Today's Message Index:
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1. 03:31 PM - Re: deck angle (bickersKEVIN)
2. 07:55 PM - Re: deck angle (taildrags)
3. 08:06 PM - Re: deck angle (bickersKEVIN)
4. 08:08 PM - Re: deck angle (taildrags)
5. 08:22 PM - Re: deck angle (bickersKEVIN)
Message 1
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Howdy Oscar,
I love a good discussion about a/c configuration, but beware of deck angle vs angle-of-incidence!
Angle-of-incidence is referenced from wing chord line to the
top longeron, while deck angle is ground-plane referenced to top longeron. So
a bigger tire will indeed increase your deck angle, allowing a greater angle-of-attack
(alpha) during t/o and landing but it will not change the angle-of-incidence.
You are absolutely right about wanting to get as close to possible
to the critical alpha during this phase for the slowest possible forward velocity.
Think of landing gear geometry (including tire size) controlling deck angle
and the cabane struts controlling angle-of-incidence. The ideal configuration
would give (deck angle + angle-of-incidence) = (critical alpha), while keeping
the angle-of-incidence at Mr. Pietenpol's recommended values.
The Piet seems to allow for some wiggle room in rigging but your missing 0.5deg
of wing angle-of-incidence (i_W) would theoretically reduce the full-up pitch
authority of your tail assuming the horizontal stab angle-of-incidence (i_H)
is rigged to plans at 0deg to top longeron. Honestly, I think a lot of these small
control characteristics are lost in the noise when comparing Piets, due to
the vast number of areas for small differences in each build!
YMMV, but in my experience a tailwheel first landing actually decreases the chance
of a porpoise when compared to a mains first touchdown. It is essentially
the same principal as a tricycle a/c, where in our case the CG is forward of the
tailwheel but aft of the mains. Inertia in the tailwheel first landing will
cause the mains to 'plop' down while decreasing alpha, reducing lift and the
possibility of said porpoise. On the other hand, in a mains first landing the
CG aft of the ground contact point will continue downwards, increasing alpha (and
lift). This scenerio is described nicely in "The Compleat Taildragger Pilot"
by Harvey Plourde, where he calls the porpoise a 'jounce' (And I know that's
not how complete is spelled but that's actually the title of the book!).
Regardless, I bet you will like your new tires! Hopefully you can do some comparison
testing in nice calm conditions to see if there is any appreciable change
in t/o and landing speed.
--------
Kevin Bickers
Tehachapi, CA
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=473198#473198
Message 2
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Kevin; my kinda guy... numbers, angles, all that stuff ;o) Believe me, I read
and re-read what I wrote so as to be as clear as I could be about the angles,
relative wind, angle of incidence, and all the rest of it. What I did *not* bring
into the discussion was laziness. Yes, I know I could rework the cabanes
to restore the angle of incidence that is called for in the original design but
it's fussy work for someone who is not a welder nor set up for metal working.
I know a superb welder and he could do it for me. However, as I posted earlier,
seeing the in-flight photos of my airplane from the eclipse weekend flight,
I didn't like the proportion of the tires to the rest of the airplane and
I was already thinking about going to a bit larger and plumper tires. Once I
got to studying the deck angle and wing incidence, it occurred to me that going
to taller tires would put the nose higher in the three-point attitude and that
might help me land with the wing nearer to critical AOA. The lazy man's work-around
for adjusting for a shortage of wing incidence ;o)
At touchdown in the three-point attitude the deck angle is the angle while sitting
static on the ground, so at that instant the AOA if the airplane is neither
climbing nor descending should be = deck angle + wing angle of incidence and
should be = critical AOA if that's the target (and it is, in this case). That's
my story and I'm sticking to it!
--------
Oscar Zuniga
Medford, OR
Air Camper NX41CC "Scout"
A75 power, 72x36 Culver prop
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=473200#473200
Message 3
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Hopefully those news tires help! What size are currently on your plane? And I didn't
mean to imply that your cabanes need any fixin' cause that's defintely no
short order! I also meant to ask, do you have a full CAD model of your
Piet??
--------
Kevin Bickers
Tehachapi, CA
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=473201#473201
Message 4
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Whoa. Back up the truck just a little bit there, Kevin! You wrote something very
interesting when you said that a little less wing angle of incidence might
reduce the full-up pitch authority of the tail assuming that the horizontal stabilizer
is rigged at 0 degrees (mounted flat onto the top longeron), which mine
is. In fact, I find that I can't really power-off stall the airplane to a
noticeable break unless I almost whip-stall it, which would seem to bear out what
you're saying. I have plenty of room to bring the stick all the way aft (others
have mentioned that being 'profound round' means you might not be able
to get the stick all the way aft), but even so all I get is mushing and nodding...
never a true stall break.
Interesting observation! Maybe the next improvement to 41CC will be having my
welder friend Jeff Sterling correct the cabane geometry. I've been threatening
to have him rework the main gear leg upper pivots on the airplane anyway, because
they have always had just a bit of slop due to the holes being ever-so-slightly
larger than the bolts, and I haven't wanted to ream them out to move up
a bolt size for fear of thinning the metal too much at the pivot/attach points.
Taxiing on rough ground, my gear makes a definite soft clunking as the gear
leg pivots bounce around on the bolts.
--------
Oscar Zuniga
Medford, OR
Air Camper NX41CC "Scout"
A75 power, 72x36 Culver prop
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=473202#473202
Message 5
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That lack of a true stall may just be a nicely behaved plane! In our Pacer I have
a similar situation where it never really breaks (just kind of a constant descent
rate mush) unless it's heavy and I really force a stall. Every Piet is
different but during the initial stall series testing in ours, I got very little
break, enough for a falling leaf maneuver (sounds like your mushing and nodding),
but nothing as sharp as say a Citabria (which is not really much of a stall
either!).
My comment about full elevator authority stems from the wing-to-horizontal relative
angle. According to your numbers earlier, you have about 1.5deg instead of
2deg relative. This could be solved by giving the horizontal stab -0.5deg angle-of-incidence
(i_H), but then you would have a non-standard deck angle. Sooo
it would probably be better to rework the wing cabanes or just accept that your
Piet has some nice gentle stall characteristics :)
--------
Kevin Bickers
Tehachapi, CA
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=473203#473203
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