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Old May 16, 2012, 05:18 PM
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PS the key factor in understanding the pendulum effect, is to consider not the direction that gravity is pulling, but rather the direction that the G-load is pulling. G-load is aerodynamic force. It does not always act straight up and down, obviously. It includes the sideforce created by a slipping airflow striking the side of the aircraft, the side of the vertical fin, etc. If this sideforce is non-zero, this will create some roll torque about the CG. The lower the CG relative to the center of side area, the more the roll torque: this is the roll component of the pendulum effect. The sideforce component of the G-load is one and the same as the tendency of a weight to displace to one side of the aircraft during a sideslip. When we attach a heavy weight to the bottom of the aircraft, and we slip, the weight contributes a pendulum effect. Unless side area is zero-- then the weight "feels" no sideways G-load no matter how much we slip, and the sideforce acting on the aircraft is zero, no matter how high above the CG is the center of side area. No pendulum effect at all in this case, at least roll-wise.

Steve
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Old May 16, 2012, 05:24 PM
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Originally Posted by aeronaut999 View Post
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.
Yes, but the wing is only effected within the prop slipstream.

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Originally Posted by aeronaut999 View Post
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.
I found a copy of the book last year and just thumbed through it quickly. I was hoping it would be more than it turned out to be. It is in one of many boxes of books in the garage because of remodeling.

IIRC, the airfoils in that book are all highly cambered. We have been discussing symmetrical and all a symmetrical wing would do is take some of the rotation out of the slipstream.(Edit) I guess I should say with how the discussed wing is mounted in relation to the slipstream.
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Old May 16, 2012, 08:57 PM
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To be frank, I still have no idea what you're saying the pendulum effect is.

Are you suggesting that any force that does not act through the center of mass causes the aircraft to behave like a pendulum (though the effect might not be seen because of other more dominant effects)?
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Old May 17, 2012, 01:11 AM
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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.
Not sure, but I think you're trying to dismiss this result because the glider was twitchy. I got up before the wind this morning and tried some glides on a local slope (gotta love southern Germany for its slopes). In still air the glide was very stable (well-damped phugoid) with no indication whatsoever that it would eventually diverge in pitch.

I've attached a short (and rather poor quality) video of one glide. The dew warped the wing before I could get the rudder adjusted for a straighter flight (the requirement for up/down symmetry makes this tough).

Bottom line. I think this free-flight glider has amply demonstrated that you can achieve stable, trimmed, upright and inverted flight without any configuration change. While the mechanism that allows this may or may not be downwash on the tail, there is certainly A mechanism that allows the plane to trim for different AOAs depending on orientation and initial conditions.

Slope.mov (0 min 7 sec)
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Old May 17, 2012, 10:53 AM
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It did exhibit self-correcting of the flight path without any divergence.
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Old May 17, 2012, 11:45 AM
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Pendulum effect

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Originally Posted by DPATE View Post
To be frank, I still have no idea what you're saying the pendulum effect is.

Are you suggesting that any force that does not act through the center of mass causes the aircraft to behave like a pendulum (though the effect might not be seen because of other more dominant effects)?
If "behave like a pendulum" means to start swinging back and forth, then no, that's not what I mean.

"Plumb bob" effect might be a better phrase-- the tendency of a heavy weight to end up on the bottom of whatever it is attached to.

I'm suggesting that over the long run, moving the CG location downward (by adding weight to the bottom of the aircraft or reducing weight on top or putting a pylon between the fuselage and the wing) relative to the center of drag and the center of side area (including fin area) creates an increase in static pitch stability, and also shifts the aircraft's "effective dihedral" in the positive direction (becoming less positive or more negative).

These changes tend to help keep the aircraft right-side up rather than upside-down, when the pilot is not making control inputs.

That's all.

Maybe a re-reading of my previous post with the above summary in mind will make my meaning a little more clear.

Some of the thoughts in my previous post are probably only of interest to hang glider pilots, who are interested in questions like "What has changed aerodynamically, when the force I need to exert to hold myself in position changes from a pull to a push"...

Sometimes I hear people say that pendulum stability doesn't exist. The term is usually used I've used it-- to signify that a low CG location tends to make an aircraft more stable in pitch and roll. It's pretty clear to me that this effect is real and does exist. Maybe the naysayers are really fighting against the idea that the aircraft somehow "feels" gravity and wants to stay oriented right-side up relative to gravity. That would be a bad description of what is happening. What the aircraft is "feeling" is not gravity but aerodynamic forces. For example if the wings are not level, the aircraft will turn, creating a difference in airspeed between inboard and outboard wingtips, yawing the nose to the outside of the turn, creating a slipping (sideways) component in the airflow. At this point all the surface area of the aircraft (as seen in a side view) located higher than the CG location tends to contribute a dihedral-like rolling-out torque.

