dgliderguy

Okay, all you aero junkies:

When I was a kid, Burt Rutan was everybody's hero, and the canard planform was the answer to everyone's quest for maximum efficiency. You know the argument, the canard up front has a lifting force in the upward direction, and the wing also has a lifting force in the upward direction, where a conventional stabilizer back at the tail must endure an efficiency-consuming download to counter the lift/weight couple. Canards resist the stall because the forward 'wing' stalls before the main wing can stall, the downwash from the forward 'wing' induces just the right downward flow for the upward swing into the leading edge of the main wing, etc., etc.

So, WHERE ARE ALL THE CANARDS???? Why do all the World Class sailplanes with a glide ratio of close to 60 to 1 have conventional tails? Why do all the pylon racers at Reno have a conventional tail? Why do all the corporate jets and fighter planes and RPVs and bush planes and flying boats and puddlejumpers and just-about-everything-else all have conventional tails?

The Rutan Vari-Eze was supposed to be the shape of things to come. In the years since, the Beech Starship has fallen into ignonimity and the Solitaire is a relic of the fanciful notions of the past. WHERE ARE ALL THE CANARDS???

Don't even get me started on flying wings!

Ollie

The fore plane produces downwash over the center of the aft wing and a vortex in the middle of each aft wing panel. This disturbed flow over the aft wing is worst at slow speeds below the best L/D, where sailplanes spend their time in thermals. At crusing speed and above, this is less of a problem and can be partly corrected by washout from the foreplane vortex to the tip of the aft wing. Hence the Voyager's ability to circle the earth without refueling. The amount of washout compensation varies with airspeed ( angle of downwash). This makes it extremely difficult to achieve efficient aft wing lift distribution (low induced drag) over a wide speed range.

Canards must have both the high speed and low speed lift characteristics of the foreplane and aft plane coordinated to avoid potentially disasterous handling problems. The fore wing must stall before the aft wing for favorable stall handling. The aft wing must go through zero lift before the fore wing to avoid violent, uncontrolled pitch down with negative G maneuvers or certain gust conditions.

A genius like Rutan can accept the risks and overcome them, unlike the design team of a large aerospace corperation.

greg-RCU

Would the downwash problem not be simply solved by placing the main wing higher than the cannard? Though I'm sure there would be other engineering issues.

Ollie

The downwash is a fairly deep flow that extends well above the wing as well as below it. There is still apreciabl down wash at a distance of two chord lengths above the fore wing. Putting the aft wing above most of the downwash would result in a configuration that was desidedly "top heavy" and had landing gear and ground handling issues. The parasitic drag (frontal area and wetted surface) of the fuselage to do it would not be a favorable trade with the benefits of avoiding the foreplane downwash.

However, Rutan's Dragon fly is somewhat in the direction you suggest. It is a tandem wing configuration which avoids some of the disadvantages by using equal spans fore and aft, putting the aftwing up some, where the downwash is not quite as strong and mounting the landing gear on the tips of the foreplane (trading the parasitic drag of the landing gear for the parasitic drag of the fuselage). The fore wing has a lot of negative dihedral to provide prop clearance. There is nothing else quite like it.

greg-RCU

Cool. Sort of like a quickie??
Love to see a pic of it.

Ollie

the Dragonfly is the two place version of the Quickie.