Friday, May 11, 2012

Accurate Three-View Drawings

In my opinion, many of the drawings available on the interweb are inaccurate in one or more respects. I recently downloaded a sample vector drawing from one "Blueprint" site to see if it was compatible with my Illustrator software. Even though it was a good looking, detailed piece of work, I discovered that elements of the side and top view were differently placed longitudinally. That's not a confidence builder.

One of the disappointments encountered in creating, and I use the word deliberately, the top or front view of a three-view drawing is very precisely tracing one side in Illustrator and then copying and flopping it to provide the other side. Almost invariably, the flopped side does not match the other side of the original. (It was possible in the good old days when drawings were created with T squares, triangles, and French curves to get the two sides the same within a line width by tracing a master but not worth the extra effort.) In any event, pre CAD, the shape of the airplane was not established by three-view drawings, per se, but by 3-D solid master models created from loft-line drawings produced by engineering that were consistent with the detail part and assembly drawings.

That said, it is possible by using manufacturer's station drawings, dimensioned three views, loft-line drawings (relatively rare, even rarer in a usable size), data (e.g. root and tip chord dimensions and wing-sweep angle at the quarter chord to establish an exact basic planform), maintenance manual illustrations,  and photos (which can be misleading due to the various distortions introduced by the camera) to create a pretty good three-view drawing. That takes a lot of time and is unlikely to be perfect but it is the best a good draftsman can do without being able to measure a real airplane, which involves significant effort and still doesn't provide more than an approximation of shape as opposed to size.

As others have noted, the primary purpose of three-view drawings produced by manufacturers was to illustrate significant dimensions, not provide model kit manufacturers with an accurate depiction of the aircraft's shape. My favorite example is this McDonnell F4H-1 top view, which provides the different lengths of the short-nose and long-nose Phantom IIs on the same drawing. (The side view and presumably this top view are the long nose version.)
A dimensioned drawing, by the way, is the only way to be pretty sure what the length dimension represents. Relying on the data in a table can lead to error. One published F4H length that I've seen is from the tip of the nose to the aft edge of the stabilators, for example, not the aft end of the fuel vent as shown above. (The length from the tip of the nose to the aft edge of the stabilators and fin was decreed by McDonnell to be exactly 56 ft to be better accommodated on the Essex-class carrier forward elevator; the fuel dump/vent was a late addition to the design.)

The major source of confusion is whether the cited length was that parallel to the water lines or parallel to the ground with the aircraft at its normal stance (there's actually a detailed definition of the variables like weight that affect the stance). In this case, the length also includes the barricade-strap deflector (to prevent the strap from hanging up on a gun barrel and jerking the airplane sideways) that extends just forward of the nose itself.
Even three-view drawings from the aircraft manufacturers known to go to some trouble to produce good ones have to be watched closely. For example, the span of the North American FJ-4 horizontal tail was decreased as a result of flight test. However, the excellent drawing that was issued depicted the original horizontal tail.
The span of the production FJ-4 horizontal tail is shown on the crude SAC top-view drawing.


Another problem with three-view drawings like those in Standard Aircraft Characteristics charts is that the creator of the original might not have reduced all of the views by the same proportion. One needs to check the common dimensions on each of the views (the wing span on the top and front views, for example) to verify that they are the same. Another problem is differential "stretch" at some point in the reproduction process, which means that the width of the copy might be proportionally different to the height of the original.

The next level up in terms of achieving accuracy is a station drawing:
 Longitudinally, the term of art is fuselage station: note that in this case, the nose extends forward of station 0; this is the result of the lengthening of the nose between the prototype XF2H-1 and the production F2H-1. Vertically the stations are called waterlines. Horizontally, they are butt (short for buttock) lines. (Waterline and buttock line date from the beginning of ship building; length was stipulated as a frame number.)

It's interesting enough when the aircraft is lengthened so the nose extends forward of station 0 (this means you have to delete the minus sign of a station forward of 0 and add it to a station aft of 0 to get the distance between the two stations) but it can get downright confusing when the manufacturer  tries to explain it, as in this note on the Grumman F9F-5 station drawing, explaining the changes due to the plug added in the F9F-2 Panther forward fuselage:
A very accurate check of part of the shape of a top view drawing can often be accomplished by finding a table of dimensional characteristics of the wing and empennage.  The essential numbers are the root chord (which is not measured at the side of body but at the center line of the fuselage), the tip chord (which results from a straight line extension of the leading and trailing edge), the wing span, and the wing sweep at the quarter chord of the wing (not the leading edge).

The best source of shape is a high-level loft-line drawing that provides cuts (cross sections):
These were based on the actual drawings used to create the master models of the external shapes from which the tooling was derived. Cross sections, and therefore shapes, created without loft-line drawings are only approximations to varying degrees. Unfortunately, these are pretty rare and we have to rely on the limited number of shapes provided in a front view or from pictures.

