Canard vs Tail Civillian

[F-14ATomcat]

canard and tail on the pegasus

 

[Nicknick]

I think the tail of the P-400t as shown is quite bizzare. When I did some googeling for it, I found several different versions, once it was a true Canard, sometimes it had three wings, sometimes no Canards at all, and the latest version on Youtube has a T-tail. Looks like, the design team couldnt agree on a final design...

 

[F-14ATomcat]

I think the tail of the P-400t as shown is quite bizzare. When I did some googeling for it, I found several different versions, once it was a true Canard, sometimes it had three wings, sometimes no Canards at all, and the latest version on Youtube has a T-tail. Looks like, the design team couldnt agree on a final design...

There are only two variants one is flying and it is the Pe-210, it is smaller and it has canards; the P-400T has a triplane configuration.

It has canards and T tail.

There was a 3rd variant but is just a model, it has never been flown and it has canards

The P-400T it has not flown, the version I posted it is supposed to fly next year.

I guess it is triplane because it is heavier and bigger than the Pe-210, I guess they decided to adapt the T tail to off set the heavier tail because it has a heavier aft fuselage.

 

[1635yankee]

The small cross section of the long easy was enabled by the pusher design.

...and tandem seats.

 

[AndersJ]

I posted pretty much the same info and text in the J7W1 Shinden thread but I think it could help to answer some questions here as well.

I don’t think anyone is denying that a canard aircraft can be made of fly.

It's just that the question that really needs to be asked is if it’s a good idea to build a propeller powered canard type of aircraft at all, as in are they better than conventional tractor type aircraft?

Below is a figure from NASA paper 2382, where they ran a full sized Burt Rutan VariEze in one of NASA’s full scale wind tunnels.

Looking at this figure, proponent of canards may say: Yeah! That’s what I’m talking about! When the lift of the canard and main wing are combined, we get a Clmax of 1.7 at circa 22 degrees angle of attack! That’s way better than on conventional airplanes!

However, if we read the fine print in that NASA report, the reference area used to derive the impressive Clmax figure of 1.7 is based on the exposed, and not (as per aerodynamic convention) the total wing area.

On a conventional tractor aircraft, the Clmax using this total wing reference area is usually around 1.35. And using the same principle on the VariEze’s figure of 1.7, this drops to 1.32.

Well you may say, that’s not a big difference is it? 1.35 versus 1.32?

But here is the catch: Look at the VariEze’s lift slope figure above again, and you can see that you can’t take the VariEze to its full potential at 22 deg aoa, since the canard stalls already at around 15 deg aoa.

Now if we read of the Clmax for the complete aircraft there, at 15 deg aoa, when the nose drops due to the canard stalling, it’s 1.4, and which I suspect is the number Burt Rutan would like us to use. However, if we instead compare apples to apples, and use the same total reference wing area for both, then the 1.4 figure for the VariEze’s Clmax drops to 1.09, which then when compared to tractor aircraft’s typical 1.35 hardly comes off as impressive.

So in summary, you can without problems design a conventional tractor aircraft to utilize the wings full potential and extract a trimmable Clmax of around 1.35 while on the VariEze this is only around 1.09.

So why is this important? Well since you usually have a landing speed requirement, the maximum trimmable Clmax determines your wing area which is intimately coupled to the drag as in smaller wing area equals less drag.

And this is not even taking into consideration the canard's problems to handle the deflection of trailing edge flaps (the absolutely best way to increase an aircraft's Clmax for landing): On a tractor the added lift due to flaps is close to the CG, i.e. easy to trim out with the tails long arm of moment. While on a canard, the added lift is far aft of the CG and the canard surface has a shorter moment of arm and is already heavily loaded as outlined for the VariEze above.

Added to this comes the canard's horrific properties when the CG is a bit further to the rear than it should be (unrecoverable deep-stall anyone?), and problems with the canard's lift under icing conditions.

In summary, I think a McDonnel Douglas engineer said it best when a journalist asked him about the best place to put a canard when he was asked this question sometime back in the seventies:

"That would be on somebody else's airplane!"

