Messerschmitt 109 Improvements

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swampyankee said:
Perhaps, but the P-51 did have stability issues, due to reduced keel area aft of the c/g, after the bubble canopy was installed.
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Milosh said:
That would be the P-51D/K but the razor back P-51B/C also had the same problem.
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It seems that the stability issues stemmed from the more powerful engine (Merlin 60-series vs Allison single stage) and the 4 blade prop. Teh stability issue wasn't solved until the -H, which had a larger fin and rudder.
 
wuzak said:
It seems that the stability issues stemmed from the more powerful engine (Merlin 60-series vs Allison single stage) and the 4 blade prop. Teh stability issue wasn't solved until the -H, which had a larger fin and rudder.
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The H also had the rear fuselage lengthened.
 
swampyankee said:
Perhaps, but the P-51 did have stability issues, due to reduced keel area aft of the c/g, after the bubble canopy was installed.
Click to expand...

Much of the reduced stability was due to the introduction of the fuselage fuel tank, rather than a reduction in keel area. If anything the removal of the "razorback" structure probably meant that the total structural weight aft of the cg was reduced, and the structure itself made stronger because it became more of an oval monocoque.
 
This is why the dorsal fin was fitted to the 'D; the earliest production ones didn't have it. in relation to the Bf 109, it'd be wise to enlarge the fin, rudder assy and if necessary fit a dorsal fin.
 
Wasn't necessary. It was only necessary to get there. Then the P-51 was more than capable of dealing with a Bf 109 since any aft CG condition was gone by the time it got there.
 
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GregP said:
Wasn't necessary. It was only necessary to get there. Then the P-51 was more than capable of dealing with a Bf 109 since any aft CG condition was gone by the time it got there.
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This was true in the tactical situation that existed in the ETO at the time. Elsewhere, or at a different time, it might have been a serious problem.

It is hardly a desirable feature in any aircraft.

Cheers
Steve
 
It is hardly a desirable feature in any aircraft.
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Yep, otherwise, if the fin fillet wasn't necessary, why did NAA add it, then go to the lengths of changing the P-51H's fin area?

I wrote
in relation to the Bf 109, it'd be wise to enlarge the fin, rudder assy and if necessary fit a dorsal fin.
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and I meant that the Bf 109 would need increased fin area; as it was its fin and rudder was too small and this was altered on later variants.
 
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What I meant to say was enlargement of the tail wasns't necessary, not the presence of the dorsal fin. That "fix" was added even to razorback P-51's. A larger tail wasn't necessary once the fuselage tank was burned off. The P-51 with the dorsal fin flies just fine.

As for not being desireable, you get a choice ... shorter range and good flying characteristics or longer range with some lateral instability until the fuel burns off. If you need the range, the choice isn't there and you go with it. In the real war, there wasn't any better escort fighter anywhere in the world. Wanting the aft CG condition to magically "go away" wasn't possible, so I don't get what is being said.

Ensuring the condition gets addressed as quickly as possible by burining the fuselage tank first seems to be quite a reasonable tradeoff. If combat is anticipated right after takeoff, then you'll be fighting a much shorter war and you simply don't fill the fuselage tanks.
 
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NAA was aware of the yaw stability issues with the B/C. The also experimented with "H" style tall vertical stabilizer but it wasn't sufficient improvement. This was clarified when the D-5 was on the production line and the dorsal fin mod was designed as field kit for B/C and installed on the D-10.

It did improve (slightly) the high speed yaw but no discernible improvement to aft cg problem. Only lengthening the fuselage of the H by ~13 inches and reducing the fuselage tank from 85 to 50 gallons solved the problem. Much focus has been placed on the reduction of the turtleback when the D was designed but subsequent experience showed that was not the issue.

The root of the issue was the increased HP and torque of the Merlin with insufficient Vert stab and horizontal stab authority to compensate - which led to subtle design changes to horizontal stab incidence as well as the taller tail and lengthened fuselage for the H.
 
Just to make sure this is clear, some pics below of the P-51 models with and without the small dorsal fin just at the front of the vertical tail. Here's a pic of the B/C without the dorsal fin.



Here's a pic of the P-51B with the dorsal fin.



Here's a P-51D without the dorsal fin.



And, finally, the P-51D with the dorsal fin.

 
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I think there is a bit of confusion here. The P-51 always had a directional (yaw) stability issue at least as far back as the B model. The problem became worse with the reduced keel area on the bubbletop (D/K) models. The solution to DIRECTIONAL stability was the fin fillet. This improved things but didn't really completely solve the problem.

The fuselage fuel tank didn't really add to the directional stability problem. The aft CoG condition resulted in a LONGITUDINAL stability issue. In other words, with even a partially filled fuselage tank, the aircraft would tend to wander a bit vertically.

These are two separate problems.

- Ivan.
 
One other change associated with the dorsal fin fillet was that the rudder trim tabs had a reverse boost linkage installed, although I don't know what effect that had.

I'm also pretty sure that the incidence on the P-51B/C/D/K tailplanes was increased and the elevators reskinned in duralumin to help alleviate the problem with longitudinal instability with the full fuselage tank, but I'll have to check Gruenhagen to be sure of the facts.
 
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The instability arising from aft cg contribution, coupled with increased torque of the merlin engine, coupled with control responses necessary to fly the airplane increased Both pith And yaw issues contributing to a.) loss of control (i.e. Snap roll and subsequent departures and b.) subsequent structural failure of the empennage.

The were several fatal accidents traced back to pitch and yaw coupling due to the aft cg - but slow rolls and snap rolls with full control authority also caused similar structural failures.
 
i have read the structural failures of the empennage but have never seen it exactly described as to what the failure was. was it a bend, break, twist, loss of structure ... the like rudder, elevator, trim tab, etc. ??
 
Bending/torsion failure for the slow roll issue. The 51 had excellent rudder response with average pedal force - easy to really crank it, so unexpected results when trying to reduce the yaw in a near terminal dive resulted in too large forces on rudder and hence to the aft fuselage/empennage. That is why the reverse boost on trim tab was installed - to make it harder to overpower the rudder.

I a slow roll, you have rudder and elevator forces controlling the airplane for asymmetric flight loads - again creating additional torsion in the aft fuselage.
 
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