Upgradeing the Powerplant

A week later, I took the P-733 out for another flight, but this time I was armed with a bit more confidence. I was told countless time to fly several mistakes high, and how altitude and airspeed are your fiend, but I kept having flashbacks to the uncontrolled climb the plane took on it's second flight. The 733 had quite a problem of stalling out, on top of almost no 'excess thrust'. This meant that I had to run at 80% throttle or else I will fall out of the sky, and my airspeed was too low to pull any complex maneuver. Unfortunately we are constricted to a small field, which means we are constantly turning giving me no opportunity to climb.

No, I was not going in for a landing.

As seen in the few photos I have of the early flights, it's always hugging the ground. There was absolutely no room for error. And as a novice pilot, I've had more than my fair share of crashes. Many of which were digging the massive wings into the ground and cart-wheeling the plane across the grass. While the wings were strong, It kept ripping the paper/tape along the top, holding the wings together. I decided to repair the repeated failure with a wooden dowel as another spar. and since I had no good access to the inside of the wing, I simply glued it to the bottom of the wing.

It was too late when I realized that my 'brilliant solution' killed my aerodynamics. I essentially put a giant damn in front of a very important stream of air. We hypothesized that the bar would not affect lift nearly as it would affect drag, since the airflow on top of the wing tends to play a bigger role in lift. With the help of my friend, Brent, we used computer programs to analyze my airfoil.

Fig.1:       Lift vs Angle of Attack    (cl vs. alpha)

In the following graphs, we compared the original airfoil (blue), with the addition of the spar (green), and if we aerodynamically covered the spar with a sheet of paper (yellow). 

Fig. 2       Glide-slope vs. Angle of Attack    ( cl/cd vs. alpha )

The results came as a surprise to us. Despite proving our initial predictions correct, the drag from the .125" square dowel was much greater than expected. Figure 1 shows the lift generated with respect to the angle of attack, and it clearly shows there was a negligible change. Note the plateau around 7 degrees, this is where the wing stalls. For comparison, most wings stall around 15 degrees.
In figure 2, we plotted the lift over drag versus the angle of attack. Lift over drag (L/D or cl/cd) can help us evaluate the  performance of the aircraft, including the maximum range and glide-slope. Naturally, increasing the L/D increases the range and gives you a flatter glide-slope.
What concerned me was the large increase in drag figure #2 was indicating from the addition of the wooden beam. I was losing 30% of my glide slope. Fortunately, the yellow line indicates that I can recover most of my cl/cd by covering the bar with an aerodynamically shaped piece of paper.

After noticing the high amounts of drag, I realized that my power plant was severely insufficient. While I could hold steady flight, there was no thrust available when I needed extra lift for a maneuver. Luckily we found a much larger brushless motor, and upgraded the battery and prop from a 3 to 4 cell and 8" to a 12" diam. prop respectively.

Comparison of the old components on top of the new.

As can be seen in the comparison, the overhaul also bought more mass. Generating enough lift was no problem, rather the new center of gravity was. If the battery was placed in the far back of the compartment, the balance was tolerable but far from optimal.

Other than needing a little bit of trim, the plane flew great. It what on that test flight, my worries finally melted away, and I learned to just have fun. The plane responded easily to any input give, and it could pull out of any mistake. I felt this plane was finally deemed flyable.