From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: Stability and Control
To: lml
Date: Mon, 15 May 2006 20:19:25 -0400
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Posted for "Rienk Ayers" <rienk.ayers@sreyaaviation.com>:

  Mark,
  thank you for the thoughtful response, and valid concerns.
  We may never see eye to eye on this issue, but I still believe that pilots
  and builders should understand more than just "lift, weight, thrust, drag."
  Pilots do not need to understand all the math behind stability and control
  derivatives (I don't), but they should at least know what they mean and how
  they apply. And certainly, the designers, engineers and manufacturers should
  know what those numbers are - and use them.
  Having considered buying both a Lancair and a Columbia, and having friends
  who own both, I have a great appreciation for these aircraft. But it is
  because I recommended a friend buy a  4P Propjet, and then have he and his
  son help pull the remains of the pilot from the wreckage of the Lancair,
  that I am concerned about these issues.
  
  I vigorously disagree that pilots have to accept a high level of risk to fly
  a high performance aircraft. Many military jets are easier and safer to fly
  than GA planes, and many high performance aircraft are easier and safer to
  fly than some that are low-and-slow. Since there are so many pilots with a
  huge number of type ratings on this list, it would be interesting to get
  their feedback as well. But to say that we can't let people get "resigned"
  about a bad design but that they should maintain the attitude that they can
  get out of anything is just wishful thinking... ideally, yes, a pilot would
  not get themselves into a situation that is beyond the airplane (say, stall
  speed)... but I continue to assert that, ideally, the plane is designed to
  be the most stable and controllable that it can be, so that when a pilot
  does something stupid, there is a better chance of getting out of the mess
  in one piece. I will also disagree with you about inanimate objects being
  safe or un-safe. For example, my kids' bicycles all have chain guards; this
  does not guarantee that their pants will not get caught in the chain, but it
  does significantly reduce the likelihood, and thus - by my definition -
  makes the bike safer. The same is true for chainsaws, pulleys on
  air-compressors, propeller cages on parawings and trikes, etc. At a
  different level, you could compare the relative safety regarding vertical CG
  and rollover of an old Jeep to a Hummer. What about PIO and the effect of
  centerline thrust on gyroplanes? What about the "perfect" proportions of the
  Questaire Venture, with decent static stability but terrible dynamic
  stability? I'm sorry, but you cannot convince me that because something is
  risky, it can never be safe. I hope to take my kids skydiving for the first
  time soon. Yes, it is risky (hurt on landing, chute fails), but there is a
  big difference between using a WWII era chute and a modern pack. It seems
  that you would contend that a jumper should not really care which chute he
  uses, as he should be able to handle (or trained for) both. I consider
  myself a safe driver, but I would still prefer a car with airbags and good
  bumpers. I put toe brakes on my Comanche, because I considered it safer,
  after almost crossing the threshold (inadvertently) because the handbrake
  couldn't hold the run-up... etc, etc.
  
  However, I will have to back down from this whole " data" issue a bit, as
  some of the sources I have for my comments - about the Lancair in particular
  - I can not name, as it would be awkward for them professionally.
  Nonetheless. the challenge that the actual numbers be made public still has
  merit. I guess all I can say about that - at the moment - is that every
  builder of ANY plane should know (about) these numbers. Thus, you are
  welcome to write off all my comments as unsubstantiated, but I would
  encourage the group to at least make the appropriate inquiries, if not
  actually push for the answers.
  Nonetheless, if you all will allow me a little latitude, I would like to
  make some additional comments about this issue - since it is invaluable to
  any owner or builder - no matter what kind of airplane. This is not intended
  as an engineering lesson (I'm only a wannabe), but a partial discussion on
  the merits of calculated and designed stability and control. I hope this is
  useful.
  
First of all, let's distinguish between the aircraft and the pilot, as they
  are two distinct issues. I do not intend to address pilot error or
  competency, as those issues are present regardless of aircraft type.
  Likewise, the inherent "risk" in any venture/endeavor/sport/activity is also
  a somewhat independent factor. However, in referring to the realm of
  aircraft design, there are mathematical formulas that determine an airplanes
  stability and control, both statically and dynamically - and these are what
  I have been referring to regarding the safety of any particular aircraft
  design.
  
