canard efficiency

[Cliff Cordy]

R. T. Jones, at the end of his 1988 book "Modern Subsonic Aerodynamics", proved that under most conditions a conventional design is more efficient than a canard design, and a tandem wing design (as in a Quickie) is always much less efficient than either. To derive this result, he assumes a feedforward interaction between the aft wing and the forward wing that I don't understand. I have no formal training in aerodynamics, and not a whole lot of airplane experience. I can't make an intelligent comment about this. I certainly can't dispute a statement by R. T. Jones. Since this is a canard site, I'm hoping someone can shed some light on this subject at a level I can understand. Since this is a canard site, I'm hoping someone will say "Jones assumptions are not valid if you design the plane like this ----. I have degrees in math and physics, and a doctorate in electrical engineering. I'm not afraid of complexity. I just don't know much aerodynamic theory.

Thanks, Cliff

[Phil Kriley]

I'm no engineer, but if you compare all 180 hp 4-place planes that exist, I'll bet that you will find that the fastest and most fuel-efficient are canard designs. If there is a faster, more fuel-efficient design than the Cozy (and sisters) then please tell me what it is, and post the performance specs.

 

As far as I'm concerned, that kinda shoots his theory.

[Cliff Cordy]

It is really hard to sort the effects of canard vs tail from good aerodynamics vs bad aerodynamics. Canard planes tend to have aerodynamic designs and their builders tend to be careful to do it right. If you look at the best of both cases, you find Klaus Sauvier flying 250 mph in his Vari-EZE powered with about 130 horses and Tracy Saylor flying his much bigger (more surface friction) aluminum (less smooth) RV-6 250 mph with a stock 180 horse engine. If you compare drag per unit surface area, it is pretty similar.

Yesterday I noticed a reply to some other thread that Rutan himself finally agreed that the canard was not the most efficient. I've never seen that before. Rutan is an aerodynamics genius. If he changed his mind about canards after 30 years, there may not be a simple explanation of the effects that R. T, Jones was writing about. It also may mean that we are all chasing our tails.

 

Cliff

[Spodman]

Haven't read your boffin's book so will not comment. Burt has been quoted that he may have been mistaken about the relative efficiency of canard designs, but there are other factors you can read about here:

 

http://www.cozybuilders.org/Oshkosh_Presentations/Nats_OSH2005_Presentation.pdf

 

The Cozy attraction for me is you get a funky looking aircraft that is efficient to fly, regardless of the potential of comparable efficiency from alternate configurations. There are no 4 seat competitors that don't require a kit. It doesn't stall (not really anyhow) and doesn't spin.

 

I did read an article once about some other boffins who were going to prove Burt wrong by moving the location of the canard around to minimise interaction with the wing but didn't actually show any benefit.

[Dick74]

I have a number of points for consideration: (assume steady state)

1. Everything written in a book is not necessarilly true.

2. Conventional wisdom can be wrong, e.g. Why an airfoil lifts.

3. Efficiency is job accomplished over effort expended.

4. Flying is lifting and propelling an aircraft.

5. Factors are Weight, Speed, Drag, and Power.

6. For comparison, Weight, and Power for each design would be equal.

7. Therefore, the fastest design would have the least Drag; is most efficient.

8. In general, Drag is proportional to Lift.

9. Conventional aircraft: Lift = wing lift - tail lift.

10. Canard aircraft: Lift = wing lift + canard lift.

11. Since canard lifts add, less area is needed.

12. Less area means less drag which = more efficiency.

13. Why do geese fly in a V formation?

14. The downward wing stroke produces a pressure bubble.

15. The bird following is lifted by the pressure bubble, e.g. ground effect.

16. Less effort is required to maintain speed and altitude.

17. I think tandem wings can be beneficial.

[Cliff Cordy]

8. In general, Drag is proportional to Lift.

Not true. In a fast plane (flying several times its takeoff speed) lift induced drag is trivial. Most of the drag is friction drag from airflow over the airframe. Much of the rest is engine cooling drag.

