a question for the aeronautical engineers among us

[Tina]

OK, so the center of gravity (except for some airplanes with really
smart computers) is going to be forward of the center of lift for
stall recovery reasons -- that makes sense. The question I have is,
for a typical GA flying machine -- take a complex single for example
-- what really is the download (I'm thinking of it as induced weight
with fuel burn consequences) supplied by the vertical stabilizer? It
has a reasonable moment arm hanging way back there, but is it as much
as say 100 pounds if the CG is near the forward limit?

[[email protected]]

On Nov 2, 7:12 pm, Tina > wrote:
> OK, so the center of gravity (except for some airplanes with really
> smart computers) is going to be forward of the center of lift for
> stall recovery reasons -- that makes sense. The question I have is,
> for a typical GA flying machine -- take a complex single for example
> -- what really is the download (I'm thinking of it as induced weight
> with fuel burn consequences) supplied by the vertical stabilizer? It
> has a reasonable moment arm hanging way back there, but is it as much
> as say 100 pounds if the CG is near the forward limit?

This isn't an aeronautical problem, its a basic mechanics problem.
The moment of the CG vs. the center of lift must equal the moment of
the tail plane.

If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
front of the center of lift, that is a moment of 2500 foot-lbs. If
the tail plane is 10 feet behind the center of lift, the force on the
stabilizer is 250 lbs.

Dean

[BT]

> wrote in message
oups.com...
> On Nov 2, 7:12 pm, Tina > wrote:
>> OK, so the center of gravity (except for some airplanes with really
>> smart computers) is going to be forward of the center of lift for
>> stall recovery reasons -- that makes sense. The question I have is,
>> for a typical GA flying machine -- take a complex single for example
>> -- what really is the download (I'm thinking of it as induced weight
>> with fuel burn consequences) supplied by the vertical stabilizer? It
>> has a reasonable moment arm hanging way back there, but is it as much
>> as say 100 pounds if the CG is near the forward limit?
>
> This isn't an aeronautical problem, its a basic mechanics problem.
> The moment of the CG vs. the center of lift must equal the moment of
> the tail plane.
>
> If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
> front of the center of lift, that is a moment of 2500 foot-lbs. If
> the tail plane is 10 feet behind the center of lift, the force on the
> stabilizer is 250 lbs.
>
> Dean
>

Now for fuel burn increases, factor in the induced drag created by the
deflected elevator to create the required down force.
Glider pilots attempt to maintain a CG at 75-85% MAC (some will go farther
aft), to keep the deflection to create the required down force to a minimum
to create the least amount of drag at cruising speed.

BT

[Tina]

I agree with your statement, but I don't know where the center of lift
is on GA airplanes, hence the question. Maybe it would have been
better to ask a center of lift question. The loading moment is
calculated from an arbitrary datum, it would be neat if there was an
easily identifed point that corrosponded to the center of lift.

On Nov 2, 5:52 pm, wrote:
> On Nov 2, 7:12 pm, Tina > wrote:
>
> > OK, so the center of gravity (except for some airplanes with really
> > smart computers) is going to be forward of the center of lift for
> > stall recovery reasons -- that makes sense. The question I have is,
> > for a typical GA flying machine -- take a complex single for example
> > -- what really is the download (I'm thinking of it as induced weight
> > with fuel burn consequences) supplied by the vertical stabilizer? It
> > has a reasonable moment arm hanging way back there, but is it as much
> > as say 100 pounds if the CG is near the forward limit?
>
> This isn't an aeronautical problem, its a basic mechanics problem.
> The moment of the CG vs. the center of lift must equal the moment of
> the tail plane.
>
> If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
> front of the center of lift, that is a moment of 2500 foot-lbs. If
> the tail plane is 10 feet behind the center of lift, the force on the
> stabilizer is 250 lbs.
>
> Dean

[Bertie the Bunyip[_19_]]

"BT" > wrote in :

