Sep 07, 2013, 05:18 AM
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Quote:
 Originally Posted by Taurus Flyer More speed Cl of the tip is negative less speed Cl is positive.
For a wing with a non-zero tip chord, Cl right at the tip is always zero, regardless of speed (or AOA).
Sep 07, 2013, 05:33 AM
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Quote:
 Originally Posted by ShoeDLG For a wing with a non-zero tip chord, Cl right at the tip is always zero, regardless of speed (or AOA).

Quote:
 Originally Posted by Taurus Flyer For the Horten the Cl =0 for the tip at the airspeed it's designed for. The bell shape lift distribution. More speed Cl of the tip is negative less speed Cl is positive.
Bell shaped lift distribution!

Quote:
 Originally Posted by aeronaut999 Coming back around to some of the questions raised in the original post-- with a swept, tailless planform, is it more accurate to say that ground effect causes the outer parts of the wing (the tip region) to operate in a reduced downwash, or a reduced upwash? Do the outboard regions of a swept wing normally operate in an an upwash, or a downwash? Steve
You may find out yourself how the lift distribution over the outboard regions looks like !

TF
Last edited by Taurus Flyer; Sep 07, 2013 at 06:03 AM. Reason: post shoe added
Sep 07, 2013, 06:11 AM
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For an untwisted wing with a taper ratio of 0.25 (first attachment), the Cl distribution along the span at 0, 20 and 40 degrees of sweep is shown in the second attachment (AoA =6 degrees).

The dashed line at the top shows the 2D Cl corresponding to 6 degrees AoA. This tells you that the entire wing is operating in downwash. As the sweep increases from 0 to 20 degrees AoA, you can see that the Cl near the tips goes up. This tells you that the sweep is reducing the downwash near the tips, but it is still net downwash there.

# Images

Sep 07, 2013, 06:41 AM
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See

# Images

 Sep 07, 2013, 02:20 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 674 Posts Yes, I see. Cl at the wing tip is zero.
Sep 07, 2013, 02:23 PM
I bail out, anywhere, anytime
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Quote:
 Originally Posted by ShoeDLG Yes, I see. Cl at the wing tip is zero.
That's because a tip according to your definition has an area of ​​0 square inch!

