How do you measure the nose and tail moments?

[Paul Taylor]

Need to get some numbers to Steve.

[Steve Berry]

With a scale! 

Ok...I usually go with front of drive washer/back of spinner to leading edge of wing for nose moment, and hinge line of flaps to hinge line of elevator for tail moment.

Clear as mud?

Steve

[Paul Taylor]

Got it.

Clear as Mississippi mud.

[W.D. Roland]

 

[jim ivey]

 Hi told me, but I forgot  do ya think it makes a difference?   look at that stunt wagon. Then look at one with  longer tail moments. Both will fly well.  I design them to balance on peak of airfoil. I found that divided in thirds seemed to work out best.  Prop to peak of airfoil=1/3rd.   2 x that= peak of airfoil to c/l of stab/elevator. I bet some in here really know, but this will work fine.  It will usually  balance right on the peak of the airfoil when finished.  jim 

[W.D. Roland]

 

[Bill Little]

For the past 50-60 years in magazines and such, the "measurement for moments" I have observed have traditionally been measured (right or wrong) as:

Nose moment= front of fuselage (nose ring,etc.) to LE of wing.

Tail moment= hinge line to hinge line for flapped plane.  Wing TE to elev. hinge line for non flapped. 

(Occasionally I would see wing TE to Stab LE on non flapped planes, and flap TE to Stab LE on flapped ones.)

At least that is what all the references I used to read were.  Like * 10" - 17" *, etc..

Of course, the scientific way is probably something different, and for design purposes it probably makes a difference if some other criteria is used.  But I guess a lot of people can actually measure from the hinge line to hinge line, etc., and understand that.  And it makes comparing one model to the other real easy.

Big Bear

[Serge_Krauss]

For the short answer, see the bold type below. The rest belongs in the engineering or design sections.

Using Jim's high point for the c.g. position in finding moment arms is a lot better than hinge-to-hinge, but rules of thumb like "Ted's Rule" for flapped planes (C.g. position in terms of %MAC = the ratio of tail area to wing area) and 15% - 19% MAC for flapless planes are better.

Regarding nose and tail moments, or rather moment arms...

A lot of these rules of thumb work somewhat, simply because the planes being compared are already quite similar. They won't work well for some categories of unconventional designs, because the amount of change in the behavior of a model is not really proportional to such dimensions as hinge-to-hinge  tail "moments". If you were to double that, for instance, you would not be doubling the actual tail moment, and the aerodynamic effect would prove too small, except for the closest coupled flapless designs. Useful or optimal tail moments are different for flapped vs. unflapped ships and are pretty much established by experience over a couple generations of stunt design. However, if you want to compare performance or predict performance changes within a category, you should use something closer to the actual tail moment arm.

Tail Moment Arm: distance from model’s center of mass (c.g.) to aerodynamic center (a.c.) of stab/elevator. This is for purely aerodynamic considerations, compromised by inertial moments of the structure. I think it's also valid to compare using the distance between the a.c.'s of wing and tail. These too are approximations in determining performance, but use valid parameters. You can find these easily by measuring your model and plugging your dimensions into neutral point (a.c.) calculators like this one. Just read the answer - no work on your part:

http://www.palosrc.com/index.php?option=com_content&view=article&id=50:cg&catid=41:ic&Itemid=50

(make the MAC balance point 25%, and the c.g. distance will tell you your neutral point or a.c. position at the fuselage. Most valid rules of thumb for c.g. concern just the wing a.c. as computed here, and this same little calculator will tell you the tail a.c. for computing the tail arm.)

If you want the entire plane’s a.c., then this site can get it for you, but it's more involved and not necessary for our approximations:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

Nose Moment Arm: Distance from c.g. of nose to c.g. of plane. This is for balance (and moments of inertia) only and has nothing to do with aerodynamics. Put the engine where it needs to be to get the c.g. right. For adjustments, it's pretty close just to work with the masses of the engine/prop and tank for good approximation. That means that you need to know how far these are from the plane's c.g.

SO...the plane's c.g. is important. You might worry about how it moves with regard to changes in the positions of these component masses. Don't. Just choose the c.g. relative to  the a.c. of the entire plane AND whether the plane has flaps. Ted's rule or the flapless c.g. limits are as good as anything though. They depend on wing a.c. only. So the c.g. is determined solely by pitch aerodynamics and if moved to adjust for line position, rudder offset, or something similar, you have the tail wagging the dog. The aircraft c.g. is determined by desire for stability vs. maneuverability in pitch. Choose it and then design around it, realizing that total aircraft a.c. changes some with choice of tail area and moment. Once the plane is designed and built, you can move the c.g. to suit your handling preferences (but not according to unrelated dimensions, like leadout positions).

SK

[Bill Little]

Hi Serge,

Not to sound like a smart alec, but like I said,

For the past 50-60 years in magazines and such, the "measurement for moments" I have observed have traditionally been measured (right or wrong) as:
(snip)
At least that is what all the references I used to read were.  Like * 10" - 17" *, etc..
(snip)
But I guess a lot of people can actually measure from the hinge line to hinge line, etc., and understand that.  And it makes comparing one model to the other real easy.

I read Paul's question as to how it is often referred to, not the best method of determining HOW the moments should be arrived at..

I admit, I build other people's designs, usually only changing the clothes on the manikin as Bob Hunt says.  But when I have asked him, or Billy, about a change to the model for an improvement they have said things like "make the tail moment 18 1/2" instead of 17 1/2" for example.  And they were referring to the hinge to hinge distance.  A lot simpler way of communicating to some folks.  If you ask me what the moments are on my Geo Juno I will tell you 10 1/4" and 17 1/2" (just guessing from memory).  You'd know what I mean.  If you ask me what the distance from the MAC to anything else, or what the tail volume is I will just tell you I don't know (or care! LOL!!). So many of those things have been worked out by others either scientifically, or by trial and error.  I quit doing Math when I retired from teaching it! LOL!!

Don't get me wrong, I am truly glad guys like you, Brett, Ted, Howard, Igor, etc., etc., can and will do the scientific work so I can steal it.  Building and flying are enough for me.

Bill

[Serge_Krauss]

Yep, Bill, I hear you! As long as the planes are similar, that works well enough as a benchmark

One thing about nose moments though: I've seen some pretty nice models that used shaft extensions on light engines like foxes in planes with longish noses, with the stated intention of reducing the nose moment. They did not realize the actual length of the arm and actually, unknowingly increased the nose moments in their designs. Even on "famous" designs, not measuring from the c.g. can cause errors.

SK

[W.D. Roland]

 

[Serge_Krauss]

David-

The flap and elevator movements do move the centers of pressure. The way I see it though is that for stability, our first consideration is for the plane not to wander or be overcontrolled around neutral. That's why these a.c. things work well. The rest is then the compromise between this stability we want and the ability to turn sharply. As Brett has pointed out, we do not seem to have the reflexes to deal with corners sharp enough to demand compromises to our ability to fly level. In other words starting or stopping turns at the right times, without bobbles, requires the neutral stability. That seems right to me. I think the 25% wing MAC position is reported to be good for well designed flapped stunters with large enough (25%) horizontal tails on long enough arms. The flapless planes seem to require smaller tails and further forward c.g.'s - like 15%-19% MAC. I think they maneuver tighter or at least quicker with shorter tail moments because of the wing's downwash being more intense there. That seems to be incorporated into the theory behind locating a planes overall aerodynamic center or neutral point. I get that from the equations suggested on Hepperle's site, the Simons Book, and some old NACA reports. Raised horizontal tails seem to be a positive influence on stability for the same reason.

