How to generate 30kh+ of lift?

[extrapilot]

Originally Posted by R_Lefebvre

Hmmm... no, this problem happens at very low descent rate on the order of just a few m/s. What else would the cause be if not VRS? Basically, I'm talking about the classic "descent wobbles".

Hey Rob-

It is probably just recirculation-based turbulence. VRS is really more of a runaway condition, where your descent rate vs outflow velocity allows for a small recirculation path- so that the outflow does not have time to diffuse its energy. In that descent rate band, you see much higher inflow velocity, so you require lots more power to generate the same thrust.

[R_Lefebvre]

Huh, I thought recirculation == VRS.

Alright. So anyway, the theory is that the angled out motors helps with re-circulation instability.

[extrapilot]

Yea, you get some in still wind OGE hover, much more in IGE hover. Normally, the effect is tiny, since the free stream path for the loop is huge, but you can see some of the effect (its reduction) in translational lift.

Once you get into a descent, you greatly reduce the mean particle path distance, which means more recirculation and less diffusion.

I don’t think there is a clear demarcation here. It is more like a tornado- where you see slow circulation, and then it tightens up and things get ugly.

Enjoy the weekend-

[R_Lefebvre]

Regardless of the aerodynamic situation, and whether or not virtual dihedral helps, it has become very clear that a huge factor in descent wobbles on multirotors is due to non-linear thrust according to the motor PWM. Basically, the Rate stabilization PIDs need to be non-linear, according to total throttle output.

We recently instituted that, and descent wobbles are now almost completely gone. I can descend with min throttle, and not wobble.

And the converse is true, high-speed shaking often occurs at full throttle, because they are overgained. So that's fixed too.

[extrapilot]

Yea, that is an interesting problem. the general relationship puts descent rate vs rotor outwash velocity, so adding outwash velocity at a given descent rate puts you at a different ratio, where you can then reduce that velocity, which then put you back where you started, etc. Maybe your approach here is the better one, so that you can permit operation in VRS onset, and even in deeper VRS (if you have the power available).

Do you have a defined descent rate for auto landing? If you get a sec, can you post your machine mass and prop diameter/count? Ill calculate the outflow and see how closely the guideline correlates with your real-world experience as regards these power changes.

I know you had a test stand setup at some point. Do you have test data for PWM vs thrust vs supply voltage for the flight config in test? With variable-speed props, it is just too difficult to infer output power from power consumed, since efficiency changes radically with RPM.

Congrats again on the progress.

Regards

[R_Lefebvre]

Yes, there's a user parameter for maximum descent rate in flight, and then descent rate for final landing speed which has more to do with contact energy rather than stability.

I don't have a lot of good data to give you regarding the other stuff. This work was done by somebody else, I'm not sure what he has. He does Matlab simulations, but I don't thing he was considering downwash aerodynamics. Just gain scheduling as required to linearize the PID output.

I do have this data I captured a few years ago:

https://docs.google.com/spreadsheets...it?usp=sharing

You can see there's a bit of S-curve to the PWM vs. Thrust output. I suspect it may be more pronounced with better data collection. One problem I had with that test was I didn't have a constant voltage power supply. I had a single battery, and input voltage was dropping with higher power levels. So I was getting thrust vs. voltage non-linearity mixed in with the thrust vs. PWM. Eventually I'll get a better power supply.

[Icanfly]

a human carrying quad, ok. I thought about this too, if a 700 can carry 50lbs and 4 would lift 200lbs so what's the problem? CONTROL. You want to go the Chinook route here.

angled out motors helps with re-circulation instability.

think of the pendulum like in a hang glider, think of the Beechcraft Bonanza V tail, in a canted rotor config one rotor will determine thrust angle while the other is lifting vertically, now you have to consider ascent/descent and wind shear, the thing is moving through high/low pressure and cross flow variants constantly.

A basic quad is a compromise of air screw/rpm and lift because of it's position in relation to gravity. A Helicopter is a compromise of rotary control surface and mechanics, more complex than a simple air screw and the effects of several variables involved make it less attractive in a multi format.

Personally, I've conceptualized a few years ago what the op is thinking to the point a working model is not far off (gimme more money, lol 10k$)

All you really need to think about is lbs per sq/in to start, then control. All the gibberish about stream tubes and fp air screw problems/cant angles and descent rates, WHERE DO YOU WANT THE COMPRESSED AIR TO GO? is all you need to think about.

[extrapilot]

How is it that in what, a week, you go from realizing that you have no idea what you are talking about as regards aero, to reversing that and posting here in #26:

“WHERE DO YOU WANT THE COMPRESSED AIR TO GO? is all you need to think about.”

You believe the rotor flow/streamtube is compressed air? Has it occurred to you that when you increase the velocity of a fluid, and you also increase its mass (by compressing it), that you have increased the mass flow across the rotor disc? Where does this extra mass come from? Magic? This theory of yours violates a core law of our universe (conservation of energy/mass). Why does the streamtube initially contract below a rotor, when you believe the static pressure in that flow has increased?

You don’t understand how flow works, how VRS works, why a helicopter/quad with its mass below the rotor in upright flight is dynamically unstable, etc. But somehow, you think that your ‘conceptualizing to the point of a working model not far off’ makes the nonsense you post here valid?

If you believe you have something special, take that idea/design to a helicopter manufacturer under NDA, or start a crowd-sourced fundraiser, or offer to a patent lawyer half the upside in return for free patent work, etc.

[Billzilla]

Originally Posted by Icanfly

a human carrying quad, ok. I thought about this too, if a 700 can carry 50lbs and 4 would lift 200lbs so what's the problem? CONTROL. You want to go the Chinook route here.

I've already got a flight controller that's (maybe ...) suitable. It's for a quad rotor variable-pitch copter.
I'm also heavier than 200 lbs in the old numbers, I'm about 105 kg.

[Icanfly]

that was a fair reply extrapilot, I'm with you on that.

Now after many days reading the main forum and coming to this just now it has helped me see that in a quad rotor config when the rotors are placed near each other there is pressure coupling at the center of the rotor wash and therefore more pressure there. The stronger down flow stream in the center will want to find a way to neutralize and like a tornado tube (outward flowing air mass in this case) will wobble around looking for the easiest path down, while unstable air is compressing it back up in the quad's relative position while in descent.

Canted rotors even out the down wash and lessen coupling between rotors. Regarding shear effects, wind? Ballooning is dependent on pitch and entry dynamics, would the quad balloon? Possibly yes while only reacting to a gust rather than anticipating it, so then in the canted position unless the entry disc is level would pitch up causing a negative pitch controller event on the entry disc and a positive event on the trailing disc to level the quad, there is a better solution which eliminates the problem completely however.

As mind exercise goes you do things at the scale level and mathematically scale them up to suite your desires. 105kg would be about 225lbs, did you see the multi 300lb lifting thing at ircha this year, wasn't a very good demo at all and showed the weakness of the platform in wind, I thought it was going to crash.