By: robban
- 18th April 2010 at 13:44Permalink- Edited 1st January 1970 at 01:00
The canards are not loaded with negative lift to counter the instability. The canards control the amount of instability on close coupled canard fighters such as the Gripen and Rafale. These two aircraft are not unstable when the canards are inactive. Although, as instability gives better performance at most speeds, altitudes and AoA, the canards are more or less active all the time.
Canards continuosly and automatically offers max avaliable lift/lowest drag throughout the flight envelope.
I don't think the above complies with a long coupled delta canard platform like the Typhoon though. But I'm not sure.
As the canards offer lift in front of the CoG(when active) they can be used as elevators. Starting and stopping a pitching moment requires very small inputs. The same can not be said about unstable fighters with a conventional tail design. Not only does the tail need to be large, but in contrary to a canard, very large inputs are needed to start, and especially stopp a pitching moment.
By: over G
- 18th April 2010 at 13:56Permalink- Edited 1st January 1970 at 01:00
Not a single solid evidence for the canard/stealth issue, just a random stupid-cool comment and the traditional disliking of the US designers for canards (for the US needs)
But that is...if a unamed guy makes a cool comment to bash international designs then it turns in a fact..because the phrase was cool enough..how was it? "the only place i want acanard is on the enemies plane"...yeah cool...
BTW, canards were seriously considered from the JSF program, despite the stealth experience from the ATF program and other LO programs.
It was not stealth the reason why canards were not considered at the end, it was the balance interaction with the liftfan, the center of lift of the main wing (with a canard), and weight balances and requirements for all the 3 JSF versions
By: Distiller
- 18th April 2010 at 14:43Permalink- Edited 1st January 1970 at 01:00
I still think the most attractive solution for the low-speed/low-AoA problem of carrier landings is blown flaps and other versions of boundary-layer control. Could lower approach speed up to 50%. Where to get the extra air from? A high bypass-ratio engine. The required high power settings could be compensated by air brakes, and the relatively high rpm count would be perfect for bolters, basically elaminating spool-up time. I also think that the maintenance aspect would be manageable.
By: sferrin
- 18th April 2010 at 15:28Permalink- Edited 1st January 1970 at 01:00
I still think the most attractive solution for the low-speed/low-AoA problem of carrier landings is blown flaps and other versions of boundary-layer control. Could lower approach speed up to 50%. Where to get the extra air from? A high bypass-ratio engine. The required high power settings could be compensated by air brakes, and the relatively high rpm count would be perfect for bolters, basically elaminating spool-up time. I also think that the maintenance aspect would be manageable.
Lots of stuff there bad for both stealth and supersonic flight.
By: em745
- 18th April 2010 at 16:42Permalink- Edited 1st January 1970 at 01:00
All US fighters after the F-16 are noi unstable. The F-18 and its newer E version are not
Stability info on the Hornets is indeed hard to come by. Legacy Hornets are probably stable, but the Shornets are said to be neutral to mildly unstable. At any rate, you'd be hard pressed to tell by looking at Hornet demos, even the legacy ones, which can, BTW, outturn an F-16 at lower speeds.
While certainly an asset to agility, I believe relaxed stability's importance is overhyped, especially among fanboys. JMHO, mind you.
Canards are used (close coupled) as substitutes for LERXs to generate high AoA vortices and stick the boundary layer to the wing but LERXs provide little benefit at low or no AoA.
(??)
LERX's provide fixed, controlled vortice management at high AoA, which is more than can be said of movable canards. What's the AoA limit of the Rafale compared to the SHornet's?
The canards are not loaded with negative lift to counter the instability.
Then tell me why in almost all the euro-canard pics I've seen of them maneuvering hard or cruising level, the canards are slightly canted downwards.
The canards control the amount of instability on close coupled canard fighters such as the Gripen and Rafale.
Wouldn't that make both inherently STABLE platforms? By definition an unstable platform is made stable thru the use of computer controlled inputs, not the other way around.
These two aircraft are not unstable when the canards are inactive.
Sure as hell seems unstable to me when the FLCS decides to call it quits.
Not a single solid evidence for the canard/stealth issue
SR-71 (mildly stealth), F-117, B-2, F-22, YF-23, F-35, T-50... Not a single canard in sight.
