Thrust line: a no-thought experiment.

It has been put forward that for an aircraft to be in trimmed out
condition with all forces in balance, the thrust line must be pointed at
the drag line.

So let's do a thought experiment...

....wait.

We don't need to do a thought experiment.

We can simply observe a couple of aircraft.

The drag line is the line of the horizontal component of the total
aerodynamic force with its origin through the aerodynamic centre of
pressure, right?

OK. The aerodynamic CoP is pretty much always somewhere in the main wing
slightly behind the centre of mass CoM.

Look at any transport jet with engines mounted beneath the wings.

Where is the thrust line?

Not clear enough?

Let's look at a powered paraglider. The CoP is some 20 *feet* above the
CoM, and thus the drag line is up there too...

....but the engine is on the pilot's back...

....right near the CoM.

Hmmm...

....perhaps a thought experiment is necessary after all.

Imagine a paraglider redesigned as a rigid aircraft. *Why* anyone would
want to, I can't imagine, but go with it. Further imagine that the pilot
is enclosed in a nice aerodynamically slippery nacelle, so that almost
all the drag of the system is in the big fat wing 20 feet above his head.

Now say you want to be able to take this thing off with a rocket to gain
intial altitude and then glide back down, so you put a very lightweight
(so that the impact on the CoM of the system is minimal) rocket motor
somewhere on this bizarre craft:

So you're on the runway with your craft and you switch on the motor.

If you put it up where the drag line is and turn it on, what is going to
happen? Right: disastrous pitch, nose down.

Now put it so that it is aligned with the centre of mass of the system.

What happens? You soar into the air!

Clear enough?

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

"Alan Baker" wrote

So let's do a thought experiment...

Keep talking; perhaps you will convince someone other than yourself.
--
Jim in NC

In article ,
"Morgans" wrote:

"Alan Baker" wrote

So let's do a thought experiment...

Keep talking; perhaps you will convince someone other than yourself.

I can't help noticing you've somehow missed the cutsie reply when I
provided an actual reference that shows that your entire notion is
bull****...

http://www.av8n.com/how/htm/4forces.html

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

"Alan Baker" wrote

I can't help noticing you've somehow missed the cutsie reply when I
provided an actual reference that shows that your entire notion is
bull****...

You have failed to understand the question. A paper that answers a
different question is no good for answering the original question.

I'm done this time. Really.

Go ahead and get the last word. I know you will.
--
Jim in NC

In article ,
"Morgans" wrote:

"Alan Baker" wrote

I can't help noticing you've somehow missed the cutsie reply when I
provided an actual reference that shows that your entire notion is
bull****...

You have failed to understand the question. A paper that answers a
different question is no good for answering the original question.

No, it deals with the facts of the situation that show that your answer
cannot possibly the correct one.

In trimmed flight, aerodynamic drag and thrust *cannot* possibly be
pointed at one another except potentially at one particular angle of
attack.


I'm done this time. Really.

Go ahead and get the last word. I know you will.

Yup. You're running away having shown no math, no references...

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

Alan Baker wrote:

In article ,
"Morgans" wrote:


"Alan Baker" wrote


I can't help noticing you've somehow missed the cutsie reply when I
provided an actual reference that shows that your entire notion is
bull****...

You have failed to understand the question. A paper that answers a
different question is no good for answering the original question.


No, it deals with the facts of the situation that show that your answer
cannot possibly the correct one.

In trimmed flight, aerodynamic drag and thrust *cannot* possibly be
pointed at one another except potentially at one particular angle of
attack.


I'm done this time. Really.

Go ahead and get the last word. I know you will.


Yup. You're running away having shown no math, no references...



But he was right, and you were mostly wrong.



--

Richard

(remove the X to email)

On Nov 14, 6:00*pm, Alan Baker wrote:

I can't help noticing you've somehow missed the cutsie reply when I
provided an actual reference that shows that your entire notion is
bull****...

http://www.av8n.com/how/htm/4forces.html

Interesting that the above article does not address thrust acting
through centre of mass. Not once. Thrust opposes drag, and acts on
mass only during acceleration or deceleration. The thrust line does
not *have* to be aligned anywhere near the CG.
Too much of this discussion has ignored the stabilizer. There's a
thing called decalage, which is the difference in angles of incidence
between the wing and the stab/elevator. The stab is usually angled
down a degree or two relative to the wing's chord line and has the
effect of stabilizing the rotational couple imparted by the CG being
ahead of the CP. This downward alignment also has the necessary effect
of lifting the nose when power is applied so that the nose will fall
into a glide all by itself if the power fails or is reduced. This
aspect of the airplane is mandated by design standards, and is the
reason lifting tails were abandoned early in WWI.
(In lifting-tail airplanes a power reduction would lead to a pitch-
up and stall, and usually an unrecoverable spin. And it still happens
once n a while when some dude overloads his airplane with all that
beer in the back, CG way aft of the limit, and has to hold the yoke
forward to keep the nose from pitching up. Death often follows,
especially on approach when power is low and the tail can't be lifted
anymore.)
So thrust lines that aren't or can't be aligned near the CG are
not a big deal. The stab incidence is adjusted so that the desired
pitching occurs. Just look at the Lake amphibians, with their very
high thrust lines; The decalage is quite pronounced, and even at that
there's some odd behavior when the throttle is opened. But it still
glides like it should when the power's taken off.
http://www.seaplanes.org/graphics/members/la4200.jpg
Note that the thrust is pointed right at the stab to help control
pitching; it's not pointed through the CG at all. THAT would be
impossible.
See the decalage he http://mars.ark.com/~dcf/takeoff2.jpg
And he http://www.seabee.info/images/teal/N6595K-03-640.jpg

Now look at the decalage on various "ordinary" airplanes that have
their props on the nose, and see that there's very little compared to
those amphibians.

