Weapon Loads and Aircraft Performance

[jc4751]

I've been googling a bit, but haven't really found much good information so far. As a rule of thumb, just how much would a bomb load reduce speed and performance of a WW2 fighter or bomb? Obviously, external vs. internal bombs would make a difference, and I would guess that no one would try any ACM while carrying a bomb load.

 

[mikewint]

Depends on many factors. Anything external increases friction and in todays world increases radar reflection. The Aircobra is a prime example, with a full ammo load it was a stable platform but as the ammo was fired off the front weight changed shifting the CofG. Many pilots reported the aircraft's tendancy to tumble under those conditions which did not happen with a full load of ammo

 

[Ivan1GFP]

Hello Mikewint,

I came to a similar conclusion regarding the Airacobra a little while back.

On the Airacobra, it isn't just the ammunition. The fuel and pilot are also in front of the CoG, so burning off a lot of fuel would also move the CoG. Although the pilot isn't disposable load, a light pilot would also tend to redue stability. The pilot weight was specified as 140 lbs, so I am not sure how much lighter you can get than that.

Hello JC4751,

One easy example that is fairly well known is the case of the J2M3 Raiden. Its maximum level speed is often listed as 371 mph but that number is from the aeroplane flown with a drop tank. Without the drop tank, it was tested at around 415 mph.

As for going into combat without a bomb load or drop tanks, early in the war, the A6M2 Reisen was often flown in combat with drop tanks. For weight purposes, a 330 liter DT and a 500 lb bomb are quite comparable in weight.

- Ivan.

 

[FLYBOYJ]

Folks, understand that to get an aircraft to tumble, even a P-39 at the aft and upper limits of it's CG envelope, you're pulling some pretty abrupt maneuvers. If the aircraft is flown with out doing any aerobatic maneuvers and flown per the POH on speed there should be no issues.

 

[Ivan1GFP]

Hello FlyboyJ,

I came across this while doing some basic research in preparation for building a flight simulator version of the P-39D Airacobra.
The information comes from aircraft manuals. I believe I got the Pilots Manual and Flight Manual from here. Attached is a load diagram for the P-39Q for general reference.

I believe you are correct that it takes a pretty abrupt maneuver to make the aircraft tumble, but note that most of the significant loads in this aircraft are well ahead of the CoG. As those disposable loads are spent, the CoG migrates pretty far aft. I can't find the spreadsheet that I used to do the calculations, but from vague memory, the CoG was at around 35% MAC which should make the aeroplane pretty wobbly considering that it starts at 28.5% according to a NACA test report on the stability of a scale model of the P-39N. (RM No. A6L27 published in 1947). This report appears to be more concerned with stability with changing aerodynamic conditions rather than CoG migration though.

Consider also that the P-39 could also be equipped with a 20 mm cannon instead of the 37 mm. With some equipment combinations, a very light pilot and less than dutiful ballasting to make up for the equipment, I can see the aircraft becoming quite unstable.

- Ivan.

 

[FLYBOYJ]

I've posted before that if you look at the P-39 W&B chart there is a horizontal and vertical center of gravity. This is common with helicopters but I know of no other aircraft that has this.

 

[Ivan1GFP]

Hello FlyboyJ,

A vertical CoG specification isn't all that uncommon from what I have found. Here is an example with the P-38. Note the CoG (A B dimensions) are listed with the Gear Up and Gear Down. I know I have a drawing of this type in the manual for the Hawker Hurricane as well, but can't find the book at the moment.

As for the original topic,

Here is a listing for the Me 109G with and without a 160 liter drop tank:
http://www.wwiiaircraftperformance.org/me109/VB-109-15-L-43.pdf

As a summary:
526 kph Clean.
522 kph with Mount but no Tank
496 kph with 160 Liter External Tank

- Ivan.

 

[GregP]

The CG change in the P-38 was fore and aft, not vertical. The top speed of the Bf 109 in various configurations is unrelated to the CG at all ... except needing to be within limits for controlled flight, of course.

In military aircraft of WWII, they knew what loads could be carried from what hardpoints or within what bomb bays with all fuel loadouts possible. That way, there isn't any figuring, you load to tech order and fly. Simple.

 

[FLYBOYJ]

Hello FlyboyJ,

A vertical CoG specification isn't all that uncommon from what I have found. Here is an example with the P-38. Note the CoG (A B dimensions) are listed with the Gear Up and Gear Down. I know I have a drawing of this type in the manual for the Hawker Hurricane as well, but can't find the book at the moment.

Ivan, that's an engineering drawing (and a nice one) not a weight and balance chart. The P-38 did not have a vertical CG that had to be calculated before flight. 35 years in aviation I could tell you first hand I've done "a few" weight and balance calcualtions on "a few" different aircraft (including warbirds) and only came across a vertical CoG on helicopters.

The CG change in the P-38 was fore and aft, not vertical. The top speed of the Bf 109 in various configurations is unrelated to the CG at all ... except needing to be within limits for controlled flight, of course.

In military aircraft of WWII, they knew what loads could be carried from what hardpoints or within what bomb bays with all fuel loadouts possible. That way, there isn't any figuring, you load to tech order and fly. Simple.