However, in response to Bill's feelings about difficulty vs. convenience, Jim's high point is a lot closer to the c.g. or a.c. positions of the wing than the hinge line, and it's easy to see and measure from. For our purposes, I thing it is a good convenient way to get a tail arm length: high point to high point (or as you said,  a bit ahead of the elevator hinge on most tails). Of course the high point at the root of most flapped stunters is pretty close to a good c.g. position, since it is ahead of the neutral point (25% MAC point). My last flapless had the quarter-chord points lined up spanwise and the high point at about 22%. So it was a reasonable place to put the c.g.

IOW, I agree with most of what you've written. Ha!

SK

[W.D. Roland]

 

[Bill Little]

I assume that the Bar Bell effect I have heard of is referring to the polar
moment of inertia?

Hi David,

Well when I first read Bob Gialdini's Olympic article, I understood what he referred to as the "Bar Bell Effect"..............  that made sense to me.

Big Bear

[Serge_Krauss]

I think after this one, we really need to move this to one of the technical sections, since we've really gove further than the original question!

...Raised tail then what happens during invert flight??
I never bought this idea for aerobatic aircraft. Cessna 150? yea.
Down wash ( Conda) would be reversed.

...25% is valid from flat plate airfoil on ward? excepting under camber?

Link to Hepperle's site?

...A question
Positive incidence elevator on C/L stunt does what and how for why?

Raised Tail: 'just to get it away from the wash, where small changes in pitch change the wake direction. I think everyone favors upright flight and uses extra effort/concentration in inverted flight. However, around neutral, where we're mostly concerned, raising it still keeps it away during inverted flight. You could lower it too. Neutral point calculations favor stability around neutral, from what I've seen. Some use slightly differing considerations, but the most specific ones seem to give varying percents of stability enhancement to vertically separated stabs. Simons, for instance, has a stabilizing efficiency factor (multiplied) of the neutral point position back along the MAC that is valued as follows:

T-Tail: .9
"Normal" Tail: .6
"Tail near wing wake": .3

The Stabilizer Volume Coeefficient (What we usually call the 'TVC') is also a factor.

Flat plate: I think it stays at 25% as do symmetrical sections  with a.o.a. changes. The c.p. moves on cambered sections with change in a.o.a. Undercambered ones have an increasingly positive pitching moment with increased a.o.a. which is unstable: the more they pitch, the more they want to do so. The right reflexed sections will want to diminish pitch - stable.

Martin Hepperle's Site, which I think addresses this: http://www.mh-aerotools.de/

Positive stab (?) Incidence: It gives a downward pitching moment to counteract opposite ones (like gyroscopic precession), some drags...) and tends to load the wrist with the need for a steady upward pressure. 'seems to me that this is advantageous in upright (favored) or inverted flight, since it alleviates the need to chase unwanted pitch changes by completely reversing the control direction. That would seem to help with "hunting" problems. I think it's like down-thrust on the engine. When I'm more awake, I might have a better answer. One thing you can do though is do a search here and especially on SSW Forum, where there is a lot of comment on this, probably even from an awake SK.

That's it for me for a while!

SK

[W.D. Roland]

 

[Ted Fancher]

For the short answer, see the bold type below. The rest belongs in the engineering or design sections.

Using Jim's high point for the c.g. position in finding moment arms is a lot better than hinge-to-hinge, but rules of thumb like "Ted's Rule" for flapped planes (C.g. position in terms of %MAC = the ratio of tail area to wing area) and 15% - 19% MAC for flapless planes are better.

Regarding nose and tail moments, or rather moment arms...

A lot of these rules of thumb work somewhat, simply because the planes being compared are already quite similar. They won't work well for some categories of unconventional designs, because the amount of change in the behavior of a model is not really proportional to such dimensions as hinge-to-hinge  tail "moments". If you were to double that, for instance, you would not be doubling the actual tail moment, and the aerodynamic effect would prove too small, except for the closest coupled flapless designs. Useful or optimal tail moments are different for flapped vs. unflapped ships and are pretty much established by experience over a couple generations of stunt design. However, if you want to compare performance or predict performance changes within a category, you should use something closer to the actual tail moment arm.

Tail Moment Arm: distance from model’s center of mass (c.g.) to aerodynamic center (a.c.) of stab/elevator. This is for purely aerodynamic considerations, compromised by inertial moments of the structure. I think it's also valid to compare using the distance between the a.c.'s of wing and tail. These too are approximations in determining performance, but use valid parameters. You can find these easily by measuring your model and plugging your dimensions into neutral point (a.c.) calculators like this one. Just read the answer - no work on your part:

http://www.palosrc.com/index.php?option=com_content&view=article&id=50:cg&catid=41:ic&Itemid=50

(make the MAC balance point 25%, and the c.g. distance will tell you your neutral point or a.c. position at the fuselage. Most valid rules of thumb for c.g. concern just the wing a.c. as computed here, and this same little calculator will tell you the tail a.c. for computing the tail arm.)

If you want the entire plane’s a.c., then this site can get it for you, but it's more involved and not necessary for our approximations:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

Nose Moment Arm: Distance from c.g. of nose to c.g. of plane. This is for balance (and moments of inertia) only and has nothing to do with aerodynamics. Put the engine where it needs to be to get the c.g. right. For adjustments, it's pretty close just to work with the masses of the engine/prop and tank for good approximation. That means that you need to know how far these are from the plane's c.g.

SO...the plane's c.g. is important. You might worry about how it moves with regard to changes in the positions of these component masses. Don't. Just choose the c.g. relative to  the a.c. of the entire plane AND whether the plane has flaps. Ted's rule or the flapless c.g. limits are as good as anything though. They depend on wing a.c. only. So the c.g. is determined solely by pitch aerodynamics and if moved to adjust for line position, rudder offset, or something similar, you have the tail wagging the dog. The aircraft c.g. is determined by desire for stability vs. maneuverability in pitch. Choose it and then design around it, realizing that total aircraft a.c. changes some with choice of tail area and moment. Once the plane is designed and built, you can move the c.g. to suit your handling preferences (but not according to unrelated dimensions, like leadout positions).

SK

Really good stuff here, Serge...as usual, I should add.

Only comment I would make would be regarding the "neutral point" of the airplane which your comments appeared to locate at the CG located at the MAC moved laterally to the fuse.  If that's what you meant it's not entirely correct with regard to the whole airplane.  The following comments will also address some of the discussion of "tail heaviness" and adjustable control handles in another post on this thread.

The "neutral point" of an airplane is the longitudinal point at which all of the aerodynamic forces acting on the airplane can be considered as "centered"!  Much as the CG is the point at which all of the mass of the airplane is centered...even though the mass is made up of a variety of parts of widely divergent individual "weight".  

Perhaps the easiest way to visualize a neutral point is to take an arrow shaft with no "head" or "tail feathers" and throw it like a spear.  Because the "impact" of resulting airflow around the shaft is uniform with respect to the CG and there is no front or back or middle to it.  It will "not" fly like a spear.  Its attitude with respect to the airflow will be entirely random--and it will not fly far no matter how hard you throw it.

