Bf109 Corner Velocity

[GScholz]

Limit load and failure load are more the result of the construction material used than design. An aluminum structure typically will fail at 1.5 times the load where it starts deforming. So if a structure starts deforming at 8G it will typically fail at or near 12G.

[drgondog]

Technically correct re: material based structural analysis.  That said, very few fighter aircraft departed from 2024/24 S, 14S etc, etc for which mil spec values for tensile strength were documented (in US). So, the engineer didn't have to look at Stress vs Deformation charts and 'guess' the elastic Yield point.

The stress analysis was performed on each component, panel and joint to check for safety factor >+ .01 for the applied loads driving the calculated maximum stress.

For US the max load was usually the max design AoA related (dive pullout), but failure occurred on empennage when the actual applied loads exceeded the design targets. For US, for example, the lateral applied load factor was 2G until the P-51H. IIRC the Brits were 1G lateral

[Gaston]

6G is blackout limit in AH. Structural limit for the 109 should be around 8G.

  A 1989 test of the F6F/FG-1/P-51/P-47D with modern instrumentation showed "The minimum 6G "Corner Speed" on all these aircraft is very close to maximum METO level Speed, so around 320-350 MPH at 10 000 ft..

  This would be roughly the same on the Me-109G... But remember this is for level turns...

  At this level of power (basically maximum continuous), the minimum speed for 6G LEVEL TURNS is quite high. This does not mean you cannot pull these Gs at lower speeds on dive pull-outs.

  However, at very reduced power settings, the Me-109G will turn its best rate of level turn at around 160-180 mph, according to Fin ace Karhila.

  Most WWII fighters similarly turned at their best rate with much reduced power outputs, particularly the P-51 Mustang and FW-190A (Red Fleet 1943: "The FW-190A will inevitably offer turning combat at minimum speed"). The trick on the Mustang involved 3 steps: Shallow flap settings, Prop put on full coarse pitch, and throttle reduced, the more reduction the tighter the turning ability.

  The assumptions these things worked like jets is nonsense: 6G in level turns meant such a rapid loss of speed it was entirely useless when you had to steadily pepper targets for hundreds of rounds before your 2% hit rate had any effect at all (this is why boom and zoom was always a marginal practice, and required high velocity or centralized guns, plus a very vulnerable target)...

  This is also why the concept of "Corner Speed" is entirely a post-war construct.

  Gaston

 

[shift8]

Couple of things here:

1: ww2 fighters cannot sustain their corner since they do not have enough power. It takes excessive down nosing in a ww2 fighter to get corner. Also 6G is not a magic corner G.

2: Reducing throttle does not make you turn  better. Best turn rate, even in ww2 birds, is ALWAYS at full power for a sustained turn. Reducing throttle in a sustained turn is only good for getting a better radius, not rate. And Radius us is primarily necessary for one-circle geometry.


HOWEVER. Corner speed is not a post war construct. It always existed, even on ww2 birds. It simply cannot be sustained on ww2 birds due to power limitations.
 
Additionally, there is no such thing as a single corner speed. Corner speed for any airplane is subject to altitude and weight. It changes based on these conditions. So if you are heavier, the corner speed generally goes up. Same for altitude. IF your lighter, it goes down. The differences can be significant.

[FLS]

Gaston,  corner speed is simply the lowest speed you can pull max g.  Your 160-180 best rate remark refers to best sustained turn which is entirely different from corner speed and as was pointed out the actual speed in either case depends on weight and altitude.

[mikeWe9a]

Couple of things here:

1: ww2 fighters cannot sustain their corner since they do not have enough power. It takes excessive down nosing in a ww2 fighter to get corner. Also 6G is not a magic corner G.

2: Reducing throttle does not make you turn  better. Best turn rate, even in ww2 birds, is ALWAYS at full power for a sustained turn. Reducing throttle in a sustained turn is only good for getting a better radius, not rate. And Radius us is primarily necessary for one-circle geometry.


HOWEVER. Corner speed is not a post war construct. It always existed, even on ww2 birds. It simply cannot be sustained on ww2 birds due to power limitations.
 
Additionally, there is no such thing as a single corner speed. Corner speed for any airplane is subject to altitude and weight. It changes based on these conditions. So if you are heavier, the corner speed generally goes up. Same for altitude. IF your lighter, it goes down. The differences can be significant.

Just one (very long-winded) correction:  reducing speed below corner velocity will not reduce the minimum turn radius (i.e. pulling max G limited by airspeed) in a level turn - it will increase it. 

  Max G is proportional to the square of velocity, as is the lateral acceleration required to maintain a given turn radius.  The actual G available to make the turn, however, is not all of the G that you are pulling.  The Geometry involved is A^2 +B^2 = C^2, where C is the total G produced, A is the G used to turn, and B is 1 (for a level turn).

  If you are pulling 2 G in a level turn, your wings are producing 1 G to remain level, and are producing 1.73 G (square root of 3) of lateral acceleration to keep you on that radius.  If you were to increase speed by 10%, you can pull 1.21 * 2 = 2.42 G, of which 2.2 G is used for lateral acceleration.  The acceleration needed to maintain the same radius would only be 2.09 G (1.73 * 1.21), so you are therefore traveling along a shorter radius.  The extreme end of this example is that if you are flying *exactly* at your stall speed to maintain 1 G, you cannot turn at all without stalling, and your turn radius is therefore theoretically infinite.

  For a sustained turn, yes, slowing down can get you a shorter turn radius.  Performance-wise, however, pulling throttle is not the way to do it unless for some reason you started out above your corner speed - you would simply pull more G, maximizing your turn rate and minimizing your radius while allowing the increased drag to slow you toward your sustained turn speed.

