Does long tail moment affect speed?

[Kenny Sharp]

From what basic understanding I have of sailboats, if you take two boats of equal size, the boat with the longest length at the water line is fastest.
Since air and water can both be treated as fluids...could this be true with sailplanes as well?

[Jurgen Heilig]

From what basic understanding I have of sailboats, if you take two boats of equal size, the boat with the longest length at the water line is fastest.
Since air and water can both be treated as fluids...could this be true with sailplanes as well?

It is not the length, but the aerodynamic shape (hydrodynamic with boats).

If you have a long tail arm, you can reduce the sizes of fin and stab, reducing drag.

:) Jürgen

[Ollie]

I agree with Jurgen.

The long tail arm increses fuselage drag but the tail reducing drag is over comes the fuselage drag.

Also, if the the tail arm is too long and it makes the fuselage too flexable, you might loose control at high speed.

I quote Don Stackhouse:
"The flight of a model sailplane is a complex phenomenon, each portion of the model seeing its own unique set of conditions at any given time, yet still having an influence on all of the other parts of the model at the same time."

[Tom Harper]

kkens4,

The 'Length at Water Line' calculation is a rule of thumb that equates wave patterns for hulls of different size. In the days before computers, it allowed performance scaling between hull models and their full sized equivalents. Boats operate in the interface between two fluids. Airplanes do not.

Your question is whether a long sailplane will go faster than a short one. If you only had a fuselage (hull) to contend with, that might be true. It would have a better fineness ratio and a higher Reynolds number. But, the large surfaces sticking out at the front and back are more of a problem than the 'hull'.

[Kenny Sharp]

Thanks to all.
It was just an idea I had while driving.
Apparently...not a good idea.

[Don Stackhouse]

From what basic understanding I have of sailboats, if you take two boats of equal size, the boat with the longest length at the water line is fastest.
Since air and water can both be treated as fluids...could this be true with sailplanes as well?

Long tail moments for sailplanes are generally (within reason) a good idea, but not for the reasons you're suggesting.

One of the biggest drag issues for a boat is "wave drag". The hull makes waves as it travels along the surface of the water. The speed at which the waves travel is proportional to their wavelength, i.e.: the distance between wave crests.

As the boat goes faster, the waves its hull makes have to get further and further apart in order to keep up with it. Eventually they get far enough apart that one is at the bow and the next is at the stern. Unfortunately they can't get much further apart than that! From that point on, any additional power applied to the boat to try to make it go faster ends up just making the bow and stern waves bigger. The boat is literally trapped between two hills in the water that are of its own making, and any attempt to escape just makes the hills bigger!

The way out of this trap is to use a "planing" type hull (as opposed to the "displacement" type), which can lift up partially out of the water, allowing it to climb up and sit on top of its own bow wave. No longer constrained by the wavelength of its bow and stern waves forced to being equal to its waterline length, it can now go quite a bit faster. However, getting enough power into a sailboat to accomplish this "getting on the step" is a pretty tough proposition that entails some other compromises, which is why we don't see really extensive use of planing hulls on sailboats. There is some, but not to nearly the extent that we see in powerboats.

The key difference between a sailboat and a sailplane in this case is the surface. Both move through fluid mediums, but the sailboat has to move along a "free surface" in that fluid, and therefore has to deal with wave action in that free surface. An aircraft does not. In that regard, it is more analogous to a submarine, not a surface vessel.

However, a long tail moment on a sailplane will generally result in smoother handling, which can often translate into better speed, among other things.

There are two kinds of stability we need to consider in the handling qualities of a model: static, and dynamic.

Static stability simply means that if you disturb the airplane (for example, pull the nose up a few degrees and then let the stick go back to neutral), it will try to return to the attitude and angle of attack it had before it was disturbed.