 

[Nicknick]

In the end, Canards, just like flying wings need to have more lift capability in their wings for landing, than what it otherwise would have been needed for cruise, due to the problems of installing flaps. A logic consequence is, that these airplanes are well suited for flying at high altitude with low induced drag. Unfortunately, flying wings are not ideal for pressurization because they don't have a round fuselage. Canards are better suited for pressurisation, but in case of the Cozy, the low frontal area was realized by being a pusher plane with an oval fuselage, also not ideal for pressurisation (but surly doable).

There has been one attempt to build a canard aircraft for this in my view ideal application, the failed “Raptor” aircraft (there was quite a lot of attention on Youtube for it). Unfortunately its builder had no clue of thermodynamics/turbocharging as well as mechanical engineering was to stubbing to react to any of the advices (none from me) which were given to him in the comment section. The two stage turbocharging with a way to small low pressure turbo (about the same size as the high pressure turbo) was robbing the power of the engine and the final belt drive with its airframe fixed propeller shaft and engine fixed drive shaft collapsed so he crash landed in a field.

Despite the unfortunate end, the idea of using two stage turbocharged Diesel engine which would (if done right) be well suited for very high altitudes and efficient pressurisation (due to the two stages compression) in combination with a canard plane was a very promising approach. If done right (easier said than done) it would have resulted in a fast and very efficient plane.

 

[HoHun]

Hi,

Rutan's varieze/longeze great performance advantage over conventional two seat GA aircraft comes from three factors: much smaller crossection thanks to cramped tandem seating, extremely smooth surfaces if built right thanks to no protruding rivets and few surface gaps or other excrescences and gross weights 50-75% of what a more common GA aircraft.

Good post! My question would be, has anyone ever built a comparable aircraft with conventional layout that delivered similar performance to the Varieze/Longeze? I don't know much about homebuilt aircraft ...

Regards,

Henning (HoHun)

 

[Scott Kenny]

Good post! My question would be, has anyone ever built a comparable aircraft with conventional layout that delivered similar performance to the Varieze/Longeze? I don't know much about homebuilt aircraft ...

My instructor with the EZ had a rivalry with a guy with a Lancair 2-seater with an O235 (so a bit more horsepower than the O200 in the EZ). The EZ needed to do some aerodynamic tricks to make up for the reduced horsepower. Dogtooth outer leading edge extensions, adding lower winglets in addition to the usual upper winglets. Had the USAF vets that had some composite experience do the work for him as extra credit, IIRC.

 

[AndersJ]

According to the OP this thread is supposed to be about civilian applications, but since both the Viggen and Gripen were mentioned earlier, and that Rutan was supposedly inspired by the Viggen when designing his civilian canards, and since many of the objection for military application applies for civilian as well, here is some input on canards from a Swede, now retired, but who got his paycheck working as an engineer on these planes for almost 15 years.

First of all, all Swedes (like me for example) are not in love with canards: In fact, Sweden’s top aerodynamicist at the time the Gripen was in the making, professor Sven-Olof Ridder of the RIT, tried to talk SAAB out of doing yet another canard after the Viggen. He published several papers on the subject, and was vociferously critical of SAAB’s decision. Not in the least after the two crashed prototypes, both caused by instability and FBW issues. His reports and papers showed that while a conventional configuration much like the F-16 is optimal at around neutral to slightly negative stability, to get optimal performance out of the Gripen’s configuration, it would have to be about 30% MAC unstable, which is a lot.

But how unstable the Gripen actually is today is AFAIK still not official, but SAAB’s chief aerodynamicist at the time, Ulf Clareaus, did a paper for the AIAA back in 1991 where he himself says that they were forced to settle for circa 15% MAC instability IIRC. This is due to limitations in the FBW system and concerns for battle damage etc. In addition, higher instability means the need for higher servo speeds on the canards, and force times speed equals power. And servo weight is basically proportional to power, so there is a limit there as well. The AIAA paper itself does not bring forward much in terms of arguments for the canard, but rather mentions how SAAB has optimized the canard design itself. IIRC in the comparison done to the other contemplated configurations for the Gripen, the only thing the canard can show as an advantage over the others is slightly lower transonic and supersonic wave drag due to the Sears-Hack area ruling being easier to end in a nice way if you don’t have a tailplane. But that advantage is very slim and in the order of a few percent, and hardly makes up for all the problem a canard causes in other area such as a low trimmable Clmax, the need for a low aspect ratio for vortex stability (means poor sustained turn performance), the need for a high degree of instability to avoid excessive trim drag. And last but not least, the canard configuration’s problems to handle trailing edge flap deflections on the main wing due to the long arm of moment for these, versus the canard surface’s.

