Overlapping Props

[ringolong]

I have been searching online for stronger mounts.

I am checking on getting a set of these since I have read that they are much better, and have 21mm arms instead of 16mm.

http://fpvlab.com/forums/attachment.php?attachmentid=51000&d=1409138715

I would really like to stay with this frame since the arms and props stay out of the picture.

 

[GJH105775]

The arms will raise a little before the opposite arm on the same font or back side when it is sitting on the floor when I pick up on any arm. Its the mount that is flexing.

Ahhh, sounds like you need a cast iron plate, or a manhole cover maybe? hahaha.

Maybe

It could be we were wrong when we said that as long as the props didn't touch it would be ok. Since you're props and motors are balanced the only thing left is the prop clearance issue. My guess, not a proven fact, is there is a shear force in action at the tips of the counter rotating props. Sort of like how a torque converter in an automatic transmission works. That is likely the cause of the arm tips vibrating and could be the cause of the arm oscillations. You have the front and rear motors changing speeds constantly while rotating in different directions. The more I think about it the more I'm convinced that we, or at least, I was wrong to think there wouldn't be an issue with the props that close.
I used 15X5.5 props on mine with no problem but I had a few inches clearance. You could try some 11 or 12" just to see if the tip vibrations go away. I think you are spot on about the mounts being the weak link and causing the arm oscillations. I have seen quite a variety of aluminum mounts listed online. Maybe there is a design that would work for you.
If the smaller props eliminate the tip vibration then I would suggest that you seriously consider extending the rear arms. You could do it by using arm extensions like the ones from Phoenix Flight Gear or by replacing the arms with ones the right length. You are using quality materials, motors and props so it is likely a design issue rather than components.
I don't have a background in fluid dynamics, but from building and racing cars and motorcycles way back when we learned a lot about turbulence and shear forces.The mounts are an obvious problem. The shear at the props tips is just my guess. I think you could fix this for far less than the new frame you are looking at.
One more thing. Like you, I no longer buy the cheap stuff. Since using only the best that I can afford (and checking the prop balance) I discovered that it is no longer necessary to use isolation materials for mounting cameras or any other equipment for that matter. Again. I think your current problem is solvable by a design adjustment. It would be rare to work perfect the first time.

It is not an uncommon problem for a frame to flex, what if you fixed a channel to act as a spar under the arms?

 

[HDtallrider]

21mm arms are huge! Those will guarantee you never have a strength issue! The mounting looks really solid.
Looking at the specs it doesn't look like they will cure your prop spacing issue. From looking at the photo it appears you would need to add 2 to 3 inches to each rear arm to get the prop clearance you need. That would actually give you enough clearance to use your 17" props if you wanted.

 

[ringolong]

Ahhh, sounds like you need a cast iron plate, or a manhole cover maybe? hahaha.

lol

 

[ringolong]

21mm arms are huge! Those will guarantee you never have a strength issue! The mounting looks really solid.
Looking at the specs it doesn't look like they will cure your prop spacing issue. From looking at the photo it appears you would need to add 2 to 3 inches to each rear arm to get the prop clearance you need. That would actually give you enough clearance to use your 17" props if you wanted.

So you think the props are too close? The props on my 680 are that close and are not causing an issue, or is that it is due to using 17 inch props?

 

[HDtallrider]

Ahhh, sounds like you need a cast iron plate, or a manhole cover maybe? hahaha.

Maybe

It is not an uncommon problem for a frame to flex, what if you fixed a channel to act as a spar under the arms?

Just from the photo's and videos I think singalong nailed it with the mounts being the issue. The frame looks to be quite rigid.

 

[HDtallrider]

So you think the props are too close? The props on my 680 are that close and are not causing an issue, or is that it is due to using 17 inch props?

I think the props being that close are causing the arm tip vibrations but I don't have any evidence to prove it absolutely. It could also be related to the flex at the arm mounts causing a sympathetic vibration at the tip of the arms. If you install some smaller props and the tip vibration goes away the you know you need more clearance. If the tip vibration continues then you are probably right about the mounts and the TBS arms are a good bet.
Ha! I just noticed my typo on the previous post. Sorry ringalong.

 

[GJH105775]

Just from the photo's and videos I think singalong nailed it with the mounts being the issue. The frame looks to be quite rigid.

Yeah, I agree with this also. If you extend your arms to keep the props from over lapping it may actually make the problem worse. Sounds like you just need a stronger way to mount the arms.

 

[photojunky]

Old thread but I found it very interesting because I accidently ordered 4x 17 inch props instead of 15 inch and now have an excuse to build something. I was thinking about overlapping as well but then after thinking about it, I was wondering if the air being propelled on to the overlapped props would cause some vibration? Anyway, I have the build bug right now and have been eyeing a pair of crutches that may work perfectly as tubing after I saw them apart. I know they will be strong enough, but they may be a bit heavy.

 

[photojunky]

I am going to use these crutches I sawed off to make a frame. They are light and strong. (30" 120 grams.) I have a lot of extra parts left over from my last build so it should not cost too much.

 

[Jackson]

I accidently ordered 4x 17 inch props instead of 15 inch

 

[holtneil]

Hi mate there are many quads like the Y6 that has motors mounted below each other so all there props will over lap not just the tips and they fly fine , I have seen a few flying with over lapping prop tips you have to use carbon props with no flexing and front or rear motors mounted a little higher to give clearance