  Allow me to use a non-aviation comparison: I love white-water rafting, which
  can be a very dangerous sport (risk). In rafting, there are different
  'classes' of rapids/rivers, rated on the severity or consequences of the
  particular rapids. However, the "safety" of a particular voyage is based
  primarily on two things: the skill and experience of the guide (pilot, in
  your point), and the adequacy and appropriateness of the equipment -
  primarily the raft. It would generally be considered "un-safe" to go with an
  inexperienced guide, but just as "un-safe" to go with inadequate equipment.
  On the equipment side, just making a raft out of "high-end" material or with
  the basic "look" of another well performing raft is not sufficient to make a
  "safe" or good handling craft. Granted, whitewater rafting has very few
  technical factors compared to an airplane, but that only makes the design of
  an aircraft even more dependent on good design principals, number crunching,
  and verification (wind tunnel and/or flight testing).
  
  Switching to an aviation example, we can look at the Bonanza: originally, it
  was probably a well balanced blend of engine size, airframe strength, and
  control surface sizing (area/volume/moments, etc) - based upon calculated
  numbers. However, as engine sizes and payloads increased, the plane
  significantly changed. The fixes were acceptable to get certification
  (springs, bob weights, some enlargements, etc), but they diminished the
  stability and controllability of the aircraft - all mathematically
  calculated - and known as 'derivatives.' Thus, the natural progression of
  wanting more power/speed and useful load ended up taking what was a fine
  airplane, and making it a marginal one (I probably angered some 35 owners,
  but it is the truth). Through the evolution of the design, fixes were
  bandaged on, most likely without review of the derivatives (it would be
  interesting to hear from someone at Beech if they ever wind-tunnel tested
  any of the various models, or re-calculated the derivatives).
  
  Pilots become so enamored with power/speed, that they easily forget or
  ignore changes to handling qualities, and end up compromising on stability
  and control, assuming that the trade off is "acceptable", only because they
  don't know that such trade offs are not necessary. It is apparent that most
  high performance pilots - including every Lancair pilot - is willing to
  trade a margin of "safety" for the performance numbers offered with the
  latest iteration (the same as Bonanza pilots).
  
  I am asserting that such trade offs do not have to be made!
  There is no reason that any plane cannot have acceptable stability and
  control derivatives (let alone good ones) and still have the performance
  that we all long for. This is not just stall/spin or landing speed issues -
  it is relative to the general "safety" and "fun" of a design. For example,
  the Envoy was pitch stable in the original configuration; but when the new
Fowler flaps were put into play, that number became almost neutral.
  Technically, it still had a "stable" number, but it was not stable enough to
  be "safe" for the average pilot (thus a change). Back to the Bonanza; if we
  assume that the original design had its derivatives in the sweet spot, but
  no major changes to the aerodynamics were made as power and weight increased
  and as CG's shifted, then the new models would become more and more
  unstable. If the original model of any aircraft were barely stable, changing
  the wing and control surface sizes to just keep up with the stability of the
  previous iteration does not help much. Regarding the Lancair (since this is
  a Lancair forum), "if" the derivatives of the original were barely stable,
  simply scaling up some surfaces will not necessarily keep up with the
  stability numbers, let alone improve them.
  
  That being said, choosing what derivatives a particular airplane should end
  up with is a subjective matter, and dependent on the mission of the
  particular aircraft. A Piper Cub and a Cessna 172 and an Extra 300 and an
F-15 have completely different mission profiles, and therefore completely
  different derivatives. Ironically, most aircraft (including many certified)
  have never had their derivatives calculated, let alone had the aerodynamics
  designed around them. Most of the numbers that are actually known for
  production aircraft are closely guarded secrets (Boeing really doesn't want
  Airbus to know what they have or are doing). However, it would behoove all
  of us in the experimental/homebuilt fraternity to make those numbers known,
  so that existing and future designs can be improved - both in performance
  and safety. That is why we are seriously contemplating building our own full
  scale wind-tunnel, so that in our design(s) we know what's really happening,
  before we let our customers become 'beta' testers.
  
  Back on track; fighter planes are specifically designed to be neutrally
  stable, or even unstable (possible only with computer controls). General
  aviation planes, on the other hand, should be very stable. Yes, it is
  possible to be too stable, but it is still feasible to design a stable plane
  that has incredible performance.There is no doubt that, like many designs
  that progress, the Lancair family of planes has made much improvement on
  different fronts (performance, utility, stability), but I would assert that
  in the area of stability, it probably can be further improved (greatly?)
  without adversely affecting the other areas.
  
