All the VTOL and quad crafts that I’m seeing have simple blades that connect directly to gears without leading/lagging joints or coning joints. Why is this?

Or more broadly, how is lift created with these VTOL blades? It seems to be pure thrust (like duct fans) more then a wing blade (like helicopters).

I’m in the early early stages of developing a larger quad, around 75lbs (thanks maxvtol), what kind of blades should I be looking into and why don’t I have to worry about leading/lagging joints or coning joints when I do want to be able to do steady forward flight (and the craft will be pitched forward to do so)? (https://www.rcgroups.com/forums/showthread.php?t=758134)

Thanks for the help guys, this is a really cool forum. (hope it's alright to start a new thread specifically about this, if not I'll take it down)

Brad

I'm not up on my helicopter aerodynamic theory, but I know lead lag and flapping hinges help compensate for the dynamics of forward flight. No faster than most quads fly and since the ccw and cw blades cancel the aero forces, they're not really necessary. Worse case, you may have to beef things up a little to compensate for the stresses if you plan on going very fast. You may want to consider something like a lead lag hinge for the blade to make it easier for disassembly or blade folding.

The quad blades produce lift like wings, they're just thinner so they can be sped up and slowed down fast for control.

For a variable pitch blade you may want to consider an airfoil that won't have a large pitching moment like an NACA 0012 or 0015. If you rotate them at their aerodynamic center (about .25 of the cord) they'll be easier to change the pitch and reduce the tendency to flutter.

You might also try searching and/or posting on the Multi Rotor Heli forum or searching Charly12's posts for some real good information.

The way I understand things, a helicopter rotor needs to be jointed for two reasons. First, control. Having just a single lifting device, the helicopter must be able to vary the centre of lift within the rotor disc to be able to maneuvre at all. If you have three or more separate rotors ju can just vary the thrust between them. Second, structural aspects. A stiff rotor (i.e. a propeller the size of a helicopter rotor) without joints would have to be incredibly over-engineered to even stay in one piece under load, and as such would be way too heavy to be practical. Of course, you can also use a material that flexes instead of joints. i believe a lot of todays helicopter use "rigid" and "semi-rigid" rotor heads, which is something along those lines.

I guess another way of putting it is that joints and hinges are complex and expensive, and on a modeling/hobby scale the costs outweigh the benefits. To build a quad (VTOL with 3 or more props), you don´t really need them.

Brad,

Also remember that any aero vehicle is bound by the laws of gravity. It's basic physics. A body (aircraft) has 6 degrees of freedom, slide (translation) in three directions (X,Y,Z axis) and rotate about the three (X,Y,Z) directions. You will have to balance those forces to have control over that body.

Let's say the Z axis is the vertical (gravity and lift)
the X axis is along the length of the helicopter fuselage (going forward and backwards)
The Y is left and right of the pilot and co-pilot seat. (side to side, right and left)
For a single rotor helicopter (getting lift from one column of air)

The main rotor is responsible for 5/6 of these degrees of freedom DOF !!
The tail rotor is responsible for one 1 DOF (rotation about the Z axis)

This is why that rotor system is so complex like rctiltwing mentioned above.

Now for a multi-rotor helicopter and muti-engined VTOL vehicle, you start to spread the 6 DOF among them.

Note that a twin rotor helicopter like the Ch-47 Chinook and the V-22, (2 columns of air) for simplicity sake, each rotor is responsible for 3 DOF, totalling 6.

For a VTOL NON-HELICOPTER rotor vehicle, for minimum stability, it has to have at least 3 propellers.... or (3 columns of air)

Once you have three or more columns of air to support your vehicle, the complexity of the rotor/propeller system needed goes down.

For example my current project the CL-84, has three propellers. They are just simple propellers, but needs the ailerons for turn control (spin about the z Axis).

Note that on the F-35 or JSF, rises on two main column of air, but is supported two smaller columns of air for roll stability (rotation about the X axis), totalling 4 columns of air.

Cheers,

Le

Another issue is that full size helis fly much faster than models, but their rotor speeds are lower (actually, the tip speeds are higher, but not much)

Therefore the difference in airspeed between the advancing and retreating blade is much more than it would on a model. Full size helis compensate for this by letting the leading blade flap upwards and lag backwards (more lift = more drag)

Model helis have a much stronger hub and blades, so they can cope with the stresses, and they have a flybar to compensate for the roll that would be caused if the advancing blade really generated more lift than the retreating blade.

Flybarless models use electronics to compensate for the roll

On a VTOL, the airspeed is very low compared to the blade tip speed and the blades are even stronger, so it's even less of an issue.

Hinges are not for control, model helis don't really have blade hinges. The cyclic control allows you to control where the lift is generated and pitch and roll the heli. However, on a full size heli, the hinges allow you to tilt the rotor disk without tilting the rest of the heli (which just hangs like a pendulum) increasing the speed of control response (at the expense of limiting the total control power, which is one reason they can't do 3D)

Interesting stuff, thanks guys!