Experimentation

After having taken in the various threads on RCO having to do with LE radius and then a debate between a couple of individuals (Ted & Al) regarding the merits or demerits regarding having the CG actually located rearward of the CL as it pertains to C/L Precision Aerobatics, I was wonder if someone here would like to paarticipate in an experiment, presumably with an old test plane regarding the merits of actually moving the CG to the rear of the C/L. There was some debate as to actually knowing where the C/L of a wing is, but I would imagine that past half cord 60 percent, would indeed be behind the C/L. My proposal is to use this unneeded airplane to test the aircrafts stability in various configurations including using fairly large 25-30% tail volumns to attempt to stabalize said aircraft when it's CG is in fact moved behind the C/L. Additionally the effect of moving the CG forward of the C/L and co-locating the CG with the C/L should be included in the experiment. Since I don't have an aircraft that I could use for such testing I was wondering if an intgerested pary that did have such an aircraft could perform said experiment and report the findings here or on RCO. Someone independent of said debatees should be the actual testing authority. Probably no one will even respond to this query, but I just thought it could bring more light to the current (and past) debate regarding the subject.

Any takers?? Bill, you out there????

Reading the debate, I wasn't sure anyone was proposing to move the CG behind the CL for a stunt model. Instead, my _impression_ was that Ted was saying "moving the CG forward will make the model more stable, and moving it back will make it less stable." In fact, they both say that.

I _interpreted_ Ted as meaning "moving the CG back towards the CL" will reduce stability; to compensate, we make the stab larger.

Later, the discussion seemed to get into canards as an example of what happens when we take this approach to extremes; i.e., moving the CG really far back and making the stab really big.

It's depressing that, for all the words written, I'm still unsure of the exact point each is trying to make
Probably my fault.

Hi Preston,

Gosh, I guess that the threads are a little hard to get the gist of. But I believe that Ted was indicating that using a large enough tail surface could mitigate instability into relaxed stability if the CG was behind the C/L. I was just wondering if a practical experiment would prove that such a case could actually be made to work in the case of a C/L aerobatic plane. Some of the arguments used are related to real aircraft aerodynamics and free flight models, but could an actual precision aero plane fly the full schedule of manuvers with this aft CG and do so adequately?

I, too, have a problem in sorting out all of the words that were written and trying to make sense of the point that each was trying to make. Sometimes people talk past each other and there is really more agreement in what they are saying than what they realize. I just have a lot of trouble understanding a lot of what is going on over there at times myself. Sort of like my doctor when he was trying to describe my condition in the hospital. Just tell me my arteries are blocked, and let's go on from there. (You need to perform how many bypasses?!!)

Al is right, however, in that there is a downward force on the tail in order to balance the "standard" airframe with the wing in front and the tail in back. This is because the center of gravity is ahead of the center of lift. On a flapped airplane, deflecting the flaps will always move the center of lift back (unless the wing, or at least the trailing edge is angled considerably forward). The problem is, a forward swept wing will decrease stability, and a forward swept trailing edge adds complications to trimming. Without going into any of this, and just assuming a more or less straight wing, any deflection of the flaps will move the center of lift back. Thus the stabilizer (aptly named) will need either a greater area or a greater movement (as a stabilator or of the elevator) in order to overcome any pitching motion.

Ted was suggesting moving the center of gravity back towards the center of lift in order to reduce this pitching motion, and that can be done by increasing the tail area. The larger the tail, the farther back one can move the center of gravity. If we increase the tail area enough, we can either end up with a tandem, where we have, in effect, two more or less equal size wings, or carry it further to where we have the wing in back and the tail in front. In the latter two cases the center of gravity will be found in between the two surfaces somewhere. And then the rear wing, along with the front surface, will both supply a positive lift. This fact did not escape the Wright brothers in the design of their first airplane. With the limited propulsion power available to them, they designed their plane to provide maximum lift (as they understood it) on all surfaces.

Although there have been limited design attempts in the use of canards, and Bob Hunt, himself, is currently working in that direction, this design in the past has always proven inadequate in the control line stunt arena. So we are back to our standard configuration.

