Chris, do you have any references as to the Neutral Point being the
same as the MAC [mean aerodynamic chord]?
All the book's I've owned tell me the MAC is calculated on the wing
platform/shape while the NP is derived from this and the tail volume
[which is composed of an arm, MAC and a area value].
Just wanted to confirm what is correct and incorrect. Also, do you
have any feedback as to how the Aerodynamic Center applies to all of
this?
Thanks
J. Johnson
235/320 55% [and holding]
> Wolfgang, et al
> The aircraft MAC (also called neutral point)
relative to CG is the
> key to
> evaluating aircraft
longitudinal stability. This is independent
> of whether the
> tail is providing an up or down force (either can be
stable).
> Longitudinal
> stability is defined by the
reaction of the entire airframe to a
> disturbance
> from
equilibrium. The size, location and pitching moment
>
characteristics
> of each component factors in (wing, tail,
fuselage
> etc.). Evaluating the
> behavior
of just the wing is not sufficient to describe the
> response of
the
> aircraft as a whole and certainly not to quantify the
response.
> Actually, a
> wing section alone will be
unstable as the pitching moment is
> negative. It is
>
stable when inverted - flying wings have negative camber for this
>
reason.
> A stable aircraft must have
a positive pitching moment when in
> equilibrium. In
> order to be stable, the pitching moment
coefficient must have a
> negative slope
> with
increasing angle of attack. This provides an increasing
>
opposing moment to
> an increasing disturbance.
> A larger tail increases the response when a disturbance
occurs.
> It is a
> function of the larger area
producing more restoring force for any
> given angular
> disturbance. The size of the horizontal
stabilizer feeds into a
> quantity called
> the tail
volume ratio - a unit-less measure of relating tail area
> to wing
area
> and wing mean wing chord to distance to the horizontal
> stabilizer. More area
> or a longer tail increase
the effectiveness in terms of stability.
> The neutral point is
fixed by the configuration of the
> aircraft. Only
> configuration changes will move the neutral point. Lowering the
> flaps, for
> example, changes the airfoil, relative incidence
angles, pitching
> moment of the
> wing and so on. In all
configurations the neutral point must
> remain well behind
> the
CG. 10% of the mean chord length is a good starting minimum.
> Once the
> neutral point is known, the incidence angles and CG
can be set.
> What will fall
> out is the trim
airspeed. That is, in equilibrium the aircraft
> will seek
out a
> specific angle of attack and the corresponding airspeed.
One can
> play around
> with different combinations
of incidence angles and CG locations
> to achieve both
> a
stable aircraft and minimum trim drag at any desired airspeed.
>
hope that helps,
> Chris
>
>
>
> Chris
Zavatson
> N91CZ
> 360std
> www.N91CZ.com
>
>
>
>
>
>
________________________________
> From: Wolfgang
<Wolfgang@MiCom.net>
> To: lml@lancaironline.net
> Sent:
Wed, July 14, 2010 10:37:18 AM
> Subject: [LML] Re: Small tail, MK II
tail, CG range
>
>
> I'm not familiar with MAC as applied
to the entire airframe, can
> you elaborate?
> I think there may
be a problem with that idea since the tail is
> typically
>
providing a down force which would move the "airframe MAC" to the
> front, not the
> rear.
>
> Wolfgang
>
> ________________________________
>
> -----
Original Message -----
> >From: Chris Zavatson
> >To:
lml@lancaironline.net
> >Sent: Tuesday, July 13, 2010 8:35 PM
> >Subject: Re: [LML] Small tail, MK II tail, CG range
> >
> >
> >Wolfgang, et al,
> ><<Any more to
the rear and you get negative stability at cruise
> and a larger
> >tail doesn't help much with that anyway.>>
> >
> >A larger tail moves the MAC rearward allowing the CG to move
> farther aft while
> >maintaining the same level of
stability.
> >There has been a lot of discussion about
Cm. We need to be
> careful to
> >distinguish between
the Cm for the wing, tail and total
> aircraft. It is the
> >later that is critical to stability and this is where the larger
> tail influences
> >the situation. The large tail
moves the MAC to the rear approx.
> 1.5 inches.
> >For
the same CG, the more rearward MAC produces a greater
> restoring force
if the
> >plane is disturbed from level flight. The practical
benefit for
> us is that it
> >allows a lot more baggage
to be thrown the rear of the plane
> before
>
>suffering stability problems. You pointed out the other benefit
> of increased
> >control authority at slow speed with full
flaps.