Dynamic stability is the ability to damp out oscillations. For example, in the example above, odds are that the model will drop its nose when the control stick is released, it will pitch over into a shallow dive in an attempt to return to its original airspeed (something it needs to do in order to get back to steady flight at the original angle of attack), but it will probably not be able to pull out of that dive before it gains a little too much speed (unless it's a Chrysalis 2-meter), so it will have to pitch up again to bleed off the excess speed, but will overshoot again and end up pitching down again, etc., resuting in a series of up-and-down oscillations.

If the oscillations get bigger and bigger, it is "statically stable" (because it keeps trying to come back to the original angle of attack and airspeed), but "dynamically unstable", because the oscillations get bigger and bigger. If the oscillations continue indefinitely, neither getting bigger nor smaller, then it has "neutral dynamic stability". If the oscillations gradually get smaller and smaller till they die out completely, then it has "positive dynamic stability".

Now, within the realm of positive dynamic stability, there are planes that overshoot and then go through these oscillations till the oscillations die out ("underdamped"), ones that when disturbed come slowly back to the original angle of attack and airspeed (the "setpoint") without any overshoot or oscillations at all (i.e.: "overdamped"), and ones that come back to the setpoint exactly as quickly as possible without any overshoot (i.e.: "critically damped").

Most models are underdamped, typically needing about two and a half cycles to damp out a moderate disturbance. The Chrysalis is slightly overdamped. In terms of being easy to handle, dynamic stability (in my experience) is actually more important than static stability. I have tested deliberately setting the C/G far aft enough on a Chrysalis 2-meter that the plane became slightly statically unstable (i.e.: pull the nose up and release the stick, and the plane wants to pull its nose up even more) and then let a first-time beginner fly it. Because the plane was so well damped, she was able to mentally stay ahead of it without trouble, despite the static instability.

So, what does this have to do with tail moment?

Static stability is linearly proportional to tail surface area, and to tail moment arm. It's also dependent on C/G location.

Dynamic stability is not so much a function of C/G, but it is linearly related to tail area. However, it's proportional to the SQUARE of the tail moment arm.

Doubling the tail area gives you twice the static stability and twice the dynamic stability. Doubling the tail moment arm gives you twice the static stability, but FOUR TIMES the dynamic stability.

Or, with a pod-and-boom fuselage construction, which aproaches the case of the extra weight and skin friction of the tail section of the fuselage being nearly negligible, doubling the tail moment and at the same time cutting the tail area in half results in essentially the same static stability, twice the dynamic stability, and slightly more than half the tail assembly's skin-friction drag.

There are tradeoffs, so it is possible to make the tail assembly too long (one of our old Monarch HLG's competitors fell into that trap, and ended up with great dynamic stability, but was about as maneuverable as a schoolbus), particularly if the rest of the design isn't properly tuned to work effectively with all that dynamic stability (which is how we avoid that problem with the 2-meter Chrysalis). However, properly managed and integrated into the overall design, a long tail can have very significant benefits.

Don
(former sailboat racer and sailboat designer, among other things)

[Kenny Sharp]

So I guess it's not a stupid thought experiement?

[Don Stackhouse]

So I guess it's not a stupid thought experiment?

Not at all. Your "what?" was on the mark, but the "how?" and the "why?" needed a little work.

Of course, thoroughly understanding the "how?", and especially the "why?" is the real secret to successful engineering.

[Griffin]

Don, Thanks for your laymans explainations. Enjoyed your long post very much!

Here's a related question for you (or others) about tail moment I've been curious about for some time. You explained the relationship of dynamic and static stabilty to tail moment--does the same relationship apply to roll stability? No doubt other aerodynamic factors play a bigger factor in roll stability, but does a longer tail or more tail surface area contribute to self righting qualities at all? I'm especially curious about three channel gliders or trainers in which use a side slip to produce a roll.

