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Old May 16, 2012, 08:52 AM
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Originally Posted by richard hanson View Post
And that also applies to T tails correct?
This business of downwash resolving the issue is -----amazing .
But you are of course welcome to your own conclusions
It will be harder, maybe impossible, to achieve pitch stable upright / inverted / no trim change w/ t tail. Because the tail position is not symmetrical with respect to the downwash. Further proving the point that the downwash has significant effects.

No one is claiming that an aircraft cannot be pitch stable if the tail is not in the downwash. We're just saying that the downwash has significant effects on pitch stability. That is just common sense. And provable- w/ the upright/inverted /pitch stable / no trim change demonstration.
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Old May 16, 2012, 09:00 AM
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You know all that ?
Can you demonstrate it or are you relying on examples written by others .
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Old May 16, 2012, 09:07 AM
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Apologies for the production quality, but here's a 0-0 glider in flight. Despite it being a bit gusty, I think it's pretty clear that you can get upright and inverted stable glides (certainly not a ballistic "lawn dart")

0 - 0 Glider (0 min 18 sec)
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Old May 16, 2012, 09:19 AM
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Originally Posted by richard hanson View Post
You know all that ?
Can you demonstrate it or are you relying on examples written by others .
Re the t tail, that is some conjecture on my part. I don't know for sure that a t-tail can't be pitch stable upright and inverted w/ no trim change. If I come across something relevant in the way of demo videos or engineering textbooks, I will post. The conventional tail situation, I knew before this thread started, because it is addressed in textbooks and also is commonly encountered by real world rc fliers. Steve
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Old May 16, 2012, 09:33 AM
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Re t tails and downwash- you have heard of "deep stall", right? The a o a is very high and the t tail is in the downwash, making a nose-up pitch torque that tends to lock the plane into the stall. This is mentioned in many textbooks. Or just google t tails and downwash if you want to learn more about t tails and downwash. Steve
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Old May 16, 2012, 09:38 AM
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Thank you -I now understand your background-
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Old May 16, 2012, 11:05 AM
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Originally Posted by Brandano View Post
For the purpose of demonstrating stability then a glide at a constant rate of descent should be enough. It's not even necessary for it to be the same rate of descent for upright and inverted flight, as long as either way up a stable glide is established where there is no acceleration in any direction, but just constant speed in one direction. When it comes to aircraft stability even a non-diverging phugoid is considered stable, so constant glide might be excessive, but I would settle for a constant glide over a distance that can be extrapolated for another distance and still result in an average constant descent rate and speed. That said, I still think it unlikely that an airplane could be designed to be stable both upright and inverted without trim changes, but I am not excluding it outright. What I have in my head is a sort of stability curve vs AOA that looks like the section of an old fedora hat, with a positive and negative AOA the plane will try to align to and a valley at 0 AOA, perhaps because the tail is exactly in the wing wake and ineffective. A plane designed like this would gradually decrease the AOA to the upright stable AOA, or, if launched at 0 AOA, do half a negative loop and reach the inverted stable AOA.
.
Yes.
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Old May 16, 2012, 11:45 AM
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Quote:
Originally Posted by ShoeDLG View Post
Apologies for the production quality, but here's a 0-0 glider in flight. Despite it being a bit gusty, I think it's pretty clear that you can get upright and inverted stable glides (certainly not a ballistic "lawn dart")

http://youtu.be/luTikIdrZiM
.
The triangular paper gliders we fold up fly just like that, with zero incidence.
Over a longer period of flight, they plummet.
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Old May 16, 2012, 02:52 PM
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Originally Posted by Sparky Paul View Post
.
The triangular paper gliders we fold up fly just like that, with zero incidence.
Over a longer period of flight, they plummet.
Of course a zero-zero plane will have a vertical dive mode, where there is no lift / no downwash/ no downwash-induced apparent decalage. At least if we add in some other "zeros" like CG is not below center of drag, so zero pendulum effect (not true of paper airplane). Still, that doesn't mean the zero-zero-zero plane can't also have a stable non-vertical glide mode. And the plane doesn't know whether it is upright or inverted if there is zero asymmetry between top and bottom. (Fin projects equal distance above and below, symmetrical airfoil, etc). So if it has a stable flight mode upright, it will also have a stable flight mode inverted. Steve
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Old May 16, 2012, 02:54 PM
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This thread http://www.rcgroups.com/forums/showthread.php?t=1650645

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Originally Posted by aeronaut999 View Post
The rc slope glider guys have chimed in resoundingly saying that many models are pitch stable both inverted and upright with no pitch trim change. The thread is currently on page 2 (now bumped to page 1) of the slope section of the glider section of this forum. It will be easier for me to post a link when I am not posting from a phone.

