the flaps more effective by reducing the boundary separation layer and providing clean air for the flaps to use, and the wing gets its lift from the following factors, as the second article states:

 

* The amount of air diverted by the wing is proportional to the speed of the wing and the air density.

* The vertical velocity of the diverted air is proportional to the speed of the wing and the angle of attack.

* The lift is proportional to the amount of air diverted times the vertical velocity of the air.

* The power needed for lift is proportional to the lift times the vertical velocity of the air.

[RTz]

Bernoulli's Principle and the idea of the air on top of the wing going faster than the air on bottom due to the shape of the wing does not explain lift in an airplane wing. If it did, how could a plane fly upside down? The long side is on the bottom, so the air on the bottom would flow faster than the air on the top, and the plane would slam into the ground, right???

 

Jon,

 

The missing piece is in the angle of attack... fly the plane inverted and the required angle of attack makes the air's path around the 'top' of the wing longer, just like when upright.

[RTz]

Jon,

 

Found a diagram of the angle of attacks' roll in speeding up the air over the top side of an inverted wing...

 

Go here and look at the second diagram....

 

http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/airfoil.html

[tube80z]

There are a number of theories that also argue that momentum has a rather large effect in explaining how airplanes fly and spoilers work. Aero is an area of interest to me but I don't know enough to have an informed opinion.

 

I can and do have data that tells me a spoiler is worth a large chunk of time even at autox speeds. And that's all I'm sharing

 

Cary

[JMortensen]

Heh, I was wondering when you Oregon flyboys were going to jump in...

 

My point is that the angle of the attack is important in producing the lift, not the shape of the wing. Maybe you were just backing me up, not sure. Also, the 2nd picture on that page show how relatively small the amount of lift coming from the tail end of the wing is, which again supports the idea that the purpose of the VGs make the flaps work better.

[RTz]

Heh, I was wondering when you Oregon flyboys were going to jump in...

 

 

My point is that the angle of the attack is important in producing the lift, not the shape of the wing.

 

While I agree with you that AOA is paramount, I'm not sure I catch your drift regarding wing shape. Its generally taught that a very small amount of ice build-up on the leading edge can reduce lift by 30% and increase drag similarly, due to the change in shape. Light frost is to be taken seriously as well as it causes early separation. Admittedly, I have only read bits of this thread so I may be off base with what you're getting at.

 

Maybe you were just backing me up, not sure. Also, the 2nd picture on that page show how relatively small the amount of lift coming from the tail end of the wing is, which again supports the idea that the purpose of the VGs make the flaps work better.

 

VG's aren't part of the curriculum, but I will go so far as to say that the purpose of VG's, as I understand it, are to 'delay' separation. I need to look into this further.

[JMortensen]

While I agree with you that AOA is paramount, I'm not sure I catch your drift regarding wing shape. Its generally taught that a very small amount of ice build-up on the leading edge can reduce lift by 30% and increase drag similarly, due to the change in shape. Light frost is to be taken seriously as well as it causes early separation. Admittedly, I have only read bits of this thread so I may be off base with what you're getting at.

I'm not sure what I'm getting at either, really... The point is that bjhines seems to be making his argument largely based on the apparently false assumptions we were taught about wings in schools; the air on top and bottom have to meet at the back of the wing at the same time, and the air on top needs to speed up to get there at the same time. The air on top does speed up, but the individual bits of air don't get to the other end of the wing at the same time.

 

The upside down thing was just an example, but the idea is that the shape of the wing doesn't create the lift, the angle of attack does. There is the upside down example to prove this point, also the balsa wood glider which produces lift despite having totally flat wings, and the stunt plane which uses wings that are the same shape top and bottom. The curved shape on top helps reduce the boundary separation layer, and the VG helps prevent that separation. At least according to what I read today.

 

There is a good section on frost and its effect on the wings on the first link, section 3.13. Basically it says that you can have ice on the wings and still fly as long as the ice is smooth. It is the rough surface at the leading edge of the wing that causes separation which slows and separates the airflow over the top of the wing, creating drag and reducing lift especially at high angles of attack.

[RTz]

There is a good section on frost and its effect on the wings on the first link, section 3.13. Basically it says that you can have ice on the wings and still fly as long as the ice is smooth. It is the rough surface at the leading edge of the wing that causes separation which slows and separates the airflow over the top of the wing, creating drag and reducing lift especially at high angles of attack.

 

I'm aware of the reg. that prohibits flight with frosted wings. I'm not aware of the 'polished frost' reg. Now I've got to look it up It does make sense though. The weight component of frost is not the issue, nor is the shape change. Just simply the roughness, like you said.

 

3.13 is purely about frost. Ice is a different animal. All of the texbooks I have read (FAA endorsed) say the same thing... ice can/does change the shape (and adds weight). It most certainly is not legal to depart with ice on the wing of a GA plane, no matter how smooth.

[RTz]

Interesting description of VG's from Wikipedia...

 

"The purpose of the generators is to stick out of the stagnant air near the surface of the wing, and into the freely moving air outside the boundary layer. This layer is typically quite thin, but dramatically reduces speed of the airflow towards the rear of the wing. The generators mix the free stream with the stagnant air to get it moving again, providing considerably more airflow at the rear of the wing and thereby providing the control surfaces with more power. This process is typically referred to as re-energizing the boundary layer. Vortex generators increase drag and delay separation and stall effects. They also improve the effectiveness of control surfaces (e.g Embraer 170 and Symphony SA-160) and, for swept-wing transonic designs, alleviate potential shock-stall problems (e.g. Harrier, Blackburn Buccaneer, Gloster Javelin)."

[RTz]

I can and do have data that tells me a spoiler is worth a large chunk of time even at autox speeds. And that's all I'm sharing

 

A-mod is proof of that. I was at Bremerton a few years back for a National Tour. One of the multi-time champs was there (sorry, can't remember his name). Unbelievably fast.