Canard-wing configuration? 10

[Slice]

I'm contemplating the use of a canard wing configuration for a high-lift, low speed model aircraft with the prime motive of maximum payload capacity.

I've heard two arguments about the canard wing configuration for this sort of purpose:

1. A canard config. will allow the canard wing to act as a stabilizer for the aircraft while adding to the net lift of the aircraft (vs. standard configuration stabilizers which work using negative lift)

2. for a canard wing configuration, the canard will stall first at a certain aircraft pitch angle; therefore, the maximum lift out of the main wing will not be attained because the canard wing reaches Clmax at its stall, then the aircraft pitches down again. (whereas in a tail-last configuration, the main wing stalls first allowing Clmax for the main wing to be achieved)

i think the first argument sounds fishy, but i just want to know if anybody has any key points to add. i'm a bit confused as to the usage of a canard wing configuration for maximum lift purposes.

 

[langford]

Gidday Slice,

I think the canard idea is a good one in terms of being an efficient lifting body as a whole but, depending on what you're considering to design (?), if you're considering a freighter, then you may be limited by the climb out angle (since they are known to fly into some fairly "plane unfriendly" strips), since you have to achieve a large amount of lift on the fwd wing in order to rotate. Some configurations couple a fwd wing with a aft horizontal tail plane to reduce the amount of sink while adding to rotation characteristics. But then there's downsides to that like and increase in both profile and lift induced drags.
I don't know about the pitching down at stall but I have flown in a small canard type aircraft and the stall characteristic is more of a "sit down" where the attitude doesn't really change too much but it just wafts down (for want of a better word). Anyway I hope this helps a little.

Wade.

 

[buzz41]

Slice -
Your number (2.) explanation will be supported by most data, I believe. A study of canard, conventional and 3 surface arrangements of horizontal surfaces I read recently had the conventional arrangement a little ahead of the others. ( Stanford aero dept, maybe )

All configurations can be OK, if need dictates. Nearly all guided missiles, for example, are canard arrangements - most practical for that purpose. If you are given a long slender piece of munitions with the CG near the middle to make an airplane from by adding surfaces to it, a canard is the likely outcome. But, if there is no particular reason for a canard, etc., a conventional layout will usually have at least a small advantage.

For your application, been there, done that. Use a conventional layout. At model plane Reynolds numbers, don't have the smallest surface working the hardest, as will be the case with a canard. All else equal, a bad idea.

 

[Rollerblades]

As Wade Langford mentionned, the canard are not used to increase lift, but they are an aircraft configuration for Delta wing airplane (whitout horizontal tails to maneuver the plane) by placing canards in front of the airplane.

 

[radomir]

Canard does increas lift when needed - during a climbing (e.g.: at take off phase when it is really crucial). The standard configuration "horizontal rudder" in fact works against plain logic: negative lift (reducing aircraft total lift) - climbing; positive lift - diving, but has some other advantages (the five bilion birds wouldn't use such configuration just for nothing).
That is the real truth about canar high lift story - (take off) pay load increase is roughly tvice the canard lift.

 

[WKTaylor]

Slice..

For a MODEL acft without flaps, canards could work OK... but "bullet-proof" piloted canard-acft designs have proven to be very, very elusive... with lots of painful lessons-learned along the way [accidents and death]. The primary causes have been poorly designed, miss-shaped and/or "unclean" airfoils [roughened by bugs, rain, dirt, paint, etc] which have caused total loss of control for various reasons/conditions.

Review the following article on the criticality of canard acft design by Martin Hollmann [FAA D.E.R.] of

http://www.aircraftdesigns.com

...

