P-51B/C/D/K G Limit at 8000lbs

[Hoggardhigh]

Hi all,

It is a well known fact that the Merlin-engined Mustangs, from the P-51B/C series onward, had a strengthened airframe compared to their Allison-engined predecessors (P-51A & below).

And yet, the maximum G limit for these later aircraft (at 8000lb) seems to be identical to that of the Allison-engined models (namely, 8G).

Anyone have any explanation for this?

 

[BiffF15]

Hi all,

It is a well known fact that the Merlin-engined Mustangs, from the P-51B/C series onward, had a strengthened airframe compared to their Allison-engined predecessors (P-51A & below).

And yet, the maximum G limit for these later aircraft (at 8000lb) seems to be identical to that of the Allison-engined models (namely, 8G).

Anyone have any explanation for this?

Hoggardhigh,

You will get a definitive answer shortly I'm assuming. However, in the interim I will fathom a guess. My guesstimate is the addition of the heavier Merlin meant beefing up the structure to hold the engine, along with anything associated with the stress of increased horsepower / heat. These "enhancements" were most likely done to keep the G limits were they were on the earlier versions instead of lowering them with the newer B/C/D/K.

Cheers,
Biff

 

[Hoggardhigh]

Hoggardhigh,

You will get a definitive answer shortly I'm assuming. However, in the interim I will fathom a guess. My guesstimate is the addition of the heavier Merlin meant beefing up the structure to hold the engine, along with anything associated with the stress of increased horsepower / heat. These "enhancements" were most likely done to keep the G limits were they were on the earlier versions instead of lowering them with the newer B/C/D/K.

Cheers,
Biff

What do you mean by, "the stress of increased horsepower /heat"?

 

[GregP]

When you increase horsepower and go faster, the dynamic pressure increases and the exhaust gas gets hotter.

Dynamic pressure is a flight limit and the heat of the exhaust that streams down the side of the airplane causes the metal to expand and contract as heat is applied and removed (when you turn the engine off or reduce power). If a major powerplant change is done, the designer must account for the increased dynamic pressure and heat applied. If the new powerplant reduces power, as in the F-16/79, then they don't have to recalculate the structure except to account for the CG change because the structure is already strong enough for a more powerful engine.

By way of example, when I said the dynamic pressure is a flight limit, we could look at the P-51 Mustang. The P-51 had a maximum permissible speed of 505 mph IAS at any altitude. The IAS converts to TAS that changes with altitude and temperature. If you exceed the 505 mph IAS, you could have structural failure from excess dynamic pressure.

The Reno Air racers sometimes come up against dynamic pressure limits, particularly when they try for a new world record airspeed, because they have to choose the atmospheric conditions so they can go fast without exceeding dynamic pressure limits. Voodoo and Strega are right up against the pressure limits when they are at full power in a straight line above 5,000 feet density altitude.

If you exceed the maximum permitted diving speed, you fall into the category of a test pilot. Good luck. The certifying authorities usually expect the max dive speed to have a 10% safety factor, but it's hard to count on that flying a modified 70-year old airplane with more than twice the originally-installed power.

 

[Hoggardhigh]

Dynamic pressure is a flight limit and the heat of the exhaust that streams down the side of the airplane causes the metal to expand and contract as heat is applied and removed (when you turn the engine off or reduce power). If a major powerplant change is done, the designer must account for the increased dynamic pressure and heat applied.

Does that affect the wings AND fuselage, or just the fuselage?

By the way, I'm also curious about one other thing: If an engine change involved an engine with the same weight as the earlier one, but with a lot more HP, would any structural changes to the airframe be necessary?

 

[Hoggardhigh]

When you increase horsepower and go faster, the dynamic pressure increases and the exhaust gas gets hotter.

Also, does that apply to level flight speeds, diving speeds, or both?

 

[GregP]

Dynamic pressure limits affect the entire outside skin and the parts attached to it, so it affects fuselage, wings, and tail.

It's very similar to a roof coming off a house in a tornado. Suppose we have a house that is 80 feet long and 40 feet wide. That's 3,200 square feet of roof, if you eliminate the roof slope. In reality, it is more, but this will do. 1 square foot has 14.7 pounds of pressure on it on a standard day at sea level. OK, suppose a tornado comes past and the outside air pressure drops by 2 pounds per square inch. There are 144 square inches in a square foot, so our 3,200 square foot roof has 460, 800 square inches. If the home owner has all the windows closed, the air pressure can't equalize, so the air inside has 2 pounds per square inch more air pressure than the outside air. 2 * 460,800 = 921,600 pounds of force going outward or, in this case, upward since the ground isn't going to move.

So, the roof is being pushed upward by almost a million pounds of air pressure and it departs the house.

The skin of an airplane experiences much the same thing as you approach the dynamic pressure limit, particularly if the airframe is well-sealed. Both Voodoo and Strega are VERY well-sealed. This is not the exact same thing as the house above, but illustrates pressure damage. In the case of the home owner, they SHOULD have closed all the windows toward the tornado and opened all the windows away from the tornado so the inside air pressure can equalize with the outside. Then, they would only have possible wind damage to deal with instead of a missing house and family.

