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rafael(at)gforcecable.com
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 Posted: Sat May 12, 2007 12:11 pm Post subject: Initial biuld decisions |
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First of all, I thank all who provided me with input in making the decision between a 9A and 7A. I have decided to build the 7A. It has not been an easy decision. Basically the balance tilted towards the 7 after considering the Vne and the added performance with the 360 engine.
Now I would like help with a couple of other decisions. First, IO-360 vs O-360. I was leaning heavily towards the injected engine, carburetor heat being the issue. However, after attending an EAA meeting this morning and talking to an RV-8 builder with a beautiful 8A with an O-360, I’m no longer sure. What advantages does the fuel injection have? Performance? Reliability? Ease of installation? Maintenance? Any other? I believe cost and weight penalty are close for either engine.
The other decision is between Constant Speed and Fixed Pitch prop. It is my understanding that the CS will provided better takeoff and climb figures and marginally better cruise performance. I’m wondering if this performance is worth the extra $6500 or so for the CS. I’m leaning towards the FP implementation, but I would like to hear comments.
Thanks and best regards to all,
Rafael
[quote][b]
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cnpeters
Joined: 24 Aug 2006
Posts: 23
Location: Bloomington/Normal, IL
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| Posted: Sun May 13, 2007 7:58 pm Post subject: Initial biuld decisions |
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Rafael,
A couple things:
1) Regarding Vne and the O-360, it shouldn't be a big deal with the
RV-9. There are quite a few O-360 powered -9's out there, and you won't
flirt with Vne in cruise. The only time it would be an issue is in a
dive/descent, a small part of your time flying. One will need to watch
for Vne with both the O-360 or O-320 in a descent - the former will make
it a bit easier to get close in a more shallow descent. Just throttle
back, that's it.
2) Between the O- and IO-360, there are many threads on this in the
archives for the various forums. Weight will be a non issue. The
injected models main advantage IMHO is to run lean of peak and achieve 1
gph improvement. A more balanced fuel mixture to each cylinder is
possible. You don't have to worry about carb icing. But, there is some
complexity, higher fuel line pressures, need for a return fuel line to a
tank and more expensive fuel selector, more difficult hot starts. There
are others on these forums that have much more real world info since I'm
still a builder.
3) FP vs CS prop - this is one of the the top three debates that rage
amongst the OBAM market (along with to prime or not, slider vs tip-up,
etc). I'm still deciding on that one, and have a year still to worry.
Frankly, I'm leaning toward an FP - lighter weight, MUCH cheaper, less
maintenance/overhaul issues. Craig Catto builds a beautiful 3 blade prop
that I have flown behind in a -9 - smooth, excellent craftmanship, and
costs $1800. He (and other manufacturers) can set you up with a prop
that is coarse pitched that will give you Van's cruise numbers.
Comparing performance numbers from various builders shows cruise numbers
about equal between a properly configured cruise FP vs a CS. You will
sacrifice climb rate/takeoff distance a bit, so if you fly into hot and
high or short fields all the time, then you need to re-think this. But
honestly, I do all my flying in Illinois and elsewhere into airports
with at least 3500' strips - really look at Van's T/O and landing
numbers with a FP prop in the -9. There are few places where you can't
go that a CS will make a difference. On the flip side a CS resell may be
better, especially with the -7. Acro and formation flying have some
benefit with a CS. One thing against the FP is slowing down and descent
rate. Coming into the pattern and trying to slow down takes a little
more forethought and earlier energy management with the FP, since the CS
can give a little drag when in fine pitch and help out. This is
particularly so with the -9. Basically, I think the argument is a little
silly, as some practice will make you competent. I fly a Dakota with a
CS, but have no trouble when I grab the glider-like Diamond DA-20 with
FP after practicing with it a little once or twice. So, for me, I'll
likely go with the FP and use the $5000 saved over the CS for about 1500
gal of avgas. That's quite a few cross-country trips!
Carl
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_________________
Carl Peters
RV-9A wings |
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