McMullan Aircraft Design

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JayBird Design Page

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Wing and Tail Sizing

I have already decided that I want to this airplane to hold two full sized adults and I also want it to have adequate baggage space. The JayBird is a conventional aircraft. I am not trying to re-design the wheel so I can learn a lot of things from other designers. By taking a look at other two-place airplanes I can come to a general idea of what the approximate wingspan on my airplane should be. There are two things that I must pay particular attention to; wing loading and aspect ratio. A lower wing loading will give increased takeoff and landing performance and it will give us a lower stall speed. A higher wing loading will give us just the opposite, decreased takeoff and landing performance with a higher stall speed. From doing some research, I want to keep my wing loading around 15 pounds per square foot. I estimate that my finished aircraft, fully loaded, will weigh between 1500 and 1800 pounds. The wing I have designed for the JayBird has a wingspan of 28 feet. The wing has an area of 111 sq ft. With a weight of 1500 pounds I will have a wing loading of 13.5 pounds per sq. ft. A weight of 1800 pounds will give me a wing loading of 16.2 pounds per sq. ft. Using the spreadsheet supplied with "Preliminary Design - Modern Aircraft Design for the Non-Engineer" makes this all very simple.

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Software provided with "Preliminary Design - Modern Aircraft Design for the Non-Engineer" by
Jay S. McMullan

The wing I have designed also gives me an aspect ratio of just a little over 7. Takeoff, climb, range and landing performance are all enhanced by a high aspect ratio wing. There are two ways you can determine the aspect ratio of a wing. For a rectangular wing with a constant chord divide the wingspan by the wing chord. For tapered and eliptical wings it is a little more difficult. In this case, divide the wing span squared by the wing area. For example. If a wing is 36 feet long and the average chord is 6 feet, you would divide the wingspan- 36 - by the average chord - 6 - giving you an aspect ratio of 6. You could also square the wingspan - 36 x 36 = 1296 - divided by the wing area - 216 - = the same aspect ratio or 6. You will notice that many of the high performance airplanes such as the Lancair IV have high aspect ratio wings. By using the software included with my book, you can quickly and easily make changes when designing your airplane. Now that we have the wing planform we can go ahead and determine the size of the flaps and ailerons.

Any modern aircraft should have flaps. The flaps will increase the lift coefficient and drag of the wing, allowing an airplane to have a lower stall speed making landings safer. There are different types of flaps that can be incorporated into the wing but for simplicity's sake, I will use a "plain" flap. The area of both of my flaps equals just over 1,741 square inches or 12.1 square feet. This gives me a flap to wing ratio of 9.2. The chord of my flap is 13.75 inches which is about 25% of the root chord of my wing and about 30%; of the chord where the flap ends. I have chosen to use the NASA NLF(1)-0215F airfoil and the designer of this airfoil, Dan Somers, recommends the flap chord to be 25% of the wing chord. Wind tunnel tests have determined the maximum lift coefficient of this airfoil to be 1.8. With flaps, the maximum lift coefficient increases to 2.2. These are useful numbers that are essential for calculating performance estimates. For more information on airfoils please refer to "Theory of Wing Sections, Including a Summary of Airfoil Data" by Ira H. Abbott and Albert E. Von Doenhoff.

Extending the ailerons past the flaps, still using the same chord makes the area of the ailerons to be just over 1,482 square inches or 10.3 square feet.

Using all of this information and a rough sketch of the airplane we can now go about calculating the volume coefficients of the tail surfaces.

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