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Old Jul 17, 2016, 10:54 PM
phil alvirez is offline
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stab away of wing wake

as wind flows over the wing it gets disturbed and the flow changes direction. then if the stab is withing this flow, it is affected as if its angle of attack is lower. not only that, there is turbulence that makes it erratic. (the so-called downwash).
a way to try to avoid this is to raise the stab, be half way as in 'cross', or t.
the question is: how high?
(it is not that i dont know, it is that i would like to hear from some1 who has experience on this)

please only comments polite, positive and to the point.
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Old Jul 17, 2016, 11:25 PM
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Phil, I'v flown a variety of gliders over the years that have the stab inline with the wing (Gentle Lady, Spectra, Risers and other such "simpler" designs and one of my own design that started at a T tail), slightly above the wing (Top Flite Metric and Bird of Time) and a highly modified RO-8 with the two fuselages that you saw in another thread.

I've read about how these things are supposed to affect the model and how having the stabilizer in the direct wake of the wing is somehow bad. But from a real world experience I have not found that any of this makes any difference that can be sensed in how the models trim out and fly.

If there is any effect it seems to get itself buried in the final trim of the tail to achieve optimum flying. So perhaps a tail in line with the wing ends up at some slight angle different from what it would be if that same tail were mounted at the top of the fin. If there is something in this I can say that it's a darn small difference. I'd say at most it would be around 1 degree and likely even less.

That possible difference in the final trim angle aside I've never detected a hint of any difference to the handling that in any way was a problem for the wing in line with the stab to one with the stab a good 3/4 chord above the wing.

Much of this direction of discussion also seems to assume that the wing wake effect extends back to the horizontal tail. Yet simulations of airflow I've seen modeled as well as videos of the wind tunnel smoke testing of airfoils seems to show what down wash there is coming off the trailing edge being restored to normal horizontal flow within at most 1 wing chord of distance behind the wing. At least that's what I've seen for lower "usable" angles of attack. When forced to very high angles the wake becomes very disturbed. But the angles at that point are well outside of what we'd encounter with a model sailplane flying at anything like normal conditions.

Now all that may well and likely does change for the more serious wing loadings found on heavier and larger models and full size aircraft. But even heavier designs in the model sailplane world tend to be quite light by comparison to most other models.

This also isn't the first time this idea has come up. And it occurred to me that if the wing has a down wash that extends very far back from the wing's trailing edge then the stab should see a very slight negative angle of attack from being located within that downwash. And that means that I should be able to produce a model with a true 0-0 wing to stab set up and with the right CG it should fly with some positive stability.

So... I built such a model as a small all sheet balsa test glider. It had flat sheet wing and tail and the idea was that when I nailed the right CG location it should be able to fly in with some slight positive stability as well upright as inverted. The result was that by the time I got it to glide at all it was neutrally stable and would not recover from even a slightly nose up launch and resulting stall either upright or inverted. The distance of the stab behind the wing's trailing edge was roughly 2.5 wing chords. Span was around 12 inches and chord about 2 inches. This was all around a year and a half ago and the test model was restored to the scrap balsa drawer afterwards.

From this I felt like it supported my previous flying experience that the stab is not flying with any great effect at all from any wing wake effects. I'm not saying there isn't any. Just that they are not significant enough to notice or create any difference in how such models are trimmed compared to a model with a high mounted stab. By "such models" I refer to regular planform gliders with 2.5 or more wing chord gaps between the wing's trailing edge and the stab.
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Old Jul 18, 2016, 07:52 AM
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terrific

thank you very much for the detailed input.
this is what am dreaming of getting from experienced modelers.



regards
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Old Yesterday, 06:52 AM
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details

but in my case i have reached different conclusions.
and this bring us to a point that Don Stackhouse made:

"As usual, "the Devil is in the details". And relying too much on "rules of thumb" and empirical formulas that contain inherent simplifying assumptions can make you overlook some of those details".

i have learned that sometimes 2 persons are right, even if they have reached differerent conclusions.
how could this be?
because, as Don says, they have reached them based on different details that are of vital importance.
i will provide my point of view on this on another post.
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Old Yesterday, 10:00 AM
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I'm sure what Don says has merit. What is also pertinent is that many people reach conclusions far too soon and often based on assumptions without actual experience. Many also jump to conclusions with their first and every following experience.

So.... I'd bet BMatthews and I experienced different details along the way gathering our empennage experiences. I'm also betting there was a decent amount of schooling along the way as well.

The question was about how high to place the stab. My experience is the answer to the question need not address the wing downwash conundrum.

