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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Add/Read: Comments [2]
Aviation | Random Stuff
Thursday, 27 October 2011 10:10:34 (Pacific Standard Time, UTC-08:00)
Sounds like excellent results and going to send this link to a friend of a friend that has a Cessna here parked at the Aztec airport.
Thursday, 21 February 2013 06:04:00 (Pacific Standard Time, UTC-08:00)
What a great story! I have learnt so much about aeroplane wings and it will help me with a few of my projects and I am in debt to you!

Thanks for the help!
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