BQM-34 wing profile



Introduction

As an aerospace engineer but an amateur aerodynamicst, I'm always amazed at the mess one gets into when trying to find the profile of an older aircraft. I've tried it for the Messerschmitt Me 163B and the Pander S4 Postjager, and it was messy and confusing to say the least. For the Firebee it was equally bad. It required this web page to sort out the bits and pieces of information that I found.



Available information on the Firebee profile

I had five pieces of information to start with:

A question mark was whether the profile was defined 'streamwise' or perpendicular to the quarter-chord. Torenbeek, page 436, states that it can be either: A wing section is formed by the external contour of a wing cross section with a plane parallel to the plane of symmetry of the wing, or a plane perpendicular to the quarter chord. From fitting profiles to photos I found out that the Firebee wing uses the second definition. It appears that this is (was) the common method in the American aircraft industry.



NACA 63-014

I wanted to start with the normal NACA 63-014 profile. Abbott & Von Doenhoff (p119-122) explains the coding system as follows:

Since the design lift coefficient is zero, it seems logical that it's a symmetrical profile, which turned out to be true.

I could not find a listing of NACA 63-014 coordinates: nothing in Abbott & Von Doenhoff, or any NACA / NASA document. I do not know whether that means it does not exist at all. In the end I decided to do a linear interpolation of NACA 631-012 and NACA 632-015 data from Abbott & Von Doenhoff (pages 337 and 338). Since they are symmetrical the interpolation result should be accurate.

                   NACA 631-012     NACA 632-015     NACA 63-014  
interpolated
      
0 0 0 0
0.5 0.985 1.204 1.131
0.75 1.194 1.462 1.373
1.25 1.519 1.878 1.758
2.5 2.102 2.610 2.441
5.0 2.925 3.648 3.407
7.5 3.542 4.427 4.132
10 4.039 5.055 4.716
15 4.799 6.011 5.607
20 5.342 6.693 6.243
25 5.712 7.155 6.674
30 5.930 7.421 6.924
35 6.000 7.500 7.000
40 5.920 7.386 6.897
45 5.704 7.099 6.634
50 5.370 6.665 6.233
55 4.935 6.108 5.717
60 4.420 5.453 5.109
65 3.840 4.721 4.427
70 3.210 8.934 3.693
75 2.556 3.119 2.931
80 1.902 2.310 2.174
85 1.274 1.541 1.452
90 0.707 0.852 0.804
95 0.250 0.300 0.283
100 0 0 0


NACA 63A014

NACA Report 903 says:

It does not literally say so, but I read that NACA 63A014 should be a modification of NACA 63-014. Although this is maybe a big assumption, I wanted to find that first. It would require a rather different thickness distribution, but I haven't anything on that subject. Instead, most of the discussion is about the mean line. Which is probably irrelevant here, since it probably symmetrical (see below).

Again I could not find a listing of NACA 63A014 coordinates: nothing in Abbott & Von Doenhoff, or any NACA / NASA document. I decided to do a linear interpolation of NACA 631A012 and NACA 632A015 data from Abbott & Von Doenhoff (pages 344 and 345). Since they are symmetrical the interpolation result should be accurate.

NACA Report 903 says: 'Ordinates and theoretical pressure-distribution data for NACA 6A-series basic thickness forms having the position of minimum pressure at 30, 40, and 50 percent chord are presented in figure 2 for airfoil thickness ratios of 6, 8, 10, 12, and 15 percent. If intermediate thickness ratios involving a change in thickness of not more than 1 to 2 percent are desired, the ordinates of the basic thickness forms may be scaled linearly without seriously altering the gradients of the theoretical pressure distribution.'

NACA 63A014 is used on the Canadair CL-41 Tutor wing root profile (see Jane's), and NACA 63A014mod is used on Fokker F27 horizontal tail (see Torenbeek).

                   NACA 631A012     NACA 632A015     NACA 63A014  
interpolated
      
0 0 0 0
0.5 0.973 1.203 1.126
0.75 1.173 1.448 1.356
1.25 1.492 1.844 1.727
2.5 2.078 2.579 2.412
5.0 2.895 3.618 3.377
7.5 3.504 4.382 4.089
10 3.994 4.997 4.663
15 4.747 5.942 5.544
20 5.287 6.619 6.175
25 5.664 7.091 6.615
30 5.901 7.384 6.890
35 5.995 7.496 6.996
40 5.957 7.435 6.942
45 5.792 7.215 6.741
50 5.517 6.858 6.411
55 5.148 6.387 5.974
60 4.700 5.820 5.447
65 4.186 5.173 4.844
70 3.621 4.468 4.186
75 3.026 3.731 3.496
80 2.426 2.991 2.803
85 1.826 2.252 2.110
90 1.225 1.512 1.416
95 0.625 0.772 0.723
100 0.025 0.032 0.030


NACA 63-014 versus NACA 63A014

If NACA 63-014 versus NACA 63A014 are plotted in the same graph, the agreement for the forward 45% is obvious. The cusped trailing edge of NACA 63-014, and the 'flat' trailing edge of NACA 63A014 are clearly visible.