Maybe these plane's engineers know something you don't?
One of the first things radar waves will hit on a canard-delta is a big-ol' movable reflector. Tell me, how is THAT conducive to stealthy design? How do you juggle VLO requirements with the very real need to ensure proper canard-wing interactions without breaking the laws of physics in the process?
By: Distiller
- 18th April 2010 at 17:19Permalink- Edited 1st January 1970 at 01:00
Not so extreme! We are still talking fighter jets here! 0.85 vs 0.35 is more what I have in mind. And bye-bye to supercruise is not a given - think of the GE YF120 variable cycle engine!
By: sferrin
- 18th April 2010 at 17:21Permalink- Edited 1st January 1970 at 01:00
Not so extreme! We are still talking fighter jets here! 0.85 vs 0.35 is more what I have in mind. And bye-bye to supercruise is not a given - think of the GE YF120 variable cycle engine!
By: over G
- 18th April 2010 at 18:00Permalink- Edited 1st January 1970 at 01:00
Sure em745, and you forgot the rest of my post
The canard has the same effect as the tailplane.
Full LO oriented designs don't have any kind of such surfaces
The US designers never liked the canard delta configuration for their requirements, for example, for their unstable aircraft (f-16) they went with a conventional tailplane,while the euros, chinese, isralits, and others went with canards
By: robban
- 18th April 2010 at 18:07Permalink- Edited 1st January 1970 at 01:00
Then tell me why in almost all the euro-canard pics I've seen of them maneuvering hard or cruising level, the canards are slightly canted downwards.
Since the canards are movable they align themselves close to the relative airstream. Here they balance the aircraft, giving optimum lift/drag ratio.
The higher the AoA, the more noticable angle is on the canard.
Wouldn't that make both inherently STABLE platforms? By definition an unstable platform is made stable thru the use of computer controlled inputs, not the other way around.
Sure as hell seems unstable to me when the FLCS decides to call it quits.
It's not easy to explain. The FCS allows the canards to create instability and provide lift ahead of the CoG improving maneuverability and pitch acceleration. So, the canard makes the aircraft unstable. In case of an FCS malfunction the canards goes into free flow mode. The aircraft is still flyable, but max G-load is lowered, allowed AoA is noticably lowered and landing/take off speeds increase. The pilot is adviced to fly the aircraft very gently.
The crash of 39.102 in Stockholm -93, shows the effects of a faulty FCS and canards going heywire. The canards are not free flowing here. Before the stall, the canards, with help from the pilots inputs overcompensates the amount needed to get control of the situation. In the end the AoA is too great, and the aircraft stalls.
By: MigL
- 18th April 2010 at 18:45Permalink- Edited 1st January 1970 at 01:00
Unstability, by definition,is easy. Take an old style delta (M3,F-106 etc.) with no fore or tail planes, it is stable only if its CoL is behind the CoG and its nose down moment is controlled by flaps which detract from lift as they need to be upturned. The M2000 was unstable because its CoL is even or ahead of its CoG, it can use down flaps to control pitch and also generate extra lift when needed most.
To determine if an aircraft is stable or not, take away the fore or tailplanes, is it departs in pitch (nose-up) it is unstable. Even a canard layout has to have its CoL ahead of CoG to be unstable. Of course as the CoL moves aft supersonically the canard layout approaches stability while the conventional tail becomes more unstable and needs larger tailplane authority for stable flight.
By: em745
- 18th April 2010 at 18:56Permalink- Edited 1st January 1970 at 01:00
Sure em745, and you forgot the rest of my post
There's a difference between ignoring and dismissing. ;)
The canard has the same effect as the tailplane.
But they DON'T have the same effect on RCS, which is the whole point here.
Sure em745, and you forgot the rest of my postThe US designers never liked the canard delta configuration for their requirements
Maybe because... Aw, forget it. I never liked going in circles. Find someone else to dance with. ("Best joke ever" indeed. :rolleyes: )
The FCS allows the canards to create instability
That's just it. That is NOT the definition of an ustable platform. You're describing a STABLE platform, like the F-15... which is stable, until a stab movement "creates [the] instability" needed for pitch. Whether that movement is initiated by the FLCS or the pilot is beside the point.
