Aerodynamic question for you engineers

D Ramapriya wrote:
On Jan 26, 5:31 am, Jim Logajan wrote:

As I understand it, the force of the tail plane's elevators typically
moves the center of lift forward and backward along the airplane's
axis as the elevators are moved up and down (as well as changing the
lift magnitude a little - though that is secondary). One presumably
enters stable flight when the center of lift is moved to coincide
with the center of gravity.


Since the CL can be altered by the wing configuration - deployment/
retraction of flaps for a given pitch, e.g., I'm not sure that the CG
and CL need to necessarily coincide for stable flight. Also, for a
body such as an aircraft, I think the CG would theoretically be
somewhere within it while the CL is a point on the fuselage, so their
coincidence may even be an impossibility.

If the total lift vector does not pass through the center of gravity, then
the resulting moment will rotate the aircraft. That is not considered a
stable situation. Here's what I hope is considered an authoritative web
site that discusses this issue:

http://www.grc.nasa.gov/WWW/K-12/airplane/acg.html

See also any text on flight mechanics and aerodynamics that has sections on
the subject of longitudinal static stability.

On Jan 26, 1:50*pm, Jim Logajan wrote:
Phil J wrote:
Actually as I understand it in stable flight the CL is aft of the CG.
The airplane remains level not because these two are in line, but
because the tail is pressing down to counterbalance the offset of the
CL.

I should have used the term "total lift" so as to avoid confusion with the
lift generated only by the main wings. What you state above appears
internally consistent and correct with the definitions you are using.

After thinking about this question some more, it strikes me that this
situation is equivalent to a lever and fulcrum. *The lever doesn't
rotate around it's CG, it rotates around the fulcrum point. *In an
airplane, this point is the center of lift.

Whether you are talking about center of total lift (that generated by the
main wings, tail or canards, and fuselage) or center of lift of the main
wings, what you state above is _incorrect_.

I know that what you wrote sounds plausible, but the problem is that the
main wings are no more a fulcrum than the tail wings. Suppose the main wing
and the tail wing are very nearly the same size and produce nearly the same
lift and all have elevator controls? Which line is the fulcrum point now
about which the airplane rotates?

Regarding the CL moving around, I think even given that complication
the airplane would still rotate around the CL.

Here's a NASA web link that explains where the rotation point is:

http://www.grc.nasa.gov/WWW/K-12/airplane/acg.html

Try to find some books on flight mechanics and look for the chapters or
sections that appear to discuss longitudinal static stability of aircraft.
They should all say that the aircraft rotates about the center of gravity.

Ok, I think it see it. There is a difference between the center of
lift and the location of the total lift vector (I guess you could call
this the net lift). In a non-canard airplane, the main wing is
pushing upward and that is the center of lift we have been
discussing. But the stabilizer is pushing downward. The net effect
of these two forces is to move the location of the total lift vector
forward to the CG location, and that results in stable flight. So a
rotational force will rotate the airplane around that point just like
a lever rotates on a fulcrum. The same thing would happen in a
canard, except that the location of the total lift vector would be
between the two wings since they both push upward.

Phil

Jim Logajan wrote in
:

Bertie the Bunyip wrote:
Jim Logajan wrote:
As I understand it, the force of the tail plane's elevators
typically moves the center of lift forward and backward along the
airplane's axis as the elevators are moved up and down (as well as
changing the lift magnitude a little - though that is secondary).
One presumably enters stable flight when the center of lift is moved
to coincide with the center of gravity.


That's exactly the case if you include the stab in the CL equation.
If you're just referring to it on the wing itself, providing the AoA
and speed remain the same it doesn;t shift. It's a matter of
definition.

Just checked one of my references[*] for proper terminology - where I
used "center of lift" it uses the phrase "total lift" with the symbol
L. For the lift of the main wings it uses Lw and for the lift of the
tail it uses Lt.


Sounds about right. I haven't read that stuff in years, though.


Bertie

"Marc J. Zeitlin" wrote in message ...
Blueskies wrote:

A canard design is just the opposite. The CG is behind the CP, and
when the canard stalls, the nose drops because their is no longer
any lift to hold it up. This is why canards must always stall the
front wing first.

