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Lifting Stabilizer (Read 3846 times)
Michael Heinrich
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Re: Lifting Stabilizer
Reply #20 - Jul 16th, 2011 at 9:38am
 
   Thanks for the correction, I've edited my post to show the correct link.
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Yak 52
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Re: Lifting Stabilizer
Reply #21 - Jul 16th, 2011 at 12:36pm
 
hepcat wrote on Jul 15th, 2011 at 4:54pm:
I love these highly technical aerodynamic discussions so I must add my two cents worth.


John! I hoped you might turn up to dig me out of this hole!  Grin

hepcat wrote on Jul 15th, 2011 at 4:54pm:
...although I would take issue with the matter of camber affecting lift slope...


Maybe I could have been more accurate there: I take it you mean that it stays the same actual SLOPE (ie 2 times pi per radian)? I was referring to the fact that the slope curve 'shifts up the graph' relative to the angle of attack compared to a symmetrical foil that makes zero lift at zero angle of attack.


Michael - I read your posts but not sure if I understood you correctly. I wasn't saying anything is irrelevant at these low Reynolds numbers... quite the contrary, these small models is where it all gets very interesting  Cool.

What I was referring to was that a flat plate is pretty poor at high Re numbers (you don't tend to see them on fullsize aircraft!) but down at low Re where all airfoils are 'sub-critical' then the flat (or curved) plate starts to look quite good again. Especially in contrast with thicker airfoils that are 'all drag' below their critical Re.

I agree with you that a thin symetrical airfoil with a proper profile to it (not cambered - symmetry and camber are mutually exclusive: probably a terminology thing) trumps a flat plate most of the time... but at peanut/embryo sizes the practicality of the flat plate wins I suppose.

I mentioned tail area but did forget to mention tail moment arm - youre absolutely right: one without the other is meaningless, hence the need for Tail Volume...


I think the point I am making in the context of this thread (and rather a large generalization!) is: If the model is designed so to have a download on the tail then there is no particular advantage (EDIT: In fact a DISadvantage) to making a 'lifting (cambered ((A-symetrical)) Cool) stab'.

Jon
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Re: Lifting Stabilizer
Reply #22 - Jul 16th, 2011 at 12:46pm
 
Just had a look at Nate's plan. I think you would definitely be right if we were talking about radio control and you were trying to widen the flight envelope... eg trying to fly very slow at high aoa.... but I'm not sure you'd ever get to use that extra aoa in free flight? I'm a bit fuzzy myself but surely tail stall isn't likely to be problem in an average trimmed model?
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Michael Heinrich
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Re: Lifting Stabilizer
Reply #23 - Jul 16th, 2011 at 12:52pm
 
   All good points, Jon. I probably make it worse by blithely misusing terminology (that most here wouldn't understand in their proper use anyway) more along their popular conventions, so when talking to a guy who actually knows, I create just this sort of trouble!
   I build mostly in the 16"-30" range, and my beliefs about 'foiled (is that word innocuous enough?) tailfeathers comes entirely from my own experience and discussions with like-minded pals.
   One other reason I like 'em is that they don't tend to buckle along the spar line, and behave a little better in the warp department. And they look right to me.
   Michael
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Re: Lifting Stabilizer
Reply #24 - Jul 16th, 2011 at 10:39pm
 
I must be missing something here..the statement is made that scale models require DOWNFORCE on a stabilizer.
In laymans terms, downforce indicates the need to force the stabilizer DOWN in order for a scale model to  fly properly...if I understand the term correctly.
In contrast, I see the need for a scale models stabilizer to do just the opposite...  i.e. LIFT.
As I see it, if NO stabilizer were present, the lifting component of a models wing would tend to make the model try to loop..continuously. Adding a stabilizer which provides lift to the tail of a scale model, balances the lifting effect of the wing and allows the resulting forces to provide equilibrium (stability).
I recognize that a flat plate stabilizer can offer the same stabilizing effect, but it is my considered opinion that a cambered stabilizer can offer an equal amount of lift  with less total area. In the case of a scale model this can assist a builder/designer to more closely adhere to direct scale proportioning, rather than enlarging the tail surfaces to compensate for model (rather than full size) dynamics.
I need someone to explain the fallacy of my argument which is based on actual, observed model performance..

