Hi Ed,
Comments interspersed below.
Regards,
WRB
On Nov 1, 2009, at 10:40, Ed Burkhead wrote:
Fred Weick said in a recorded interview that having the fairing on the
nose gear made almost exactly one (1) mph difference in speed.
What we didn’t explore was, isn’t that 1 mph difference comparing the
nose gear all the way extended with the scissors snugged up to the
back of the strut tube?
In that case, the scissors wouldn’t make much extra drag as it is
hiding behind the tube.
I would believe that Fred spoke with strict reference to that with
which he was familiar and had measured. In that context, his
comparison would logically have been with a 75 hp coupe, a 73" long
propeller and without the fairing and presuming full nose oleo
extension (no snubber). In this context your observation is excellent
and original in its detail.
When the strut extension is limited, then you have some shortening of
the tube, hence less drag. But you also have the scissors hanging way
out where it commits drag all on its own.
Could there be as much as two (2) mph difference between having the
nose gear extended with the fairing versus having it 2/3rds down with
the tube and fairing both providing their maximum aerodynamic drag?
I would speculate that the propwash extends sufficiently beyond the
actual propeller diameter as to perhaps increase drag with a snubber
even though the frontal area is reduced and the fairing would become an
angled scoop as you describe. I would also mention in passing the
additional "issues" of decreased steering capability during cross wind
takeoffs and landings and decreased oleo range inevitably associated
with limiting original oleo extension.
Giving the FAA bureaucrats due consideration, I'm not sure what the
proper procedure would be to run the simple flight tests necessary to
secure "real world" figures. No one is going to get a 337 for each
described configuration...perhaps a log book entry would be considered
sufficient. If such log book entries were inadvertently delayed until
after testing was complete and, in retrospect, deemed unnecessary (if
no permanent change was made), there might be no "foul" of record (as
to the pilot/owner name or N# or Serial# of the coupe utilized) if data
were derived and submitted anonymously.
For purposes of pure and abstract speculation, it would seem that any
curious couper with a GPS and the original 75 hp engine and a 73" metal
prop (most common today) of standard pitch could easily fit a temporary
snubber and measure the difference at (a) at "normal" cruise rpm and
(b) at maximum rpm) on a nice day at a given altimeter setting, rpm,
altitude and heading, recording each resulting indicated air speed and
OSA temp. Of course there are few snubbers "out there" that are the
same length, so I would suggest that the "test length" snubber be the
M10 one that is sold. It is ??" in overall length. Let us call this
"Snubber Drag Test #1".
It would then be logical to remove the strut fairing (as most with the
snubber do in the field) and perhaps also the rubber bumper (since
there is no longer the possibility of the scissor contacting the oleo
when strut extension is limited) and record airspeeds, etc. that
configuration as "Snubber Drag Test #2".
If that couper wanted to get us some more useful information, he/she
might then substitute a 71" metal prop of standard pitch and repeat
preceding tests as "Snubber Drag Test #3 and #4". The result of these
latter tests reveals (a) how efficient the smaller prop is at the same
cruise rpm (obviously less so) and (2) if any power/speed actually
results from any increased maximum power rpm possible with the smaller
prop.
The "pudding on the cake" would be to then put the 85 hp venturi and
jet in the Stromberg carb and repeat tests #3 and #4 which, in this new
configuration would be "Snubber Drag Test #5 and #6".
Available Continental performance data suggests, at least to me, that
prop length, venturi and jet changes contribute little, if any,
performance. This would be because ERCO intentionally chose original
75 hp props that would permit the engine to turn rpm commensurate with
producing 85 hp.
Comparing the ERCO performance data in Ercoupe Service Memorandum No.
37, Appendix A, with what most of us achieve today it would appear that
the 73" 75 hp prop and the 75 hp jet and venturi in the Stromberg was
actually a more efficient setup than the smaller 71" one currently in
fashion.
Fred Weick said this, although carefully expressed in terms that would
not clearly challenge ERCO's decision to "officially" offer an 85 hp
engine commencing with the 1948 415-E Model. He "wrote the book" on
Aircraft Propeller Design in 1930, and information presented therein is
also consistent with this conclusion. (My copy, purchased from a used
technical book seller in Los Angeles several years ago, bears a stamp,
etc. indicating it was once a reference in the Technical Information
Center of the Lockheed-California Company, Burbank, California, Central
Library!)
Best Regards,
William R. Bayne
.____|-(o)-|____.
(Copyright 2009)
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Just wondering.
Ed
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