In article
,
Alan Baker wrote:

In article ,
wrote:

On Fri, 14 Nov 2008 16:52:24 -0800, Alan Baker
wrote:

In article
,
Bob Kuykendall wrote:

On Nov 14, 4:10Â*pm, Alan Baker wrote:

...Where must the axis of the thrust vector be?

Through the centre of mass. Period.

Ah, I see. All those self-launch gliders and amphibs with pylon-
mounted engines must be figments of imagination. Thanks for clarifying
that.

Bob K.

Bob,

Let's put back what you snipped:

Now: when you start the engine and add its thrust into the equation,
what is the one direction in which you can apply that thrust and not
cause the aircraft to pitch?

I'm not saying that you can't have an engine mounted with the thrust
line pointed off the CoM. You can, but when you change engine power, the
aircraft is going to experience a pitch change.

I'm saying if you don't want pitch changes with changing engine thrust,
you have to mount it with the thrust line through the CoM.

And I'm right.

But if you don't believe me, will a reference from Princeton suffice?

http://www.princeton.edu/~stengel/MAE331Lecture9.pdf

"Pitching Moment due to Thrust
€ Thrust line above or below center of mass induces a pitching
moment"

Note: no mention of drag line...


Let's step back and take another look. The plane is trimmed for level
flight.All forces are ballanced. The aerodynamic drags (parasitic and
dynamic) and lifts (main wing and rear horizontal stab) balance out,
so the center of mass is what the plane will rotate around when
something other than aerodynamic trim causes the plane to pitch.
Moving the center of mass forward or backwards from the center of lift
will cause a pitching motion, as will adding thrust (or drag) which is
not applied through the center of mass.

Correct. And given that there can be at most precisely *one* AoA at
which the thrust line acts through the CoM, changing thrust will
normally cause a pitching moment.

Note, you can simplify your statement by removing the division between
parasitic and dynamic drag and by removing mention of the aerodynamic
trim. A change in that just changes the vector (and technically, the
position by a very small amount) of total aerodynamic force.

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