Tuesday, February 12, 2008

Why Vortex Generators?

N252Q's project has gotten quite a bit of attention while she's been at Tejas. Folks from Mooney Corp. have taken some pictures of her, some of my blogging friends have visited her and she's even created quite a buzz on the field & in the shop. Of course, I suspected that she would get some attention. 252Q is being transformed into one of the best, if not the best, 252's in the world after all.

However, I was not expecting this... 252Q's Makeover has caught the eye of Flying Magazine. Not because of her fabulous paint scheme or Technically Advanced Avionics, but the Vortex generators we recently installed. It turns out that Flying Mag has been looking for a good example of a Mooney that has had Vortex generators retrofitted for quite some time. They want to test fly one and write about their experience. If all goes as planned, 252Q's maiden voyage may actually be by the editor of Flying Magazine! How cool is that?

After speaking with the folks prepping for the article, I was surprised to hear that not many Mooney owners have installed VG's. I thought it would be good write a bit about the benefits of vortex generators and walk you thought the process I went through when I decided to install them on 252Q.

What is a Vortex Generator?
NOTE: Portions of the following description and photos were taken from the Micro Aerodynamics website:

Vortex generators are small metal blades placed in a spanwise line aft of the leading edge of the wing and oftentimes the horizontal and vertical stabilizers. They are designed to control airflow over the surface of the airfoil by creating vortices (spinning air) that energize the boundary layer (area between the airfoil surface and moving air). This results in improved performance and control authority at low airspeeds and high angles of attack.

When several of these small "wings" are applied to an airfoil in a very specific pattern and location they create tiny vortices in the airstream that are spun downward toward the surface of the airfoil. These vortex energize the normally stagnant boundary layer of air on the wing's surface. An energized (active) boundary layer is more resistant to flow separation than a stagnant boundary layer. The result is that airflow "sticks" to the wing better, permitting flight at lower airspeeds with improved control authority.

Why do people install Vortex Generators?
There are two camps of people that install VG's. One camp is the bush pilot or someone that needs to greatly improve performance so they can get in and out of short fields (STOL). The second group are the ones that want to enhance safety by enhancing the aircraft handling characteristics.

In more specific terms, the key reasons someone would install VG's are to:
  • Decrease Lift Off Speed,
    ** You can get out of a much shorter runway at max gross weight if you can takeoff sooner.
  • Lower Stall Speeds,
    ** You can slow down and fit in behind a 152 on a long slow final without having to go around.
  • Improve Controllability at all airspeeds,
    ** A VG equipped airplane will handle the same, or better, at slow speeds as they do in cruse.
  • Improve Handling Characteristics,
    ** It takes less control input to counteract P-Factor and crosswinds.
  • And, VG's Improve Safety,
    ** You can still control roll and direction even if the airplane is in a stall.
Why don't more Mooney's fly with VG's?
My guesstimate is that some of the reluctance to install VG's stem from being nostalgic and thinking that if Al Mooney didn't install them then I don't need them! They did everything they could to improve performance afterall! That was certainly true when our beloved Mooney's were built, but lots of things have changed since then. I also suspect that owners have heard that VG's may slow them down or they are reluctant to invest in something that others haven't.

Here are some additional facts that may make naysayers reconsider...

1) It is a proven fact that aircraft with VG's are safer and have fewer accidents. In fact, it is rare that you would ever get into a corporate jet, airliner or modern high-performance airplane that doesn't have VG's as standard equipment.

2) I sincerely believe the VG's will provide the best dollar per dollar benefit of anything I've done to 252Q thus far. There are not many mods you can do to an airframe that has virtually zero impact on performance while systematically enhancing safety. They improve low speed handling characteristics, reduce stall speed, shorten takeoff distance, improve climb performance and materially improve wing efficiency in cruise.

3) VG's greatly improve landing safety margins. The #1 cause of accidents in the M20 are takeoffs and landings. It is all too common to read about Mooney pilots that land too fast and overrun the end of the runway or they takeoff on a short field a bit too heavy and run off the end of the runway! In fact, I did a quick analysis of the NTSB Aircraft Accident/Incident database. What I found was that there have been 2509 Accidents/Incidents reported on M20 Aircraft and 692 of them were due to a landing incident of some-sort. In other words, 27.6% of the NTSB M20 incidents were landing related! If you read them you will find that the vast majority are not from gear up, but form landing too fast, trying to force the plane onto the ground and ultimately overrunning the runway. VG's permit you land slower without sacrificing control!

4) NASA spent millions researching and developing VG's. If they can build an international space station, I think they may know a thing or two about aerodynamics ;)

5) They are dirt cheap! Installed I paid a whopping $1400!

Need I say more...?

I sincerely hope that I've got you thinking about installing VG's on your Mooney, or any airplane that doesn't have them. Do your own research and I bet you will be as convinced as I am. I also bet that when Flying Mag does their research they will come up with similar findings.


Anonymous said...

I can understand your logic for the installation of the VG's, but I have the feeling you are trying to convince yourself that you made the right decision. The "what if" scenario of decreased performance has to be on your mind, correct?

Jim Kerr said...

I suppose everyone has in the back of their mind the question of did they do the right thing whenever they take a chance to do something not many have. I'm as speed hungry as any red-blooded Mooniac. I certainly wouldn't want to intentionally do something that would materially impact cruise performance. However, even if we loose a few knots the safety margin on landing and slow speed control was paramount in my mind when deciding to install VG's.

Not many M20's crash in normal cruse flight unless they run out of gas, have an engine seizure or fly into weather. Fortunately these types of in flight accidents are rare in a Mooney (statistically much lower than our c/p/b buddies); however, the NTSB reports are quite clear on our akilies heal... 1 out of 3 Mooney's crash due to landing issues, which is significantly greater than our c/p/b flying mates. Why, because we Mooney pilots tend to land faster than we should in order to maintain comfortable control authority.

