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TUFT ENOUGH?

After an extended period of down time, my VariEze is back flying. Typical small bugs are getting sorted out.

One item that may be of interest to other VariEze drivers .... I have never been completely satisfied with the low speed aileron authority at 80 knots and below. And now it seems to be worse since return to service. First question ... Any changes that might account for the degradation? Two possibilities come to mind:

1. I had sealed the hole thru the wing root wall around the aileron control tube to prevent cooling air loss. The rubber seals had developed typical "old rubber rot", and I removed them without replacement (note; the seals are not in the plans, they are a Lambert mod).

2. I installed Berkut style armpit scoops. My symptoms were moderate adverse yaw with poor roll response (aileron). It is my belief that both of the above items aggravate the problem, but are not the root cause. We will first discuss what I believe to be the main problem, then discuss why I feel the above two items deepened the severity of the problem.   Ed note: wavy stripes are chase plane canopy distortion.  Not Johns paint job! To help diagnose the problem, Bob Greider taped yarn tufts to the top and bottom of one wing. We flew at various speeds with Bob flying chase in his Grand Champion Long-EZ to observe what was going on behind me. Greider's diagnosis ... Spanwise flow in front of the ailerons on the top only, at low speeds. This accounted for both the adverse yaw and the poor aileron authority.

I found my source for the rubber aileron seals, installed them, and went flying. Nice improvement, back to the not so great performance, that I had always previously
had.

I talked the problem over with Harry Abbott and John Dee. The end result was installing new tufts to the tops and bottom of both wings (and some other areas I
was curious about), and again went flying. This time with Harry and John in the "Blue Goose" shooting pictures. We flew at speeds ranging from 120 knots down to
my minimum airspeed of a gentle pitch bucking 55 knots. We started at 120 and worked the speed down in 10 knot increments. They shot pictures from "Goose
Chase" at each speed step.
Air flow is straight back on the bottom surface of the wing. Different story up top. Air flow is reasonably straight at higher speeds.  As you slow up, the deck angle goes up, and with it, the main wing angle of attack. At 90 knots, you can just start to see the yarn begin to turn outboard on both upper wing surfaces. Straight wing ... no problem. But on the swept VariEze wing, it wants to turn. At 80 knots, the air flow separation occurs well aft of the thickest part of the cord just in front of the aileron. The down aileron is in clean higher pressure air. The up aileron is in lower pressure disturbed air flowing at a right angle in front of it. Tripping may be  aggravated by higher pressure air bleeding up through the aileron hinge gap. The two items I mentioned at the beginning of this article, I feel further aggravated the problem by blowing additional air through the aileron tube clearance hole and sideways out across the aileron, tripping the span wise flow sooner, and with more severity. Seals missing allowed air to pressurize the aileron boxes, and the armpit scoops increased the higher pressure flow nearer to the aft wing root near the aileron tube holes.

(A friend mentioned he had much the same problem with his VariEze when he lost some cowl fasteners in that area, allowing the top cowl to lift and leak air much the same way.)

We landed, and discussed the problem. John and Harry had suggested vortex generators across the front of the ailerons. It was going to be my precious butt flying in front of those vortex generators, so I suggested (insisted) on a limited number, and the same number and roughly the same location on both sides. Harry installed them in just a few minutes, made and quickly applied with aluminum tape. These were located on the wing at the root end of the ailerons only.

Again, with the "Camera Goose" in chase, we flew the same test in 10 knot increments. Higher speeds produced heavier control pressures for roll response ... A step backwards, however at 80 knots control was positive ...  it felt great. Pictures reveal straight tufts behind the vortex generators, with spanwise flow starting outboard ofthe tufts. More testing is in order. I will attempt to organize some of the local VariEze's to fly with tufts on a weekend to get a better feel for how consistent the separation is from airframe to airframe. Will let you know the results, and any fix that improves the bottom end, without undue impact on the top end. Happy takeoffs and soft landings!