As previously noted, even three-view drawings produced by the manufacturers can't be trusted without close examination. In the case of the P-80, available Lockheed drawings were clearly inaccurate in some respects, e.g. the side view in the TO-1 SAC had inlets that weren't even close to the real thing as well as a TO-1 drawing with a P-80A canopy instead of the early F-80C canopy that it had. Another disappointment was the fact that even after correcting for slightly different horizontal and vertical sizing of the umpteenth-generation copies, matching key points as precisely as possible, etc. the outlines of three different side-view drawings are not identical.
Nevertheless, any one of the outlines would be recognizable as a P-80 and look right when compared to a photograph, just like either side of the top view. Well, not the inlet and canopy on the TO-1 SAC drawing ...

So the trick is somehow determining whether a given drawing is fairly accurate in shape, basic dimensions, location of major components, etc. so it can be used to evaluate the accuracy of a kit and correct any objectionable differences. If you don't have access to manufacturers drawings, the primary way to determine whether or not a drawing is to be relied on is to compare the basic dimensions to a dimensioned drawing and the shape to photos (with the caution that photos can be misleading due to perspective distortion, etc.).

Monday, May 7, 2012

The Spey-Powered Phantom Changes

18 November 2016: For a note on F-4K XT-595 first flight markings, see http://tailhooktopics.blogspot.com/2016/11/f-4k-first-flight-markings.html

17 November 2016: I apparently noted but forgot to add this excellent description of empennage detail differences: http://phantomphacts.blogspot.com/2014/05/differences-between-phantom-fg1-and-fgr2.html

11 October 2015: Added additional information about engine inlet change

10 October 2015: Added nose landing gear illustration of difference between J79 and Spey F-4K/FGR.1 Phantom

21 February 2014: I added a comparison of the difference between the boundary-layer-removal hole patterns on the forward variable ramp.

15 December 2013: I added some more information on the fuselage changes and the afterburner shroud.

13 December 2013: I added some information about the F-4K changes required so it fit on the elevators of the British carriers.

11 December 2013: I revised the comparison of the area aft of the afterburner nozzle to show that the absence of a "step" in the upper line of unpainted metal was on the prototypes XT595 and 596 and the first production F-4K, XT857 only. The M prototype retained the step.

Note that I refer to the Spey-powered Phantoms as the F-4K and F-4M instead of FG.1 and FGR.2 respectively, the proper British designations.

The key points are these:

1. Air inlet width increased by three inches on each side (Derek Bradshaw). Note that the original inlets were about 43" deep and each had an inlet area of 500 square inches. Widening an inlet by only three inches would increase the inlet area by about 25%, somewhat more than the reported increase of 20%.

2. Afterburner shroud outside diameter of 43 inches; diameter of shroud at aft edge, 38.5 inches (from the circumference measured by Rob as reported on Britmodeler)

3. Wider and higher "nacelles" (bulkhead comparison drawings from Craig Kaston)

3. No change in wing span: the location and width of the trailing-edge flaps did not change and their inboard edge established the width of the F-4K/M fuselage at that point (as measured, the F-4K/M fuselage was slightly wider at the trailing edge of the wing)

4. No change to fuselage aft of the arresting hook attachment except that the outer mold line of the heat-resistant shingles was lowered and extended forward to fit the Spey afterburner shroud.

5. Increase in angle of incidence of the engine installation from 5.2 degrees (J79 measured on McDonnell drawing) to 6.75 degrees (F-4K/M specification)

6. The aft Sparrows were mounted the same distance apart as in the J79-powered Phantoms but were angled at 3.75ยบ nose up consistent with the increase in depth of the lower aft fuselage

Note: It has been reported that the F-4K/M stabilators had reduced anhedral. This is incorrect; they had the same anhedral as every other Phantom.

A common error is to assume that the lower auxiliary air doors on the F-4K/M are in the same location as on the J79-powered Phantoms; they are not. See bottom view illustration.

I've used McDonnell-generated data, three-view drawings, lines drawings, and illustrations to create the comparison shown below, using the pretty-good McDonnell Blue Angel F-4J drawing as a baseline. Unfortunately, although it otherwise adheres pretty closely to the McDonnell B/J lines drawing, it is not exactly correct with respect to the longitudinal location or size of the J79-GE-8 engine afterburner.

An excellent set of walk-around photos of F-4M (FGR.2) XV497 by Graham Platt can be found here: http://www.britmodeller.com/forums/index.php?/topic/234949864-mcdonnell-fg1-fgr2-phantom/

Although photos have limited use in establishing much more than basic shape or position relationships, this picture of XT-595 is as good a side view as you'll see:

The Spey was bigger around than the J79 because it was a bypass engine and had a higher mass flow. The air inlet and engine "nacelle" were enlarged accordingly, the former in width and the latter in width and height. Since the relationship of the thrust line to the location of the airplane's center of gravity is pretty important and McDonnell apparently didn't want to change the location of the arresting hook or make any significant changes to the fuselage aft of station 515 (subcontracted for the F-4K/M to Short Brothers), the Spey had to be installed at a slightly greater angle than the J79, requiring a deepening of the lower aft fuselage. (Other considerations as to engine location were the weight/cg of the Spey versus the J79 and the effect of the exhaust on the flow field around the stabilator.) Close examination of photos leads me to conclude that the top of the Spey's afterburner shroud was located at or very slightly below the top of the J79's afterburner and the inboard sides of the two engines were probably the same distance from the fuselage center line (note the weasel-wording).

