2
$\begingroup$

I originally asked a variant of this on Worldbuilding.SE, but they didn't seem to like it. So I apologize if it's out of place even here.

I ask, because I've been forming in my head an alternate history or two that would logically involve a completely fresh start to aviation; no Wright Brothers, no Otto Lilienthal -- the entire world looks slightly different on a political map.

Because the world is different, it would be lazy of me to just import elements of our timeline without understanding why things developed the way they did. Our world, in general, appears to have rolled with canard planes and later adopted the tailplane for just about every conceivable function for fixed-wing aircraft.

That alone seems to imply that the tailplane is the superior choice to any other design, but is it really? Or was it just a case of technological and economic inertia?

Since 'superior' can be subject to opinion, I'll try to narrow down some criteria:

  1. The design has comparable advantages in stability, lift production and maneuvering.
  2. The design has comparable economic advantages; it doesn't necessarily require more time and resources to build and maintain.
  3. The design does not (necessarily) incorporate concepts that require an advanced and well-developed understanding of fixed-wing aerodynamics. These would be among the early aircraft designs, or at least a short time into successful flight, so things like vortilons, fly-by-wire, etc. would not be present.

There are definitely plenty of wacky creations like Burt Rutan's Quickie, the V-173 and Miles M.39, but I'm sure there are good reasons none of them ever became popular...right?

$\endgroup$
5
$\begingroup$

If you write an optimizer which modifies the wing spans of a virtual aircraft, you will end up with a big wing in the middle and a smaller one at the tail if your goal was to achieve good performance, a wide cg range and docile behavior. So yes, a concurrent evolution would also result in a conventional layout.

Why no canard?

  • The forward wing is more highly loaded for stability. Adding control surfaces there would reduce the possible control effectiveness. Putting them at the large rear wing will result in higher stick forces and higher lift changes for the same pitch moment changes.
  • For the same reason, the wing cannot utilize its full lift potential because the canard has to stall first.
  • Since the wingspan of the canard is smaller than that of the wing, the wake of the canard will hit the wing and disturb the lift distribution over span there.
  • For directional stability a tail is still needed, even though it is only used for vertical surfaces. Alternatively, the canard wing can be swept and winglets be used for directional stability, but adding sweep will reduce the efficiency of the wing.

This all puts the canard at a distinct disadvantage. It works, but comes at a price.

Why no flying wing?

  • Flying wings have low pitch damping and a very limited center of gravity range.
  • They cannot have trailing edge flaps for extra lift during slow flight, so their take-off and landing speeds are higher than those of a comparable tailed aircraft. Or the wing loading is higher, which hurts at high speed.
  • Even though a good flying wing design has sweptback wings, they are poorly suited to high transsonic speeds. Their wing airfoils cannot have rear loading, so the critical Mach number at the same lift coefficient is much lower than for conventional configurations.
$\endgroup$
  • $\begingroup$ The SWIFT (Swept Wing Inboard Flap Trim) ultralight glider did use a flap. I wonder why the concept wasn't applied to other flying wing designs? $\endgroup$ – John K Feb 21 at 23:27
  • $\begingroup$ @JohnK: A small inboard flap is possible with enough aspect ratio and sweep, but its effectiveness is rather limited, especially when compared to the possibilities of the conventional configuration. $\endgroup$ – Peter Kämpf Feb 22 at 5:11
4
$\begingroup$

The canard layout has serious limitations that the down-lifting tail does not. The biggest one is getting the desirable pitch-vs-speed response to give good pitch stability, with the proper stick-free speed-seeking characteristics, while having adequate low speed authority.

With the tail at the back in the airplane-as-a-seesaw configuration, lifting down, it's easy to get the proper responses (speed up, pitch up, slow down, pitch down, etc.).

With a lifting surface at the front, (airplane-as-a-table-configuration) the required pitch response to speed has be obtained by using a canard airfoil with a steeper lift slope than the main wing, so when you speed up, the airplane pitches up, and vice versa. With a regular tail, the airfoil can be a sheet of plywood and it still works fine.

The Rutan designs early on used a canard airfoil developed by the University of Glasgow that had the required lift slope characteristic. Unfortunately this airfoil was very sensitive to laminar disturbance and flying in rain could have a huge effect on trim (they would pitch down in rain, not out of control, but enough to be a problem). A bandaid solution for this was adding Vortex Generators to the canard. Later a new airfoil was developed that didn't have the rain sensitivity.

