Forward CG Experience

(Corky Scott) wrote in message ...

The A-20's were immediately extremely effective, but Pappy was unhappy
with their performance, feeling that he needed a bigger bomber with
more capacity. Enter the B-25 strafer.

I've got to stop, I could go on about this for a while longer. ;-)

Go ahead, Corky, it's cool!

Very fond of our local B-25 "Show Me"

Cheers,
Sydney

In article zR5tb.196731$HS4.1666204@attbi_s01, Jay Honeck
wrote:

As we crossed the numbers, I noticed things just didn't "feel" right.
Pulling back on the yoke was having very little effect as far as changing
the angle of attack, yet the speed was still bleeding off. This was weird,
but -- as it was all happening in the last few seconds, there wasn't much
else to do but add a touch of power and pull back some more.

Subtract the weight of the fuel burned enroute and recalculate you W&B
and C/G.
Which way does the moment move? Fore or aft?

"Jay Honeck" wrote in message
news:zR5tb.196731$HS4.1666204@attbi_s01...
Recently we flew with a friend who weighs over 320 pounds. With he and I
in the front seat [...]

Argh! "With him and me in the front seat". Sorry, pet peeve.

Anyway, some comments:

* Wind only reduces groundspeed. It has no effect on control effectiveness
for a given airspeed.
* As you found, forward CG translates into increased drag, which means
airspeed drops off quicker than usual. You can either carry more airspeed
on final (scrubbing off to normal touchdown speed in the flare, but doing so
more quickly than normal) or you can use more power (using thrust to offset
the extra drag).

The only thing aft CG isn't good for is stability. Otherwise, it's a good
thing. I prefer to fly my airplane with the CG as far aft as practical (and
legal, of course).

Pete

Argh! "With him and me in the front seat". Sorry, pet peeve.

Ooooo. That *was* bad, wasn't it? :-) Sorry.

* Wind only reduces groundspeed. It has no effect on control
effectiveness
for a given airspeed.

Yes. I was trying to say that the wind would help with a gentle landing by
allowing a slower touch-down speed.

As I've told my son, don't listen to what I *say*, dang it, listen to what I
*MEAN*.
;-)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

* As you found, forward CG translates into increased drag, which means
airspeed drops off quicker than usual. You can either carry more airspeed
on final (scrubbing off to normal touchdown speed in the flare, but doing so
more quickly than normal) or you can use more power (using thrust to offset
the extra drag).

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.

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?

Alex

"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

On 2003-11-14 06:42:39 -0800, "Jay Honeck" said:

I opted for two notches of flaps.

That probably did a lot to help your landing. Flaps add a nose down pitching moment - something you don't really need with a forward CG.

We arrived firmly, in a flat attitude. The nosewheel and mains hit nearly
simultaneously, and the end result was more like a mush into the ground than
my usual "flare, chirp, chirp, settle...". It was surprisingly smooth, but
I realized that if I hadn't given that extra burst of power, and a firmer
yank on the yoke, we might well have landed on the nose gear, with possibly
expensive consequences.

Not necessarily. Sometimes, this is as good as it gets. I've had lots of nosewheel first arrivals in a Seneca (before I figured out the little flap trick). It's only a problem if you let it get out of hand by chasing the oscillation.