Vortex Generators

[aviatoreb]

Fair enough question. Aerodynamics (to a lesser extent physics) is the answer, but, at the speeds we're traveling (compressibility of air type speeds), probably is fairly irrelevant.

Two types of flow, laminar and turbulent.

As the researchers explain, an object's boundary layer starts out as laminar, or smooth and orderly. As the object continues to fly through the air, small disturbances create instabilities and, above a critical value, the laminar flow regime transitions to a turbulent one. The transition can easily result in an order of magnitude increase in skin-friction (part of parasitic) drag on an aircraft.

Parasitic drag is inversely preportional to induced drag.

The thing is, turbulent flow adheres to a wing much longer (and better) than laminar flow... Which is why VG's drastically increase high AOA performance and slow speed maneuvering... But the price you pay is an increase in skin friction drag across the wing. Particularly when we're talking about a "laminar flow wing" design. But really it's probably only 2-3 KTAS in practice at our speeds. Not really significant for every day usage. But that's part of the reason why you don't see VG's on fighter jets.

Edit: those numbers IRT mooneys are just a guess on my part based on the VG size and position.

Double edit: after trolling a "c-word" brand .org, it seems the "expert consensus" says 1-2 KTAS loss of top speed for a C-140 or C-172. Cleaning the wings seems to be of more concern for those guys, though!

Triple edit: The TKS install on my missile certainly doesn't let it hit book numbers.., but last weekend I was flying down in the lowlands (Seattle), and had to depart ifr due to cloud layers- they (atc) held me down at 4k for what seemed like FOREVER- I had let the missile stay making full power expecting the follow on climb that didn't come for 20 miles... Anyway, ROP, 2650RPM at 4K I was seeing 182KTAS. Sweet!!!! Fuel burn? 20GPH ish.... Not so sweet, I should have leaned it out to 16ish or so, but I was expecting that climb.... Have I mentioned how much better I like VFR flying?

 

I respect that you are saying all the right things - but aerodynamics are strange - this is why they do wind tunnel testing, and now a-days, CFD computations before that, and all that often before they ever build something to fly on an airplane - because the best engineers are often surprised, not necessarily entirely but at least in magnitudes and even signs of effects.  I would not believe any guesstimates of magnitude of change or even sign of change (improve, hurt or neither).

 

I claim you simply cannot guesstimate your way to the right answer on this.  The claim is that many see no speed difference and I believe it is plausible - but who knows if it is true - some flight test engineers I am sure - but we wouldn't believe them as advertising self interested sales department stand between us and them.

 

The physical claim is that the VGs sit below the boundary layer when the airplane is fast as the boundary layer is thicker, and sticks out in the actual flow above the boundary layer which is not so deep when the airplane is slow.

[carusoam]

Erik,

Do you have any recent rainy day flying experience?

I recall seeing how large (area) of an effect the speed brakes have from a video posted on line...

If the VGs have no effect at cruise, you would not see anything while flying...

It's been years since I flew in rain. I don't recall how visible rain is on the wings....

Does TKS fluid get colored for improved visibility of it being distributed...? (Flow tracing experiments)

Best regards,

-a-

[khedrei]

On 4/18/2015 at 1:06 AM, LANCECASPER said:

The most effective thing that VG's do is remove $1500-$2000 from your wallet which decreases your sink rate on short final.

ROFL

[aviatoreb]

On 4/19/2015 at 1:15 PM, carusoam said:

Erik,

 

Do you have any recent rainy day flying experience?

 

I recall seeing how large (area) of an effect the speed brakes have from a video posted on line...

 

If the VGs have no effect at cruise, you would not see anything while flying...

 

It's been years since I flew in rain. I don't recall how visible rain is on the wings....

 

Does TKS fluid get colored for improved visibility of it being distributed...? (Flow tracing experiments)

 

Best regards,

 

-a-

Hah - I see this question right to me Anthony from 5 years ago.

Rainy day flying - some - I have noticed nothing.

TKS - seems invisible and looks like just wet stuff on the wing.

I had a big bug once stand on my wing through part of a flight - clearly enjoying a boundary layer.

