# What is the major difference between this VTOL setup and an actual helicopter, in terms of degree of freedom?

I am studying QNET VTOL(Vertical Take off & Landing) system. I want to know how it is different from an actual helicopter . According to my understanding, an actual helicopter has 6 degrees of freedom while this VTOL system has only 1 degree of freedom as it pitch is the only parameter here that is variable.I am also attaching snapshots of top & front view of this system and also i am giving below link of a relevant youtube video.

Please correct me if my understanding is not correct

• I guess it is controlled by some other microcontroller but i am pretty much sure that aurdino is not being used here. It is something else Nov 2, 2022 at 3:35
• Ok, thanks for the answer :) Nov 2, 2022 at 8:33

The differences are endless, the only thing in common is the name VTOL

A helicopter has indeed 6 degrees of freedom. The helicopter is controllable by 5 input variables: lateral & longitudinal cyclic, collective, anti-torque pedals and power). But non of the inputs correlate purely with a single degree of freedom; there is a lot of cross talking between the control variables.

Change any one of the inputs and you disturb the balance of forces and moments so that you have the change something else as well.

For example, if you change the collective to climb, you will also require more power to avoid the rotor from slowing down. More power means more torque on the rotor, so now you also need to control the anti-torque pedals to balance that out. But the tail rotor will then change the side force on the helicopter, so now you need lateral cyclic to compensate for that. That upsets the vertical equilibrium slightly and so we are back at the start.

Another major difference is the stability. Helicopters need constant control inputs to stop them from diverging from the initial state. The QNET VTOL would very likely find a balance point for a given control input setting. A helicopter, being unstable, would diverge from its initial state and crash, not matter what constant input you would give. It needs constant input adjustments to keep it in check.

From a control system point of view, this QNET VTOL system seems to be a nice tool to get practical experience with modeling, system parameter identification and PID control. However there is still an enormous step to go before one can design a helicopter control system. An intermediate step would be controlling a quad-copter which is in terms of control system complexity somewhere in the middle (but still very far from the QNET VTOL)

• A key difference here is stability - I would expect a fixed power level would damp to a fixed position even without electronic assistance on this due the presence of a ground effect. VTOL aircraft are infamously unstable so poor control system setup will diverge and crash quickly. If the setup allowed the arm to go vertical that might be a closer example of flight control. Oct 31, 2022 at 13:22
• @GremlinWranger good point. I'll add a paragraph about stability Oct 31, 2022 at 13:25
• "...the only thing in common is the name VTOL." Which is a misnomer because that machine is neither V, nor TO, nor L. Oct 31, 2022 at 17:21
• @MichaelHall absolutely agree. There is nothing that this thing and a helicopter have in common. Oct 31, 2022 at 18:48

What is the major difference in terms of degree of freedom?

In terms of degrees of freedom (dof) a helicopter's fuselage has indeed 6 dof but then you have to add the dof of the blades (each of them pitches, lead-legs and flaps) and the dof of the tail rotor (pitch of its blades).

That if you consider the structure as being rigid, otherwise the dof due to the flexibility should be considered as well... helicopters are quite complicated :-)

By extension also your experiment has actually 2 dof: its angle in respect to the hinge and the speed of the propeller which, I suppose, is controlled via the voltage applied to the electrical motor driving the propeller.