Yet in a sense, the aircraft is indeed "feeling" gravity, at least as far as roll dynamics go. In an uncommanded turn (a turn not created by control inputs, the flight controls are centered), the slip-skid ball will displace to the low side. A hanging pendulum in the cockpit will also displace to the low side. These actions are the result of the aerodynamic sideforces generated by the slip, but they also do give a clue as to which way the aircraft is turning, relative to the horizon, i.e. relative to gravity.

So in theory, you could tell which way an aircraft was turning even in clouds by watching a hanging pendulum. If the air was smooth. And if the rudder and ailerons were centered-- if you were not making rudder or aileron inputs to make competing slip/ skid effects that would mask the slight slip from the turn. It wouldn't work well in practice because any slight uncoordination in the pilot's rudder and aileron inputs would make a larger slip or skid that would mask the slip from the turn. Also, using the rudder to coordinate the turn would, obviously, mask the slip that would otherwise be generated by the turn. But it would work in, say, a free-flight model airplane with no moving control surfaces. A video of a pendulum hanging in the plane would reveal a displacement that was in synch with the direction that the plane was turning, especially in smooth air. At least in the case where the rudder was centered and the plane was symmetrical with nothing to induce a turn, the turn was induced only by a brief disturbance in the air that rolled the plane into a bank. Following this disturbance, as the aircraft turned, a pendulum would displace toward the low wing. Because the aircraft is slipping. The slip would presumably interact with dihedral to roll the aircraft back to level, because the the aircraft presumably has lots of dihedral, being a free-flight model. But in the time before the aircraft reached wings-level, a pendulum inside the plane would be displaced toward the low wing.

I've kind of gotten off topic. Yet not really. Because both the actions of an actual pendulum or weight hanging in the plane, and the actions of a slip-skid ball, and the stability contribution of the "pendulum effect" that I've been talking about, all reflect the way that the aircraft "feels" the aerodynamic forces, and the way that these forces are different in turning flight than in non-turning flight, in a way that is clearly related to the direction of turn.


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Old May 17, 2012, 11:59 AM
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Steve, could you clarify whether your thoughts below do support the idea that a glider-- such as a glider with a symmetrical airfoil and zero decalage-- could be pitch-stable both inverted and upright? Would the stability be confined to a narrow range of conditions? (The stability would have to be somewhat confined, for example the aircraft would clearly have no capability to pull out of a vertical dive.)

Steve

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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 17, 2012, 03:26 PM
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Originally Posted by aeronaut999 View Post
Steve, could you clarify whether your thoughts below do support the idea that a glider-- such as a glider with a symmetrical airfoil and zero decalage-- could be pitch-stable both inverted and upright?
With a symmetrical wing it seem to me that for a trim setting that holds the same line upright or inverted it would have to be zero-zero, or no decalage. So, look at that slope plane in the video. How do you or the plane know which side is right side up?

http://www.rcgroups.com/forums/showp...7&postcount=35

I believe it will have the same level of stability right side up or down.


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Originally Posted by aeronaut999 View Post
Would the stability be confined to a narrow range of conditions? (The stability would have to be somewhat confined, for example the aircraft would clearly have no capability to pull out of a vertical dive.)

Steve
I don't know what you are asking "range of conditions." To your statement, I think we can agree stunt glider have very low stability and would not pull out from that very quickly.
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Old May 20, 2012, 03:27 AM
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Back to the OP's original question...

A999, I don't think you need to approach the fundamental question in your post by examining peripheral considerations. You can "build up" any lift distribution on a wing/aircraft by superimposing a collection of horseshoe vortices. It appears you have the background to be able to consider a simple horseshoe vortex (with stream-wise trailing legs) and go through the exercise of actually calculating the air's rate of momentum change. This can be done, and I think it will provide some insight to how a wing's interaction with the air results in a pressure footprint on the ground.
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Old May 21, 2012, 04:00 PM
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Originally Posted by aeronaut999 View Post
Steve, could you clarify whether your thoughts below do support the idea that a glider-- such as a glider with a symmetrical airfoil and zero decalage-- could be pitch-stable both inverted and upright? Would the stability be confined to a narrow range of conditions? (The stability would have to be somewhat confined, for example the aircraft would clearly have no capability to pull out of a vertical dive.)

Steve
I've been away for the last five days doing other things so this one got by me.

The test glider I made up to try this showed enough promise at flying the same both upright and inverted that I "think" it can be made to do so and show a stable self pitch correcting glide. But it is VERY sensitive to the slightest change as we can expect due to balancing at the neutral point. The glider is also very close to being symetrical with the fin, wings and tail all split so the center lines are even every which way. The idea was to avoid ANY confusion of forces by providing any manner of pendulum stability effect. My only mistake was using a fairly small stick I had handy which turned out to be somewhat banana shaped. So I'm going to try another glider.

I also need to wait for dead calm and DRY conditions. Something which has been all too rare around here recently. But I promise I have not forgotten this whole thread. The model is sitting beside me here by the computer. I'll post a picture later.