Unstable platforms inherently want to pitch over, all by themselves (like throwing a dart, flight-first). It's the FLCS that forces them into stability thru constant adjustments of the flight surfaces (stabs, flaperons, canards, whatever). When a pitching moment is desired, the FLCS "relaxes" these corrections and lets the nose of the jet loose for a short while. THAT's the very essence of an unstable platform. You have it backwards.
Before the stall
Stall?? That was a violent over-pitch, caused by FLCS failure. Any unstable jet that undergoes a complete loss of FLCS goes through similar motions, regardless of whether they're wing-tails or canard-deltas. A stable jet would just keep flying (more or less) nose-first, straight/down (again, planform design doesn't matter).
By: sferrin
- 18th April 2010 at 20:34Permalink- Edited 1st January 1970 at 01:00
Maybe because... Aw, forget it. I never liked going in circles. Find someone else to dance with. ("Best joke ever" indeed. :rolleyes: )
You should see the contortions I went through to dispell the "US is scared of canards" myth. Talk about a monumental waste of time. Some just want to believe what they want to believe and reality be damned.
By: robban
- 18th April 2010 at 21:46Permalink- Edited 1st January 1970 at 01:00
That's just it. That is NOT the definition of an ustable platform. You're describing a STABLE platform, like the F-15... which is stable, until a stab movement "creates [the] instability" needed for pitch. Whether that movement is initiated by the FLCS or the pilot is beside the point.
I'm hardly an expert on the subject. From what I've read, it is the canards that makes the aircraft unstable. The elevators on an F-15 for example doesn't destabilize the aircraft like the canards on the Gripen does.
An F-15 begins a pitching moment by forcing the tail down. In order to maintain the turn rate the tailplane must continue to force the tail down, otherwise the turn stops by itself. In other words an F-15 needs to continue to kill lift in order to turn. An F-22 also kills lift initiating a turn. The difference here is that the F-22 doesn't have to continue to force the tail down during the turn, at least not with its tailerons.
F-22 initiates a pitch up moment. Notice the angled tailerons.
Gripen initiates a pitch up moment. Notice the very slightly tilted canards. And no downforce on the elevons.
· Stable detached leading edge vortex flow, high maximum lift coefficient. · Positive trim lift on all lifting surfaces. · Floating canard offers stable aircraft if EFCS fails.
· Good field performance (take off and landing), enhanced by special aerodynamic breaking mode.
Unstable platforms inherently want to pitch over, all by themselves (like throwing a dart, flight-first). It's the FLCS that forces them into stability thru constant adjustments of the flight surfaces (stabs, flaperons, canards, whatever). When a pitching moment is desired, the FLCS "relaxes" these corrections and lets the nose of the jet loose for a short while. THAT's the very essence of an unstable platform. You have it backwards.
You are right of course. I'm guessing that had the canards been fixed the aircraft would be unstable/unflyable, but since the canards are fully movable, the amount of instability can be controlled?
By: sferrin
- 18th April 2010 at 22:13Permalink- Edited 1st January 1970 at 01:00
I'm hardly an expert on the subject. From what I've read, it is the canards that makes the aircraft unstable.
No, what makes an aircraft unstable is having the CP ahead of the CG (or even just clost to it). Doesn't matter if it's a canard, a tail, or niether. F-16 is unstable, the Mirage 2000 is unstable as is th Typhoon. You could have stable canards. The X-10, Hound Dog, and Navaho were all stable canards.
By: robban
- 18th April 2010 at 22:24Permalink- Edited 1st January 1970 at 01:00
No, what makes an aircraft unstable is having the CP ahead of the CG (or even just clost to it). Doesn't matter if it's a canard, a tail, or niether. F-16 is unstable, the Mirage 2000 is unstable as is th Typhoon. You could have stable canards. The X-10, Hound Dog, and Navaho were all stable canards.
I didn't mean that a canard makes an aircraft unstable by default. Their position in relation to the CP and CG is of course very relevant. As shivering said, the Viggen wasn't unstable and it has canards.
New
Posts: 489
By: kiwinopal
- 18th April 2010 at 23:14Permalink- Edited 1st January 1970 at 01:00
That's only true with stable wing-tails, not unstable ones. In unstable wing-tails (like the F-16, F-22 & F-35), the stabs produce lift to prevent the nose from overpitching. In stable wing-tails (like the F-15), the stabs do indeed produce downforce to keep the nose from drooping in level flight.
