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Dec 06, 2011, 09:41 PM
An Ordinary User
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Build Log

48" ufo

Hello! Greetings!

I finally got around to finishing my 48" UFO build. It took longer that I thought because I had a few setbacks, but first let me describe the project.

After building 2 successful UFOs and looking at lots of plans for Mantas, bi-planes with disc wings, and other oddly shaped aircraft that could be reduced to a flying disc at the most basic level I decided I wanted to try something large.

I knew the wing loading could be low - maybe 3oz / sq ft - but how low was to be seen. My first step was to buy the sheet of 8 ft x 4 ft 1/2" Dow bluecor insulation at Lowes for $12. The plan was to make this a KFm2 with the step on top being a semi-circle of 48" diameter.

I cut the full 48" dia circle and the semi-circle quickly. After fitting them and doing some tests I realized that the weight of the back half caused it to flex and bow and bend. Like a cattail on the end of a reed has a lot of leverage when the wind blows.

Then I realized I needed a strong spar of some sort to hold the back half of the craft straight and level with the front half. I used a 3 ft 1/4" wooden dowel from Michaels for this. Melted a trench into the center panel all the way along the center thrust line and glued the dowel in with hot glue and gorilla glue. Didn't have to be neat, just straight down the center. It would be covered later with the fuselage.

After glueing the spar down the center it was time to cover the front half with the 1/2" KFm step. Used 3M Super 77 adhesive for this (as always) and got it lined up pretty nice. Don't worry too much, will trim and sand to perfection later.

Once I had the top step in place I test it for strength. Pretty good, no flex. BUT, it's not level across. The Dow foam baord had a little bit of bow in it. The best way t straighten it out at this point is to turn it over and glue another layer on the bottom so it can't bow. So I did. The bottom layer is 6mm FFF foam, not 1/2", because it's thinner and lighter. This thing already weighs about 1000g with no electronics!

Now it has turned into a KFm4 airfoil. No problem, though. I trim the layers evenly and then use a sanding block to round the leading edge 180 degrees around the circle. Didn't do the back half.

Did the electronics next. The heaviest motor I have is about 100g. The heaviest battery is a 2200mah 3S. I put them all up front and did a balance test. It was crazy tail heavy. Looking around the house for something heavy I found a bunch of bananas. I tried several bananas and found that 3 bananas + the electronics up front balanced it nicely. The bananas weighed close to 500g.

After cutting the palce for the electronics and getting the motor mounted, I moved down to the servos and elevons. I had to custom make some control rod guides for the large control rods and the deep foam. I ended up using some servo control arms drilled out glued vertically into the foam. This did a great job preventing the control rods from bowing.

There was another problem, however. The weight of the large elevons was too much for the first pair of servos. One stripped out farily quickly becuase when I would lift the UFO off the ground the elevons would drop and pull the servo arms. One too many times I guess.

I installed some larger servos and was more careful about lifting it after that. The elevons still drop and I can hear the servos moving but they have held up so far. Also, I glued some plywood pieces under the servo tabs and screwed the servos down instead of glue. This has eliminated the servo "wobble" that I noticed when they were glued into the foam pockets. These servos are mounted firmly and elevon movement is more precise (no slop).

The easiest part of the whole build was cutting the fuselage and glueing it on the craft. No problem at all with that...

After everything was installed and the UFO was ready for a maiden flight, I still had to balance it at 25% (1 ft) from the nose. My 3 bananas were black and soft so I had to find something else heavy and compact. Aha! I had some D-Cell batteries in a drawer. 4 of them placed in the battery tray and taped securely to the top balanced it out.

I would have preferred a heavier motor and maybe a 4000mah battery but I used what I had and made it work. Anyway, I ended up using a Turnigy EDF motor rated for 300W @ 60A. I only had a 40A ESC so I propped down to about 30A using my watt meter.

With the Turnigy motor, I used a 2200mah 3S lipo, 40A ESC, 11x4.7 APC style prop, and 2 HXT900 servos.

Here are some pictures and a short video. The hat cam stops recording a lot and I don't know why it does that. Sometimes I don't get any usable video at all from this thing.

48" UFO (2 min 51 sec)
Last edited by GooberSB; Dec 06, 2011 at 11:32 PM.
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Dec 06, 2011, 11:33 PM
Watt Waster
Tsavah's Avatar

Lik'em Big!

If your battery was a little too warm, that would suggest the amp draw was a bit on the high side. Since the motor was hot I would think the propeller was a bit much for the motor. I would try the same length of propeller, but down one on the pitch. If it still runs hot, I would try the next size shorter with the lower pitch. At some point the motor should run warm (acceptable) and the battery pack just barely warm. That is a good indication of a good propeller, motor, and battery match. The ESC should be only slightly warm also, or it may be necessary to go up a few amps in rating and at least 5-10 C more on the battery pack rating. The question now is does the propeller combination provide enough thrust to equal at least 80% of the weight of the ready to fly model. If not, the flight characteristics might be limited and disappointing. It would also mean you could only fly on very calm days since the propeller combination wouldn't provide enough thrust to push into a head wind.

Having said all of that, I lik'em big and I tend to go for a slow flyer that can stay up for at least 10 minutes. So, what is your next scheme in the evolution of this 48" UFO design? If you are thinking it would be wise to stick with your propeller, motor, ESC, and battery pack for now, why not consider giving the 48" a trim to 44" to see if what you have will handle the slightly smaller size better? If you would rather build a new foamie and try some weight saving ideas, let me know. There are a lot of ways to trim the fat/weight of a foamie and make the round frame stiffer. One, real simple method involves thin, 1/2" wide balsa strips in strategic locations. We can talk more if you are interested. I am interested in this project because it mirrors one of mine I have on hold for the moment while I work on others. So naturally I am very interested in your UFO project. I lik'em round too.
Latest blog entry: Center of Gravity - Airplanes
Dec 07, 2011, 12:22 AM
An Ordinary User
Thread OP

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,

Dec 08, 2011, 10:51 AM
An Ordinary User
Thread OP
Thanks! I like your artwork. Did you airbrush the paint or use some decals?

Also, is that a working rudder? It seems stationary but some movements in the video seem to be caused by rudder control.

It seems light. Is that about 36" diameter?

Dec 11, 2011, 08:35 AM
Registered User
Originally Posted by GooberSB
Thanks! I like your artwork. Did you airbrush the paint or use some decals?

Also, is that a working rudder? It seems stationary but some movements in the video seem to be caused by rudder control.

It seems light. Is that about 36" diameter?

Hi GooberSB, sorry that I'm late with the answer.

No, the skin is the model film, "Cab" color. The smile is black film.
The word on the back is printed on usual paper, glued just at several points and then covered with transparent adhesive tape.

The rudder is quite powerful. Not for usual contol, but for some tricks, like quick snap rolls or turns about the wingtip.
There is also a rudder at the fore fin, which works opposite to the main rudder, but it's just for fun, it makes possible hovering on the airscrew and funny inverted flat spins at about 4 revolutions per second. It's powered with a separete servo and can be switched off when there in no need for crazy flight style.

The diameter is 37.4''.
Mar 11, 2018, 09:21 PM
Watt Waster
Tsavah's Avatar
Dang, it has been a while since anyone has posted, but this is still a good, easy to build design that needs a bump.
Latest blog entry: Center of Gravity - Airplanes

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