Effects of CG on cruise speed

[gman]

Hello All,
Does anyone know if the location of the CG effects the cruise speed in
something like a Skyhawk of Skylane? In other words, given the same
weight, will the cruise speed at 75% power be higher or lower if the CG
is moved from near its forward limit towards its aft limit.

I think the aft CG will result in a higher cruise speed because the arm
between the center of lift and CG will be reduced (assuming CL is
behind the CG) and the tail will have to do less work.

Any ideas?

[Bob Gardner]

Higher. Moving weight aft means that the download on the horizontal
stabilizer is reduced.. Less elevator deflection means less drag.

Bob Gardner

"gman" > wrote in message
oups.com...
> Hello All,
> Does anyone know if the location of the CG effects the cruise speed in
> something like a Skyhawk of Skylane? In other words, given the same
> weight, will the cruise speed at 75% power be higher or lower if the CG
> is moved from near its forward limit towards its aft limit.
>
> I think the aft CG will result in a higher cruise speed because the arm
> between the center of lift and CG will be reduced (assuming CL is
> behind the CG) and the tail will have to do less work.
>
> Any ideas?
>

[Bob Moore]

gman wrote
> I think the aft CG will result in a higher cruise speed

That's the way we did it in the airline business.

Bob Moore
PanAm (retired)

[Maule Driver]

Yep. And you'll know you went too far back when 1) you've exceeded the
CG range in the POH and 2) when the a/c gets uncomfortably squirrly in
all axis or axii or whatever.

....seeking the middle of the range keeps things comfortable and allows
for some shifting, weighing errors, and such.

Bob Gardner wrote:
> Higher. Moving weight aft means that the download on the horizontal
> stabilizer is reduced.. Less elevator deflection means less drag.
>
> Bob Gardner
>
> "gman" > wrote in message
> oups.com...
>
>>Hello All,
>>Does anyone know if the location of the CG effects the cruise speed in
>>something like a Skyhawk of Skylane? In other words, given the same
>>weight, will the cruise speed at 75% power be higher or lower if the CG
>>is moved from near its forward limit towards its aft limit.
>>
>>I think the aft CG will result in a higher cruise speed because the arm
>>between the center of lift and CG will be reduced (assuming CL is
>>behind the CG) and the tail will have to do less work.
>>
>>Any ideas?
>>
>
>
>

[Kingfish]

Bob Moore wrote:

> > I think the aft CG will result in a higher cruise speed
>
> That's the way we did it in the airline business.
>
> Bob Moore
> PanAm (retired)

Heh. "This is the captain speaking - would all fat *******s please take
up the empty seats in the back of the plane, please...Your cooperation
is greatly appreciated"

[B A R R Y[_1_]]

gman wrote:
>
> I think the aft CG will result in a higher cruise speed because the arm
> between the center of lift and CG will be reduced (assuming CL is
> behind the CG) and the tail will have to do less work.

I would think the CG that requires the least amount of trim would be the
most efficient. If that is in fact true, the most efficient CG would
vary for different aircraft.

[The Visitor]

Maule Driver wrote:

> ...seeking the middle of the range keeps things comfortable and allows
> for some shifting, weighing errors, and such.


Don't forget fuel burn. But does anybody know how much faster if you do
load it farther aft?

[Bob Gardner]

Was it really necessary for me to say that the CG should be kept within the
operating envelope? Are newgroupies so anal that everything must be spelled
out? The manufacturer provides loading information so that pilots can choose
where they want the CG to be depending on their needs...the OP is obviously
opting for speed, and loading to the aft edge of the envelope is just as
valid as a more central loading.

Bob Gardner

"Maule Driver" > wrote in message
m...
> Yep. And you'll know you went too far back when 1) you've exceeded the CG
> range in the POH and 2) when the a/c gets uncomfortably squirrly in all
> axis or axii or whatever.
>
> ...seeking the middle of the range keeps things comfortable and allows for
> some shifting, weighing errors, and such.
>
> Bob Gardner wrote:
>> Higher. Moving weight aft means that the download on the horizontal
>> stabilizer is reduced.. Less elevator deflection means less drag.
>>
>> Bob Gardner
>>
>> "gman" > wrote in message
>> oups.com...
>>
>>>Hello All,
>>>Does anyone know if the location of the CG effects the cruise speed in
>>>something like a Skyhawk of Skylane? In other words, given the same
>>>weight, will the cruise speed at 75% power be higher or lower if the CG
>>>is moved from near its forward limit towards its aft limit.
>>>
>>>I think the aft CG will result in a higher cruise speed because the arm
>>>between the center of lift and CG will be reduced (assuming CL is
>>>behind the CG) and the tail will have to do less work.
>>>
>>>Any ideas?
>>>
>>
>>

[Maule Driver]

Com'on Bob, Chill.

