No one is "wrong" in this thread. The classic and accepted way of looking at the wing loading is simply weight divided by surface area. When I'm calculating the wing loading of a new model that's how I do it as well. But that doesn't mean that other factors are not at work in a flying model and that those effects don't come into effect.

The load on the wing (in S&L flight) is equal to the weight of the airplane PLUS the load necessary to counteract the downward force on the tail. The reason canards are beneficial is because they have positive lift versus the classical negative lift on a tail at the rear of the airplane. This means the lift is additive and the load on the main wing is less (ergo AOA is less)... therefore overall induced drag is less. The change in wing loading is not significant structurally, but it is for speed, range, and indeed most importantly, fuel economy (i.e., everything related to drag).

Example:

Your plane's weight: 200 oz.
Your plane's main wing area: 5 sq. ft.

Classical Wing Loading: 40 oz./sq. ft.

Downward force required on tail to counteract main wing's lift induced nose-down moment: 5 oz.
Required load on main wing for S&L flight: 205 oz.

Actual Loading on Main Wing: 41 oz./sq.ft.
(And slightly higher AOA required than you would expect if you only considered 200 oz.)

Replace classic tail with a canard...
UPWARD force required on canard to counteract main wing's lift induced nose-down moment: 5 oz.
Required load on main wing for S&L flight: 195 oz.

Actual Loading on Main Wing (Canard Configuration): 39 oz./sq.ft.
(And slightly less AOA required than you would expect if you only considered 200 oz.)


With both a traditional tail and a canard, the induced drag on the tail (or canard) surface is nearly the same, however the induced drag on the main wing is reduced with a canard configuration due to the reduced required AOA (which is also reflected in the reduced actual wing loading).

Relating this to the original thread question, if you move the CG further forward, you make the plane more stable, but you also increase the tail load required and therefore the actual wing loading, required AOA, drag, etc. Flying the plane with the least stability manageable maximizes (milks) efficiency in speed and range. This is all separate from control surface "effectiveness", though.

I hope this brings the gnat's ***** further into focus.

Just agreeing with the folks who say tail load is real and increases actual wing loading.

Tail load IS considered in designing structures and in computing performance and stability data. It seems a small factor in level flight, but if you consider the additional load the wing has to support due to tail down-force, at the most forward CG, and at the limit load factor (worst case), it can be significant to the structure, even for a model.

However, designers of model planes have little control over what the actual tail load of a given model is going to be, since they don't know the exact finished weight and CG that the end user will impose on it. So it's just easier to design for some worst case and specify simple wing area on the kit box. Imagine the confusion at the hobby shop if tail down-load information was printed on kit boxes! Full-scale airplane manuals normally don't even include such information.

The effect of the CG location and resulting AOA was used by a number of full-size aircraft pilots. If I recall correctly, there is a bit of lore among Mooney pilots that when the airplane is lightly-loaded, with only two aboard, you can gain a few extra knots by having the person in the right seat slide it back a few notches. Of course, this is for just a FEW knots, but bragging rights are important to a lot of people. Mooney aircraft have been known for their excellent cruise speeds versus their engine size.

so we have actual static wing loading ( size n weight )
then we add induced weight (can I say that?) to actual weight and lump it all together as wing loading.
The induced weight is really a by product of added downforce at stabilizer (where ever that may be ) requirement- which is actually an induced load .
I don't see any comments about this induced load being additional drag.
Which it is --as far as I can see. which ain't too far anymore ----
The Mooney Mk21 - is a noisy but fast SOB and I speak from first hand experience -- also trim is reasonably sensitive -
I have setup elevator trim for cruise -(pain in the ass )- then just put one arm forward on the wheel-- and I had to retrim.
done it a number of times . My ol buddy who was an instructor ( now dec), let me steer the deers whenever I wanted to .
I am not a licensed pilot - just a passenger - but I thot the trim sensitivity was interesting .

Additional force on the wing due to tail force isn't specified to pilots because it varies so much: For a given airplane, it depends on airspeed, load factor, CG position and gross weight. Simple wing area divided by weight is constant. It's a design consideration in a quantitative sense, and a general performance consideration for pilots (aft CG means slightly better performance).

Induced drag is increased with an increase in tail down-force (or up-force). Up-force on the tail usually increases overall induced drag because the horizontal tail usually has a lower aspect ratio than the wing, therefore, generates more induced drag taking it's share of the airplane's weight, than the additional induced drag the wing would have generated carrying all the weight itself.