  It should never be acceptable that a pilot who had hundreds or thousands of
  hours flying safely in a certified plane, shouldn't be just as qualified to
  fly a high performance plane along the lines of the Lancair - if the design
  has been maximized. Again, every plane is a series of compromises (including
  the Envoy) that is based on the reality that - barring new discoveries - the
  rules of physics and engineering as we know them cannot be cheated. Some
  designers are just plain lucky, and others seem not to be... what is the
  tipping point? Only one thing - the difference in the derivatives (stability
  and control). Why is it that so many pilots and builders think that they
  have no choice but to give up "safety" (as I define it) for other features?
  There is no reason, other than the designers ignore or don't know about the
  importance of these numbers. That being said, aerodynamics are still too
  complicated (too many variables that are interdependent) for us to be able
  to predict everything by the numbers (computers), so wind tunnels and test
  flight programs are still necessary. And as long as a plane doesn't crash on
  its first flight, all sorts of "fixes" can be made in order to tame the
  negative surprises that rear their ugly heads. But I assure you that most of
  that (including the trial and error of attempting to get things right over
  several iterations of design and building) can be avoided from the get-go.
  And believe me, I have learned this the "hard" way.
  
  The reality is that, after spending so much time, effort and money on
  design, tooling and fabrication, most manufacturers cannot/will not afford
  the delay necessary to make the appropriate changes, which can have drastic
  consequences (a prime case in point is the Jim Bede design, which became the
  Grumman Yankee... it never started out as a particularly stable plane, and
  the fixes incorporated barely made it usable/safe). The reality is, there
  are very few airplanes out there where the designers and manufacturers
  wouldn't want to improve the design if it weren't for the economic
  constraints upon them... the "If I could do it over" syndrome. When people
  are building a one-off experimental plane, that is to be expected; but when
  a company is producing a virtually ready-to-go kit, the proper homework
  should be done. There are only two ways for an airplane to be a good design
  and successful in the marketplace - either do the appropriate homework in
  the design process, or just get lucky. There are many planes that got lucky
  and are able to maintain momentum because of the unique market niche they
  created (maybe Lancair for speed and sex appeal; Aerocomp for size and
  simplicity, etc). Others end up eventually dying, because their appeal was
  quickly eclipsed by their drawbacks or lack of progress/improvements
  (consider Glassair, Avid, etc).
  
  Let's face it, what saved Lancair from a potential slow death was the timely
  introduction of the L4. But the L4 has never been and will never be a true
  four place plane - it can't be, and the derivatives would prove it. But as
  the Legacy seems to have proven, such a design is an awesome two place plane
  - and if I were to ever buy a fast, conventional two place, the Legacy is
  the only thing currently on the market that is worth considering (but again,
  that is why I am so enamored with the Phoenix... 52" wide cabin, great
  visibility, comfort, and fighter like handling, with cruise speeds of
  230-250kts on 170-200 hp? At 30-40 mpg? All of which has been wind tunnel
  verified? That is exciting!). Do a simple test... figure out where the CG
  range is on both a L4 and a Legacy, and then compare the physical size and
  locations of the lifting and control surfaces in the side and plan view...
  that would give you a hint of what might be going on. comparisons of CG to
  quarter chord distances, along with size, is everything (static stability is
  directly related to distance; dynamic stability is directly related to the
  'square' of the distance).
  
  But I digress. Often, the real reason that a craft stays successful for so
  long is because something else new or better has not yet come along. There
  isn't a lot of impetus for a company to change a proven formula (until
  something new comes along). A good company will change to meet the new
  standards; a Great company will be a major player in creating those new
  standards. Lancair is the former, and striving to be the latter. I wish them
  success, because it helps us all in the long run. However, they will only do
  so if, in their design process, they keep their eyes focused on what is
  important... smooth lines, laminar flow, new composites and building
  techniques, new engines - they are all important; but to quote Curly (from
  "City Slickers") they aren't the "one thing." The One Thing regarding
  aircraft is stability and control.
  
  I don't bring it up to try to negatively impact Lancair, but rather to
  remind them that their long term success depends on this - and so do the
  lives of their pilots.
  You may disagree with me on this, but then that is why they have horse races
  - and so many different kinds of airplanes - isn't it great !?!
Frankly, this issue would be put to bed if Mr. Bartell were to post those
  numbers, since they are the only thing that can refute my concerns. As I
  said, the Lancairs may not be "un-safe" airplanes, but they are not
  necessarily "safe" either... the derivatives can tell the story. Since I
  have so many friends that either fly or want to fly Lancairs, I would love
  to be proven wrong.
  
  Blessings, RA
  
  PS - the point about derivatives is true of any plane that anyone might
  consider buying or building, not just Lancairs.
  
  And by the way, I am fairly confident that the only reason that Columbia
  went through all the changes they did to their wing was because it was a lot
  cheaper and faster than fixing the real problem - the tail. That should make
  you wonder about any similar plane you fly that doesn't have those same
  fixes as the FAA required.
  
  Along the same vein, the AOA indicator IS the simplest (and best?) way to
  keep any plane - especially a poorly designed one - from getting you into
  serious trouble
  (remember, I admit to being opinionated :)