Lancair Legacy Design Flaw?

In article
,
Alan Baker wrote:

Pusher or puller doesn't affect pitch forces. What affects pitch forces
is the length of the moment arm between the centre of mass and the
thrust line.

Or center of drag vs thrust line?

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

Pusher or puller doesn't affect pitch forces. What affects pitch forces
is the length of the moment arm between the centre of mass and the
thrust line.

Or center of drag vs thrust line?

The problem with that is "centre of drag" changes.

Better to take all the separate moments for the various components about
the centre of mass.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg

Very interesting post, keep it up guys.

Monk

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

Pusher or puller doesn't affect pitch forces. What affects pitch forces
is the length of the moment arm between the centre of mass and the
thrust line.

Or center of drag vs thrust line?

The problem with that is "centre of drag" changes.

And, as it does, so would pitch resultants.

Better to take all the separate moments for the various components about
the centre of mass.

Which will change as airspeed/AOA/etc. change, right?

Come to think on it, the total moments of drag don't care about any
masses, just the shape(s) of the aircraft exterior.

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

Pusher or puller doesn't affect pitch forces. What affects pitch forces
is the length of the moment arm between the centre of mass and the
thrust line.

Or center of drag vs thrust line?

The problem with that is "centre of drag" changes.

And, as it does, so would pitch resultants.

Better to take all the separate moments for the various components about
the centre of mass.

Which will change as airspeed/AOA/etc. change, right?

No. The centre of mass *never* changes (in the context of this
discussion.


Come to think on it, the total moments of drag don't care about any
masses, just the shape(s) of the aircraft exterior.

You have to take moments about something that isn't going to shift,
Steve. Centre of mass.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

Pusher or puller doesn't affect pitch forces. What affects pitch
forces
is the length of the moment arm between the centre of mass and the
thrust line.

Or center of drag vs thrust line?

The problem with that is "centre of drag" changes.

And, as it does, so would pitch resultants.

Better to take all the separate moments for the various components about
the centre of mass.

Which will change as airspeed/AOA/etc. change, right?

No. The centre of mass *never* changes (in the context of this
discussion.

No, mass (other than fuel onboard) doesn't. But drag components can
change with changes in speed, AOA, flap positions, landing gear, cooling
flaps, etc.

Come to think on it, the total moments of drag don't care about any
masses, just the shape(s) of the aircraft exterior.

You have to take moments about something that isn't going to shift,
Steve. Centre of mass.

I know that that doesn't change (ignoring fuel burn), but things like
flaps' contribution to drag moments changes with changes in
configuration.

Else we'd never see pitch changes as we raise or lower the flaps, or
changes in speed as landing gear are retracted or extended.

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

Pusher or puller doesn't affect pitch forces. What affects pitch
forces
is the length of the moment arm between the centre of mass and the
thrust line.

Or center of drag vs thrust line?

The problem with that is "centre of drag" changes.

And, as it does, so would pitch resultants.

Better to take all the separate moments for the various components
about
the centre of mass.

Which will change as airspeed/AOA/etc. change, right?

No. The centre of mass *never* changes (in the context of this
discussion.

No, mass (other than fuel onboard) doesn't. But drag components can
change with changes in speed, AOA, flap positions, landing gear, cooling
flaps, etc.

Right. So take moments about the thing that doesn't change.


Come to think on it, the total moments of drag don't care about any
masses, just the shape(s) of the aircraft exterior.

You have to take moments about something that isn't going to shift,
Steve. Centre of mass.

I know that that doesn't change (ignoring fuel burn), but things like
flaps' contribution to drag moments changes with changes in
configuration.

Else we'd never see pitch changes as we raise or lower the flaps, or
changes in speed as landing gear are retracted or extended.

Right. But trying to take moments about a centre of drag that is
changing because of the very thing causing you to take the moments in
the first place is a recipe for madness.

Just take them about the centre of mass!

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

Pusher or puller doesn't affect pitch forces. What affects pitch
forces
is the length of the moment arm between the centre of mass and
the
thrust line.

Or center of drag vs thrust line?

The problem with that is "centre of drag" changes.

And, as it does, so would pitch resultants.

Better to take all the separate moments for the various components
about
the centre of mass.

Which will change as airspeed/AOA/etc. change, right?

No. The centre of mass *never* changes (in the context of this
discussion.

No, mass (other than fuel onboard) doesn't. But drag components can
change with changes in speed, AOA, flap positions, landing gear, cooling
flaps, etc.

Right. So take moments about the thing that doesn't change.

Looks to me that we're in something resembling violent agreement.

The center of mass doesn't change (ignoring fuel burn).

The drag components measured from that center certainly do, however.

"Alan Baker" wrote

Right. But trying to take moments about a centre of drag that is
changing because of the very thing causing you to take the moments in
the first place is a recipe for madness.

Just take them about the centre of mass!

It matters not what you take the moments from, as long as it is from a
stationary reference on the plane.
--
Jim in NC

"Steve Hix" wrote in message
...
In article
,
Alan Baker wrote:

You have to take moments about something that isn't going to shift,
Steve. Centre of mass.


Steve replied ...

I know that that doesn't change (ignoring fuel burn), but things like
flaps' contribution to drag moments changes with changes in
configuration.

Else we'd never see pitch changes as we raise or lower the flaps, or
changes in speed as landing gear are retracted or extended.

Semantics can really make things confused. In aerodynamics and aircraft
design we simplify things to make the
math a bit easier. Instead of dealing with forces that move all over the
place and make it difficult to superimpose
mathematically to calculate their resolution and the resultant vectors which
would get relatively intractable, we
pick a point more or less at random and say "The force acts HERE!" Then we
figure out the moments about that
point contributed by our other factors. Then we merely sum up the moments,
using the appropriate arithmetic for
summing up vectors of course, and determine the resultant forces relatively
easily. To make this simplification
work we need to pick arbitrary points that don't move around. Things like
the "center of mass", or the "center of
lateral area", or the "center of gravity", or the "quarter chord line of the
lifting surface" and then we find how the
moments change about that point. We can even play additional simplification
games and normalize the actual moments
into a dimensionless coeficcient. If you look at a plot of the forces on a
wing you will see alongside the lift/drag
polar, another line labeled "moment coefficient." This moment coefficient
tells you how the center of lift moves about
relative to the quarter chord line of the wing. The neat thing about the
NACA 23012 airfoil section is the moment
coefficient is approximately zero throughout the normal angle of attack
range. This means the center of lift doesn't move
around very much on the wing. This is what made possible the monospar wing
on the DC-3! :-)

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )