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

Larry Fransson wrote in message news:2003111412455475249%newsgroups@larryandjenny .net...
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

Not necessarily. Depends whether you fly a low or high wing aircraft.

* 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

******Not necessarily. Depends whether you fly a low or high wing
aircraft.****

Not necessarily. The old Piper Cherokee wing pitches down with flaps. The
newer Warrior wing pitches up with flaps.

Has NOTHING to do with wing placement!!

Karl

"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

"karl gruber" wrote in message
...
******Not necessarily. Depends whether you fly a low or high wing
aircraft.****

Not necessarily. The old Piper Cherokee wing pitches down with flaps. The
newer Warrior wing pitches up with flaps.

Has NOTHING to do with wing placement!!

Well, actually...

It has a little to do with the wing placement in that wing placement
certainly can affect whether flaps cause a nose-up or nose-down trim change.
Other factors are involved as well, so knowing the wing placement alone
won't tell you what flaps will do. But certainly, all else being equal,
moving the wing can change the way flaps affect trim.

Now, all that said...I don't think that's what Larry was talking about. The
trim change is usually a result of a combination of things, including
airflow over the horizontal stabilizer and elevator. But if I recall
correctly, flaps extended below the wing *also* create a pitching down
moment, always. This moment may be enhanced or obscured by other factors,
but it always exists.

Since trim changes often become negligible at lower airspeeds, this moment
can become more significant during the flare, even if the net trim change at
the normal flap extension speed would be nose-up.

At least, I think that's the point Larry was trying to make. I could very
well be wrong about what point he was trying to make, and I might have
misremembered this particular aspect of the aerodynamics of flaps.

Pete

***** Has NOTHING to do with wing placement!!

Well, actually...

It has a little to do with the wing placement in that wing placement****


So true. I must remember, the words "nothing, always, never,...Etc" don't
get very far on usenet.

Best

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

On 2003-11-14 21:55:21 -0800, "Peter Duniho" said

At least, I think that's the point Larry was trying to make. I could ver
well be wrong about what point he was trying to make, and I might hav
misremembered this particular aspect of the aerodynamics of flaps

That was my point exactly

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.