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What Are Canards, And Why Don't More Aircraft Have Them?

primary-eurofighter Peter Gronemann

You've probably seen a canard - the small, forward wing you see on a Eurofighter Typhoon or the Rutan Long-EZ. A canard can serve two purposes; it can improve aircraft control, which you often see on combat aircraft. It also can contribute to lift, replacing the horizontal stabilizer and - theoretically - reducing overall drag.

How does a lifting canard reduce drag? It decreases the total amount of lift that your aircraft needs to produce, which decreases drag. However, it does this at a cost - a canard can be destabilizing and decrease stall recoverability.

Increasing Performance By Decreasing Lift

Most aircraft use a horizontal stabilizer on the tail to maintain stability, like on a Cessna 172. With the center of gravity ahead of the wing's center of lift, the aircraft wants to pitch nose-down. However, the horizontal stabilizer on the tail acts as a mini-wing, generating lift downwards (called tail down force) and pitching the nose back up.

172-lift

There's a downside here - the wing needs to generate enough lift to oppose both the weight and the stabilizer's tail down force. So, if the aircraft weighs 2000 lbs. and the tail generates 250 lbs. of tail down force, your wing needs to generate 2250 lbs. of lift. That extra 250 lbs. of lift creates more drag - decreasing performance and efficiency.

The canard is essentially moves your horizontal tail up to your nose, and places the wing's center of lift behind the center of gravity. To balance the natural nose down tendency, the canard generates an upward lifting force - which helps oppose weight.

long-ez-lift

If your aircraft weighs 1300 lbs., and the canard generates 250 lbs. of lift to balance the aircraft, the wing only needs to generate 1050 lbs. of lift. The Cessna has to generate 2500 total pounds of lift to fly (2250 lbs. from the wing, and 250 lbs. from the tail) - which is 500 lbs. more than the total weight, as compared to the 1300 lbs. of lift needed to fly the 1300 lbs. Long-EZ. The Long-EZ should have less drag, which means more performance.

rutan-2 Martin Wippel

It's Not That Simple - Stability And Stalls Get In The Way

If canards were that simple, every aircraft would use them. However, aerodynamics are never that simple. Things get complicated in a stall.

The Canard Must Stall Before The Wing

On a Cessna 172, if the wing stalls before the tail, you'll still have elevator controllability to pitch down. If the tail stalls before the wing, the aircraft will naturally pitch down. In either case, stall recovery is natural.

However, if your aircraft has a canard instead of a tail-mounted horizontal stabilizer, you're in real trouble if the wing stalls first. In this case, the center of gravity would drop the wing and tail, pitching the nose up. The aircraft now enters a deeper stall and becomes unrecoverable.

animation-wing-first-canard-stall

So, how do you solve this problem? You need to make sure that the wing is always further away from the critical angle of attack than the canard. You can accomplish this by using a larger wing on the aircraft. This ensures that the wing never gets close to it's critical angle of attack before the canard stalls. However, this larger wing adds weight and drag, reducing the design's efficiency

Stability

Canards can also make an airplane unstable. Simply put, if a wind gust briefly increases the angle of attack on a Cessna 172, the aircraft tends to pitch nose down and return to it's original attitude. In the Cessna's case, the increased angle of attack increases the wing's lift. However, it actually decreases the tail down force, because it decreases the horizontal stabilizer's angle of attack.

172-stability

However, the canard can actually make your aircraft pitch up further. The increase in angle of attack causes both the canard and the wing to generate more lift. If the canard's increase in lift is greater than the wing's, the nose will pitch further up.

long-ez-stability

To solve this problem, designers use high wing loading on the canard. This means that the canard generates more lift per square foot than the wing. At high wing loading, an increase in angle of attack causes a smaller increase in lift than at low wing loading.

But here's the downside - high wing loading generates more induced drag. To counter this, designers often use a high aspect ratio canard - which means it's long and narrow. That decreases the drag, but makes a large canard hard to build.

rutan-3 Martainn MacDhomhnaill

The Tail - Where Is It?

Since the wing's farther back, you'd probably guess that you can eliminate the aft fuselage, which decreases weight and drag, right? Not quite - you still need a vertical stabilizer, which needs to be set back from the center of gravity.

Either you still need a long fuselage (which adds back the weight and drag), or you can sweep the wings and put the vertical fins on the wingtips. Burt Rutan does this with his VeriEze and the Long-EZ designs. Again, there's a drawback - swept wings decrease slow-speed performance as we explain in this article.

rutan-1 PARSHOTAM LAL TANDON

Nothing's Ever Perfect

A canard looks great on paper; but as you can see, it adds a lot of design complexity to an aircraft. Rutan's mastered the concept on the VeriEze and the Long-EZ series of aircraft; but for many designs, the complexity outweighs the benefit. One thing that's absolute, though, is the look. A canard equipped aircraft is striking - and generates a lot of attention on the ramp...

eurofighter-1 elanaiba

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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