Message-ID: <19980922174138.2042.qmail@www04.netaddress.usa.net>
Date: Tue, 22 Sep 1998 17:41:38
From: Dan Schaefer <dfschaefer@usa.net>
To: lancair.list@olsusa.com
Subject: C/S vs fixed prop, battery as filter, gross wt
Mime-Version: 1.0

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To Jeff Chipestine asking about the relative merits of different 
battery technologies as pertains to noise filtering. The easy 
answer is they're all about the same, i.e., they look like HUGE
capacitors right across your main bus. Of course, this isn't even
quantifiable if your battery is near it's end of life. One of the 
parameters that goes to pot in a battery about ready to give up 
the ghost is internal impedance (the parameter that, when really
low, kills noise). Even so, it's a secondary effect, any battery
will probably quit cranking your starter before you notice much loss
in filtering. How you wire your airplane, in particular minimizing 
common ground returns from different pieces of equipment, will 
probably have a more notable effect.

To Pieter Pienar: I changed from a fixed pitch wooden prop to a C/S
MT on my 235 for a number of reasons. First and foremost was that
I was tired of scaring myself (and the pilgrim in the right seat)
half to death when taking off high, hot 'n heavy. I don't know 
where you fly but some of the places I go here in the Western US are
around 5-6,000 ft with summer temperatures into the 90's. The wood
props I had tried (three), when pitched for a decent RPM in cruise,
would turn up no better than 2350 RPM for take off at SL. With a 
O-235 up front, even with hi-compression pistons, I wasn't making
a lot of horsepower at that RPM. About the only thing good about
that situation is that I've probably got more rejected take-off
practice in a Lancair than anybody! With the C/S prop, I now get
right close to 2700 RPM on take-off, which, according to an old 
text book, is approx. 14% more HP. (You Physics professors out there
correct me if I'm wrong, but HP= Torque x RPM / 5250. Torque should
not be too much different at these two RPM's, so it appears that
horsepower is (nearly) linearly related to RPM. Conclusion: it all
depends on how much money you want to spend.
On the gross weight discussion, two things: Seems that the steel
parts of the gear are stronger than the mounting for same - A friend
(who insisted on checking himself out in his 320) landed about 
eight feet above the runway and poked his main gear struts up thru
the top of the wings when gravity reared it's ugly head and showed 
him where the surface really was. Nothing else broke (in the airframe
structure that is) so I guess these things are stronger than it 
might seem. (My friend's plane is fixed and flying again). Lastly,
what you put on the paperwork (within reason) is a legal thing -
and one you shouldn't exceed if you don't want your insurance company
to disown you if you should ever need them. So I put a higher number
on my application (1550 lb gross vs the factory specified 1440) than
I ever plan to use when I fly. I know this doesn't answer the guys
wishing to actually fly at higher weights, but it's something to
think about.

Cheers,

Dan Schaefer


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