If, however, you add weight to the "nose" or feathers to the tail you've introduced a "moment" between the CG and all of the air "impacting" the shaft and it will tend to fly with either the weighted end forward or the feathered end aft.  Put enough weight in front and you can enter the spear chucking event at the next track meet...and throw it a couple of hundred feet.  Take the weight off and you'll be lucky to throw it 10 feet.  Same principle with the feathered shaft and if you combine the two you get an arrow that flies almost literally true to the target.

This happens because the CG is forward of the "neural point" (think of it as the center of aerodynamic pressure affecting the entire vehicle).  As long as the CG is forward of that location the object will be "stable" with respect to displacements for whatever cause, maneuvering, turbulence, etc. and will, therefore, self correct to fly "straight".

One more test.  Now take the weight off the front of the arrow and put it behind the tail feathers.  Depending on the ratio of weight to feathers this new vehicle will either resemble the unadorned shaft (not stable in any attitude) or, if the weight is great enough will be moderately stable if thrown "backwards".  Don't you just hate this aerodynamic stuff.

Now the neutral point of a conventionally configured stunter will be aft of the Aerodynamic Center of the wing alone (how much aft will depend to a large degree on the size of the horizontal tail but will also be equally affected by all surfaces of the entire aircraft [the fuselage for instance is the exact same thing as the sides of the arrow shaft).  The important thing to know as a stunt designer and trimmer is that the stability (the desire of the airplane to go forward and to correct itself to go forward if displaced) is a function of the distance between the CG and the Neutral Point.  That distance is called the "Static Margin".

As long as there is a Static Margin (CG forward of the NP) the airplane will be technically "stable" and will react in the manner we've discussed.  However, as the static margin decreases so does the magnitude of the restoring forces.  Thus, the suggestion that we use an adjustable control handle spacing to "calm down" the response as the Static Margin gets shorter.  This will likely make some of you ponder the question of whether to trim your stunters with a smaller "static margin" and tame it with the handle.  

I don't recommend doing so.  Here's few sentences on why not.

The reason I (and others) strongly advocate the use of a 25% MAC CG in close formation longitudinally with the Aerodynamic center of the wing (for symmetrical wings also at 25% of the MAC) is because doing so eliminates forces that develop between the two which can change while maneuvering and require constant control adjustments as G forces change in maneuvers (especially a problem when flying in winds and a great contributor to the tendency for the airplane to "wind up" during consecutive maneuvers).  As the CG is moved forward of the AC the required control forces will increase with increased G loads and if moved aft of the AC they will decreases.  That is because the lift generated by the wing (centered at the AC) develop a "moment" between the two and the lift tries to "rotate" the airplane about the CG.  That is the pits when you're trying to fly rulebook sized and shaped maneuvers!

A primary design goal (and trimming that new stunter as well) is to achieve that symbiotic relationship between those two forces.

If you've managed to hang on this long, one more important "Neutral Point" bit of wisdom.  As the tail gets bigger the Neutral point goes aft.  If the tail gets so big that it's bigger than the wing (otherwise known as a canard) the NP will eventually be moved well aft of the forward surface and the CG will have to move commensurately otherwise the static margin will become TOO great and maneuvering in pitch will become difficult (eventually impossible).  As you approach that "impossible" nose heavy condition you will experience control loads increasing when flying maneuvers in the wind to the point that you will not be able to continue flying those loops because doing so will run you out of altitude and ideas...never a good idea.

The flip side of that aft movement is that the NP on a canard can "NEVER" be as far aft as 25% of the Mainplane (the rear surface that is now the "wing").  In order to be "stable" the CG must be forward of that point to maintain the required "static margin".  If you move the CG to the heretofore desirable 25% MAC in line with the Aerodynamic Center of that main plane you will have built yourself that (long ago discussed) arrow with the nose weight moved behind the tail feathers.  It just will not go in a straight line and will "never" return to straight ahead flight on its own!  

The net result is that there will always be a "moment" between the CG and the AC of the mainplane of a Canard and that is why you don't see Variezes or their various cousins competing with Yaks, etc. in aerodynamic events.  The primary efficiency factor of a Canard is the fact that it maintains a stable condition with both surfaces lifting (the canard surface keeps the nose from dropping...which it wants to do because the CG  is [must be] forward of the point at which the wing is lifting the bulk of the vehicle's mass.   This "dual lifting surface" means increased fuel efficiency as no "down load" on the tail needs to be held aloft by the wing.  I that same vein, Airbus has sold a lot of fly by wire airplanes that have CG ranges well aft of where they've historically been located and have better specific fuel consumption as a result.  This is made possible in part by the computerized flight control systems.  A very, very big deal in the airline biz.

IMHO, this is why you're never going to see truly competitive CL Stunt Canards.  Oh, you can design and trim one to fly pitch maneuvers pretty darn well under a certain set of conditions.  When the wind blows however and the G loads build up forward of the mainplane's AC the control forces required to fly the same size and quality maneuvers are going to generally increase but even more importantly, vary considerably as the maneuvers are flown.  The Canard that will fly competitively in the wind will be trimmed to sensitively to fly well in the calm and vice versa.

Of course, having put all this stuff out in public Bobby Who will probably finally finish his Canard and kick butt in Muncie this summer!  Wouldn't be the first time he's done something like that.

Sorry to run on but this is an important subject for stunt fliers.

Ted Fancher

p.s. Sparky may want to move this post to the stunt design thread...in fact the whole thread has evolved to where doing so might be appropriate.  On the other had it has attracted a lot of attention her on the "Front Page"!

[Bill Little]

Hi Ted,

I think we will leave it here for the time being.

Bill

[Tim Wescott]

25% MAC is what I recently decided to use for ground zero  from now on as some of the
airfoils I am trying have very rearward high point with the thinking that the Flap
increased Conda effect will be improved through smoother flow across hing line and flap.

25% is valid from flat plate airfoil on ward? excepting under camber?

Per the aerodynamics books that I've read, 25% is valid for anything.  Instead of viewing lift as acting at a center of pressure that wanders around on the wing with angle of attack, you can pretty much split the action of a wing section into a lift force at 25% MAC plus a diving (or lifting) moment*.  The moment is a function of the wing section, while the lift force is a function of the wing section and the angle of attack.  Work out the math, and you'll find that a moment that tends to twist the wing up will be stabilizing, while a moment that tends to twist the wing down will be destabilizing.  Most wing sections have a destabilizing moment or none at all, because that makes for a more efficient airfoil -- airfoils that are designed for flying wings have a stabilizing moment, and as a consequence they have a narrower angle of attack range over which they work, and they have greater drag.

So if you want a stable airframe, you make this great big wing with a neutral or destabilizing moment, then you stick a little wing on the back of the fuselage (or the front), you call it a "stabilizer", and you arrange it to lend a stabilizing moment to the airframe as a whole.  You do the same thing whether you're using a wing with undercamber, or with no camber at all.  You might need more stabilizer area for an undercambered wing, but that's more because it needs more force to overcome the undercambered wing's destabilizing moment, and you really, really don't want your stabilizer to stall.

So are you gonna build a stunter with undercamber or something?

* Why, then does a symmetrical airfoil have a CP that travels?  Well, that's why I said "pretty much".  The first-cut theory of lifting sections says that you'll see a pure force and a pure moment** -- but in reality the effective camber of the wing changes a bit with angle of attack, and screws up all the pretty math.  They mention that in the books, too.

** And when I say "moment" here I mean a mechanical engineer's "moment" -- a torque, not a "moment arm".

[W.D. Roland]

 

[Ted Fancher]

Glad your going to leave it here Bill!
You know if I leave the main page and get lost you will have to talk me out!