Canard Aircraft Designs by Martin Hollmann

In the May issue of Custom Planes I wrote an article on FLYING WINGS, CANARDS or BIPLANES? In this report, I wrote what I know about John Denver's death in a Long EZ. I only wrote a part of what I know. Here is a little more. Under no circumstances is the following an assault on Long EZ pilots or Mr. Burt Rutan who, beside myself, is one of the most prolific aircraft designers in the world today.In the early 1980's my good friend and aerodynamicist, Rick McWilliams, started building a Long EZ and he asked his good friend, famous aerodynamicist, Dr. R.T. Jones* "What is the best aircraft configuration?" R.T. replied, "I do not know." Rick spent a year researching this topic to find out that certain characteristics could be improved on with this aircraft. One such item was the GU25 airfoil used on the canard (the front wing) of the early Long EZ's. This airfoil has 70% laminar flow on top and 70% laminar flow on the bottom, which gives it very low drag but also gives it a bad stall characteristic especially if it becomes contaminated by small specs or bugs or rain on the leading edge. When that happens and the canard is stalled, it may take a long time to recover. ACCORDING TO R.T., the canard airfoil should use a robust airfoil such as the NACA 101 or the Goettingen 387 which allows a quick recovery after stalling even when contaminated. R.T. also stated that the GU25 airfoil could be dangerous since the nose of the Long EZ could drop (called a deep stall) and the aircraft loose a substantial amount of altitude before the pilot recovers. As such, in 1986, I put a note on page 40 in my book "Modern Aircraft Design, Vol. 1" that this airfoil is "No good." I did not elaborate further. In retrospect and from the accidents that have happened, I should have.A number (3) of fatal accidents had also occurred in Florida in which Long EZ's had dove in. A number of lawsuits followed.After Denver's accident in 1997, I talked to one of my Stallion builders, Jon, in Florida. He built the first plans built Long EZ and he told me that he had heard of the problem with GU25 airfoil. When talking to him I had not mentioned anything of what I knew of the problem with the GU25 airfoil.He had found out that a John Murphy at Merritt Island was working on a fix.He visited Murphy who told him that, "he was not allowed to talk about the problem but that Jon should look on his drawing board." The large cusp on top of the trailing edge of the GU25 airfoil was filled in with a straight line. Jon modified his airfoil accordingly.Because of these problems another aerodynamicist, John Roncz, was hired in the early 1980's to design a new rain canard called the Roncz 1145 airfoil for the Long EZ.Some GU25 Long EZ pilots have also modified their canard by locating vortex generators just ahead of the cusp.Denver's Long EZ was serial no. 54. It was one of the early canards. George Peterson of the NTSB who investigated Denver's accident claimed that the Roncz airfoil was used. He gave me a name and phone number of the person that told him that. I called that person and he denied it.One Long EZ pilot writes, "When I purchased my Long EZ in 1989, I flight tested it completely according to the Owners Manual. In addition, I flew into rain showers to check reaction of the original canard and loss of lift problem when wet. The first test was a sudden loss of 500 feet of altitude as the stick pulled out of my hand." He also states that he has 1,000 hrs of trouble free flying in Long EZ's since then. By-the-way, John Denver was flying at 400 feet.Yes, many Long EZ's are flying safely with the GU25 airfoil but accidents still occur.Following is a tragedy that occurred in March 1999, one and a half years after Denver's accident. It was reported by John McAvoy in EAA Chapter 62's newsletter on April 1999. John and several other Long EZ pilots flew to Baja, Mexico from CA in Long Ez's.The accident pilot's name was Gus and the aircraft a Long EZ. They lost track of Gus and found his wreck several days later in the ocean. After the Long EZ wreck was recovered, McAvoy inspected the wreck. In McAvoy's words, "We were also met by another EZ builder who was accompanying a representative of the news media. Both he and I made a thorough visual inspection of debris and we both came to the same conclusion; the impact was not survivable. Our basic agreement is the aircraft hit the water inverted. The failures and stresses indicate the pilot restraint failed due to very high G loading. The pilot was ejected through the canopy."Was this accident caused by a deep stall of the canard? You be the judge. There are many other similar accidents!THE SOLITAIREThere is more to canards than meets the eye. At the 1982 Homebuilt Sailplane Association meeting at Tehachapi, CA, two pilots Einar Enevoldson, famous NASA test pilot, and Walt Moonie evaluated the canard Solitaire. One