Almost all of the canard's theoretical benefits were negated in the real world, which is the real reason the configuration is rare. It's not some anti-canard conspiracy; they just don't work as well in the balance of compromises that makes an airplane.

Yes they can't stall/spin, but you can make a normal airplane do that as well (Ercoupe). The VariEZe/Long-EZ have high take off and landing speeds and yes you can't spin it, but if you put it down after an engine failure you are probably going to get hurt anyway.

Rutan developed a sailplane called the Solitaire that used a canard surface. You'd think that would optimize the canard advantages to make the perfect sailplane. It was unsuccessful because it developed high sink rates at thermaling speed (you are normally turning just above the stall, at min sink speed). He's a brilliant guy, but all Rutan's designs are homebuilts or specialty aircraft where the limitations can be lived with.

What about mass production? Well, you have the Beech Starship. A catastrophe for Beechcraft, nearly bankrupting them. The only place you see canards in the production world are as supplemental surfaces to the primary stabilizing surface, the horizontal tail.

The Wrights put a lifting surface at the front because it seemed like the logical thing to do at the time. The surface moved to the back pretty quickly as airplanes progressed.

$\endgroup$
1
$\begingroup$

As far as the rear mounted horizontal stabilizer, nature's inexorable trail and error over unlimited time has yielded the answer. Canards do work very well with deltas, but deltas are not nearly as efficient as straight wings in the all important lift/drag analysis. Birds have over 100 million years of history. Theirs are behind the wing.

But for human flying objects, arrows have been around much longer and indeed form part of the understanding of flight. Archers are able to maintain considerable hitting power at long range by setting the CG forward so the arrow pitches down to maintain speed, just like an airplane. The rear mounted tail plane also helps to pitch the nose down as the plane sinks. Both these factors help regain airspeed and proper Angle of Attack in the event of a stall.

Vertical stabilizers have more possibilities and can be mounted on the wing tips as well as on the rear of the fuselage. But with active onboard trim computers, we become more like birds, finely controlling our aerodynamic surfaces. This will help eliminate the need for ponderous, drag producing stabilizers and lead to greater fuel efficiency.

Planes that "eat like a bird".

$\endgroup$
0
$\begingroup$

Different applications have different optima, in aviation as in most other things. If you're maximizing one combination of overall performance, pilot forward vision (in the nose-up landing and taxi attitude of a conventional gear design, which was nearly all there was before 1930), and cost/weight of construction, it's pretty easy to see you get pretty much the designs we had then, which lead to the designs we have now.

That said, if something had been invented earlier or later than it was, it might have changed the end result fifty or seventy-five years down the line. If tricycle gear had become popular before aircraft carriers won the Pacific theater of naval combat, we might have seen designs in service by 1945 that were abandoned in our history -- aircraft like the Curtis XP-55 Ascender, for instance, or the broadly similar German and Japanese designs that never saw service, despite promise of better performance and flight characteristics than the tractor engine, conventional layout designs that went before them.

Other factors might as readily have resulted in tailless aircraft coming to the fore (like the Cutlass jet fighter of the 1950s, or YB-49 bomber) -- either those or canards can be made stable enough and give enough performance for a given task, in general, but the industry has so much more experience with conversational designs that designers and customers alike are more comfortable with the elevator in the rear and propeller in front, and never mind how well Rutan's odd-looking designs fly.

$\endgroup$
0
$\begingroup$

That alone seems to imply that the tailplane is the superior choice to any other design, but is it really? Or was it just a case of technological and economic inertia?

A bit of both. And beside the technological features discused in other answers, one should not forget about history here. The tail plane/front engine has its disadvantages, but at the same time an incredible development advantage due WW2. Standard setup planes allow very agile configurations, something fighters depend on. And while many other configurations have been tried during the war, most attention was given to standard ones. As a result knowledge for their construction was widespread after the war and used for civil developments.

It's a bit like the question why the once leading electric car was put out of business by petrol cars - again, WW1 played the major role. While loading batteries isn't a hassle in a developed civil environment, it becomes almost impossible under war conditions - here petrol holds an incredible logistics advantage as moving a few barrels of petrol is easy done, in contrast with setting up power generators to load batteries.

It's not always the war, as economic scaling works as well - like why our modern computers almost all reassemble at the core simplified mini computer constructions - and huge efforts are now added to 'de-simplify' them again.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.