Three Surface vs Canard (Two Surface) Discussion

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wsimpso1

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Flaps could be "added" to a canard if you planned them from the start. If his plan was to change out the elevator tracks at the time of adding flaps it could work. If, and that is a big if, the design of the flaps and elevator had been done such that they could be geared to deploy together and keep the CL vs Alpha curves of both wings in the safe zone through the entire range it could work. Chances of that having been done looking at all else is minimal. Without some sort of verification of the CL vs Alpha curves design would be tricky. And the flaps and elevator would have to be somehow physically geared together. A dual electric deployment without a very sophisticated transducer/position sense and fail system would be suicide.

Has this ever been done on any production or kit aircraft?
Beech Starship has flaps on the main wing and variable sweep on the canard. As the flaps go down, the canard sweeps forward... The static margins (stability) and main wing stall margins (flat stall prevention) are maintained throughout the range of these gadgets. It can be done. Demonstrated on the Scaled Composites designed and built protoype, then built full size. I do not know if the flaps and canard sweep were geared together or linked electronically or both.

Now, the question comes up, can this bird do the same thing with only flaps?

Well, just pulling numbers out of the air for the basic configuration: Current airplane with stall at CL of 2.0 on the canard and 1.0 on the wing. Now say you want to drop the stall speed 15%, that is going to take 1/0.85^2 = 1.38 or a 38% increase in CL on BOTH the main wing and on the canard. Wing CL would have to get to 1.38, canard would have to get to 2.76. I know that CL for the whole wing of 1.38 is not too hard to achieve but whole canard of 2.76? You can barely achieve that with slotted flaps at the section, much less when you get 3-D effects reducing the Cl/alpha slope. Even if you could achieve it, I am skeptical that it will behave well with the flaps that far out and wiggling it as an elevator control surface. Add a bit more elevator to raise the nose as you cross over the number, the canard stalls, the nose drops... this happens at 35 feet above TDZ, it is a bad day. You must have control margins around the CL that you fly down final or fly shortly after take-off.

We did not even get into a couple more issues. Deploying flaps drives more nose down pitching moment due which requires even more CL from the canard. And then there is still the whole issue of the neutral point has to stay put behind the CG while we move around the lift.

If the Raptor is to get lower landing speeds, that is best achieved by disciplined weight reduction and maybe a variable sweep canard.

Billski
 

wsimpso1

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I'm pretty sure the Avanti has some kind of syncronized flap system on both the main wing and it's front lifting surface.
The Piaggio Avanti is a three surface airplane. Instead of the big wing being substantially behind the CG, a three surface has the wing just behind the CG. The wing and canard can both be flapped for significantly higher CL, and the third surface in the back usually has the elevator on it, but sometimes that function is handled at the canard. Then you can trim and/or flight control with both canard and aft tail, flap both the main wing and the canard, etc. More good examples of three-surface airplanes besides the Avanti - Rutan Grizzley is in the Museum at OSH, and it has big Fowler flaps on canard and main wing; Rutan Catbird is also notable and usually visits OSH during the big party. There are others.

Billski
 
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Himat

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Flaps could be "added" to a canard if you planned them from the start. If his plan was to change out the elevator tracks at the time of adding flaps it could work. If, and that is a big if, the design of the flaps and elevator had been done such that they could be geared to deploy together and keep the CL vs Alpha curves of both wings in the safe zone through the entire range it could work. Chances of that having been done looking at all else is minimal. Without some sort of verification of the CL vs Alpha curves design would be tricky. And the flaps and elevator would have to be somehow physically geared together. A dual electric deployment without a very sophisticated transducer/position sense and fail system would be suicide.

Has this ever been done on any production or kit aircraft?
The SAAB AJ/JA/SF/SH 37 did have flaps both on the canard and main wing if I remember correct.
 

TarDevil

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The Piaggio Avanti is a three surface airplane. Instead of the big wing being substantially behind the CG, a three surface has the wing just behind the CG. The wing and canard can both be flapped for significantly higher CL, and the third surface in the back. Then you can trim and/or flight control with both canard and aft tail, flap both the main wing and the canard, etc. More good examples of three-surface airplanes besides the Avanti - Rutan Grizzley is in the Museum at OSH, and it has big Fowler flaps on canard and main wing; Rutan Catbird is also notable and usually visits OSH during the big party. There are others.

Billski
Never understood why this configuration panned.
 

Tom Nalevanko

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Incredibly irritatable noise signature due to engine/prop placement. We had a bunch at KCMA until one took off without one elevator. Flew to Nevada and did two stops before discovered.. That was the beginning of the end... No one laments their demise.

