VG Gurus / Aeronautical Engineers

[N395V]

I know VGs have been discussed many times here and the consensus is they are of little value relative to RVs. My purpose for this thread is education and information.

I understand how they work and what they supposedly do. I understand when you put them on the wing you also put them on the underside of the horizontal stabilizer so as not to have the elevator stall before the wing. Is this correct?

Also many VG kits claim you can increase your aft CG range by installing them. If this is so should they not be installed on the upper surface of the horizontal stab?
In most installations I also see them installed on the vertical stab, is this to enhance spin recovery?

Since one of their purposes is to decrease stall speed would benefit be incurred by putting VGs on your flaps?

[gpiney]

VG's are put on the underside of the HS to improve elevator authority.
For everything you want to know about VG's, contact Art at http://www.vortexgenerator.net

[plehrke]

Originally Posted by N395V

I understand when you put them on the wing you also put them on the underside of the horizontal stabilizer so as not to have the elevator stall before the wing. Is this correct?
Also many VG kits claim you can increase your aft CG range by installing them. If this is so should they not be installed on the upper surface of the horizontal stab?
In most installations I also see them installed on the vertical stab, is this to enhance spin recovery?
Since one of their purposes is to decrease stall speed would benefit be incurred by putting VGs on your flaps?

The answer to all your questions are MAYBE.
VG are very configuration specific. Any general statements, like you are asking, need to be tested on your specific application/airplane. They may or may not work from one airplane model to the next. They also may or may not work by moving them slightly, making them taller, or making them longer. The only way to make claims they work is to have wind tunnel or flight tested for your exact configuration. (note: Wind tunnel test must be large scale) I would ask for the data before buying or ask to flight test them for him and buy them if they work. Anybody that says any different is just trying to sell you something like the snake-oil salesman of the 1800s.
I have seen and tested lots of stuff in the wind tunnel and am always amazed and what will work great on one airplane and makes things worse on a different one. Fairings and wing tips are good examples.

[noelf]

...in the 1999 1st edition of the RVator and this article is also in the 27 Years of the RVator. Also, the current issue of Kitplanes (Oct '09) has an article by Barnaby Wainfan on the uses of VG's. And there is also a small writeup on VG's in the soft cover book "The Illustrated Guide to Aerodynamics".

Hope that helps...

[cguarino]

I read an article that compared the drag of -4 style gear leg and the -8 style. The drag on the -8 was tremendously less than the -4 and the article contributed this to he intersection between the gear leg fairing and the wheel pant. On the -4 this area sort of makes a "V". Could VGs help in this area? If so where would you put them?

[Ironflight]

Originally Posted by cguarino

I read an article that compared the drag of -4 style gear leg and the -8 style. The drag on the -8 was tremendously less than the -4 and the article contributed this to he intersection between the gear leg fairing and the wheel pant. On the -4 this area sort of makes a "V". Could VGs help in this area? If so where would you put them?

Now THAT's an interesting idea! I have no idea where you'd put them exactly - probably need to do some tuft testing and have a video camera record what you get. The 737 has some little VG's on the inboard, forward end of the engine nacelles (near the wing) - I wonder if they are there for drag reduction purposes? If you can keep the boundary layer attached, it is a good thing!

Paul

[mike newall]

Re - 737's etc

http://www.aerospaceweb.org/question...cs/q0255.shtml

They also have smaller ones further back, I guess these are for pylon wing drag reduction.

[pierre smith]

Originally Posted by cguarino

I read an article that compared the drag of -4 style gear leg and the -8 style. The drag on the -8 was tremendously less than the -4 and the article contributed this to he intersection between the gear leg fairing and the wheel pant. On the -4 this area sort of makes a "V". Could VGs help in this area? If so where would you put them?

...because the -4 in my hangar has Fairings-etc intersection fairings BECAUSE of that "Vee". We dabbed oil dots in front of the vee and went flying. I can assure you, there's no turbulence on ours and doesn't need VG's.

Besides, veegees on wings just make the wash rag hang up, add weight and reduce cruise speed

Regards,

[Bill Wightman]

VG's work by mixing air deep inside the boundary layer (moving slower; having less momentum energy) with air further away from the surface that has higher energy/velocity.

By mixing higher energy flow into the boundary layer velocity profile, the result is the flow then has more momentum energy to work against an adverse pressure gradient, such as is found on engine nacelles, high curvature areas on the aft end of a fuselage, or over a wing operating at high AOA.

The "payment" for producing the vortices is drag caused by the VG's extracting energy from the flow. Its just a big parasite drag hit. There are times where taking the drag hit from adding VG's is better than living with separated flow, but that's not the case for us in the RV aircraft (however, the idea of using some boundary layer control over the wheel pants is interesting!)

[cropdusterengineer]

Originally Posted by pierre smith

...because the -4 in my hangar has Fairings-etc intersection fairings BECAUSE of that "Vee". We dabbed oil dots in front of the vee and went flying. I can assure you, there's no turbulence on ours and doesn't need VG's.

Besides, veegees on wings just make the wash rag hang up, add weight and reduce cruise speed

Regards,

Pierre, Reading your post reminds me of a good Air Tractor story. A customer had installed VGs on his older model Air Tractor, and called Mr. Snow to ask why the factory didn't install VGs on all new airplanes. Mr Snow said "if you want to go slower, just buy an Ag Cat!"

CDE