 

10. Canard aircraft: Lift = wing lift + canard lift.

11. Since canard lifts add, less area is needed.

12. Less area means less drag which = more efficiency.

This is the same argument I've been using for 25 years. BUT, it ignores stability. I understand stability only in the most general terms. Stability involves center of lift, CG, and this feedforward effect between the wing and canard, which I don't understand at all. The Q2 canard carries 3/4 of the total weight of the plane, but it is only slightly bigger than the wing. Thus, the takeoff speed is high (75-80 mph) and most of that wing is not doing anything but causing drag and preventing stalls. Until I read Jones, I assumed this was true because they couldn't figure out how to get the CG further back with the front engine. Now I suspect it is necessary to make the plane stable, which my Q2 isn't quite (because I always fly it with CG near aft limit in order to get onto the runway with the tailwheel at least somewhere near the ground). There may not be a simple explanation to this, but if there is, I'd sure like to see it.

 

Cliff

[rpellicciotti]

According to the CAFE Foundation, a canard airplane is the world's most efficient airplane:

 

http://cafefoundation.org/v2/pdf2/WMEA.pdf

[Marc Zeitlin]

According to the CAFE Foundation, a canard airplane is the world's most efficient airplane:

 

http://cafefoundation.org/v2/pdf2/WMEA.pdf

Ummm, no, according to CAFE, a canard airplane is the most efficient aircraft they had tested up to that point. There's a LARGE difference. The worlds most efficient powered aircraft at this point in time (late 2006) is the Global Flyer, which is a conventional aircraft. See:

 

http://www.virginatlanticglobalflyer.com/

 

Of course, comparing aircraft with different engines, different load capacities, etc. is apples to oranges. If you want a comparison of canard to conventional, you need to design them to have exactly the same power and weight. No one has yet done this, so all the arguments are theoretical.

 

However, to the O.P.'s question, do a google search on "canard" and "David Lednicer" - he's written some reasonably comprehensible explanations over the years as to why canard aircraft are not quite as efficient as conventional or 3-surface aircraft.

 

Basically, it has to do with the fact that you want the lift distribution of the aircraft as a WHOLE to be elliptical (NOT just the lift distribution of each wing individually), and the downwash from the canard on the main wing makes achieving this VERY difficult in a canard aircraft - you end up with a lot of lifting surface that isn't doing much. I may have some emails from David at home in which he goes into more detail about this - I'll try to remember to look for them this evening after I vote.

[Marc Zeitlin]

I may have some emails from David at home in which he goes into more detail about this - I'll try to remember to look for them this evening after I vote.

Here's what David said in response to a question regarding elliptical lift distributions:

 

"Actually, the overall elliptical sum comes from Trefftz, who theorized that the induced drag could be measured best in the far field. To do this, he used a survey plane a long way back from the vehicle. At this survey plane, nothing is known about what produced the wakes. What matters is the distribution of downwash at this location. Typically, the Trefftz stuff is taught for a single wing, but if you have more than one, it still works the same."

 

A Google search for:

 

Trefftz overall elliptical sum canard

 

turns up some useful links, the most useful of which would be an AIAA paper that you'd have to pay for to get, but it's title is:

 

"A fundamental comparison of canard and conventional configurations"

 

by Tad McGeer and Ilan Kroo.

 

Anyway, this should point you in the right direction.

[Ron Springer]

A Google search for:

 

Trefftz overall elliptical sum canard

 

turns up some useful links, the most useful of which would be an AIAA paper that you'd have to pay for to get, but it's title is:

 

"A fundamental comparison of canard and conventional configurations"

 

by Tad McGeer and Ilan Kroo.

 

That paper would be a good read. AIAA sells it for $25. Another option is to get a copy at the library since it was printed in the 1983 Journal of Aircraft. Just need to find a library that carries it. It am almost certain it can be found at the NASA Langley library, so I'll volunteer Wayne Hicks for the task!

[Richard Riley]

"A fundamental comparison of canard and conventional configurations"

 

by Tad McGeer and Ilan Kroo.

 

 

You also might look up the "C-wing" configuration by Kroo. It would unload the canard in cruise. It would be a radical change for an EZ that should only be done with the intense involvement of highly trained people and a lot of testing, but it's something I've thought about.

