Monday, 24 October 2011

A few weeks ago a friend and I installed a kit full of micro vortex generators (VG’s) on the wings and tail section of my airplane, a 1975 Piper Cherokee Warrior PA28-151, N639MR. As I described in some detail before, the VG’s modify the flow of the air over the surfaces of the wings, making the air “stick” better, resulting in smoother air flow and better air flow where it’s needed – over the control surfaces at the trailing edge of each wing.

Micro Vortex Generators on Piper Cherokee Warrior PA28-151Well, I’ve now flown the plane several times since installing the VG’s and the results are in:  It’s truly amazing the difference they make.

In a nutshell, here are the results of the modification. I’ll start out with takeoff, talk about in-flight changes, and then finish up with benefits at landing time.

  • From the beginning, the plane gets off the ground sooner. A lot sooner, in fact. And it doesn’t need as much airspeed to initially get into the air. That translates into shorter ground roll and a much sorter takeoff. I’ll have to measure it to see exactly what the distances are.
  • The plane climbs faster. In situations where I used to get 500 feet per minute I often now get 600-700. In those situations where I used to get 700-750 feet per minute, it’s not unusual to get 800-1000. And if I want to convert some airspeed to altitude, pulling back and riding it up is fast and fun.
  • The airplane is faster in cruise flight. Truly faster. You’d think that adding nearly 200 metal tabs to the flying surfaces of the wings and tail would create drag and reduce top speed, but nope. Apparently the improved airflow over the wing is a great tradeoff – Add a little bit of drag as a result of adding the VG’s, but reduce overall drag over the wings and tail thanks to improved laminar airflow. Net result is higher airspeeds at the same engine RPM.
  • Related to that, I can now fly at a cruise speed around 120 mph at a lower RPM than before, which translates into burning slightly less fuel going cross-country. It used to be a real chore to get the plane up to 120 mph in cruise and maintain it there at 2500rpm. But now it’s often flying well over 120mph even at 2400 rpm. That 100rpm difference makes a real dent in fuel consumption, believe it or not. And if I want to fly at around 115 miles an hour, the difference in RPM required is even greater. So, I can get there faster on the same fuel as before, or take my time and burn even less.
  • In flight, one of the critical tests you put an airplane through when you are becoming familiar with the way it flies is aerodynamic (wing) stalls. Needless to say, I have been stalling this plane more times in the past few weeks than is typical as I get to know the new flight characteristics. Again, the difference is substantial: It’s almost impossible to get it to drop it’s nose and stall, one wing or two. In power-on stalls, much of anything beyond a buffet is very difficult to make happen. It just keep on flying and buffeting along at 44 miles an hour or even slower -- which is a lot slower than it used to stall before the VG's. In a power-off stall configuration I’ve flown it in a slight headwind down to 40mph indicated airspeed, and all it really wants to do is drop the nose a little then and keep flying. I literally flew it power-off at 45mph in a buffet for half a minute, no stall. Of course, eventually it will drop, but it sure hangs in there, and loses very little altitude. Compared to before the VG’s, stall speed is at least 5 mph lower, probably more like 7-8 mph less.
  • The control surfaces respond quickly, sharply and with authority in flight. I thought the plane rolled left and right pretty quickly before the VG modification, but it’s much cleaner and more responsive now. Similarly, although not as important or pronounced at cruise speeds, pitch changes happen quickly and yaw is solid with good authority thoughout.
  • Steep turns (45-degree bank or more) are so much more fun now. The plane carves and holds it’s altitude in steep turns, and is so easy to control you just have to wonder if you jumped in the wrong plane by mistake… Nope, this is my plane. Okay, cool. :)
  • As a final note regarding in-cruise flight, the plane generally feels much smoother and more solid moving through the air. It's hard to explain, but it's noticeable when you fly.
  • The rest of the major differences are seen when preparing for landing and during the landing itself. This phase of flight is so different than pre-VG’s that some people will tell you that you'll have to learn to land all over again, and they’re right. At lower the speeds flown in preparation for landing, the control surfaces respond much more sharply and with more authority than before the VG’s. Not only that, the plane simply won’t descend as quickly anymore. It just wants to fly. So, careful reduction of speed to under 80 mph is needed to make sure you’re descending soon and fast enough in the landing pattern.
  • Landing pattern turns are clean and sharp. The sluggish, slightly-mushy sort of feeling is associated with rudder at the slower speeds in turns and when slipping on approach is gone.
  • In the landing flare, one simply must pay attention and fly the airplane slower than used to be the case, since the plane just floats along over the runway like nobody’s business if you’re too fast. Cherokees – especially the tapered wing models like the warrior – are kind of famous for floating, but now the effect is VGs - how they work - from Micro Aerodymanicseven more pronounced. In fact, I’m flying almost 10 miles per hour slower over the numbers at the approach end of the runway than before I installed the VG's, and the plane settles to the runway at a substantially lower speed than before - and quite a bit below the lower end of the white arc. Makes for some smooth, short, nose-high landings – which is great.
  • One of the chief complaints some people have about Cherokees at landing time, when the plane is slow, is that the rudder (which controls yaw, or the direction the nose is pointing) and the stabilator (which controls pitch, or how high or low the nose is pointing) lose their effectiveness. With the tail section VG’s installed and the improved flow of the air over these surfaces at landing time, rudder and stabilator authority is much improved in a very noticeable way.

