vortex generators

[Russell Knowles]

Knots 2 U advertise a vortex generator kit for both the single and twin comanche at around $1450.00. Does anyone have any information about whether this is a worthwhile modification?

Russell Knowles

[Jay]

I know that Karl Hipp has them on his Miller PA30. You might give him a call, he's in the Pathfinder.

Jay

[Don Nelson]

We saw an installation on a single at the Laughlin seminar a week or so ago, but I can't remember who's it was. Maybe the single featured in the seminar. Ask Lorne Harmon, or Cliff Willewski.

Best, Don

[Russell Knowles]

Thanks Jay and Don will do.

Russell

[T210DRVR]

The placement of the VG's on the wing is critical. While I don't have experience with them on my Comanches, from Cessna 182 experience it seems that the 182 RG which shares similar placement to a standard 182 may have as much as a five to six knot speed penalty. On a standard 182 it's probably a knot or less as advertised.

Our laminar flow wings are very efficient, and I'd be concerned about whether there would be a speed penalty. That said, on my 182 they are very effective at improving low speed handling in both pitch and roll. Stall speed reduction is difficult to measure as the difference between CAS and IAS widens at higher angles of attack. I can tell you that stall speed on my 182 is reduced, but more important is the crispness of the controls at low airspeeds.

I did my BFR this year in my highly modified 182. The instructor doesn't know me very well but knows that I fly often, generally well over 300 hrs per year so I think he was feeling pretty relaxed. When he asked me to demonstrate a short field landing I asked him if he wanted a true short field landing or just a kind of short field landing. I think the question left him a bit perplexed, but he asked for a short one. Well, in my modified 182 that involves indicated airspeed on short final of 50 MPH with the stall warning blaring. Well, I saw him tense as I plopped her down and could have stopped in the first 500 ft of runway. In the interest of the brakes I coasted to the 1,000 ft turn off. He was rather surprised and asked if that was how I always fly my short fields. I answered yes, when I do a short field it's a short field.

Anyway, my point is that they are highly effective in the low speed envelope in a 182.

[Russell Knowles]

Thanks everyone for your input. I think it would be prudent to speak to someone who has fitted these devices to a comanche before making a decision about the installation of VG's.

Russell

[Scott R Morris]

I have them on my single. See my comments on the chat forum side. In short, I definitely agree that you need to take the time to measures several times the placement of the template on the wing and BOTTOM of the stabilator prior to install. I did not notice any change on the top end but I wouldn't doubt if there was 1-2 mph penalty. They definitely work on the bottom end. I think my stall speed was reduced 5-7 mph and I do have better aileron control. For me it was worth the money. I'm a believer.

I can say that it took me longer than a day to install the VGs as I went very slow to ensure I placed each VG exactly where it was supposed to be and cleaned the excess epoxy with a swap after each VG install to minimize the clean-up. Also, I found the most difficult portion of the install was getting the rudder template to line up correctly.

If you have any questions about the install call or email me.

Scott
N7610P

PS from looking at the knots2u website, I suspect the knots2U VGs are actually the ones from microaero. www.microaero.com. These are the VGs that I put on my plane.

[SLIMDREDGER]

A retired NW airlines pilot friend with a standard PA30 put on VGs and is very pleased with the better aileron control at slow speeds. He did not feel they did much to lower his stall speed and doesn't worry about what they did or did not do to his top speed. He had flown my counter rotated, Robertson modified airplane before and remarked at the time that he felt the Robertson very much improved the aileron control.

That said, does anyone have any performance figures or experience on a counter rotated, PA30 Robertson with VGs ? I talked to the VG people at Anacortes, Washington about this,and they had none to offer. I was surprised that it would be legal to install VGs on any airiplane without extensive testing and numbers approval by the FAA, but they showed me a copy of the STC for the PA30 that approved VG installation with a rather wide open proviso that it in essence left it up to the pilot to test and accept the compatibility of VGs with any other modifications on the airiplane. (!!)

I like my airplane with its current mods, but I would certainly like to know what the numbers would look like if I added VGs.

Al Powers ICS 2978

[Jay]

Hi Al:

It's kinda of a dark secret of STCs that it's up to the "operator" to determine STC compatibility. You can legally "stack" most STC'd mods to your hearts content, but you get to be the test pilot.

The only STC's I can think of offhand with conflicts are those that increase gross weight. For example, if you have the Robertson STC you get a 200 lb gross weight increase. But if you put on the Osborne (Brittain) tip tanks, any weight above 3,650 lbs must consist of fuel in the tip tanks. In other words, you can't get the Robertson 200 lb increase and then the Osborne 150 lb increase on top of that for a total gross weight of 3,950 lbs.

It does make a certain amount of sense. If an STC applicant had to test his STC in conjunction with every other STC that an airplane could have it would make an already difficult process impossible.