[steingar]

I had VGs on my Cherokee, they were really cool.  Couldn't stall that thing, and it was perfectly controllable at any speed.  I could land anything in that Cherokee, it just didn't matter.  Then again, I landed a 2200 foot strip in my stock Mooney just the other day with tons of room to spare. 

[firelog1101]

15 hours ago, khedrei said:

ROFL

I think he ended up putting them on his ovation?

[LANCECASPER]

On 7/5/2020 at 10:40 PM, khedrei said:

ROFL

How things change in 5 years. I moved to an airpark to live with a higher elevation and shorter runway. I did a lot of research and talked to pilot's who have installed the micro vg's and I have to eat crow and say i was wrong. They do make a difference. I've had them on my Bravo over a year and they allow me to come in at a slower speed and still have full authority. I still am not thrilled with the looks on the laminar flow wing but I like what they do.

[khedrei]

How much do they affect your cruise speed.  I have a hard time believing they dont slow you down. Nothing is free. 

[PTK]

59 minutes ago, khedrei said:

How much do they affect your cruise speed.  I have a hard time believing they dont slow you down. Nothing is free. 

In cruise they do nothing but add drag which for a given angle of attack reduces the aerodynamic efficiency of the wing. Putting them on a beautiful Mooney wing is stupid imo. 

[Mcstealth]

11 hours ago, LANCECASPER said:

How things change in 5 years. I moved to an airpark to live with a higher elevation and shorter runway. I did a lot of research and talked to pilot's who have installed the micro vg's and I have to eat crow and say i was wrong. They do make a difference. I've had them on my Bravo over a year and they allow me to come in at a slower speed and still have full authority. I still am not thrilled with the looks on the laminar flow wing but I like what they do.

Was the install before or after the Bravo conversion? Do you have an apples to apples comparison real world numbers?

[LANCECASPER]

2 hours ago, khedrei said:

How much do they affect your cruise speed.  I have a hard time believing they dont slow you down. Nothing is free. 

I was told before I installed them that it wouldn't affect cruise, but didn't really believe them. I was expecting a 2 knot loss and willing to live with that, but I haven't experienced a loss. On a light wind day I feel comfortable coming over the fence at 65 knots now, so for me it was well worth it. 

 

More info from the developer: 

 

[exM20K]

Just now, LANCECASPER said:

I was told before I installed them that it wouldn't affect cruise, but didn't really believe them. I was expecting a 2 knot loss and willing to live with that, but I haven't experienced a loss. On a light wind day I feel comfortable coming over the fence at 65 knots now, so for me it was well worth it. 

 

You may have just cost me $1600.  Thanks. :-)

-dan

[LANCECASPER]

6 minutes ago, Mcstealth said:

Was the install before or after the Bravo conversion? 

After the Bravo conversion.

[LANCECASPER]

Just now, exM20K said:

You may have just cost me $1600.  Thanks. :-)

-dan

Tell them you were planning on buying them at Oshkosh and see if they'll give you the show special.

[ArtVandelay]

I was told before I installed them that it wouldn't affect cruise, but didn't really believe them. I was expecting a 2 knot loss and willing to live with that, but I haven't experienced a loss. On a light wind day I feel comfortable coming over the fence at 65 knots now, so for me it was well worth it. 
 
More info from the developer: 
 

VGs disturb the air flow, this requires energy, the energy has to come from somewhere unless you are breaking the laws of physics. There is a Noble prize and trillions of dollars waiting for the person who can create energy out of thin air.

[EricJ]

Just now, ArtVandelay said:

VGs disturb the air flow, this requires energy, the energy has to come from somewhere unless you are breaking the laws of physics. There is a Noble prize and trillions of dollars waiting for the person who can create energy out of thin air.

They definitely use up some energy, but whether it is enough to be noticeable or not is a question.    The Lear 23 in our school hangar had these on it from the factory.   

 

[steingar]

Just now, ArtVandelay said:

VGs disturb the air flow, this requires energy, the energy has to come from somewhere unless you are breaking the laws of physics. There is a Noble prize and trillions of dollars waiting for the person who can create energy out of thin air.