That's only true with stable canard-deltas, not unstable ones. ;) In unstable canard-deltas (like the EF), canards produce downforce to keep the nose from overpitching.
As to why canards make for poor VLO designs, a major reason is canards having to be precisely sized, positioned and shaped for optimum aerodynamics and airflow interactions with the main wing. With rear-mounted stabs, engineers have much more leeway when it comes to VLO optimized shaping. Look at the shaping of the F-22's stabs:
Also look at the F-22 head-on--the stabs and wings are on the same plane. Not so with any of the canard-deltas:
you are totally right, the canard needs to be above the wing to be in a better drag lift rate, it works better with a dihedral positioning, these factors make it more dificult to align it with the wing and create planform alignment
Increasing canard dihedral causes multiple effects including aircraft lifting length enhancement to attain a target equivalent area for low sonic boom. Increased canard dihedral also enables a pilot to exploit asymmetry in control of canards on opposing sides of the aircraft for directional control.
The canard dihedral can also be structured and the canards may be differentially controlled to enable yaw control and roll control, thereby facilitating lateral and/or directional control of the aircraft.
The canard is usually position above or in line at the same level of the wing for the canard to downwash the wing and enhance lift and vortex interaction
the shape of a canard has more to do with high and low aspect ratios and their interaction with the wing, high aspects are prefered for low drag while low aspect for lift Canard aircraft designs (and Rutan ones in particular) are "tuned" so
that the canard stalls before the main wing. This results in the need
for a high aspect ratio canard surface, to get the correct lift curve
slope on the canard relative to the main wing. Hence, you are stuck
with the narrow chord, long canard.
By: em745
- 19th April 2010 at 00:41Permalink- Edited 1st January 1970 at 01:00
The elevators on an F-15 for example doesn't destabilize the aircraft like the canards on the Gripen does.
The F-15's stabs "stabilize" it in the sense that they keep the nose up for level flight.
F-22 initiates a pitch up moment. Notice the angled tailerons... Gripen initiates a pitch up moment. Notice the very slightly tilted canards. And no downforce on the elevons.
The point is not how the control surfaces INITIAITE the pitching moment, it's what happens with those surfaces AFTERWARDS. Your pic talks of canards that "STABILIZED the AoA"... "STABILIZE the pitch rate"... "BALANCE the aircraft"... All clever metaphors that tell the same tale: the canards are applying DOWNFORCE to prevent the nose from departing.
In a tight, sustained turn (or vertical loop), canards must apply downforce to prevent over-pitching--THAT's the so-called "stabilizing" effect. With an unstable wing-tail, that same stabilizing effect is accomplished by the stabs producing UPLIFT to keep the nose down... And since lift is the centripetal force that holds an aircraft in tight turns, more lift=GOOD.
Look at what the stabs and canards are doing in the following pics...
As a baseline, here's a (stable) Tomcat cornering hard; stabs canted downwards to "force" the plane into the turn (i.e. fighting against the plane's inherent stability):
The two favorite eurocanards in various states of cruising, climbing and banking turns (note the canards' downward slant):
Now, the F-22 turning, hard in some cases (note the stab angles):
Posts: 448
By: robban - 18th April 2010 at 13:44 Permalink - Edited 1st January 1970 at 01:00
The canards are not loaded with negative lift to counter the instability. The canards control the amount of instability on close coupled canard fighters such as the Gripen and Rafale. These two aircraft are not unstable when the canards are inactive. Although, as instability gives better performance at most speeds, altitudes and AoA, the canards are more or less active all the time.
Canards continuosly and automatically offers max avaliable lift/lowest drag throughout the flight envelope.
I don't think the above complies with a long coupled delta canard platform like the Typhoon though. But I'm not sure.
As the canards offer lift in front of the CoG(when active) they can be used as elevators. Starting and stopping a pitching moment requires very small inputs. The same can not be said about unstable fighters with a conventional tail design. Not only does the tail need to be large, but in contrary to a canard, very large inputs are needed to start, and especially stopp a pitching moment.