Your explanation of everything else was good, but this is incorrect.
For ALL aircraft - conventional, canard, tandem - doesn't matter - the
CG MUST always be ahead of the aerodynamic center (center of lift,
center of pressure, neutral point - all terms are sometimes used
interchangeably) for positive static stability in the pitch direction
to exist. Put the CG behind the AC and the plane becomes unstable in
pitch - the further back, the more unstable and harder to fly.


Eikes! You are correct of course. I know this, and that is part of the reason the canard configuration is more
efficient; all the flying surfaces are countering gravity...
Well, I get a B- on this one ;-)


In ALL aircraft, the front wing must stall first to avoid deep stalls
and maintain control in the stall to allow recovery. In canards (one
of which I built and fly), as in all planes, the stall is not a
complete loss of lift, but either a leveling off or a slight drop in
lift as the AOA increases.

This has been a bizarre discussion for this engineer, because by definition, if you separate translations from
rotation, all rotation occurs about the CG of a mass.

And with respect to the four-bar linkages in cabinet hinges (and car trunk hinges, etc.), depending upon the design,
center of rotation of the moving member (door, trunk, etc.) can be continually changing.

And for our friend from OZ, statements like "totally wrong", "clueless" and your insult of Gerry Caron don't really
add anything to the conversation.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2008 http://www.mdzeitlin.com/Marc/

D Ramapriya wrote in news:7e76f9d7-32f5-4585-
:

On Jan 26, 4:12 pm, Stealth Pilot
wrote:
On Fri, 25 Jan 2008 19:15:13 -0800 (PST), D Ramapriya

wrote:

Since the CL can be altered by the wing configuration - deployment/
retraction of flaps for a given pitch, e.g., I'm not sure that the
CG
and CL need to necessarily coincide for stable flight. Also, for a
body such as an aircraft, I think the CG would theoretically be
somewhere within it while the CL is a point on the fuselage, so
their
coincidence may even be an impossibility.

Ramapriya

totally wrong.

Stealth Pilot


While the CG is unchanging - ignoring CG travel due to fuel burn and
pax moving around - the CP (CL) changes with the AoA. I think it keeps
moving forward as the AoA increases. Thus, so long as the CP (CL) is
close to the CG, stable flight should be possible and their
coincidence isn't a sine qua non.

Still all wrong?


Look, you've obviously got a narrow grasp on this, but you're all over
the place with definition. Now, to be fair, there are conflicting texts,
(lots of them) but it all works the one way.
You're trying to split hairs with a chainsaw.
In short, you've got a slim but perverted grasp of the physics but
you're not speaking the right language to learn any more here.
Go get a decent book on it if you real ywant to know and then come back
and ask again. You need to swap the chainsaw for a scalpel


Bertie

wrote in news:532e84e3-c288-47df-b2dc-
:


Kerschner likely has something to say on it. I'll have a look.

Kershner is great. I used to teach from his book, but it's not a physics
book...


Bertie

On Jan 26, 9:52 am, Phil J wrote:
After thinking about this question some more, it strikes me that this
situation is equivalent to a lever and fulcrum. The lever doesn't
rotate around it's CG, it rotates around the fulcrum point. In an
airplane, this point is the center of lift.

Regarding the CL moving around, I think even given that complication
the airplane would still rotate around the CL.


This might hold if the CL (or CG as some would argue) is
rigidly fixed. But in flight, we're in a rather elastic medium, and
things move around, even leaving out forward motion.
Imagine, for example, two kids on a seesaw or teeter-totter or
whatever name by which you know that playground thing. Two kids, same
distance from the pivot, same weight. The board rotates around the
pivot. The CG is at the pivot. No argument there. But suppose we had a
different mounting for that pivot, one where the pivot was suspended
by a couple of springs. Same kids, same weight, board level and kids
motionless. (Yeah, right: motionless kids.) Now I walk up to one kid
and shove down on him; where will the board *really* pivot? As i push
down, the pivot point will move down some, too, because of the mass
and inertia of the kid at the other end. Now the real point of
rotation is somewhere along the board between the pivot and the far
kid, and it'll move back toward the pivot as that kid starts to move
upward. At any instant in this process it's somewhere besides the
original CG.
We could complicate things: A heavier kid near the pivot, a light
kid at the other end, but this light kid is a little too light, so we
have a small spring pulling down under his seat, just enough to keep
the seesaw level. Just like the engine in our airplane (big kid near
the pivot), the mass of the airplane behind the CG (light kid) and the
elevator's downforce (little spring). The main pivot, still on big
springs (wing in the air) will still move downward at the instant I
shove down on the light kid and the real rotational point will be
somewhere on the big kid's side of the pivot.
Rotation about the CG works if we ignore all the other
variables. Trouble is, those variables are with us every time we fly.
We can watch an aerobatic airplane twisting around in the air,
appearing to rotate around its CG, but is it really? Can we see the
small displacement of that point (do we even know exactly where it is
just by looking at the airplane?) at the instant of any change in tail
forces or flight path?
Like I said earlier, CG is probably good enough for our puddle-
jumper purposes, but I think the guys who study advanced aerodynamics
would have something to add to it. I don't think it's really all that
simple.