Bob
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« Last Edit: Jul 16th, 2011 at 10:42pm by Duco_Guru »  

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Re: Lifting Stabilizer
Reply #25 - Jul 17th, 2011 at 3:43am
 
This may not have too much bearing on the effects in free-flight models, but I built a 30" biplane (RC) back in the '70s that called for a (full) symmetrical stab.

I didn't want to cut THREE sets of ribs, so I made the stab as a flat plate (sticks) - saved a bit of weight in the process.  Built the rest as per plan and article - could NOT get the buggar dialed in.

After a few letters & phone calls to the designer, I mentioned the modified (flat) stab.  His responce: "build a new stab per plan and try it".

I did, and the CHIGGER flew like it was on rails - with no other changes.  I STILL have the plane and it is now undergoing E conversion.

For what it's worth...
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Re: Lifting Stabilizer
Reply #26 - Jul 17th, 2011 at 3:50am
 
Michael Heinrich wrote on Jul 16th, 2011 at 12:52pm:
    I probably make it worse by blithely misusing terminology (that most here wouldn't understand in their proper use anyway)
   

No, some of us aren't very bright. Sorry.
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Michael Heinrich
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Re: Lifting Stabilizer
Reply #27 - Jul 17th, 2011 at 7:32am
 
Pete Fardell wrote on Jul 17th, 2011 at 3:50am:
Michael Heinrich wrote on Jul 16th, 2011 at 12:52pm:
    I probably make it worse by blithely misusing terminology (that most here wouldn't understand in their proper use anyway) 

No, some of us aren't very bright. Sorry.

   Nononono, Pete, the rest of the statement from which you take this says just WHO isn't very bright, and it is ME. Misuse means misuse, and I don't use the lingo right. My point is, the convention of usage on this blog often leads us into just this sort of confusion.

   A graphic case in point is Yak's use of the word "downforce" in post #4, which Bob has rightly jumped on--because to narrow the focus down to that one word completely ignores the larger relationship of "decalage," where the wing and stab work in concert, aided by prop thrust line, to create the flight dynamic we typically employ. A stab set at minus three degrees incidence to wing zero, which we often use as our baseline setup in low-wing models (see Bob's and Mike Isermann's "Setup" DVD for a good explanation of this) may look like "downforce" UNLESS you consider that the stab is not an independent element--it's there to keep the wing at its essential angle of attack, hence the name "stabilizer." It works with the wing, and provides a NET GAIN of lift: the stab doesn't give "downforce" any more than the wing does "UPforce," they work together as parts of a whole.
   The problem gets worse in a case like this, where our "conventional" decalage setup changes with the use of a lifting stab. This is why I posted Bill Henn's drawing, to illustrate a successful arrangement that we all could look at.
   Citing Bob's DVD again, a good application of the three-degree decalage for a mid- or shoulder-wing setup would be wing plus two, stab minus one. I can't get back into that DVD right now to check, but my own sense would call for three degrees downthrust as my starting point for this setup--Bob, comment?
   To the best of my early-morning bleary measuring, I see the setup on the Chambermaid plan as wing plus 1/2, stab minus one, AND a callout on the plan for two degrees downthrust. Now, these figures are all flatter than our "conventional" setup, and you have to take all three settings into the mix (and more, which I'm not going into now--this is a point of discussion and not a how-to, so readers do not quote me out of context!)--and the difference is, the inclusion of cambered, lifting, stab.
   Changes the whole equation.

   In all this discussion, we've completely ignored such essentials as the relationship of wing's and stab's zero-lift planes, just for starters. We usually can get away with doing so by just adapting the simple "three degrees incidence" homily and not thinking further, because, CONVENTIONALLY, the simpler setup takes all these things into account and we can just plug in the numbers and fly. And, sadly, that hasn't happened here--you change one element, you've just changed the model. Now, you can rightly plug in a NEW setup, but you gotta do it with eyes wide open.