Posts: 1,838
By: over G - 18th April 2010 at 13:56 Permalink - Edited 1st January 1970 at 01:00
Not a single solid evidence for the canard/stealth issue, just a random stupid-cool comment and the traditional disliking of the US designers for canards (for the US needs)
But that is...if a unamed guy makes a cool comment to bash international designs then it turns in a fact..because the phrase was cool enough..how was it? "the only place i want acanard is on the enemies plane"...yeah cool...
BTW, canards were seriously considered from the JSF program, despite the stealth experience from the ATF program and other LO programs.
It was not stealth the reason why canards were not considered at the end, it was the balance interaction with the liftfan, the center of lift of the main wing (with a canard), and weight balances and requirements for all the 3 JSF versions
Posts: 4,674
By: Distiller - 18th April 2010 at 14:43 Permalink - Edited 1st January 1970 at 01:00
I still think the most attractive solution for the low-speed/low-AoA problem of carrier landings is blown flaps and other versions of boundary-layer control. Could lower approach speed up to 50%. Where to get the extra air from? A high bypass-ratio engine. The required high power settings could be compensated by air brakes, and the relatively high rpm count would be perfect for bolters, basically elaminating spool-up time. I also think that the maintenance aspect would be manageable.
Posts: 9,683
By: sferrin - 18th April 2010 at 15:28 Permalink - Edited 1st January 1970 at 01:00
Lots of stuff there bad for both stealth and supersonic flight.
Posts: 40
By: em745 - 18th April 2010 at 16:42 Permalink - Edited 1st January 1970 at 01:00
Stability info on the Hornets is indeed hard to come by. Legacy Hornets are probably stable, but the Shornets are said to be neutral to mildly unstable. At any rate, you'd be hard pressed to tell by looking at Hornet demos, even the legacy ones, which can, BTW, outturn an F-16 at lower speeds.
While certainly an asset to agility, I believe relaxed stability's importance is overhyped, especially among fanboys. JMHO, mind you.
(??)
LERX's provide fixed, controlled vortice management at high AoA, which is more than can be said of movable canards. What's the AoA limit of the Rafale compared to the SHornet's?
Then tell me why in almost all the euro-canard pics I've seen of them maneuvering hard or cruising level, the canards are slightly canted downwards.
Wouldn't that make both inherently STABLE platforms? By definition an unstable platform is made stable thru the use of computer controlled inputs, not the other way around.
http://www.youtube.com/watch?v=OVr6QJzW094
Sure as hell seems unstable to me when the FLCS decides to call it quits.
SR-71 (mildly stealth), F-117, B-2, F-22, YF-23, F-35, T-50... Not a single canard in sight.
Maybe these plane's engineers know something you don't?
One of the first things radar waves will hit on a canard-delta is a big-ol' movable reflector. Tell me, how is THAT conducive to stealthy design? How do you juggle VLO requirements with the very real need to ensure proper canard-wing interactions without breaking the laws of physics in the process?
Say bye-bye to supercruise, then.
Posts: 9,683
By: sferrin - 18th April 2010 at 16:45 Permalink - Edited 1st January 1970 at 01:00
Hell, say bye-bye to supersonic speed period.
Posts: 4,674
By: Distiller - 18th April 2010 at 17:19 Permalink - Edited 1st January 1970 at 01:00
Not so extreme! We are still talking fighter jets here! 0.85 vs 0.35 is more what I have in mind. And bye-bye to supercruise is not a given - think of the GE YF120 variable cycle engine!
Posts: 9,683
By: sferrin - 18th April 2010 at 17:21 Permalink - Edited 1st January 1970 at 01:00
That's not "high bypass".
Posts: 1,838
By: over G - 18th April 2010 at 18:00 Permalink - Edited 1st January 1970 at 01:00
Sure em745, and you forgot the rest of my post
The canard has the same effect as the tailplane.
Full LO oriented designs don't have any kind of such surfaces
The US designers never liked the canard delta configuration for their requirements, for example, for their unstable aircraft (f-16) they went with a conventional tailplane,while the euros, chinese, isralits, and others went with canards
Posts: 448
By: robban - 18th April 2010 at 18:07 Permalink - Edited 1st January 1970 at 01:00
Since the canards are movable they align themselves close to the relative airstream. Here they balance the aircraft, giving optimum lift/drag ratio.
