Vortex Generators

[Vev]

Geoff,

 

 

Thanks for your contribution.

 

 

Can you tell us typically what has been your long experience in fitting VGs in terms of performance improvements and are there any down sides?

 

 

Regards

 

Jack

 

 

[Student Pilot]

Actually the underside of the elevator is the place to put them for maximum effectiveness. On my CH701 I have them placed right at the bottom when the elevator is at full up deflection.

Doesn't the 701 have a full flying tailplane? Normally the generators go onto the stabilizer not the elevator. There are lots of people saying how much they improve things, the only actual tested results (as against "Jimmy says they lower the stal speed 30 knots" type stuff) I have seen say they only improve the stall speed 1 knot if at all. The difference they make is to the feel at lower speeds, a more solid less wallowy feel. So you can fly closer to the actual stall speed thereby giving a shorter TO and landing distance. With the Chiefton kit it ups the gross by a bit and lowers the blue line speed.

 

 

[Vev]

Hi Student Pilot,

 

 

It is sometime very hard to sort between perception/impression of performance and scientific fact. However I think there is a lot of evidence out there to support most of the claims made by manufactures, albeit generic and non-specific information associated with a particular aircraft.

 

I guess the good thing about this forum is it allows one to test assumptions against actual experience, of which many contributors have a great deal.

 

 

Here are a couple of interesting things to read re VGs

 

 

http://www.nasa.gov/centers/langley/news/factsheets/Micro-VG.html

 

 

http://www.flxsys.com/Applications/Active%20Flow%20Control/Vortex%20Generator/

 

 

http://www.avweb.com/news/reviews/182564-1.html

 

 

Regards

 

Jack

 

 

 

[Student Pilot]

Yep, read those links. The NASA stuff had some research but not real world figures. Apart from the manufactures claims there are no actual before and after figures. I have seen some done extensively with GPS and calibrated instruments rather than just looking at the airspeed and saying "That's about 5 knots". The actual real world decrease in stall speed was in the 1% region, as I said the advantage was it feels more solid closer to the stall so people can fly slower more safely.

 

 

[Walter Klatt]

Hi Walter… Many thanks for your input.It sounds like you must have stuck the VG’s on with some sort of double sided tape to try out the optimum position? If this is the case, what did you use and did you loose any of them in the process?

 

Still keen to hear from someone with a Jab that has used VGs ???

 

Cheers

 

Jack

Yes, I tried them first with the double sided thin carpet tape, and then glued them on with Loctite 401.

Walter

 

 

[vk3auu]

Just a comment about rate of climb with VG's fitted.

 

If for instance, the stall speed has been reduced from say 40 knots to 35 knots, then the safe approach speed which in my case is roughly equal to the best climb speed will be reduced by about 6.5 knots. (I said roughly) That will mean that the best rate of climb speed should drop from 52 to 45.5 knots. That will mean a drag reduction of about 30 percent which will mean some more power available to climb, if you adopt the lower speed.

 

As to the best place to put them, forget about percentages, it depends on the wing upper surface profile and should approximately be right on the top when the wing is just starting to stall which will generally be at around 14 degrees depending on the washout.

 

Finally, the CH701 has an all flying rudder but the elevator is separate from the stabiliser, although it has the unusual upside down profile.

 

David

 

 

[facthunter]

Rate of climb.

 

Dunno if it is that simple. while the current approach speed is approx. the same as the best ROC speed, the reconfigured wing may change all that. I do not agree with the reduction in drag as a simple function of reduced stall speed. Your ability to climb is a result of surplus energy available when the thrust is sufficient to cover the drag and then some. You can either accelerate the aircraft (gain kinetic energy), or you can climb and have it as potential energy.(energy of position), ie. extra height. The maximum ROC will be obtained at minimum drag speed for your weight. This is the speed when your whole aircraft achieves it's best L/D ratio. Nev..

 

 

[Vev]

Just let you know I tried fitting the Land Shorter VG’s to my J160 and thought I would provide some feedback.

 

 

Firstly, I fitted them as per the suggested instruction both on the wing and the underside of the horizontal tail… the following was observed:

 

 

Pros

 

• Improved stall by about 5 knots
   
   
  
• It gave a more positive feel at slower IAS
   
   
  
• Gave more elevator authority at lower speeds
   
   
  
• Shorter take off
   
   
  

 

Cons

 

• Reduced cruise speed by 4-5 knots for the same power setting
   
   
  
• Reduce max S&L speed by 5 – 6 knots
   
   
  
• No real detectable improvement in climb
   
   
  
• Had an negitive impact on normal trim settings and increased drag
   
   
  

 

 

I then tried various adjustments to the placement of the VG, within the limitations of the instructions, which made some small differences to the stall characteristics but still caused an unacceptable loss of speed. I reduced the amount of VG’s and then eventually I removed them from the horizontal stabiliser as there was a noticeable increase in forward trim with the VG's fitted to maintain S&L flight.. I presume the improved air flow efficiency over the horizontal stab needed to be off-set by trim which was causing drag… the removal of the VG off the horizontal stab did improve the cruise by 2-3 knots.