-John Lambert

Tuft testing for aileron flow without and with vortex generators

Airflow change with dimishing speed near the trailing edge of N8VE		Airflow with the addition of three pairs of VG's mounted ahead of the inboard third of each aileron
	100 Knots
	90 Knots
	80 Knots
	70 Knots
	60 Knots	60 Knot photo with VG's Not available but the chase plane pilots impression was that the flow was still straight back although it appeared that they were raised off the surface and rotating.

TUFT ENOUGH !- PHOTOG REPORT

What we did:

Ailerons on the VariEze are located inboard and airflow separation over them at low speeds was thought to be the most likely culprit.(ed: of inadequate low speed
aileron response) Harry Abbott recommended to check out the boundary layer flow behaviour using tufts taped to the wing's top rear surface on, ahead, and
outboard of the ailerons. And so we did.

We departed from 27 Ramona as a flight of two on Sunday morning and climbed into a clear, cloudless sky to about 7500' to start the fun and games.

We photographed the right wing's tufts at 10 knot intervals from 100 down to 60. The left wing's tufts behaved symetrically, so we didn't shoot them. The runs were
completed just as it was coming onto lunch time over the Anza-Borrego Desert, so we landed and parked by the airport's "new" restaurant. There we enjoyed good
food, costing only a little more than our Ramona beaneries, with nice atmosphere and we ran into a few friends from our home base. We recommend it as a
refreshing stop.

Over too many coffee refills we proceeded to solve the world's problems, arrived at no consensus, and departed for Ramona. On landing at Chuck Hall Aviation, Harry retrieved some stiff, sticky-backed aluminum metal tape and with a pair of scissors fashioned a set of instant VG's. After a bit of negotiation with the aircraft's builder-owner, Harry located six new VG's about three inches ahead of the right wing's inboard hinge. For control response symmetry, six were also added to the left panel as well. The VG location was based on the fact that the flow there had changed from longitudinal at 100 knots to spanwise at 60 knots.

Once again we departed from 27 Ramona, climbed to altitude and repeated the series shots from 100 down to 70. Harry could not match John's increased sink rate
at 60 knots with VG's, but could see that the tufts aft of the VG's were lifted off the surface at a significant angle and were rotating, suggesting that flow separation
over the aileron was resulting in a higher sink rate. But also, since the tufts were rotating and still in trail rather than spanwise, the VG's seemed to be attempting to do
their job of penetrating the separated flow region via energizing vortices. Harry's camera was a digital Sony Mavica, a very convenient, compact camera having
a very good zoom range and recording on 3-1/2" floppies instead of film. On landing, I drove home (located under the 45 for 27 Ramona) and made floppy copies
for all. We looked at the pics on the computer (no time lost for developing!) and here is what we saw!

What we saw:

Without VG's, the big picture seems to be that the boundary layer flow at the aileron was longitudinal at 100 knots and stagnated with spanwise drift at 60 knots aft of the hingeline. That implied flow separation over the ailerons at the lowest speeds. With VG's, the boundary layer flow downstream of them remained longitudinal at all speeds; ie, the downstream flow remained attached. That observation was consistent with John's noting that the stick lateral response had become stiffer.

Without VG's, as the speed was reduced, the initiation point for trailing edge flow separation progressively advanced inward. Spanwise boundary layer flow first became evident where there was no hinge, suggesting that the hinge attachment rivets might have been tripping the boundary layer, introducing enough turbulence to somewhat energize the flow downstream. (As an aside, note that the first tuft outboard of the aileron has a spanwise component at all speeds, probably due to air bleeding up through the aileron end gap. Also, the most inboard tuft vibrated rapidly at all speeds, which doesn't show in a still shot, but resulted in fraying and finally knotting itself to it's nearest neighbor.)

All in all, we had a great day with our "Experimental" aircraft, and learned a bit more about applied aerodynamics. And by the way, at Williams-Gateway we saw a
Learjet that had VG's installed just ahead of it's ailerons! So, we're not the first to try to solve an aileron control problem that way!

- John Dee & Harry Abbott

If you have a question or comment about this article, send email to the EZ Flyer editor John Dee