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
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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