> > wrote in message
> oups.com...
>> On Nov 2, 7:12 pm, Tina > wrote:
>>> OK, so the center of gravity (except for some airplanes with really
>>> smart computers) is going to be forward of the center of lift for
>>> stall recovery reasons -- that makes sense. The question I have is,
>>> for a typical GA flying machine -- take a complex single for example
>>> -- what really is the download (I'm thinking of it as induced weight
>>> with fuel burn consequences) supplied by the vertical stabilizer? It
>>> has a reasonable moment arm hanging way back there, but is it as
>>> much as say 100 pounds if the CG is near the forward limit?
>>
>> This isn't an aeronautical problem, its a basic mechanics problem.
>> The moment of the CG vs. the center of lift must equal the moment of
>> the tail plane.
>>
>> If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
>> front of the center of lift, that is a moment of 2500 foot-lbs. If
>> the tail plane is 10 feet behind the center of lift, the force on the
>> stabilizer is 250 lbs.
>>
>> Dean
>>
>
> Now for fuel burn increases, factor in the induced drag created by the
> deflected elevator to create the required down force.
> Glider pilots attempt to maintain a CG at 75-85% MAC (some will go
> farther aft), to keep the deflection to create the required down force
> to a minimum to create the least amount of drag at cruising speed.
>

huh?
75 to 85% MAC? I began to build one years ago and it's CG was in the
"usual position" about 25% to 30%, depending on how much you had for
lunch
I can't see any of the competition gliders being able for a CG in that
position either. not with the stab area that they have. A model glider
with a massive stab, yes, but not any full size as far as I can see.
Anything with a CG that far aft on the main plane is leaning more
towards being a tandem wing aircraft than a conventional one.
Can you provide an example of one of these?


Bertie


Bertie

[BT]

Bertie... perhaps I miss spoke..
correct.. not 85% MAC... but 85% towards the AFT CG limit..
BT

"Bertie the Bunyip" > wrote in message
.. .
> "BT" > wrote in :
>
>> > wrote in message
>> oups.com...
>>> On Nov 2, 7:12 pm, Tina > wrote:
>>>> OK, so the center of gravity (except for some airplanes with really
>>>> smart computers) is going to be forward of the center of lift for
>>>> stall recovery reasons -- that makes sense. The question I have is,
>>>> for a typical GA flying machine -- take a complex single for example
>>>> -- what really is the download (I'm thinking of it as induced weight
>>>> with fuel burn consequences) supplied by the vertical stabilizer? It
>>>> has a reasonable moment arm hanging way back there, but is it as
>>>> much as say 100 pounds if the CG is near the forward limit?
>>>
>>> This isn't an aeronautical problem, its a basic mechanics problem.
>>> The moment of the CG vs. the center of lift must equal the moment of
>>> the tail plane.
>>>
>>> If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
>>> front of the center of lift, that is a moment of 2500 foot-lbs. If
>>> the tail plane is 10 feet behind the center of lift, the force on the
>>> stabilizer is 250 lbs.
>>>
>>> Dean
>>>
>>
>> Now for fuel burn increases, factor in the induced drag created by the
>> deflected elevator to create the required down force.
>> Glider pilots attempt to maintain a CG at 75-85% MAC (some will go
>> farther aft), to keep the deflection to create the required down force
>> to a minimum to create the least amount of drag at cruising speed.
>>
>
> huh?
> 75 to 85% MAC? I began to build one years ago and it's CG was in the
> "usual position" about 25% to 30%, depending on how much you had for
> lunch
> I can't see any of the competition gliders being able for a CG in that
> position either. not with the stab area that they have. A model glider
> with a massive stab, yes, but not any full size as far as I can see.
> Anything with a CG that far aft on the main plane is leaning more
> towards being a tandem wing aircraft than a conventional one.
> Can you provide an example of one of these?
>
>
> Bertie
>
>
> Bertie

[Bertie the Bunyip[_19_]]

"BT" > wrote in :

> Bertie... perhaps I miss spoke..
> correct.. not 85% MAC... but 85% towards the AFT CG limit..

Ah, 85% of the range, you mean?

OK.

In airliners we actually go for the aft limit, if possible. There's a
double benefit from this. One, the drag from the stab is lowered and the
effective weight of the aricraft is also lowered. But perhaps more
importantly, the lower wing load resulting from the aft CG allows a wider
buffet margin and therfore a higher allowable cruising level.
One of the reasons that manufaturers want FBW systems is to permit flying
with what would be an extreme aft CG for a standard airplane. This allows
almost silly cruising levels at very, very low buffet margins to be had.
The reason of course is the higher you go, the more fuel and $$$$ you save.



