Airflow debate

[DucktileMedia]

Anyone who has known me for the last few years knows I have not given up on making a torus shaped heli. There are a few reasons I like this idea but another one came to mind. If you imagine the wind flow of a prop over our typical radial arm designed helis, you can imagine that the airflow is disturbed(much like myself) when it passes over the arm. But what about the fact that it is disturbed ONLY on one half of the blade? This must make some kind of vibration where the blade passes over thin air on one side and the other is conflicted with the resistance of the arm in its way. So let's move forward to my idea of the ring/torus idea. You would have double the amount of surface in the way of the props as each time the blades passed over the frame, they would both be encountering an aerodynamic obstacle. So the question is, is it better to have more flow resistance but equal on each blade simultaneously or is it better to have less resistance but unequal loading on the prop?

View attachment 15424

 

[Carapau]

Interesting idea but as the frame doesn't move symmetrically through the prop disc, the problem is still there? I would be genuinely interested to hear why you think the arm would cause vibes as it is not affecting the blade at all, only the prop wash unless of course you are looking at an X configuration. It would be interesting to see some slo mo footage of the air passing over a multi's props.

 

[RotoTwit]

Your radial idea is nice and simple, but why not reduce any interference symmetrical or not?

How about 'vertical distance' motor mounts, 1",2",3" or more, if you were to put vertical distance between any structure, radial or straight tubes and motor/props, it can only help and at some point, would not have any appreciable interference ?

Use lightweight cylinders to space or even just spacers on bolts, OR use much smaller arms, but with structural triangulation for strength coming from below, think 'V' but more shallow.

IMO, the more complex the harder to repair after those crashes!! Simple is usually better.

 

[RotoTwit]

Another idea is to add aero sleeve (up down, not fore/aft) only where props interference would occur on tube/arms, a bit of tape or glue would keep from rotating. Could make your own.

Some real planes and ultralight planes and hang gliders use this approach but mostly to increase efficiency fore/aft which effectively is what you are after.

Could work on co-axial motors using symmetrical aero form.

Just a couple of thoughts.

 

[bruce365]

Hi IrisAerial, with that circular shape the 2 prop blades are going to reach the tubular frame at different times so I can not see an advantage, one blade will still be over open space while the other is over the tube body unless you make a hexagon shape for 6 motors.

I am inclined to agree with RotoTwit "How about vertical distance". I found that in my experiance with hovercraft distance from objects is good, in particular from the inlet side, as air from the intake side of the fan/propeller is drawn in from a larger area, therefore velocity is lower. The air from the output side of the prop is very direct and high velocity.

So in my opinion have the propeller mounted below the supporting structure and give it a bit of distance to make the whole assembly more efficient and perhaps reduce prop cycling vibration.

regards - bruce

 

[DucktileMedia]

I don't necessarily feel that this added vibration from either of these situations really hinders our performance. But more a theoretical question. I didn't think about the fact the blades aren't actually over the tube simultaneously due to the curve. Good point. But they are much much closer. In regards to raising the prop, yes, no doubt that is a good solution. I've always liked the ring because it is easy to make, super rigid, potentially transmits vibrTions in a way that doesn't focus them on the vital areas. I'm just on the search for something different I guess. The typical radial multi is getting old.

 

[Carapau]

The typical radial multi is getting old

So is the design of a bike wheel or a bicycle's gearing system but that is because simplicity works well I guess

 

[DucktileMedia]

Sure it does, but certainly no harm in thinking outside the box is there? I am the kinda guy that would totally put a square wheel on my dirt bike if it didnt break into pieces.

 

[Carapau]

Very true, very true. Looking at your design above, it looks like it would need minimal hardware in order to build the frame making this one light beast. Perhaps a better idea would be to have the two crossbars in a cruciform formation to stop flexing of the ring in flight?

 

[DucktileMedia]

It doesnt flex much as is. I made it from .060" 1" 6061 tubing. Problem is more logistics of wires. It isnt apparent at first glance but since the FC needs to be in center-ish of the heli, you really have an increase inwiring going all over the place. And that is 1 of 2 reasons this hasnt taken off. The other is the gimbal which convetionaly mounts under the body. Since this has no body the idea was to use 3-4 cables with rubber isolators to a gimbal. My idea was strictly to try something different, not reinvent the wheel. But then again, you just never know what works until you try. It is very cheap tp make and very strong, so far the best perks. It has quote a few other drawbacks though like the size is kinda awkward being that the arc of the tube goes beoynd that of the outer extremities of the motors. Initially I wanted to make this fold but then that prevented wires from running inside the tubes. And so on...

Hey! I got an idea! How about a helicopter with 1 large prop in the middle that changes angle and keeps a constant head speed, it could have some kind of tail rotor to stabilize it. Oh wait.

 

[ChrisHigs]

I plan to build myself a quad at some point. I too was pondering the torus shape. Not for the prop wash but I was wondering if straight and would flex more than a torus and maybe have better stability. It obviously adds more weight to the design tho as the diameter of the ring is greater than the length of 4 arms.

I was also wondering if the prop height above the center would make a difference so would angling the arms upwards towards the motors, thereby keeping the heavy battery and such below give better balance by altering the COG?

 

[jes1111]

I plan to build myself a quad at some point. I too was pondering the torus shape. Not for the prop wash but I was wondering if straight and would flex more than a torus and maybe have better stability. It obviously adds more weight to the design tho as the diameter of the ring is greater than the length of 4 arms.

You mean circumference - yes but not by much: total length is 2*Diam for arms and 3.14*Diam for torus. Assuming same tubing diameter used, the fuselage plates at the centre of the conventional frame would undoubtedly make it heavier than the torus. Torsional rigidity should be good but perhaps not quite as good as arms since unsupported length is greater (0.5*circumference as opposed to 1*radius).

