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packardpursuit
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- Dec 3, 2008
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Well, technically, the B/C retrofit of Dorsal Fin Fillet, and early D retrofit with up locks were not factory included items, although they were to become so, to one extent or another. B/C dorsal retrofit was covered by same opriginal combat TO which required them on P-51D's and only last block P-51C's had them factory installed. It's pretty clear now the D's canopy and" loss of side area" had nothing to do with the official reason for the DFF (and other related tail fixes).
I've not head anything abouta beefed up vertical attach fitting, but seems plausible.
I would respectfully question your ascertion "H solved the problem" What problem exactly are we talking about? The B/C/D/K's apparent penchant for high speed yaw instablilty and resultant structural failure? And the need for a few aerodynamic, structural, and mechanical fixes? Or the H's apparent need for more and more vertical area?
Improved yaw stability was achieved in combination with tail redesign and lengthening the fuselage. The resultant yaw stability improvement at high speed also apparently led to better take off characteristics vis a vis rudder pedal forces as well as less vertical stab trim required.
The primary problem with the B/C/D/K was inadequate rudder surface area as well as inadequate structural integrity to compensate for the increase in torque due to the Merlin.
One would think adequate P-51H tail strength and sufficient vertical area factors would have been addressed while in design, especially given the problems believed associated with immediate earlier types. Tail strength seems to have not been a problem . However the H's Vertical stab got added to at least once and its rather miniscule DFF was never altered.
Charlie - The H tail increase to both height and area was specifically to provide for better yaw control with less rudder input at high speed (IIRC). NAA finally figured out that the ventral fin did not improve yaw stability to the degree expected, and experimented with higher Vertical stab, with inconclusive results, which never was put in production. But that higher, 'skinny' tail was the fore runner for the H.
Increasing the static margin by 'extending' the fuselage some 13" was the key to providing good design response to the increasing torque output of the Merlin 1650-9.
The key to reducing the asymetric structural load due to increased rudder deflection and engine torque was to focus on a design which contributed most to reducing rudder input from pilot to control the yaw - both in high speed dive as well as rolling manuevers. The increase in fuse length was probably more important than the slight increase in Vertical Stab height and area. The change in moment arm from cg to AC of the Stab/Rudder automatically provided more lateral stability with even a D tail and the increase for the H contributed more positve benefits.
It has been my observation nearly ALL high speed aircraft developed during WWII got progressivly larger vertical surfaces, as combat speeds rose. IMHO, Mustangs, including H, fall into that general category, while the apples vs oranges specifics between the two can be harder to quantify.
Yes. increasing tail area easier than extending fuselage.. but sooner or later you have to redesign when you make a dramatic mod like either a much more powerful engine - or moving the Cg around - Fw 190D is a good example of both, and the P-51B/C/D was another example.
The 109K tail grew for the same reason but in each case some new limits must be placed on manuevers because the Fuselage/Tail structure aft was not designed for the increased aero loads imposed with a delta in rudder deflection/load due to the increased torque.
NACA tested additional fin area first on P-51B and then a D. British appear to have also tested larger fin and rudder on a Mustang III (p-51B) Niether types were adopted during WWII. The NACA fin cap WAS included on some late Cavalier Mustangs, becoming standard in the 60's. Interesting thse often mistaken as having an H fin and rudder.
Having a bit of a problem following your logic. It could be me.
Specifically what, the benefits to extending the fuselage so that the wing and cg could also move slightly aft and still derive an extended moment arm to the tail MAC's? Or?
I decided to check the distance differences between aircraft under discussion. So far very surprising. Will have to triple check, but I'm finding some interesting figures that may color our understanding of supposed"givens".
Since both B/C/D and F share almost same lengths (32'-2.9" vs 32'-3") I've included the later. P-51H is 33'-3.25". So right off we can appreciate that total fuselage length is 12-1/4" longer than earlier types.
Sounds right on the H, although my recollection was "~13" with no precision
How about distance over spinners to 25% chord line? According NAA dimensioned 3-views: F is 130.25" while H is 139.25" and BD is133.437"
For stability comments, knowing the location of the cg relative to the MAC of the wing (and horizontal/vertical Stab mac's) is critical but we can assume for the moment that NAA engineers would keep the 25% chord/c relationship the same - assuming the new wing Moment Coefficient was the same as the D.
Distance from 25% line to rudder hinge line is most revealing: 231.063" for F. 235.500" for H and 228.11" for B/C/D.
Please double check my math. front of B/C spinner is approx 34.436" infront of Fuse. Sta. 0. 25% wing datum is located Fus.Sta.99, and rudder hinge line is at Fus. Sta.327.11 (34.436+ 99=133.436") (327.11"- 99"= 228.11" )
I get the same results
I arrived at distances between 25% and Rudder hinge lines of P-51's F&H by subtracting the over spinner to 25%, plus rudder width dimensions from the overall lengths, in inches.
I don't recall what design specific design changes were incorporated in the H tail. If the plan view of the horizontal stab/elevator is essentially the same then we can conclude that measuring to the respective hinge lines will maintain a similar relationship with 25% chord of the wing.
By my calculations the moment arm from respective 25% chord lines (which is also 25% of all M.A.C.'s and not an unreasonable common CG for calculation purposses) of the H is 7.39" farther aft than B/D. That 12.25" inches longer is not looking that effective to me. BTW- comparative lengths for P-51A is about 1/8" less than for B/C/D but all fore'aft surface locations identical.
The combination of moving the H cg aft, along with reducing fuel from 85 gallons to 50 would be very significant in removing the aft cg issue of the B/C/D for full fuel load out.
