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Old Jul 29, 2008, 11:05 PM
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San Diego
Joined Nov 2004
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Question
Aerodynamic effects - moving wing forward and back

Greetings fellow free flight friends:

I'm wondering if the following explanation holds water and figured this would be the place to get some opinions. This concerns a free flight stick glider with an adjustable main wing - by that i mean I've built it so that I can slide the main wing forward or backward as one means to adjust trim. Nothing else on the plane moves or changes when i slide the wing forward and back.

My original question was: Why does the plane climb and try to loop with the wing forward and fly straight or dive with the wing in the rearward position. Is this the correct explanation?

In the forward position, the CG is moved forward, although it falls further back in terms of where it lies in terms of percentage of the chord. With the wing moved to it's rearward position, the CG is further back, but it falls further forward in terms of percentage of the chord. When the wing is forward, since the cg is further back in terms of percentage of chord, then it falls further behind (relative to the situation with the wing in the rear position) the aerodynamic center of the wing (assumed to be at about 25% chord). This gives the CG a longer moment arm (and thus more power) to counter the pitching down moment of the main wing, and thus prevents dive, or causes climb, and a slower flight speed once it settles out.

With the wing in the rear position it then follows that the CG is too close to the aerodynamic center to stop the pitching moment of the wing and thus the plane dives and flies faster.

The secondary question is: is sliding the wing fore and aft a better way to adjust CG than by adding weight to nose or tail. Since we're always trying to reduce weight, i figured perhaps it is a good way to go.

Any comments, corrections or elaborations would be most appreciated.

Thanks,

Paul
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Old Jul 30, 2008, 02:27 AM
Grumpy old git.. Who me?
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Paul,

What matters in regard to the stability of a tailed aircraft (I assume yours has a horizontal stab) is the overall Neutral Point of the aircraft, not the main wings aerodynamic centre alone. The neutral point (NP) on a conventional tailed aircraft will be quite significantly behind the 25% centre of pressure position for the wing. For some models with large tails the NP actually sits behind the wing's trailing edge. The NP can be thought of as the point where the 'overall' lift of the aircraft (including the tail) acts.

For a stable model your CG must always be ahead of the NP. When you slide the wing forward and back you are adjusting the distance between the CG and the NP. This distance is called the 'Static margin' and it determines how stable the model is in pitch.
Slide the wing back and the Static Margin increases because the wings rearward movement causes a similar rearward movement of the NP, whereas the CG moves back less, so the gap between than increases. Slide the wing forward and the static margin decreases, slide it forward far enough and the CG and NP will coincide, this is a Neutrally stable model and for freeflight that's bad, a model with neutral stability wont recover when it goes into a dive. Slide the wing further forward and the CG moves behind the NP producing negative Stability; this is very bad and will be unflyable even with R/C assistance.

Having a big gap between the CG and the NP (large Static Margin) produces a strong nose down pitching moment (because the point the lift acts at (NP) is well behind the point that the weight acts (CG)... so this model tends to dive. This is why your model dives when you slide the wing back. The opposite applies when you slide the wing forward.

Sliding the wing is a nice easy way to adjust static margin and is quite effective within reason. If the wing goes back too far the moment arm distance between the wing and tail gets too short which can cause issues but providing the wing is always in a reasonable position it should work fine.

Does this help or only further confuse?
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Old Jul 30, 2008, 02:49 AM
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Couldn't quite follow the explanation, but I think you could neglect the effect of the wing weight to the overall CG, and consider that the CG stays at a fixed position on the fuselage. Thus, moving the wing forward will increase the moment arm of the lift centre of lift corresponding to the CG, make that moment larger, and thus increasing the nose pitch up moment of the wing. And vice versa.