   In my clumsy way, I'm trying to bring some of those relationships back into the picture we're making here, and like all of us I risk focusing too narrowly on one word or one concept. So, Pete, that risk and any failure incurred are all mine.
   Michael
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« Last Edit: Jul 17th, 2011 at 8:55am by Michael Heinrich »  
 
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Re: Lifting Stabilizer
Reply #28 - Jul 17th, 2011 at 9:03am
 
Okay, now that we all have a pretty good understanding of the relationship between the wing and the stabilizer angles, and the effect of the thrustline's axis on them, is there a place in the discussion for the aircraft's longitudinal axis? Can anybody even figure out where it actually is? and in Free Flight, does it matter at all? (I've always understood it to be a line thru the CG, from front to back, around which the plane rolls; what's that got to do with an uncontrolled craft?)
I know that Mr. Heinrich, in the past, has opined that you want to see the the plane flying flat and level. But they don't, necessarily. Anybody who has trudged forward up the aisle of a 747 in cruise can tell you that. And if you look at the bottom of your Gollywock from a hundred feet under it, you can't tell if it's flying nose up or nose down.
a.
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Re: Lifting Stabilizer
Reply #29 - Jul 17th, 2011 at 9:22am
 
   What I've tried to say is, I personally like my models to look like I imagine the real thing to fly, relative to ground plane. My favorite example is the Bf109, which in films of The Day looks like it's a little tail-high: that's because of the taper of the tailboom, I think, but I try to copy it. I would claim there's less parasitic drag in a more in-line orientation, but that's purely my musing.
   What's missing in these discussions is the actual flightpath of the model in air. We establish the decalage/thrustline relationship based on a reference line in the drawing, but that line doesn't exist in the air--it's just for us to look at when we talk about this stuff. The actual relationship of the model to the mass of air it's moving through, we can't "see"--not without help of some instruments we normally don't trudge out on the field.
   It's another of those factors we conveniently sidestep by adapting a simple-to-apply "recipe" to our layout, saving us a good bit of number-crunching maybe but also falling somewhat short of accuracy.
   Michael
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Re: Lifting Stabilizer
Reply #30 - Jul 17th, 2011 at 9:39am
 
Reference reply #21.
Yak, no need to dig you out of a hole, that was the point I was making about the lift slope.  Perhaps it would be useful to explain a little more for other readers:

S’funny how often people talk about ‘a good lifting section’ where in fact they all lift about the same.  It was calculated long ago in aerodynamic theory and backed up by thousands of wind tunnel tests that all wing section increase their lift coefficient by about 0.1 for each 1 degree increase of angle of attack. (This is the lift curve slope that Yak mentions.)  Now,  we all know that if the angle of attack is increased too far the airflow breaks away, the airfoil stalls and we get no more increase in lift coefficient.   However if camber is added to an airfoil then, at a high angle of attack, the airflow will more easily flow around the airfoil, the stall will be delayed, and a higher lift coefficient can be reached.  Sadly, of course, although the camber improves the airflow and stalling angle at high angle of attack it does just the opposite at low and negative angles of attack where the stall will be early and sharp.

I am sure some of you will be saying: ‘if all airfoils lift the same why are there so many different ones to choose from?’  The quick answer is to try to reduce the drag over the angle of attack range normally used.

There is another point to be amplified.  The ‘lift slope’ of 0.1 increase in lift coefficient for each 1 degree of angle of attack is for a wing of high (infinite) aspect ratio and reduces as aspect ratio is reduced.  If you think of the usual case of an aeroplane with a tail aspect ratio lower than that on the wing then, if there is an increase in angle of attack, the wing will increase its lift more rapidly than the tail so a low tail aspect ratio is not good for stability.  Don’t think of this as a major  factor compared to tail area and moment arm.

Reference reply #10
Michael, Piker?  You’re correct, I may be able to do all sorts of calculations but I am shy of giving a definite answer.  However I will do my best to give an opinion – which I don’t think varies much from what others have already said.