Thanks Ted  
Question:
NP= CP of entire airframe surface?

Ok, I am finally convinced that I need to do some tail volume numbers on the FW.

Your post was perfect timing as Stab/Elv parts cutting was fixin to commence!(tonight!)

Will read you post again(thanks!) and now I see Tim's post to read also.

David

EDIT:
Tim,
Does the sections moment(torque) come from the moment arm lenght between the CP and CG?
torque equals force times distance?

Taill volume is penetrating gray matter........................................

You're welcome, David,

Just to be clear, btw, a "moment arm" is the distance between a force applied and the axis of its rotation (the CG in our stuff).  A "moment" is the actual leverage applied, the load multiplied by its "arm" in, for instance, inch ounces of torque.  Arm means distance and moment means the torque applied to the point about which the force acts.

The moment between the Aerodynamic Center of the wing and the Center of gravity is a function of the distance between them multiplied by the lift applied at the AC and/or the G loads applied at the CG.  that's why its important to keep these two "force generators" close together.  The further apart they are the greater will be the required control forces to overcome the moment that multiplies between them.  In a hard corner you get the worst of both worlds, the G forces go up dramatically and the lift required to support those G loads has to be sufficient to support the load.  increase the distance between them and it's like putting a sumo wrestler on one end of a teeter totter and then you, the pilot, has to put in enough control to simulate another sumo wrestler just to break even; more if you want to turn a tight corner--it just aint' gonna work!  If, however, the first sumo guy is sitting right on top of the fulcrum (the CG) the additional "flab" forces have no "arm" to fight the pilot's input and the problem largely goes away.  This really, really works.  A stunt ship with the proper relationship of CG and AC is little affected in rates of turn when accelerating or decelerating in high winds and wildly varying G loads.  The rate of turn is largely unaffected and, assuming the pilot can keep up, fine patterns can be flown because the airplane isn't fighting the pilot.  when you hear a top pilot talk about hoping for bad winds you can bet his airplane turns just as predictably in bad air as good.  It's a "huge" advantage.

Cambered wings also have a "pitching" moment that is a function of the camber of the wing.  Unless the wing is "reflexed" (trailing edge turned up like an elevator; necessary on flying wing planforms) cambered wings have a negative pitching moment (leading edge down) when generating lift.  The stabilizer must produce a down force to "stabilize" a cambered wing's inherent tendency to pitch down when producing lift.  This is why when the pushrod breaks between the flaps and elevators and the pilot reflexively gives full "up" elevator the flaps camber the wing and that negative moment pitches the airplane nose down and you crash anyway.  (This is also, by the way, why flaps don't, in an of themselves, cause a stunter to turn tighter.  In fact,  when flaps are added to an airplane without flaps you'll actually lose corner and it is also the reason that flapped airplanes [properly configured with respect to tail volume] require a further aft CG location for competitive rates of turn...see Serge's comments about a 25%MAC for flapped airplanes and 15%MAC on unflapped designs). 

By the way, the pitching moment due to camber is the reason that even airplanes with the CG and AC co-located still have increased control inputs required under high G conditions...because the higher speed (and its accompanying higher G loads) requires more lift to maintain the required turn radius and, thus, more flap deflection producing more camber induced pitching moment.  This is, literally, only a fraction of the moment produced as the CG and AC part company.  Finely, because the CG/AC pitching moment is largely eliminated the actual speed up in maneuvers is reduced dramatically which, in and of itself, serves to mitigate the camber induced factor.  It's a win win!

Too much?

Ted

[Howard Rush]

Work out the math, and you'll find that a moment that tends to twist the wing up will be stabilizing, while a moment that tends to twist the wing down will be destabilizing.

I did, and that's not what I found.  There are people who write books about aerodynamics, yet don't know the difference between pitching moment and rate of change of pitching moment with angle of attack.  

To elaborate, some authors use "stabilizing" for moment, rather than increasing negative rate of change of pitching moment with angle of attack, which runs the poles to the left.  Other authors know the difference, but explain it funny. 

[jim ivey]

Hi was thinking a raised Tail getting it out of wing wash was going to make it more effective. When I first saw that I didn't like it. He had harsh words for my criticism. I told him Hi you dont fly! Those us who do, Know if you want the plane to perform the same inverted as right side up it needs to be symetrical, Tats gonna be a tail swinger on outside maneauvers, and it was drasticly so. Any of you who have built one know this is so. Dennis never entered  a contest with his other than that disasterous 58 nats. When rob asked me about changes I'd suggest. The second suggestion was lower the stab n elevator to C/L of thrust. The old Detroiters that were so sucessful, Engine wing and stab al the same centerline. If the plane is not symetrical you have to fly it different one side to the other. That makes coffin corners that much more difficult.  Gee serge thanks fo the vote of confidence. Like I said mine was seat of the pants theory The c/g = highpoint of airfoil just seemed logical to me. also it seems to work for me.  jim

[Howard Rush]

The "neutral point" of an airplane is the longitudinal point at which all of the aerodynamic forces acting on the airplane can be considered as "centered"!  

If the airplane has no pitch acceleration, the point at which all the aerodynamic forces acting on the airplane can be considered as centered is the CG.  My people define neutral point as the CG position where rate of change of pitching moment with angle of attack (or lift) = 0.

The reason I (and others) strongly advocate the use of a 25% MAC CG in close formation longitudinally with the Aerodynamic center of the wing (for symmetrical wings also at 25% of the MAC) is because doing so eliminates forces that develop between the two which can change while maneuvering and require constant control adjustments as G forces change in maneuvers (especially a problem when flying in winds and a great contributor to the tendency for the airplane to "wind up" during consecutive maneuvers).  As the CG is moved forward of the AC the required control forces will increase with increased G loads and if moved aft of the AC they will decreases.  That is because the lift generated by the wing (centered at the AC) develop a "moment" between the two and the lift tries to "rotate" the airplane about the CG.  That is the pits when you're trying to fly rulebook sized and shaped maneuvers!

I've read stuff like this you've written before.  You seem to be describing real phenomena, but the explanations don't seem to match.  Maybe it is because I don't understand stunt talk.  Is "wind up" the tendency of an airplane to go faster in consecutive loops, the tendency of consecutive loops to get tighter with the same control deflection, or something else?

[Howard Rush]

By the way, the pitching moment due to camber is the reason that even airplanes with the CG and AC co-located still have increased control inputs required under high G conditions...because the higher speed (and its accompanying higher G loads) requires more lift to maintain the required turn radius and, thus, more flap deflection producing more camber induced pitching moment.

That's not the reason for the increased control input requirement.  Lift and "centrifugal force" both increase as airspeed squared, so control deflection won't change.  Hinge moment for a given control deflection does increase with airspeed.  That's why you have to torque the handle more when the airplane goes fast. 

[Howard Rush]

Hi was thinking a raised Tail getting it out of wing wash was going to make it more effective. When I first saw that I didn't like it. He had harsh words for my criticism. I told him Hi you dont fly! Those us who do, Know if you want the plane to perform the same inverted as right side up it needs to be symetrical, Tats gonna be a tail swinger on outside maneauvers, and it was drasticly so. Any of you who have built one know this is so.

I can see why Hi thought that, and I think you are correct.  With programs like Profili you can now easily find downwash for any wing, flap deflection, and angle of attack.  It might be interesting to fiddle with it and make some plots. 