I won't be able to help you in providing you with an airframe (or data) for your experimentatons, but I can say this much. We have long been advocates of moving the center of gravity back as far as practical on all of our designs. The last three of four planes that Matt has built have had tail weight boxes designed into them. On his current design he added tail weight in small increments in order to increase the maneuverability of the airplane until he lost stability and "groove" which is also necessary for the stunt pattern. He eventually lightened the nose and removed all tail weight to keep the airframe as light as possible. This still left the center of gravity reasonably far ahead of the center of lift, and it was determined that there was too much flap effect for the weight of the plane and the size of the tail (23% of the wing area). This was solved simply by adjusting the elevator throw (an adjustable horn was used in the design) to be greater than the flap throw which reduced the pitching motion and added leverage to the tail.

What you are proposing would be a design where one could easily replace the tail with a progressively larger one. There would also need to be provision to easily change the center of gravity, and one would also need to be able to adjust the leadout position beyond the confines of the wing. Maybe what one should do is build a standard configuration, then a new creation with the tail, perhaps, 2/3 the size of the wing, and then a canard with the front surface 2/3 the size of the rear. If these could be built all of approximately equal size, it would make an interesting comparison in the areas of stability and maneuverability for controline stunt.

It does, however, take a lot of time to reinvent the wheel. Most of us are better off spending our time with a proven design and simply practice to hone our flying skills unless we are truly one of those who enjoys building and designing more than flying.

I pretty much lost interest in that discussion for several reasons. I thought I would post this here because I don't want to get involved there and it is probably irrelevant anyway.

Back in the '50's there were a couple of articles in MAN on how CG position in a FF controls climb angle, and effects of tail volume vs CG position on stability. Basically with the CG forward the model should climb, with the CG back it should dive (which I did not believe until I tried it).

I went out one day with a FF and a roll of solder. I varied the CG and made the model climb vertically, fly out level at head height, and various angles in between (did not make it dive). One thing I did, which was not mentioned in the articles, was to retrim the model for glide with each CG change. I.e. when I moved the CG forward, I put more negative in the stab, and vice versa. So I am not sure how much of the change was due to CG movement and how much was due to incidence changes for glide trim. It would be interesting to try this with a VIT airplane where glide trim would not affect power flight.

CG movement between vertical climb and level flying was less than 10% of the wing chord.

You probably answered your question, here, when you mentioned changing the stab incidence. If the plane is trimmed for a flat glide with a little "Down" force on the stabilizer (same as giving "up" elevator) and then you increase the speed, the plane will begin to climb. With a glider you can bring the cg behind the center of lift and have a lifting stabilizer to trimmed to make it glide level. But if you now add speed (power on) the stab will lift too much in relation to the position of the center of lift and cause the tail to rise and the plane to dive (or outside loop). So I don't think these principles really apply to our designs in the same way.

But it does show why you need to fly a non-flapped plane at a higher speed in order to maintain the same type of maneuverability. Adding speed will give a great affect to any deflection of tail surface in relation to the center of lift.

Rereading the debate (well some of it), I conclude that my early interpretation was mistaken.

I now believe that Ted was making the argument that increasing the size of the stab effectively moves the CL for the plane (the entire plane, not just the wing) back. Hence Iskandar's comment about a continuum, ranging from

- large wing/small stab, to
- large wing/arge stab, to
- 2 large wings, to
- canard

And if the CL for the plane is moved back, the CG can move back too.

So maybe (I emphasize the "maybe" because I'm very much a beginning student here) people have their cart and horse reversed. Maybe a long time ago, people made bigger stabs to try to turn more quickly. And then found that they had to move the CG back too (since the plane's CL had moved back). After all, to turn quickly, you still need the CG to be close to the CL.

Of course, all this motion backwards conflicts with Leonard's reminder that we like short noses (prefer the engine to be close to the CG).
Unless you build a mid-engine canard, like Bob Hunt appears to be doing, where the engine might be right on the CG (and CL).
Or a combat plane, with the engine set back into the LE and a relatively small stab.

If the center of lift on the wing moves back, then the center of lift on the whole plane moves back. And if the tail plane becomes large enough to move the center of lift back far enough, it will need to become a lifting device as on a tendem or canard. And I think here is where Ted and Al were talking past one another, because Al was saying the tail plane provides a negative (downward) force. but he was sepaking only of a normal configuration, such as we normally fly. Certainly Al would not dispute that the canard wing and tail provide a positive (lifting) force when in neutral stability and flying level. But with all the words and technical language I got lost a bit with most of the rest of the crowd.

And my comment about shorter noses is in reference to using nose length merely for balance--especially with some of our more modern and heavier engine/muffler combinations. There are no "sacred" numbers involved with nose moment or nose length other than this. Sometimes a long nose is needed. And sometimes, as on a canard, the engine may need to end up somewhere in the middle in order to provide proper balance.