NACA 63A014.63

Abbott & Von Doenhoff reports in '6.6 Modified NACA Four- and Five-digit Series Wing Sections' (page 117): 'The suffix -63 indicates sections very nearly but not exactly the same as the sections without the suffix'. Now this probably only applies to four- and five-digit series profiles. Plus, without factory information, we can never find out how Ryan modified the NACA 63A014 profile for the Firebee.

Alternatively, because there's a dash and not a decimal preceding the '53', plus pointing at the Q-2A 'NACA 63, A014.6' airfoil tidbit, it could be that Ryan used a 14.63% thick version instead of the 'standard' 14% thick version. Because of this possibility, I also calculated the coordinates for the 14.63% thickness.

                   NACA 63A014  
interpolated
  NACA 63A014.63  
interpolated
      
0 0.000 0.000
0.5 1.126 1.175
0.75 1.356 1.415
1.25 1.727 1.802
2.5 2.412 2.519
5.0 3.377 3.531
7.5 4.089 4.276
10 4.663 4.876
15 5.544 5.798
20 6.175 6.458
25 6.615 6.919
30 6.890 7.205
35 6.996 7.315
40 6.942 7.257
45 6.741 7.043
50 6.411 6.696
55 5.974 6.238
60 5.447 5.685
65 4.844 5.054
70 4.186 4.366
75 3.496 3.646
80 2.803 2.923
85 2.110 2.201
90 1.416 1.477
95 0.723 0.754
100 0.030 0.031


Intermediate result

What we now have is the wing profile of the Q-2A (USA) / KDA (Navy), the first generation Firebee. The Q-2C / BQM-34A wing was modified with a drooped leading edge extension.



Messing with the chord

I overlayed the photo with the 14.63% thickness version, and the fit was excellent. Note that the wing skins of the center box are fairly thick at 0.190" / 4.8 mm; therefore the profile drawing is positioned on their outside edges.

The transition between the NACA 0009.932 nose section and NACA 63A014.63 original profile is quoted to be at 26.4% chord. But what chord is that? It's either the original chord or the new extended chord. That extended chord can be calculated to be 23.62" / 20.51" = 115.15% of the original chord. A quick photo analysis made it clear that it was the latter.

The 'Structural Repair Instructions USAF model Q-2C target drone' (T.O.1Q-2C, 1 November 1960) contains a cross-section drawing that shows the positions of the spars webs. The drawing strangely ignores the leading edge droop, nor does the profile represent NACA 63A014.63. I used three dimensions to do another check on the photo, and the agreement is reasonable but not perfect.




NACA 0009.932

NACA 0009.932 is another mysterious profile definition - what does the '.932' mean? A 9.932% profile thickness is possible but unlikely, mainly because three digits behind the decimal point is definitely not standard.

Next I found 'extensions' for NACA profile codes in 'Aerofoil Sections - Results From Wind-Tunnel Investigations - Theoretical Foundations' by Riegels (1961). Riegel explains (page 7) their use to describe variations of the nose radius, using '1.1', '0.825', '0.55', '0.275' extensions. But there is no mention of a '.932' extension.

Time for a very wild guess. The table with coordinates in Abbott & Von Doenhoff page 314 has an open space for the 0.5% chord coordinate. Maybe .932 would fit? It gives a slight V-shape to the extreme leading edge, something that I see in the photo too.

In the end I decided to list the NACA 0009 data, plus the extrapolated 9.932% thick version.

                   NACA 0009  
  NACA 0009  
9.932% thickness
      
0 0.000 0.000
0.5 (0.932) (1.029)
1.25 1.420 1.567
2.5 1.961 2.164
5.0 2.666 2.942
7.5 3.150 3.476
10 3.512 3.876
15 4.009 4.424
20 4.303 4.749
25 4.456 4.917
30 4.501 4.967
40 4.352 4.803
50 3.971 4.382
60 3.423 3.777
70 2.748 3.033
80 1.967 2.171
90 1.086 1.198
95 0.605 0.668
100 0.095 0.105


Fitting the extended leading edge

The nose section had two variables: what chord definition was used, and what thickness was used. I made the following drawing to test all four options, ready for test-fitting on the photo.

The smallest option was a bad fit.

Skipping options 2 and 3, it was only the largest option that had a good fit. However, the section was rotated 7.5 degrees instead of the expected 5.85 degrees.

A close-up view of the fit. If my fit is accurate (quite unlikely), I would expect a kink in the sheet metal of the nose extension on the lower side, so it can connect properly with the 'secondary' flange of the front spar. The 'primary' flange would be the one that connects with the thick center wing skins (see also the picture from the manual, six pictures up). However, photos do not show the kink in the sheet metal.

If the kink is to be avoided, a part of the NACA 0009.932 nose extension must be modified. This is drawn in green, and it fits well with the structural details of the wing. I think this is the best fit that can be achieved within the limitations of this analysis.



The reconstructed BQM-34A wing profile

The result is the following profile, in two variations. The top one retains the chord line of the original 63A014.63, and I think this is how the real wing relates to the Firebee's reference axis system. The bottom one has a new chord line that has a horizontal chord line between the leading and trailing edges; it is rotated 1.85 degrees nose up. The top one has a thickness of 12.71%, the bottom one is 12.86%. One could also draw the 'streamwise' profiles from them, by reducing the thickness to 70.71%.


A number of assumptions were made in the above analysis. The profile codes were strange but fitted well. I was unhappy that the nose extension rotation angle differed considerably from the given value.





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