On Sat, 29 Nov 2003 18:20:53 -0500, Larry Smith wrote:


"Kevin Horton" wrote in message
news
On Sat, 29 Nov 2003 14:17:48 -0500, Larry Smith wrote:


At its website Cessna explains why it uses exclusively aluminum
construction. Cessna says there are too many unknowns regarding
glass and that aluminum is better. Well, aluminum IS better in some
respects,
but glass is also better in many respects too. Fiberglas is easy to
repair, it lends itself nicely to compound curves, it does not corrode
or
fatigue like aluminum, it is capable of absorbing more shock than
aluminum, and its strength-to-weight ratio cannot be beat.


There is no doubt fibreglas construction has its advantages, but I'm not
so sure that light weight is one of them. My impression from comparing
specs of similar aircraft is that aluminum construction is usually
lighter. For example, if we look at two seat, side-by-side fixed gear
aircraft, the RV-6 seems to come out at least 100 lb lighter than a
Glasair II TD if we have similar engines and props. And the RV-6 has a
lot more wing (110 sq. ft. vs 81 sq. ft).

http://www.airsport.com/kits/ksuper2.htm
http://www.vansaircraft.com/public/rv-6spe.htm

The Zenair CH2000 and the Diamond DA-20 were both designed to meet the
same requirements (JAR-VLA). The aluminum CH2000 is about 100 lb
lighter than the composite DA-20:

http://www.newplane.com/amd/spec.html
http://www.diamondair.com/contentc/c1spec.htm

--
Kevin Horton RV-8 (finishing kit) Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/ e-mail:
khorton02(_at_)rogers(_dot_)com


I don't disagree with you here at all. It is true that most composite
kitplanes and most composite factory-built aircraft are a little heavier
than similar aluminum aircraft. However, you will find that careful
layups like those in the lighter Longezes and Variezes, and indeed in
Rutan's Voyager, will produce an aircraft lighter and stronger than
aluminum. Matter of fact you can't really make a comparison because of the
variations and the dissimilar advantages and disadvantages in each method
of construction.

Let me ask you something. Do you believe an aircraft like the
around-the-world Voyager could have been constructed of aluminum? Has
anyone ever built an aluminum Quickie or Cozy or Velocity?

I'll give you another example --- Mike Arnold's 213 mph world champion
speedster, the AR-5. Do you believe that same airframe, which is very
light, btw, because of judicious (but not vacuum-bagged) layups, could
have been made of aluminum? I don't. The AR-5 defeated the previous
world record-holder, which was an aluminum BD-5. Aluminum and compound
curves don't mix. Aluminum and laminar flow airfoils don't mix either.
So I'd say that the composite aircraft 100 pounds heavier than the RV-6 is
faster on the same engine and prop combination. I may be wrong. At
least you can hide antennas inside the airframe.

Not taking anything away from 2024-T3, of course. Duralumin is still a
miracle material for aircraft construction. And, having recorded the
"From the Ground Up" series with Joe Schumacher and Mark Annick, I'm
envious of your RV-8 project.

I certainly agree that if your design needs very smooth exterior surfaces
or compound curves that you need to use some sort of composite.

And yes, the Voyager was a very light design. But was it really
fibreglas as you were originally talking about? I thought it had
graphite skins.

http://www.compositesengineering.com/Pages/Links.html

Some aluminum aircraft manage a good speed. Kent Paser made a long series
of incremental mods to his Mustang II and eventually had it doing 239 mph
at 8,000 ft on a 160 hp O-320. I wonder what the fastest fixed gear
Glasair does at 8,000 ft with an O-320.

So, lets agree - if you want the fastest speed, composite is probably
better. If you want the lightest weight, a review of similar
designs shows that aluminum usually wins.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com