Decalage: http://en.wikipedia.org/wiki/Decalage

Dan

wrote


D: Interesting that the above article does not address thrust acting
through centre of mass. Not once. Thrust opposes drag, and acts on
mass only during acceleration or deceleration.
*********************

J: Once again, I did not have the 's pop onto this reply. Every once in a
while that happens, and I have no idea why. I'll use D: for the post I am
replying to,
and J: for my posts.

J: Yes, that (Thrust opposes drag, and acts on mass only during
acceleration or deceleration) is the key point that Alan could not seem to
understand. The center of mas is ONLY an important factor in understanding
where a rotating body rotates, ONLY while it is actually rotating-never mind
what forces caused the rotation. Once the rotation ceases, nobody needs to
think about where the center of mass actually is, other than to keep it as a
factor in the calculations to make sure ballanced flight is taking place,
and that the plane behaves in an expected way.

J: While the plane changes speed due to a change in thrust, if the thrust
line is not somewhat through the center of mass (wherever it is at that
moment) the plane would rotate for a couple seconds, _ _ if_ _ the pilot did
nothing to oppose it with control surface changes, until the plane settles
on its new speed.
*********************

D: The thrust line does not *have* to be aligned anywhere near the CG.
**********************

J: That should be very obvious for anyone to see, if they only think about
a few different designs, for just a few seconds.
***********************

D: Too much of this discussion has ignored the stabilizer. There's a
thing called decalage, which is the difference in angles of incidence
between the wing and the stab/elevator. The stab is usually angled
down a degree or two relative to the wing's chord line and has the
effect of stabilizing the rotational couple imparted by the CG being
ahead of the CP. This downward alignment also has the necessary effect
of lifting the nose when power is applied so that the nose will fall
into a glide all by itself if the power fails or is reduced. This
aspect of the airplane is mandated by design standards, and is the
reason lifting tails were abandoned early in WWI.
(In lifting-tail airplanes a power reduction would lead to a pitch-
up and stall, and usually an unrecoverable spin. And it still happens
once n a while when some dude overloads his airplane with all that
beer in the back, CG way aft of the limit, and has to hold the yoke
forward to keep the nose from pitching up. Death often follows,
especially on approach when power is low and the tail can't be lifted
anymore.)
*********************

J: You are correct, but many things have not been mentioned, and perhaps it
is because the whole subject of aeronautics is such a complex subject, with
so many things being interrelated to each of the other factors, that whole
books sometimes fail to address all of the different related factors. Your
mention of the factors above is right on, but should not be necessary to
understand the "thrust line" discussion taking place, but perhaps it will
help some people see what is happening.

D: So thrust lines that aren't or can't be aligned near the CG are
not a big deal. The stab incidence is adjusted so that the desired
pitching occurs. Just look at the Lake amphibians, with their very
high thrust lines; The decalage is quite pronounced, and even at that
there's some odd behavior when the throttle is opened.
*********************

J: EXACTLY true; it rotates _ _around the center of mass_ _
while and only while the plane is _ _actually rotating_ _. Once
the new speed is established, and the plane is in ballanced flight, the
thrust line (wherever it relates to incidences, and everything you have
mentioned) can be anywhere and it only serves to keep everything ballanced,
and the plane behaving properly.

D: But it still glides like it should when the power's taken off.
http://www.seaplanes.org/graphics/members/la4200.jpg
Note that the thrust is pointed right at the stab to help control
pitching; it's not pointed through the CG at all. THAT would be
impossible.
See the decalage he http://mars.ark.com/~dcf/takeoff2.jpg
And he http://www.seabee.info/images/teal/N6595K-03-640.jpg

Now look at the decalage on various "ordinary" airplanes that have
their props on the nose, and see that there's very little compared to
those amphibians.

Decalage: http://en.wikipedia.org/wiki/Decalage
******************************

J: Excellent post. You explained things well. The changed thrust line's
relation to the center of mass was a "blue herring", I think, in the whole
discussion. While it is true that if the thrust line is not through the
center of mass that the plane will tend to rotate about the center of mass,
that rotation attempt will be compesated for, and be prevented by control
movements. Those movements opposing the rotation are only necessaary until
rotation ceases; if it ever did even rotate. Once speed is stabilized, and
no rotation is taking place or trying to take place, the thrust line has no
relation to the center of mass. Only to all of the aerodynamic forces being
in ballance in trimmed flight.

J: I hope I did not confuse anyone with my response to your fine post. I
hope the subject is now fully explained and understood, and it can now be
put to bed.
--
Jim in NC

Morgans wrote:

"Alan Baker" wrote


So let's do a thought experiment...


Keep talking; perhaps you will convince someone other than yourself.


Not me.


--

Richard

(remove the X to email)

In article ,
cavelamb himself wrote:

Morgans wrote:

"Alan Baker" wrote


So let's do a thought experiment...


Keep talking; perhaps you will convince someone other than yourself.


Not me.

That's only because (apparently) I was asking too much when I expect you
to..

...you know...

....think.

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