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Old Jan 31, 2003, 11:07 PM
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Denton, TX
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Tail up or nose up... which is it?

It seem that I have begun to be confused about what indicates lift. I went out flying today in 10 - 15mph wind(typical of where I live) and noticed that sometimes I found lift when I saw the tail pop up. Example.- launch 300ft. -trim out - plane comeing down constant rate - continue up-wind and boom, the tail goes up. The plane is in a dive - I recover and begin a turn at 100ft. The plane is in a fast high bank. I've got the elevator pegged rearward, but the plane is not loosing any altitude. I continue to let the plane drift in the turn till I had to bail out in order to fight my way back up-wind. I got about 25- 30 turns at about 45 deg bank while climbing. I know this was a thermal, but did I just luck my way into it? Or is the old nose up thing wrong? JS
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Old Jan 31, 2003, 11:26 PM
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The tail going up is a result of the air going up, and the plane reestablishing a new attitude relative to the new direction of the air. It appears to be nose-down/tail-high relative to the ground.
Then in circling you need to pull a little elevator anyway to keep the plane from descending.
The other usual indication is a wing-rock without any control input. The wing that goes up is in rising air.. turn that direction.. some guys do a quick 270 to enter the center of the thermal.
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Old Feb 01, 2003, 06:14 AM
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vario's Avatar
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please,no flames!

someone will step in and flame me but ,,if the glider is flying straight and level [roll] and the tail picks up ,it usually means that you have flown straight into the thermal in the centre,buy that i mean;;;if you had entered it at an angle the wings would have rocked [roll]

i have found from full size soaring that if that happens you can usually turn either direction and still be in lift

sparky is right,the glider is adjusting its angle of attack to the new found rising air
,,,vario
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Old Feb 01, 2003, 11:13 AM
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Quote:
Originally posted by Sparky Paul
The tail going up is a result of the air going up, and the plane reestablishing a new attitude relative to the new direction of the air. It appears to be nose-down/tail-high relative to the ground.
I have to disagree here. If the glider is in uniform rising air, it has no knowledge of the vertical motion, and will remain in its usual level orientation relative to gravity. The only thing a ground observer will see is that the glider is rising rather than descending.

This situation is exactly equivalent to having the glider fly in a large upward-moving elevator. Surely the glider will not fly tail-high inside the elevator, since there is nothing inside the elevator which indicates vertical motion. This is Galilean invariance at work.

What does make the glider change pitch attitude is flying through a lift GRADIENT. This is because the lift gradient makes the wing and tail see effectively different AoA's, which is equivalent to a decalage change or an elevator input. The glider effectively gets up-elevator when flying into lift, and down-elevator when flying out of lift. The closer the glider is to neutral stability (aft CG), the stronger its lift-gradient pitch response will be. See the CG article at www.polecataero.com for a little cartoon of this.
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Old Feb 01, 2003, 01:12 PM
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Quote:
Originally posted by markdrela
I have to disagree here. If the glider is in uniform rising air,..
.
It isn't. Until it encounters some. The GRADIENT mentioned below.
.
Quote:

it has no knowledge of the vertical motion, and will remain in its usual level orientation relative to gravity. The only thing a ground observer will see is that the glider is rising rather than descending.
.
Everyone sees the tail go up.
.
Quote:

This situation is exactly equivalent to having the glider fly in a large upward-moving elevator. Surely the glider will not fly tail-high inside the elevator, since there is nothing inside the elevator which indicates vertical motion. This is Galilean invariance at work.
.
The glider will be smashed into the floor when it
enters the rising elevator.
.
Quote:


What does make the glider change pitch attitude is flying through a lift GRADIENT. This is because the lift gradient makes the wing and tail see effectively different AoA's, which is equivalent to a decalage change or an elevator input. The glider effectively gets up-elevator when flying into lift, and down-elevator when flying out of lift. The closer the glider is to neutral stability (aft CG), the stronger its lift-gradient pitch response will be. See the CG article at www.polecataero.com for a little cartoon of this.
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Old Feb 01, 2003, 01:40 PM
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Quote:
Everyone sees the tail go up.
[/B]
Not everyone. I see the nose go up when it flies into the rising air column, and then the nose goes back down (tail goes up) when it flies out of the column.

