7e7
Join Date: Dec 2001
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So the 7E7 gets the Comets´ nose section, and the nice rounded VC-10 horizontal stabilizer, guess I know where all the british aircraft designers went to after their compamies were shut down 
Old Aero Guy,
as induced drag is nothing that can be calculated for one wing alone, you can´t state that the canard works with higher induced drag just because of it´s lower span. Aspect ratio is the important value, not span.
You can state in general, that for static stability reasons wing loading of the canard must alwys be higher as the wing loading of the main wing. This means your canard always operates at higher Cl than the main wing.
This implies, that the high lift devices on the canard must be able to allow higher Cl than on the main wing, so you probably end up with slats and tripple slotted flaps, making elevator design a real challenge. If you look at the Tu-144 canard, it has double slotted slats, and tripple slotted flaps, a maintenance nightmare !
Comming back to induced drag, as Cl is influencing the induced drag to the second, one can assume that therefor the induced drag should be very high. But looking at the theory in detail, you see that induced drag is coming from the difference in local angle of attack in comparison to the angle of attack at CG. Multiplying Cl with the differential angle gives you induced Cd, and differential angle is a function of Cl and aspect ratio. As the wing tip vortices induce a downdraft on the wing, differential angle is negative, therefor the induced force is in drag direction. For any multiple wing system, differential angle is not only influenced by the wing´s Cl, but also by the Cl of all other wings in the same flow field. And the main wing induces an updraft on the canard, resulting in a positive differential angle, or an induced force in thrust direction. Therefor the main wing produces ´induced thrust´ on the canard, making it indeed more efficient than it would be in free stream. On the other hand the canards downdraft produces enormous additional induced drag on the main wing inner part, but also produces ´induced thrust´ on the outer main wing. As for structural reasons the inner main wing normally carries more load as the outer, and the outer wing updraft induced by the canard is lower than the inner wing downdraft, overall influence on the main wing is negative.
Canard configuration aerodynamics is nothing easy (I´ve worked on it for about 7 years) and it is hard to give a general statement whether it is the ´better´ configuration. It all depends on the overall layout of the plane.
Old Aero Guy,
as induced drag is nothing that can be calculated for one wing alone, you can´t state that the canard works with higher induced drag just because of it´s lower span. Aspect ratio is the important value, not span.
You can state in general, that for static stability reasons wing loading of the canard must alwys be higher as the wing loading of the main wing. This means your canard always operates at higher Cl than the main wing.
This implies, that the high lift devices on the canard must be able to allow higher Cl than on the main wing, so you probably end up with slats and tripple slotted flaps, making elevator design a real challenge. If you look at the Tu-144 canard, it has double slotted slats, and tripple slotted flaps, a maintenance nightmare !
Comming back to induced drag, as Cl is influencing the induced drag to the second, one can assume that therefor the induced drag should be very high. But looking at the theory in detail, you see that induced drag is coming from the difference in local angle of attack in comparison to the angle of attack at CG. Multiplying Cl with the differential angle gives you induced Cd, and differential angle is a function of Cl and aspect ratio. As the wing tip vortices induce a downdraft on the wing, differential angle is negative, therefor the induced force is in drag direction. For any multiple wing system, differential angle is not only influenced by the wing´s Cl, but also by the Cl of all other wings in the same flow field. And the main wing induces an updraft on the canard, resulting in a positive differential angle, or an induced force in thrust direction. Therefor the main wing produces ´induced thrust´ on the canard, making it indeed more efficient than it would be in free stream. On the other hand the canards downdraft produces enormous additional induced drag on the main wing inner part, but also produces ´induced thrust´ on the outer main wing. As for structural reasons the inner main wing normally carries more load as the outer, and the outer wing updraft induced by the canard is lower than the inner wing downdraft, overall influence on the main wing is negative.
Canard configuration aerodynamics is nothing easy (I´ve worked on it for about 7 years) and it is hard to give a general statement whether it is the ´better´ configuration. It all depends on the overall layout of the plane.
Join Date: Apr 2001
Location: Sanford, FL, USA
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boeing
amazing,
all this talk of "i'll be impressed when they have something new". yet when boeing was proposing their close to mach 1 airliner that was totally new and different, everyone here shot it down as too radical. so boeing proposes a fairly conventional midsize aircraft with 25% savings and that is no good.
do you see a pattern here?
what do you want boeing to produce? maybe a mach 6 aircraft with a lower cost than a 747?
all this talk of "i'll be impressed when they have something new". yet when boeing was proposing their close to mach 1 airliner that was totally new and different, everyone here shot it down as too radical. so boeing proposes a fairly conventional midsize aircraft with 25% savings and that is no good.
do you see a pattern here?
what do you want boeing to produce? maybe a mach 6 aircraft with a lower cost than a 747?
