C G

[bigmak]

On great planes p-39 they stated that the p-39 in testing out turned the 109 and the spitfire, and the reasoning they cited was that the center of gravity was more aft than the others.

Does anyone have confirmation on the report to which they were reffering. 

Also could someone in simplish terms explaine the effect CG has on turn performance please.

 

[SCTusk]

No doubt someone will be along shortly with a bucketful of scientific gobbledegook but I think this might help you out more; you can easily move the C of G around by adding/removing weight or shifting weight, in a glider (for instance) it's common to add ballast up the front to compensate for the more diminutive or lightweight pilot. All this does is keep the C of G located within acceptable fore and aft limits for safe flight. I actually forgot to do this once and in addition to a tendency of flying dangerously nose high (requiring full forward trim and significant forward stick pressure) pitch sensitivity was also increased. Something to do with the C of G having moved closer to the C of L (ift) abit like a more delicate balancing act if you like, less stable. Similar experiences with radio control models further supports this observation.

So disregarding any other considerations (which you can rarely do in reality) I think yes, a more rearward C of G could improve turning. But the a/c you mention are so dissimilar that to suggest this as the singular reason for one to out turn another seems an over-generalisation, for example wing loading and weight would play just as important a role.

As for the accuracy of the claim I do believe the P39 was a favourite of Chuck Yeager, he knows a thing or two about a/c so I wouldn't be suprised, but I don't know for certain. One for the researchers.

 

 

[Stoney]

On great planes p-39 they stated that the p-39 in testing out turned the 109 and the spitfire, and the reasoning they cited was that the center of gravity was more aft than the others.

Does anyone have confirmation on the report to which they were reffering.  

Also could someone in simplish terms explaine the effect CG has on turn performance please.

  

[Disclaimer]  The following explanations use some generalities, approximations, and skips some details in an effort to make complex concepts easier to understand.  In places where I have glossed over some details, there is an asterisks (*).

1.  "Out turned".  This term gets thrown about quite a lot, but it can mean different things to different people.  It could mean "smallest turning radius", "fastest turning rate", or, in some anecdotal comparisons from the WWII era, it could even refer to quickest roll rate enabling a hard turn.  So, that's the first issue.  When they say "out turned", what do they mean?  Unfortunately, for the History Channel, et al, they rarely even know themselves--they merely just repeat something they heard from a pilot or saw in their cursory research.

2.  CG and its effect on turning.  Typically, the CG of an aircraft is not considered to have an effect on an aircraft's ability to "turn"*.  It does have a huge impact on the stability* of the aircraft.

So, in sum, the TV shows are often full of crap.  It is conceivable in some situations that a P-39 could "out turn"* the 109 or Spit, but that had little* to do with the fact that the P-39 had a CG that was "further aft" than its peer aircraft.

[Dawger]

Having experienced the effects of moving the CG out of limits on real aircraft I can tell you exactly what it does.

Aft CG doesn't help the plane perform better. There is a slight benefit due to decreased tail down force and less drag as a result but that would not effect turn performance.

What aft CG does do is make the pitch axis more sensitive. This is the instability Stoney refers to.

Stability in an aircraft is its tendency to return to where it was when you displace the controls.

The aircraft rotates around the CG. The center of lift of the wing is aft of the CG and the horizontal stabilizer center of lift is even further aft. The wing lift opposes the tail lift.

The further the CG is from the center of lift the greater the stability due to longer moment arms.

Get the CG near the center of lift and the stability becomes neutral. The aircraft stays where you last left it. CG aft of the center of lift and stability becomes negative and the aircraft accelerates in the direction of control input.

I've flown  airplanes with the CG very close to the CG and it scared the devil out of me every time.  Very easy to over control the pitch axis and enter an accelerated stall or over-G.

So...aft CG will increase the pitch response and make the aircraft "lighter" on the controls but it doesn't actually improve performance. It only changes the inputs required by the pilot to achieve a given result.

[hitech]

There is a slight turn performance gain as the cg moves aft. If the planes main wing cl is aft of  cg  the the elevator will have to produce a down force for level flight. This down force will have to be compensated for with more lift (I.E. more aoa) to maintain level flight. This same concept applies to a max performance turn. You will have less net lift in the direction of the turn because the tail force is working against your turn. As you move the CL back the net lift will increase do to the less DOWN lift required by the tail.

HiTech

[Soulyss]

Unfortunately that show is rife with errors, I remember the P-39 episode and I groaned out loud a couple times at some of the things they said.

However a look at some of the numbers is rather interesting.  Prototypes of the P-39 stacked up well against the 109 on paper, they had slightly more HP, slightly larger wing area, slight higher rated speed and a slightly lower/better wing loading.

There's only one problem, in this comparison we have the XP-39 prototype and the Bf109E which is a production plane.  If you compare the P-39D-1, a combat/production you see an increase in empty weight from 4,545 to 5,601 lbs and the numbers don't stack up as well anymore.  The 109 now enjoys a better wing loading as well as a  roughly 330ft/min advantage in climb rate.

[Stoney]

There is a slight turn performance gain as the cg moves aft. If the planes main wing cl is aft of  cg  the the elevator will have to produce a down force for level flight. This down force will have to be compensated for with more lift (I.E. more aoa) to maintain level flight. This same concept applies to a max performance turn. You will have less net lift in the direction of the turn because the tail force is working against your turn. As you move the CL back the net lift will increase do to the less DOWN lift required by the tail.

HiTech

Is this slight turn performance gain enough to be noticeable?

[bigmak]

I am wondering is the effect a direct weight issue,
 or a precentage of weight issue?

For example would the effect of 100kg 1 meter aft of CG
be the same on a 6000kg aircraft as it would on a 11000kg aircraft?

I am curious as to how the effect manifests itself.

 

[hitech]

Is this slight turn performance gain enough to be noticeable?

stony 1k airplane doing 6 g's = 6000lb lift.

say 1 ft cl behind cg = 6kft/lb torque

say 15 ft tail 6k / 15 = 400 lb extra lift needed so would 5.6 / 6.0 less lift then with a neutral cg.

HiTech

[Stoney]

Would the increase in pitching moment that occurs as the center of lift moves aft help counter-act this force?

[hitech]

Would the increase in pitching moment that occurs as the center of lift moves aft help counter-act this force?

if cl is moving aft, it would worsen the turn.

HiTech