Likewise, when circling on the edge of a thermal, the glider will "balloon" when flying towards the core, and dive down when flying away from it. These pitch cues allow me to adjust the circle to move over towards the thermal core thusly indicated. Works for me.

Quote:
The glider will be smashed into the floor when it
enters the rising elevator.
What does this have to do with the tail going up or down?

As the glider enters the "elevator" (or thermal), the wing will see the updraft a moment before the tail does. This will briefly impart an increase in the effective decalage, and the glider will nose up. The opposite happens as the glider leaves the thermal.

Such decalage changes may be small, but a glider with its CG at the aft limit is very sensitive to decalage changes (infinitely sensitive in theory). So the visible effect on pitch trim can be considerable.
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Old Feb 01, 2003, 01:50 PM
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Re: Tail up or nose up... which is it?

Quote:
Originally posted by wingsnapper
It seem that I have begun to be confused about what indicates lift. I went out flying today in 10 - 15mph wind(typical of where I live) and noticed that sometimes I found lift when I saw the tail pop up.
I think that when you saw the tail pop up, you just exited the strongest part of the thermal. If I saw that, I would try to come back to the spot just BEFORE where the tail went up. That should be where the strongest lift is. Paul may disagree here.
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Old Feb 01, 2003, 01:52 PM
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i see up

you see up sparky???
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Old Feb 01, 2003, 04:39 PM
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What I see...
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Old Feb 01, 2003, 05:48 PM
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Quote:
Originally posted by Sparky Paul
What I see...
Sorry, I can't agree with this diagram. If you're maintaining AoA and airspeed, the glider's pitch angle relative to gravity cannot change. Draw a force vector diagram if in doubt. The uniform vertical airmass velocity does not affect this.

If the glider did nose down like in your diagram, it would accelerate to some higher airspeed, just like in still air. The only way such behavior might occur is if you held some down-elevator after entering the rising air. There is nothing else in the picture which would provide the necessary pitch input to put the nose down (this assumes that the rising air is uniform, so there are no lift gradients).
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Old Feb 01, 2003, 07:03 PM
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I have read these same comments about what indicates lift and have witnessed my models displaying all of these different indicaters at one time or another. I like to fly with my c/g adjusted slightly to the stable side of nuetral. I believe that there are many factors that determine how the model reacts when it runs into lift. Such as, are you flying into, across or downwind. Trimmed for slow flight into the wind or with a little "down" cutting across sink when you run into lift. I think that the model reacts differently in these different circumstances. So, when my model bumps, twitches, rocks, rolls, rises or dives I realize it is telling me "something". If you have been in lift and start losing alt. you are probably in sink and need to get out or circle to try to get back into the lift. Generally, for any other reaction, I assume that it may be some lift and I make a cirlce. It is not always succesful but many times it is. I read someplace, maybe it was "The Old Buzzard", when in doubt, circle. What have you got to lose? The more you fly, the better you will become at determining what your model is telling you.
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Old Feb 01, 2003, 07:57 PM
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Quote:
Originally posted by markdrela
Sorry, I can't agree with this diagram. If you're maintaining AoA and airspeed, the glider's pitch angle relative to gravity cannot change. Draw a force vector diagram if in doubt. The uniform vertical airmass velocity does not affect this.

If the glider did nose down like in your diagram, it would accelerate to some higher airspeed, just like in still air. The only way such behavior might occur is if you held some down-elevator after entering the rising air. There is nothing else in the picture which would provide the necessary pitch input to put the nose down (this assumes that the rising air is uniform, so there are no lift gradients).
.
There IS a lift gradient- a TRANSIENT- condition! The glider goes from STILL air to RISING air. With no adjustment it will seek a flight path -relative to the rising air- which will be a different attitude -relative to the fixed observer- on the ground.
I presume sticking a wingtip into rising air also has no effect, if this is incorrect.
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Old Feb 01, 2003, 08:33 PM
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As stated above, without the gradients you wouldn't be able to detect the thermals at all, other than sometimes the glider would be getting higher, and sometimes not.

I too have seen *both* the nose high and tail high indicators for lift. I'll be flying along nicely and poof, the nose pops right up, I wait half a second, throw in some camber, crank the rudder over and start circling, looking for that ballooning to tell me where the core of the thermal is and adjust accordingly.

