Vortex Generators

[MJEPilot]

What are vortex generators for? I overheard a conversation that a couple pilots were having the other day at my flight school about vortex generators. I have read somewhat about them in books and on the internet, but I am still a bit confused. Thanks.

 

[Cornelius]

VGs are primarily always upstream of a control surface that you want high energy air over (ailerons, rudder, and so on). People argue whether they are used primarily for low speed flight to delay boundary layer separation due to critical angle of attack, or whether they are used primarily for high speed flight where you can have boundary layer separation due to a shock wave forming on top of the wing midchord. Either way, it increases the energy of the boundary layer helping it stay contoured and attached to the surfaces.

Just remember if you go out to your aircraft and see a VG missing on one wing but not the other. Go ahead and break the other VG off so the two wings have the same number of VGs. I'm just kidding about this last statement, I thought it would be funny.

 

[avbug]

Vortex generators serve very specific functions, and there shouldn't be any arguement as to their effect, or purpose. In some aircraft the purpose is to affect high speed handling characteristics, and in others, low speed characteristics. They perform both functions.

VG's are used to modify the behavior of airflow near the surface of a wing or stabilizer.

On light aircraft, generally VG's are used to effect greater aileron or rudder authority at low speeds, increasing the utility of the airplane, and increasing controllability at the extreme low end of the flying spectrum. VG's may also be used to introduce airflow farther back along the chordline of the wing during high angle of attack operations (slow flight), permitting the airplane to fly slower. In some respects, this performs the equivilent of an aerodynamic poor man's slotted wing.

VG's are found on small and large aircraft, low altitude and low speed aircraft, and high-altitude, high speed aircraft. the specific function of the VG's varies with the application. You can usually recognize them as a series of raised fins on the leading edge of the wing, but may also be found on the horizontal or vertical stabilizers. They may also be found in the form of attachments to the wing which are less recognizable. Sometimes these are known as "boundary layer energizers," or BLE's.

Other devices such as stall strips are also used along the leading edge of airfoils to modify the local airflow to produce specific flight characteristics.

 

[TurboS7]

You see VG's on a "straight wing" Lear 24 or 25, also you will see them on modified PA-31, and C-400 series aircraft. With the 400 and Navajo they increase gross weight and reduce VMC. With the Lear they let it fly like a bandit at 450 and mach.81.

 

[bigD]

Turbo - how do VG's reduce Vmc? I'm not saying they don't, I'm just curious what the reason is...the only thing I could think of is putting some on the vertical stabilizer to maybe increase rudder effectivess?

 

[DC9stick]

Dig D, You are correct the VG's on the verticle stab keep the airflow attached to a lower speed thereby giving the rudder more authority at low speed. I've been flying a 310N with VG's and the flight manual supplement shows a reduction of 3-5 kt in all the critical speeds. They are also supposed to make it easier to land but I still thunk it on.

 

[atpcliff]

Hi!

On the aircraft I was supposed to be flying (J41-I'm furloughed) we have VGs. There were rules governing the number of missing VGs we could have and still fly.

There were a certain number that could be missing total, a certain number per group (we have VGs near the wing root, and near the tip), and we couldn't have 2 missing next to each other.

There's a lot to learn about in flying!

Cliff

 

[TurboS7]

How they do it? I don't know maybe someone with a lot more aerodynamic savvy can fill me in also. I was going to buy the mod for our PA-31 so I was looking at all the "goodies", I was interested in the increased gross weight but it was interesting that they also claimed to reduce VMC. I also forgot to mention the VG's on the stab of the 727, American designed them so their eagle could climb to the top of the perch to rest.

 

[avbug]

The VG's don't increase rudder effectiveness at low speeds, so much as they increase the effectiveness at high sideslip angles. It's the angle that is the critical difference, rather than the speed. As indicated before, vortex generators serve to sever the seperation effect at high angles of attack, for a given airfoil, and the ensuing vortex returns some of the boundary layer energy to the boundary region, farther down the chordline of the airfoil. This is important at high angles, when the airflow separation would otherwise rob a control surface of it's effectiveness, or airflow separation would lead to a stall. Interruption of the airflow separation delays this effect. In the cockpit, we see it as the ability to fly slower with the same control.