My experience is a T-tail suffers more from structural problems than it benefits from reduced drag and it doesn't benefit at all from avoidance of wing downwash. I had to scratch up my own T-tail as finding a model plan/kit/etc wasn't in the cards.

As for locating the stab anywhere from midpoint down.... No experience of discovery of any performance enhancement or detraction that pointed to location.

If you'd like some insight into stab location and design (note the V tails numbers) take a look at where technology has led glider design.

http://www.espritmodel.com/3D-slope-...ailplanes.aspx

note the difference in chord width and tail moments between the old BOT and the new stuff. and of course, how far up the stabs aren't
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Old Yesterday, 10:30 AM
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I played in the world of turbine jets for a long time , I can guarantee that if the tail on a T-38 wasn't out of the wing wash with anhedral thrown in for good measure , it would be a bear to handle if you could handle it at all . It will porpoise violently at speed . You can damp it only if slow it down significantly . Seen it incorrectly built , then rebuilt correctly , problem completely gone .



As opposed to this L-13 Blanik which could probably care less
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Old Yesterday, 12:21 PM
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There are two different "wing wakes" at play. First of all there is a thin sheet of turbulent air coming off the wing that is basically the remains of the wing's boundary layer. I have seen some cases where a model had "hysteresis" in pitch due to the horizontal tail passing through this layer. Note, "downwash" is very real, and it flows downhill behind the wing, so your tail is most likely to encounter this layer as low speeds and higher angles of attack. Normally, in a more level attitude, the horizontal tail will be sitting above this layer.

HOWEVER, there is also the overall wing wake and vortex flow, which is far higher, although the intensity of it varies depending on where you are within the vortex. The height of that is roughly equal to the wing span. I've attached a pic. Note, in this pic the center of the vortex is actually a little above the wing tip, due to ground effects. At higher altitudes it would be a little below the wing tip, flowing downhill behind the wing. In full-scale glider training, one of the standard exercises was "boxing the wake" behind the tow plane, which included flying down below the tow plane far enough to go through the propwash and the wing wake enough to get below them, then out to the side and back up to above the tow plane, back to the middle behind the tow plane in "high tow" and then back down to the normal "middle" position (which typically needed either spoilers or a forward slip to prevent picking up speed and overtaking the tow plane, which would be very bad).

One of the really startling observations in this exercise was just how far the wake was below the tow plane. And going through it, you could feel the prop wash, which was quite different from the feel of the wing vortex. We never stayed in either one, but just passed through them. The rolling effect from the wing vortex on either side could overpower the ailerons if you stayed in the vortex too long.

The point is that for a tail at any reasonable moment arm, there is no way to get the tail completely clear of the vortex-associated downwash, althoguh the intensity will vary with angle of attack. Generally this is more of a problem for T-tails, because that means the T-tail gets into the heart of the vortex at higher angles of attack than something with the tail lower. In extreme cases this could mean near stall, where immersing the T-tail in the high intensity max-Cl downwash can hold the tail down, making it impossible to recover from the stall. Bombardier (Lear Jet) lost a regional airliner prototype and its test pilots that way, it got trapped in a deep stall and pancaked in. The pilots did not get out.

The effects on models are generally not strong, but can be significant, showing up in particular with non-linear pitch control characteristics. T-tails seem to be the worst (especially near stall), conventional tails less so but generally at more level pitch attitudes. V-tails seem to be the least sensitive to these effects, in no small part due to the dihedral of the tail means that different parts of the tail get inmersed in the wake by different amounts at different pitch attitudes, so the entire tail does not see these effects at the same time. If only a thin strip of tail at any given time gets immersed, then the overall effect tends to get "averaged out' by the rest of the tail.
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Old Yesterday, 01:44 PM
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That appears to be the wake of a Pawnee . Been in it many times having to learn to "box" it .
It's real and will wake you right up.
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Old Yesterday, 05:29 PM
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t or cross

california, here i come: my experience with all kinds tells me that, for my planes, t tail or cross works better. in both cases i think that what matters is that the stab is higher than the wing. period.
with these (t and cross), my planes behaves better: more stable, and response is more precise. and about the structure, i have found a way to make the tails strong enough and without adding too much weight. i may post pics in a future.
also, i have tried servos in tail, and with them the response is even more precise due to no drag in the sleeves of pushrods, but that is another story.
so even if i am not an advocate of t and cross, i have not experienced any bad habit at all.
luck? maybe, but that is my experience.

so, many things depend of the details.

and for that, some times there is more than 1 solution to a problem, depending of how we face it.
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