This is one reason canard configurations are still rare - it is possible to design an aft-tail airplane that is more effecient than some possible canard configurations, with the same payload. That little canard can't be too big because it is destabilizing. Therefore, it must operate closer to it's CL_max than the wing does at all times (to ensure it stalls before the wing), and therefore must generate relatively high induced drag, even though it's only carrying a small percentage of the airplane's weight (depending on the CG location). That, in turn is a primary reason canard surfaces have high aspect ratios - to make them generate less induced drag for the lift required of them.

The most efficient aft-tail arrangement is for the horizontal tail to only be providing stability - not lifting up or down. So a well designed aft-tail airplane will come close to that during heavy loading conditions at cruise.

I have some Mooney time too. They take a while to slow down.

Years ago, bored on a long cross-country in a C-152, I leaned forward and back in my seat to change altitude slightly. Not surprising for such a light airplane.

There are so many flyers out there that do not really understand how performance is effected by cg shift. I see it all the time at the field, a couple of guys would lift a plane by the wingtips, fingers on the spar, the nose plops to the ground -"A-OK, shes nose heavy, lets fly". Then they wonder why it lands like a rocket. Many people are under the assumption that there is no such thing as 'too far forward'. Its the extream cases on heavy, under powered airplanes where I see the problem most. All too often, when someone doesnt like the performance or feel of a plane, they are quick to move the weight forward, creating another set of problems.

Ted

Big planes, yeah, in some form, but I don't remember anything other than general information about performance data vs. CG in the manuals for more than two dozen models of full-scale aircraft I have flown. Cruise and endurance info is given at a specified CG (worst-case, most forward normally), but I don't remember seeing charts plotting CG vs airspeed in airplane manuals. Gross weight has a bigger effect on cruise speed than CG normally. Of course I'll be the first to admit that I don't remeber every page of all two-dozen plus manuals I've used[sm=wink.gif].

If you ain't current - it don't matter -

This is a fascinating thread, but I didn't see (or maybe I overlooked) the answer to the original question: Why would a tail-heavy airplane have sluggish aileron response? That was one of the first Articles of Faith imparted to me by my instructor 'way back when, and I have observed it myself. Never understood why, though.

Not that it matters a whole lot, I guess ... I just dial up the throw as needed. R/C has spoiled us! Free flight rules!

Mr. Pitch, Troll, & Yaw

sluggish?
I don't think so -
IF the plane is trimmed with aft CG - it will fly at an attitude of lesser wing angle (tail will be higher) - the aileron response should be better -
a whole lot of trimming pattern setups over many years says this is a demonstrable setup.

When I think of an airplane with high wing loading (e.g. F-111), one of the first characteristic that comes to mind is an airplane that is not gust sensitive. In other words, if the airplane experiences an up or down gust, it will respond with a small load factor (and therefore flight path) excursion. If my back-of-the-envelope analysis is correct, the change in load factor for a given vertical gust is proportional to CL_alpha*qS/W, where:

q = dynamic pressure (1/2*rho*v^2 - one half density times true airspeed squared)
S = reference wing area
W = aircraft weight
CL_alpha = rate of change of Aircraft Lift Coefficient with change in Angle of Attack

The choice of S is somewhat arbitrary, but CL_alpha*S will remain constant regardless of your choice for S.

As long as the wing remains on the linear part of the lift curve slope, none of the above factors are affected by CG is position. So, while it is obvious that changing the CG will affect the load carried by the wing, I would not say that CG position affects "wing loading" in the sense that it is traditionally used.

Oh but I am current. I'm picking up a Navajo with a friend this weekend from Nashville.

I can agree with that.

BTW, I have sometimes thought I have noticed an increase in aileron effectiveness when CG is moved aft, with certain models. But it's hard to say for certain.

<edit> added qualifications

the math says the same thing I was harping on -
CG really does not affect wing loading -as it is commonly addressed .

CG does however - affect the total drag ( efficiency)of the aircraft - and the interpretation of that effect -is where I ,personally don't subscribe to the "increased wingloading " analogy.
Call me picky---Most of what I have learned about the CG shift --comes from spending hours flying pattern models I designed and marketed for years .
In my ongoing search for truth-I found that a lot of the "factual" info about aerobatic model design -and some "rules"- just plain bunk.
The reasons why certain designs tucked or yawed or dove or twisted - were not peculiar to a particular design but were simply due to these designs being setup incorrectly.
A very forward CG (10%) for example - made for a very good high speed model - it was easier to hold in a predictable line
Conversely a very aft cg (30+%) worked great on lightly loaded, low speed models
extremely agile.

