Vortex generators

[garybcollins]

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 of 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. (http://www.microaero.com/) 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. 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 of the 170. 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 airplane or the wing tips, no fat leading edge to clean. The only possible down side it 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 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 one short on the horizontal stabilizer. After installation it is difficult to recognize the flat ones and I ended up putting a strait edge on all 76 mounted on the wing, to find them. They were removed by gently heating them with a heat gun. After cleaning the mounting location and the bottom of each VG they were reinstalled which 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 per the instructions, put it on with a Q-tip. Use another Q-tip to put the adhesive on the VG. This almost elimintates 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 and consistent. 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, providing 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 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 seemed 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 20 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 my 1600 foot strip is much easier. I can routinely land and stop in 400 feet flying solo. Speed control on final is more important with the
VGs since if you are fast it will float. Take-offs with out flaps work better than before. High altitude performance seems better. 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 110 mph, 5 mph higher than before. The nose down pitch in a full flap slip seems to be gone but I want to check it more before I try any at low altitude.

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.

[zero.one.victor]

Gary,I really enjoyed your report on the VG's.Informative,yet concise.Excellent!

Eric

[JDH]

Gary, you don't say where you fly out of; elevation?? Must be high, cause 1600' is lots of runway at sea level.
Full flaps side slips? Does the airflow using the VG's make that possible safely?
I liked your report, very good insight.
I had VG's on a Cherokee, only 4 per wing and they did work very well. Problems I had is passengers would occasionaly step on one of them and bend them (shouldn't be a problem with the 170, although, not sure about my 8 year old...) The other thing I found tough was putting on and taking off wing covers without tearing up the fabric or catching an edge. Now with 76 of these puppies on each wing would make wing covers an impossible task and how about snow while tied down, how tough would it be to clean off the wing I wonder.
Just wondering out loud.
JD

[Bruce Fenstermacher]

I love my VGs and highly recommend them.

The Micro Vortex kit is one of the most complete kits I have ever seen. Besides the paint, alcohol and paper towels or rags you can drive right out to the plane with just their kit and a step ladder and you'll have everything needed to complete the installation

It took about 6 hours to install them by myself but I did not paint them.

I did not notice any shorter take off except of course that you could leave the ground at about 45 mph comfortably if you choose too. The airplane may not fly any slower (I think it does) but you will feel much much more comfortable doing it. I have no problem demonstrating this. At altitude I can pull the nose up till I have 0 airspeed showing and with about 1700 rpm just stear the plane around the sky with ailirons and a little rudder. I have set the plane up with this nose high atitude and with 0 airspeed showing and very little if any forward speed over the ground had a desent rate between 300 and 500 fpm with very comfortable control feel. In fact what VGs end up doing is improve the effectivness of the ailirons in all flight envelopes. I now fly my base at 60 turn final and slow to 55 or more and come over the numbers around 45 and end up using very little runway. Control feel is great thoughout.

Since installing the VGs on the elevator it has become much much more difficult to blank out the elevator in a full flap slip (I have B model wings and flaps) and I routinely (with some caution) slip with flaps deployed.

I have lost a few VGs when I've knocked them off refueling and have just glued them right back on.

The disadvantages of VGs that I found are that they are sharp and you must be more careful refueling and washing the plane. Also if you de-ice or de-snow your wings in the winter the VGs make this task more difficult.

Anybody ever in the Quakertown, PA (just north of Philadelphia) I'd be happy to give them a demonstration flight.

[hsjrev]

It sounds great. I wonder how the performance compares to the Sportsman STOL cuff? They make some pretty impressive claims on their website but I've never heard from an owner who has actually flown with it. Anybody out there have any comments on the Sportsman kit?? Total distance to clear a 50' obstacle reduced from 1625' to 780' sounds pretty great.

[N1478D]

Gary,I really enjoyed your report on the VG's.Informative,yet concise.Excellent!

Eric

Me Too! Super Report!

