{names}EM215 Section 1November 23, 2014

Lab 2a and 2b: Lifting Surfaces – Wings!

Objectives

The aim of this lab was to research an airfoil, create and test a prototype of the airfoil, and design and test the performance of the airfoil in the form of a flyer similar to that of the Wright Brothers.

Background

The Wright Brothers’ journey started with Orville’s letter to another early 20th century researcher, Octave Chanute. He starts off with, “For some years I have been afflicted with the belief that flight is possible to man.”1 The Wright Brothers were the first aeronautical researchers to create procedurally sound research and testing of the popular ideas on unpowered manned flight.

To optimize wing design, also called the ‘air-foil’, the Wright Brothers created the first wind tunnel in which they tested the drag and lift of small-scale prototype air foils. During these tests, they analyzed different combinations of the major aspects involved in an air foil such as chord and span (the width and length of the wing, respectively). The Wright Brothers also tested different cambers (the distance from the top of the wing to the chord line). Cambered surfaces are used to increase the maximum lift coefficient and the stall angle, and minimize the stalling speed.

Ultimately their tests revealed that the optimum combination for a low speed stable flyer is a longer and thinner wing with a moderate camber. Figure 1 shows the high ratio of lift force to drag force that their design produced.

The raw data points collected and analyzed for this experiment are described in Figure 2.

1

Figure 1.4

2

These raw measurements allow for the calculation of the overall performance of an airfoil which is considered in terms of the coefficient of lift

CL=2 L

ρA v2 (1)

and the coefficient of drag, where D is the drag force (lb).

CD= 2 DρA v2 (2)

Another key consideration is the relationship between angle of attack and the coefficient of lift. As defined by NASA, “The angle between the chord line and the flight direction is called the angle of attack and has a large effect on the lift generated by a wing.”3 As the angle of attack is increased, the lift also increases until it reaches the stalling point in which the air flow around the wing becomes turbulent and the wing ineffective. Figure 3 demonstrates the forces acting on an airfoil included with the angle of attack.

Figure 3.4

2

3

4

Figure 2.4

The airfoil design used in this report was pulled from an array of designs based on previously established aspect calculations from the online Airfoil Investigation Database.

Figure 4.5

Procedures

The project was conducted with four people. Each member of the team was independently tasked with researching an airfoil design for low speed flight. Factors that were considered for stable low speed flight include the camber and the maximum coefficient of lift including the coefficient of lift vs. angle of attack and the coefficient of lift vs. drag force. A plethora of airfoils were found at the online Airfoil Investigation Database which included those factors listed above as well as additional data such as stall angle, thickness, and trailing edge angle.

The airfoil data was extracted from the database into Excel and imported into SolidWorks where the airfoil design was reproduced. One airfoil was selected by group decision to be submitted and to be used for the remainder of the lab. Lab technicians created an initial prototype from this design with a 4-inch chord which was tested in the wind tunnel. Wind tunnel testing was completed to collect data between the angle of attack, the velocity of the wind over the foil, and the normal and axial forces. This data was used in Matlab operations to plot the coefficients of lift vs. angle of attack and the coefficient of drag vs. angle of attack. Matlab operations were also used to predict the total lift force at 30 mph wind speed, the net lift force at 30 mph, and the wind speed at which the flyer will have zero net lift. These were used to predict the lift force of the full size wood flyer at four different air speeds at a given angle of attack.

A second foil with an 8 inch chord and lightening holes was then also sent to the technicians to be manufactured out of wood. A final flyer was then constructed and tested in the wind tunnel.

5

GOE 482 Air Foil

Results, analysis, and discussion

The other designs were as follows:

However, these designs had lower lift coefficients.

Lightening holes and spar spaces were then designed into the chosen airfoil design.

As explained in the Procedures, data from the Airfoil Investigation Database were extracted in .DAT files. These .DAT files were manipulated with Excel and Notepad which were then used to insert the data points into SolidWorks. SolidWorks then generated the curves which are the different airfoils seen in Figure 4 and Figures 5-7. These curves were then scaled by a factor of four for their possible construction as foam foils. Figure 4 was decided upon because it had the highest coefficient of lift and was subsequently made by technicians into a foam foil. This foam file was used in wind tunnel testing where data was collected and then used in Matlab to predict lift forces. The other designs, Figures 5-7 were discarded at this point. Eventually, the scaled foil was scaled again by a factor of two resulting in its overall scale factor of 8. Lightening holes and spar spaces were then added to this airfoil, Figure 8, which was made into wooden ribs by technicians. These ribs were used in the construction of the flyer.

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9 shows how the coefficient of lift changes as the angle of attack increases. Generally coefficients of lift versus angle of attack plots are concave down. The max value of the parabola corresponds to the angle of attack right before the wing will stall. The y-value at this point is the maximum coefficient of lift the flyer can obtain. With this particular wing (‘Student Flyer’) began to stall around 10 degrees but then stabilized while the coefficient of lift continued increasing. The stall point for this wing could not be determined due to the maximum angle of attack the wind tunnel can simulate.

Figure 9 also overlays a plot of the Eiffel 10 – 1903 angle of attack versus coefficient of lift for comparison purposes. The Student Flyer greatly exceeded the Eiffel 10’s maximum coefficient of lift and angle of attack. For the Eiffel 10, the maximum coefficient of lift was 0.76 at a 10 degree angle of attack. The Student Flyer had a .76 coefficient of lift at only a 3 degree angle of attack, with a maximum coefficient of lift of 1.94 at a 19.9 degree angle of attack.