The only CG factor that may affect aircraft speed is to fly with a rearward CG, and the speed increase will only be a few knots (if any)

 

[FLYBOYJ]

Page 20A shows a P-39 weight and balance chart.

http://www.ww2aircraft.net/forum/other-mechanical-systems-tech-/39q-pilot-manual-5086.html

Page 17 and 19 shows a W B chart for an RAF P-38. There is not a Vertical W B calculation.

http://www.ww2aircraft.net/forum/other-mechanical-systems-tech-/british-38-lightning-11637.html

 

[stona]

The Aircobra is a prime example, with a full ammo load it was a stable platform but as the ammo was fired off the front weight changed shifting the CofG. Many pilots reported the aircraft's tendancy to tumble under those conditions which did not happen with a full load of ammo

Knowing how much work the RAF put into ensuring that their aircraft remained safe for an average pilot to fly in combat with various load outs (and also that the Luftwaffe spent a lot of time and effort doing the same) I find it surprising that the Americans allowed such a potentially unstable aircraft, if it was indeed such, into service.
Cheers
Steve

 

[FLYBOYJ]

Knowing how much work the RAF put into ensuring that their aircraft remained safe for an average pilot to fly in combat with various load outs (and also that the Luftwaffe spent a lot of time and effort doing the same) I find it surprising that the Americans allowed such a potentially unstable aircraft, if it was indeed such, into service.
Cheers
Steve

You want a fighter NOT to be entirely stable (discussion for another story). In actuality what needed to be determined is how bad this condition was was and did it make the aircraft unsuitable for combat service. Evidently some in the USAAF felt this wasn't and issue, and again, if the aircraft was flown within the envelope and non aerobatic (no turns in excess of 45 degrees, no maneuvers pitch up or down in excess of 30 degrees) were done, all should be fine.

The XP-77 and the XP-55 both had bad spin/ stall characteristics and were both cancelled (one of many reasons).

 

[Shortround6]

There may be a difference between a plane departing controlled flight ( severe oscillations or swings ) and actually turning or pitching more than 90 degrees from the flight path so the plane is going tail first. "Tumbling" may require a full 180 degree flip depending on your definition. What it looks like from the ground or from another plane may not be what it looks like from the cockpit

Once a pilot is in something like an inverted flat spin he is a lot more interested in getting out of it or getting out of the airplane than trying to recall the exact sequence of airplane gyrations that got him there.

 

[Ivan1GFP]

Hello All,

I believe we have a pretty good group here in this discussion but one major point some of us are missing is that we are actually discussing TWO DIFFERENT SUBJECTS. The OP was just asking about performance reductions with external loads. Post #2 introduced the Airacobra with its reputation for tumbling which really other than a peripheral connection with induced drag has nearly no connection with a performance reduction due to external stores. I don't mind reading about either subject, but I do find it amusing that we have connected them.

FlyboyJ,
You are probably correct about Pilots manuals W&B charts. I believe my Hawker Hurricane manual is actually a shop manual but the diagram is nearly identical to what I posted for the Airacobra. The location of the Hurricane's datum point surprised the heck out of me: It is the hole for the starter crank handle in the lower cowl. I need to find the manual.
The flight restrictions on pitch and roll you listed for "Normal Flying" seem to be way too limiting for combat flying though.

Regarding stability limits allowed for service aircraft, many times the requirement for additional fuel would make an aircraft sacrifice stability in order to be able to fly the missions. The Mustang and Spitfire fuselage fuel tanks are good examples of this.
As for optimal stability, you want an aircraft to be positively stable about all axes but not overly so. Negative stability is pretty hard for a human pilot to deal with.

- Ivan.

 

[FLYBOYJ]

The flight restrictions on pitch and roll you listed for "Normal Flying" seem to be way too limiting for combat flying though.

Not really. Unless a pilot chooses to engage in an all out turning and twisting dogfight, those flight maneuvers could probably accomplish most missions. Most people think that you're always going to be twisting and turning while in combat, sometimes far from the truth. Go into a 30 degree dive at a 45 degree bank and it will get your attention.

If Biff reads this I hope we get his perspective...

 

[Shortround6]

I've been googling a bit, but haven't really found much good information so far. As a rule of thumb, just how much would a bomb load reduce speed and performance of a WW2 fighter or bomb? Obviously, external vs. internal bombs would make a difference, and I would guess that no one would try any ACM while carrying a bomb load.

getting back on topic

Not all bombs of the same weight had the same shape/drag.
Not all bomb racks/sway braces had the same drag.
Not all bomb installations using the same bomb affected/interfaced with the surface of the aircraft the same (well out on wing, under fuselage, next to engine nacelle).

External drag was high enough that most designers went for internal storage when they could (post war bombs are another story, the US bombs being much lower drag than WW II bombs)

even the Curtiss Seahawk was supposed to carry it's bombs inside the float

But the couldn't get the bay doors to seal properly and had to revert to under wing carry.

AS far as air combat goes. The Mustang, for example, was stress for 8 "G"s at 8,000lbs or 64,000lb. hang a pair of 1000lb bombs under it and now you are rated at 6.4 "G"s at BEST. Local overloads on structure may restrict things even more. ( and a P-51D at 8,000lbs was carrying darn little fuel or ammo). At a more reasonable 10,600lbs ( rear tank empty and a bit of fuel burned off in warm and take-off) teh max G loading is down to 6.03 . Please remember than even in a nominal 2 "G" turn that goes 180 degrees there are momentary peaks and valleys of "G" load from one second (or tenth of second) to another as the pilot adjusts the the turn with the elevators. Some of the peaks can go over 4-5 "G"s for a fraction of a second or so.