> my comment about shorter noses
> is in reference to using
> nose length merely for balance

I was thinking of the barbell effect.
If we can balance a plane with a shorter nose (i.e., weight carried closer to the CG), then I would think it would turn (and stop turning) better.

Yup. That was where Al Rabe made his major mistake when he added 16 ounces of weight to his Mustunt and then said the plane flew fine even with the extra weight. If a plane were built that heavy because of materials or finish, then the mass would be moved closer to the extremities. The wing might be able to carry it, but the turn would be drastically affected because of the barbell affect.

>If the center of lift on
>the wing moves back, then
>the center of lift on
>the whole plane moves back.

I'm having trouble here. What moves the CL of a wing? I was assuming the wing was fixed and the only thing we were doing was growing and shrinking the tail plane, which would move the CL of the plane, but not the wing.

> And if the tail
>plane becomes large enough to
>move the center of lift
>back far enough, it will
>need to become a lifting
>device as on a tendem
>or canard.

Again, this seems backwards. The tail plane _is_ a lifting device, isn't it? I.e., whenever you are getting lift out of the wing, you are getting lift out of the tail plane. And if it's relatively large, it has a relatively large effect on the CL of the plane.

>>If the center of lift on the wing moves back, then
>>the center of lift on the whole plane moves back.
>
>I'm having trouble here. What moves the CL of a wing?
>I was assuming the wing was fixed and the only thing
>we were doing was growing and shrinking the tail plane,
>which would move the CL of the plane, but not the wing.

If the (main) wing is the only lifting surface on the plane, then the center of lift would be located somewhere on the wing, itself. If, however, there is more than one lifting surface, then the center of lift would be somewhere between the two (or more) surfaces, as on a tandem or canard. Certainly the location of the wing is fixed. We aren't moving that. But even the movement of flaps would move the center of lift on the wing (rearward). Also, the movement of the elevator, as it becomes a positive or negative force, will, change the center of lift on the whole airplane. Making the tail plane larger will allow us to move the center of gravity back, but I don't think this will affect the center of lift as long as the tail plane, itself, remains a negative force in level flight.

>>And if the tail plane becomes large enough to
>>move the center of lift back far enough, it will
>>need to become a lifting device as on a tendem
>>or canard.

>Again, this seems backwards. The tail plane _is_ a lifting
>device, isn't it? I.e., whenever you are getting lift
>out of the wing, you are getting lift out of
>the tail plane. And if it's relatively large, it
>has a relatively large effect on the CL of the
>plane.
>
>Student

I may not have stated the above too accurately, but in our "normal" airplanes (wing in front, smaller tail in back) the center of gravity is always set ahead of the center of lift. Without the tailplane, this would cause the plane to go into an outside loop (or hit the ground). The stabilizer (aptly named) provides a downforce (a negative lift). It presses down on the aft portion of the plane to stabilize the lifting force whose center is behind the center of gravity. As the rear surface becomes larger and is turned into a lifting surface (as on a tail first canard) then both the center of gravity and center of lift are moved rearward. The center of gravity still remains ahead of the center of lift, however, and then the front mounted stabilizer must become a lifting surface to counter the pitching moment of the "behind the center of gravity" center of lift.

Just remember, in level flight, tail in back presses down, tail in front lifts up. And both perform the same purpose of stabilizing the airplane--or destabilizing it as we give "up" or "down" elevator in order to perform our maneuvers.

I'm trying not to butt in but....

Try not to get your nomenclature confused. When "student" talks about the Cl of the entire plane he is discussing the Neutral Point. The Neutral Point can also be called the Center of Pressure of the entire airframe which is probably a bit more descriptive.

The Center of Pressure can be logically thought of as the point at which "ALL OF THE FORCES ACTING ON THE AIRFRAME" are centered. Lift is thus just part of the forces determining the actual CP of the airframe. For our discussion of stability and pitch maneuverability the foreplane and tailplane are the "major" contributors and thus we speak of the Neutral Point as being that point that allows an airframe to be "pitch stable" as long as the Center of Gravity is forward of that location.

To illustrate a forward "CENTER OF PRESSURE" envision an otherwise normal aicraft with a "donut of flat plate area just behind the prop. Although the Neutral point for stability reasons might be anywhere, the CP will be located well forward depending on the size of the flat plate donut. If it is large enough the aircraft will be unable to fly "forward" as the CG is aft of the CP and the ship will try to swap ends.

