Forward CG Experience

"Koopas Ly" wrote in message
om...
I would think that the only source of increased drag from a forward
c.g. condition is profile drag due to a more nose-up elevator trim tab
or elevator.

There are several factors that translate into increased drag:

* Drag from the trim, if used (as you noted)
* Drag from the elevator itself (as you noted)
* Increased induced drag from the horizontal stabilizer/elevator due to
increased lift on that airfoil
* Increased induced drag from the wings since the increase in lift on the
horizontal stabilizer translates into added weight for the aircraft, which
has the exact same increase in induced drag that adding physical weight to
the aircraft would have

How do you see a forward c.g.'s extra drag translating into premature
airspeed bleeding? Sink rate and angle of descent would
increase...but airspeed?

I'm not sure I understand your question. Is this a continuation of the "why
is there increased drag?" question? Or are you asking, even if one assumes
increased drag, why does the airspeed bleed off quicker?

If the former, I hope my earlier bullet points answer your question. If the
latter, that should be obvious. For a given configuration, deceleration is
strictly related to the net difference between thrust and drag. When thrust
is greater than drag, you accelerate. When thrust is less than drag, you
decelerate.

Furthermore, the rate at which you decelerate is directly proportional to
that net difference. For a given thrust, more drag means a greater rate of
deceleration. Moving the CG doesn't affect thrust, but it does affect drag.
Moving CG forward increases drag (as noted above) and thus increases the
deceleration rate.

Pete

Pete,

Comments in your text.


I would think that the only source of increased drag from a forward
c.g. condition is profile drag due to a more nose-up elevator trim tab
or elevator.

There are several factors that translate into increased drag:

* Drag from the trim, if used (as you noted)
* Drag from the elevator itself (as you noted)
* Increased induced drag from the horizontal stabilizer/elevator due to
increased lift on that airfoil
* Increased induced drag from the wings since the increase in lift on the
horizontal stabilizer translates into added weight for the aircraft, which
has the exact same increase in induced drag that adding physical weight to
the aircraft would have

Agreed.


How do you see a forward c.g.'s extra drag translating into premature
airspeed bleeding? Sink rate and angle of descent would
increase...but airspeed?

I'm not sure I understand your question. Is this a continuation of the "why
is there increased drag?" question? Or are you asking, even if one assumes
increased drag, why does the airspeed bleed off quicker?

If the former, I hope my earlier bullet points answer your question. If the
latter, that should be obvious. For a given configuration, deceleration is
strictly related to the net difference between thrust and drag. When thrust
is greater than drag, you accelerate. When thrust is less than drag, you
decelerate.

Furthermore, the rate at which you decelerate is directly proportional to
that net difference. For a given thrust, more drag means a greater rate of
deceleration. Moving the CG doesn't affect thrust, but it does affect drag.
Moving CG forward increases drag (as noted above) and thus increases the
deceleration rate.

I agree that your deceleration is equal to (Thrust - Drag)/mass.

Even though the airplane momentarily decelerates due to the increased
drag, I ideally presume that the airplane's trimmed angle of attack
has not changed (if you consider that the forward c.g. shift occured
in flight). The assumption is probably invalid since, as you
mentioned in your last point, the wing needs to develop more lift to
offset the increase in tail downforce. The differential lift would
require a change in either trimmed speed or angle of attack.

However, ignoring this fact, if the airplane was originally trimmed
for level flight, I contend that you would only start experiencing a
slight descent rate at an airspeed no different than prior to the
forward c.g. shift.

Your thoughts?

Have a good weekend,
Alex

I contend that you would only start experiencing a slight descent
rate at an airspeed no different than prior to the forward c.g.
shift...Your thoughts?

Very good!

How about this: since the increased drag leads to an increased
descent rate with the power off, you will have to increase your angle
of attack at a greater rate during your flare in order to maintain a
constant altitude above the runway. Since you're increasing your AOA
more rapidly, your airspeed will be falling more rapidly.

"Koopas Ly" wrote in message
om...
However, ignoring this fact, if the airplane was originally trimmed
for level flight, I contend that you would only start experiencing a
slight descent rate at an airspeed no different than prior to the
forward c.g. shift.

If you ignore that fact, sure. But you can't ignore that fact and still
have a correct understanding of the situation. I fail to see the relevance
of a hypothetical situation in which things aren't as they actually are in
real life. It won't help you understand what's happening in real life.

It's like saying "if you ignore the fact that there's gravity, we could fly
with a lot less power required than we do now". Sure, it's a true
statement, but it's not terribly useful.

Pete

Pete,

I can see where I set myself up nicely for your latest moral
redressing.

My previous description pertaining to constant airspeed with a sink
rate after the forward c.g. shift was valid only if you didn't touch
any controls.