I am current in the F-18F - and in that airplane it still don't matter. I've come home with the airplane weighing only 60% of what it weighed when I left, and the flight control system is good enough to make any changes in flying qualities (except in roll) nearly transparent. I've never noticed a significant connection between CG and aileron effectiveness, at least not in "controlled flight".

Dick, the wing is subjected to an increase load, hence the wing load is increased. Once again, I'm not refering to the 'traditional' or 'conventional' definition. That force acting on the wing is actually there, its not an interpretation, nor an analogy. The overall drag increase you are mentioning does not change the fact that the wing is in fact experiencing the additional load.

ted

I understand that - - any additional load - be it weight or drag - requires more work to lift and move it.

Sounds like we're all singing the same tune more or less. It's a beautiful thing.

Have a great weekend all!

Suppose you have 2 geometrically-identical airplanes flying side-by-side. Airplane 1 weighs 10 pounds, and its CG is placed such that there is no up or down load on the tail in level flight. Airplane 2 weights 9 pounds, but its CG is placed such that there is a down load of 1 pound on the tail in level flight. In both cases, the wing has to lift 10 pounds in level flight. Is the wing loading of these planes the same? I say absolutely not. Suppose that these 2 airplanes experience the same vertical gust as they are flying side-by-side.

As long as the wing and tail of both airplanes remain on the linear part of the lift curve slope (a good assumption in most cases), the change in lift due to the gust will be exactly the same for both airplanes. However, the 9 pound airplane will experience more vertical acceleration and more flight path deviation than the 10 pound airplane (a = F/m). If you were watching these airplanes fly past on a gusty day, the heavier airplane would have a perceptibly smoother flight path. Why? Because its wing loading is HIGHER, not the same.

I will buy that
To go a bit further --the "induced" loading (I hope this term is not offensive to the academically trained) changes the way the plane responds- primarily (I think) because drag is shifted more to rear of aircraft.
Sorta like having an arrow where the feathers could be slid along the shaft and change the center of pressure vs the center of gravity.

Our own cut n try approach to solving questions bears this out.

Wrong. Wing loading and inertia are two different things.

Both planes you describe, by your very own definition, will experience 10 lbs. of load on the wings. I certainly hope you aren't the one designing the main wing spar of the 9 lb. plane if you think it is experiencing a lighter load than the 10 lb. plane... if indeed it has a natural CG bias as you posit.

When people consider wing loading in such esoteric evaluations such as reaction to gusts and if it's a floater or not, it is only a rule-of-thumb that is generally useful, although not exact, with a certain class of plane. As has been talked much about in other threads, a wing loading of 100 would seem hardly flyable and if so, then very sturdy for the average RC model, but for even a small full-scale plane, a wing loading of 100 would be the most fluttery floater you ever saw.

Another mistake in an earlier post is the notion that nothing changes in the lift/drag equations when you move CG. This skips about six chapters in aeronautical design. In order to understand the lift you need, you need to know the static margin, thus the down force on the tail, thus the load on the wing (including the weight of the plane). THEN, you can look up what your requried AOA will be for a given airfoil (using CL curve). The required CL and AOA WILL be different (higher) if you shift the CG forward. Of course, as AOA increases, not only does CL go up, so does CD, so there's a "drag-bucket" evalation that needs to be done in optimizing the planforms, etc. THEN, you can go back and re-evaluate dynamic stability (which is generally improved with a forward CG). And it's really not this sequential- in reality, there are interdependencies between the different characteristics of an airplane, so the design process is iterative.

Starting to repeat some of the things that were explained before, so I'll give up...

Actually, no, not wrong at all. I was just pointing out that an airplane that is loaded by the tail will fly differently from one that is loaded by weight. From your earlier post about the induced drag "benefits" of canards, it is clear that your understanding is limited, so I'm not surprised you didn't understand my point.

Not to insult your obviously unlimited knowledge, but you did not simply point out differences in flying characteristics. Your above statement is incorrect and I would like to point that out to others reading the post- others can/will believe what they want.

And if you think my knowledge is limited and my thesis on canards is incorrect at all, by all means point out the faults in my explanation. I made the explanation for those who'd like to know, but please share your less limited knowledge of the subject so that we may all learn- I hope I haven't led anyone astray, Sensei.

Your suggestion that the overall induced drag for a canard configuration is not correct. I think Drela's post from a while back explains it well...

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