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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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