How was CG balancing handled in WW2 aircraft?

[MachFly]

Recently I read a conversation on a warbirds forum where it was pointed out the owner of a P51 wanted to remove the guns. So he had a mechanic take them out and he went up for a ride. Seems the mechanic did not add weight to the tail and the owner of the P51 had a very scary ride and landing along with a visit from the local FAA rep.

Is this real in terms of P51's and removing the guns in the first place? Secondly, how was this handled with all the external ordinance many aircraft carried for specific roles? 109's and their gondola. 47's/51's and drop tanks, bombs, rockets. 190's and removing the outboard cannon. Tiffy firing rockets and so on.

What happened to the CG balance when the weight of ordinance suddely was gone and the pilot had to dogfight?

When you drop bombs (or make any change to the weight of the aircraft) your CG changes. Normally aircraft are designed to with a large envelope so when you drop a bomb the aircraft will continue being flyable. Looks like this P-51 (or all) was not designed to be flown without guns so it's CG was in the tail which made it less stable.



Here is the difference in forward and aft CG:

Forward:
Longer T/O roll
Longer Landing distance
Harder to rotate
Slower cruise TAS
Higher fuel burn rate
More stable
Less maneuverable
Easier to stall
Easier to recover (from a stall)

Aft:
Shorter T/O roll
Shorter Landing distance
Easier to rotate & flare
Higher cruise TAS
Lower fuel burn rate
Less stable
More maneuverable
Harder to stall
Harder to recover (from a stall)

[nrshida]

What is the source of this information please?

Perv is that your model Sopworth?

Concorde could pump its fuel fore and aft to maintain CofG, I think you can do the same with your Camel and your enamel tea mug 

[MachFly]

What is the source of this information please?

Are you referring to my post?

[Rob52240]

Were the guns de-milled?  Did the P-51 owner have a class 3 license and a half dozen M2 machine guns?  I'm confused. 

[nrshida]

Are you referring to my post?

Yes.

[MachFly]

Yes.

Oh that's just something I wrote out based on physics and aerodynamics. If you have any questions I'll be glad to explain it.


If your really interested in it here is an FAA book that describes effects of CG: http://www.faa.gov/library/manuals/aircraft/media/FAA-H-8083-1A.pdf

[nrshida]

Thanks, I'll read that.

[RTHolmes]

unless anyone can show that WWII fighters were ballasted for different loadouts I'll stick with my answer. elevator trim was used to compensate for changes in C_G, because they were designed to operate within a range of C_G and normal loadouts would keep the aircraft within this range. bombers and transports, another matter.

[Oldman731]

unless anyone can show that WWII fighters were ballasted for different loadouts I'll stick with my answer. elevator trim was used to compensate for changes in C_G, because they were designed to operate within a range of C_G and normal loadouts would keep the aircraft within this range. bombers and transports, another matter.

But the question was "how was CG balancing handled," which (to me at least) means "how do you keep the plane's center of gravity within its limits, given different loads?"  That requires the crew to figure out where on the plane to put different weights - if you can put them on/in the plane at all.  This is important for any flight in a plane loaded to or near its max weight (and sometimes even if you're not near the max).  Otherwise you'll be out of your CG limits and no amount of trim will save you.

- oldman

[BravoT]

unless anyone can show that WWII fighters were ballasted for different loadouts I'll stick with my answer. elevator trim was used to compensate for changes in C_G, because they were designed to operate within a range of C_G and normal loadouts would keep the aircraft within this range. bombers and transports, another matter.

Trim only effects control forces felt by the pilot.  It only compensates for changes in CG in the sense that a pilot can use trim to make his life easier.

In other words, any aircraft can fly within its CG limits regardless of whether it has elevator trim.

[BravoT]

Thanks, I'll read that.

You'll probably find it somewhere in that link, but the way to think of "why" is to think of the tailplane as an upside down wing, meaning that the lift vector generated by the tailplane is down.  So if an aircraft is tail heavy, the tailplane is generating less "down lift" , so performance characteristics which benefit from increased "up" lift will improve.

That said, never, ever, ever exceed the aft CG limit (or the fore one, for that matter) in a real world aircraft.  Never.

[BravoT]

But the question was "how was CG balancing handled," which (to me at least) means "how do you keep the plane's center of gravity within its limits, given different loads?"  That requires the crew to figure out where on the plane to put different weights - if you can put them on/in the plane at all.  This is important for any flight in a plane loaded to or near its max weight (and sometimes even if you're not near the max).  Otherwise you'll be out of your CG limits and no amount of trim will save you.

- oldman

Again, the flight manuals for all aircraft have Weight & Balance charts.  They tell you exactly how much weight you can put where.  So balancing is handled by knowing the weight of various things, and then knowing where you are going to put them, and then looking to the charts to see if you're still within the permitted CG envelope.

And, again, trim has nothing to do with balancing CG or staying within CG limits.

[RTHolmes]

ok so thats Wmaker, MachFly and BravoT who think that WWII fighter pilots DIDNT use the elevator trim to trim the aircraft for level flight to compensate for changes in C_G due to fuel use and ords release. anyone else?

[BravoT]

ok so thats Wmaker, MachFly and BravoT who think that WWII fighter pilots DIDNT use the elevator trim to trim the aircraft for level flight to compensate for changes in C_G due to fuel use and ords release. anyone else?

Pilots use trim so that their arms don't get tired.  The same way they use trim when dropping flaps on final approach.  The same way they use trim if they make a change in power.  Pilots also use trim in all phases of flight, and not just level flight.  That's why every real world pilot knows what PAT (or APT) means.

I didn't say that they didn't use trim to compensate for a change in CG.  I said that pilots use trim to compensate for forces on the controls, and that elevator trim isn't used to "manage" CG.

[Oldman731]

because they were designed to operate within a range of C_G and normal loadouts would keep the aircraft within this range.

Heh.  I think this is the statement we have problems with.  It may be possible that you can hang 2x500 pound bombs and full MG ammo on a P-51 with only 50 gallons fuel in the main tanks and still be within CG limits, but I'd want to work through those calculations before I believed it.

- oldman