> >
> >Chris Zavatson
> >N91CZ
>
>360std
> >www.N91CZ.com
> >
> >
>
>
> >
> ________________________________
> From:
Wolfgang <Wolfgang@MiCom.net>
> >To: lml@lancaironline.net
> >Sent: Tue, July 13, 2010 2:51:23 AM
> >Subject: [LML]
Small tail, MK II tail, CG range
> >
> >
> >The
quest continues.
> >
> >I'm checking further into the data
on these questions and am
> coming to question
> >the need
for a larger tail. I'm not sure a larger tail by itself
> will solve
the
> >problem. After doing some static and in flight measurements,
it
> looks like the
> >tail authority is not a big problem,
if a problem at all.
> >
> >Static measurements of N31161
have shown "vanilla" parameters.
> 2.5º incidence
> >between
the wing root at full reflex and the tail and a 1.3º
> washout. Put the
> >flaps at 0º and you get an additional AoA of 1.8º at the root for
> a total
> >incidence of 4.3º . . . . not radical at all.
> >
> >What is interesting is the POH (Dec. 1994 pg. VI-3)
gives the CG
> range as 24.5"
> >to 30.3" aft of the rear
face of the fire wall and the MAC at 15%
> to 20%
> >
> >. . . well . . . no . . . that range is more like a MAC range of
> 15% to 30% - -
> >- a good range made touchy only by the
small size of the air frame.
> >
> >After going over the
plan view kit drawings, I come up with a CG
> range of
>
>23-1/4" to 29-1/4" for a MAC range of 15% to 30%
> >That range
is about 1-1/4" forward of the book and fits better
> with first hand
> >flight experience.
> >
> >
> >Any
more to the rear and you get negative stability at cruise and
> a
larger tail
> >doesn't help much with that anyway.
> >
> >Negative stability makes pitch control a real chore. As Scott K.
> has indicated,
> >going to 0º flaps helps under that
loading condition.
> >
> >Too far forward and landing
becomes "interesting". A larger tail
> can help there
> >. .
. or don't use as much flaps.
> >
> >I think understanding
these conditions can help everyone.
> >
> >. . . The quest
continues . . . Comments welcome.
> >
> >Wolfgang
>
>
> >
> >
>
________________________________
>
> >From: "Wolfgang"
<Wolfgang@MiCom.net>
> >Sender: <marv@lancaironline.net>
> >Subject: Small tail, MK II tail, CG range
> >Date: Sat,
10 Jul 2010 21:01:11 -0400
> >To: lml@lancaironline.net
> >The LNC2 uses the NLF(1)-0215F airfoil. A lot can be found by
> doing a Google
> >search on that number.
> >More
detail can be found by going to Google for "NASA Technical
> Paper
1865".
> >
> >I have not taken the time to reverse
engineer the CG range of the
> LNC2 but let
> >me offer some
observations.
> >
> >The airfoil used has long been touted
as "the greatest thing
> since sliced bread"
> >for General
Aviation and it definitely has some advantages. But
> it's not new.
> >Compare this airfoil to the P-51 airfoil and you will see some
> close
> >similarities. The LNC2 being composite
construction instead of
> aluminum lets the
> >airfoil show
more of it's theoretical advantages.
> >
> >It's a laminar
shape with a good drag bucket. That bucket can be
> made to move to
> >the lower Cl (lift coefficient) ranges with reflex allowing
> noticeably lower
> >drag at higher cruise speeds. Along
with reflex, the Cm (moment
> coefficient)
> >goes positive,
the center of lift of the wing travels forward
> giving a nose up
> >force requiring down trim. This is in addition to the usual nose
> up force that
> >goes with most all airfoils at high
speed before considering flaps.
> >
> >With down flap, the
drag bucket will move to higher Cl's making
> slower flight
>
>more efficient. And, of course, the Cm goes negative giving a
>
nose down force
> >requiring up trim.
> >
> >. .
. and appropriate variations in-between . . .
> >
> >
> >So, the rear CG limit is determined by high speed flight and
> available control
> >authority,
> >and the
forward CG is determined by low speed / landing flight
> and available
> >control authority.
> >
> >What is becoming
clear here is that the center of lift does quite
> a bit of
>
>traveling fore and aft which is exaggerated by allowing negative
>
or "cruise"
> >flaps. Since you can't shift the CG during flight,
you need a
> large amount of
> >pitch authority from the tail
to cover that range of lift travel.
> >
> >You have
two choices in the LNC2, live with the limitations or
> install a
larger
> >tail to give that extra pitch authority.
> >. .
. A larger tail area can also help with abnormal
>
attitude recovery.>
> >Wolfgang
> >
>
>
>
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