With the non-stop rain around here, I haven't been able to do any flying, so I decided to start a new project. Since I want to do FPV eventually, I went with an H frame layout. Not much to show yet, but all the bits are collected, so it should go together fast.

 

Decided to make the arms a bit longer than needed to act as prop protectors.

Components right now:

3/4" Square aluminum tubing for arms

1/8" G10 Fiberglass body plates

HobbyWing Quattro 4-in-1 20 Amp ESC

4 x Exceed Optima 850KV motors

APM2 with external GPS (the newer version of the Mediatek from 3DR)

3DR Radio telemetry

FrSky RX

 

Views: 1173

Comment by criro1999 on December 22, 2012 at 8:51pm

My first quad was a diy H-Frame like yours. For me the main reason was to avoid radio interference when flying FPV. H-Frame allows you to assure that minimum distances for radio interference.

The first issue I did have with my h-frame was twisting frame. Someone did suggest me to try to raise one single arm until the next arm start to raise itself. The vertical distance should be lass than 2 cm (for my frame).

I am not sure I understand "make the arms a bit longer than needed to act as prop protectors."

Can you elaborate, please.

Comment by Gary McCray on December 22, 2012 at 10:14pm

Hi I hope you don't mind my tossing in a few observations.

First: Using 3/4" square extruded aluminum tubing is a great idea, but the wall thickness on the tubing in your photo seems to be a lot thicker and heavier than you'd need. If you get T6 hardness aluminum tubing (most extruded tubing) half the wall thickness you have would probably be plenty. There are quite a few online suppliers of appropriately hard square aluminum tubing and I would get the thinnest wall you can find.

Second, Extending the tubing out to past the edge of the Prop Arc doubles the amount of Prop wash resistance and on square tubing that is flat plate area as well as increasing weight significantly. Also, the practical extra protection you get isn't worth either of those negatives.

Third, 2 G10 plates interconnecting by themselves is going to be strong enough, but too flexible unless you add additional vertical stiffeners. Otherwise your front and back arms can and will twist in relation to each other and are likely to induce a low frequency oscillation in flight under various conditions. You could add stiffeners or "box" it in some way to achieve torsional resistance.

I am an engineer, though an electronic one not mechanical although I do have quite a bit of practical experience with mechanical design and I build and fly multicopters.

These are just some thoughts that might help.

Comment by Jake Wells on December 23, 2012 at 9:59pm

I agree with Gary. The square tubing is overkill that you'll be disappointed with. And the twisting is definitely going to cause you issues. The tubing will exaggerate the twisting. But hey, you can call it the chubby checker.

Comment by Scott Berfield on December 23, 2012 at 10:26pm
Thanks for the feedback. We'll see how it goes. With the spacers in place, there is not much twist. The tubing has the advantage of being locally available and cheap. If it proves to be a problem, it will be easy enough to do up new arms out of other material.
Comment by Jake Wells on December 23, 2012 at 10:46pm

Yeah I can only get that 3/4 inch square tubing local too. I went with 5/8in Channel instead. It works great. Although I'd prefer to use 5/8in square. Gotta go with what I could get.

Comment by Jake Wells on December 23, 2012 at 10:53pm

Just saw that you live in WA. We are moving to Sequim in Feb. Looking forward to flying the forests up there. 

Comment by Scott Berfield on December 23, 2012 at 11:45pm

It's beautiful over there. FPV through teh forest would be amazing.

Comment by criro1999 on December 27, 2012 at 7:01pm

one solution you can try for arms is to use aluminum U-shape instead of square and fill that U shape with square balsa stick. Balsa will reduce significant the aluminum vibration. I did initially on my H-frame. But I had to go back to balsa only arms (3 layers) because of weight. With balsa 3 layers (glued), the weight for each arm was 3 time less. Indeed, less strong as aluminum, but no vibration. And, in case of a hard landing when prop touch ground, I prefer to broke the balsa arm ($3 for 2 arms) instead of bent the motor axes.

Comment by Scott Berfield on December 28, 2012 at 11:43am

Thanks, criro1999 - I considered using the channel, but was concerned about the twist. Body flex I don't worry so much about given the typical dynamics of a quad, but arms twisting around could be trouble. I considered filling the channel with expanding foam (hadn't considered balsa), but ultimately, I am not super concerned with weight yet. If it looks like an issue, I have a fair number of options including cutting out a good bit from the G10 plates and drilling out the arms some. Also considering filling the arms with foam if vibration is an issue, but I don't think it will be too bad with balanced motors and props. 

Comment by criro1999 on December 28, 2012 at 1:04pm

Hi Scott, you said you not super concerned with weight. Do you have an estimation of your total weight with all gears? What props are you planning to use and what Lipo? And this would give you how long time to fly?
On my H-Frame quad, with all gears for FPV and Hero2 and 1 x Lipo 3S 2200MAh, 20C, motors 1000kv, prop 10x6 I have like 8 min to fly (not acrobatic), with alarm at 10v. With 2 Lipo in parallel, I can have like 12-13 min. Would you expect longer time to fly?

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