Self Loathing Froggy
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Just to add my grain of salt to the anti-canard topic (sorry about thread hijacking).
Did you notice that an ovewhelming majority of gliders still use the classical wing-tailplane combination. There have been some rather unsuccessfull canards (Dick Rutan's Solitaire is probably not his best achievement) or flying wings, but basically none achieved better performance with equivalent handling.
This has to do with one of the facts pointed out by volume:
This is especially bad for gliders who spend a significant time flying at low speeds, hence close to max Cl for the wing.
Note : before someone starts flaming me, I know that gliders are not airliners, but there are some commonalities there (efficiency requires high L/D, hence high aspect ratio and similar "tricks" to operate close to max L/D in cruise, altitude for airliners, ballast for gliders)
Did you notice that an ovewhelming majority of gliders still use the classical wing-tailplane combination. There have been some rather unsuccessfull canards (Dick Rutan's Solitaire is probably not his best achievement) or flying wings, but basically none achieved better performance with equivalent handling.
This has to do with one of the facts pointed out by volume:
You can state in general, that for static stability reasons wing loading of the canard must alwys be higher as the wing loading of the main wing. This means your canard always operates at higher Cl than the main wing.
Note : before someone starts flaming me, I know that gliders are not airliners, but there are some commonalities there (efficiency requires high L/D, hence high aspect ratio and similar "tricks" to operate close to max L/D in cruise, altitude for airliners, ballast for gliders)
Last edited by Bre901; 4th Mar 2004 at 16:20.
Join Date: Feb 2001
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Volume,
A good way to assess relative induced drag is to consider the far field.
Plot the total configuration lift for all lifting surfaces in units of force across the span of the configuration. Lift at the tips should be near zero.
Minimum induced drag for a given span implies a smooth distribution with the minimum slope at any given spanwise location, with an elliptical loading the classic "ideal".
As we both agree, the canard has to be highly loaded, so it tends to cause a lump of lift in the middle of the wing, with a high slope discontinuity at its tips.
You can reduce this slope break by unloading the inboard wing and the canard downwash will help you do it.
But this means you waste the inboard wing area as it loafs along, not carrying much lift. Wasted area means unnecessary skin friction drag.
A section of inboard wing with inverse taper might help you out, but that induces structural problems since you probably still need to put the gear in the wing.
I glad you mentioned the need for canard high lift devices and their complexity.
Unless the wing is over sized (more wetted area), it probably needs its own high lift devices, simple trailing edge flaps and maybe outboard slats.
In any case, you now have high lift devices on two lifting structures to maintain as opposed to the conventional configuration needing them on only one.
Hardly a way to achieve greater efficiency.
I'm not really anti-canard, but I've been through this trade a couple of times in my career. Canards are great if you want an airplane with gentle stall and a limited CG range.
They can also are great de-stabilizers for fighter maneuverability.
Canard configurations don't work well for airliners where you need a highly efficient cruise capability, relatively simple high lift devices and the ability to have a wide CG range for loading flexibility.
A good way to assess relative induced drag is to consider the far field.
Plot the total configuration lift for all lifting surfaces in units of force across the span of the configuration. Lift at the tips should be near zero.
Minimum induced drag for a given span implies a smooth distribution with the minimum slope at any given spanwise location, with an elliptical loading the classic "ideal".
As we both agree, the canard has to be highly loaded, so it tends to cause a lump of lift in the middle of the wing, with a high slope discontinuity at its tips.
You can reduce this slope break by unloading the inboard wing and the canard downwash will help you do it.
But this means you waste the inboard wing area as it loafs along, not carrying much lift. Wasted area means unnecessary skin friction drag.
A section of inboard wing with inverse taper might help you out, but that induces structural problems since you probably still need to put the gear in the wing.
I glad you mentioned the need for canard high lift devices and their complexity.
Unless the wing is over sized (more wetted area), it probably needs its own high lift devices, simple trailing edge flaps and maybe outboard slats.
In any case, you now have high lift devices on two lifting structures to maintain as opposed to the conventional configuration needing them on only one.
Hardly a way to achieve greater efficiency.
I'm not really anti-canard, but I've been through this trade a couple of times in my career. Canards are great if you want an airplane with gentle stall and a limited CG range.
They can also are great de-stabilizers for fighter maneuverability.
Canard configurations don't work well for airliners where you need a highly efficient cruise capability, relatively simple high lift devices and the ability to have a wide CG range for loading flexibility.