Dan

On Sat, 26 Jan 2008 10:00:43 -0800, "Marc J. Zeitlin"
wrote:



And for our friend from OZ, statements like "totally wrong",
"clueless" and your insult of Gerry Caron don't really add anything to
the conversation.

did Gerry?

wrote in message ...
On Jan 26, 9:52 am, Phil J wrote:
After thinking about this question some more, it strikes me that this
situation is equivalent to a lever and fulcrum. The lever doesn't
rotate around it's CG, it rotates around the fulcrum point. In an
airplane, this point is the center of lift.

Regarding the CL moving around, I think even given that complication
the airplane would still rotate around the CL.


This might hold if the CL (or CG as some would argue) is
rigidly fixed. But in flight, we're in a rather elastic medium, and
things move around, even leaving out forward motion.
Imagine, for example, two kids on a seesaw or teeter-totter or
whatever name by which you know that playground thing. Two kids, same
distance from the pivot, same weight. The board rotates around the
pivot. The CG is at the pivot. No argument there. But suppose we had a
different mounting for that pivot, one where the pivot was suspended
by a couple of springs. Same kids, same weight, board level and kids
motionless. (Yeah, right: motionless kids.) Now I walk up to one kid
and shove down on him; where will the board *really* pivot? As i push
down, the pivot point will move down some, too, because of the mass
and inertia of the kid at the other end. Now the real point of
rotation is somewhere along the board between the pivot and the far
kid, and it'll move back toward the pivot as that kid starts to move
upward. At any instant in this process it's somewhere besides the
original CG.
We could complicate things: A heavier kid near the pivot, a light
kid at the other end, but this light kid is a little too light, so we
have a small spring pulling down under his seat, just enough to keep
the seesaw level. Just like the engine in our airplane (big kid near
the pivot), the mass of the airplane behind the CG (light kid) and the
elevator's downforce (little spring). The main pivot, still on big
springs (wing in the air) will still move downward at the instant I
shove down on the light kid and the real rotational point will be
somewhere on the big kid's side of the pivot.
Rotation about the CG works if we ignore all the other
variables. Trouble is, those variables are with us every time we fly.
We can watch an aerobatic airplane twisting around in the air,
appearing to rotate around its CG, but is it really? Can we see the
small displacement of that point (do we even know exactly where it is
just by looking at the airplane?) at the instant of any change in tail
forces or flight path?
Like I said earlier, CG is probably good enough for our puddle-
jumper purposes, but I think the guys who study advanced aerodynamics
would have something to add to it. I don't think it's really all that
simple.

Dan



In a sense it is that simple. The CG does move due to accelerations of the aircraft in flight (your spring analogy is
close), but the aircraft still rotates around the center of mass at any given moment (no pun intended!).

Dan also...d²

On Jan 26, 11:50 pm, Nomen Nescio wrote:

From:

Like I said earlier, CG is probably good enough for our puddle-
jumper purposes, but I think the guys who study advanced aerodynamics
would have something to add to it. I don't think it's really all that
simple.

Yea, it's REALLY all that simple!

There's a flaw in your seesaw example that makes it distinctly different
from an aircraft. Figure out the flaw, and reality will fall right into place.

You're no help at all. Maybe you could point out the flaw: I
would be pleased to know what it is so I can retract my analogy if it
IS wrong.

Dan