I was also wondering if the prop height above the center would make a difference so would angling the arms upwards towards the motors, thereby keeping the heavy battery and such below give better balance by altering the COG?

Having the vertical centre of gravity close to the plane of thrust improves responsiveness (moments of inertia are lower) - but all that goes out the window as soon as you hang a gimbal under the thing. "Arm dihedral" has a theoretical stability advantage (viz. fixed-wing aircraft) but why bother when you have a FC doing all that for you?

 

[ChrisHigs]

Yes, I do mean circumference. I don't think my brain had woken up properly this morning. For a hex/octocopter the circle would be only slightly more or even less.

I'm not basing any of this on anything scientific just curious about binding something different. My thought was if the thrust is quickly changed on a straight arm the arm might flex then the rest of the body follow. Depends how rigid they are. And the size I guess. And would a ring deflect as much/differently? No idea.

 

[DucktileMedia]

The flex issue was in my mind as well. Another thing is that I'm not sure how far off center you can place the FC and get away with it. If you lay this out on paper the lengths of wire involved are disturbing which is the main reason this has not yet come to fruition. I then worried that I might be making some sort of electro-magnet.

 

[gtranquilla]

There are several of us thinking the same way regarding this issue. Glad someone took the initiative to put in in writing and draw it up.
By utilizing the bicycle wheel concept but replacing the tire with a carbon fiber hoop would resolve nearly all issues with structure, vibration and airflow.
In my case the hoop would simply be a thin CF band running the circumference. The hub would be held rigidly by means of spoke tension.
And since there is no compression issues as occurs with a bicycle wheel, the number of spokes can be greatly reduced.

Anyone who has known me for the last few years knows I have not given up on making a torus shaped heli. There are a few reasons I like this idea but another one came to mind. If you imagine the wind flow of a prop over our typical radial arm designed helis, you can imagine that the airflow is disturbed(much like myself) when it passes over the arm. But what about the fact that it is disturbed ONLY on one half of the blade? This must make some kind of vibration where the blade passes over thin air on one side and the other is conflicted with the resistance of the arm in its way. So let's move forward to my idea of the ring/torus idea. You would have double the amount of surface in the way of the props as each time the blades passed over the frame, they would both be encountering an aerodynamic obstacle. So the question is, is it better to have more flow resistance but equal on each blade simultaneously or is it better to have less resistance but unequal loading on the prop?

View attachment 19296

 

[Mojave]

There are several of us thinking the same way regarding this issue. Glad someone took the initiative to put in in writing and draw it up.
By utilizing the bicycle wheel concept but replacing the tire with a carbon fiber hoop would resolve nearly all issues with structure, vibration and airflow.
In my case the hoop would simply be a thin CF band running the circumference. The hub would be held rigidly by means of spoke tension.
And since there is no compression issues as occurs with a bicycle wheel, the number of spokes can be greatly reduced.

How about a figure '8'? It would give you a central point for your gimbal and open a field of view for the camera like an XY8. The center would be a weak point though. I do like this conversation it seems to be closer the the flying saucer concept. Outside the box is good.

 

[RotoTwit]

I plan to build myself a quad at some point. I too was pondering the torus shape. Not for the prop wash but I was wondering if straight and would flex more than a torus and maybe have better stability. It obviously adds more weight to the design tho as the diameter of the ring is greater than the length of 4 arms.

I was also wondering if the prop height above the center would make a difference so would angling the arms upwards towards the motors, thereby keeping the heavy battery and such below give better balance by altering the COG?

This 'upward angled' beams would be referred to as 'dihedral' (at least in airplanes) and it increases stability BUT at the expense of agility! So, you use that sort of thing depending on what you want. This will put a bit more strain on the motors (battery length) to do the same movement as a straight arm copter.

Ex. The most agile jet fighters are designed to essentially be unflyably unstable, and by using 'fly by wire' ie-computer controls which would be completely impossible for a human pilot, makes the plane not only flyable but VERY agile.

So, if agility you want, put arms DOWNward (anhedral), then dial INsensitivity into controls !!

Torus: You could use much smaller beams with lower triangulation arms (total weight less?) aligned to landing gear, and small diameter torus beam (does NOT need to be large dia. for torsional strength) and maybe weight would be less, center area should at least be not larger/heavier than straight beam copter.

This design would be cool looking and unique, and certainly more UFO looking, add flashing LEDs, watch old movie 'Close encounters of the 3rd kind' !!

BUT, more work to fix upon crashing!!

Have fun!

 

[RotoTwit]

I plan to build myself a quad at some point. I too was pondering the torus shape. Not for the prop wash but I was wondering if straight and would flex more than a torus and maybe have better stability. It obviously adds more weight to the design tho as the diameter of the ring is greater than the length of 4 arms.

I was also wondering if the prop height above the center would make a difference so would angling the arms upwards towards the motors, thereby keeping the heavy battery and such below give better balance by altering the COG?

Germans have a solution! Look at this:
http://www.cnn.com/2013/11/27/tech/innovation/volocopter-18-propeller-electric/

 

[RotoTwit]

How about a figure '8'? It would give you a central point for your gimbal and open a field of view for the camera like an XY8. The center would be a weak point though. I do like this conversation it seems to be closer the the flying saucer concept. Outside the box is good.

I think this would be a nice compromise, especially if you could make it essentially 1 piece! OR could use diff dia. tubes for straight/crossover part of '8', smaller curved parts, should be strong torsional, but up depends on those straight tubes.

 

[jes1111]

I knew I'd seen one before - just took some searching to find it: http://www.youtube.com/watch?v=POorwqxmJ-4

This is, I believe, Simon Jardine: one of the early "names" in multirotor history