The higher the AoA, the more noticable angle is on the canard.
It's not easy to explain. The FCS allows the canards to create instability and provide lift ahead of the CoG improving maneuverability and pitch acceleration. So, the canard makes the aircraft unstable. In case of an FCS malfunction the canards goes into free flow mode. The aircraft is still flyable, but max G-load is lowered, allowed AoA is noticably lowered and landing/take off speeds increase. The pilot is adviced to fly the aircraft very gently.
The crash of 39.102 in Stockholm -93, shows the effects of a faulty FCS and canards going heywire. The canards are not free flowing here. Before the stall, the canards, with help from the pilots inputs overcompensates the amount needed to get control of the situation. In the end the AoA is too great, and the aircraft stalls.
Posts: 190
By: MigL - 18th April 2010 at 18:45 Permalink - Edited 1st January 1970 at 01:00
Unstability, by definition,is easy. Take an old style delta (M3,F-106 etc.) with no fore or tail planes, it is stable only if its CoL is behind the CoG and its nose down moment is controlled by flaps which detract from lift as they need to be upturned. The M2000 was unstable because its CoL is even or ahead of its CoG, it can use down flaps to control pitch and also generate extra lift when needed most.
To determine if an aircraft is stable or not, take away the fore or tailplanes, is it departs in pitch (nose-up) it is unstable. Even a canard layout has to have its CoL ahead of CoG to be unstable. Of course as the CoL moves aft supersonically the canard layout approaches stability while the conventional tail becomes more unstable and needs larger tailplane authority for stable flight.
Posts: 40
By: em745 - 18th April 2010 at 18:56 Permalink - Edited 1st January 1970 at 01:00
There's a difference between ignoring and dismissing. ;)
But they DON'T have the same effect on RCS, which is the whole point here.
Maybe because... Aw, forget it. I never liked going in circles. Find someone else to dance with. ("Best joke ever" indeed. :rolleyes: )
That's just it. That is NOT the definition of an ustable platform. You're describing a STABLE platform, like the F-15... which is stable, until a stab movement "creates [the] instability" needed for pitch. Whether that movement is initiated by the FLCS or the pilot is beside the point.
Unstable platforms inherently want to pitch over, all by themselves (like throwing a dart, flight-first). It's the FLCS that forces them into stability thru constant adjustments of the flight surfaces (stabs, flaperons, canards, whatever). When a pitching moment is desired, the FLCS "relaxes" these corrections and lets the nose of the jet loose for a short while. THAT's the very essence of an unstable platform. You have it backwards.
Stall?? That was a violent over-pitch, caused by FLCS failure. Any unstable jet that undergoes a complete loss of FLCS goes through similar motions, regardless of whether they're wing-tails or canard-deltas. A stable jet would just keep flying (more or less) nose-first, straight/down (again, planform design doesn't matter).
Posts: 9,683
By: sferrin - 18th April 2010 at 20:34 Permalink - Edited 1st January 1970 at 01:00
You should see the contortions I went through to dispell the "US is scared of canards" myth. Talk about a monumental waste of time. Some just want to believe what they want to believe and reality be damned.
Posts: 448
By: robban - 18th April 2010 at 21:46 Permalink - Edited 1st January 1970 at 01:00
I'm hardly an expert on the subject. From what I've read, it is the canards that makes the aircraft unstable. The elevators on an F-15 for example doesn't destabilize the aircraft like the canards on the Gripen does.
An F-15 begins a pitching moment by forcing the tail down. In order to maintain the turn rate the tailplane must continue to force the tail down, otherwise the turn stops by itself. In other words an F-15 needs to continue to kill lift in order to turn. An F-22 also kills lift initiating a turn. The difference here is that the F-22 doesn't have to continue to force the tail down during the turn, at least not with its tailerons.
F-22 initiates a pitch up moment. Notice the angled tailerons.
Gripen initiates a pitch up moment. Notice the very slightly tilted canards. And no downforce on the elevons.
Below is an excerpt from here http://www.mach-flyg.com/utg80/80jas_uc.html
The direct fall-out of relaxed static stability are:
· Higher trimmed lift.
· Reduced lift dependent drag.
· Reduced supersonic trim drag.
