TALESPINNERS - June, 2002

The Micro Vortex Generator Kit for Cessna 170B
by Gary Collins

I have owned my 1954 Cessna 170B since 1987.  The 170 is a relatively big airplane that does a remarkable job on just 145 hp.  I soon learned that at near gross weight and if it is hot, you have to be careful on the shorter strips I like to use.  From the very beginning I have been investigating how to make it perform better, particularly in takeoff, climb and cruise.  At first the various modifications using a leading edge wing cuff held my attention.  But the more I investigated the less appealing they were.

The leading edge cuff adds a larger radius, somewhat drooped leading edge to the wing.  It actually adds wing area.  After modification the transitions at the wing root and at the wing tip never looked good to me.  The wing tip transition could be fixed by new wing tips but they were fiberglass and were also heavy.  But those concerns were minor compared to the next two.  First, the cuff adds a lot of weight, between 15-20 pounds.  The 170 does not need to be made heavier.  Second, the cuff adds some undesirable flight characteristics.  While they allow the wing to fly at greater angles of attack and lower airspeeds and make normal stalls very gentle, all of the STC's using a leading edge cuff are prohibited from spins.  Apparently it is difficult to get a plane equipped with cuffs to spin but once in a spin they are difficult to get out.  A less serious concern is bugs.  I get tired of cleaning bugs off the normal leading edge.  With the cuff the leading edge is bigger and there is more surface to collect bugs.

Several years ago I moved to a hangar on a 1600 foot strip.  Summer operations there made the 170 a two place airplane and even then takeoffs were too exciting at times.  I began look for other ways to improve take off performance.  Something that was light and did not involve an engine change.  When Micro Aero Dynamics Inc.  got the STC for the Cessna 180 I started inquiring on a regular basis about when they would have the micro vortex generator kit for the 170.  (They now have STCs for the whole line of single engine Cessnas) This approach to making the wing lift more at low speed and high angles of attack utilizes small "micro" vortex generators across the whole top of the wing.  There are 76 of them on the wing and 36 on the bottom of the horizontal stabilizer.  They are aluminum and all 112 of them weigh only 3 oz.  The performance improvements are similar to the leading edge cuff without the negatives; no increase in weight, no changes to the flight manual (spins not prohibited), no changes to the fit of the wing to the fuselage or the wing tips, no fat leading edge to clean.  The only possible down side is that the plane is not airworthy if five or more of the VGs are missing and a placard to that effect is supplied with the kit.  A repair kit is also supplied which contains extra VGs, instructions, and the adhesive for installation of missing VGs so unless there is some strange accident, missing VGs should never ground you.

Literature supplied by the company says the VGs can be installed in one day.  That is probably true for the second airplane but it took me 15 hours not counting reading the installation instructions 3 times before I started.  It would speed things up if you had a 3 foot high scaffold the length of a wing.  I spent a lot of time moving stepladders.  The VGs come anodized in a dull, gold color and can be ordered painted to match your plane for an extra $100.  I decided to do my own painting and that process led to one of my time consuming mistakes.  There are two different VGs.  The ones that go on the wing have a curved lower surface to fit the wing surface.  The VGs for the stabilizer are flat bottomed.  In the painting process I got them mixed up and I installed four of the flat ones on the wing.  I did not discover that until I found I was short of the flat bottomed ones on the horizontal stabilizer.  After installation it is difficult to recognize the flat ones and to find them I ended up putting a strait edge on all 76.  There were only flat ones on the wing and they were removed by gently heating with a heat gun.  Correcting this little mistake took about 3 hours.  The lesson is to carefully keep the flat ones separate from the curved ones.  It became obvious as I read the instructions that a VG would probably end up on the landing light lens.  An email to the company confirmed that was the case and also provided a handy tip.  Instead of spraying the adhesive accelerator, put it on with a Q-tip.  Use another Q-tip to put the adhesive on the VG.  This makes for much less mess to clean-up. 

A friend said you are not paying for the VGs.  You are paying for the "map" for where to install them and all the work it took to make that map.  Placement is critical and Micro AeroDynamics has perfected that process.  A jig is provided to make locator marks on the wing.  Working from those marks, a string is stretched along the leading edge of the wing in two sections.  The first from the wing root to the flap-aileron junction covering the constant chord part of the wing and another from that point to the wing tip which covers the tapering part of the wing.  The VGs are mounted in pairs with reference to the line.  This is where the templates provided in the kit save the day.  The templates make placement of the VGs along the length of the wing easy and very precise.  Once the templates are in place it takes about 45 minutes per wing to actually apply the adhesive to install them.  The kit is very complete and includes the placard for the instrument panel, a preprinted entry for the logbook and preprinted 337 forms for the FAA.  All adhesive, string, pencil, gloves, Q-tips, etc.  are provided.

Now you want to know how the plane performs with the VGs.  At this time I have only made one test flight to comply with the STC requirements.  Slow flight is no lower on the airspeed than before but it takes more elevator to get to 40 mph indicated.  The stall is near 35 mph indicated, also not much different.  Flying solo, which puts me near the forward edge of the CG range, the stall did break but it was gentle with no wing drop.  Although not carefully evaluated yet, cruise seems just the same.  The major difference noted so far is that lift off comes about 150 feet sooner and the plane is very solid and will accelerate to 60 quicker and then climb well at that speed.  The take off and landing remind me of a Cub, just very gentle and solid.  It does take a little more nose up trim for the take off setting.  Landing also seems very solid and controlled and it floated a bit more than before.  I may have to reduce the over-the-fence speed a bit.  It will be interesting to check out the performance changes this coming summer at higher temperatures and with heavier loads.  So far so good.

Addendum: I now have about 10 hours with the VGs.  They make the elevator heavier and I find I do more with the trim wheel than I did without them.  They definitely do work and I find that getting into and out of the 1600 foot Hutchinson strip is much easier.  I had an excuse to overfly the local Class B and I ended up at 11,500 to stay VFR and getting that high was much easier than before the modification.  Indicated airspeed up there was about 5 mph higher than before.  Now about the bugs --- the VGs seem to make the air "see" a fatter leading edge.  I get bugs further back on the bottom side and a few actually leave a streak on the top of the wing behind the vortex generators.

VAN’S RV FACTS / FIGURES

As of 05/31/2002, more than 2,908 RV aircraft have been completed and flown!  Listed by Model:

Young Eagles Weekend - June 8, 2002
Editorial by Mike Griffith

Another successful Young Eagles weekend was held, June 8, 2002.  The weather was cool, foggy, and slight haze, but with the work of all the Chapter members we were able to fly over 95+ Young Eagles.

The value of this program will not be realized for some years, but it should create a channel for young boys and girls to become interested in flying as a hobby or as a career.  Only time will tell.

As for the Chapter, we may have started someone on the enjoyment of a lifetime.  The experience is in the future of these events and how the first ride for someone effects them for life.

Thanks to all the Chapter 174 member’s for the volunteering spirit, time and expense to make this happen again this year with no incidents or problems.  Other than slight oil leak I’m not aware of anyone having any mechanical problems and that speaks well to the preparation by all involved.