[Tim Wescott]

I did, and that's not what I found.  There are people who write books about aerodynamics, yet don't know the difference between pitching moment and rate of change of pitching moment with angle of attack.  

To elaborate, some authors use "stabilizing" for moment, rather than increasing negative rate of change of pitching moment with angle of attack, which runs the poles to the left.  Other authors know the difference, but explain it funny. 

If you take the wing section behavior as I explained it, then when you adjust the center of gravity to bring a wing with positive pitching moment and positive lift coefficient into static equilibrium, the center of gravity will be forward of the MAC (and hence neutral point) of the airfoil.  Hence, the thing will be stable.

(If you want to argue this further, it'll be stable because the moment from the center of gravity leading the lift acting on the MAC is a pitch-down moment that varies with airspeed and angle of attack, while the moment from the wing depends only on airspeed.  Decreasing the angle of attack acts to decrease the pitch-down moment, and so you have the negative feedback needed to restore the angle of attack.)

I'd be interested in seeing the math and/or reasoning that led you to a different conclusion.

[Tim Wescott]

Tim,
Does the sections moment(torque) come from the moment arm lenght between the CP and CG?
torque equals force times distance?

Taill volume is penetrating gray matter........................................

Not really -- the moment's going to be there regardless of where the CG is.  I'm more presenting the 'single-point force & moment' model because it gives me a very convenient model for thinking about stability.

Really, neither the 'force at a moving center of pressure' model or the 'force at MAC plus a moment' model is the whole story.  What's really going on is that there's a pressure distribution over the entire chord, with some parts of the airfoil being pushed, and others being pulled.  That pressure distribution is a lot closer to the whole story -- but it's a lot of needless detail if you just want to do stability analysis.

[Serge_Krauss]

Really good stuff here, Serge...as usual, I should add.

Only comment I would make would be regarding the "neutral point" of the airplane which your comments appeared to locate at the CG located at the MAC moved laterally to the fuse.  If that's what you meant it's not entirely correct with regard to the whole airplane.  

Ted-

Thanks for the vote of confidence!

I re-read my post and am not sure about the impression of the CG located at the MAC "moved laterally to the fuselage" vs N.P. for the whole airplane. Perhaps my justification for using the "ted's rule" was insufficent, but, as you probably remember, I did graph out its prediction vs. static margin for the whole plane on SSWF and felt it to be plenty "close enough."  That, for this section of the forum, was my intended point. Probably because when we write, sometimes we see more what we mean than what we actually write. Ha!

Anyway, I would disagree on the idea of the canard making a good stunter. I've posted some stuff about canards, but the key factors are interference and the requirement not to use the "main" wing to it's fullest extent in order to load the canard sufficiently. This eliminates the canard configuration's reputed lift advantage. That said, I did watch Mike Melville (I think), or perhaps Dick Rutan, do aerobatics with a VariEze at Oshkosh. That was good, but the Pitts, Eagles, and Extras were much more impressive. I think 3-surface planes, like Dennis', are more viable.

SK

[Howard Rush]

If you take the wing section behavior as I explained it, then when you adjust the center of gravity to bring a wing with positive pitching moment and positive lift coefficient into static equilibrium, the center of gravity will be forward of the MAC (and hence neutral point) of the airfoil.  Hence, the thing will be stable.

That's right.  What's doing the stabilizing is putting the CG in front of the neutral point.  The negative camber (or a tail that pushes down) balances the couple, as you said.  Etkin, in his first book, used the same argument: calling a negative camber stabilizing because you need a negative camber to get a stable flying-wing airplane with the moments balanced.  Some authors refer to the negative moment of a forward CG as being stabilizing.  I just think it's a little confusing, particularly when stated without mentioning dCm/dCl.   

[Howard Rush]

Anyway, I would disagree on the idea of the canard making a good stunter. I've posted some stuff about canards, but the key factors are interference and the requirement not to use the "main" wing to it's fullest extent in order to load the canard sufficiently. This eliminates the canard configuration's reputed lift advantage. That said, I did watch Mike Melville (I think), or perhaps Dick Rutan, do aerobatics with a VariEze at Oshkosh. That was good, but the Pitts, Eagles, and Extras were much more impressive. I think 3-surface planes, like Dennis', are more viable.

I think conventional stunters are more viable, but I've had a canard notion for 35 years or so that I haven't got around to building yet.  Yes, you can have flaps at the back of the wing that go in the right direction. 

Four ways of getting around the destabilizing effect of the canard surface are: 1) a really low aspect ratio canard, 2) a canard with a perverted airfoil operating in a range where it has a flat lift curve slope, 3) a rotating-cylinder canard, and 4) a floating canard.  Number one is too practical.  It may be wrong, too.  Somebody explained to me why Rutan's canards have high aspect ratios, but I forgot.  Number two is what the Tu-144 uses.  I can't figure out how to do it in both directions, as one must for stunt.  Number three might work, but square corners and transitions in eights would be clumsy if you couldn't reverse the rotation rapidly.  Number four is my favorite.  You hinge the canard at the quarter chord and drive it with a tab or stabilator on the back of the canard. 

[Howard Rush]

Undercambered ones have an increasingly positive pitching moment with increased a.o.a. which is unstable: the more they pitch, the more they want to do so. The right reflexed sections will want to diminish pitch - stable.

Here, too, this is true if you have a tailless airplane and put the CG where it needs to go to the airfoil's pitching moment.  For a given CG location, camber doesn't affect stability, except for the tiny effects that Tim mentioned that don't go along with the simplified theory.  Flaps don't destabilize a stunt plane.  There is, however, an effect that I suspect is causing a phenomenon that Ted has mentioned.  If the place on the flap where flow separates changes as angle of attack changes, it could reduce (for inside loops) the wing's negative Cm0 as angle of attack increases.  Hence the airplane wanting to "dig in" in turns (if I'm using the same definition of digging in). 

[Bill Little]

Hi Howard,

You briefly spoke to the "inline" configuration in your discussion of Jim Ivey's/Hi Johnson debate.

What are your applied theories to having all C/Ls lined up in our stunt craft?  On the surface it appears that the forces acting will actually be the same for both upright and inverted, whereas the offset C/Ls would appear to be unable to perform the same both upright and inverted.  Is my limited thinking of this "symmetrical world" flawed by the forces acting on our tethered models?  It appears that the "logical" answer would be that having everything "lined up" would create a situation where the model plane would not care if it is upright or inverted.  I am not aware (obviously) of all the aerodynamic principles that would upset the apple cart in this scenario.

Thanks!
Bill
(open to Ted, Serge, and all the others versed in these subjects)

[Tim Wescott]

Here, too, this is true if you have a tailless airplane and put the CG where it needs to go to the airfoil's pitching moment.  For a given CG location, camber doesn't affect stability, except for the tiny effects that Tim mentioned that don't go along with the simplified theory.  Flaps don't destabilize a stunt plane.  There is, however, an effect that I suspect is causing a phenomenon that Ted has mentioned.  If the place on the flap where flow separates changes as angle of attack changes, it could reduce (for inside loops) the wing's negative Cm0 as angle of attack increases.  Hence the airplane wanting to "dig in" in turns (if I'm using the same definition of digging in). 