So, is it worth the time and money? I spent almost $1500 on the kit and a couple evenings installing them (under the supervision and with the approval of my IA). And the plane flies great – faster, more responsive to control inputs and more efficient, plus a longer glide and the capability of going to and departing from noticeably shorter fields. The Warrior isn’t intended to be a back-woods airplane, but shorter-field capability is definitely welcome and valuable.

So, yes – It’s definitely worth it and I’d do it again in a heartbeat.

Other items addressed lately by the aircraft shop include a new landing light – a Whelan LED model that will never burn out (which beats the heck out of 30-hour or so per halogen bulb) and a newly-rebuilt attitude indicator (the gyro had seen it’s better days, and was in desperate need of repair before I start my instrument training). Needless to say, I won’t be dumping any more money into the plane for a while (at least not voluntarily), since it’s emptied my wallet this summer, to be sure!

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Monday, 24 October 2011 17:44:00 (Pacific Standard Time, UTC-08:00)
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 Saturday, 01 October 2011

A couple months ago I dropped by Micro Aerodynamics in Anacortes, Washington after putting together a big fireworks show there for Independence Day. Micro Aerodynamics makes kits of micro vortex generators that can be installed on aircraft to improve performance in a variety of areas. Under the supervision of A&P What-VGs-domechanic and IA (plus all-around-good-guy) Danny from Twin Oaks Airpark in Hillsboro, I’m installing the micro vortex generators on my airplane, N639MR, a 1975 Piper Warrior PA28-151.

What the heck are vortex generators (VGs) and why would I want them? Good question.

VGs can reduce stall speeds and improve an aircraft’s aerodynamic performance. They allow the wing to develop more lift and fly at lower airspeeds, as compared to not having VGs installed.  In turn, this can reduce takeoff speed and improve the rate of climb.  VGs also help to retain effective aileron (toll) control and enhance your rudder (yaw) authority in higher angles of attack.

As air flows over a clean, efficient wing the air "sticks" or adheres to the surface of the wing – a function called “laminar flow.” This clean, laminar flow of air over the properly-shaped wing's surface results in a high pressure zone underneath and a low pressure zone above the wing, which is how lift is produced – The wing moves toward the low pressure zone as the difference is equalized. If the air flowing over the wing surfaces (especially in the low-pressure zone on top of the wing) loses its laminar flow, wing and flight performance can suffer in the form of increased drag, loss of lift and higher fuel consumption.

NASA researchers developed micro VGs to control this flow delamination by producing miniature, controlled spirals of air, called "vortices." The spirals of air laminate well to the surface of the wing and as a result airflow over the wing is more efficient and “sticks” better across the entire surface, including at lower air speeds and higher angles of attack. The result is reduced drag and increased wing efficiency and lift (or you can think of it in terms of DSC00401less engine power being required to produce the same amount of lift). In a perfect world, the end results for the pilot are shorter take-off distances due to more efficient creation of lift, lower aerodynamic stall speeds, ability to land slower and therefore in a shorter distance, snappier and more responsive control inputs for roll, pitch and yaw at all speeds (including critically slow speeds such as in landing configuration), and in some cases even increased top cruise speeds and smoother ride due to the resulting aerodynamic improvements (in the case of especially inefficient wings).

Anyhow, I dropped about $1450 on the kit, and after discussing with my mechanic and discovering I could install them under his guidance and supervision I adopted a measure-twice-mark-once methodology and a friend joined me in the hangar to begin the process of installing the micro VGs on my airplane.

The Micro Aerodynamics kit, first of all, is incredibly complete and well put-together. It includes literally everything you need, with the exception of a couple items I needed to pick up at the local store (90% or higher concentration rubbing alcohol, and in my case some paint and blue painter’s tape, since I would be painting the VGs myself). The VG kit has all the thread, masking tape, adhesive, of course the nearly -200 aluminum VG pieces, self-adhesive patterns to stick on the wings, DSC00432abrasive pad, razor knife, a metal measuring tape – even a sharpened pencil all ready to go. I imagine the only reason I had to buy the rubbing alcohol on my own was because it’s not normally easy to ship that in the mail.

Preparation consisted first of a thorough washing of the entire aircraft to remove all the dirt, dust and bug crud, especially from the wings and tail control surfaces. Another good friend helped me with that a few days prior. On the day we started installing the kit, a lint-free cloth and some rubbing alcohol removed any final layers of crud from the areas where the VGs will be applied – The wings, stabilator (horizontal stabilizer on the tail) and the vertical stabilizer (the upward fin portion of the tail).