Thanks! I hope it's not too far off the thread starters question. -Steve

[Sparky Paul]

The Lockheed T-33 was found to be faster than its parent, the P-80.. with the same wing and motor. The difference was in the fuselage length, the T-33 having a section added in for the second cockpit.
I liked the explanation of bow waves,... and why a model,totally immersed in air can't use that property. :)

[BMatthews]

Actually there IS a connection but not a model related one. Do a Google on "area rule fuselage" for info on the coke bottle shape that was developed for some of the early supersonic aircraft. It's not directly hull speed related but it is sort of the airplane version of it.

PS: Pauls note about the two seater T33 being faster may be part of this as well.

[Sparky Paul]

The longer fuselage, or "fineness ratio" has been used a few times.
For some reason, the exact opposite idea was promoted by Lockheed and Boeing. A -shorter- fuselage was proposed as more efficient. Lockheed's L-1011-500, and Boing's 747SP were the result.
Only Pan Am bought the SP.. Just before they went under.
There's several -500s flying, some with the RAF.
The "area rule" only works for marginally powered planes that just inch past Mach 1. It's a way to get a smooth transition in total aircraft cross section from front to rear.
Overpowered planes don't bother with it.
Craig Breedlove had a Lockheed Flight Test Engineer design one of his land speed record cars using the area rule. ???? There weren't any wings whose cross-section needed to be considered for the real area rule.
I -almost- asked the FTE why he'd done it that way, as it had no purpose.. but native fear kept me quiet. :)

[biber]

Static stability is linearly proportional to tail surface area, and to tail moment arm. It's also dependent on C/G location.I can't see a direct connection between static stability and tail surface area or tail moment arm.
That is because there is no tail needed for static stability.
In my eyes, only the CG as giving the actual static margin is determining the static stability.
That being said, I would of course admit, that adding a tail (or changing its place and size) will shift the planes' NP and with the CG held constant, the static margin would actually change.

biber

[Don Stackhouse]

I can't see a direct connection between static stability and tail surface area or tail moment arm.
That is because there is no tail needed for static stability.

Not true. Read on...

In my eyes, only the CG as giving the actual static margin is determining the static stability.
That being said, I would of course admit, that adding a tail (or changing its place and size) will shift the planes' NP and with the CG held constant, the static margin would actually change.

Well, you've sort of answered your own question.

A tail is needed to get static stability, which is not the same thing as static equilibrium.

It's possible to find a location of the C/G that will put all of the various forces and moments in balance. An airplane (even a bare wing with a conventional positive-cambered airfoil) in this condition will continue to fly in a steady flight path as long as it is not disturbed. This is static equilibrium.

However, the moment that you disturb it, the plane will diverge and not try to recover unless it posesses positive static stability. For positive static stability, you have to have something that gives it a net recovering moment, so that a pitch-up results in a desire to pitch down, and vice-versa. This generally requires a tail of some sort, even in the case of a flying wing.

In a plank-type flying wing, you could consider the reflexed portion of the trailing edge to be the horizontal tail. In that sense, a plank is just a conventional-tailed aircraft where the tail moment arm is so short that the leading edge of the tail coincides with the trailing edge of the wing. You can even use the theory of tail volume coefficients to predict how much reflexed portion you need.

In the case of a swept flying wing with washout, the wing tips act as the horizontal tail.

As you implied, the neutral point ("NP", the location of the C/G that results in exactly neutral static stability, with no tendency to either recover or diverge further if the plane's attitude is disturbed, i.e.: the plane goes exactly where you point it) is dependent on both the wing and the tail, including both the tail moment arm and the tail area.

The difference between the resulting NP location and the C/G location (typically expressed as a percent of the wing's Mean Aerodynamic Chord) is the "static margin", the standard measurement of static pitch stability. If the C/G is ahead of the NP, then the plane will have positive static stability, and the more static margin it has, the stronger it's static stability.

Of course if the C/G is behind the NP, then the plane is statically unstable, and the greater this distance is, the more negative its static margin and the more strongly unstable the plane becomes.

