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Thread: [Acro] Acro Prop Comparison Article Rev2 (9 De ...

Message: [Acro] Acro Prop Comparison Article Rev2 (9 Dec 03)

Follow-Up To: ACRO Email list (for List Members only)

From: Andrew Boyd <aboyd at qnx.com>

Date: Tue, 09 Dec 2003 20:11:34 UTC


Message:

 
Thanks to everyone for their feedback!  Following is
the revised version of the prop comparison article,
in which I have tried to incorporate your comments.
    
Please review it, and if I have made any mistakes,
let me know!  I want to get this off to _Sport Aerobatics_
  
-- cut --



An Informal Survey Of Aerobatic Propellers for Six-Cylinder Pitts
-----------------------------------------------------------------

I want to mention right off that this article, unless otherwise
explicitly notated, is merely a collection of subjective observations
by one pilot.  The following is only relevant for the six-cylinder Pitts,
not the lower-horsepower four-cylinder Pitts or a lower-drag monoplane.

If you do discover any objective fact or data deduced below by any
scientific method, it's inclusion was clearly an error on the part
of the author's  :-)


  Hartzell HC-C2YR-4CF/FC8477A-4 Aluminum 2-blade Prop
  ----------------------------------------------------

Now that we've got the weasel-worded disclaimer over, let's
start by discussing the metal 2-bladed Harztell propeller that
was installed on most Pitts S-2B's when they were new.

One could write an entire article just about the Hartzell
2-blade hubs, but briefly, they are available as "no suffix"
(now "E suffix"), "A suffix" and "B suffix" hubs which refer
to the letter at the end of the serial number of the hub.

The gory details are available in Hartzell Service Bulletin
61-227 Rev 2 which is available at:

  www.hartzellprop.com/pdfs/sb227r2.pdf

and if you own a 2-blade metal Hartzell prop, it is well
worth reading.  Here is the an online copy of the corresponding
Airworthiness Directive:

  www.tc.gc.ca/aviation/applications/AD-CN/documents/US2001-23-08.HTM

The original "no suffix" or "E suffix" hubs could crack under severe
aerobatic usage, so AD 2001-23-08 requires that these hubs be
eddy-current inspected every 150 hours.  If no cracks are found,
you can keep flying, until you hit the 2000 hour limit discussed
below.

In the early 90's, to address the cracking problem, Hartzell
came out with the new "A suffix" hub design.  Many of these
propellers were installed as new equipment on S-2B's.  The
problem is that the "A suffix" hubs cracked too, and in a
place that was hard to inspect.  AD 2001-23-08 specifies
that "A suffix hubs" may not be used after December 25, 2007,
and you may not overhaul an "A suffix" prop - if it has to
go back to the prop shop, you must purchase a new "B suffix"
hub now.  Thanks to John Blum for pointing that out!

The "B suffix" hubs appeared in the late 90's, to address the
cracking problems of the "A suffix" hubs.  The "B suffix" hubs
have no AD's hanging over them - yet!

It's important to realize that all of the above Hartzell hubs and
aluminum blades are life-limited to 2000 hours by AD 2003-02-20:

  www.tc.gc.ca/aviation/applications/AD-CN/documents/US2003-03-20.HTM

I was talking to a Pitts owner the other night - he had an early
90's S-2B with low time.  However, his prop had been replaced in
the mid 90's after an accident with an "A suffix" hub and used blades.
Well, from the above we know that the "A suffix" hub is good until
the end of 2007, but his blades had 3000 hours on them when they were
quite legally installed on his aircraft in the mid 90's.  So even
though his hub was still legal, his blades were not legal to fly any
more after AD2003-02-20 was issued, effective March 11, 2003.  Oops.

The moral of the story is that the aircraft, hubs and blades can
each have different hours accumulated.  Don't assume they're the same.

In a nutshell, that's a very brief history of the 2-blade harztell
aerobatic prop as installed on six-cylinder Pitts.  I've glossed
over many important details which may or may not apply to you.

But how does it perform?  The two-blade Hartzell is marvellously
tough - you can fly through rain without worries, and it pushes
and pulls pretty darned well.  However, at slow speeds, the gyroscopic
precession of those heavy metal two-blades requires a delicate
touch with forward stick to avoid an inverted spin during a
hammerhead, and a bootful of right rudder is required during pushes.

During certain aerobatic maneuvers involving high rates of pitch
and yaw, such as high-speed snaps, tumbles, torque rolls and
tailslides, and power-on spins, the tremendous gyroscopic
forces produced can be enough to crack the crankshaft and pound
the front bearings into the crankcase.  For this reason, the
2-blade in my personal opinion is really only suitable for gentle
aerobatics - perhaps up to and including Intermediate category.


  MTV-9-B-C/C-190-18a MT 3-blade Prop
  -----------------------------------

The next propeller that was available for the certified S-2B
was the 190cm (or 75 inch diameter) MT 3-blade, which appeared
in the mid 90's as optional equipment on new S-2B's - they are
installed not by an STC, but by a revision to the type certificate,
if you are interested in such obscure trivialities.