[Tom Nalevanko]

Cliff,

All wings have a circular regeneration and thus each wing affects the other and vice-versa. The best book that I can suggest is "Introduction to Aerodynamics" by Gale M. Craig (available from Amazon). This book explains aero in terms of Newton's laws and goes into adequate detail. It is a great book for understand aero basics.

Blue skies,

Tom

[brinesharks]

Can you define "efficient"? Payload, range, speed, etc?

 

Here's a quote from "Aircraft Flight" (Barnard & Philpott , 1995)*:

 

Since both surfaces on a canard produce positive lift, the overall area, total weight and drag can be lower than for the conventional arrangement.

 

They go on to discuss the arguments for and against the view that canards are 'unstallable'. Also:

 

The main problems with the canard configuration stem from interference effects between the fore-plane wake and the main wing. In particular, the down wash from the fore-plane tilts the main wing resultant force vector backwards, thus increasing drag. By careful design however, the advantages can be made to outweigh the disadvantages and highly successful designs by Burt Rutan such as the Vari-eze....provoked renewed interest in the concept.

 

So, it comes back to efficiency for a given mission. The canard design is also "efficient" for fighter aircraft that need to do tight manoeuvring as the canard produces immediate lift (unlike a tailplane which usually produces a down force).

 

An interesting thing to throw into the mix is that a lot of the canards discussed on this site have pusher engines. D Raymer in his book "Aircraft Design a Conceptual Approach" highlights that having the propeller too close to the trailing edge of the wing will significantly reduce it's efficiency and therefore overall performance will suffer and vibration problems may occur.

 

*BTW this book is a great introduction to aerodynamics and stability - blows away a lot of myths about aircraft flight.

[rlapelle]

Gentlemen,

You all need to accept that any aircraft design is a series of compromises. The question of efficiency is dependent on attitude and airspeed of the aircraft in the airstream. The value changes as attitude and airspeed changes because the airstreams change their positions as the conditions of flight change. There is not any right or worng answer that always applys. A good article on the canard vs wing air flows (60+ pages) can be found at:(http://www.mecheng.adelaide.edu.au/~marjom01/Aeronautical%20Engineering%20Projects/2006/group14.pdf). I suggest you take a look at it for your answers.

[Marc Zeitlin]

A good article on the canard vs wing air flows (60+ pages) can be found at:(http://www.mecheng.adelaide.edu.au/~marjom01/Aeronautical%20Engineering%20Projects/2006/group14.pdf). I suggest you take a look at it for your answers.

You realize, of course, that you're replying to a 2 year old thread?

 

Also, having perused this article, it's a mishmash of truths, partial truths, and complete nonsense (undergraduate projects sometimes are). The largest complete nonsense part is their claim that canard aircraft are inherently unstable and need automatic control systems to maintain pitch stability. They've either got a sign error in their equations or an inherent misunderstanding of what's going on. There are thousands of VE's, LE's, COZY's and Velocities, etc. flying that are pitch stable and require no "automatic control systems" to maintain it.

 

There is no difference in the equations for canard or conventional two surface aircraft - as long as the CG is ahead of the AC, and the Cl of the front wing is lower than the Cl of the rear wing, the aircraft will be statically stable.

 

This is a pretty large error, even for a bunch of undergrads.

 

Plus, their "history" of Burt Rutan's aircraft development is incomplete at best.

[raiki]

This quote from that report shows how much this group of students really know about aircraft and aviation "Since the Wright Brothers’ first flight in 1904".

 

Pretty sure that was 1903.

[Marc Zeitlin]

This quote from that report shows how much this group of students really know about aircraft and aviation "Since the Wright Brothers’ first flight in 1904".

 

Pretty sure that was 1903.

It was the other side of the international date line.

[Niels Nielsen]

Hi Cliff, Niels here in Corvallis. Are you still in Thailand? Is your plane over there? When you next are in Corvallis, come see the car I built out of the hull of a crashed VariEze. For my next ground car conversion project I want to start with a non-airworthy q2 hull so I can carry two people and not wind up with an excessive wheelbase length. Please contact me at niels.nielsen@imtech-or.com sometime especially if you know where I can get a derelict quickie or q2!

 

-Niels, still crazy after all these years