Delta canard’s inherent good aerodynamics are:
· Stable detached leading edge vortex flow, high maximum lift coefficient.
· Positive trim lift on all lifting surfaces.
· Floating canard offers stable aircraft if EFCS fails.
· Good field performance (take off and landing), enhanced by special aerodynamic breaking mode.
You are right of course. I'm guessing that had the canards been fixed the aircraft would be unstable/unflyable, but since the canards are fully movable, the amount of instability can be controlled?
Posts: 9,683
By: sferrin - 18th April 2010 at 22:13 Permalink - Edited 1st January 1970 at 01:00
No, what makes an aircraft unstable is having the CP ahead of the CG (or even just clost to it). Doesn't matter if it's a canard, a tail, or niether. F-16 is unstable, the Mirage 2000 is unstable as is th Typhoon. You could have stable canards. The X-10, Hound Dog, and Navaho were all stable canards.
Posts: 85
By: shivering - 18th April 2010 at 22:14 Permalink - Edited 1st January 1970 at 01:00
Just a small point, probably irrelevant but.....
canards do not make an airframe inherently
unstable....example, the Saab Viggen.
Posts: 9,683
By: sferrin - 18th April 2010 at 22:20 Permalink - Edited 1st January 1970 at 01:00
Now you've done it. Someone's going to come by and have a freak out that the Viggen's canard isn't a canard. ;)
Posts: 448
By: robban - 18th April 2010 at 22:24 Permalink - Edited 1st January 1970 at 01:00
I didn't mean that a canard makes an aircraft unstable by default. Their position in relation to the CP and CG is of course very relevant. As shivering said, the Viggen wasn't unstable and it has canards.
Posts: 489
By: kiwinopal - 18th April 2010 at 23:14 Permalink - Edited 1st January 1970 at 01:00
you are totally right, the canard needs to be above the wing to be in a better drag lift rate, it works better with a dihedral positioning, these factors make it more dificult to align it with the wing and create planform alignment
Increasing canard dihedral causes multiple effects including aircraft lifting length enhancement to attain a target equivalent area for low sonic boom. Increased canard dihedral also enables a pilot to exploit asymmetry in control of canards on opposing sides of the aircraft for directional control.
The canard dihedral can also be structured and the canards may be differentially controlled to enable yaw control and roll control, thereby facilitating lateral and/or directional control of the aircraft.
http://www.freepatentsonline.com/7416155.html
The canard is usually position above or in line at the same level of the wing for the canard to downwash the wing and enhance lift and vortex interaction
the shape of a canard has more to do with high and low aspect ratios and their interaction with the wing, high aspects are prefered for low drag while low aspect for lift
Canard aircraft designs (and Rutan ones in particular) are "tuned" so
that the canard stalls before the main wing. This results in the need
for a high aspect ratio canard surface, to get the correct lift curve
slope on the canard relative to the main wing. Hence, you are stuck
with the narrow chord, long canard.
http://yarchive.net/air/canard.html
Posts: 40
By: em745 - 19th April 2010 at 00:41 Permalink - Edited 1st January 1970 at 01:00
The F-15's stabs "stabilize" it in the sense that they keep the nose up for level flight.
The point is not how the control surfaces INITIAITE the pitching moment, it's what happens with those surfaces AFTERWARDS. Your pic talks of canards that "STABILIZED the AoA"... "STABILIZE the pitch rate"... "BALANCE the aircraft"... All clever metaphors that tell the same tale: the canards are applying DOWNFORCE to prevent the nose from departing.
In a tight, sustained turn (or vertical loop), canards must apply downforce to prevent over-pitching--THAT's the so-called "stabilizing" effect. With an unstable wing-tail, that same stabilizing effect is accomplished by the stabs producing UPLIFT to keep the nose down... And since lift is the centripetal force that holds an aircraft in tight turns, more lift=GOOD.
Look at what the stabs and canards are doing in the following pics...
As a baseline, here's a (stable) Tomcat cornering hard; stabs canted downwards to "force" the plane into the turn (i.e. fighting against the plane's inherent stability):
The two favorite eurocanards in various states of cruising, climbing and banking turns (note the canards' downward slant):
Now, the F-22 turning, hard in some cases (note the stab angles):