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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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