Trailing Edge Fences
 2005.11.04
 

Trailing Edge Fences
Several hundred hours with the TE Fences has made me a believer. The most noticeable difference was a more comfortable, lower approach speed. While not specifically quantified, I agree with those that report a shorter takeoff roll and no speed loss on the top end.

Recently I made new set of TE fins with half-inch base flanges. The flanges were fabricated while standing up comfortably the whole time, rather than having to spend half the time turned upside down under the wing. Maybe everyone has already figured this out but me. Am sending a note to Terry Schubert for the CSA Newsletter.

The fence patterns were made using yard sticks laid aft on top and bottom of the wing with about a four inch high trailing edge, so the fin top and bottom edges are almost parallel. They extend aft of the wing about an inch and a half. They are spaced about twenty inches from the winglet and each other.

Klaus’s TE Fences are probably one of the most simple and effective improvements that have been developed. They are obviously beneficial.

But I seldom get their full advantage in the slow speed areas where they work best. Grass strip operations would seem to get the most benefit, where you want to get off the ground as soon as possible and land as slow as possible. But that’s not my arena. If I moved to a short strip, they’d be my best friend.

Operational Considerations
The Trailing Edge Fences didn’t produce an extra five knots (heck) or make me Superman. My wife says heck.
After initially installing the fences I made quite a few practice short field landings, at home base with the usual sunset run fuel load. All set. Then on a couple of cross countries, I learned not to arbitrarily throw in a short field approach without due consideration, checking first, for example, for a crosswind over structures or trees.

I accidentally tipped the 62 inch carbon prop a couple of times, resulting in the tip folding over a little. The practice approaches at home had been under ideal circumstances. Both of these firm landing events were actually the result of burble and an unnecessarily slow approach speed.

Both events involved landing on shorter runways than I normally use. On one the slight crosswind was coming over buildings adjacent to the touch down point. The other was from air tumbling off the end of the embankment at the end of the runway.

Both times I had a passenger and had flown for about an hour and a half, so not much fuel was burned off. A short-field type approach was made both times. Both times the burble caught me on the down side of its rotation and spread the gear a little. Both times the approach was flown slower than necessary, with no problem slowing down afterward on the runway.

As a result I now add ten mph in turbulent or crosswind landings. You want plenty of air over the control surfaces anyway and the TE Fences are still an asset.

Power is often added just at touchdown. Interesting discussions can develop here, with concern that adding power at touchdown will extend the ground roll.

I like to listen in on the Power vs Attitude discussion. Power controls altitude, right? Power allows the rate of descent to be reduced without having to increase the already proper nose attitude. Nicer for the prop tips. The follow-on phrase, attitude determines speed, points to the advantage of holding a constant attitude during touchdown to avoid adding speed and increasing the ground roll.

A normal takeoff around here is also accomplished without getting maximum gain from the TE Fences. A short ground roll is not the target, but rather a good energy state over the last half of the runway, past the abort point.

I disagree with trying to get the nose off early. Fine for grass strips. When flying from hard surfaces the early nose lift causes the plane to waddle along sluggishly rather than accelerating briskly. The Pilot Operating Handbook works good here. In addition I prefer to set the elevator trailing edge about ¾ inch low and let the plane accelerate and lift off when it’s ready. Even on a shorter runway. This results in more energy throughout the takeoff evolution.

On takeoff I normally see 150 to 160 mph at 200 feet over the end of our 6000 foot runway here. On the turn-back decision, I can use 120 mph instead of just altitude. If making a forced turn back to the runway the canard is happier at 120 than at 80, as are the control surfaces.

If I’m low and slow over the runway climbing out at 85 mph and the noise stops, I’m looking over the canard, partly because like it or not I’m most likely going to touch down out there somewhere in or ahead of its span. And if I have any maneuvering options, I’m looking over it because I’m at least thinking that the nose needs to go over to 100 mph if possible.

Whether landing straight ahead or madly maneuvering we need a 22 mph buffer speed above “minimum speed” to flare and arrest the rate of descent. I purposefully didn’t use the word “stall speed”. Properly built and CG’ed, there ain’t one. It’s actually the canard’s “angle of maximum-lift” speed. And it’s comfortable there. And because of it’s design, span and cord, it ain’t going any higher. This works out nicely to be the aircraft’s “minimum speed”.

Well, I did it. The “stall” word popped out.
I just pulled out a couple of pages here on stalls, and EZs, and sink, and put it into a note for next time, after I’ve gathered my composure.

In any case, even with the need for speed, I am glad to have the advantage of the TE Fences.
Bill James, Fort Worth VariEze

PS- Several notes ago I did a touch and go off of why we are so stinkin’ determined to get out there in these planes. Thanks to #1 son Trey, I am now reading what I was thinking, in Wild at Heart by John Eldredge.

 

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