Hi Guys
I hope everyone's Thanksgiving was good.
I've had several questions about how I created the profile shapes for the P-51D as well as the Parts. Here are the answers and a few pictures to help with the explanation.
The exact shape of the P-51 was defined by its ordinates, which were finalized in a wind tunnel and then either destroyed or lost to history. What are ordinates? They are fixed points in space as they relate to the X, Y, & Z axis. The X-Axis representing the Horizontal plane, the Y-Axis representing the Vertical plane and the Z-Axis representing the depth plane.
Fortunately, I was able to find the coordinates used to define “most” of the Station 75.5 Firewall. The example below represents just 5 of the 35 2D points that were used to define that shape. Unfortunately, the ordinates in the drawing stopped about three-quarters of the way up for the part. The points below represent the 2D (Horizontal & Vertical) profile shape. In this example, the first number is the X Point (horizontal) by units and the second number is Y Point (vertical) by units, as they are measured from the defined starting point of 0,0 in space.
16.000,5
16.102,4
16.199,3
16.283,2
16.369,1
16.445,0
Here is what the Firewall Ordinates look like when plotted out in AutoCAD.
Then since the 0,0 point is also listed as the “intersection” of the Frame Reference Line (FRL) and the Ships Centerline (SCL), I can then add in these lines by point 0,0. Then it’s an easy task for AutoCAD to Mirror those Ordinate Points on the right by the 0,0 location, to create the left side ordinates, as shown below.
Now we come to the ENORMOUS PROBLEM. The complete "Ordinates" used for the P-51D overall profile have been either lost, destroyed or misplaced to history over the years. Fortunately, the Firewall drawing does tell me the vertical length of the part. This measurement provides the “missing” ordinate for the Top of the part. With that point, I can then add in the positions of the Upper and Lower “longerons” and then create a tangent curve from the Top Profile point to the existing side ordinates points, to complete the full profile shape shown below.
Next, I can add in the Cockpit Plan’s Floor Location and Upper and Lower Longeron locations, and then finally the “Part” itself. In this case, I am showing the Station 89.75 Rib parts with the overall profile. Note, the ribs are based on the original rib drawing, then modified for the Cockpit Plans while retaining the original outside and inside profiles, as well as specific details per the original drawing for holes, curves, connections, etc. This is showed in the picture below.
The complete ordinates for the P-51 A-C are available on old scanned drawings of very poor quality. When you are entering 1,000's of 5-digit numerical number sets, quality and readability are imperative for accuracy. Thus today, the only place I know where a true ordinate set exists, is on Hugh's Technote Site, where Huge recreated from the few readable ordinates across various drawings for the P-51A-C and then mathematically completed the rest.
The P-51 A-C and D models are "basically" identical from the Upper Longeron down. The P-51D's shape changes mostly above the upper Longeron. This means we can use the data from the P-51A-C for most of the shape. As of this point today, everything calculated above the upper longerons is an estimate unless you have a real P-51D available to measure for 3D points...which most of us don't.
The good news is that we do know several key measurements from the Ship Center Line and Fuselage Reference Line to the Top of the Firewall, Windshield Canopy Frame, Canopy Top and the Longerons Center line locations. These provide the data to replicate the part locations to a very precise point, which I estimate to be close to 99.9% accurate. The reasons we cannot get it perfect is because we don't have all the ordinates and many of the drawings contain errors in dimensions, as everything was done with a pencil, triangle and a scale ruler. This means the Cockpit Plans should be accurate to a minimum of 1/128th of an inch or better overall. This equates to 0.0078125 for you decimal guys out there, which is pretty darn close.
The last thing is the overall Top Profile of the P-51 is virtually straight from Station 75.5 to about Station 146, as shown in the pictures below. The main profile changes are above the Upper Longeron, which I have calculated with the actual part(s) dimensions, for everything above the Upper Longeron.
Thus, with my Cockpit Plans, I am using the same outside profile for Stations 75.5 through 146. I do use all the specific data for the inside profile and mounting hole locations from the original parts for accuracy. This makes for easier building and ensures the shape accuracy.
I hope that sheds a little light on the P-51D’s profile and how its represented by the Cockpit Plans.
Please let me know any questions you have and enjoy the weekend.
Stang out