Have a nice day Shoe, I bail out!
Last edited by Taurus Flyer; Sep 07, 2013 at 02:35 PM.
 Sep 11, 2013, 02:52 PM Registered User The Willamette Valley, Oregon Joined Dec 2008 841 Posts Thanks for the comments Shoe, I think I've learned a few things here... After an afternoon of hands-on experimenting, I've pretty much given up the idea of actually measuring a change in pitch trim due to ground effect and/ or wind gradient, and likewise the idea of measuring a change in "effective dihedral", for any given pitch attitude or angle-of-attack, due to ground effect and/or wind gradient. Absent a wind tunnel or a moving platform and some assistants, it is too difficult to make precise measurements and observations, to discern any such effects that may be present. It does seem that one effect is strikingly present near the ground that is absent in normal flight: there is a strong roll-due-to-bank effect. Absent other compensating factors, the upwind wingtip wants to rise. I don't see this in normal flight. I attribute this effect to the wind gradient. Unfortunately I don't have an easy way to test this hypothesis with zero-wind tests. I don't care to actually fly very near the ground in banked, turning flight, especially with no wind to reduce my groundspeed! Elaborating: in ground-handling tests of a hang glider on flat ground, with wind strong enough to lift the glider's full weight at zero groundspeed, I intentionally kept the nose yawed cross to the wind at a constant angle. At high angles-of-attack, the "effective dihedral" was positive and the upwind wingtip wanted to rise. I could bring the glider back into balance, roll-wise, by raising the downwind wingtip. At the correct bank angle the glider would balanced in roll and would require zero roll torque from the pilot to hold the bank angle constant. On the other-hand, at low angles-of-attack, the "effective dihedral" was negative and the downwind wingtip wanted to rise. I could balance the glider in roll by keeping the upwind wingtip higher than the downwind wingtip. This roll-torque-due-to-bank effect means that once a wingtip starts rising, it tends to keep rising. The roll-torque-due-to-bank is also evident when the nose is pointing directly in the wind, rather than yawed to the side. I welcome any more comments re the original questions... thanks Steve Last edited by aeronaut999; Sep 11, 2013 at 03:07 PM.
 Sep 12, 2013, 01:22 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 674 Posts I can offer a description of what might be going on that can explain what you're observing. When you roll the wing, are you doing it at constant angle of attack, or at constant pitch attitude? When you roll the downwind wingtip up, you are converting some of the angle of attack into sideslip (assuming you perform the roll at constant pitch attitude). It's not too hard to imagine that if you raise the downwind wing far enough, the wind will eventually be "blowing on the top of the wing". When you roll the upwind wingtip up, you are converting some of the sideslip into angle of attack (again, assuming you roll at constant pitch attitude). In the case where you started at high angle of attack, you are reducing the angle of attack by rolling the downwind wingtip up. In the case where you started at low angle of attack, you are increasing the angle of attack by rolling the upwind wingtip up. Since the glider exhibits positive effective dihedral at high angle of attack, and negative effective dihedral at low angles of attack, presumably there's an intermediate angle of attack where the effective dihedral is neutral. It's likely that what you are doing by rolling the glider in both cases is "rolling the angle of attack" to this intermediate value where the effective dihedral goes to zero and the side slip no longer causes a rolling moment. Last edited by ShoeDLG; Sep 12, 2013 at 01:54 PM.
 Sep 12, 2013, 03:06 PM Registered User The Willamette Valley, Oregon Joined Dec 2008 841 Posts Shoe, that is an interesting point in the above post. It's worth noting that a hang glider doesn't offer the pilot good visual references for easy, precise monitoring of small changes in pitch attitude. In some tests I attached some bubble levels to the glider for this purpose. Your question is raising some other questions in my mind. Let's say we take a full-scale airplane and run a giant barbecue skewer through from nose to tail and put the ends of the skewer on supports that allow the aircraft to roll left or right around the skewer. We face the whole apparatus so that the aircraft's compass heading points directly into a steady wind and adjust the height of the supports so that the wing meets the air at a 5-degree angle-of-attack. Let's assume the wing has zero incidence relative to the fuselage and skewer so that the skewer and fuselage have a 5-degree nose-up pitch attitude. If we then roll the aircraft around the skewer to a 45-degree bank angle, has the pitch attitude gone more nose-high? If the pilot were looking through a gunsight, wouldn't the distant horizon line drop further below the pipper, at the point where the distant horizon line crosses a "vertical" (canopy-to-belly) line inscribed through the center of the gunsight? If we then roll the aircraft around the skewer to a 90-degree bank, doesn't the pitch attitude become undefined? The distant horizon no longer crosses the "vertical" line in the gunsight at all. It seems to me that this is not a roll with a constant pitch attitude. But it is a roll around the aircraft's longitudinal axis. It looks to me like that is the situation you were describing as a possible explanation for my results. Is this really a roll with a constant pitch attitude? Meanwhile, the angle-of-attack is not constant either-- the angle-of-attack drops to zero at the 90-degree-bank point. Is there a different scenario that could be more accurately described as a roll with a constant pitch attitude? If the barbecue skewer were exactly parallel to the wind, then angle-of-attack would stay constant as the aircraft rolled. Only in the case where the skewer is exactly parallel to the horizon, does the pitch attitude stay constant as the aircraft rolls (in this case the pitch attitude may be zero as seen through the pilot's gunsight, but doesn't need to be, if the skewer is not parallel to the axis of the gunsight. Or, substitute "artificial horizon" for gunsight".) Getting back to the application of these ideas to my ground-handling experiments-- I would say that as I held the aircraft on a fixed heading, offset from the wind, and a roughly fixed angle-of-attack, I was mainly keeping the pitch forces constant as a cue that the angle-of-attack was staying constant. While keeping the pitch forces roughly constant, I adjusted the bank angle to neutralize the roll torque. So, I think that the angle-of-attack (say as measured at the wing root) was staying roughly constant as I changed the bank angle. The bank angles involved were modest, always less than 30 degrees. It still seems to me that the wind gradient must have been the cause of the observed roll-torque-due-to-bank angle. Would a true roll with constant pitch attitude, involve a constant pitch indication in a gunsight or artificial horizon? It seems that this could not involve a rotation that was solely around the aircraft's longitudinal axis, unless the aircraft's longitudinal axis happened to be exactly horizontal. Steve S Last edited by aeronaut999; Sep 12, 2013 at 03:27 PM.