I don't know whether the Elevator mac of the H was the same as the D nut, if so, Seven + inch movement aft of the tail mac from the cg is important for pitch stability. Ditto for the H Vertical Stab/Rudder in context of improving yaw stability
I checked Gruenhagen's area charts and find them a bit confusing.Will have to sort that one out, as well. Initial production P-51H's had 75.5" tall fin. Later, added cap to 82-9/16".
Again, I see nothing in the design of the P-51H that specifically addressed the problems of the P-51B/D. It was a different design entirely athough laid out in a similar general premis. General dynamic fixes were incorporated and yet the un-anticipated directional issues of the H were addressed by a fix tried out on B/D, but not deemed important enough to fit or even retrofit, during WWII, and some 11 years later.
Charlie - I have a couple of questions that occurred to me after the last post.
Since both B/C/D and F share almost same lengths (32'-2.9" vs 32'-3") I've included the later. P-51H is 33'-3.25". So right off we can appreciate that total fuselage length is 12-1/4" longer than earlier types.
How about distance over spinners to 25% chord line? According NAA dimensioned 3-views: F is 130.25" while H is 139.25" and BD is133.437"
Distance from 25% line to rudder hinge line is most revealing: 231.063" for F. 235.500" for H and 228.11" for B/C/D.
By my calculations the moment arm from respective 25% chord lines (which is also 25% of all M.A.C.'s and not an unreasonable common CG for calculation purposses) of the H is 7.39" farther aft than B/D. That 12.25" inches longer is not looking that effective to me. BTW- comparative lengths for P-51A is about 1/8" less than for B/C/D but all fore'aft surface locations identical.
Questions -
1. are you measuring to 25% chord point at CL Root chord? Or 25% Chord point at the WS of the MAC? Or 25% of the wing chord as WS where the fuselage intersects the wing?
If the latter, then the comparisons between the Cg should be close between the models.
If the second condition I would assume that the MAC would be close between D and H wings relative to WS and the respective chord at MAC.
If the former, then I have to ponder the significance of the much larger length of the CL Root chord of the D/K to that of the H.
I checked Gruenhagen's area charts and find them a bit confusing.Will have to sort that one out, as well. Initial production P-51H's had 75.5" tall fin. Later, added cap to 82-9/16".
Again, I see nothing in the design of the P-51H that specifically addressed the problems of the P-51B/D. .
My perception of our conversation thus far, is that we both tend to see-saw a bit, first one of us is speaking in generaliiies and then switching over to specifics. The other reciprocating in turn. Quite a dance. LOL
Only by you providing the specific details regarding STA locations was it possible for me to be specific regarding the effect of different design changes on the H as far as weights and balances and moment arms to help me explain why the changes to the H geometry help solve the stability issues inherent in B/D due to the Merlin upgrade.
Nobody I know (other than you) has the full set of aircraft specific lines/WS/Fuse STA details to flesh out this discussion or any other regarding 51 configurations.
I believe you tend to see the H as a natural evolution and extention of the earlier variants. It is only natural to compare the the newer to the earlier D variant, as it is a most obvious one to make. USAAF did same early on.
Charlie - from my perspective the H was, as you state below, a near totally different aircraft if one defines 'nearly the same' as having much in common - per my comment below.
I tend to see the H as a totally new airframe with no direct physical connection whatsoever with earlier production types (which clearly have a direct evolved developement),except for basic configuration similarities, and more advanced thinking incorporated (to which I humbly, now, stipulate did in fact address some issues that plaued B/C/D. And yet, as a new design, had problematic issues with stablity of its own).
I guess what I'm chaffing at is how we tend to think in absalutes, aided by our 20/20 hindsight. For all the problems we pecieve tody, about the stablitiy issues of the Merlin being adapted to the basic Allison type airframe, the fact is, the manufacturer, in concert with official tech oversight and THOUSANDS of flight test hours, did not feel an immediate change to the Fin/rudder area was warranted.
I was thinking that too. It could go in any number of directions.
One question that has bothered me is the implimentation of the boundry layer air gap, as developed for P-51B radiator air inlet. According to Gruenhagen and others, this was instituted when the XP-51B's were being tested and entailed considerable effort before arriving at a fianlized successful design. There is a glaring problem with the accounts that has not, IMHO, been addressed satisfactorily. As I see it, it concerns NAA engineers in a case of "what did they know and when did they know it?"
If we recall, the first Mustangs had a rather short carb intake situated on top of the cowl, about a foot aft of the spinner. I n rather short order this was modified by moving the intake to actually overhang the spinner rear by approx.1" Anothe feature of new configuration was a raised lip boundry layer by-pass that allowed dirtier air to escape injestion,below the intake itself.
Somehow I find it a bit difficult to understand how they developed a succesful fix for a boundry layer issue in the spring of 1941, but seemed to have (if the accounts are to be believed at face value) entirely forgotten the lesson for a year and a half, while deciding on a similar fix for the radiator intake.
I suspect that both adjustments to the carb intake design (Pre-Merlin) were more trial and error than analytical approach. Practical analytical prediction of boundary layer characteristice when immersed in a turbulent prop stream tube was not really possible in those days
While it is possible that they really didn't understand what was happening with those early Xp-51B radiator intakes, I find it a bit far fetched that no one, apparently, appreciated the great similarity of issues, earlier on. There HAs to be more to the story or it wasn't the earth shaking discovery purported.
Weather by anylitical or emperical means the solutions to both problems resulted in near identical configurations. It's the apparent year and a half difference that doesn't make sense to me.
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