REFLECTOR: Engines and High DA

[Scott Derrick]

>I'd expect the most effective way to reduce takeoff roll would be going to a
>variable pitch prop, no?
>
Maybe, maybe not.  There are wins and losers for an MT, the only prop 
with a history  and its checkered.  You do gain in a reduced take off 
roll.  You lose in a 65-85 lb CG demon on the end of the engine.  I've 
been told to expect at least a 100 lb loss in net weight once you add 
weight up front to get the cg in line.  They break easily & often.  They 
are horribly expensive to get fixed.  They take forever to get fixed.  
Many on this list have more time on there spare fixed pitch then there 
MT.  You also loose 5 to 10 knots on the top end, something I don't want 
to do.  And of course there is the added complexity for flying and 
maintenance.

>  For a fixed pitch cruise prop a good portion of
>the blade is in stall condition until the speed gets up, and adding more
>power would mostly just make more noise.  I'm just guessing here; have no
>data.
>
>  
>
The data I  have gathered says your wrong.  Its not just noise.  Its 
called Horse Power.  The density altitude alone  here in Grants reduces 
my HP by 20 %.   I've been taking to a fellow with an E-Racer with a 
TIO540, using a Catto Prop.  He breaks ground in less than 1000 feet at 
gross weight.  With his old IO360 it was 2500 ft.  Sounds like a goodly 
portion of those 300+ HP are pushing it along down the runway.

When I'm at sea level my plane performs much better.  Why, because the 
engine is making 20% more power, if as you say it was being just turned 
into noise I wouldn't see that big of a reduction in runway requirement. 

I've talked at length with Craig Catto about this. He has extensive 
experience, compared to us,  about big HP engines with fixed pitch 
props.   Even turbo's engines with fixed pitch props.  Its all about 
designing the prop for a specific mission.  My mission is to reduce my 
takeoff roll by 50% and cruise 25% faster.  After running the 
speed/RPM/HP tests on my plane for Craig he thought it as easily 
attainable with the TSIO520 running it at 60% power at altitude, and  
90% power at takeoff. We'll see.

Scott