I'm not even sure if it's that.  Flaps don't destabilize a plane (at least not by any aerodynamic theory that I know), but they do add a healthy amount of "anti-elevator".  When you pull up elevator, the elevator is trying to pitch the plane up, but the flap is trying to pitch the plane down.  This has nothing to do with the plane's stability, but it has a whole lot to do with the plane's pitch sensitivity, and the track that the plane is going to take.  Making a big tail makes it easier for the elevator to overwhelm the flap action, and so turn the plane when it needs to turn.

Two extremes that I can think of are no flaps, where you pull up elevator, the whole plane turns, and that creates lift (causing a delay, and unsightly turns), and no elevator, where you pull up "elevator" and the plane hops up from extra lift, but immediately starts pitching down.  Somewhere short of that you may conceivably have a plane with no pitch control, but which you could make rise and fall with control input, and somewhere closer yet to the 'no flap' case you'd have a stunt plane, that gives nice, balanced-looking clean corners.

Someone will read this and say "Ahh, but why, then, do small-tailed flapped stunters like the Nobler have such a narrow acceptable CG range?".  Well -- I dunno.  I strongly suspect, however, that because of the anti-elevator effect of the flaps, you need to move the CG back to get any turn at all, but if you go too far back then you no longer own a stable plane.  Making the tail bigger gives the tail more authority to drag the plane through a turn, and moves the neutral point back -- both of these effects would tend to increase the CG sweet spot.

[Bill Little]

While waiting on Howard's response to the above I have another question (or three) dealing with this discussion.

Just how stable do our models need to be?  I know the answer will deal with the individual pilot's skill level and preferences (the average raw beginner cannot fly as tail heavy as say Billy Werwage), but..........  

And how does our control systems compare to/impact the "nose/tail moment", TVC, NP, CG, variations we can trim into a model?  (maybe too vague a question?)

Finally, how does wing loading come into play?  Let's say we have to "identical" models in all aspects except total aircraft weight.  One is 6 oz. heavier.  What, if anything does this do aerodynamically to the situation?

Big Bear

[W.D. Roland]

 

[Howard Rush]

What are your applied theories to having all C/Ls lined up in our stunt craft? 

Beats me.  The Yatsenko Shark has them lined up and flies just fine.  I didn't notice if it has stab incidence or elevator bias. I think Brett may have calculated the stab incidence required to compensate for the prop gyroscopic effect in level flight and decided it's about how much incidence people use.  I hope to know more when I get to switch prop rotation direction on my airplane and when somebody plots the downwash field behind wings at different flap deflections and angles of attack.

I think the Simons numbers on tail effectiveness look suspicious.  Maybe he worked on the BAC 1-11.

[Howard Rush]

Someone will read this and say "Ahh, but why, then, do small-tailed flapped stunters like the Nobler have such a narrow acceptable CG range?".  Well -- I dunno.  I strongly suspect, however, that because of the anti-elevator effect of the flaps, you need to move the CG back to get any turn at all, but if you go too far back then you no longer own a stable plane.  Making the tail bigger gives the tail more authority to drag the plane through a turn, and moves the neutral point back -- both of these effects would tend to increase the CG sweet spot.

Yep.  Small tails lead to narrow CG ranges on other planes, too, like transport airplanes and F2D models.

[Howard Rush]

Just how stable do our models need to be?  I know the answer will deal with the individual pilot's skill level and preferences (the average raw beginner cannot fly as tail heavy as say Billy Werwage), but..........

That's a good question.  There's more to it than just static margin. I don't think anybody has calculated the pitch dynamics of a stunt plane (which includes the weight stuff you mentioned).  I have had the privilege of flying a few Nats-winning stunt planes, and my first impression was that they would make good trainers.  They were very easy to fly. 

[Serge_Krauss]

A couple points/reminders...

1) As Howard hinted, my remark about "unstable" (positively cambered) wing sections was limited to just that: sections or airfoils. The stabilizer stabilizes. The problem with trying to confine oneself to simple answers off of the technical forums is that they don't deal with the entire picture.

2) A symmetrical 0-0-0, inline plane is not symmetrical dynamically, because of asymetrical forces from prop rotation (precession and P-factor) and other parts that contribute drag asymmetrically (e.g. fin, l.g.). I also agree that there's a danger of wing wake causing abrupt changes with elevator deflection about neutral. Also the effectiveness of the stabilizer seems to be diminished, according to N.P. calculations and formulae I've found (as posted above). So it would appear that an in-line stunter might need a larger stab, except that wake effects might then be magnified and any building inaccuracy might aggravate such problems. My one in-line model seemed "OK", but some have reported otherwise. Diverse opinions on performance of these models have been posted.

3) I see that Bill just posted. The Simon stab efficiencies seem severe, but I have seen this kind of degradation entered in others' computations concerning aircraft N.P.

4) I don't think that ANY planes perform the same upright and inverted. Those that come closest in pilots' perceptions may be just more predictable. For "stability" I think that we are most concerned with level flight with controls near neutral and then consistency in things like round maneuvers and the ability to exit a maneuver or reverse direction without bobbles.

[Howard Rush]

A couple points/reminders...

1) As Howard hinted, my remark about "unstable" (positively cambered) wing sections was limited to just that: sections or airfoils. The stabilizer stabilizes. The problem with trying to confine oneself to simple answers off of the technical forums is that they don't deal with the entire picture.

2) A symmetrical 0-0-0, inline plane is not symmetrical dynamically, because of asymetrical forces from prop rotation (precession and P-factor) and other parts that contribute drag asymmetrically (e.g. fin, l.g.). I also agree that there's a danger of wing wake causing abrupt changes with elevator deflection about neutral. Also the effectiveness of the stabilizer seems to be diminished, according to N.P. calculations and formulae I've found (as posted above). So it would appear that an in-line stunter might need a larger stab, except that wake effects might then be magnified and any building inaccuracy might aggravate such problems. My one in-line model seemed "OK", but some have reported otherwise. Diverse opinions on performance of these models have been posted.

3) I see that Bill just posted. The Simon stab efficiencies seem severe, but I have seen this kind of degradation entered in others' computations concerning aircraft N.P.

4) I don't think that ANY planes perform the same upright and inverted. Those that come closest in pilots' perceptions may be just more predictable. For "stability" I think that we are most concerned with level flight with controls near neutral and then consistency in things like round maneuvers and the ability to exit a maneuver or reverse direction without bobbles.

Another couple of points and reminders:

1. Except for some subtle stuff, there is no difference in stability among normal airfoils below stall angles of attack.  Stability is not a function of camber or leading-edge radius.  You can safely assume that the aerodynamic center is at the quarter chord. 

2. Sharks work a treat.

3. Is there some NACA data, for example, on this?  Anything I've seen theoretically, experimentally, or practically (e.g. the Shark) suggests those numbers are wrong.

4. And maybe symmetry isn't that important.  I flew a famous stunt plane that had way asymmetric control sensitivity.

[Serge_Krauss]

Another couple of points and reminders:

1. Except for some subtle stuff, there is no difference in stability among normal airfoils below stall angles of attack.  Stability is not a function of camber or leading-edge radius.  You can safely assume that the aerodynamic center is at the quarter chord... 

3. Is there some NACA data, for example, on this?  Anything I've seen theoretically, experimentally, or practically (e.g. the Shark) suggests those numbers are wrong...