pilot would fly the Solitaire and the other a Schweitzer I-36. Performance between the aircraft was similar at low G's but when the pilots landed the one in the Solitaire claimed it was really rough. The I-36 pilot claimed it was smooth. The pilots traded places and the same thing happened with the canard pilot claiming rough air. As Walt Mooney explained it to me, "the canard enters the gust first causing the nose to pitch up and then the wing to push the aircraft up." With a conventional aircraft, the aircraft only translates up and down since the tail keeps it level. Furthermore, when making high banked turns in the Solitaire the sink rate would increase greatly as shown in the Pilot Report "The Rutan Solitaire" in November, 1982 Soaring by famous NASA test pilot Einar Enevoldson and Marta Bohn-Meyer, famous NASA aeronautical flight test engineer.THE CM-44In the fall of 1987, California Microwave Inc. (CMI) came to me and asked me to fix a problem on their CM-44 designed by Rutan. The CM-44 was a canard aircraft and at 95 Kts it would yaw 45 degrees opposite to the direction of the turn which was very uncomfortable to the pilot. No one knew what the problem was but I felt I could solve it with the help of my friends; Dr. R.T. Jones, Rick McWilliams, Jim Phillips from NASA, Dr. Ilan Kroo from Stanford. I agreed to help. Dr. Kroo set up a lattice vortex model of the existing CM-44 and showed that (during a turn) the downward moving winglet was stalling causing the upward moving winglet to push the nose in the opposite direction of the turn. Figure 1, not shown, shows the Cl distribution on the wing, canard, and winglet. John Roncz designed a new canard airfoil. Using the lattice vortex program we resized the aircraft, designed and built new wings, winglets and a canard and the aircraft, now called the CM-44A, was test flown. As CMI reports "We have a winner." Not only was the yaw problem solved but the take off distance was shortened to about half. The wing taper ratio had been the culprit and reducing it kept the wing tip and the winglet from stalling. We could now use a larger canard area which allowed the aircraft to fly slower. I sent a list of items to CMI that needed corrections to the CM-44 prior to getting a contract to design and build the CM-44A. These items include:· The wing is only twisted 2.5 degrees. I should be washed out 6 to 7 degrees. · The winglet is aligned with the wing leading edge so that the pressure peaks coincide. The winglet should be moved aft as far as possible to keep the pressure peaks from aligning. · The airfoil of the wing tip and winglet should be none laminar flow, high lift airfoils which will no stall at high angles of attack. · The present CM-44 airfoils have poor stall characteristics. · The wing taper ratio on the CM-44 is too high. The tip chord is too low in comparison to the root chord. · The angle of incidence of the winglets must also be properly selected. BEECH STARSHIPWhen the canard configured Beech Starship was conceptually-designed my friends and I were surprised that flaps and a variable canard were going to be used.When Rich McWilliams and I held technical design classes at Beechcraft in the early days I told Rick not to say anything negative about canard aircraft. When we had lunch with our clients and students we were surprised that they knew what we knew. Despite of them not convincing their management to make changes, they did a fantastic job in designing and certifying the structure of the Starship.We all knew that using flaps on the Starship would not reduce the stall speed by any significant amount since the lift coefficient and lift of the canard dictates the stall speed of the aircraft. The added pitching moment of the wing from the flaps cause the lift load on the canard to increase and the canard to stall a bit earlier.Many years later, I talked to my good friend, Ed Hooper, the project manager of the Starship at Beechcraft. Ed had flown the Starship 200 times and he stated the stall speed on the Starship stayed the same, 69 Kts, with or without flaps down. We were not much surprised.MICROLIGHTAgain I want to reiterate that I am not trying to reflect a poor image on Mr. Burt Rutan. On the contrary, I consider Mr. Rutan an ambitious and prolific aircraft designer willing to take risks to explore new designs. Such people must be encouraged and supported if we are going to advance the state of the art. If I did not feel that way, I would not have turned down an offer from attorneys to work on behalf of the plaintiffs in a lawsuit against Mr. Rutan in the canard Microlight aircraft accident in which the owner and test pilot of this aircraft were killed.CONCLUSIONIt is important to recognize mistakes so we or others do not make them again. After all, the life we save might be mine or maybe even yours. Saving someone’s life would certainly not be the first time that I have done that.

