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Was Monowheel Performance with fixed pitch prop & 912S... No

 
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ptag.dev(at)tiscali.co.uk
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PostPosted: Thu Mar 30, 2006 4:25 pm    Post subject: Was Monowheel Performance with fixed pitch prop & 912S... No Reply with quote

Hi! Duncan, All.
Just a point of reflection .....I have quite a lot of Constant Speed
experience having flown the MT Prop for going on 600 hours. The knob
twiddling and log filling is of no worry or pressure what so ever!
However the TBO is of major concern for the pocket or "plastic"!
Especially so when you have a fully certified CS Prop.!
It is of paramount importance to remember the phrase "with the benefit
of hind sight" when examining my experience. I would briefly remind
readers that perhaps as long as 7 years ago (before I could even fly!) a
very qualified airline pilot made a statement to me ..."if ever I was to
have my own aircraft it would be a major priority to have a CONSTANT
SPEED PROP!"
Just about that time Europa Aircraft were trialling .....a CONSTANT
SPEED PROP on G-KITS...... In perhaps a fit of naivety I made a hasty
decision to order the same prop. on the premise that you wouldn't buy a
high performance car and put a crap transmission or gear box on it would
you?
I then became very enthralled with the concept of a new engine on the
scene, not having a gear box, but somewhat untried (in fact it was on
display at Cranfield Rally that year and caused immense interest, it was
Serial No. 005 !) I bought it being flush at the time! .... But the only
advice the promoter gave me about propeller choice was that until I
became proficient as a flier I should not risk such an expensive prop.
but take his Jabiru bog standard fixed pitch "cheapo" especially since
they were intending to develop their own range of adjustable and CS
props. To go with their engine!
He made not once any mention of the science of Polar Moment of Inertia
or limits of the crank shaft or strain resonance etc. and got my bullish
response that the MT CS Prop. had cost an arm and a leg and as such it
would be the only prop.it would fly with!
Only then began a 12 month battle with the PFA Engineering, MT
Propellers Gmbh and the Jabiru Chief. With me the "meat in the
sandwich". I became embroiled even in almost a racialist dispute between
Germans and "Ausies" when it was alleged that the Jabiru Propeller
Flange was a "fatigue waiting to happen" and that there was no way 120
HP could be transmitted by 6 No. 3/8" cap screws fixing a flange to the
available friction surface at the end of the crank shaft without the
threads being in shear. The waist undercut at the end of the shaft would
fail with the use of a 32lb Propeller with a Polar Moment of Inertia
some 33% more that the notional figure slapped on the engine by the
Jabiru Technical People!
Thanks to the late Barry Mellor (The major designer of the Europa
Fuselage Bless his soul) we ploughed through all obstacles like needing
to produce special cold rolled aircraft quality ground shanked cap BOLTS
with which to fix a special German made flange (a flywheel in its self!)
c/w aircraft bushings which weighs about a further 5lbs and the need to
hand reem out 5/16" OF THE THREAD FROM THE HOLES IN THE END OF THE CRANK
to provide dowelled drive without the threads in shear.
We then had to have the aircraft trailered to Bavaria for the MT factory
to do free static resonance tests within the first 50 hours to honour
their warranty. Although I by now had a PPL from a trip to Florida I
hadn't yet been converted to a Europa or even flown it solo in the left
seat (and MT didn't feel competent to fly) This test produced strain
information which prohibited continuous use between 2100 and 2400 RPM
(the Constant Speed facility was therefore to initially to prove it's
worth in that whilst I was always aware of the restriction the
controller watched out for me in a long descent. It also ensured there
would be no overspeed of the propeller limit of 3000 RPM. It was by the
way that I had to request MT for yet another free factory test (two
technicians 3 days)to establish where the "avoid" range moved to in
flight! (in fact it narrowed to 2150 to 2250)Incidentally a further
freebie they kindly extended me was another visit to provide a very
professional sound certificate (which our authorities don't accept as
bonafide!)
The Constant Speed facility gives a brilliant MAUW take off run in Fully
Fine Pitch and a slow RPM in the cruise but necessitates the use of a
Manifold pressure gauge to monitor the engine power, this was a source
of major concern since Jabiru would never provide Power/Manifold
Pressure graphs and since ambient pressures vary and pressure varies
with altitude it was always difficult to know the engine loading or fuel
usage.
So you have the background to my current dilemma the Calendar TBO will
be up in September and it is a small fortune! Viz:- I can buy three
fixed pitch props.for the price! (although it has to be said MT
Propellers Gmbh have been very professional throughout and in spite of
the initial cost they don't "owe me a penny" especially with the
"freebies" but the TBO cost just isn't viable)
During this time I have endured lots of banter about fuel usage etc. and
have slowly become used to employing a higher RPM engine setting mostly
towards say 2650 RPM so improving fuel usage the CS is set to a maximum
2800 RPM.(so my CS requirement for fully fine takeoff and coarse cruise
is somewhat now negated until I get a hot humid high wet grass field to
depart from!)
In preparation for the inevitable date with expense I yesterday flew to
Southery (Under the Marham Matz) to take a look at a Sensenich two blade
wooden prop. ...nice looking gear! On the way back I was able to engage
Manual Control of the MT prop and take it up to it's max. 3000 RPM
limit. Believe it or not I've never done this previously, in spite of
the MT Pilot having momentarily taken the combination to 3,300 on the
last test.
I flew most of the return journey in this 3000 RPM mode with less than
usual MP down to say 20" (it is usual to "square" MP with RPM ie:- 24"MP
with 2400 RPM)and was surprised to find cruising at 120kts a piece of
cake except oil temps were needing control.
I had during later years managed to keep fuel usage down by limiting
cruise speed to say 110kts.
So the moral in the story is "horses for courses" perhaps I hadn't
needed the CS prop all these years but the "hind sight" factor is still
there!
Yesterday I did notice though that with the CS Controller in "Manual"
(nearly like a fixed pitch) needing to respect the maximum prop.speed of
3,000RPM called for a big input in throttle control of the RPM,
something I've never needed to watch what so ever over the last 6 years
!
So if it's not one knob you need to twiddle it's another !!!!!
Hope this hasn't been boring and helps those who have yet to decide
which way to go?
First decide the type of flying you intend..... short field and or grass
operation needing a "fully fine" setting changing to cruise at lower RPM
..or longer paved runways where take off thrust isn't so critical or
conversely the operating temperatures and height of the runway too....
Then closely examine the depth of your "Pocket" or "Plastic".... Then
the needs of the engine and output and climb rates.... then when you
have a picture formed look at what's available!
I have only experience of the one type that I have described but
Airmaster seems to get good reviews and for what it's worth I have lots
of respect for the Woodcomp having visited their factory and seen their
technical capability (but Mark Burton has produced a more compact
controller for panel mounting considerations).
You do need a check ride to convert to Constant Speed or In Flight
Adjustable procedure though and the controller limits of maximum and
minimum need precise setting to allow "go round" in the event of the
pitch change mechanism failing.
Finally at the expense of flouting the forum rules ..... should anyone
fancy an MT CS Prop. it is quite likely I'll have one we could do a deal
on! After all your engine type or range of operation might be more
suited?
Regards
Bob Harrison G-PTAG Europa MKI/Jabiru 3300

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