I believe that the effect of the wing's downwash on the tail is the key to understanding how this can be possible.

Steve
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Old May 16, 2012, 03:22 PM
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Back to the original topic of the thread, this is an interesting read...
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Old May 16, 2012, 03:24 PM
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Decalage is not even in the stability equation. Stability requires the tail to apply a corrective force at the CG that is greater than the force change between the wing and CG after an upset. This is accomplished by the volume of the horizontal stab and the distance between the wing and stab.

A symmetrical wing section with the CG at the aerodynamic center does not have an up or down load at any trim setting. The elevator angle, or decalage, determines the wing AOA. If the CG is moved back the stab must now support the load increase at any trim setting. As I pointed out before, moving the CG rearward does the same thing as up elevator and as long as the CG is still ahead of the neutral point it will still be stable, only less so. So the conclusion I come to is zero-zero gliders just use CG location to trim the AOA.(Edit) I don't mean with a moving mass while flying. I mean by adjusting on the ground for the current wind strength.

So… if the AOA is the fuselage reference line in relation to the glide slope and a propeller axis is also inline, what happens to the direction of the airstream as power is applied? It goes from glide slope to reference line for part of the wing and most of the stab... trim change.

If everything is symmetrical, it is the same either side up.
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Old May 16, 2012, 03:38 PM
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I am not sure I fully follow you. I think you are saying that applying power decreases the aoa of the wing and tail due to the slipstream direction being different than the relative wind direction.

Interestingly, in Frank Zaic's "Circular Airflow and Model Aircraft", he opens by talking about a case where the slipstream was curved by the wing's downwash to exert a powerful downforce on the tail. Causing the model to loop under power. This was with a high-mounted engine on a pylon above a wing with lots of dihedral. Putting the engine on the nose and putting the wing high above the fuse on a pylon avoided this problem as the slipstream was no longer so much curved by the downwash. Or maybe the point was that the slipstream was curved down to pass below the tail? My copy is out in the car, I can't quite recall that detail. Seems weird that the plane with the higher thrustline was much more prone to looping, but it was.

If this seems wrong to you, check the book and see if I have gotten anything wrong or if you disagree with the author's explanations...

Steve


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Originally Posted by Steve Anderson View Post
Decalage is not even in the stability equation. Stability requires the tail to apply a corrective force at the CG that is greater than the force change between the wing and CG after an upset. This is accomplished by the volume of the horizontal stab and the distance between the wing and stab.

A symmetrical wing section with the CG at the aerodynamic center does not have an up or down load at any trim setting. The elevator angle, or decalage, determines the wing AOA. If the CG is moved back the stab must now support the load increase at any trim setting. As I pointed out before, moving the CG rearward does the same thing as up elevator and as long as the CG is still ahead of the neutral point it will still be stable, only less so. So the conclusion I come to is zero-zero gliders just use CG location to trim the AOA.

So… if the AOA is the fuselage reference line in relation to the glide slope and a propeller axis is also inline, what happens to the direction of the airstream as power is applied? It goes from glide slope to reference line for part of the wing and most of the stab... trim change.

If everything is symmetrical, it is the same either side up.
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Old May 16, 2012, 03:52 PM
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Originally Posted by aeronaut999 View Post
pendulum effect
Can you describe what you mean by pendulum effect (in the context of how you used it, of course)?
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Old May 16, 2012, 04:55 PM
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Pendulum effect

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Originally Posted by DPATE View Post
Can you describe what you mean by pendulum effect (in the context of how you used it, of course)?
Sure.
(I knew that someone would ask!)

CG is far below center of drag. In steep dive, drag provides a nose-up pitch torque, because drag acts high above the CG.

Because of the pendulum effect, the high-wing configuration is inherently more stable than the low-wing configuration, for the same decalage.

Consider hang gliders. Normally the pilot has to apply a pull force on the control bar to maintain a high airspeed. At moderately high speed, the wing normally generates an net aerodynamic pitch torque that wants to pitch the nose up, and the pilot is giving a nose-down pitch torque to offset this and hold the aoa constant.

