Thread: Build Log 48" ufo
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Old Dec 06, 2011, 11:22 PM
GooberSB is offline
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United States, VA, Fluvanna
Joined Jan 2011
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Your comments are well taken. The motor I'm using is this one from HK

I've read prop data for this motor and it will output about 1650g of thrust with a 60A ESC and 11x5.5 prop. I went down to 11x4.7 for thrust over speed and to lower the amperage. As you surmise, the motor was a bit warm (not too hot to touch though) and my first impression was I need a smaller prop! I suspect a 10x4.7 would take some of the load off but at the cost of thrust.

As you see in the video, even with the 11" prop it could fly level into the wind just fine (and gain speed while doing so) but did you notice what happened when the wind got under it and started pushing it to a vertical position? I could not power out of it and go vertical. To recover, I basically did a stall turn and levelled out travelling with the wind until I gained enough airspeed to turn around and come back into the wind again.

I estimate the UFO to weigh about 2000g and I suspect the thrust with that motor and prop was about 1000g-1200g. Enough to move it on the horizontal axis but not the vertical axis.

What would I do different? I would use larger servos with metal gear, a motor that can output 2000g of thrust without getting hot, and a 4000mah or so battery. That is, if using the same airframe.

If I ever do it again, I'll drop to a 40" diamater. Making the "lever" down the thrust line shorter will mean you don't have to stiffen it as much as there won't be as much lever effect. Changing the length of a lever just a little has a large effect on it's leverage. Shorten it by 8" inches on each end and I may not even need a spar.

Taking 8" off all the way around will remove a fair amount of foam. Even though it's light, less of it means less drag and a less powerful motor is needed to move it along. If you take off less than 8" the result wont be as dramatic and, in the end, neither will the outcome.

Also, with this design, 50% of the weight must be in the first 25% of the airframe if everything is distributed pretty evenly. So I had to add an extra 500g of dead weight to get it to balance. If it was 40" or less, the total weight would have been less. The 400g of electronics might have been sufficient to balance it since the total AUW would be several hundred grams less. If not, it would probably only require an addional 100-150g of weight, which could easily be met in the form of a larger battery. See what I mean?

There was no way I could come up with an extra 500g of usable weight that would work with the power system I used.

It would be much easier to make weight if it was 40" or less. Again, I'm talking about what I should have done to make the best aircraft for the motor, prop, and battery I used.

If I had a bigger motor and battery that a) weighed 300g-500g more than what I'm using and b) provided about 2000g of thrust this aircraft would be great at 48". I don't have that, but if I did ...

Thanks again for your interest,

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