Actually I think it is a good design.
Best,

Tom
 

wsimpso1

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The SAAB AJ/JA/SF/SH 37 did have flaps both on the canard and main wing if I remember correct.
The Saab Viggen line of airplanes had a digital flight control system. Usually this means that inherent flight stability has been relaxed and stability is then augmented by the digital flight control system. With this digital stability, similar wing loadings can be carried on wing and canard, and it will be stable without stall or spin issues.

While the Saab Viggen line of airplanes is an interesting canard aircraft, it only has lessons for the Raptor canard configuration if the Raptor team find relaxed stability aerodynamics important, and then would need electronic flight controls.

Billski
 

rbarnes

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The Saab Viggen line of airplanes had a digital flight control system. Usually this means that inherent flight stability has been relaxed and stability is then augmented by the digital flight control system. With this digital stability, similar wing loadings can be carried on wing and canard, and it will be stable without stall or spin issues.
While the Saab Viggen line of airplanes is an interesting canard aircraft, it only has lessons for the Raptor canard configuration if the Raptor team find relaxed stability aerodynamics important, and then would need electronic flight controls.
Billski
Not to mention the rocket powered seat ready to shoot you out of the plane it comes equipped with too....

That's interesting info on the 3 lifting surface design. That has always been my dream experimental plane to design/build ....2 seat, 3 lifting surface, pusher twin with a pair of Rotax 915iS .. about the size of a Velcoity SE.. like a mini-Avanti
3 lifting surface twin engine always seemed like the perfect airplane to me. VMC benefits of a canard twin engine without the deep stall problems since it has the elevator mounted like a traditional configuration.
 

Himat

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The Saab Viggen line of airplanes had a digital flight control system. Usually this means that inherent flight stability has been relaxed and stability is then augmented by the digital flight control system. With this digital stability, similar wing loadings can be carried on wing and canard, and it will be stable without stall or spin issues.

While the Saab Viggen line of airplanes is an interesting canard aircraft, it only has lessons for the Raptor canard configuration if the Raptor team find relaxed stability aerodynamics important, and then would need electronic flight controls.

Billski
I am not sure, but at least I have never heard that the SAAB Viggen had relaxed stability. On the other side, it may have had limits on control input depending on flight mode. And of course, as rbarnes point out, ejection seat for the pilot if the need to step of arised.
 

thjakits

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Aaaaaah!! You had to say the "Viggen" word!!
.....you brought this upon yourself!

https://en.wikipedia.org/wiki/Saab_37_Viggen
"...Saab and Honeywell co-developed an automatic digital flight control system for the JA 37 Viggen, which has been claimed to be the first such system in a production aircraft...."

https://saabaircraftindustry.com/en/roads-to-new-capability/development-skills/technical-capability-development-for-control-systems/ ...scroll down a little and you can see all control system concepts of all Saab fighters! The Viggen was mechanical with digital input - there was one test-platform with a Fly-by-wire + mechanical back up (I don't know if this one used independent surfaces control....)

Warning! Going to this site WILL cause significant loss of time due to "acute need to see more!":

https://www.thedrive.com/the-war-zone/6237/saabs-viggen-could-stick-a-landing-and-takeoff-again-like-no-other-fighter

:cool: thjakits
 

Scheny

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Off topic, but here for the 2-seater 3 lifting surface aircraft you dreamed for in the last page: The swiss Aceair Aeriks 200

Unfortunately it was built around the Austrian twin rotor Wankel engine with 110hp (originally intended for the Diamond DA20) which got cancelled due to bad consumption and reliability. Up to date, they only produce the single rotor version with 50hp and a conversion to a Rotax 914 somehow was not feasible with the fuselage design. The single rotor Wankel is used for the Schiebel helo drone, which is a camera platform and the Wankel has almost zero vibration (and there are no 50hp turboshaft engines available).

It flew quite well and it won a bunch of prizes, but still it got cancelled in 2004.
 

Staggermania

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Off topic, but here for the 2-seater 3 lifting surface aircraft you dreamed for in the last page: The swiss Aceair Aeriks 200

Unfortunately it was built around the Austrian twin rotor Wankel engine with 110hp (originally intended for the Diamond DA20) which got cancelled due to bad consumption and reliability. Up to date, they only produce the single rotor version with 50hp and a conversion to a Rotax 914 somehow was not feasible with the fuselage design. The single rotor Wankel is used for the Schiebel helo drone, which is a camera platform and the Wankel has almost zero vibration (and there are no 50hp turboshaft engines available).