There are some other factors that come into play, but in general, the NP will be at about the same location as the aerodynamic center of the complete aircraft (basically the wing plus the tail, inflenced somewhat by their airfoils and relative aspect ratios, with usually minor contributions from the fuselage, possibly significant ones from propellers, and further influences from any other appendages that interact with the airflow). Generally for models the wing and the tail are the main ones, and the others are usually fairly minor. If you find the aerodynamic center of the wing, the aerodynamic center of the tail, then use the ratio of their areas to find the approximate aerodynamic center of the two together, you'll probably be in the ball park.

Note, there is a static margin and NP for yaw, just as there is for pitch, and the two are not necessarily the same. In the case of the yaw NP, it's the interactions between any side-lifting areas that matter.

For example, in the performance measurements that Mississippi State did on the Horten H-IV flying wing sailplane, they tried to "improve" it by installing a fairing over the nose skid. This added side area ahead of the C/G, and they had to fly with a much more forward C/G in order to avoid yaw instability. Running the C/G that far forward forced them to carry bunches of up elevator (or elevons in this case) to keep the plane in static equilibrium ("pitch trim") at the desired flight speeds, which totally screwed up the plane's spanwise lift distribution, which then caused a big penalty in induced drag. This was one of the factors (there were a number of others, many also related to ill-advised "meddling" with the original design) that caused their measurements to fall short of the measured performance the plane displayed during flight tests back in Germany during its original flight tests in Germany.

Yet another classic example of how a seemingly minor change in an airplane's design can have significant "ripple effects" throughout the rest of the design!

[Don Stackhouse]

...You explained the relationship of dynamic and static stabilty to tail moment--does the same relationship apply to roll stability? No doubt other aerodynamic factors play a bigger factor in roll stability, but does a longer tail or more tail surface area contribute to self righting qualities at all? I'm especially curious about three channel gliders or trainers in which use a side slip to produce a roll. ...

Yes and no.

There isn't a direct connection between tail moment arm and static or dynamic roll stability.

However, there is a relationship between dihedral and yaw stability. If you have too much dihedral and not enough vertical tail effect, the plane will be prone to "dutch roll". Conversely, too much vertical tail effect and not enough dihedral effect will give the plane negative "spiral stability"; in other words, in a turn it will want to steepen up the bank all by itself, into a "graveyard spiral".

The mass distribution is also a factor. There is a sort of "sweet spot" in the combinations of dihedral and vertical tail in between having a dutch roll problem and a spiral stability problem. In general, the more mass you have out at the extremities, the nose, the tail and especially the wing tips, the smaller that sweet spot will be. In extreme cases, the sweet spot could actually become negative, meaning that there is no combination of vertical tail and wing dihedral that doesn't result in either dutch roll, spiral instability, or both!

This is one reason why having a longer nose to reduce the amount of ballast required to get the correct C/G is not necessarily always a good idea. Besides adding projected side area (yaw) and projected horizontal area (pitch) ahead of the C/G (which then requires more tail area and tail mass), it also moves the weight further forward of the C/G, which then makes this balance of vertical tail and wing dihedral more critical.

Isn't airplane design fun??!! Just no end of new parameters to keep track of, and new details to think about!

[Don Stackhouse]

Regarding "area rule", this is something that applies to transonic and supersonic airplanes.

An airplane with local supersonic flow (which actually begins while the plane's "free stream" airspeed is still slightly subsonic, what's referred to as "transonic") makes shock waves and resulting "wave drag", that are a little analogous to the problem that boats have with their wave drag in the surface of the water.

Anything that delays the formation of these shock waves or that reduces their intensity will reduce the plane's wave drag, letting it go faster for a given amount of thrust.

Sweeping the wings and other flying surfaces will delay the formation of the waves, which is why we see swept wings on transonic airplanes. However, if you're not going at speeds that cause sonic shock waves, then sweeping the surface will not influence the wave drag, since for all practical purposes there isn't any.

