Not much flying was done at the end of the fall semester, because there was a lot of repairs to be done, and school work picked up. So at least that time was put to good use. Aerodynamics, structure, and vibration control was my primary focus, so the '733 was tucked away safely in the closet as I buckled up for my final exams.
After what felt like an eternity, I was back home in Seattle. with my own transmitter bought and shipped there ahead of time, I set off to build a simple and easy trainer plane to show off/recruit more people into the R/C hobby. With all my rambling about RC planes on my phone calls home, I figured my parents needed to see what all the hype was about.
Due to the short amount of time I had at home, I used premade plans from "Flite Test", specifically the 'Old Speedster'. As previously stated, I wanted a trainer for other people to fly, I did not want another lengthy design project. The plane was a slow 3 channel flyer, meaning it I had an elevator to pitch my nose up and down, and a rudder to roll and turn. With large wing area, and a thin , curved airfoil, perfect to fly in the smaller local parks in western Washington.
Between build time, part shipments, and weather delays, I finally got to fly in the last week of break. I showed 3 people the plane, and got the same response each time. Honest laughter. They were shocked that this piece of foam board and hot glue actually took off. Despite my audience being impress by the plane's lazy demeanor, they were hesitant to fly. Now I must acquire a second transmitter so I can set up a "buddy box" system.
On my final day of break, I was flying at a local field when my power plant quit out on me. the motor started screeching and the prop would not rotate. I had no thrust. My mom put down her camera in shock, as my dad tensed up.Mid turn, about 30' off the ground, I was able to smoothly glide it in and perform a textbook landing. It just goes to prove that airplanes are quite stable, and want to be in the air.
It was later discovered that it was the brushless motor, as opposed tot he Electronic Speed Controller (ESC), that quit out. Turns out that I overloaded my motor with an over sized 9" diameter prop (9x4.7 instead of a 8x4.5).
With that tragedy to end my vacation on, I packed up my electric components, and flew back to Virgina Tech.
Grobelny Aerospace Project
Monday, March 3, 2014
Monday, January 27, 2014
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.
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.
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).
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.
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.
No, I was not going in for a landing. |
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 ) |
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.
Friday, November 8, 2013
The Maiden Flight
Still learning how to wire up the plane, let alone assemble it. |
I was truly nervous going into my
first flight. I poured countless hours into building the model, and now it all
could be rendered useless in seconds. I had no idea how it would fly, as the
P-733 was conceived from scratch.
Weather was mostly windless, but
there was a heavy overcast. While I felt at home in the Seattle weather, I
meant it would be hard to track the white plane amongst the white clouds.
No turning back now, as Brent tosses the '733 for her first flight |
Just like the Kitty Hawks first
flight, I opted for a short straight flight, just to prove that I did in fact
have control. I found a flat section grass and we lined up for the launch. With
shaking knees, I timidly power up the motor.
POP!!!
And the engine mounts slides out of
the craft and dangles by the wires. After the laughter settles down, I discover
I forgot to put the pin that holds the motor in. Ops...
So we repeat the set up procedure
for launch, and the tension rises back up. The next thing I know, the '733 is
lifting away from Brent. My heart races as I realize I am in control of the
monstrosity I built. Ailerons: online, elevator: online, rudder: online, all
control surfaces nominal. Everything checked out, so I prepare to land. With
the 'runway' drawing shorter every second, I try to bleed off as much speed as
I can before I set it down. What I thought would be a perfect landing proved
otherwise. As the plane skids to a halt, I see the wings stay level, but the
tail twisted off and was lying upside down.
Despite the crash, I was quite happy that the P-733 flew |
Despite
the crash, I was quite happy that the P-733 flies.
|
When I approach the crash site, the
plane was indeed in 2 pieces. The pink foam of the fuselage was sheared
perfectly in half. I was more than prepared for a crash, so I begin the
repairs.
The metal wires act as rebar |
It appeared that shear was the killer of the fuselage, as the cut was fairly clean and there were no signs of compression from bending. To help transfer the load, I added short metal skewers along the center of the foam. To help keep the fracture from splitting again from tension, 2 panels of the foam poster board was glued on either side.
Much to my advantage, the glue dried quickly, as I was eager to fit in one more flight. The field repairs looked sturdy, so it was back to the skies for. But this time, it was going to be a real flight.
Proof that the '733 actually flies. |
With a hefty toss, the '733 was back
under my control. The first thing I noticed was the lack of power. Weighing in
around 2.5 lbs., with a wingspan of 60", this is a big plane. Now
unfortunately, the power-plant consists of a single 8" prop, a 3 cell
battery and a lack luster brush-less motor. As I try to climb to a safe
altitude I find myself stalling out. I see the nose pull up, but the plane
fails to climb. I was able to remain in control of the craft, but gaining
altitude was a struggle. I was satisfied with the large about of yaw authority
I got from the rudder. Roll control was fine as far as I could tell; the high
wing placement provided enough stability. The elevator on the other hand gave
me some trouble.
When I moved the tail so far back, I
also gained a lot of control authority. I noticed I could easily nose up, and
stall out the craft. Especially with my lack of thrust, this proved to be a
problem when trying to gain altitude. I was able to make 2 passes until
disaster struck.
What you can't hear is everyone yelling "Pull Up!" Belive me when I say I yanked the stick as far back as it goes. |
I was trying to climb to a safe
height when I pulled a little too hard. This is where the above clip starts. I
tried pushing the stick down, but no response, it kept nosing up. With not
enough power to recover, the plane sank. The good news is that with the forward
CG, and the large tail, it nosed down where I could then regain control. Sadly,
I was too low and underpowered that I did not get enough airspeed. What really
shocked me was the lack of damage the P-733 took in the crash.
I thought it was quite peculiar that
I had a sudden loss of control like that. In the blink of an eye, the elevator
bugged out on me. In a later investigation, I found out that I did not push the
servo wire extensions in all the way. The lessons I learn from failures
The good news is there were
practically no repairs necessary when I returned home. After 2 more successful
flights, I really needed to upgrade the powerplant...
The last photo I have of the '733 with the small motor and 8" prop. I added decals to help identify the plane in the clouds. |
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