Regards, Wil Taylor

 

[BAHarvey]

I would like to take this opportunity to comment on a few of the inaccuracy in the Mr. Hollmann’s article quoted previously. While I am not familiar with all of the instances that he has alluded to I would like to correct the items I am. First “bad stall characteristics” is a very subjective term with respect to how the GU-25 airfoil behaves when contaminants spoil the laminar flow. From what I’ve read, it is more accurate to say the maximum coefficient of lift (CL max) is reduced. This does not mean that the loss of lift is more abrupt which is what is commonly though of as a “bad stall characteristic”. The pitch change that some canard aircraft experience was not consistent through out the fleet. Some experienced significant pitch change while other experience none. This pitch change was attributed to a reduction in lift of the canard with respect to the lift of the main wing. This required increased backpressure on the elevator to overcome either manually or via trim. Mr. Hollmann is using a different definition of “deep stall” than the rest of the canard community. This behavior is when the main wing stall before the canard which force the aircraft into and ever deepening stall because the canard, which dictated pitch, continues to increase its angle of attack. If the canard was stalling and thereby loosing lift it would never be capable of reaching a deep stall.
The NTSB report I read on the Denver accident stated that he lost control while reaching for the fuel selector to prevent fuel starvation during departure. What exacerbated this problem was that the selector was not located according the Long-Ez plans between the pilots legs. The builder had relocated it to the shoulder support. This unfortunately located required the pilot to loosen their shoulder belts to turn around to actuate it. Additional if have heard that the selector had be sticking and required a “Vice-Grip” to actuate. Sticking has be notes as a common problem due to age with some of these selectors, which lends credibility to this comment. I have no familiarity with the remainder of Mr. Hollmann’s comment but the fundamental inaccuracies I have noted leads me to question them. However this is just my opinion you be the judge.

 

[WKTaylor]

BAHarvey...

Any characteristic (or combination of characteristics) of an aircraft that can cause loss of control for a reasonably skilled pilot in adverse conditions is cause for extreme concern and is a "bad characteristic".

The potential for unpredictable stall characteristics is due to surface contamination... that will NEVER be consistent across a surface... affects BOTH pitch and roll(-off) characteristics. As a pilot and engineer, this is aspect of the older canard designs is enough to qualify as a "bad characteristic" [widow-maker].

The over-all philosophy(s) of canard design has matured over time. The undesireable characteristics of the older designs are being eliminated by hard experience and advanced design techniques. Hollman's article was a direct attempt to enlighten the community... with sober info/facts. He is a DER... and like most DERs... he is conservative in favor of predictability and safety.

BAHarvey... what is YOUR canard experience?

Regards, Wil Taylor

 

[BAHarvey]

I did not mean for my statements to be take as a personal attack against you or Mr. Hollman. My intent was just to correct what I believe to be inaccurate. This was not done to dispute your statements but rather inform the causal reader about the nature of this flight behavior.
As for my background on this subject, I don’t mean to infer that I am an expert but I’ve done a considerable amount of research with regards to the pitch change phenomena and deep stall in canard aircraft. I have read every Canard Pusher, the Rutan Aircraft Factory newsletter, publish from it’s beginning until 1996 and quite a number of Central States Association newsletters, which is an independent canard aircraft publication. As well I have read the documents published on research done in the 80’s at the NASA Langley low speed wind tunnel on a full scale Vari-Eze (GU-25 canard), which among other things looked at these issues. There is also a series of articles published in Sport Aviation that present finding on this issue as well. However I am not ashamed to say that all of my information is purely academic, as I have never personally experienced pitch change due to precipitation.
Mr. Hollmann sited only two accidences as direct evidence of a problem with the GU-25 airfoil as it applies to canard aircraft. However both of these accidents were found by the NTSB to be a result of pilot error. Mr. Denver exhibited poor judgment on the day of his death, why else would anyone ever depart from an airport with only a few gallons on board in an aircraft that holds 52 gallons. This accident per the NTSB finding had nothing to do with the aircraft, only the pilot. By the way the pervious owner of Mr. Denver Long-Ez stated to the NTSB (National Transportation Safety Board) as documented in their report that the canard airfoil was the Roncz 1145MS not the GU-25. The other accident mentioned, the crash of Gus Sabo, was a result of a VFR (Visual Flight Reference) rated pilot flying into IMC (Instrument Meteorological Conditions). As I’m sure you are aware this kind of lapse in judgment has killed a vast number of pilots independent of the type of aircraft they were flying. As a side note a friend of Mr. Sabo’s said he used Roncz airfoil the as well.
With regards to my credentials, I am a mechanical engineer, canard aircraft builder, and a pilot. I’ve also had some experience developing DER packages for submittal to the FAA.