This is also true for most propellers, including the vast majority of model propellers. Contrary to popular belief, the vast majority of props (NASA's Propfan and a few others excepted) do not operate at transonic or supersonic speeds, and so the vast majority of those props with sexy-looking swept tips are for the most part just a marketing ploy.

However, getting back to planes that do have to deal with shock waves, the shock waves tend to form anywhere there is a sudden change in the aircraft's cross-sectional area. If we chop the plane up into a series of spanwise slices from the nose to the tail, measure the cross-sectional areas of those slices and then plot them on a graph, any place there is a sudden kink or bump in the graph will likely be the location of another shock wave, with its additional contribution to wave drag.

The wing is one of the biggest contributors to this problem. The fuselage's cross section is fairly smooth and uniform, till all of a sudden the wing pops into the picture, adding a bunch of additional cross section at that spot, then taking it away again at the trailing edge, in the process making two more shock waves.

However, if the wing is fairly thin (something we need for good behavior in sonic airflow anyway), we can slim down the fuselage in the area of the wing, taking away enough fuselage cross section to at least partially offset the cross section of the wing, which smooths out the hump in the cross-section graph, and reduces the wave drag at the wing. This is the idea of the "area ruled" fuselage (a "Coke-bottle" or "wasp-waisted" fuselage). This is the primary difference between the F-102 "Delta Dagger" and the F-106 "Delta Dart", and it made a very significant contribution to the plane's performance.

Likewise, adding area ahead or behind the wing to smooth out the cross-section plot can have similar benefits. The longer upper deck on the 747 helped fill in the gap between the end of the old upper deck and the leading edge of the wing, reducing the plane's wave drag and resulting in an increase in cruise speed. Likewise, those "speed fairings" added along the trailing edge of the old Convair 990 airliner helped fill in the cross-section plot behind the wing, again reducing wave drag.

However, unless your model is going to operate at Mach numbers approaching 1.0 (in which case you'd better make sure all your speed record paperwork for submission to FAI is in order), none of those techniques are going to help your model go faster. In fact in all probability they will add wetted area, thereby increasing skin friction, and will not reduce any wave drag (since you don't have any wave drag to begin with), so they will tend to decrease your model's performance.

Then of course there's the whole issue of how swept flying surfaces tend to dramatically complicate that whole problem of dutch roll/spiral stability balance (they tend to make the plane have one problem at some airspeeds AND the other problem at other airspeeds), as well as tending to have truly awful stall characteristics. With scale models it may be a necessary evil, and with most non-scale models it's something usually best avoided altogether.

[biber]

Of course I meant stability, not equilibrum.
And I still don't see the tail directly connected to static stability,
since you can also have an unstable plane with lots of tail moment arm
and heaps of tail surface area.
Only the static margin makes static stability,
regardless of the number and configuration of wings of an airplane.
But anyway, Don, even though I'm not convinced of the way you explain that particular point,
I have to admit that in general your postings are some of the best quality stuff
on aeromodeling I found on the net.

biber

[Don Stackhouse]

...Only the static margin makes static stability,...

Yes, static margin is one very common quantitative measurement of static stability. It's not the only one (use of stability derivatives are another method), and it doesn't apply in all cases (for example, the flying wing I designed and successfully tested that has conventional positive-cambered, non-reflexed airfoils with nose-down aerodynamic pitching moments, essentially zero washout, elliptical lift distribution, and the C/G at 52% of mean aerodynamic chord, yet has positive static and dynamic pitch stability), but static margin is one of the more common and generally accepted means of measuring static stability.

However, static margin is the distance between the C/G and the neutral point, so it is not just a function of C/G location alone.

Neutral point location is a function of tail area and tail moment, among other things.

Since you can keep C/G location exactly constant, and then move the NP by changing ONLY the tail surface area and/or moment arm, and thereby change the static margin, then static margin is in fact directly related to the tail area and moment arm.

Any time you have a parameter (static margin in this case) that depends on two other independent parameters (in this case on C/G location, and on the NP location, which in turn is directly influenced by tail design), then the parameter in question is a function of all of those other parameters.

