From: zwork@starfighter.den.mmc.com (Michael Corvin)
Newsgroups: rec.aviation
Subject: Re: Homebuilt Fly-by-Wire
Date: 4 Sep 92 00:30:27 GMT
Organization: Martin Marietta Astronautics Group

I have not seen the articles in _Kit Planes_ September or October
issues, but here are a couple thoughts on fly-by-wire and homebuilts.
(It's a bit of an epistle...)

The main reason to use fly-by-wire (or fly-by-light) flight
control systems is that this allows one to add a significant amount
of stability augmentation and/or autopilot functionality to
the system.  In some aircraft it may also offer a useful amount of
weight savings (I believe that there is a smaller blimp currently
flying that uses "fly-by-wire" electric actuators for its elevator
and rudder to save weight.  These are, however, simple servo systems.).

Stability augmentation is required in aircraft that have reduced
stability margins, either to enhance maneuverability (like the F-16)
or efficiency (like some newer airliners).  I'm not so sure homebuilts
should be built with this characteristic (although some designs
have turned out to have them... ;-} ).

The problem with building this stuff is designing it and then
validating it - i.e making sure it works.  Also, one likes to design
and build it so that it is fail-safe to a large degree.  Fault
tolerance is, to say the least, a gnarly subject.  Fly-by-wire 
systems are typically computer-based, which brings in the need
to design, implement and test reliable software, another nontrivial
issue.  One can build analog fly-by-wire control systems - the F-16
has one (at least it did originally), as does the F-14 - but these 
have their own complications.

Fly-by-wire systems also must have actuators to move the controls.
Hydraulics are used on most systems today, although electric actuators
are under development.  Either requires a hefty, engine-driven power
source and the actuators must provide fail-operate, fail-safe
behavior.

So, while a fly-by-wire homebuilt would be possible to design, the
design probably would be very costly due to all the added systems.
Construction complexity would be greatly increased as would be the
need for systems testing and maintenance involving complicated avionics
and actuators CRITICAL to life and limb...

Homebuilts are getting complex enough as it is - take a pressurized
Lancair IV for example, not to mention the BD-10!!  Actually, the 
BD-10 is a good example for this discussion.  The Venerable (J.) Bede
(any English History buffs out there in rec.aviator.land?!) et al
explicitly designed that bird to have a simple, all mechanical,
manual flight control system.  This is _not_ an easy thing to do
for an aircraft with its, uh, largish range of operating speeds.
(I'm looking forward to hearing more about their flight tests and
how well they have succeeded in achieving their handling and 
control goals!)

To be cute, I think the best idea for a homebuilt is to put the smarts 
into the design, not into the plane.  Fly-by-wire is most useful
when the the aircraft and it mission _require_ smarts on-board
in the autopilot/flight control system.

A nice set of pushrods and maybe a couple of springs and bob weights
is going to be a lot more reliable and easier to build than a
fly-by-wire system.  In a plane like the BD-10 this means one has
to be rather clever with the aerodynamics and the design of that
system of pushrods, etc., but it's alot easier overall than making EAA
mean Electronic Aircraft Assoc. :-)...

I think the place where advancement using the latest in sensors,
electronics and computers would be useful is in small, light weight,
reliable and (of course) inexpensive autopilots for homebuilt and
GA aircraft - autopilots working in conjunction with the primary,
mechanical pilot controls.  With tie-ins to nifty, GPS-based,
integrated nav and collision avoidance systems, of course...

Okay, I guess I'd better quit expending bandwidth...
(BTW, I work on guidance & control systems for rocket-type things that
would (and sometimes do) perform dramatic aerobatics (space-batics?) if
their (fly-by-wire) flight controls decide to hiccup...) 

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Michael Corvin 	      PP-ASEL, PP-G		zwork@starfighter.den.mmc.com
just another spaced rocket scientist at    Martin Marietta Astronautics Group
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===============    My views, not Martin Marietta's   ========================
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Newsgroups: rec.aviation