Solved Sizing a motor

[Inq]

I'm wondering if anyone has any advice, rules-of-thumb or experience with sizing a minimal motor for a sailplane. I'm thinking of something that is hand launched so it doesn't have to overpower wheel drag and I only want it to have the motor powerful enough for a gentle climb with the expectation that I can cut it off and soar for a while. I'm thinking of using 1S micro receivers with built-in linear servos and brushed ESC. Something like...
- https://www.ebay.com/itm/324764860333

I'd like to think that powering a motor as small as...
- https://www.amazon.com/dp/B087C9LTCR/?tag=lstir-20

...with a 350 mAh battery might be possible???

I'm planning on something on the order of AUW of 250 mg. Do you think this is even remotely possible?

 

[Merv]

...I'm planning on something on the order of AUW of 250 mg. Do you think this is even remotely possible?

That setup is for an indoor plane, think closer to 50 to 100 g.

 

[flyboa]

Looks like that receiver has an option to run a brushless motor. You could then use a 10g or 16g brushless motor that would power a 250g glider.

I have done this mod in the past.

 

[Inq]

Looks like that receiver has an option to run a brushless motor. You could then use a 10g or 16g brushless motor that would power a 250g glider.

Yeah, I noticed that. They also have a few models in that line that have the brushless ESC on the board as well. Is there some rule of thumb for how much trust to weight ratio I might need? When I asked this same question for my trainer, I was surprised that the recommendations came back from the forum over 1:1 for a trainer even though full size plane like a Cessna is down in the 0.25:1 range.

For a sailplane and merely to get it up to altitude in a lazy way (I don't need it to climb out at 45°) what kind of thrust to weight should I be looking for?

 

[flyboa]

I would stick with 1:1 then if you have power to spare make a bigger plane.

 

[flyboa]

A simpler option would be to make a 2ch pure glider and not bother with a motor. If you find it hard to launch then try a micro Hi-start.

 

[Inq]

I would stick with 1:1 then if you have power to spare make a bigger plane.

Actually I'm already shooting for a 10' wingspan.

I'd have to get new batteries and motor for the experiment. And it's just an thought/paper experiment at the moment. I have either A2212 or these little indoor motors and nothing in-between. With the A2212 gear, there is 250 grams just for the gear (no plane at all). With the micro equipment, the gear alone is only 19 grams.

I'm also basing the wings/spars on only carting the micro equipment. As soon as the wings have to carry a body with 250 grams, the wings and spars have to get beafed up and the design spirals up to a kilogram.

IOW, I'm still looking for someone with the minimum thrust to weight ratio that they've used in a soaring type plane.

 

[Merv]

...what kind of thrust to weight should I be looking for?

The amount of thrust needed depends on how you want to fly.
A slow flying trainer, the thrust will need to be 60-80% of the weight of the plane.
A typical plane, 80-100%. A war bird or one with a little zip will be 100-150%. A 3D plane will need 150-200% thrust to plane weight.

Here is a FT thread with more information.
https://forum.flitetest.com/index.php?threads/thrust-vs-power-to-weight-ratios-for-prop-planes.5102/

Watts per pound is another useful guideline. You'll need to weigh the plane and estimate the weight of the motor and battery.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

 

[flyboa]

I made a 10' motor glider with a 2730 motor and it was a hoot. Just too big to store in the house.

 

[Inq]

The amount of thrust needed depends on how you want to fly.
A slow flying trainer, the thrust will need to be 60-80% of the weight of the plane.
A typical plane, 80-100%. A war bird or one with a little zip will be 100-150%. A 3D plane will need 150-200% thrust to plane weight.

Here is a FT thread with more information.
https://forum.flitetest.com/index.php?threads/thrust-vs-power-to-weight-ratios-for-prop-planes.5102/

Watts per pound is another useful guideline. You'll need to weigh the plane and estimate the weight of the motor and battery.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

This does not need power for maneuvering or stunts or 3D. It is simply to gain height while going strait. So even the 50-70 category is assuming for maneuvering speed. I'll have to measure the wattage on this micro motor. I want to say its around 20 watts and if I get the plane down to 250 mg ==> that gives me 36 watts/pound.

 

[Inq]

I made a 10' motor glider with a 2730 motor and it was a hoot. Just too big to store in the house.

Good gosh! I bet that climbed like a rocket with that big a motor. I plan to make the wing break at the center so they can fit inside on the truck back seat. I definitely have the house storage problem. Note to self - make sure it can go up through the attic trap door.

 

[flyboa]

Good gosh! I bet that climbed like a rocket with that big a motor. I plan to make the wing break at the center so they can fit inside on the truck back seat. I definitely have the house storage problem. Note to self - make sure it can go up through the attic trap door.

It was kind of a pig with a fairly high lift wing so I used a small 3s battery.