QED.

[Kenny Sharp]

This generally requires a tail of some sort, even in the case of a flying wing.

In a plank-type flying wing, you could consider the reflexed portion of the trailing edge to be the horizontal tail. In that sense, a plank is just a conventional-tailed aircraft where the tail moment arm is so short that the leading edge of the tail coincides with the trailing edge of the wing. You can even use the theory of tail volume coefficients to predict how much reflexed portion you need.

In the case of a swept flying wing with washout, the wing tips act as the horizontal tail.


First, allow me to say thank you for all the information...and yes, it is fun to consider aerodynamics, and all the implications and effects, one design change can cause.

My next question comes from the above mention of swept wings and tail moment....Why do turbine jets have "elevators" located near the front, instead of the rear? And they do move opposite of rear elevators, right?

[Don Stackhouse]

...Why do turbine jets have "elevators" located near the front, instead of the rear? And they do move opposite of rear elevators, right?

Yet another seemingly simple question, with no simple answer...

There's a bunch of discussion of topics like these in the "Ask Joe and Don" section of our website:

www.djaerotech.com

There's over 400 articles on various aeronautical topics there, and a search engine to help you find the ones that deal with any particular subject. We also have all of the articles in a .ZIP file that you can download and save for later reference.

I believe what you're referring to are "canards". Essentially it's just a two-surface aircraft with the "horizontal tail" mounted at the front instead of the back. It's a concept as old as powered flight (the original Wright Flyers, Santos Dumont's 14 BIS, many of Burt Rutan's designs such as the VariEze, LongEze and Voyager, etc.). It is not limited to jets, although the reasons for using them differ in different categories of aircraft.

"Canard" is French for "duck" and for "hoax". Both meanings could be considered applicable.

As far as elevator travel is concerned, yes, when you deflect the canard-mounted elevators downwards, it tends to raise the nose, the opposite of the elevators of an aft-mounted tail.

From an aeronautical engineering standpoint, there is really no fundamental difference between a canard and an aft-mounted tail. You use essentially the same methods to determine the neutral point of the aircraft, and then locate the C/G based on the same static margin criteria. The two approaches merely represent two different regions of two-surface aircraft with different relative sizes of the forward and aft flying surfaces.

The reason commonly cited is that supposedly a conventional tail lifts downwards, making the wing's job more difficult, while a canard lifts upwards, supposedly helping the wing. In actual practice this argument is generally false.

The canard is smaller than the wing (if it wasn't, then it would be the wing, not the stabilizer!), and because of that, it is a less effective lift producer than the wing. Taking away work from the best lift producer and giving it to another surface that is not as efficient reduces the overall efficiency of the whole combination. And, since in order to maintain static stability, the work done by the forward surface in proportion to its area generally needs to be greater than that of the aft surface, the canard is taking an even bigger portion of the total job of supporting the airplane than its area alone would suggest.

In addition, it's very important that the canard stalls before the wing, or else some really ugly things can happen (which is how the Curtiss XP-55 "Ascender" earned the alternate pronunciation of its name). Typically the plane will try to swap ends in flight, which is usually followed by a shower of shattered control surface parts and a series of epithets from the test pilot about the species of the chief engineer's mother. Because the wing cannot be allowed to stall, there has to be some safety margin in the lift coefficients it's allowed to achieve, so you therefore can't use its full lift-making capabilities. This then requires that the wing be larger than what you would need for a conventional tail-aft layout, resulting in more wing weight and more skin friction.

Furthermore, the idea that having both surfaces lift upwards is somehow more efficient then having a smaller aft one lift downwards is itself a myth. A flying surface makes lift by grabbing chunks of air and shoving them downwards, and (due to "action and reaction" as described by Newton), the air responds by shoving the airplane upwards. (No, I'm not going to get into an argument about Newton vs. Bernoulli; there is no conflict between the two explanations of lift, Bernoulli is simply one way to explain how the wing manages to grab hold of the air in order to do that shoving.)