1) OK, you've confused me here, Howard. Did you think I meant that the instability of a cambered section was important to the behavior of a stunter with deflected flaps? I didn't. The added negative pitching moment from deflected flaps far surpasses the instability of the section from those deflected flaps as it pitches, and of course the smothered accelerated pitching tendency is in the desired direction anyway. However, I don't understand the statement about stability not being a function of camber or the relevance of where to assume the aerodynamic center to be. 'seems to me that a constant a.c. with a pitching moment is equivalent to a center of pressure somewhere else (even off the wing) acting about that point.  Data shows that the more any positively cambered section is pitched up, the more it wants to pitch further. That's a definition of instability.

3) I looked only at what was close at hand in my collection - 'didn't go looking at the NASA site. I just found TR 133, The Tail Plane (Munk, quite historical and early); TR 293, Two Practical Methods for Calculation of Horizontal Tail Area Necessary for a Statically Stable Airplane (Diehl, 4/6/28), andTN 587 Notes on the Computation of the Minimal Horizontal Tail Surface for Airplanes Equipped with Wing Flaps(Soule', 1937), based on Diehl's report. Munk complains about lack of (propeller) slipstream data at that time on aoa and says he must oversimplify. He does take into acount the rate of change of lift with aoa, mentioning wing wake, but gives the trusty "It will be sufficient in most cases to assume..." a certain angle of degradation. Soule' seems to ignore vertical placement assuming a value for the overall tail derivative and computing area. TVC again ranks high, being modified by these approximate tail CL values. Diehl uses "subsidence of down wash in the horizontal and vertical planes", but it is hidden in a factor computed from other constraints. He says it is accounted for, but does not give input anything expressing stab height relative to the wing.

However, in his book Engineering Aerodynamics, Diehl does give some efficiency factors. They are more conservative than Martin Simons', but not very compelling, ranging from .70 for triangular planform on fuselage center line to .95 for elliptical on fin. The intermediate values are all consistently increasing as height of stab increases. Millikn somehow avoids  specifics. Torenbeek shows some practical stab heights and configurations, but gives no numbers or theory in his book on aircraft design. Remember that I  brought up my own question on downwash angles above and aft of a symmetrical airfoil recently, saying that data seemed scarce, and no one was able to cite better data.

   

[W.D. Roland]

 

[Howard Rush]

1) OK, you've confused me here, Howard. Did you think I meant that the instability of a cambered section was important to the behavior of a stunter with deflected flaps? I didn't. The added negative pitching moment from deflected flaps far surpasses the instability of the section from those deflected flaps as it pitches, and of course the smothered accelerated pitching tendency is in the desired direction anyway. However, I don't understand the statement about stability not being a function of camber or the relevance of where to assume the aerodynamic center to be. 'seems to me that a constant a.c. with a pitching moment is equivalent to a center of pressure somewhere else (even off the wing) acting about that point.  Data shows that the more any positively cambered section is pitched up, the more it wants to pitch further. That's a definition of instability.

3) I looked only at what was close at hand in my collection - 'didn't go looking at the NASA site. I just found TR 133, The Tail Plane (Munk, quite historical and early); TR 293, Two Practical Methods for Calculation of Horizontal Tail Area Necessary for a Statically Stable Airplane (Diehl, 4/6/28), andTN 587 Notes on the Computation of the Minimal Horizontal Tail Surface for Airplanes Equipped with Wing Flaps(Soule', 1937), based on Diehl's report. Munk complains about lack of (propeller) slipstream data at that time on aoa and says he must oversimplify. He does take into acount the rate of change of lift with aoa, mentioning wing wake, but gives the trusty "It will be sufficient in most cases to assume..." a certain angle of degradation. Soule' seems to ignore vertical placement assuming a value for the overall tail derivative and computing area. TVC again ranks high, being modified by these approximate tail CL values. Diehl uses "subsidence of down wash in the horizontal and vertical planes", but it is hidden in a factor computed from other constraints. He says it is accounted for, but does not give input anything expressing stab height relative to the wing.

However, in his book Engineering Aerodynamics, Diehl does give some efficiency factors. They are more conservative than Martin Simons', but not very compelling, ranging from .70 for triangular planform on fuselage center line to .95 for elliptical on fin. The intermediate values are all consistently increasing as height of stab increases. Millikn somehow avoids  specifics. Torenbeek shows some practical stab heights and configurations, but gives no numbers or theory in his book on aircraft design. Remember that I  brought up my own question on downwash angles above and aft of a symmetrical airfoil recently, saying that data seemed scarce, and no one was able to cite better data.

1. The positive-cambered one would pitch up; the negative-cambered one would pitch down.  As Tim said above, you can split the forces and moment on an airfoil into two parts: lift coefficient at the quarter chord, which is a function of angle of attack, and moment coefficient (Cm0), which is constant.  What counts for static stability is the sign of dCm/dAlpha of the airplane.  The contribution of the constant airfoil Cm0 to this is zero.  The airfoil's contribution to stability is dCl/dAlpha * the distance from the CG to the quarter chord.  An airfoil's lift at any given angle of attack will be a function of camber, but the lift curve slope, dCl/dAlpha, is the same for all.  Mind you, the usual caveats about low Mach number and below-stall alphas apply.

3. Inspired by the effort you went to, I actually did some looking myself.  The usual stability and control books mention the reduction of dynamic pressure in the wing wake, but aren't much help in determining it.  One of them referred to http://naca.central.cranfield.ac.uk/reports/1939/naca-report-648.pdf , which is pretty cool.  Interestingly, it's a function of wing section drag, which figures.  There are a bunch of plots, but also some equations-- a bother in 1939, but no big deal with Excel.  If you want to do the ciphering yourself, assume .011 for level-flight Cd0 and about .03 for corners. Those come from Gary James's Impact airfoil Profili runs.   I had been assuming that the Simons efficiency numbers you were talking about were the ratio of wake dynamic pressure to free-stream dynamic pressure.   That's the standard definition of efficiency, but he might have thrown in some d epsilon / d alpha into those efficiency numbers.  Different authors do it different ways.  The NACA report has both, so it will keep you amused for awhile.  At a chord or two behind the wing, you wouldn't lose much tail effectiveness in level flight from dynamic pressure loss, confirming what Orestes seems to experience with his in-line airplane.  I didn't look at d epsilon / d alpha. 

In other news, the Revolution seems to be going OK.  We got one of guys running Egypt ( http://www.purdueexponent.org/city/article_b56b0b3c-466f-11e0-988e-00127992bc8b.html ).  Pete Super thinks that as a Kentucky Colonel, I would be a plug-and-play replacement for Colonel Gaddafi.  No dice unless the chick bodyguards come with the deal.  On the other hand, I could be on both the Libyan F2B and F2D teams.  I could get Yatsenko planes with the oil money.  This is coming together.   

[Serge_Krauss]

Howard-

'just checked in to skim the comments, but am leaving town in less than an hour. Your comments on #1 seem to match my understanding of things. I'll read again when I return.

SK

[W.D. Roland]

Distractions removed

[Ted Fancher]

That's a good question.  There's more to it than just static margin. I don't think anybody has calculated the pitch dynamics of a stunt plane (which includes the weight stuff you mentioned).  I have had the privilege of flying a few Nats-winning stunt planes, and my first impression was that they would make good trainers.  They were very easy to fly.  

I agree with Howard's assessment regarding top stunters and I think I know the reason why.  Particularly with respect to stability fitting hand in glove with maneuverability.  "SURPRISE"!  Ted thinks it has a whole lot to do with the (maybe I've mentioned this before) relationship between the AC and the CG and the "adequate" static margin that allows such a configuration.