 Sep 12, 2013, 03:23 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 674 Posts I wouldn't get too wrapped around the definition of pitch attitude. Take your example where you skewer the airplane nose-to-tail. Imagine you start with the skewer pointing directly into the wind. If the wing has zero incidence then its angle of attack and side slip angle will be zero. Now raise the front of the skewer 5 degrees. The wing will be at 5 degrees angle of attack. Now change the heading of the skewer by 5 degrees. The wing will now be at 5 degrees angle of attack and 5 degrees sideslip angle. If you roll the airplane about the skewer by lifting the downwind wing, eventually the wind will be blowing on the top of the wing. In other words rolling the airplane about the skewer by lifting the downwind wing is reducing the angle of attack (to the point where the angle of attack will eventually become negative). If you are rolling you hang glider as if it were on a fixed skewer, then you are adjusting both angle of attack and sideslip angle as you roll.
 Sep 12, 2013, 03:47 PM Registered User The Willamette Valley, Oregon Joined Dec 2008 841 Posts Shoe, since your response is recent, I may have edited my last post after you read it. I don't think that I was changing the angle-of-attack as I changed the bank, because I cueing off the pitch trim pressure, not off of a visual reference that allowed me to keep any reference line on the glider in a fixed attitude in space. The skewer example was a counter-example to what I think I was actually doing. it's interesting food for thought though. I do see that there would have been some change in sideslip angle, because the glider's heading stayed roughly constant. Steve
 Sep 12, 2013, 03:52 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 674 Posts Imagine that an airplane has an attitude indicator with graduations on it. If you start from wings level and raise the nose 15 degrees, the "dot" representing the nose of the airplane on the attitude indicator will sit on the 15 degree graduation. If you then roll the airplane as if it were on a fixed nose-to-tail skewer, the dot would remain on the 15 degree graduation. In other words, a pure roll about the airplane's nose-to-tail axis is by definition a roll at fixed pitch attitude (unless you have a whacky definition of pitch attitude such that the elevation of the dot on the attitude indicator doesn't correspond to the airplane's pitch attitude).
 Sep 12, 2013, 03:53 PM Registered User The Willamette Valley, Oregon Joined Dec 2008 841 Posts Another relevant point-- in the version of the experiment where the nose was pointing into the wind, heading-wise, banking the glider still made the high wing want to keep rising. If the bank adjustment had involved a significant decrease in angle-of-attack, the glider would have likely no longer supported its own weight, at least in some of the trials where the wind was barely strong enough to support the glider's weight. Again, I think I was responding to the pitch trim pressure in a way that kept the angle-of-attack roughly constant during the changes in bank angle. Steve
 Sep 12, 2013, 04:07 PM Registered User The Willamette Valley, Oregon Joined Dec 2008 841 Posts Shoe here's perhaps a more fundamental point-- Thinking through the crosswind case in more detail--say a crosswind from the right-- if we are rolling the glider as if it is attached to a skewer, which is kept in a fixed position in space, then-- and assuming horizontal airflow--(tests on flat ground or in horizontal flight)-- and assuming negligible roll rate, we are dealing with the results of being banked, not the results of the rolling motion per se-- 1) If the axis of rotation is inclined nose-up (e.g. roughly parallel to the mean wing chord, with the wing at a high angle-of-attack), then rotation either left or right reduces the angle-of-attack of BOTH wings, does it not? 2) If the axis of rotation is inclined nose-down (e.g. roughly parallel to the wingtip chord, on a highly washed-out-wing in a part of the flight envelope where the root is at a low angle-of-attack), then rotation to a banked attitude, either to the left or to the right, makes the angle-of-attack of any part of the wing, whether the left wing or the right wing, become more positive or less negative, does it not? The 0-bank and 90-degree-bank are the easiest cases to visualize. The other bank angles are harder to visualize but I'm not seeing that the placing the aircraft in a banked attitude in this fashion would increase the lift of one wing and decrease the lift of the other wing. Am I missing something? You gave an example where the wing might be hitting the top of one wing-- the upwind wing-- bearing in mind the anhedral geometry. True, but by the same taken the other wing is also seeing its angle-of-attack become less positive, or in the most extreme cases, becoming negative, by exactly the same amount. The delta aoa due to the rotation seems the same magnitude and sign on each wing, as far as I can see, regardless of the fact that there is a crosswind. Steve
Sep 12, 2013, 04:31 PM
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Quote:
 Originally Posted by ShoeDLG Imagine that an airplane has an attitude indicator with graduations on it. If you start from wings level and raise the nose 15 degrees, the "dot" representing the nose of the airplane on the attitude indicator will sit on the 15 degree graduation. If you then roll the airplane as if it were on a fixed nose-to-tail skewer, the dot would remain on the 15 degree graduation. In other words, a pure roll about the airplane's nose-to-tail axis is by definition a roll at fixed pitch attitude (unless you have a whacky definition of pitch attitude such that the elevation of the dot on the attitude indicator doesn't correspond to the airplane's pitch attitude).
Shoe, I don't think this is the case. Or I should say, it is only the case if the axis of rolling is exactly horizontal. Well, hang on a minute... OK I guess I see your point, but I would point out that there are other sensible ways (if not standard ways) to define pitch attitude. It really is just a matter of definition.

