William Walker
New Member
- Joined
- Oct 11, 2021
- Messages
- 1
Hi all, my name is William Walker,
I am developing a new type of VTOL flying car. I have just built a full scale unmanned prototype and we have begun to test it. It weighs 150 lbs + batteries, which is about 20% more than we expected. I originally designed the vehicle to have a thrust to weight ratio of 2. The vehicle uses four 10kW motors with 32in x 6.5 in props which generate 64 lbs thrust each, 2 in the front and 2 in the back that are mounted coaxially thrusting through a hole in the wing. We expected 80 lbs thrust out of each motor, so it is 20% less than we expected. Here is a link to see pictures of the vehicle. The cockpit is not mounted yet:
Full Scale Vehicle Final Contruction - Google Drive
I have built a 1/6 scale vehicle that flies very well:
https://drive.google.com/file/d/1q6iPsVydYqC_2YI6IAgRftKsNPcW9C_0/view?usp=sharing
Here is a link to see a flight simulation of the vehicle flying:
https://drive.google.com/file/d/1AAvC3BLvut7xjlhY81S9EqD8OW0Qw-kS/view?usp=sharing
The vehicle can lift the front end with 2 batteries in the vehicle. Unfortunately the motors can not lift the back end, unless we manually lift the back end first. We think this is because, when on the ground, the 6 ft x 2 ft opening in the back under the wing is not big enough to vent the thrust and the pressure is building up. Because one of the props is mounted in the hole, we think the pressure is causing this motor to fail, which then affects the top motor, which is mounted on top of the hole. Pictures attached.
When we place 24 cm high crates under each wheel, motors are able to lift the back end by them selves. We could just cut the bottom of the vehicles side panels so they allow the thrust to vent better. But this is not a solution, since the situation will get worse when we build the manned vehicle, which will be heavier and will require even more thrust. Instead I think that the prop motors need to be placed on top of the hole, so that the motors develop their full thrust before it is directed into the hole. This is how the 1/6 scale model is configured and it works very well. The pressure under the wing from the thrust will then just help lift the vehicle.
I did a simple test with a small hand held coaxial motor setup attached to a stick. As I lowered the prop motor setup into a hole in a box without a bottom (placed on a table top), I noticed the setup getting lighter as it approached the hole and getting heavier when the bottom motor is in the hole. When I lifted the box a little off the table, I observe the same effect but with less changes in weight. If I lifted the box completely in the air, I noticed no change in weight when I repeated the experiment. These results led me to the above mentioned conclusions.
I have been doing a lot of reading and it looks like the coaxial motor loss will be about 25%. If this is true, then this explains why the back end of the vehicle is not lifting. According to our calculations, even if we remount the motors on top of the hole this will not be enough thrust. We could connect the two motors mechanically together, but it would still not generate a enough thrust to be able to maneuver. So we have decided to order another more powerful 40KW back motor with a 3 bade prop with higher pitch 32in x 12in. It is coming from China and will be here in a month. Hopefully this will solve the problem.
What do you think? Do you agree with my thinking about the back motor situation?
Your input would be appreciated, Thank you.
I am developing a new type of VTOL flying car. I have just built a full scale unmanned prototype and we have begun to test it. It weighs 150 lbs + batteries, which is about 20% more than we expected. I originally designed the vehicle to have a thrust to weight ratio of 2. The vehicle uses four 10kW motors with 32in x 6.5 in props which generate 64 lbs thrust each, 2 in the front and 2 in the back that are mounted coaxially thrusting through a hole in the wing. We expected 80 lbs thrust out of each motor, so it is 20% less than we expected. Here is a link to see pictures of the vehicle. The cockpit is not mounted yet:
Full Scale Vehicle Final Contruction - Google Drive
I have built a 1/6 scale vehicle that flies very well:
https://drive.google.com/file/d/1q6iPsVydYqC_2YI6IAgRftKsNPcW9C_0/view?usp=sharing
Here is a link to see a flight simulation of the vehicle flying:
https://drive.google.com/file/d/1AAvC3BLvut7xjlhY81S9EqD8OW0Qw-kS/view?usp=sharing
The vehicle can lift the front end with 2 batteries in the vehicle. Unfortunately the motors can not lift the back end, unless we manually lift the back end first. We think this is because, when on the ground, the 6 ft x 2 ft opening in the back under the wing is not big enough to vent the thrust and the pressure is building up. Because one of the props is mounted in the hole, we think the pressure is causing this motor to fail, which then affects the top motor, which is mounted on top of the hole. Pictures attached.
When we place 24 cm high crates under each wheel, motors are able to lift the back end by them selves. We could just cut the bottom of the vehicles side panels so they allow the thrust to vent better. But this is not a solution, since the situation will get worse when we build the manned vehicle, which will be heavier and will require even more thrust. Instead I think that the prop motors need to be placed on top of the hole, so that the motors develop their full thrust before it is directed into the hole. This is how the 1/6 scale model is configured and it works very well. The pressure under the wing from the thrust will then just help lift the vehicle.
I did a simple test with a small hand held coaxial motor setup attached to a stick. As I lowered the prop motor setup into a hole in a box without a bottom (placed on a table top), I noticed the setup getting lighter as it approached the hole and getting heavier when the bottom motor is in the hole. When I lifted the box a little off the table, I observe the same effect but with less changes in weight. If I lifted the box completely in the air, I noticed no change in weight when I repeated the experiment. These results led me to the above mentioned conclusions.
I have been doing a lot of reading and it looks like the coaxial motor loss will be about 25%. If this is true, then this explains why the back end of the vehicle is not lifting. According to our calculations, even if we remount the motors on top of the hole this will not be enough thrust. We could connect the two motors mechanically together, but it would still not generate a enough thrust to be able to maneuver. So we have decided to order another more powerful 40KW back motor with a 3 bade prop with higher pitch 32in x 12in. It is coming from China and will be here in a month. Hopefully this will solve the problem.
What do you think? Do you agree with my thinking about the back motor situation?
Your input would be appreciated, Thank you.
