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Why Lifting Stab?

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Why Lifting Stab?

Old 11-06-2005, 12:06 PM
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Default RE: Why Lifting Stab?



Not every plane has down-wash issues - take a look at any T-tail design.
While a T-tail can get the horizontal stab out of the propwash, I think it would have to be awfully tall to escape the wing's downwash. The low pressure area over the wing doesn't just lift the wing up, it also pulls the air above the wing down resulting in downwash. You would have to get that tail nearly a half wingspan above the wing to be clear of the downwash.

Any one who has raced sailboats is familiar with the "safe leeward position", a boat that's ahead of you and within a mastheight down wind (leeward) can outpoint you because you are in his sail's zone of downwash, well sidewash in this case. Sailors call it backwind or "dirty air". BTW, I took first place in the Hobie 14 Nationals in Oklahoma City in 1989.
OK, on that point... let's not forget the change in pressure around the entire plane as it flies. This pressure envelope is measured many times the distance of the span. We don't really care about it because its so insignificant, that the effort to stay clear of it is pointless. Sure a T-tail (and by that I'm obviously referring to sailplanes - unless you have a DC-9 kit) will encounter down-wash from the air blanket higher up, but it's effect is so minimal, it's pointless to worry about. My point was not to say "there is absolute clean air just a little higher up", but that "cleaner" air is.

A sailplane lacks one dirty feature found in many well known wing designs. That being a flat bottom design. The dirty flat-bottom wing goes through air like a blade on a lawnmower. The aggressive nature of it's design begs the surrounding air to succumb to it's will at the cost of drag and loss of power. Then everyone using it wonders why they experience swelling in a turn. The foil on a sail plane is thin and sleek - almost symmetrical at points, making only nominal lift in comparison to a flat bottom foil. The down-wash from this wing is very minimal. The needed lift is generated from it's immense span, gaining it the efficiency it needs.

With a wing like that, putting a tradition tail behind it would only experience down-wash at a tiny fraction found otherwise. The blanket of air flowing over a foil with such a minimal cord - in conjunction with the tail being so very far back from the wing, would make most people question if there was any down-wash at all. But pilots like my friend Peter Schober doesn't stop there... no, they make use of a T-tail on top of all that. The level of down-wash your pointing out is like saying your receiver antenna is the cause of poor performance due to drag. So I still maintain "Not every plane has down-wash issues". If you consider the amount of down-wash on a sailplane an "Issue".

BTW, Peter is veteran sailplane champion 6 times over (1/3 scale).

I suspect that the real benefit of a T-tail is that it is slightly cleaner than a conventional tail, less interference drag due to surfaces meeting a right angles etc. I doubt that sailplanes use a CG location that has the tail lifting to any significant degree.
Ask most sailboat racers why they set up their boats a certain way and you usually hear some variation of "that's how the guys that win do it". Do they win because of it or in spite of it? I don't know. There's a lot of "monkey see monkey do" in competition.
One thing that a veteran sailplane champion would likely agree with is that all the high tech engineering in a competition sailplane is moot if you chase after a thermal that isn't there.
Old 11-07-2005, 12:11 AM
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Default RE: Why Lifting Stab?

And hence, the whole variable of competing. Nothing is a mechanical process repeatable on demand. If pilots didn't have an angle they stick to, then there wouldn't be much fun in competing. It's the same in any hobby/sport/activitiy. The choice of personal taste.

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