[garybcollins]

All the comments about washing, removing frost or wing covers are valid questions and the VGs make them more difficult. My strip is a 910 feet elevation but the engine is getting tired and I am about to assemble a new one. Maybe that will make my performance better. I have been thinking about why the take off is not improved more. I think it is because the plane can not be put at high enough angle of attack---it needs longer gear legs. With the VGs it is no problem to land gently with the tailwheel on the ground and the mains in the air. Add a little power and I have been able to "fly" down the runway for a 100 yards with the tail wheel on and the mains off. BUT on take off you can only get the tail so low. The VGs are helpful in low speed control and owners who are really pushing limits will add VGs to a plane that already have the STOL cuff mod. Seems like gilding the lilly to me but if it helps, it helps. Gary C

[zero.one.victor]

180 gear legs & a shortened tailspring would increase your 3-point angle of attack .I have 2 friends who swore by the shortened tailsprings on their 140's.

Eric

[AKbushpilot]

Great article on the VGs. I have a Sportsman STOL kit on my 53B and it was without doubt the best money I invested in my aircraft. The performance is incredable and it is really like flying a different wing. I put Horner wing tips on with the cuffs as they will deflect the wingtip vorticies away from your wing tips. (see their web site for the details on that).
I routinely fly into short strips here in Alaska and without that cuff I would never be able to get out of some of them. Take a look at the numbers on their website and believe them all. I currently have a C-145 and fly the airspeeds supplied by them. (The FAA refuses to recognize their published airspeeds and it would take several full-time aeronautical engineers to convince them and they still would not approve it). This cuff allows me to fly slower on approach, touch down slower, and I'm definetly airborn earlier. I get airborne around 40 kts and on a 65 degree day I will get 1200-1500 FPM climb. On a -25 degree day I can almost get 2000FPM up to pattern altitude.
The only aspect that is hard to get used to is the point around 60kts. It loves to fly right here and will hover aat this airspeed. Use a little backpressure/throttle decrease to get it below 60kts. Once you do this, it will slow right down and be extremely stable all the way to the flare. With me (210lbs), 75 lbs of survival gear (a huge gun included), and 1/2 tanks of gas. I can fly safely at 45-50 kts all the way down final and then slow to 37-40 over the numbers. I usually touch down at around 35 kts. If I really need to land short, I will fly 35-37 all the way downand plop it down on brick one. I routinly practice these approach speeds but they are obviously higher as the gross goes up. I picked up about 4-6 knots at altitude. I DO NOT slip the aircraft with full flaps as my tail will still blank out despite the STOL kit. Apperantly the VGs will solve this problem.
I have a neighbor who has a Sportsman cuff in his 52. He flew like this for a few years before adding the VGs. His appraisal was favorable as it made the aircraft much more stable at slow airspeeds and his tail will now fly as long as his wings. Now he does not suffer the tail drop at high AOA. He beats my approach speed by about 2-3 knots so his landing roll is less than 250' fully laoded. (confirmed by the use of a laser rangefinder..his takeoff roll is about 280' as he has an O-360 and his aircraft is much lighted than most of ours)
Either of these kits is worth the money but unless your want to fly at much slower airspeeds, the VGs are probably suficient. If you are willing and want to fly slow then buy the Sportsman kit and add the VGs later. I spent about $6000 for the Sportman kit, the wingtips, Whelan strobes and the installation of all. As I said, it was by far the best money I spent.
For those of you who have to "confer" with the spouse, use the argument that the aircraft is vastly safer now as you will be going slower when you have to put here down in an emergency. Much better to hit the ground at 35 than 50-60 kts.
In my case I actually prevented a serious accident and the injury of my pink body by having the Sportsman kit on my airplane. I got hit by a huge gust of wind right when I flared. It blew me sideways and spun me about to the left. By the time I got my act together I was 30 degrees off runway heading and NOT over the runway anymore and pointed at 80" trees. Full throttle, 38Kts, and hanging on the mighty thrust of the Continental C-145, is how I cleared the trees by just under the propeller. (Distance proven by the thump and as the top of a tree hit my right wheel). Without that kit, I would have had to put it into the trees to prevent me from crashing into the Flight Service Station, or into all the parked airplanes in the parking area.
After I landed (delay of 15 minutes to calm down), the witnesses to this incident all told me they thought I was going to crash and could not believe the Cessna 170 could put on such an airshow. The lady in the FSS said she could see the entire bottom of my airplane out her front window as I passed overhead. To them it all ocurred in a few seconds but to me it seemed like an eternity to reach those trees. Thats my selling point and full recommandation for the Sportsman STOL kit. Happy and safe flying from the North.