 

 

In the end I went through the process of increasing the spacing of the VG’s on the wing and then eventually removed them altogether as I could not find a spot or spacing combination to recover my speed loss.

 

 

On a more esthetical note, the VG’s don’t do very much for the nice clean lines of the Jab smooth wing and look somewhat ugly…. Which was a shame to spoil such a clean looking a’craft?

 

 

In my view, (after many hours) the Land Short VG’s do improve stall as claimed but at a significant cost to speed and economy on the J160. I think these VG’s are only useful if you have a requirement to get in and out of a short runway regularly and don’t do a lot of cruising and fuel economy isn’t a big factor in your flying consideration.

 

 

Hope this is of use?

 

 

Regards

 

Jack

 

 

[Walter Klatt]

VGs and cruise speed

 

Just let you know I tried fitting the Land Shorter VG’s to my J160 and thought I would provide some feedback.

 

• Reduced cruise speed by 4-5 knots for the same power setting
   
   
  
• Reduce max S&L speed by 5 – 6 knots
   
   
  

 

the removal of the VG off the horizontal stab did improve the cruise by 2-3 knots.

 

Regards

 

Jack

This is very interestesting and is the first time I have heard of a significant change in speeds with using VGs. While I am not familiar with the J160 cruise speed or wing airfoil (am in Canada), I have used VGs in two of my airplanes and found no detectable difference in cruise speeds.

 

Can you give us more info on exactly how you measured your cruise speeds and corresponding power settings?

 

One of my planes is a Murphy Rebel amphib which cruises at 120 mph and I did extensive tests with and without the VGs to determine the difference. While my stall was reduced by 5+ knots, I could not detect any difference in cruise speeds.

 

I used fuel flow (have a fuel flow monitor) and RPM at the same airspeed (and weight) for my tests. I know this is very accurate for my airplane as fuel flow will change immediately with any changes in weight or flaperon settings.

 

For example, there is a .5 gph reduction in fuel flow from neutral to the reflex flaperon position to maintain the same airspeed. Conversely, to maintain the same fuel flow, there is a 5 knot difference in airspeeds between the neutral and reflex positions.

 

So any drag or cruise speed differences in my airplane are easily detectable. But I could not find any such differences with the VGs on or off.

 

Maybe it has to do with the airfoils. The Rebel has a modified 4415 with a 30 ft span. I do not use the VGs on the tail, just the wings.

 

Walter

 

 

[Yenn]

Sounds as if they may be useful on J230, as the pilots I know seem to have trouble landing them short.

 

 

[Vev]

Hi Walter,

 

 

My cruise speed was very simply measured by setting the same RPM that I normally cruise at (2900 – 3000 rpm) @ 15.8 lt per hour (I have a flow meter too) which gives me around 104 – 105 knots in solo configuration with 60-80lt of gas in the wings. I tend to use the fuel consumption rate as more of an indicator for cruise configuration as this is a bit of a sweet spot for CHT management. With the VG’s fitted at the same power settings and weight as above I was pulled back to 100 – 101.

 

 

After I removed the VG’s off the tail plane I recovered most of my cruise at 103 – 104 knots, again with the same power/weight settings, but I had still lost 1-2 knots.

 

 

I was of the view that the VG’s improved the air flow over the tail plane (which is an inverted wing) and was requiring more forward trim to compensate for this effect causing drag. Once I removed the VG from the tail plane my trim went back to normal settings and the speed went up.

 

 

Interestingly too, I find when I am fully loaded close to MTOW I can added a small amount of flap 2 or 3deg and my cruise will jump up around 2-3knots as well, however this doesn’t work in solo configuration … I presume when adding flap it lifts the tail slightly and reduces the drag. The guys at Jabiru gave me this tip at a fly-in last year…. I’ve also heard others confirm the same experience too.

 

 

Regards

 

Jack

 

 

[Walter Klatt]

I guess it all depends on the wing. I looked the Jab site, and see that the 160 has a pretty small wing compared to the Rebel. Interesting too, that you actually gain some speed with slight flap. The Rebel is the opposite. It gains speed with reflex.

 

Well, that's the nice thing with these VGs. With that double sided carpet tape, you test it first, not only for location on the wing, but if you don't like the performance, pull them off.

 

My other plane (912 Chinook) didn't benefit much from the VGs, only about 2 kt stall reduction, but again no change in cruise speeds.

 

The only reason I wanted the VGs on the Rebel, was to improve water take-offs, and they do that, so am happy. Otherwise wouldn't bother. Stall speed was good enough before, for my type of flying. When on floats, you really can't utilize a low stall speed on landing, because the angle of attack is too severe (would drag the float tails).

 

Walter