If I understand your assertion, you want to maintain altitude without
augmenting power so I concur that you would have to reduce your
airspeed through a control input to meet the higher drag.

I note that the above would be invalid on the backside of the power
curve since the increase in angle of attack further compounds the drag
accumulation. In such case, only a power augmentation could be the
remedy.

Can we agree on this?

Alex

"Koopas Ly" wrote in message
om...
I can see where I set myself up nicely for your latest moral
redressing.

Not sure what you mean.

My previous description pertaining to constant airspeed with a sink
rate after the forward c.g. shift was valid only if you didn't touch
any controls.

My previous comments were with respect *only* to the increase in drag, and
resulting increase in deceleration rate. Constant airspeed and sink rate
are irrelevant to those comments.

If I understand your assertion, you want to maintain altitude without
augmenting power so I concur that you would have to reduce your
airspeed through a control input to meet the higher drag.

Not sure where you got the impression that I "want to maintain altitude".
All I "want" to do is explain why airspeed scrubs off more quickly when the
CG is farther forward.

I note that the above would be invalid on the backside of the power
curve since the increase in angle of attack further compounds the drag
accumulation. In such case, only a power augmentation could be the
remedy.

No, a decrease in angle of attack in that situation would reduce drag.

Can we agree on this?

We can agree on any number of things. But you would need to stay on topic
and not make up purely hypothetical but physically impossible situations for
us to do so, at least in this thread.

Pete

I can see where I set myself up nicely for your latest moral
redressing.

Not sure what you mean.

What I mean are your consistent unfriendly didacticisms. The
defensiveness you've displayed in your last posts is unwarranted, as
is your gratuitous stern tone. I am neither attempting to provoke you
nor challenge your knowledge. In fact, I respect it. However, for
reasons unbeknownst to me, you've set upon a course to systematically
dismiss every one of my comments without the least consideration for
merit. The gist of your reply revolves around the irrelevance of my
comments with respect to the deceleration due to drag with a forward
c.g. condition. In contrast, I believe that my subsequent comments
are very much relevant digressions that expound upon your original
thread.


My previous description pertaining to constant airspeed with a sink
rate after the forward c.g. shift was valid only if you didn't touch
any controls.

My previous comments were with respect *only* to the increase in drag, and
resulting increase in deceleration rate. Constant airspeed and sink rate
are irrelevant to those comments.


Constant airspeed and sink rate are in fact relevant to those
comments, as they succeed the latter. With no control inputs after
the forward c.g. shift, you will experience a lower pitch attitude and
a subsequent sink rate at constant airspeed.



If I understand your assertion, you want to maintain altitude without
augmenting power so I concur that you would have to reduce your
airspeed through a control input to meet the higher drag.

Not sure where you got the impression that I "want to maintain altitude".
All I "want" to do is explain why airspeed scrubs off more quickly when the
CG is farther forward.


I was wrong. I assumed that you wanted to maintain altitude, and I am
sure you know what happens when you assume.



I note that the above would be invalid on the backside of the power
curve since the increase in angle of attack further compounds the drag
accumulation. In such case, only a power augmentation could be the
remedy.

No, a decrease in angle of attack in that situation would reduce drag.


A decrease in angle of attack would change your altitude, an excursion
I was trying to prevent.


Can we agree on this?

We can agree on any number of things. But you would need to stay on topic
and not make up purely hypothetical but physically impossible situations for
us to do so, at least in this thread.

Pete

With no control inputs after the forward c.g. shift, you will
experience a lower pitch attitude and a subsequent sink rate at
constant airspeed.

Moving the CG forward, with no control inputs, will change the
equilibrium lift coefficient for the aircraft, making it smaller. The
aircraft's velocity will increase, using gravity for thrust.

You can see this in a 152. Both pilots lean forward, and the aircraft
descends and speeds up. Both pilots lean back, and the aircraft
climbs and slows down.

If you move the CG forward, and want to keep the same airspeed, you
will have to increase the tail down force, i.e., nose up trim. In
this scenario, you will have the same airspeed, but slightly higher
drag and will incur a slight descent.

"Koopas Ly" wrote in message
om...
What I mean are your consistent unfriendly didacticisms. The
defensiveness you've displayed in your last posts is unwarranted, as
is your gratuitous stern tone.

I submit that you may want to find a different forum, if you have found my
posts defensive or gratuitiously stern. You appear to be far too sensitive
for strictly textual communications to participate in Usenet. I have simply
attempted to answer the original question, while correcting elements of your
posts that were not true.

It is unreasonable of you to post your hypothesis, ask whether your
hypothesis is valid, and then get offended when you are told it is not.

Pete