Unapproved Design Changes

From Cozy Newsletter #59, copied verbatim.

We referred earlier to design changes which we haven’t approved, which we think are ill-advised, and which in some cases may even be dangerous. Here are design changes to avoid:

  1. Do not change the way the elevators are constructed or installed. The top surface of the elevators must be reflexed, as we show in the plans, as for example in the cross sections we show in Chapter 11, pages 5 & 6. If yours have not been built this way, scrap them and start over. A false rumor has been circulated that there is a “dead spot” in elevator response. This is not true if built according to plans, and we have proved it and can demonstrate it with our “plans-built” Cozy Mark IV. It has been reported that the same person circulating this rumor has a problem with pitch trim. This could be a symptom that there is something more seriously wrong with his airplane, if not the shape of his elevators, it could be that his canard is not set at the correct angle of incidence, and his airplane is unstable in pitch. Our flight tests apply only to a Mark IV built according to our plans. You should be aware that the fuselage is a lifting body, and the canard span and incidence needs to be adjusted for the amount of lift contributed by the fuselage. If you change the shape of the fuselage, either by making it wider, higher, or using a different canopy, or you change the angle of incidence on the main wing, there is only one way to determine whether you have a safe airplane, and that is to install a traveling 135 lb. weight, like we did, and go up to 11,000 ft. with a parachute, and determine whether the point of neutral stability is well aft of the aft c.g. limit, and whether the c.g. range is safe at the aft c.g. limit and at least 1.2 inches beyond.
  2. Do not limit elevator trailing edge up (nose down) travel to 11 degrees rather than the 15 degrees we specify. If you should ever remove your vortilons, or miscalculate your c.g., or set the canard at too low an angle of incidence, or change the fuselage, and do stall tests and find the main wing start to sink at the same time the airspeed starts to fall, you will want as much nose down authority as you can muster to keep from becoming a statistic. The plans say 15 degrees trailing edge up, and that is what you should have.
  3. Do not use “hard shelling” (Note: applying a mixture of micro to the foam, letting that cure, then applying glass). This results in a poorer bond between the fiberglass and the core. On airfoils, the top surface is usually in compression. A poorer bond means that the airfoil (canard or wing) skin can delaminate, buckle and fail sooner, that is, fail at a lower g-loading, than an airfoil built according to plans.
  4. Do not use “tri-ax” cloth. We are not aware of any static load testing that has been done with Cozy or Long EZ wings to prove that it has as much torsional, tensile, and compressive strength as the individual layups specified by Burt Rutan, and which we use. Furthermore, it is much harder to work with because it does not conform as well to curved surfaces and can cause air bubbles (poor lamination), and it has a tendency for one or more layers to wrinkle when squeegeed, which it is not possible to correct. Also, our plans call for one less layer of glass where the ailerons will be cut out. You can’t do that if you are using tri-ax. Ailerons covered with tri-ax cloth will not balance leading edge down, with the very likely possibility of “flutter” which can cause catastrophic failure of an airframe, unless you sand off at least one layer of glass, which is more work than if you had used the correct glass cloth layups in the first place.
  5. Do not increase the chord length of the ailerons. There is no possible justification for doing this. It not only makes it much more difficult to balance the ailerons, but it reduces the wing cross section at the point where the bending load is the greatest. In other words it weakens your wings both in drag and in bending loads.
  6. Do not increase the span of the ailerons. Our 3-place Cozy has excellent roll response. The Mark IV has even greater roll response because the 2 ft. greater wingspan puts the ailerons farther outboard which results in a greater roll moment, and also farther outboard of the downwash from the canard. There is no possible justification fo this change.
  7. Do not buy “fast build” wing kits where the foam cores have been cut for wider chord and greater span ailerons, and the “kink” in the trailing edge of the wing has been eliminated. The latter changes the angle of incidence of the airfoil, which makes it different from the airplane we have tested, and causes problems in fitting the cowlings.