All the above verbiage doesn’t amount to much more than saying that adding camber to an airfoil will bias the lift curve so that it will give a higher maximum lift coefficient in one direction from the zero lift line than the other.  (Perhaps I should mention here that turning a flat plate into a symmetrical section will improve the lift curve a little in both directions.)So how best to use this fact?

First let me say that I am with Bob in not liking the very forward CG position.  It may suit some full size aircraft in that if a pilot is inattentive the ‘plane will more likely dive than stall.  On a model it wastes efficiency and the extra incidence difference required will make it much more difficult to trim for both gliding and powered flight.  So, I agree with Bob that most models will adopt a more rearward CG such that the tailplane is lifting in normal flight.  This demands some angle of attack above the zero lift line of the tailplane and as some of the lift coefficient range has been used in this initial setting then it seems logical to extend the lift coefficient range in that direction by cambering the tailplane airfoil.

There is a further point on which I have no proof but it niggles at me; I feel that when a model is disturbed by a gust or turbulence that it more often stalls than dives.  This seems to me to be a further reason to camber the tail airfoil – curve uppards of course.

John    






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Re: Lifting Stabilizer
Reply #31 - Jul 17th, 2011 at 9:48am
 
Michael Heinrich wrote on Jul 17th, 2011 at 7:32am:
Pete Fardell wrote on Jul 17th, 2011 at 3:50am:
[quote author=4471737B69526574747871100 link=1310730405/23#23 date=1310845970]    anyway) 





 
   Citing Bob's DVD again, a good application of the three-degree decalage for a mid- or shoulder-wing setup would be wing plus two, stab minus one. I can't get back into that DVD right now to check, but my own sense would call for three degrees downthrust as my starting point for this setup--Bob, comment?


 
   Michael

Michael: You are quite correct on the recommendations that Mike and I espoused on the DVD. Basically, a DIFFERENCE of 3 degrees decalage and a combination of 3 degrees down and right thrust(for conventional, single engined  models featuring CCW prop rotation).
In my thread on trimming I emphasize the importance of trimming for the initial heavy burst of power on fully wound/powered models.
There are MANY modelers who read these notes who NEVER wind their models to capacity  and therefore never observe the response that a fully wound motor can have on models flight performance.
Per Michaels previous comments and Art's observance of an aircrafts "sit" in the air, I can offer my own opinion and that is that I do not pay much attention to the sit...only the total powered AND glide performance. If I were to hazard a guess, I would opt that most models sit somewhat in a slight nose high attitude in the glide and predominately nose high in climb..

Bob
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Re: Lifting Stabilizer
Reply #32 - Jul 17th, 2011 at 5:24pm
 
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Re: Lifting Stabilizer
Reply #33 - Jul 18th, 2011 at 2:55am
 
Michael Heinrich wrote on Jul 16th, 2011 at 12:52pm:
so when talking to a guy who actually knows...


Embarrassed I feel I ought to point out: I'm no Aerodynamicist, just an extremely average modeller with an interest in the how and the why. I owe all of my understanding to other modellers (Hepcat, Jetplaneflyer, Don Stackhouse, Martin Simons)


Duco: I wasn't stating anything catagorical about scale models, but compared to sport models (with big tails) they are more likely to fall into the area where a downforce on the tail is needed.



As I said before, the condition of the stab (either tail lifting or with a downforce) is a sliding scale dependant on various factors. It just so happens that us free flighters often favour the lifting end of the spectrum most of the time. And so a cambered stab is preferable in this condition.

But fly that same model fast enough or with the CG forward enough and it will require a tail downforce at some point - regardless of what airfoil you put on the stab.

The other sliding scale we are discussing is that of Static Margin ie Stability. For any model to be stable the CG must be slightly ahead of the NP, the neutral point at which all the aerodynamic forces act. This gap between the CG and the NP is called the Static Margin. The neutral point is rather complex to find as there are many factors that affect it, eg the propeller spinning, the fuselage length, even putting wheel pants on a model will shift the NP slightly... but fortunately the two biggest factors are the wing area and the combined effect of stab area and fuselage length. In otherwords: Tail Volume.

Increasing the Tail Volume, (ie making the stab bigger or moving it further from back from the wing to make it more effective) will move the Neutral Point aft. This gives us more 'breathing space' to move the CG back.

