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Chris,
Hmmmmm, as I remember, a problem could/would sometimes arise with aft CG if
one skidded the airplane in landing configuration at slower speeds
where the nose would bob up and down. As an experiment, I
found it very hard to produce significant bobbing whilst at a more fwd
CG and in the landing configuration.
Not a problem for me as I always crabbed in a crosswind until just before
touchdown. So many ways to skin a cat ...............
Scott
BTW, only the top half of the chart came through ???
In a message dated 9/18/2013 10:18:41 P.M. Central Daylight Time,
chris_zavatson@yahoo.com writes:
Scott,
Our little Lancairs with a wing aspect ratio around 6.6
are definitely in the Hawk category - so you do have an appropriate
call-sign.
Horizontal stabilizers have a few other issues. They live in a
terrible place aerodynamically speaking. First there is the
intersection and proximity to the fuselage, then the downwash
off the wing that diminishes their effectiveness. Then, at high
angles of attack, they can enter the wake of the wing. This low energy
air further diminishes their effectiveness and hinge moments start to
fall off. It is this region that is of most interest. This is
where all the anecdotal reports of stick force reversals in the flare get
started. The stories about the nose pitching up on its own,
told at the Lancair fly-in after folks arrive with the plane packed full of
people and baggage .
I think the most important chart in all of the reports is Figure 12
in the stability and control write-up. It shows a well behaved
increase in AoA with stick force throughout, even with an aft CG.
Chris
Chris Zavatson
N91CZ
360std
From: "Sky2high@aol.com"
<Sky2high@aol.com>
To:
lml@lancaironline.net
Sent:
Tuesday, September 17, 2013 12:35 PM
Subject: [LML] Re: Lancair 320/360
performance and stability
Chris,
You're talking to GrayHAWK, not Al Batross. I.E. (from
Wikipedia):
High aspect ratio wings abound in nature. Most birds that fly long
distances have wings of high aspect ratio, and with tapered or elliptical
wingtips. This is particularly noticeable on soaring birds such as albatrosses
and eagles. By contrast, hawks ... have wings of low aspect ratio ... for
maneuverability. Everything is aviation is a compromise.
I hope a small tailed 360 steps up.
Scott
In a message dated 9/17/2013 2:18:50 P.M. Central Daylight Time,
chris_zavatson@yahoo.com writes:
Scott,
Thanks. Examining the 360 (MkII) performance and
characteristics in greater detail as been very interesting.
The small tail has a very low aspect ratio and may indeed be
subject to higher drag if the stabilizer incidence requires significant
elevator input to trim. The MkII tail adds about 2 sqft, but more
significantly has a much greater aspect ratio. My stab was well
aligned for the sweep of flap settings as the elevator deflection was about
0.5 degrees TE down. In fact, all of the points were inside
of 0.1 degrees of elevator movement.
The concept of aft CG being more efficient is by reducing
trim drag. It is used quite successfully in aircraft that adjust the
entire stabilizer for trim. A fixed stab angle that is too far
from neutral in the aft CG or in the 'super-reflexed'
cruise condition could negate any benefit. In my case
the plot of flap setting vs. airspeed showed that I had not yet reached a
peak. Extrapolating the curve gives me another 2 kts at 12 degrees
reflex. Extrapolating is a bit dangerous with any polynomial curve,
but on the other hand this one has an exceptionally
well behaved 2nd order trend. -7 degrees certainly provides
a large portion of the benefit.
It would be very interesting to run through the same series of
tests with a small tail at the same static margins for a side by
side comparison.
Chris
Chris Zavatson
N91CZ
360std
From:
"Sky2high@aol.com" <Sky2high@aol.com>
To: lml@lancaironline.net
Sent: Sunday, September 15, 2013 12:26
PM
Subject: [LML] Re:
Lancair 320/360 performance and stability
Chris,
Great research.
In my small tailed 320, increased flap reflex experimentation did
not result in increased top end speed. The nose up pitch was
increased, requiring increased nose down trim - probably resulting in
greater empennage drag negating any reduction in drag from the
greater reflex. Of course, we would have to discuss the angle of
incidence of the small tail and its relationship to the elevator correcting
for nose down pitching ( my incidence was at -.9 degrees).
By moving weights forward and aft in the same flight, forward CG
was better for maximizing speed - unlike some aircraft that see max
speed when the CG is at the neutral point, probably a consequence of more
standard wing/tail design that saw drag from wing/horizontal +/- lift
factors more balanced and minimized.
For me, the biggest gain in speed came from adding $140 worth
of gap seals to both sides of all control surface and the flaps
(remember Greenameyer made his flaps part of the wing in his very fast
Reno Legacy - he didn't need no stink'n flaps). I believe that I lost
no speed when I removed the upper seals from the flaps last year. The
seals were curved Mylar seals often obtainable for gliders. The
gain was from 6 to 8 KIAS, depending. Controls were more responsive
and the wee rudder was effective about 5 KIAS sooner.
I didn't follow up on a cockpit controlled diffuser for managing cooing
drag as I saw about 7" H2O upper to lower cowl at about 135 KIAS (climb
speed), but 13" at 200 KIAS and such pressure was not necessary as the
engine ran cool there.
I found the small tail had enough control - I only wanted my little
engine to pull everything along as quickly as possible.
Scott Krueger
In a message dated 9/13/2013 7:32:54 A.M. Central Daylight Time,
chris_zavatson@yahoo.com writes:
N91CZ has been a flying laboratory for most of this year. Below
are links to three reports that may be of interest to the Lancair
community, in particular 320/360 flyers.
The first takes a look at the effect flap position has on
total aircraft drag using the NLF(1)-0215. The numbers are
quite impressive in terms of drag coefficients. In the end, it
looks like we could benefit from a little more reflex beyond -7
degrees.
The second report looks at the neutral point differences and static
margins of the small and large tail 320/360 models. A large portion
of the document is a tutorial of sorts on longitudinal stability. It
deals only with static stability, but is a good lead-in to the third
report.
The third report looks at the stability of the 360 MKII in much
greater detail. It include dynamic stability in both cruise and
landing configurations, as well as, stick force gradients and elevator
effectiveness all the way down to stall speed.
Chris Zavatson
N91CZ
360std
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