Bertie
>

[Larry Dighera]

On Fri, 02 Nov 2007 18:12:58 -0700, Tina > wrote
in . com>:

>OK, so the center of gravity (except for some airplanes with really
>smart computers) is going to be forward of the center of lift for
>stall recovery reasons -- that makes sense. The question I have is,
>for a typical GA flying machine -- take a complex single for example
>-- what really is the download (I'm thinking of it as induced weight
>with fuel burn consequences) supplied by the vertical stabilizer?

Of course you mean the horizontal stabilizer.

>It has a reasonable moment arm hanging way back there, but is it as much
>as say 100 pounds if the CG is near the forward limit?

I would estimate, for a C-172, sans cabin occupants, it takes about 50
lbs of down force on the horizontal stabilizer to lift the nose wheel
from the pavement.

I'm no engineer, and I know this is not the answer to you question,
but it gives you a feel for the force involved. You can try this
yourself the next time you're at the airport; just make sure you are
putting your hand over the spar on the inboard portion of the
horizontal stabilizer near the fuselage to prevent permanently
deforming any aluminum.

[Tony]

We've moved small Cessnas by lifting the nose wheel that way, and of
course the mains are aft of the CG. I don't know where the center of
lift is, though: that would be the question. I'd guess -- and it's
only a guess -- it might be a third of the way along the wing chord
for a straight winged airplane.



€€On Nov 3, 1:34 pm, Larry Dighera > wrote:
> On Fri, 02 Nov 2007 18:12:58 -0700, Tina > wrote
> in . com>:
>
> >OK, so the center of gravity (except for some airplanes with really
> >smart computers) is going to be forward of the center of lift for
> >stall recovery reasons -- that makes sense. The question I have is,
> >for a typical GA flying machine -- take a complex single for example
> >-- what really is the download (I'm thinking of it as induced weight
> >with fuel burn consequences) supplied by the vertical stabilizer?
>
> Of course you mean the horizontal stabilizer.
>
> >It has a reasonable moment arm hanging way back there, but is it as much
> >as say 100 pounds if the CG is near the forward limit?
>
> I would estimate, for a C-172, sans cabin occupants, it takes about 50
> lbs of down force on the horizontal stabilizer to lift the nose wheel
> from the pavement.
>
> I'm no engineer, and I know this is not the answer to you question,
> but it gives you a feel for the force involved. You can try this
> yourself the next time you're at the airport; just make sure you are
> putting your hand over the spar on the inboard portion of the
> horizontal stabilizer near the fuselage to prevent permanently
> deforming any aluminum.

[Bob Noel]

In article >, BillJ >
wrote:

> Tina wrote:
> > OK, so the center of gravity (except for some airplanes with really
> > smart computers) is going to be forward of the center of lift for
> > stall recovery reasons -- that makes sense. The question I have is,
> > for a typical GA flying machine -- take a complex single for example
> > -- what really is the download (I'm thinking of it as induced weight
> > with fuel burn consequences) supplied by the vertical stabilizer? It
> > has a reasonable moment arm hanging way back there, but is it as much
> > as say 100 pounds if the CG is near the forward limit?
> >
> Download created by vertical stabilizer?? Maybe in a steep turn?

well, the drag vector of the vertical stabilizer is probably above the CG in
the vertical plane, so it probably provides some download. I was
pretty sure Tina meant the horizontal stab.

--
Bob Noel
(goodness, please trim replies!!!)

[BillJ]

Tina wrote:
> OK, so the center of gravity (except for some airplanes with really
> smart computers) is going to be forward of the center of lift for
> stall recovery reasons -- that makes sense. The question I have is,
> for a typical GA flying machine -- take a complex single for example
> -- what really is the download (I'm thinking of it as induced weight
> with fuel burn consequences) supplied by the vertical stabilizer? It
> has a reasonable moment arm hanging way back there, but is it as much
> as say 100 pounds if the CG is near the forward limit?
>
Download created by vertical stabilizer?? Maybe in a steep turn?