 

[Miper]

A canard is not a good arrangement for maximum lift. You are right that the canard surface adds to lift instead of subtracting from it the way aft tails do. But it is a little more complicated than that, as noted above -- and I have not carefully read everything above, so I am not commenting on any of that here.

Basically, if you have a given payload with a given CG range and want to take off from a given runway, fly a given distance at a certain altitude with a certain level of comfort, the canard is not the right way to go. Keep in mind that, although the original Wright Flyers were canards, the Army hired Groever Loening to put the tails on the back so they wouldn't be so hard to fly (twitchy in pitch). Canards can have the ride quality of a bucking bronco - very sensitive to gusts.

The argument for a canard goes a follows:
At stall, the download on an aft tail is about 15 - 25% of the airplane weight. This download adds to the weight that the wing must lift at stall. So the wing has to be about 20% bigger for a given weight and landing speed, just to carry the extra tail load. On the other hand, a canard surface lifts up. If it works as hard as an aft tail, a canard surface lifts 20& of the airplane weight, so the wing can be 20% smaller. Now 20% smaller seems a lot better than 20% larger. In fact, this means that a canard's wing should be roughly 40% smaller than the wing of a similar aft tail airplane.

Nevertheless, the canard cannot have effective flaps, because flaps cause the main wing to stall well before the forward tail, which causes the airplane to suddenly flip over backwards. Consequently, few existing canard airplanes have flaps at all, and none has effective flaps. This means that a canard is limited to a max lift coefficient of about 1.5; whereas aft tail airplanes achieve overall lift coefficients of more than 2.

Consequently, for a given landing speed and weight, the aft tail airplane has less total wing+tail area.

In addition, since a canard wing is smaller, its available fuel tank volume is less than half that of an aft tail airplane. Furthermore, the wing of a canard configuration is at the tail of the airplane, so the fuel weight is much too far aft. The fix has been to put strakes along the fuselage to hold the remaining fuel and to move the fuel CG forward. These strakes add more weight and drag.

Canard arrangements have interesting styling effects, and they can be made to be safe with a lot of extra effort; so they might be ok for sport airplanes and certain special purpose airplanes, but they are not recommended where the ultimate performance, efficiency, comfort, and utility are desired.

There is a designer in California who has made a name fore himself designing unusual-looking airplanes with canard surfaces. No one else seems to have had any real success with them. It is not because his designs are better than anyone else's, but because he has very narrow and limited goals for his airplanes. If you want an airplane with the normal, broad utility of other airplanes, don't design it with a canard surface.

ek

 

[radomir]

Gentlemen (in particular wktaylor, BAHarvey and Miper) since you seem very knowledgeable concerning the canard matter I would like to raise a question. It is bugging me for quite a while but being unable to find a satisfactory answer you are my last hope.

All the latest European fighter aircraft are with canards: Swedish - Saab JAS 39 Gripen, French - Rafale and Anglo-German-Italian(-Spanish) - Typhoon (former Eurofighter). On the other hand none of USA counterparts has it. What do you think? Are those Europeans all wrong or you are being too cautious (I said “you” because supposedly you are all Americans while I am a Canadian)?

Maybe the question should be rephrased, since I do know that JAS 39 had some problems initially but I am not aware about the other two, to read as follows:

Are Europeans encountering so big difficulties that US designers opted not to be caught in the same trap?

 

[matt00000]

WRT the Eurofighter(Typhoon)I think the canards are for greater manoeuvrability and are not essentially needed as the aircraft is primarily a delta config.
Very interesting thread this...
I am looking forward to Wills retort wrt BAH's excellent comments!

 

[chasbo]

I recommend you get a copy of Andy Lennon's book on R/C aircraft, which covers canards and has some canard designs in it. He teaches that you have to be careful selecting the zero lift angle of attack of the canard and main airfoil differently to have stable flight in pitch. This generally means a more highly cambered airfoil for the canard than the main wing. This should reduce the flight roughness described in earlier posts.

Canard design seems to be stuck in a first generation sort of situation, sort of like the way monoplanes were when they first came out. (Maybe I should say 2nd generation, since Burt does postdate Orville and Wilbur.)