If the pilot has to exert a push force on the bar to maintain a constant body position and a constant airspeed, that means the wing has run out of aerodynamic pitch stability and wants to pitch down not up. But the pilot's push force on the bar is holding nose up. So the aoa stays constant.

At the point where the pilot has to apply a push force not a pull force just to hold his body in a constant position, he has suddenly started contributing a stabilizing (nose-up, pendulum) effect rather than a destabilizing (nose-down, antipendulum) effect. The wing has run out of aerodynamic pitch stability and only the pendulum effect of the wing's drag vector acting high above the CG of the pilot/glider combination is preventing the angle-of-attack from decreasing more. Beware! The glider's stability margin is becoming thin.

Of course there is also a geometrical solution to the pitch attitude where the pilot has to pull rather than push to hold himself in a constant position. So there is a geometrical limit to the steepest steady-state nose-down pitch attitude that is possible in a hang glider where the wing has positive aerodynamic pitch stability independent of the pendulum effect.

There's no way to say that this pendulum nose-up pitch torque does not exist. As the pilot exerts whatever force on the bar is needed to hold himself in a constant position, he can feel in his arm muscles that he is exerting a nose-up pitch torque on the control bar! That is the nose-up pitch torque contributed by the pendulum effect of the pilot's body, as it is forced by the pilot's arm muscles to stay in a fixed position relative to the rest of the glider. That is the same as the nose-up pitch torque created by the wing's drag vector, acting high above the CG of the glider-pilot combination. The pendulum effect by the wing's drag acting high above the pilot's body which is locked in fixed position on the control bar, and the force exerted by pilot's arm muscles on the control bar as he holds himself in a fixed position on the bar-- they are one and the same.

If the pilot lets himself swing freely, of course, the pendulum effect contributed by his body disappears. So in one sense there is no pendulum effect independent of whatever control inputs the pilot chooses to make. Replace the pilot with a steel cannonball bolted to the bottom of the control bar, and that is no longer true. Now the pendulum effect acts through the bolts connecting the cannonball to the rest of the glider, rather than through the pilot's arm muscles.

To a very modest degree-- an extremely modest degree-- the pendulum effect operates in roll, too. A banked, turning aircraft tends to slip (in the absence of corrective rudder inputs) (because the outside wingtip is moving faster and generating more drag than the inside wingtip), meaning that if the center of side area is above the CG, there will be a stabilizing roll torque as the slipping flow strikes the side area. But the slip angle is typically so modest that the pendulum effect's contribution to roll stabilty is extremely small. (Especially in a hang glider, with minimal cross-sectional side area, i.e. minimal tendency of the pilot to fall to the low side of the control bar in a slip!) But in the hang gliding case, to the minimal extent that this effect does exist, it will again be expressed in the force that the pilot needs to exert through his arm muscles to hold himself in fixed position on the control bar. So it does not exist independent of his roll control inputs. That is not the case if the pilot is seated in a chair bolted in fixed position on the control bar-- or if we replace the pilot with a cannonball bolted to the control bar-- now his weight does exert a slight stabilizing pendulum effect in roll, no matter what his muscles are doing. Again, the roll torque created by the sideways flow striking the side area high above the CG, and the roll torque created by the cannonball's tendency to fall to one side, are one and the same. If the aircraft has no side area to "feel" the slipping flow, the cannonball will have no tendency to "fall" toward the low side of the aircraft and will exert no side force on the bolts holding it to the glider, so there will be no roll torque contributed by the cannonball via the pendulum effect. Disclaimer: the above does not consider the floating keel. This turns the pendulum effect into a destabilizing aileron-like control input-- so modern hang gliders get no roll stability at all from the pendulum effect.

In roll, the pendulum effect is generally vastly overshadowed by other aerodynamic consequences of the slipping flow-- such as the roll torque generated by anhedral/ dihedral-- which acts at a much greater moment-arm from the CG than does the pendulum effect. But really, as far as roll goes, the pendulum effect is inseparable from dihedral/ anhedral effects-- we can say that lowering the CG makes the aircraft's "effective dihedral" become more positive or less negative, because all the side area of the fuselage, fin, etc that is above the CG makes a dihedral-like roll torque in the presence of a slipping airflow. And raising the CG makes the aircraft's "effective dihedral" become less positive or more negative, because all the side area of the fuselage, fin, etc that is below the CG makes an anhedral-like roll torque in the presence of a slipping airflow. It's just that these effects act at a shorter moment-arm than the roll torque created by bending the wings up or down to create dihedral or anhedral.

Steve
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