The afterburner shroud moved aft to position the afterburner "petals" fully open for afterburner operation and forward when the afterburner was shutdown.
At the moment, I don't have any information on the forward and aft location of the shroud or its position when the engine was shut down. Note that XV497, the subject of Graham's walkaround referenced above, does not have the afterburner petals installed.

The red area in the following illustration corresponds to the dip in the fastener line in the next illustration.
 (Fuselage station 515 is shown as A in the following pictures and "F-19" on the side view below; fuselage station 493 is B and "F18".)
Some have questioned whether there were any changes to the structure aft of FS 515. The heat-resistant shingles were different on the Spey-powered Phantoms from FS 515 to approximately FS 576. Moreover, the panel that incorporated the trough for the thermocouple wiring on the J79-powered F-4s was moved forward of FS 515 on the Spey-powered Phantoms and replaced with a titanium shingle.
Minimizing the airframe width increase at mid-fuselage due to the bigger compressor appears to have been addressed by toeing the Speys in slightly (0.55 degrees). This also had the benefit of reducing the yaw-moment arm of the higher-thrust engines, which meant that no increase in rudder control power would be required with one engine inoperative on climb out or wave off. (It also might have allowed retention of the location of the pockets for the upper fins on the aft Sparrows; the original F4H was pretty dense.)
I've somewhat refined the fuselage side view changes and added additional cross sections.


Another difference between the B/J and the K/M is the location of the lower auxiliary air doors.
Note that because of the bigger diameter of the Spey afterburners, the "wedge" just aft of the tailhook attachment point between the afterburners was not as wide since it is on the same waterline on both airplanes. This isn't a very high resolution picture, but it illustrates the change compared to the drawing above:

As it happens, my first job after college was working for McDonnell Aircraft as a junior flight test engineer on the F-4K/M Phantom program. Here is a picture of "my" airplane, YF-4K Number 1, XT-595, in June 1966, taxiing in from the first attempt by McDonnell Chief Test Pilot Joe Dobronski at first flight (the F-4K/M project pilot Bud Murray is in the aft cockpit).
I also worked with XT-596, which is now esconced in the Fleet Air Arm Museum at RNAS Yeovilton, Ilchester, Somerset, UK (http://www.fleetairarm.com):

A fairly direct comparison of a J79-powered fuselage with the F4K/M was obtained from the Robert F. Dorr collection:
Note the additional auxiliary air door on the side of the aft fuselage and the low-pressure compressor bleed port just ahead of and below it. Both are shown in red on the side view illustration.

Another example is this side-by-side picture taken aboard Independence in 1975:

The additional extension of the F-4K nose landing gear for launch was necessary for operation from the smaller British aircraft carrier. Note the added auxiliary torque arm or "scissors" at the bottom of the strut.
The F-4K/FGR.1 nose landing gear was also raked aft three degrees.

The F-4K/FGR.1 also featured a barricade-strap cutter located on the top and bottom of the nose just aft of the radome.

Another set of changes required for compatibility with the British carriers was a folding radome and shorter fuel vent.
The radar antenna folded with the radome.
(This is from an early McDonnell brochure so details might vary.)

The fuel vent was shorter and the outlets modified into a shallow "V".
The tail light presumably moved down to the parachute housing cover when a large antenna was added to the top of the vertical fin; I don't know when or why the vent was added.

I think the bottom line is that modifying an F-4J kit into an F-4K or F-4M model is doable but may involve more changes to the fuselage than most would want to make. Frank Mitchell is not one of those people: http://hyperscale.com/features/2002/fgr2fm_1.htm

"Chek" brought another difference to my attention that I hadn't noticed. The hole pattern on the forward variable ramp (the holes suck off the slow-moving boundary layer air) is different between the F-4B and subsequent J79-powered Phantoms and the Spey-powered ones:
On the J-79-powered ramp, there are thin strips of holes between the larger areas of holes and the pattern of the top-most segment is very different. (There is a possibility that the top of the forward variable ramp is angled up slightly more on the Spey-powered Phantom but if so, it's impossible to verify in pictures taken from different distances at different angles with different lenses.)

The inlet ramp and inlet itself were changed as well. Because the inlet was widened by three inches, its lip was moved aft about four inches to keep the shock off the leading edge of the fixed ramp properly positioned on it at supersonic speeds. Moving the inlet lip back necessitated moving the leading edge of the variable ramp (which was hinged to the trailing edge of the fixed ramp) back about three inches for the same reason. This meant that both the fixed and variable ramps were slightly wider.

More later...