Figure 9

Figure 10 gives the predicted lift force (in pounds) across the range of angles of attack based on varying wind speeds. The fixed line shows where the predicted wind speed at which the flyer will have zero net lift force. The net lift force is calculated by subtracting the total lift force by the force required to lift the Student Flyer (1.115 lb force). The figure shows that the net lift force occurs at 6.5 mph. The actual Student Flyer was flown in the wind tunnel at a speed of 11 mph. This figure shows that at that speed, the predicted lift force is around 3.2 pounds of lift force.

Figure 11

Figure 10

Figure 11 plots the Coefficient of Drag vs. Angle of Attack. It is noted that the coefficients of drag changed linearly with the angle of attack. As the angle of attack of the foam flyer increases so does the coefficient of drag and ultimately the drag force. This is because as the angle of attack increases, the flyer exposes more of its surface area to the wind stream which acts as drag. It makes sense that for the data points before the stalling point, that the coefficient of lifts for the same angle of attack would be greater than the coefficients of drag. Lift must be greater than drag for the flyer to fly and indeed as proved in the lab the flyer did fly.

Figure 12 is the finished product flyer. The frame of the flyer is made out of wood and the foils are made of fabric. The airfoils were drawn up in SolidWorks and were then saved as a DXF file which the machine shop used to create the foil ribs. The foil ribs contained spots for spars, both on the leading edge and trailing edge as well as on the top and bottom of the foil which were used then to connect the ribs together. Wood glue was used to adhere the wood. Next fabric was cut and placed over the ribs to give the flyer a full surface for which air to contact it. The foils, both top and bottom, were connected by hooks and wooden dowels. Between the dowels centered in the aircraft, fishing line was attached to act as control lines to give tension to the supports for a stable flyer structure. Subsequently, a tail was added to the center dowels with both string and hot glue to give the flyer added stability and lift.

Figure 12

Figure 13 shows the finished wooden flyer gliding in the wind stream behind the low speed air tunnel in Rickover Basement. The data from the flyer reflected that even at high angles of attacks (15-25 degrees) this is the range in which it generated the most lift. Resolving the geometry from the image inserted above, at this moment, the flyer was flying with an 18 degree angle of attack.

Zooming in on the the left foil of the flier the geometry for the angle of attack was able to be resolved. Using a protractor and lining up the top of the blue shelf in the picture, which is a level surface, with the base of the protractor, the angle up to the left foil could be measured with the protractor which was indicated at 18 degrees. It is noted that the way the picture was taken the left foil appears as the lower foil in the picture.

In the last day of the lab, the finished wooden flyer was tested in the Rickover basement low-speed wind tunnel to see if the final design would indeed fly. Wire was attached via a hook through the eyelet of the anterior-distal dowels (the dowels closest to the leading edge at the ends of the flyer) at both the top and bottom. Students then climbed a ladder and tested the flight of the flyer in the wind stream produced by the tunnel. The use of the wires as indicated allowed the students to slack the line and allow the flyer to edge back into the wind stream beyond the conduit pipe on the ceiling proving the design was flight competent.

Initially, the lab called for using a force dynamometer to measure the actual lift force in this wind tunnel test. This measurement would allow for comparison between the predicted values in the Matlab code of lift force vs. angle of attack with the experimental

Figure 13

value measured in the lab. The angle of attack (18 degrees) is found from the geometric resolution of the angle in the picture taken. From the Matlab plot, the associated lift force with this 18 degree angle of attack is found to be 2.75 lbs. of force. Since the weight of the flyer is 1.15 lbs. and the lift force in the tunnel experiment is 2.75 lbs., this explains why the flyer flew so well even when the wires were let out and the flyer still flew fifteen feet from the front of the tunnel.

Although the dynamometer was not used, the flyer was tested in its lift force in a more qualitative manner. With two students on the wires for either side of the flyer, another student held the flyer above his head behind the wind tunnel. Suddenly, the student let the flyer go and it immediately caught the wind and since the lift force was greater than the flyer’s weight it ascended and stabilized in the wind stream.

Conclusions

The end result of this lab was the comprehensive designing, constructing, and testing of a Wright Brother’s style flyer. From the experimental data recorded, Lift Coefficient and Angle of Attack have been found to increase together by a third order relationship until the stalling point, similar to the relationship between Total Lift Force and Angle of Attack. The flyer presented here had a maximum coefficient of lift of 1.94 at 19.9 degrees which is 2.5 times that of the Eiffel 1903 that had a maximum Coefficient of Lift of 0.76 at 10 degrees. In the final wind tunnel tests of the flyer, an average angle of attack was determined to be 18 degrees by geometric analysis of the picture. At this angle of attack, there were 2.75 lbs. of lift force and a Coefficient of Lift of 1.62. Apart from the technical processes and subject matter now familiar to the team, an appreciation for the work of the Wright Brothers and their systematic method of scientific inquiry also has transpired. The Wright Brothers were the first in flight, paving the way for air transportation, air warfare, space travel, and bachelor’s level aviation experiments by engineering students.

References

Anon. Dayton, May 13, 1900. Dayton, May 13, 1900 [Internet]. [cited 2015 Nov 22]. Available from: http://invention.psychology.msstate.edu/inventors/i/wrights/library/chanute_wright_correspond/1900/may13-1900.html

Anon. Inclination Effects on Lift. Inclination Effects on Lift [Internet]. [cited 2015 Nov 22]. Available from: https://www.grc.nasa.gov/www/k-12/airplane/incline.html

GOE 482 Airfoil. Airfoil Investigation Database. Web. 13 Nov. 2015.

Ratcliffe C. Professor Ratcliffe's Data Analysis. PDF.

Ratcliffe C. Professor Ratcliffe's Lab 2 - A Set of Lifting Surfaces or Wings. PPTX.