I am Charles Linquist, a semi-retired (previous) CTO of a tech company in Silicon Valley. I wanted time to explore my hobbies and do some other, personal things that I have neglected over the years. Working part-time gives me that opportunity.
I wanted a challenge, something that would utilize my skills as a mechanical designer, an Electrical Engineer, a firmware writer, and a guy interested in general science
I have 4 quadcopters, a hexacopter and a 3D printer, so something using those seemed like a good start. Quadcopters use all the disciplines mentioned above AND can benefit from the use of 3D printed parts. I had an idea.
People have set records with the length of time a quadcopter could hover. I believe the current record is almost 90 minutes. Those times were set with stripped-down frames, very large propellers and slow motors. This long hover time took a Herculean effort in weight reduction. Before you get off on the wrong track - adding batteries is not the answer. Yes, more batteries increases the power available, but it also increases the weight. At some point, it is a losing proposition - or very, very close to one. Weight is the enemy. NASA realized this very early on. Putting one extra pound into orbit takes 9 pounds of extra fuel. Now you know why rockets are so large!
But rather than set a record for the longest hover time, I wanted to do something different - I wanted to see how FAR I could fly a quad. That seems like a simple thing: If you are in the air for 90 minutes, and you can go 30 miles/hour then you could go 45 miles. But wait! Going "sideways" is much more than simply hovering. The entire craft has to tilt in order to provide a sideways thrust vector, and when it tilts, the downward thrust vector (= lift) decreases. This means that the motors must spin faster just to hold altitude. And the more you tilt the craft to go faster, the more lift you lose. Since a 'quad' normally hovers at about half throttle, when it is tilted at a 45 degree angle, it has lost half its lift and must run full throttle just to stay in the air.
The sideways speed is determined by the tilt angle, the RPM of the props and their pitch, since it has to "screw" its way forward.
Quadcopter propellers are classified by length (generally 6-16") and pitch (generally 4.5-6"). The length is self-explanatory. The pitch is the distance the propeller will "screw" through the air with each revolution, and is basically the same idea as the pitch on a screw.
It is generally known that a large propeller, turning slowly is more efficient at producing lift than a small propeller turning fast. To prove this "old flyer's tale" I turned to ecalc.
This is a very helpful tool that seems to be quite accurate. I recommend that - if you use this tool, you pay the $5 fee and get full access. The extra features that are enabled with the "full version" are worth it.
After some research I found that a very good combination would be the SunnySky V3508 motors (the number refers to the diameter - 35mm and the height - 8mm). and a 16 X 5.5" pitch propeller. The motor has a KV rating of 380, which means that every volt fed to the motor will give an unloaded 380 RPM. These motors are expensive - $58 each, which is about 2.5 times that of most quadcopter motors. But the SunnySky motors are known for their good bearings and high efficiency.
Using eCalc, I found that if I used this motor, I could hover for some amazing times - 40-50 minutes if my airframe was light enough. But there was a slight problem: Slow turning propellers - while they may do wonders for hovering, simply do not go fast sideways. Their slow speed doesn't screw through the air fast, so even if you tilt the craft 45 degrees, you move slowly. Even if I could stay in the air for 20 minutes at full throttle, I could only go about 13 MPH, which would allow me to travel a paltry 4.3 miles. My goal is 18 miles!
I have a plan to eventually reach my goal, and I will describe that in future posts. In the meantime I realized my largest home-built quad could handle 16" propellers, so I mounted the SunnySky motors and the big props this morning. I figured that this craft would be a good test bed to get started. The picture of this test bed is below.
It definitely flies, but I have to change some of the controller parameters, since the props are so large they have a lot of inertia.