I shot a few “before” pictures of the airplane a jotted down some recent performance numbers to help my memory. Typical cruise speed is 117 to 120 MPH properly trimmed at about 2500 RPM and typical sustained climb with full tanks and just me in the plane is about 600-700 feet per minute on a standard-ish day. The airplane stalls with full flaps and in landing configuration (power off) at a pretty low speed – around 50 miles per hour or less. Under full power it’s hard to get it to do a full stall at all, but seems like it’s about the same speed in slow flight at altitude (3000 feet). But ultimately the test will be flying the airplane, hands on the controls and butt in the seat, and seeing how it flies.

Installation consists of following a set of provided diagrams and instructions, and carefully measuring parts of the plane, making marks where indicated, stretching black thread between points to define reference lines, and then applying a DSC00449bunch of patterns made of self-adhesive contact paper. The pattern templates have cut-outs where the VGs will go, as well as notches you line up with the thread lines and the various reference marks you made in the measurements phase.

Placement is important, and the templates make it pretty easy to get it right. In fact, the creator of the Micro VGs told me one customer, who happens to be an airplane mechanic, gave the kit to his 12 and 14 year old buys to install on his airplane (supervised of course, and with great success). Measuring twice and having a helper to provide a second set of eyes will ensure you get everything in the right place. In fact, there are many parts of this project that are much better done with two people.

We completed the full installation in two evenings over about . The first evening was spend measuring, marking with a pencil, stretching thread lines and putting the contact paper templates in place. Then the measurements had to be re-checked carefully, since the adhesive that’s used to stick the small aluminum VG pieces is basically permanent. There’s no moving them once they’re on there.

DSC00464Day two consisted of finish painting the VGs and allowing them to fully dry (I’d actually recommend doing this the day before you actually install them, though) followed by prepping the surfaces where the VGs would be glued down. Pre consists of using a Scotch Brite pad (supplied in the kit) to break the glossy paint barrier, and then wiping the surface clean using rubbing alcohol.

The adhesive in the kit comes in two parts: A small aerosol can with chemical activator that is sprayed on the surface of the airplane skin where the template cutouts are, as well as a syringe of adhesive material, which is applied one drop at a time to the bottom tab on each of the 196 VGs. Needless to say, it takes a while and is some careful, tedious work to glue nearly 200 little metal tabs.

Probably the easiest to mess up and least-forgiving part of the whole project is the process of getting  the right amount of adhesive on the bottom of each VG. It’s easy to get too much on there, and the result is glue squeezing out from underneath. When it cures, it tends to turn from clear to DSC00472a brownish color, so you don’t want that stuff left over when you’re done – It will just make your wings look cruddy. So, a package of 100 cotton swabs is also included in the kit, along with the razor knife, to allow you to clean the excess adhesive before it becomes a problem that can only be solved with a Dremel tool.

The first few VGs we applied were not pretty – I ‘ll just admit that up front. I had to scrape enough adhesive off that it took paint off the VGs, so I will be spot-painting those in the next day or two so they look proper and nice. But after applying a few, my friend and I got in the swing of things and discovered exactly how much it takes. One thing the kit doesn’t have that I think would be of huge benefit is a few practice VGs and a template and piece of aircraft sheet metal. For someone who’s never done this before, a small amount of practice could be really helpful before defacing the skin of a real airplane. But that’s just an idea, and in the end this is not rocket science.

The adhesive cures quickly, and we adopted the recommended method of applying a little at a time in stages. Activator first, then glue on a group of VGs (maybe 20 or 30 or so). Let the previous set cure and harden while you install the next set. Once cured, you just peel the contact paper templates off from around the VGs, DSC00476clean up any excess adhesive and tape and crud, and then move onto the next section. Applying all the VGs took us about three hours of solid work as a team. My friend Matt applied the glue to each VG, while I sprayed on the activator and then placed each VG on the plane in the gaps provided by the templates. On the wings the VGs go on top of the wing surface a few inches aft of the leading edge. On the tail, it’s different. The horizontal stabilizer VGs actually go underneath the wing, and on the vertical stabilizer they go just in front of the “rudder” surface, in the middle of the stabilizer structure. You need to be careful to make sure the ones you apply to the vertical surfaces don’t slide out of place due to gravity – a few of mine wanted to, so I had to make sure they stayed in the right spot until they cured, which takes only a couple minutes. Less is more when it comes to adhesive, we found. But too little and you’re also in bad shape, so really it’s all about the art of getting just the right amount on the base of each VG.

The end result is an airplane that looks pretty darn different – Those little tabs really give the old plane teeth. Next up is a final inspection and (hopefully) signoff by the shop, completion of some required FAA paperwork, and then I’ll get to test fly it, which will be fun. Until then, just have to wait!

Update: The plane was checked and given the green light on Sunday and I flew it for an hour or two. Results were great, with a number of pleasant surprises. Will post more info soon.

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Friday, 30 September 2011 23:52:51 (Pacific Standard Time, UTC-08:00)
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