So we have a model that has it's neutral point at say 40% of the wing chord so we put the CG at 30% and have a staic margin of 10% - nice and stable. Moving the CG back to 35% makes it less stable but still flyable. Put the CG at the same place as the NP and all bets are off - the model has neutral stability, ie no corrective forces to realign the flight path. Move the CG back again until it is aft of the NP and the model is positvely unstable, not good!

So we take the model and put a bigger stab (or longer tail) on it and by doing so we move the Neutral Point back to 50% of the wing chord. Now we can have the CG back at 40% and still have a nice big Static Margin of 10%. However we can't do this on a scale model without making it look rubbish!

So finally we come to my overall point ( Shocked):  For most models the two sliding scales (stability margin and tail lift/downforce) have a nice big overlap. We can move the CG back to the point where the stab is making lift (not downforce) and still have plenty of Static Margin left for stability. In these cases a 'lifting (cambered) stab' is a GOOD thing, reducing the drag. John (Hepcat) has mentioned some of the advantages of this condition... the reduced incidence and lower stability all aid trimming, efficiency and help the model cope with the massive variations in power we get with rubber motors.

However, for some models, especially those with limited tail volumes (ie some scale planes) when moving the CG back, the point of instability is reached BEFORE the tail is lifting, while it still has a downforce. These models can't benefit from a cambered stab - it just makes the tail more inefficient and draggier.



On another point you raised Michael: just looking at the 'decalage' doesn't tell you that the stab has a downforce. The tail can be making lift with negative incidence or vice versa... the tail is flying in the downwash of the wing so it's rather more complex than just what one sees on the plan...

Hepcat put his finger on it when he mentioned aspect ratio. The difference between the aspect ratio of the tail (lower) and the wing (higher) is probably the single biggest factor in the way the two surfaces interact to provide pitch stability.

Jon


Oh, and here is another good explanation from Don Stackhouse of DJAerotech: http://www.djaerotech.com/dj_askjd/dj_questions/lifttail.html
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Re: Lifting Stabilizer
Reply #34 - Jul 18th, 2011 at 3:55am
 
hepcat wrote on Jul 17th, 2011 at 9:39am:
...There is another point to be amplified.  The ‘lift slope’ of 0.1 increase in lift coefficient for each 1 degree of angle of attack is for a wing of high (infinite) aspect ratio and reduces as aspect ratio is reduced.  If you think of the usual case of an aeroplane with a tail aspect ratio lower than that on the wing then, if there is an increase in angle of attack, the wing will increase its lift more rapidly than the tail so a low tail aspect ratio is not good for stability.  Don’t think of this as a major  factor compared to tail area and moment arm.


John, I just realised that I kind of contradicted you there... I hadn't picked up on the point you made but, having thought about it a bit:

The low tail aspect ratio IS destabilizing for a lifting tail. But in a conventional aircraft (with a tail download) I think it is a major factor in pitch stability.

With a downloaded tail, the low aspect ratio does 'react slower' to aoa changes as you said, but bacause the lift is in the opposite direction (down) this causes a nose down pitch correction.
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Re: Lifting Stabilizer
Reply #35 - Jul 18th, 2011 at 4:17am
 
Duco_Guru wrote on Jul 16th, 2011 at 10:39pm:
I must be missing something here.....

.... I see the need for a scale models stabilizer to do just the opposite...  i.e. LIFT.
As I see it, if NO stabilizer were present, the lifting component of a models wing would tend to make the model try to loop..continuously. Adding a stabilizer which provides lift to the tail of a scale model, balances the lifting effect of the wing and allows the resulting forces to provide equilibrium (stability).



Bob, the only thing missing is that when a wing airfoil has camber (ie asymetry) as well as making lift there is also a significant nosedown pitching moment present.

This used to be explained in terms of a moving 'centre of pressure' but it's more easily visualized as a nose down pitching moment that acts at 25% of the wing chord.