  8. Do not eliminate the extra layups on the top of the wing which we show in Chapter 19, page 6, Fig. 32. These provide added strength to the wing where the bending and drag loads are the greatest.
  9. Do not increase the span of the rudders. They are even more effective on the Mark IV than on our 3-place because of the increased wingspan. Very little rudder input is required in flying a Mark IV. Furthermore, it was discovered on the Varieze that too much rudder authority could cause a Varieze to lose control. You wouldn’t want this to happen on base or on final.
  10. Do not leave the lower winglets off or make them smaller. We found in our very thorough aft c. g. flight test program that the lower winglets shown in our plans give you a 0.5” c.g. safety margin protection against a main wing stall. They also provide more roll stability at slow speeds and high angles of attack. Again, something that is important on base or on final.
  11. Do not eliminate the NACA scoop in the fuselage bottom and use arm pit scoops instead. The NACA scoop gives better cooling of the engine when parked nose down, and probably also in flight. Better cooling of the carburetor or throttle body means fewer problems (if any) with hot starts. Also, if you notice, the Cozy Mark IV is quite clean underneath. We believe this (and better cowling contour) contributes to the extra 20 mph we get as compared to another aircraft with arm pit scoops, fixed gear and the same horsepower.
  12. Do not cover the top of the canopy. The compliment we receive most often on the Cozy and Cozy Mark IV is its wonderful visibility. If we had gull-wing doors, we couldn’t have a full bubble, but with a side-hinged canopy it is a wonderful feature. Install the full bubble first. Then, if sun bothers you (it helps to keep you warm at high altitudes in the winter), you can always wear a baseball cap. Then, if you are still bothered by the sun, you could use cling-type plastic shades, which are moveable, over your head. If you really like reduced visibility, you can always paint part of the bubble. The latest pictures we have seen of the Velocity show that they removed most of the cover over the top of their canopy. We consider this to be a safety consideration.
  13. Do not hook your fuel tanks together. Having two separate tanks is a safety feature. If you lose one fuel cap with both tanks hooked together, all of your fuel will be siphoned out, and you will become a statistic.
  14. Do not eliminate the sumps under each tank. They make all of your fuel usable, and allow you to fly for a little while after the gauge shows empty. Although not recommended, you can run one tank completely dry, switch, and keep on trucking. We have done this (accidentally) twice in the last 12 years.
  15. Do not locate your fuel selector valve remotely. Experience with the Varieze was that the remote location of the fuel valve was the cause for more than one emergency landing.
  16. Do not buy components from custom shops without calling us first. Custom shops are in business to make money. They may use unskilled labor and take shortcuts, and sometimes their parts don’t fit or are unairworthy. They will probably claim their parts are better, but then ask you to sign a waiver relieving them of any product responsibility or liability, and will not guarantee the airworthiness of parts they supply. We have heard of some very bad experiences some builders have had with custom shops. If you are not willing to build the parts yourself per plans, you would be better advised to build a Velocity. The Swings have a pretty good record.
  17. Do not install retractable main gear. Our airplane was not designed for it. Most people agree the benefit (if any) isn’t worth the cost (and we aren’t just talking about dollars).
  18. Do not change the size or shape of the engine cowlings (in other words, use the cowlings we approved from Feather Lite). Our cowlings were contoured to have nice clean airflow back to the prop, to avoid a vacuum or reverse airflow at the prop. It was interesting to hear that one 4-place canard aircraft (we won’t mention the name), which has been highly advertised, and which has a very blunt rear end, has not yet been able to meet its performance goals of useful load and speed because it cannot generate enough thrust after becoming airborne. We suspect that it is because the prop is spinning in reverse airflow or a vacuum (you know, like behind a semi).

[A bunch of text to backup that unapproved design changes tend to have bad results]

Lastly, if you do not build your airplane according to plans, the Cozy Mark IV Owner’s Manual does not apply, and we ask that you not register or insure it as a Cozy Mark IV!


Which is why I’m not building a Cozy Mark IV, I’m building a Coz-E, which happens to be largely similar to the Mark IV, but is not the same airplane.

Last updated: 2021-08-07 08:24:43 -0700