The Mooney wing is not designed to fly low and slow, which is why I decided to install VG's on 252Q. There is no disputing the fact that VG's will help me slow down on landing, save my tires/brakes, enable me slide in behind the 152 on a long slow final and safely get me in/out of shorter fields at max gross weight.

That said, however, I am as curious as all of you about the safety/speed trade-off. After I get the plane back I will do a through in flight test and publish the results (good or bad). Flying Mag will also likely do their own numbers, which is why they want to fly 252Q. We shall soon see if I made the right decision...

bparker said...

Where did you purchase the VG's and who installed them? Also, will you be going to Sun-N-Fun?

Jim Kerr said...

The VG's were purchased from Micro Aerodynamics (www.microaero.com). The install kit comes with everything needed to mount them. Tejas did my install, but any A&P can do the work. They have an STC for virtually every M20 airframe, so all it takes is an IA completing a FAA Form 337 to return the aircraft to service.

I am going to try to make it to Sun n'Fun, but the project may not be complete by then. I will be in Oshkosh all week this year. I will post the location of where 252Q is parked on this blog.

bparker said...

I am curious if you have any feedback on the Vortex Generators since you picked up your plane? It looks awesome. I hope to see it in person @ Osh Kosh.

Thank you,
Bryan Parker

Jim Kerr said...

I will be in OSH this year. I've asked Tray with MAPA about parking in front of their tent. This would make it easier for everyone to see. I'll let you all know where she will be parked and soon as the location is firmed up.

I will be writing a full article on the VG's, but haven't had a chance to do my flight tests. So far I have not noticed any material speed reduction, but I have noticed a significant improvement in low-speed control. I can land well below marked stall speed with the horn blearing, yet I still have full control authority. I can also land the plane like a Cessna and hold the nose off of the ground as I decelerate on roll-out. I've never been able to do that before since the 252 has a heavy engine and a forward CG.

Anonymous said...

I'm in the process of moving back to Mooney flying from 10 years in a C210. Besides economy one big factor is safety, and a M20 is just rock solid in turbulence compared with brand C, which not surprisingly has a much higher breakup rate. One safety concern for me about brand M though is stall-spin, and VGs could make a big difference there. What I wonder though is if Mooney's with VGs actually have a lower stall spin accident rate. Anyone know?

Jim Kerr said...

There probably isn't enough data to quantify the safety of the M20 from a stall/spin perspective. I am one of a few, if not the only, Mooney's out there with VG's installed. That said, however, the VG's are not only installed on the wings, they are also installed on the horizontal and vertical stabilizers. Having VG's on all control surfaces leads me to believe it would be easier to fly out of a spin. Not to mention the fact that it would be more difficult to get into one in the first place since stall speed is lowered and the ailerons continue to function while the aircraft is in a stall.

Anonymous said...

Other Mooney owners HAVE installed vortex generators on their Mooneys and have found that VGs extract a significant penalty in cruise flight because they disturb the normal laminar flow for which the Mooney wing was designed, and does not significantly increase the margin of safety in at lower airspeeds compared to a properly flown Mooney by a competent Mooney pilot.

Since most folks buy Mooneys to "go fast" at the most efficient fuel burn, VGs simply slow the airplane down in cruise without much in return at the lower airspeeds.

This topic has been discussed in detail in past on the Mooney Listserve by owners who have installed VGs and then took then off.

Jim Kerr said...

lford, your comments make since from a layman's perspective; however, the actual science does not back up your position. NASA has a ton of research on the effects of vortex generators and laminar flow. Here’s a link to some of the NASA Technical Reports: http://ntrs.nasa.gov/search.jsp?No=10&N=4294871474&Ns=ArchiveName%7C0.

VG’s are not drag components like just about anything else that disrupts air flow. They are tiny airfoils that improve airflow adhesion to the boundary layer. In other words, the vortices that are spinning around hold the air closer to the wing, which actually improves the laminar flow characteristics of the wing. My flight tests have proven this fact as well. I have not seen any perceivable decrease in speed, but I have seen marked improvement in slow speed flight and the reduction of stall speed.

Your comment about trained Mooney pilots landing properly is certainly an accurate statement. Properly trained pilots land the space shuttle and airliners every day without incident as well. The sad fact of the matter is, however, that there are more fatalities in the M20's on takeoff and landing than in any other phase of flight. VG’s may not be the magic bullet, but they most definitely improve slow speed and landing performance. To me it was worth the $2K investment.

Anonymous said...

A friend of mine installed VG's on his Cessna 177RG and lost 4 knots or more in cruise. I can't imagine that VG's on a Mooney would substantially make any difference. The wing would basically ignore the VG's because the airflow separates much further back on the wing than on a Cessna or Piper wing. YOUR PLANE IS BEAUTIFUL, HOWEVER, AND YOUR WEBSITE ROCKS!!! Well done!

Vortex Generators cruise speed said...

we should install Microaero Vortex Generators on a Mooney M20K 231 of a customer. He wants to know if this installation has a negative effect on cruise speed. There are rumors that the plane might loose something like 5 knots indicated after the installation of vortex generators. However we do not know if this is true.

Would be nice to get your "real world" experience



Jim Kerr said...

The VG's do slow the plane down a bit. It is somewhere between 3-5 KIAS. One benefit that is not widely discussed, however, is that VG's also improve climb performance. I've gained 200-300 FPM in a normal cruise climb. Getting to altitude faster is a nice fuel savings benefit and helps offset the speed decrease.

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