Back to the example of a gunsight with a fixed pipper-- imagine that the axis of rolling is the same as the aircraft's longitudinal axis, and the aircraft is initially pitched nose-up 30 degrees. The horizon line is 30 degrees "below" (in aircraft's reference frame) the pipper of the gunsight. The horizon line intersects the "vertical" crosshair of the gunsight at the 30-degree tickmark. As the bank angle increases, the horizon drops further "below" (in the aircraft's reference frame) the pipper, and when the bank angle is vertical, the horizon line does not pass through the "vertical" crosshair through the gunsight pipper at all (which can be interpreted as meaning the pitch angle has become infinite? Or perhaps 90 degrees? I'm not sure.)

Think about the 90-degree bank case. The horizon line is "vertical" in the aircraft's reference frame, and not passing through the pipper at all.

Translating to an artificial horizon-- I guess I'm thinking in terms of reading the instrument by looking to see where the horizon line intersects an imaginary "vertical" (in aircraft's reference frame) line though the center of the face of the instrument. This reading will increase as we roll toward a 90-degree bank. If we read the instrument in the more conventional way, by looking at the gradation under the dot, then the reading will be much lower, and will remain constant as the aircraft rolls around the longitudinal axis, or whatever axis is precisely aligned with a zero pitch reading on the artificial horizon.

I guess what we are getting at, is that an "attitude indicator", with its horizontal gradations and central dot, doesn't really indicate the aircraft's pitch attitude, if you read it at whatever gradation is under the dot. Instead, it appears to indicate the angle by which the aircraft's longitudinal axis (or whatever axis the pipper has been aligned with) is inclined above the horizon, as seen by an outside observer. It looks to me like the true pitch attitude should be represented by the point where the horizon line passes through a vertical line drawn through the face of the instrument. But, there is inevitably some distortion in representing three-dimensional reality with a mostly-flat instrument face, it's an imperfect representation of the gunsight analogy I was giving above.

For example in the situation illustrated by the artificial horizon in the upper right of this illustration (front page) http://flighttraining.aopa.org/magaz...technique.html , I would say the true pitch attitude is actually much higher than 15 degrees. But the longitudinal axis is 15 degrees above the horizon as seen by an outside observer.

Fundamentally it simply boils down to which direction we mean by "above". "Above" the horizon as seen by the pilot and airplane, or "above" the horizon as seen by an outside observer? "Above" as in how far below the pipper the horizon intersects the "vertical" crosshair of the gunsight, or "above" as in measuring from the pipper to the horizon in a direction that runs perpendicular to the horizon? (That's what the artificial horizon does.) That's all. Yes, as you implied above, it is simply a matter of definition. OK I realize now that I was thinking of measuring the pitch attitude in a rather non-standard way, but a way that still makes a lot of sense from the standpoint of what's really going on with the aircraft.

All interesting food for thought. But as to how all this relates the original experiment, I would say--

1) I think I was not rolling the aircraft in a manner that changed the angle-of-attack, because I was cueing off of pitch control pressure.
2) I think that rolling around a "constant pitch attitude", i.e. a barbecue skewer fixed in space, would change the angle of attack of each wing in the SAME direction, not in opposite directions. Even in a crosswind situation.