One interesting issue not mentioned is free rotating canards like the all flying tails on Pipers versus canards with elevators, like the tails on Cessnas.

 

[kb9wle]

Slice:

I have read the replys to your question and think that some good info was presented there. I joined eng-tips just now to add my $1 worth.

First off I must assume that the purpose of the planform is to compete in a model airplane payload contest such as the SAE event? If not what I say should still apply.

Having studied this problem for 8 years, I agree with a previous post that the Reynolds number is way too low to allow a high performance model airplane canard to work. To provide for maximum performance, I recommend a medium aspect ratio "nuerfluegel" -tailless aircraft- planform with 20-25 degrees sweepback and low washout. If the aerodynamics are carefully calculated the aircraft will be unable to stall due to the the bell-shaped lift curve of the planform, it will simply lose lift and "mush". The reynolds number can be tackled because of the larger overall scale of the aircraft. An added bonus is that the weight of the aircraft can be evenly distributed throught the airplane - meaning less structure is required. Many engineers dislike the tailless planform mainly because they do not understand it fully. Those very few that do hesitate to use it because of lack of overall utility.

In any case, I recommend that you find a copy of "Nuerfluegel" by Reimar and Walter Horten. The Hortens designed and built tailless aircraft before and during WWII and produced a 650 mph fighter jet made from 4.2mm plywood and steel tube.

Good luck,
john

 

[cabanestrut2002]

there are a lot of model canards, scale and non scale fun designs , both large and small, unfortunately they are very difficult as models. i have been building/flying models for years and i have never seen a first time succesful canard model.

 

[edmacs]

Hi:
Canards seem to be very effective for Ekranoplans, called also WIG planes (wing in ground effect). It seems that canard improve their stability, which is ekranoplans primary disadvantage.
Edmar
Eng. Edmar Silva

 

[Minifloresta]

Slice,


Let me comment about the two arguments that you mentioned:

1 - The canard wing does not act as a stabilizer. As a matter of fact it is destabilizing. The stabilizing element in a canard aircraft is the wing itself. The puspose of the canard is control only, not stability.
Yes, it adds lift to the aircraft. But one thing is for sure: for an aircraft to be stable one normally needs "longitudinal dihedral", that is, the front surface must have a higher CL (Lift Coefficient) than the rear surface. This concept is well discussed in a book by UK's Darrol Stinton, "The Design of the Airplane". This however does NOT mean that on a conventional aircraft the tail produces downward lift. One can easily have CG positions that will make the tail of a conventional aircraft produce positive lift. And it can - also easily - be stable provided that the tail area is appropriate.

2 - The belief that a canard configuration is better because the wing never stalls is not exactly correct. First because there are combinations of airfoils and ticknesses that can make the wing of a canard aircraft stall before the canard; and even if it is a properly designed canard aircraft, in severe turbulence there may arise a situation where both surfaces stall - and in that scenario a canard aircraft is usually far nastier. However in normal flight of a good canard aircraft the canard will stall first. The design must be such that it stalls at an angle of attack close to the wing's maximum angle of attack, or the takeoff and landing speeds will be too high due to control limitations.

 

[JohnFortier]

We went into this a while back in a long and at times rather acrimoneous thread concerning the Piaggio P180 Avanti, a three lifting surface design. Actually a misnomer in the case of the Avanti, since the third, tailplane, surface contributes neither positive nor negative lift.

I'm a telecommunications engineer and as such do not profess to be expert on matters aerodynamic, but it strikes me that for maximum lift with minimum drag tne three lifting surface design has it all. The Avanti has a high aspect ratio wing for minimum drag at its cruising speed, the forward lifting surface loses lift before the main wing in the stall, but, since it is not a control surface, the pilot is not left trying to control his aircraft using a stalled airfoil, control remaining with the, unstalled, rear surface, and the wing position leads to an extremely spacious cabin. With no negative lift anywhere, the wing can be more lightly loaded and drag can be reduced, leading to near jet speeds at turboprop economies.

A further three surface design, which is now in production is the Swiss Aericks (hope I spelled that right!) which also was mentioned in the previous thread.

Others are now pursuing this design for medium size private aircraft. You might consider this for your model, slice, since it seems to answer your problems.

Regards

John