For the second question. The combination of CG location, decalage (the angular difference between wing aln tail), and tail moment has multiple effects on the model balance, stability (both static and dynamic), performance and behaviour in turbulent conditions and lift. So there is no straightforward way to say, whether it is better to move CG or decalage to trim the model. If you know the optimal decelage, set that fixed and move the CG to get required trim. If you know where the CG should optimally be, fid that and alter decelage for the trim. If you do not know either, then experiment :-)
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Old Jul 30, 2008, 12:16 PM
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Quote:
Originally Posted by JetPlaneFlyer
Paul,


Having a big gap between the CG and the NP (large Static Margin) produces a strong nose down pitching moment (because the point the lift acts at (NP) is well behind the point that the weight acts (CG)... so this model tends to dive. This is why your model dives when you slide the wing back. The opposite applies when you slide the wing forward.


Does this help or only further confuse?
First of all, your explanation does help, but it also confuses me a bit based on what I've read about static margin and neutral point. Above you say that sliding the wing back increases the static margin, and thus the plane dives. Of course this is what happens, but what I've read about static margin is that the larger the static margin the more stable the model. To me a more stable model is the one that corrects for the dive and recovers. Your explanation implies that the model becomes more stable as the static margin decreases, which is counter to what I've read.

Since sliding back the wing (increasing the static margin) does not in this case make the model more stable, is it perhaps that with the wing all the way back the static margin is too large?
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Old Jul 30, 2008, 01:38 PM
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It's not the effect of the static margin that's causing the dive, it's the CG being too far forward for the angular difference between the wing and tail. Increase the incidence/decalage[angle between the wing and tail] until the a/c glides nicely. CG and incidence are related.

The static margin is related to wing and tail size along with tail moment. When you're sliding the wing, youre changing the tail moment and therefor the tail volume coefficient[Tvc], which all help to determine the neutral point.

In general, the greater the distance the CG is forward from the neutral point[greater static margin], the more an a/c will return to the speed its trimmed for. Moving the CG to the rear will mandate a reduction of incidence and make the a/c lesss likely to return to "normal'[trimmed speed].
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Old Jul 30, 2008, 02:15 PM
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I think there is some confusion between 'stability' and 'trim' here. Any object that has it's CG in front of it's NP will be 'stable' i.e. if disturbed it will return to it's original trajectory. The larger this static margin then the quicker the object will return to it's original trajectory... but the original trajectory could be straight down! A finned bomb dropped from an aircraft is 'stable' but it does not fly level.

For a trim on an aircraft you need a positive pitching moment (nose up moment) which balances the negative moment (nose down) caused by the static margin. Positive pitching moment can be the result of adding a tail or can (on a tailless aircraft) be the result of using a special (reflexed) airfoil or wing twist.

When you slide the wing back you actually have a 'double whammy'... you increase the static margin which creates a stronger nose down pitch moment (this is true Daphela), but you also (slightly) decrease the positive pitch moment from the tail (because the moment arm is now shorter). The model tends to dive. It's actually 'more stable' because of the static margin increase but now it's trimmed for a higher speed which it must dive to attain.

Move the wing forward and you decrease static margin plus you increase nose up pitching from the tail. Providing the CG is still ahead of the NP the model will still be stable (but less so than before) but it may be now trimmed for a flying speed thatís below stall speed; if so the model will fly in a series of stalls.

Does this help fill the gaps?
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Old Jul 30, 2008, 02:59 PM
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Yup, that does it. Thanks to all of you for helping me to make sense of this. Just one more question. What little I know about all of this comes from my experience with models and full scale gliders, and a book called Model Aircraft Aerodynamics by Martin Simmons. I'd love to find other great sources of information to take me to the next level so if you know of any you really like, please let me know. Thanks again.
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Old Jul 30, 2008, 04:50 PM
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USA, CT, Hamden
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I think Simons book is great!
Everything JetPlaneFlyer points out is true. Everything, and I agree.

The only a/c I have with a sliding wing is a "Pinkham stick". When I move the wing back 1" the CG moves about 1/4"+. When I run a neutral point calc on it I see the NP move only 2% of the 4" wing chord. I see this as a much larger change in the trim arrangement than a change in stabilty.

Either a change to the CG after shifting the wing or a change to the angle between the wing and stab after the movement will retrim the plane. I doubt there will be much change in the stability.

I'm sorry I can't follow this post anymore as Im sure good will come of it BUT, I'm off to Muncie and the AMA/NFFS Nat's!
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