The end result oif all of this is that the air behind a wing or canard is flowing downwards, something we refer to as "downwash". In the case of a conventional downward-lifting tail, it is typically flying in this downdraft created by the wing, which actually improves the tail's efficiency. Conversely, in a canard arrangement, the wing has to fly in the downdraft created by the canard, making the wing's job more difficult.

There are induced drag penalties incurred with either arrangement, but the net result is that for a given amount of pitch stability, the induced drag penalties are about equal.

There is also the frequent argument that a canard has natural resistance to stalls and spins, since the wing is never allowed to stall. In actual practice this is not true. This does not happen naturally, you have to very carefully design the canard arrangement to behave this way, and woe betide you if you don't! It is also possible, via several different approaches, to make a conventional aft-tailed arrangement equally stall and spin resistant, and the penalties involved in making this happen are not any worse (and could even be better) than for a canard arrangement.

So are there any advantages to a canard arrangement? In the case of a subsonic airplane, there can be, but they involve getting into the minuscule details of the design. For a pod-and-boom tail design where the mass and wetted area of the tail section is negligible, there isn't a significant benefit, but for a tail section on a more conventional tail structure like a typical light aircraft's, there can be a lot of sheet metal and surface area in the tail cone. Because of C/G considerations it isn't possible to make much use of this volume, so it ends up being mostly wasted space.

Conversely, a canard's C/G is between the wing and canard, so the fuselage structure neeeded to connect them to each other is the same fuselage structure needed to carry equipment, fuel and payload, thereby possibly eliminating most or all of the structure and wetted surface that would have been needed for a tail cone. Of course you still need to have sufficient yaw stability, and you might end up putting all of that weight and surface area back in while providing sufficient amounts of vertical tail and the structures required to support it.

In addition, for at least some canard arrangements, the bending moments in the fuselage from the various loads such as engine weight, payload weight, lift forces, and wing and canard aerodynamic pitching moments can be made to at least partially cancel each other, reducing the stress in the fuselage. This can result in some additional weight savings, but ONLY if you take the trouble to truly optimize the fuselage design. Since the loads on the fuselage tend to be fairly low to begin with, often making mere surface durablity (i.e.: resistance to dents and such from handling in everyday use) the deciding factor in the fuselage structure, realizing any real weight savings in a structure that already has to be overbuilt just to avoid "hangar rash" could be difficult or impossible.

So, the case for canards on subsonic aircraft tends to be a little shaky at best.

For supersonic aircraft, particularly fighters that use active stabilization (rather than being naturally aerodynamically stable) for extra maneuverability, canards can in some cases be used to an advantage. Going to a three-surface layout (a canard in front, wing in the middle and a horizontal tail at the back) can also help maneverability. Also, the massive amount of equipment in a jet fighter's fuselage can make the whole issue of potential fuselage structural weight savings I described above a little more valid in this particular case.

In addition, it's possible to arrange the canard and the wing such that the wing uses the supersonic shock wave coming off the canard to help it make lift. In effect, the plane is "surfing" on its own shock wave.

So, if you're designing a supersonic jet, there could be some advantages, although nothing is certain by any means. For a subsonic aircraft the possible advantages are even more nebulous and the possibility of ending up losing more than you gain is that much greater. In addition, for a canard to have decent handling qualities requires significantly more effort on the part of the designer unless you plan to just "wing it" and rely on sheer luck for a good outcome (an approach not generally well received among competent engineers).

Canards can be advantageous in some cases, but if their advantages were all that significant and clear-cut, we would see a lot more of them. Go ahead and give them a try, if you're thorough and clever you could come out ahead with them, but it is by no means a certainty. As with all aircraft design decisions, it depends on the details of each individual design, and on how well you deal with them.

[Kenny Sharp]

It appears that I've stumbled upon a gold mine!...Thanks.
Regards,
Ken Sharp