Being totally symetrical in every way the model will rely on the wing's trailing downwash for the stabilizer to see some amount of negative angle of attack. But to produce a trailing downwash the wing has to be producing lift. If released at a vertical angle with the nose down the wing will not be forced to produce lift such as when I launch it horizontally. So one would expect it to simply lawn dart itself. On the other hand if the air direction the tail sees depends on the wing and assuming that the most virtual "decalage" occurs when flying at a higher lift coefficient is this even a self stabilizing setup at all? We will see once the weather cooperates and I can do more videos of the flights.

But yeah, I'd say that the stability of such a setup is going to rely on the manner of the air flowing at the model.
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Old May 21, 2012, 05:35 PM
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I would not expect it to lawn dart because as soon as there is a disturbance the wing will have an AOA , create a downwash and bring the plane in a "pitch stability valley". Which means that if you launch it at 0 AOA you should have even odds that it will either pick up an angle and glide or do half a negative loop to a glide or to an inverted stall. Dropping it vertically from an altitude would probably bring it in a sequence of loops/stalls resembling a falling leaf. But for it to just lawn dart would require a CG well ahead of the neutral point, that as I understand is not the case here.
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Old May 21, 2012, 05:40 PM
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Originally Posted by BMatthews View Post
Being totally symetrical in every way the model will rely on the wing's trailing downwash for the stabilizer to see some amount of negative angle of attack. But to produce a trailing downwash the wing has to be producing lift. If released at a vertical angle with the nose down the wing will not be forced to produce lift such as when I launch it horizontally. So one would expect it to simply lawn dart itself. On the other hand if the air direction the tail sees depends on the wing and assuming that the most virtual "decalage" occurs when flying at a higher lift coefficient is this even a self stabilizing setup at all? We will see once the weather cooperates and I can do more videos of the flights.

But yeah, I'd say that the stability of such a setup is going to rely on the manner of the air flowing at the model.
Bruce,
Please re-read my post #192 and watch ShoeDLG's video in post #199, It pulls the nose up just fine. it may do it slowly, but it does. So lets call it a high performance plane?

You guys, decalage is just a trim setting and these gliders are flying at a low Cl of lift. Anything trimmed to fly at low Cl has little decalage and will most likely need an elevator input to pull out of a vertical dive. But if the CG is forward of the neutral point it is stable. Again, any high performance plane that is trimmed to a low Cl that is put into a vertical dive will most likely need an elevator input to pull out.

Another example. The only way a plane with lots of decalage will hold a vertical dive is to move the elevator setting where the wing AOA is for zero lift, that is zero decalage to the zero Cl line of the wing airfoil. To pull out it has to have another elevator input, like releasing the stick.

Take that same plane with lots of decalage, if the CG is 1/4" behind the neutral point it is not stable... but it has decalage. It will pull out from vertical when you release the stick, but it is an unstable plane with decalage.
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Old May 22, 2012, 10:31 AM
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From the earth's point of view it makes a difference-- it is part of the way that the plane pushes down on the earth, so that a scale under a block of air containing the plane will feel the plane's weight (or more precisely, will feel the plane's (weight x G-load.)

But, I'm no longer arguing that the downwash is the only way that the plane can exert a downward push on the earth.

As far as whether what happens behind the wing makes a difference to the wing-- I'm really not saying that downwash is the "cause" of lift. I'm just saying that the wing cannot make lift without exerting a downward force/ shove/ push on the air. Steve
Too bad - you should be claiming that downwash is the cause of lift - whenever air is turned, the object doing the turning will react to having turned the air. Wings turn air downward and upward - but more downward than up - thus there's a net lift vector. Downwash is the result of turning the air. Note that if a system turns the air, and then deflects the downwash somewhere other than down, that ultimately changes the lift vector's direction for that system. So the turning points where the downwash is generated add up to the lift vector for a system - but if the air in the downwash is turned again, those new thrust vectors from the new air turning points have to be taken into account - so the downwash is important, in that it must not be redeflected while it's leaving the wing after it's been turned downward by the wing.

Lift is a Newtonian reaction by a wing to turning air downward - ie the momentum of the air is changed by the wing - it's accelerated and pointed downward by the wing. The wing reacts to this by lifting.

It's that simple.

For those who know nothing about this - here's some proof - a chopper has rotating wings. A chopper hovering over a scale shoves enough air downward to make the scale read the chopper's dead weight. That's Newton. Planes do the same - but since they move about one cannot measure the Newtonian lift with an airplane, as we can with a model chopper - hence also the confusion produced by our textbooks putting forth all the various absurd Bernoulli explanations of lift.

There are some who think that downwash can be created by magic - without any associated lift reaction by the object creating the downwash. This is bogus physics. Nothing more.
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Old May 22, 2012, 11:08 AM
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Heli downwash on a scale:
http://www.youtube.com/watch?v=49wPl...9&feature=plcp
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Old May 22, 2012, 12:40 PM
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!!!RUN FOR THE HILLS!!!

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