Mike

[shift8]

Just one (very long-winded) correction:  reducing speed below corner velocity will not reduce the minimum turn radius (i.e. pulling max G limited by airspeed) in a level turn - it will increase it. 

  Max G is proportional to the square of velocity, as is the lateral acceleration required to maintain a given turn radius.  The actual G available to make the turn, however, is not all of the G that you are pulling.  The Geometry involved is A^2 +B^2 = C^2, where C is the total G produced, A is the G used to turn, and B is 1 (for a level turn).

  If you are pulling 2 G in a level turn, your wings are producing 1 G to remain level, and are producing 1.73 G (square root of 3) of lateral acceleration to keep you on that radius.  If you were to increase speed by 10%, you can pull 1.21 * 2 = 2.42 G, of which 2.2 G is used for lateral acceleration.  The acceleration needed to maintain the same radius would only be 2.09 G (1.73 * 1.21), so you are therefore traveling along a shorter radius.  The extreme end of this example is that if you are flying *exactly* at your stall speed to maintain 1 G, you cannot turn at all without stalling, and your turn radius is therefore theoretically infinite.

  For a sustained turn, yes, slowing down can get you a shorter turn radius.  Performance-wise, however, pulling throttle is not the way to do it unless for some reason you started out above your corner speed - you would simply pull more G, maximizing your turn rate and minimizing your radius while allowing the increased drag to slow you toward your sustained turn speed.

Mike

I think maybe you are either misunderstanding what I meant or you are perhaps leaving some important factors out. Either way, take one look at a EM diagram for any modern fighter and you will see what I mean. Less speed = smaller radius, but worse turn rate for sustained turns. Less speed also means a shorter reletive radius to someone who is flying faster, sustained or no.

Take for example a scissors, which is a textbook example of a 1-circle geometry fight. In a one Circle Geometry, Radius is paramount, not rate. This is why people who are in nose to nose fights dump flaps and an chop throttle. Getting slower means a smaller radius, and this is why the person with the larger radius will end up out front: and lose.

If you are nose to tail, rate is king, and so long as you are not above corner speed you should be a max power.

[FLS]

Shift8, Mike is agreeing with you. Note his last paragraph.  For a sustained turn, yes, slowing down can get you a shorter turn radius.

[shift8]

Yes it appears we are talking past each other lol

[drgondog]

The Corner velocity:

Vc = SQRT[ (2Nmax*W)/(rho*CLmax*S)]

For a P-51D, 2N= (2*6.3) at GW=10,200 pound full internal combat load. The factor "6.3" for maximum G load is derived from the Limit load envelope, and of course far less than the 8G design Limit Load for the fully loaded Mustang I/P-51-1 at 8,000 pounds. Allowable "G" for the P-51D then is extracted from 8*8000/10200= 6.27

The P-51H was designed for 7.3G Limit load at fully loaded internal combat gross weight of 9500 pounds.

No WWII came close to achieving level Corner Velocity . Among the factors were a.) CLmax derived for two dimensional airfoil section and experimental data gathered in level flight stalls. in a high G turn CLmax is degraded by virtue of asymmetric lift distribution between 'high' and low wing. b.) The Free body forces on a turning fighter are difficult to calculate with precision as the balance between Thrust and Drag as well as true thrust of a max power engine/prop system in that medium airspeed envelope. Additional Drag calculation requires introducing trim drag due to rudder and aileron deflections.

When you look at data from turning tests - say from VVS testing of Allied and Axis fighters - and back out the CLmax based on turn rates and time to complete 360 degrees of 'level' turns, you should find about 10-20% lower CLmax than Stall CLmax.

I've been wrong before, but I don't get too excited when folks get lathered up about Corner Velocity and Calculated Turn performance for the above reasons.

   

[FLS]

We use 6g as the practical limit for corner speed because that's when the pilot blacks out in Aces High.

[shift8]

Just in case anyone is curious, I spend alot of time measuring this sustained level turn times of AH planes at sea level with equivalent fuel loads a max power. The fuel loads are as close to equal as possible in terms of time available to fly at MIL, not percent or weight. The planes are in order of best to worse. Where the time in seconds is the same, the former plane has a slight advantage somewhere between seconds. The P-38 and Corsair times are with Full flaps deployed. The reason for this is that they are the only two planes on the list that get sustained turn rate improvements in level flight with any degree of Flap. All other planes on this list ultimately have worse turn rates with any degree of flap if the turn is sustained and level. Turn radius improves on all planes with flaps in level turns. These figures are accurate, as each test as painstakingly done over and over again to assure accuracy.

Zero: 13 seconds
P-38L: 16 seconds.
Spit 16: 16 seconds
Corsair-1A: 17 seconds
Yak-3: 17 seconds
La-7: 18 seconds
Bf109G-14: 18 seconds
P-51D: 19 seconds
FW190A5: 20 seconds
FW190D-9: 20 seconds
P-47D25: 21 seconds

[FLS]

You might want to test the Corsair with 2 notches flaps. I believe that gives the Corsair it's fastest sustained turn.

Were you riding the stall buffet for the tests?

[shift8]

You might want to test the Corsair with 2 notches flaps. I believe that gives the Corsair it's fastest sustained turn.

Were you riding the stall buffet for the tests?

Yes I was right on the edge of the envelope. That is why I did at least 10 runs for each. Also now that you mention it the corsair was at 3 notches, more than that and it fell to 18 seconds.

[FLS]

Testing is a pain. Good job.     Equivalent fuel is different. We usually use 25%.  If you look at the climb and speed charts in the hanger they show weights that often don't match any loadout. I assume they use weights to match published charts from the actual aircraft.