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

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)

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)

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

In article ,
cavelamb himself wrote:

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.

No. He was completely wrong and I was completely correct.

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

"Alan Baker" wrote in message
...
In article ,
cavelamb himself wrote:

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.

No. He was completely wrong and I was completely correct.

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

No, you are wrong!

The OP, Clare, was concerned about the effect of a deviation from the plans
for the aircraft he is building, and sought advice on this forum.

You, in turn, have fixated on a particular geometric relationship as though
it is always critical-which it is not. It is certainly interesting, and it
has been used to advantage in a few designs--such as the Ercoupe. The
easiest way to verify this is simply to watch an Ercoupe taxi past--it looks
"weird" because so few designs have their engine geometry adjusted to
trivialize the effects of propeller thrust.

However, in most single engine land airplanes, the effect is modest and
easily managed--although a slight nose up tendency with power is admittedly
the norm.

In the case of the Pegazair, which was the subject of the original question,
I can not even begin to guess whether a slight increase in the height of the
thrust line would cause a slight nose up or down tendency as compared to the
original design. My experience with Cessna 150s and 172s suggests that the
camber of the wing exerts a large influence on the horizontal
stabilizer--and I can not guess whether that effect would be increased or
decreased.

Peter