On the other hand, on days where there's a lot of sink surrounding the thermals, I often see the tail high thing. I'll hit some bad sink, and the tail goes down faster than the nose, I may push a little down elevator to get out of the sink faster, and when I plow into the next thermal, the whole glider goes up, but the tail goes up faster. I pull back and initiate a normal thermal turn procedure. If I fly through the thermal I may see the tail drop hard again. Maybe it's just me on the stick, but we're talking miniscule amounts of elevator movement, while visually the tail rising or falling is very pronounced. I've had other people comment on it unprompted. "Look at the tail pop up? What's that mean?" Usually it's a sign of *big* lift. I see it when I hit a thermal that's taking a paraglider up fast.

Something about the elevator analogy bugs me too, but I'm having a hard time putting my finger on it. Not that lack of gradients (we've covered that), but ignoring accelleration and momentum of the glider. The main wing is generating lift, and the glider balances on it. The horizontal stabilizer generates zero to some negative lift. So here's what's bugging me. Say you've got this big massive elevator filled with air. Wherever the elevator goes, the air goes (assume no induced turbulence). What happens if you're gliding along happily and the elevator starts accelerating upwards. My thought is that since the CG and center of lift really aren't in exactly the same place, the upward acceleration will induce a pitching moment. If the CG is in front of the center of lift (which is normal slightly stable setup) then the glider should pitch *down*, as the air in which it's flying is accelerated upwards. Perhaps pitching up, is a result of crossing a gradient, and pitching down is a result of finding oneself in air that's being "uniformly" accelerated (not just rising, but accellerating) upwards. That makes sense to me, because I *know* that the tail down attitude is most common in broad and brutal sink. "The sky is falling" kind of sink. It may also be more pronounced the more stable your glider is set up (CG a bit more forward).

BTW. There is a time when the entire sky seems to go up at once, with no visible turbulence or gradient at all. It's "magic air" and occurs when soaring over the middle of a valley (even relatively close to the bottom), usually after sunset. A whole bunch of cool air slides down the sides of the valley underneath the valley air, which gets pushed straight upwards. If your glider is up above it, then it seems as though the entire sky is rising at once. It really is magic. I've seen it so big that paragliders had to pull big-ears to get down out of it before it got totally dark. I've taken a Zagi combat glider up and flown it until it got so dark I almost couldn't see it flying magic air. Oh.. not uncommon to not feel a breath of wind when it's doing this too.

ian
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Old Feb 01, 2003, 10:46 PM
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Judgeing by the range of comments, it seems I've touched on a topic the proves that we all see things slightly differently. I have talked to Ray Hays and read Thornburg's Old Buzzard's book. Both state tail high = lift. Also, both sources have been flying model sailplanes longer than I have been breathing air. I have to admit that I have also seen it go both ways though. This is the point of my confusion and I think the only way to sort it out is to fly the same model as often as I can.
One thing I did do before left the field is use that 15-20 wind as a kind of simulation of lift. I held the fuse with the tips of my fingers at a neutral angle (glider was vertical- pivoting on my fingers) and turned so that the wind was hitting the bottom of the wing and stab. Guess which end swung back.... the tail. When I tried the opposite the glider seemed to try to stay vertical. It seems that during this crude experiment that the glider did try to seek the least resistance and "fly" into the wind.

Try this and I'd love to see your comments.
Ps. I fly my glider tail heavy/ maybe a nose heavy model would have a different reaction.
JS
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Old Feb 01, 2003, 11:19 PM
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Quote:
Originally posted by markdrela
Sorry, I can't agree with this diagram. If you're maintaining AoA and airspeed, the glider's pitch angle relative to gravity cannot change. Draw a force vector diagram if in doubt. The uniform vertical airmass velocity does not affect this.

If the glider did nose down like in your diagram, it would accelerate to some higher airspeed, just like in still air. The only way such behavior might occur is if you held some down-elevator after entering the rising air. There is nothing else in the picture which would provide the necessary pitch input to put the nose down (this assumes that the rising air is uniform, so there are no lift gradients).
next time your nose drops and your tail lifts,,set down your slide ruler and pay attention,if you cant understand that i glider can climb with a low angle of attack [sorry, tail high!] then i suggest you get [DELETED INAPPROPRIATE LANGUAGE] in a real sailplane and see for yourself

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INAPPROPRIATE, DISRESPECTFUL LANGUAGE WILL NOT BE TOLERATED



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