The most telling example to ponder as you try to "decipher" the hyperbole between Al and me is the recognition of the transition of "flyable" planforms from "flying wing to flying wing and all the various sized secondary planes (surfaces) in between.

Any of the infinite varieties between the extremes of "mainplane/tailplane:foreplane/mainplane" configurations can be made both stable and capable of aerobatics. Any of our conventional stunters is an example of the former and Bobby Hunt's Canard an example of the latter. From a pitch perspective a stable planform will always have the Neutral Point located between the Aerodynamic Centers of the two surfaces. If there is only one surface (a flying wing) the Neutral Point will be located at the Aerodynamic Center of the wing.

As the area of the smaller surface (tail or foreplane) gets larger the Neutral point will move toward it. As long as the CG remains ahead of the resulting Neutral Point the craft will be stable.

Whatever the configuration, as the distance between the CG and the NP decreases it will become less stable (and more responsive)and as it increases the reverse will occur. This, again, is called the "static" (stability) margin. At the point that the CG moves aft of the NP the craft then becomes "un"stable and will be incapable of stablized flight.

Finally, here is the crux of the discussion as represented by the examples of the conventional and canard planforms.

The Center of Gravity of a conventional layout "MAY" be as far aft as the Aerodynamic Center of the wing...may in fact be further aft if the tail is large enough.

Contrarily, the CG of a canard can "NEVER" be as far aft as the AC of the mainplane since, by definition, the NEUTRAL POINT is always forward of the Aerodynamic Center of the Mainplane. Because we recognize the definition of the Static (stability) margin to be the necessity to have the CG forward of the Neutral Point, a canard can never have the CG and AC of the mainplane co-located.

This is probably the reason no high performance aerobatic ships utilize the canard configuration. There will "ALWAYS" be a moment arm between the CG and the AC of the mainplane of a canard and, thus, the commensurate pitching out of a maneuver when G loads increase is unavoidable.

Ted

Oi vey, that probably made it worse, didn't it.

"Oi vey"? Must be the political climate that is getting to you, Ted. As I said, I got lost in a bit of the debate between you and Al Rabe, but you are always welcome to "but in" here (your teminology). And, as always, I appreciate your input.

One thing, we don't have to understand everything about a television set or a computer in order to put it to work. Same thing goes with our little toy airplanes. As long as we stay within the envelope of proven "numbers", our planes are likely to fly OK.

I like a comment Bob Gieseke made once (maybe more than once) while he was still flying his "Gieseke Nobler." Every year he built a new one and changed something--bigger, smaller, longer, shorter. But he said, "I don't know if I ever improved it."

>"Oi vey"? Must be the
>political climate that is getting
>to you, Ted. As
>I said, I got lost
>in a bit of the
>debate between you and Al
>Rabe, but you are always
>welcome to "but in" here
>(your teminology). And, as
>always, I appreciate your input.
>
>
>One thing, we don't have to
>understand everything about a television
>set or a computer in
>order to put it to
>work. Same thing goes
>with our little toy airplanes.
> As long as we
>stay within the envelope of
>proven "numbers", our planes are
>likely to fly OK.
>
>I like a comment Bob Gieseke
>made once (maybe more than
>once) while he was still
>flying his "Gieseke Nobler."
>Every year he built a
>new one and changed something--bigger,
>smaller, longer, shorter. But
>he said, "I don't know
>if I ever improved it."
>
>
>Leonard Neumann

Excellent points, Len. (Happy to see you back to your normal combative self, by the way. Glad our prayers were answered!)

I've been doing this a long, long time and probably the only significant change we've made in airplane design (not to be confused with the "miracle cure"...adjustable everything) is the use of large tails and aft CGs. Utilizing this "G-Force stabilizer" in the wind is a real and quantifiable increase in performance. The fact that it is of negligible to nonexistant aid in dead or decent air explains a lot of its lack of totally universal acceptance.

A good straight Nobler (or variant) with a good powerplant and a good pilot is still a competitive machine in the vast majority of cases. I'm still tempted to trash all the "heavy" machinery every time I put in a contest flight on the Chizler.

Yeah, Al and I have been arguing the number of dimples on a golf ball for all the effect the CP/AC/CG/CL baloney has on the average stunt flight. It was the same thing with our GP and P-factor "discussion" a dozen years ago. Almost nobody gave a d*^&% but us, but it was great fun, nonetheless.