Thomas Parkes wrote:

Would this be a good time to point out that pretty much everybody
 flying a rotary now & in the near future is/will be using a fixed pitch
prop?
;-)
If it's pitched to allow 'full power' on takeoff, any fast airframe is
going to have very poor performance & fuel efficiency at cruise. Engines
on f/p prop planes virtually never run at 'full power', certainly never
on takeoff. Fixed pitch props on fast planes (RV's, M-II's, etc)
typically turn 10-25% *faster* in cruise than on takeoff. Is anyone
prepared to run their 13b rotary at 10,000 cruise rpm?  (I know that
Tracy runs his at 40% on cross countries, but he hasn't yet experienced
the joyous requirement of minimum time between bathroom stops.)

The fast stuff is usually has such a great power to weight ratio that
65-70% power on takeoff just isn't a problem. The slower, heavier
airframes tend to solve their own problem because the prop will be
pitched flatter & the engine can come closer to 'full power' rpm on

takeoff.

So, shouldn't we just tune for 80-100% of design cruise rpm?

Paul & others with high-gross-weight, fast planes really do need that
c/s prop.

Charlie England

Tom Parkes wrote:

This topic of c/s versus inflight adjustable versus fixed pitch props has
been well covered in previous discussions.  My own experience with my turbo
20B with inflight adjustable IVO Magnum 76" prop confirms Charlie's points,
and those of many other contributors, but there have been some completely
unanticipated developments that underline the desirability of a c/s prop.

First off, with my IVO prop set at mid-twist, full throttle and 32" MP only
4500 engine rpm was attainable during initial take-off roll.   With twist
set at full increase rpm at brake release, the same MP would generate 5740
engine rpm--about ideal as prop will be turning 2645 rpm.

Obviously, the prop unloads as the aircraft accelerates, and immediately
after liftoff the rpm increases fairly rapidly toward 6000+ engine rpm as
the aircraft accelerates toward 100-120 knots.  The pilot with a 76" prop
must be prepared to keep the rpm under 5800 to avoid transonic tip speeds.
This increases pilot work load, but is manageable.  What I hadn't expected
was the rapid rise in EGT that accompanies the increase in rpm.  That's when
I briefly encountered 1800 degree EGTs on my first couple flights.

A constant speed prop would solve this problem--even if it is electrically
controlled, like the $12,000+  MT prop I wanted.  A fixed pitch prop
probably wouldn't overspeed and wouldn't overtemp--unless it isn't pitched
correctly.

Incidently, I am using the full range of the IVO "twist" control from
takeoff through cruise.  Requires 20 seconds to adjust from one extreme to
the other.

Tom Parkes
N44AZ 30.5 hrs flown, 9.5 to go.

Tom, what top speed are you seeing so far?
What is your test weight and max rate of climb seen so far?

Paul Lamar
 
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