Finally, it’s a myth that the Swedish basing system utilizing public roads for dispersion and the resulting short take-off and landing requirement more or less forced SAAB to adopt the canard configuration. You can accomplish the same with a conventional configuration which Ridder did in his F-16 like Gripen alternative which you can find on my homepage under this tab.

And with regards to the Viggen: While it has decent instantaneous turn performance, the sustained turn performance is abysmal: In fact, Chuck Yeager, who on a visit to Sweden flew it and while certainly expressing praise with regards to many of its systems, commented that he had never flown a plane the slowed down so much when you tried to turn with it.

 

[Nicknick]

I dont believe, that Burt Rutan took any inspiration from an instable military airplane for his civil canards. He designed them all to be stable, otherwise they would have been completely unsuited for homebuilt airplanes!

Without the need for stability, canards could be much more efficient (as well as conventional designs), because all the limitations mentioned before would irrelevant. An unstable Canard aircraft means, you don't need the higher wing loading on the canard and prevent a deep stall by electronic controls.

 

[Scott Kenny]

I dont believe, that Burt Rutan took any inspiration from an instable military airplane for his civil canards. He designed them all to be stable, otherwise they would have been completely unsuited for homebuilt airplanes!

Without the need for stability, canards could be much more efficient (as well as conventional designs), because all the limitations mentioned before would irrelevant. An unstable Canard aircraft means, you don't need the higher wing loading on the canard and prevent a deep stall by electronic controls.

IIRC, the first Rutan canard design was the Vari Viggen, a wooden airframe instead of the EZ series fiberglass-and-foam.

 

[AndersJ]

I dont believe, that Burt Rutan took any inspiration from an instable military airplane for his civil canards. He designed them all to be stable, otherwise they would have been completely unsuited for homebuilt airplanes!

The Viggen is not unstable. In fact it's very much stable as in the canard surface being heavily loaded.

Without the need for stability, canards could be much more efficient (as well as conventional designs), because all the limitations mentioned before would irrelevant. An unstable Canard aircraft means, you don't need the higher wing loading on the canard and prevent a deep stall by electronic controls.

And this was exactly what I wrote: In order to provide decent performance, a canard needs to be very unstable. The Gripen should for optimal performance be about 30% MAC unstable but it's only in the order of 10-15% MAC unstable.

Then about deep stall: This is still a problem for a canard even with FBW because on swept wings the tips stall before the inner parts of the wing. And since the tips are well aft of the aerodynamic center (AC) at normal AoA, a loss of lift on the tips at high AoA leads to a forward shift of the AC and a pitch up.

You can see this in the second Gripen crash: The FBW tries to get the nose down by unloading the canard and applying full downward deflection of the tailerons but it's still not enough since the tips are stalled.

 

[AndersJ]

I should add that the FBW software version that was running in the Gripen at the time of the second crash had known limitations, but it was given an airworthyness clearence for a public display anyway.

But with the benefit of hindsight, this proved a grave mistake given that the crash was caused by an unfiltered overlay of both the pilot's and the FBW's separate signals that due to a pitch down disturbance just prior to the accident resulted in both the pilot and FBW commanding a pitch up independently. Unfortunately this led to a too high pitch acceleration, and when the FBW system became aware of this and commanded full down on both canard and tailerons, this pitch rate was already at such a magnitude it was unstoppable and led to the violent pitch up.

The FBW SW running now is of course much more mature and would not allow this. However, in rare cases like an extreme wind shear etc. bad thing like this could still happen, but would of course be rather unlikely. But it could still occur, and it is in fact said that a Gripen from an operational wing almost crashed due to passing through the vortex wake of another during ACM training.