I almost wrote something like "I just wanted to add my 2 cents to the
ol' sage's answer"

I was just rambling with my thoughts on rearward CGs. Your answer was
accurate and complete as always. Heck, you wrote the book.

Bob Gardner wrote:
> Was it really necessary for me to say that the CG should be kept within the
> operating envelope? Are newgroupies so anal that everything must be spelled
> out? The manufacturer provides loading information so that pilots can choose
> where they want the CG to be depending on their needs...the OP is obviously
> opting for speed, and loading to the aft edge of the envelope is just as
> valid as a more central loading.
>
> Bob Gardner
>
> "Maule Driver" > wrote in message
> m...
>
>>Yep. And you'll know you went too far back when 1) you've exceeded the CG
>>range in the POH and 2) when the a/c gets uncomfortably squirrly in all
>>axis or axii or whatever.
>>
>>...seeking the middle of the range keeps things comfortable and allows for
>>some shifting, weighing errors, and such.
>>
>>Bob Gardner wrote:
>>
>>>Higher. Moving weight aft means that the download on the horizontal
>>>stabilizer is reduced.. Less elevator deflection means less drag.
>>>
>>>Bob Gardner
>>>
>>>"gman" > wrote in message
oups.com...
>>>
>>>
>>>>Hello All,
>>>>Does anyone know if the location of the CG effects the cruise speed in
>>>>something like a Skyhawk of Skylane? In other words, given the same
>>>>weight, will the cruise speed at 75% power be higher or lower if the CG
>>>>is moved from near its forward limit towards its aft limit.
>>>>
>>>>I think the aft CG will result in a higher cruise speed because the arm
>>>>between the center of lift and CG will be reduced (assuming CL is
>>>>behind the CG) and the tail will have to do less work.
>>>>
>>>>Any ideas?
>>>>
>>>
>>>
>

[Jose[_1_]]

> Heh. "This is the captain speaking - would all fat *******s please take
> up the empty seats in the back of the plane, please...Your cooperation
> is greatly appreciated"

Fat ******* #1: I'll do it for $300. How much is fuel per gallon?

Jose
--
The monkey turns the crank and thinks he's making the music.
for Email, make the obvious change in the address.

[Peter Duniho]

"B A R R Y" > wrote in message
...
>> I think the aft CG will result in a higher cruise speed because the arm
>> between the center of lift and CG will be reduced (assuming CL is
>> behind the CG) and the tail will have to do less work.
>
> I would think the CG that requires the least amount of trim would be the
> most efficient. If that is in fact true, the most efficient CG would vary
> for different aircraft.

I agree that there's a secondary effect with respect to control surface drag
related to the trim position. However, the primary issues are a) downforce
balancing the CG (presumably forward of the center of lift), which has to be
compensated for in the form of more lift (so more induced drag) and b) the
induced drag from the horizontal stabilizer/elevator itself (adding to the
total induced drag directly). Both of these directly correlate to the CG
position.

The parasitic drag due to trim position is only likely to be significant
when the CG is already relatively far aft and the increase in induced drag
isn't large. So yes, the *exact* place where the least total drag occurs
may not be precisely at the rear-most CG position available, but I suspect
in most cases it is (hopefully airplane designers aren't normally creating
trim systems that require a non-neutral trim position for a neutral elevator
force), and when it's not, it's not going to be far from there.

Pete

[[email protected]]

gman wrote:
>
> I think the aft CG will result in a higher cruise speed because the arm between the center
> of lift and CG will be reduced (assuming CL is behind the CG) and the tail will have to do
> less work.
>
> Any ideas?


I know that this is off-topic but regarding the dynamics of loads in
flight, CG, loading patterns, etc., there's a really nice 6.3 MB
publication by Airbus that I'd be happy to mail any aviation student if
they wrote in privately.

Cheers,

Ramapriya

[Doug[_1_]]

You will go a little faster with rear CG, but in a plane like a
Skyhawk, it will only be a knot or two, at MOST.

[B A R R Y[_1_]]

Peter Duniho wrote:

> The parasitic drag due to trim position is only likely to be significant
> when the CG is already relatively far aft and the increase in induced drag
> isn't large. So yes, the *exact* place where the least total drag occurs
> may not be precisely at the rear-most CG position available, but I suspect
> in most cases it is (hopefully airplane designers aren't normally creating
> trim systems that require a non-neutral trim position for a neutral elevator
> force), and when it's not, it's not going to be far from there.

Thanks for the explaination.

For me, it's time to break out the water bottles and experiment across
my particular range.

Thanks to the OP for posting the question!