Says you.   Science is often far more complex than can be easily comprehended by the layman.  My Cherokee was one of the fastest ones I've seen, had a problem leading a formation of Cherokees because I'd run away from them.  Won't claim to understand the aerodynamics, ain't my biz.  

[aviatoreb]

3 minutes ago, ArtVandelay said:

VGs disturb the air flow, this requires energy, the energy has to come from somewhere unless you are breaking the laws of physics. There is a Noble prize and trillions of dollars waiting for the person who can create energy out of thin air.

How are VGs creating energy?

So goes the theory - the thickness of the boundary layer depends on speed.  At low speed the boundary layer is thin and the VGs stick out into the wind and change the flow over the wing.  At high speed the thick boundary layer hides the VGs in the relatively still air closer to the wing, so no change in friction if they are sitting in mostly stagnant air.  They only disturb the airflow at low speed but not much to measure at high speed.

The proof is in the experiment.  Do they create a speed loss or not.  Unfortunately I have never measured before and after so I cannot say.

I can say that once I notice a big insect on my wing at the hold short line during run up.  SO then I departed and I was very impressed as the nice insect seemed not worried about sitting on a wing while I flew at 150mph.  But he was walking around and at some point caught into some moving air and he disappeared instantly.

 

[mike_elliott]

1 hour ago, exM20K said:

You may have just cost me $1600.  Thanks. :-)

-dan

The V I stalled that had them stalled at 52kts about 200 under gross, Dan.

[carusoam]

1) Then we need to look at their effects at the altitude we cruise at...

2) a lot seems to get lost in the science of where flow is and isn’t occurring...

3) How long the air stays attached to the wing...

4) How thick the boundary layer is... and how far back it exists...

5) And how a nice, clean, waxy surface can have less drag....

6) Or a simple frost covered surface has terrible lift challenges...

7) The extra few knots of stall speed change really makes extra sense for short runways....

 

Oddities of flow studies... hard to get really good data... 
 

PP summary of some of the variables involved...

Best regards,

-a-

[exM20K]

1 hour ago, mike_elliott said:

The V I stalled that had them stalled at 52kts about 200 under gross, Dan.

You're not helping, Mike.

In addition to better short field landing performance (approx 50% of my flights) lowering the Vso by 6 KIAS (10-ish %) offers a pretty impressive (17% ish) energy reduction in a forced landing.

-dan

[LANCECASPER]

17 hours ago, exM20K said:

You're not helping, Mike.

In addition to better short field landing performance (approx 50% of my flights) lowering the Vso by 6 KIAS (10-ish %) offers a pretty impressive (17% ish) energy reduction in a forced landing.

-dan

My main motivation was if I lose my engine on take-off out where I live. Being able to put it down a few knots slower without stalling might be the critical difference. 

[LANCECASPER]

20 hours ago, ArtVandelay said:

VGs disturb the air flow, this requires energy, the energy has to come from somewhere unless you are breaking the laws of physics. There is a Noble prize and trillions of dollars waiting for the person who can create energy out of thin air.

When Piper was working with Micro VG on the gross weight increase for the Meridian In 2001, Piper engineers found that even at cruise the air is so well directed with the VGs that the increased efficiency of the airflow mostly negated the parasitic drag of the VGs. I flew the Meridian before and after the VG installation and the slow speed handling was noticeably better - well worth it even if there was a couple knots lost in cruise. If I get to my destination a couple minutes later I’m still alive. Usually if I stall/spin the airplane I’m dead.

[aviatoreb]

4 minutes ago, LANCECASPER said:

My main motivation was if I lose my engine on take-off out where I live. Being able to put it down a few knots slower without stalling might be the critical difference. 

That was exactly the thought that was on my mind when I had VGs installed on my Mooney within weeks of purchase over ten years ago.  As a safety feature for just this scenario.  That we all hope not to think too much about.

[firelog1101]

It sounds like if there is a decrease it's <2 knots.  I've searched around the forum and haven't heard anyone who put them on saying they have seen even a 5 knot difference.  I could see <2knots being a pretty reasonable trade off for better slow flight characteristics.