I used to rent Pitts S-2B's out at Livermore, California a few
years ago.  Miraculously, they were available for solo renting,
which I simply was unable to pass up - I was spending a lot of
time in northern California during the wonderful tech boom of
the late 90's.  Anyways, one of the S-2B's at KLVK had a metal
2-blade, and one of them had the 190cm MT 3-blade.  Having heard
so many good things about the superior performance of 3-bladed
propellers, you much imagine my astonishment when I discovered
that the 190cm MT did not subjectively pull or push anywhere near
as well as the Hartzell 2-blade.

If you look at an S-2B flight manual from the mid 90's when
the 190cm MT was optional equipment, you will note the increased
distances (and hence decreased performance) for takeoff ground run
and 50 obstacle clearance, when the 190cm MT was installed vs
the 2-blade.

This blew me away, and was contrary to all the hype I had heard
about supposedly superior 3-bladed props.  From a performance
standpoint, the 190cm MT would appear to suffer from a skinny blade
chord, and a reduced total diameter - 75 inches vs the 80 inches
of the metal 2-blade.

The 190cm MT has two advantages, though - the smaller diameter
reduces the speed at the blade tip, and is thus quieter, and
the reduced gyroscopic precession of the lighter blades means
that you won't likely break the crankshaft, and slow-speed
maneuvers like the hammerhead are a whole lot easier to fly.
The nose has a very "loose" feel compared to the 2-blade.

Ron Spencer (www.splitimageaerobaticteam.com) just bought a
beautiful low-time S-2B with a 190cm MT 3-blade prop, and
noticed this on the ferry trip home:

  "At 2400 inches, 2100 rpm, used 12.9 gph which indicated a
   solid 160mph, (10 mph better than the 2 blade metal at
   similar power settings!)"

I should mention that the MT blades, which appear to my untrained
eye to be a laminate of wood and fiberglass with a metal leading
edge, are tremendously delicate and must be babied.  Flying in rain
is a no-no, in my opinion, but Martin Albrecht of MT Propeller
(Martin.Albrecht at mt-propeller.com) strongly disagrees.  He said:

  "With an MT Prop. you can fly through rain, ice etc without
   any problem.  Our blades are certified on turboprops and twins
   and a lot more IFR certified electrically de-iced commercially
   operated airplanes."

MT props habitually sling a tiny bit of grease on the blades,
which appears to be mostly a cosmetic annoyance.  There was a problem
with lag bolts some time ago, but that seems to have been fixed in
recent years.


  MTV-9-B-C/C-203-46 MT 3-blade Prop
  ----------------------------------

After the Hartzell "claw" (see below) was introduced, MT responded
with an updated version of their propellor, which is installed as
per STC SA00457DE.  It used the same hub as the 190cm version but
the blades had a wider chord and a larger total diameter - 203cm
or 80 inches, the same as the metal Hartzell 2-blade.  The 203cm
MT is constructed similarly to the 190cm so the concerns about
the delicacy of the blades is the same.

How does it perform?  Good question.  A couple years ago, I
upgraded one of our S-2B's to a 3-bladed 203cm MT prop, and we
immediately went out and flew a formation aerobatic sequence,
with the other S-2B with a 2-blade metal Hartzell.

It didn't work very well.  The differences between the props
drove us crazy, in fact.  When the noses pointed up, the 3-blade
203cm pulled away from the 2-blade.  When the noses pointed down,
the 2-blade Hartzell walked away from the 203cm MT.  After we
landed, we ordered another 203cm MT prop.

As the ever-helpful Dave Swartz has observed, The slow speed
(below 120 mph) thrust of the 203cm MT is superior to the
2-blade.  However, at high speed (esp over 180 mph) the
2-blade is superior to the 203cm MT.  With the 2-blade prop,
I used to be able to nudge 200 mph in straight and level
flight.  With the 203cm MT, I'm not going to get much
over 180 mph.  But the reduced gyroscopic precession, which
makes it so much easier (and different) to fly, plus the
increased slow-speed thrust, is worth the tradeoff for me.

It's not a bad prop for $12K, and it uses your existing
governor.  Dave Swartz thinks the low-power (eg 150 mph)
cruise is actually 5 mph faster than the 2-blade, which
I have trouble believing, but Dave is the sort of guy who
notices details like that, and Ron Spencer reported a
similar cruise speed increase with the 190cm MT above, so
there must be something to that.


  Hartzell HC-C3YR-1A/7690C/E 3-blade "Claw"
  ------------------------------------------

In the mid 90's, people began installing the 3-blade Hartzell
"claw" on S-2B's.  It was later added to the type certificate and
shipped by Aviat on the very last of the S-2B's and all of the S-2C's.

It is worth mentioning that for the first few years, the 3-bladed
"claw" prop was an adapted version of a "normal" propeller.  The
problem with using a normal prop for aerobatics is that when oil
pressure is lost, which can happens from time to time during
aerobatics, the blades go to fine pitch and engine overspeed results.

