BD-5 Wing Reprofile Specs

The BD-5 airplanes as originally designed have a well documented, undesirable sharp stall characteristic that has caused numerous accidents, many of them fatal. The main cause of the problem is the use of the infamous NACA 64-212 airfoil section at the wing root. This airfoil is low-cambered and only 12% thick, which largely accounts for its low maximum lift coefficient and also the sharp loss of lift at the stall, followed by a hysteresis loop during stall recovery. That is, the angle of attach must be decreased considerably in order to obtain flow reattachment after stall separation has occurred., usually with a considerable loss of altitude. Also, the stall is often accompanied by loss of lateral control, resulting in a wide departure. Other factors contribute to the problem, such as the extremely low Reynolds number of this airplane at the stall, less than 1.5 million. All in all, the airfoil selection was especially poor for this application, and this is inexcusable.

Short of a complete wing redesign, various methods are available to improve the BD-5 stall performance. For example, BD Micro Technologies offers dropped leading edge nose ribs (and other components) for wings that are still under construction. For completed airplanes, however, several methods of reprofiling the wing shape have been used with good results. Seth Anderson has modified his well-known BD-5 with an upper forward surface reprofile of his own design, extending back to approximately 40% of chord on the top surface, and this has been flying for several years with excellent results. This modification is minimal and relatively simple and, like other upper surface buildup schemes, stiffens the upper skin to prevent wrinkling. Unfortunately, this partial reprofiling essentially reverts the wing profile to a turbulent section, resulting in a slight drag increase. This is only a minor disadvantage, and is of little concern, considering the marked improvement in flight safety that results.

Another scheme for upper surface reprofiling is shown on the attached sketch, dated 11/20/92(Note: Sketch will be available later on when I am able to scan it, in the meanwhile if you need it, send me an email with your FAX number; however, it makes more sense to send Harry the $10 for the full-size sketch). In this scheme the complete upper surface is built up from the leading edge to the flap/aileron well lip, at approximately 80% of chord. The flaps and ailerons require no modification and are reused as is. Also, the existing tip section, NACA 64-218, while improvable, has acceptable stall performance due to its 18% thickness. This, a reprofile template is furnished for the wing root only, and the reprofile buildup feathers out to nothing at the wing tip, and at the leading edge and at the flap/aileron well lip. Maximum buildup, at the root, is 1/2 inch additional thickness.

The "GA" reprofile improves the all-important leading edge profile, adds a small amount of thickness and camber, and moves the loading forward slightly so that the pitching moment coefficient is not affected. The result is a softer stall, higher maximum lift coefficient with and without flaps, and a wide laminar bucket. In addition, the new profile has been designed to maintain constant velocity flow to approximately mid-chord, preserving laminar flow (and low drag). CG limits for the airplane are not affected.

Flight experience with this modification has been excellent. A typical example is the attached report from Bobby Farmer of Odessa, Texas.

Another option for BD-5 projects under construction is simply to assemble the wings as designed, and then reprofile the top surface before flight.

In the interest of improving flight safety, I offer a full-size template drawing for $10.00 (ten dollars) to cover the cost of printing and mailing.

Note: The attached standard table of ordinates can be used to generate the profile for the BD-5 root template (31 inch chord), of for any airplane using the NACA 64-212 airfoil.

Station	Upper	Station	Lower
`0.000`	`0.000`	`0.000`	`0.000`
`0.238`	`0.836`	`0.200`	`-0.532`
`0.713`	`1.426`	`0.582`	`-0.925`
`1.188`	`1.835`	`0.841`	`-1.105`
`2.375`	`2.588`	`1.353`	`-1.379`
`4.750`	`3.705`	`2.618`	`-1.846`
`7.125`	`4.540`	`5.132`	`-2.491`
`9.500`	`5.214`	`7.636`	`-2.967`
`14.250`	`6.270`	`10.135`	`-3.352`
`19.000`	`7.076`	`15.128`	`-3.945`
`23.750`	`7.686`	`20.114`	`-4.376`
`28.500`	`8.128`	`25.097`	`-4.680`
`33.250`	`8.413`	`30.079`	`-4.871`
`38.000`	`8.540`	`35.059`	`-4.948`
`42.750`	`8.481`	`40.039`	`-4.910`
`47.500`	`8.218`	`45.018`	`-4.703`
`52.250`	`7.741`	`50.000`	`-4.377`
`57.000`	`7.093`	`54.984`	`-3.961`
`61.750`	`6.314`	`59.971`	`-3.477`
`66.500`	`5.432`	`64.961`	`-2.944`
`71.250`	`4.478`	`69.955`	`-2.378`
`75.047`	`3.700`	`74.953`	`-1.800`
`80.045`	`2.825`	`79.955`	`-1.233`
`85.038`	`2.054`	`84.962`	`-0.708`
`90.027`	`1.303`	`89.973`	`-0.269`
`95.013`	`0.604`	`94.987`	`+0.028`
`100.000`	`0.000`	`100.000`	`0.000`

Like I told you, I would let you know how the (top surface) scab-on wing airfoil rework came out (on my BD-5B).

It lowered my stall speed from 80 mph to 67 mph. The stall is now something like an Ercoupe, kind of "mushing" like. Top end, the same -- no loss there. The warm day takeoff distance was 2,500 ft., and is now about 1,600 ft. I still hold it down until I get about 85 mph, and then lift off. However, I now notice that the acceleration is quicker from 60 to 85 mph than it used to be, probably because the wing is now generating more lift on the takeoff roll than before, thus the takeoff roll is shorter. Time to rotation is now only 16 seconds @ 1,600 ft., with OAT @ 86 degrees, density altitude 6,200 ft. Our airport is at 3,000 ft. ASL.

For the rework, I left the wings on the ship, and cleaned the top surface of the wing of all paint, to the bare aluminum, and scrubbed this with APX. I used 2 lb. density foam, 1/2 inch thick, covered with two layers of glass cloth. I put on the first layer lengthwise, and the second layer 90 degrees to the first, and finished off with two coats of polyester. I then reinforced the leading edge with two layers of fine weave 3 inch glass tape, staggered, which extended back about 2 inches from the leading edge on the lower surface. The total cost of all materials was $105 from Wicks Aircraft, and it took me 44 hours to put it all together. The airplane empty weight was 467 lbs., and is now 471 lbs., so the rework added only 4 lbs. to the empty weight.

My ship is a BD-5BDH, S/N 19, N6782F, which I built myself. It has the standard (non-stretched) body length. The wing span is 21.5 ft. It has a non-turbo Honda engine with a 46 x 66 Heggy prop. It has Matco wheels and disc brakes.

P.S. I also wrote to Rich Perkins and gave him all the info and pictures for the BD-5 Network newsletter.

An Upper Surface Wing Re-Profile for the BD-5