Jay

[farmer]

Microaero vgs were installed on my pa 30 in 2004 and I would recommend them for much improved low speed aileron control with no noticable cruse speed loss. This is some of the best money I have spent on this plane. The final approach is rock solid to landing.

Charles Allen

[Hansmeister]

Back in 1990, when I was a bit more "adventurous", I entered the Cafe 400 Triaviothon, which required a best rate of climb out of Santa Rosa, CA, followed by max speed at 6000 feet, then a 180 turn, and slowest possible speed. The score was ROC * max speed/min speed. I won in my weight class.

The ROC and speeds were determined by their pitot boom recording barograph. With the barograph temporarily mounted on the right outboard wing, I was handed a temporary airworthiness certificate by the FAA at the event.

Being temporarily experimental, I mounted my own vortex generators using double back tape. Their shape was like a 1/2 delta wing offset at 15 degrees to the airflow at the skin splice at the spar (40% chord). Empty weight of the plane was around 2850 pounds.

The results were: ROC was about 1850 fpm, top speed was 213 mph, and minimum speed was 59 mph. The minimum speed was a bit scary. Full power, gear down, flaps down, and the nose waaaay up. It took full right rudder to keep the ball centered. I might have gotten a little more out of it, but I was worried that if a stall occurred, the plane would go into a spin.

Previous to the event, I experimented with the vortex generator placement, including tufting the wing. My reduction in stall speed in the normal flight configuration was about 6 mph lower at a modest power setting. Since the vortex generators were at 40% chord, my speed reduction was minimal, since that is the location where the airflow turns turbulent anyway.

And yes, they do work in reducing the stall speed. The current STC configurations I have seen place the vortex generators on the leading edge, and in that configuration, cruise and top speed will suffer a bit since the forward section of the wing should be in laminar flow, and with the generators, turbulent flow is introduced. The difference in drag between laminar flow and turbulent flow on wings is 5 to 1.

In the intervening 22 years, more improvements have been made to the plane, and I now normally cruise at 208 mph (180 knots) in smooth air at altitude, provided the plane is clean.

[SLIMDREDGER]

Hans:

Thanks for the factual comments and data points. Your cruise speeds are impressive.

I assume my Robertson cuff destroys the laminar flow characteristics of the unmodified Comanche wing????? I felt I lost 3 mph +/- when I had the mod done. Subsequent Lopresti and Knots 2U have regained the speed I lost.

I converted to Robertson back in the days that PA30 VMC demos were frequent and fatal and I had experienced a 1/2 turn spin with an instructor demonstrating VMC to me.

If I added VGs on top of the non laminar flow Robertson wing, do you think I would still lose 5 mph or so on the top end cruise?

Any thoughts on what VGs effect on the stall speed would be? Currently with counter rotated props Vs is 63 mph.

Warm regards, Al Powers

[John Fuller]

Put VG's on my PA-39 three years ago since my son was going to get his multi in the plane and I thought a little extra safety was a good idea. Then I tried to stall it at 9500' with one engine at max throttle and the other about 11" to simulate feathered. Easy to control and keep from yawning or rolling, it acted like a Cherokee. I'm a believer. Top speed may be off a knot or two, but it's worth it.

[SLIMDREDGER]

John:
Very interesting report! Did you actually stall it with the extreme eccentric thrust on the engines?

Having experienced (as a passenger) a fast 1/2 to 1 turn spin before I bought my airplane, when the demonstrator pilot feathered one and had normal cruise power on the other engine to show me what VMC was, I am impressed that your VGs apparently allow you to prevent a spin.

After I bought my airplane, there were a number of fatal training accidents resulting from pulling engines on takeoffs and the PA30 reputation suffered. I was concerned about the problem and when the Robertson mod appeared I flew their demonstrator and was impressed with the ability to fly out of below VMC spin entry. "Push the nose down and stand on the opposite rudder" worked. My airplane was the second or third to receive the Robertson mod.

What loss (if any) of speed was there on yhour cruise speed?

Warm regards, Al Powers ICS 2978

[John Fuller]

Al,

Yes, I stalled it with one engine at cruise power at 9,500'. Plenty of rudder and even some aileron required, but vmc is clearly below stall speed at that power setting. Nothing dramatic at all. I got my multi back in 1968 in a Twin Comanche when all the hoopla was going on and stalls (with both engines running) were a lot more dramatic in that plane. VG's do seem to be magic.

I'm sure I lost a knot or so in cruise speed with the VG's, but it's hard to tell since I put them on while I was having the engines overhauled. I still see about 163 kts plus or minus at 9,500' depending on weight (my paint is very rough but I have lots of speed mods - and tip tanks).

I had the Robertson STOL kit put on an Aztec I had once. Very impressive improvement in already good runway skills, I was also hoping for an improvement in single engine performance.

John