To wit: "Stability and maneuverability are optimum when the least required amount of change in control input is required to fly consecutive round loops in a variety of conditions; i.e. when the appropriate control can be given and the airplane feels like it locks in to the desired radius and then just goes round and round and then exits cleanly and simply flies straight ahead in level flight".  (Edited addition)  I meant to add that, as Howard stated, in level flight such an airplane will fly straight and level exactly as you'd like a trainer to do, requiring conscious input to make it do something else such as--oh, let's see--doing tricks almost as easily as flying level!

One of the great and abiding "gospel" trim  advocacies of the early stunt years was the almost ubiquitous phrase "...if your airplane responds too slowly add weight to the tail and, if it is too responsive, add weight to the nose".  Yes, doing so on a particular airplane will have that effect under a given set of conditions and, therefore, appears to be valid.  It is, however, much too simple and, if the more complex factors are not addressed, could be entirely the wrong thing to do.  (It is worth noting, by the way, that control sensitivity is also an important factor in  this discussion and, in the olden days, the widespread use of Veco control horns with a max arm of 1/2" combined with three inch bellcranks and [generally] four to five inch EZ Just and or U-Reely handles was a recipe designed to make these issues critical.  This is a great part of the reason that it took really good drivers to fly great patterns in those days and why the level of flying in general, with more up to date choices in control systems, is so much better--top to bottom.

Howard (later in this sequence) addresses the issue of too small tails and their much more critical CG ranges in every thing from bumble bees to A380s--ooops, B747-8s.  This is a very true statement and it relates directly to the impact of the NP on performance in a variety of areas.  Many old stunters, for instance, had much smaller tails than today's "norm".  As a result their static margins were smaller and the response effect of small movements of the CG were dramatically greater.  If the tail is small enough (say in the 15% or wing area range which was sort of the lower extreme) the response rate from full tank at the beginning of a flight to empty tank at the end had a dramatic effect on the response rate.  Modern tail volumes are much less affected by even much larger amounts of fuel being burned out.  

These are all functions of the factors we've been discussing and is the basis on which I've convinced myself that the propinquity of the CG and the AC are most important factors to both stability and maneuverability.  If your airplane feels as "locked in" flying loops in calm air and windy conditions I think it is a safe bet that the airplane has that harmonious relationship.

Ted

Edit.  I've highlighted the phrase "the more complex factors" above because it was a poor choice of words.  The additional factors are neither complex nor difficult to include in a competent design, although they would be difficult to do with an airplane that wasn't designed and built with them in mind from the start.  All it requires is the commitment to locate the design (longitudinal location of the CG at 25% of the MAC (average chord is close enough) and design a tail large enough to accommodate a CG at that location.  As Serge mentioned above Ted's "rule" is a perfectly adequate standard.  If you make the tail of any remotely "normal" stunter equal to 25% of the total wing area putting the CG at 25% MAC will be just fine in terms of both stability and maneuverability.

Oh, what the heck, Edit again.  Commit yourself to that CG location and use your adjustability factors to "trim" the airplane to suit your taste rather than simply throwing weight around the front and back ends.  Flap/elevator ratios, control sensitivities and handle adjustments.  The airplane "WILL" be happier with the CG located at or near the AC.  All that trick stuff you built in is for the purpose of making the pilot happy with his happy airplane.  (This, of course, is not as simple as I've tried to make it sound but I'm trying to reinforce the validity of the concept.  Trimming isn't easy but starting out in the correct ballpark goes a long ways towards getting the job done right)

[Tim Wescott]

Oh, what the heck, Edit again.  Commit yourself to that CG location and use your adjustability factors to "trim" the airplane to suit your taste rather than simply throwing weight around the front and back ends.  Flap/elevator ratios, control sensitivities and handle adjustments.  The airplane "WILL" be happier with the CG located at or near the AC.  All that trick stuff you built in is for the purpose of making the pilot happy with his happy airplane.  (This, of course, is not as simple as I've tried to make it sound but I'm trying to reinforce the validity of the concept.  Trimming isn't easy but starting out in the correct ballpark goes a long ways towards getting the job done right)

Ted -- you just told me that I can take some of the absurd excess (it's almost 4oz in a 36oz plane) of nose weight off of my Waiex.  That's way cool!  Presumably this means that I also need to move the lines way in on my handle, because it's currently set for "not too fast, not too slow" with the spacing I was running at.

This is the nice thing about having lots of experts weighing in on a question -- sooner or later, one of them will say what you want to hear, and you can latch onto that and run with it.

[Ted Fancher]

Ted -- you just told me that I can take some of the absurd excess (it's almost 4oz in a 36oz plane) of nose weight off of my Waiex.  That's way cool!  Presumably this means that I also need to move the lines way in on my handle, because it's currently set for "not too fast, not too slow" with the spacing I was running at.

This is the nice thing about having lots of experts weighing in on a question -- sooner or later, one of them will say what you want to hear, and you can latch onto that and run with it.

Tim,

That's certainly a start.  I caution you, however, that you can't just take part of what we've been talking about and jump on that as "the" solution to a problem.  I don't want to suggest you don't know the following, but just in case, let me discuss some of what also needs to be addressed if you make that big a change.  Also, the following assumes the airplane has flaps of "normal" configuration, i.e. modest chord (20% of the chord or less) and full span or a little less.  Not a deal breaker if much different but if it ain't got flaps at all we've got to go a different route!

Be sure, first, to do a little basic math to insure you meet the underlying parameters, i.e. a large enough tail to stabilize the plane with the CG at the 25% MAC location.  That will insure you've got adequate "static margin" to be predictably flyable.  Here's a little crutch if you don't have a tail that's 25% of the wing area.  Figure out what percentage of the wing area you do have and then set your CG at that same % location of the average chord  (Got a 20% tail, CG at 20% of the average chord, etc).  I can't tell you that is optimum (I'm not at aero engineer and can't do the math...heck, don't even know the right questions!) but I can tell you with assurance that a CG that matches the "raw" tail volume will be flyable and stable.  I do know that as you go aft of that point the possibility for having inadequate static margin (and a difficult to control airplane) increases.

Recognize that as the CG goes aft you'll have to relocate the leadouts appropriately as well.  I don't recall the span of the Waiex but a three degree sweep from the CG at the Fuse will be a good place to "center" the leadouts at the tip.

The controls, as you've mentioned will need to be slowed down significantly if it was already  "stuntable" with that much lead in the nose.  Ideally, a lot of that would be done by adjusting the internal controls (longer horn arms or shorter BC pushrod location) but, if they aren't accessible, narrowing the handle will accomplish much the same thing (the only downside from the pilots perspective is that narrowing the handle spacing gives you much greater mechanical advantage over the control forces and you will lose some feel.  If you are able to adjust the "rate" with the BC and Horns and allow your self to still use ~3.5 to four inches of handle spacing you'll retain some of the feel...this helps refine maneuvers and is valuable at the highest levels of competition.)

Just one additional comment about the handle spacing and feel.  I'm aware that the control forces themselves don't change (a function of the amount of deflection of the flaps and elevators) but the fact that the force required to displace the controls will be applied at a greater distance from the "hinge" of the handle means that you'll be able to feel the loads more with your fingers.

I think the above should get you in the ballpark for flying with the big CG change.  There are some other refinements that will improve your scores (proper landing gear placement, primarily) but they can wait until you get happy with the airborne changes.

Ted

[Howard Rush]

My current airplane's CG is at 25% MAC, coincidentally.