The downforce on the tail is one way of tackling this nose down pitching force. That's the way conventional full size aircraft do it. They keep the CG at 25% (where it has no pitching effect) and make sure the tail has a down load. This is safe and stable. (In fact I seem to remember that its a CAA requirement to have a download on the tail...)

The way we tend to do it with our models is to use the CG to tackle the nose down pitching tendency. By moving the CG aft of 25% of the wing chord, we are using it to create a nose up pitching moment that counteracts the wings natural tendency to pitch nose down.

At the point where the nose up moment of the aftwards CG balances the inherent nose down moment of the wings camber - the tail HAS NO LOAD on it at all. (Ignoring for a moment the other pitching effects of drag, thrust etc.) This is actually the most efficient flight condition from the drag point of view because the stab makes no lift and therefore no drag, no tip vortices etc...

We tend to go even further than that, putting the CG still further aft, so that even with the nose down pitch of the wing, the greater effect of the aftwards CG is creating a net nose up pitch... which has to be  balanced by a lifting tail.

The problem is that the nosedown pitching effect of the wing varies with airspeed. Flying faster increases it, slower decreases it (Ignoring the stall at the moment!) So it's a moveable feast...

A model with a lifting tail at 'cruise' speed will quite possibly have a download on the tail at higher speed in a steep dive.


Right, that's enough theory for me  Cheesy I'm off to build something and get some more PRACTICE!  Cool

Jon
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Re: Lifting Stabilizer
Reply #36 - Jul 18th, 2011 at 2:24pm
 
Yak 52 wrote on Jul 18th, 2011 at 2:55am:
On another point you raised Michael: just looking at the 'decalage' doesn't tell you that the stab has a downforce. The tail can be making lift with negative incidence or vice versa... the tail is flying in the downwash of the wing so it's rather more complex than just what one sees on the plan...
   Yes, yes, and yes. I started to go into this at one point, and forgot. (I seem to do that a lot lately, or else I just get tired of typing. Dunno which is worse.)
   And then there're the models whose stabs are out of the wingwash. Or in some odd propwash. And And And. Jeez, I wish Zaic's CIRCULAR AIRFLOW was still available, that's the one that got to me best...not everyone will "hear" the same source the same, each person might come to this understanding through any number of texts.
The thing I keep saying, be prepared to go into YOUR particular model like it's the first time; don't get stuck in "well, HE said THIS," because you may need a different application of the prevailing beliefs. Or not. Only YOU'll know for sure, and you'll know because your model's flying.
   Michael
   
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Re: Lifting Stabilizer
Reply #37 - Jul 18th, 2011 at 3:57pm
 
I'm gonna start needing pictured here!
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Re: Lifting Stabilizer
Reply #38 - Jul 19th, 2011 at 12:50am
 
Dave - here is a graphical representation of the three tail load conditions at a constant airspeed... (nothing special - just knocked up in MS Paint I'm afraid)

The pics show what happens as you move the CG back from the 25% chord - known as the Aerodynanmic Centre of the wing.

Of course as you move the CG back, you are losing stability all the time - reducing the Static Margin as the CG gets closer to the Neutral Point.

For some aircraft with small tail volumes where the Neutral Point is quite far forward, the CG 'runs out' of Static Margin before you get to the third condition (lifting tail)...
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Re: Lifting Stabilizer
Reply #39 - Jul 19th, 2011 at 2:45am
 
Here's a pic showing Static Margin - the gap between CG and NP.

The Neutral point is not the same as the Aerodynamic Centre of the wing but is the point at which aerodynamic forces act for the whole aircraft.

The Neutral point is the limiting factor on how far back the CG can be. The CG cannot be aft of the neutral point.

The main factor influencing the position of the NP is the size of the stab and it's tail moment arm (fuselage length). Changing the airfoil on the stab will not move the NP back. To move the NP back you need a larger tail area or longer tail moment arm.

It can be seen that for some aircaft, as you move the CG back the point of instability is reached before the point where the tail neeeds to make lift.

A large Static Margin (forward CG) is very stable but requires a large download on the tail. Too much stability isn't always a good thing in a FF model - it actually causes 'power stall' under excess thrust at the beginning of a rubber motor run.
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