 

[Shortround6]

Does that affect the wings AND fuselage, or just the fuselage?

By the way, I'm also curious about one other thing: If an engine change involved an engine with the same weight as the earlier one, but with a lot more HP, would any structural changes to the airframe be necessary?

Lets see,
More hp means more torque so stronger engine mounts and attachment points/supporting structure are needed.
You are going to need a larger propeller to transmit the increased power. larger diameter or more blade area or both.
Increased torque reaction (and propeller swirl) can need larger tail surfaces to maintain control.

The aerodynamic forces on the airplane go up with the square of the speed. A 10% increase in speed means 21% more force trying to tear off the wings and other bits and pieces.

How close was the original design to a limiting speed?

 

[drgondog]

Hi all,

It is a well known fact that the Merlin-engined Mustangs, from the P-51B/C series onward, had a strengthened airframe compared to their Allison-engined predecessors (P-51A & below).

And yet, the maximum G limit for these later aircraft (at 8000lb) seems to be identical to that of the Allison-engined models (namely, 8G).

Anyone have any explanation for this?

The Load Calculations for the airframes for All Mustangs (except XP-51F/G/J and P-51H) Design Limit Accelerations were 8g Angle of Attack for symmetrical pullout and 1G Lateral for 8,000 pound gross weight. The XX-73 Static Test Model was tested to 8g and 12G for AoA and 1G to 1.5G for Lateral loads to test empennage assembly and engine Mount assembly.

The design Load for the airframe (X73/NA-73) was as noted above for Design and Ultimate Accelerations at Gross Weight of 8000 pounds.

As the airframe morphed from NA-73/83 to NA-91 to A-36/P-51A the significant design changes included strengthening the wing to support 1000# bombs and Fuel tanks, and for the A-36 - to sustain loads imposed by dive brakes. For the P-51B-1 the wing was beefed up on the aft spar to enable the additional angle of aileron deflection. The aft spar was modified (slightly) again to hang three hinges instead of 2 for the aileron. WS 75 was beefed up with a doubler to reinforce the wing during pullout. Never was there a part by part re-examination using higher Design Limit loads to determine if the wing was in fact deemed 'safe' for design AoA load conditions for higher gross weights.

Nothing so modified ever raised the wing/airframe to exceed 8G Limit/12G Ultimate Angle of Attack Loads imposed on an 8000 pound Gross Weight Mustang. If anything the modifications to the wing (and fin spar) Enabled the airframe to attain the Design Limit loads.

Airframe structures in 1940s was a science, but when performing Stress Analysis using WWII methodology on individual parts there is a degree 'art' when analyzing complex geometries to determine centroids and moments of inertia - or shear panel load distribution to predict buckle/oil can effects on a fuselage subjected to bending and torsion. Wing Torsion analysis was in its infancy and zero knowledge was applied to practical aeroelastic phenomena. Modern Computational methodology (pioneered by grid relaxation techniques and CAD/CAM geometry) revolutionized airframe structures in the very late 1960s and 1970s when computational power of computers grew exponentially.

IMO nobody in the airframe business was interested in devoting the man hours in completely re-examining a major component (i.e. Wing) for new modification although a major re-examination was applied by NAA on the P-51D-1-NA wing when the 1000 pound bomb load was validated for stress loads due to not only the additional weight (over 600 pound limit for B/C/D pylon load) but also look in detail to dynamic pressure loads in high speed cruise imposed on the bombs - affecting the pylon, pylon support structure, leading edge spar and main spar for 3G accelerations (IIRC). None of the load envelope examinations remotely approached an 8G dive pullout on the 8,000 pound airframe.

 

[Hoggardhigh]

WS 75 was beefed up with a doubler to reinforce the wing during pullout.

Was this modification done in the field or at the factory? Or both?

 

[drgondog]

Was this modification done in the field or at the factory? Or both?

Both - The Technical Order retro-fitted the WS Doubler in the field below the production break insertion.

 

[Hoggardhigh]

For the P-51B-1 the wing was beefed up on the aft spar to enable the additional angle of aileron deflection.

As you've noted elsewhere on this site (and also as noted on pages 124-125 of this book), the XP-51 was tested with greater aileron deflections similar to those of the P-51B/C and later Mustangs. No structural modifications to the XP-51 wings for these tests is mentioned in either of these sources.

Can you or someone else provide an explanation for this?

 

[drgondog]

As you've noted elsewhere on this site (and also as noted on pages 124-125 of this book), the XP-51 was tested with greater aileron deflections similar to those of the P-51B/C and later Mustangs. No structural modifications to the XP-51 wings for these tests is mentioned in either of these sources.

Can you or someone else provide an explanation for this?

Yes - I can. That said I devoted a lot of print starting on page 124. What wasn't clear to you?

The aft spar and aileron hinge design was changed for B-1, and changed again for rest of P-51B/C and D/K to provide3 hinges for the aileron.

 

[fubar57]

Good luck with this one Bill