It flew quite well and it won a bunch of prizes, but still it got cancelled in 2004.
Great looking aircraft. I’d like a side by side version.
 

wsimpso1

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I relocated some posts on the Avanti and three-surface airplanes here as well as the Saab Viggen, rather than further drift the threads they were in.
 
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Aerowerx

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I have a paper around here somewhere* that talks about that little canard surface.

It isn't necessarily a control surface. On transsonic swept wing aircraft it is put there to do nice things to the air before it hits the main wing. And there is an optimum location for best effect. The Saab is specifically mentioned,

------
*I really need to take an extra lifetime so I can catalog and organize all my papers!
 

Scheny

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An aerodynamicist once explained to me, that in a 3LS design the canard is off-loading the elevator. So what you are gaining, is the induced drag that the elevator normally would have created. But this comes at the price of another interference drag at the canard and fuselage junction plus turbulent air behind the canard. You can avoid the turbulent air by careful wing placement, but not the interference.

So, his conclusion was, that in most cases it does not help to have a 3LS vs a conventional design.

The canard can be used for trimming and due to the unloaded elevator, it can be a.) built smaller and b.) has less drag in all envelopes.
 

Topaz

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An aerodynamicist once explained to me, that in a 3LS design the canard is off-loading the elevator. So what you are gaining, is the induced drag that the elevator normally would have created. But this comes at the price of another interference drag at the canard and fuselage junction plus turbulent air behind the canard. You can avoid the turbulent air by careful wing placement, but not the interference.

So, his conclusion was, that in most cases it does not help to have a 3LS vs a conventional design.

The canard can be used for trimming and due to the unloaded elevator, it can be a.) built smaller and b.) has less drag in all envelopes.
Essentially yes, most of this. A three-surface design can, theoretically, reduce trim-related induced drag of the main wing to zero. There will still be induced drag of the canard and horizontal tails, because they're creating lift, but since that lift is equal-but-opposite, the main wing sees no increase in load due to trim, as it does with a conventional wing-tail airplane, and less reduction in operating CL than it would see with a pure canard design.

Whether or not this is worth the added parasite drag of the additional surface (the canard) depends entirely on the design mission and cruise wing loading of the aircraft. If the design is operating at a low cruise CL, has a long tail boom so the horizontal tail creates minimal downforce to trim the aircraft (think sailplane), or has a relatively short design range, the slight reduction in induced drag a three-surface design can achieve is likely not going to be worth the additional parasite drag - there will be an overall increase in total drag. On the other hand, if the main wing is relatively highly loaded at cruise, the fuselage is relatively short, and the aircraft has a very long range, the three-surface design might show a measurable reduction in fuel use over the course of the flight, if the reduction in induced drag is of greater magnitude than the increase in parasite drag. Most homebuilts tend to fall in the former group, and three-surface design does little or nothing to improve range performance. Bizjets, however, meet most of the criteria for seeing some level of benefit from a three-surface configuration. Whether that's enough to compensate for the increased cost and construction time is up to the designer and user.

However, that's not the only reason to choose a three-surface design. Packaging is the best one for small airplanes up to bizjet-sized aircraft, in that the canard allows the designer to place the main wing spar carrythrough aft of the occupant cabin without resorting to forward sweep. Another reason to adopt a three-surface design could be to include stall-resistance, if that's desired.

"Pure" canard designs always wind up with a somewhat larger total lifting area than if they were, instead, a conventionally-tailed aircraft. Because of the constraints stability and stall resistance place on the design, the main wing has to operate and a lower CL than it might do with a conventional tail, and consequently has more area for the given load than might otherwise be the case. The result is an increase in parasite drag due to the larger wetted area of the wing. The relatively large area of the canard generally means an overall increase in either induced drag or wing aspect ratio over a "conventional" design, because induced drag is most heavily related to span, and with some of the lifting area moved to the canard, the wing is either shorter span or higher aspect ratio to get the same span. Again, you might pick a canard design for other reasons than flight efficiency - packaging and stall resistance again being chief among them.

Maximum flight performance in light aircraft will be achieved with a conventional wing-tail combination. Again, look at sailplanes, which have to perform with no motor at all. For most homebuilt-size-and-performance airplanes, the overall choice of configuration will be made for the secondary reasons I mentioned, not flight performance. A three-surface or canard design will not be able to realize any significant advantage over a conventional design in terms of performance, but may see packaging benefits and stall resistance, if that is desired for the design.
 
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