To deal with the engine overspeed problem, an engine oil
accumulator was installed, and when oil pressure drops
below 25 psi, the accumulator steps in and supplies oil.
It is worth mentioning that the original installation had the
accumulator mounted on the engine, and it had a habit of coming
loose.  Aviat SB#31 specifies the relocation of the accumulator
to the firewall:

  http://www.aviataircraft.com/bulletins/sb31.pdf

Later, a counterweighted-blade version of the "claw" became available,
and was installed on new S-2C's.  Aerobatic propellors (such as
the 2-blade Hartzell and MT) generally have counterweighted
blades, which when they lose oil pressure, have the blades go
to the coarse position to avoid overspeed.

Some people in the aerobatic community frown upon the use
of non-counterweighted props for aeroobatics, but one pilot
had this to say on the subject of engine overspeed due to
loss of oil pressure:

  "I spent almost all of one entire week on the 3 blade
   Hartzell trying to get 0 oil pressure under all sorts
   of conditions to begin an overspeed when it first came
   out.  Unless you are stupid and slam the throttle full
   forward after a long slightly negative vertical of over
   3500 feet straight down I think it would be almost
   impossible."

If you look at Aviat's web site, they actually still sell both
versions of the claw:

  http://www.aviataircraft.com/promotion.asp

It's got an 80 inch diameter and an impressively wide chord,
with a tough metal leading edge on the composite blade.  It's
the same technology used on Hartzell's props used on commuter
aircraft.  You won't any worries about flying this prop through
rain.  You will peel paint off the leading edge of your Pitts
before you'll hurt these blades by flying them in heavy precipitation.

As far as durability during severe aerobatics goes, Sean Tucker
(www.poweraerobatics.com) flies the claw, and if you've ever seen
him fly, I think you'll agree that he beats the hell out of it.
Sean sends it off to Hartzell every year, and every year Hartzell
takes it apart, inspects it, changes the grease and o-rings and
send it back to him.  A very tough propeller indeed.

Sounds like an incredible propeller, doesn't it?  The problem is
that it has an incredible price tag.  Hartzell wants $47,000 for
one.  Aviat has a price contract and will resell you one for $25,000
for the counterweighted version (no accumulator, same governor).

Durability and price differences aside, an interesting question is
how does it perform, compared to the 203cm MT?  The other day, I
was able to answer that question, at least to my own satisfaction,
because we purchased a third S-2B for our formation aerobatic
airshow team, and it had a Hartzell claw.  It didn't take us very
long to do a head-to-head comparison by flying formation aerobatics,
one S-2B with a 203cm MT and one S-2B with a claw.

Departing the airport, we both set 2700 rpm and 20 inches of
manifold pressure, and incredibly, both aircraft hung side by
side, with no perceptible speed delta.  Then, it was onto
formation loops and hammerheads, with the 203cm MT S-2B and
claw S-2B alternating lead and wing.  Amazingly, both aircraft
appeared to deliver nearly identical performance.  Frankly, it
wasn't what we were expecting - we thought the extremely wide
chord and tapered tip of the claw should have delivered better
thrust than the 203cm MT, with it's smaller chord.

Kevin Kimball (www.jimkimballenterprises.com) had this to say
on the subject of comparing the 203cm MT to the claw:

  "We did the STC certification test flights for the 203 here
   at our shop.  Our test showed basically the same performance
   comparisons as you found to the claw.  One thing we found was
   the vertical line of the MT was slightly better than the claw
   at same power settings in formation pulls to vertical.  We also
   found that the top speed was slightly greater with the MT. If
   the cowl flap was open on the MT airplane and closed on the claw
   airplane, the top speeds would match more closely."

It's worth mentioning that airshow pilot Jim LeRoy
(www.bulldogairshows.com - check out his incredible video
clips) who beats up his Pitts S-2S at least as hard as Sean
Tucker does, flies behind a 203cm MT.

So, it's up to you, which 3-blade prop you want to fly behind
on your certified Pitts.  The 203cm MT and claw deliver nearly
identical performance, but the more expensive claw appears to
be subjectively tougher and will likely require less maintenance
over the years.  I know of Pitts S-2B's with the claw that have
gone an incredible 10 years between prop overhauls, but that
probably wouldn't be a good idea with the MT.

Subjectively, I think the claw has more ramp appeal - the wide,
aggressive claw blades simply reek of testosterone  :-)


  Whirlwind 400C 3-blade Prop
  ---------------------------

I should mention the Whirlwind propellers.  They are NOT certified,
but they are very attractively priced - around $12,500 (pretty
much the same as the 203cm MT) for a prop which appears nearly
identical to the Hartzell claw - same tough carbon fiber
counterweighted blades, nickel edge and very wide chord blades
with a tapered tip:

  www.whirlwindpropellers.com/html/aviation/aviation_400c_series.shtml

If you look closely at the specs, you will notice the diameter
is only 78 inches, which does not bode well for performance,
but might be quieter at the same rpm.

I cannot offer any personal information as to the durability or
performance of the Whirlwind propeller, but if I was flying
an experimental six-cylinder Pitts such as the S-2S, I would be
looking closely at the Whirlwind 400C and talking to the various
airshow performers (such as Skip Stewart) who fly behind the
Whirlwind.  It looks like a lot of propeller for the money.

--
aboyd




                


© Dr. Günther Eichhorn
Retired
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