Some Technical Questions

[HeliSmelly]

Ok, so here are the latest tecky questions I've come up with.

What is "Throttle Curve"?
What is Idle 1 and Idle 2?
What is "mixing" i.e. mechanical mixing or digital mixing?

Muchas Gracias,
Tim

[hawkman]

Throttle curve:

if you can imagine a box...with the stick movement ranging from 0 to 100% on the bottom and 0 power (idle) to 100% power from the bottom of the box (say left side to the top of the box) you can have a 'curve' so the whole thing looks like a graph.

for beginners you would be looking at a line...linear from idle to 100 % power...this would look like a 45 degree line going from bottom left to top right of the graph.

50% would be half stick where the heli should be hovering...with 50% pitch and 50 % power.....this curve can be altered to suit whatever setup you have....bering in mind you have pitch curve and throttle curve....most radios can do a 5 point curve...where the stick positions can be altered...or rather the percentages can be altered to suit whatever application you want.

Idle up?

this is for stunt flying....You would have 'normal' , idle up 1 and idle up 2 so that switch is 3 position.....the idle ups mean you can have different throttle and pitch curves....so you can do stunt flying etc.....for inverted flight you would have a 'v' curve for throttle....so at the mid stick position the power would be 50% but as you bring the stick down the power increases....by which time the pitch range would be negative....so you can fly upside down.

I use normal and idle up 1 as I dont have a use for idle up 2....some flyers do and others dont....it all depends on what we are doing...or what we want from the heli.

Mixing? this varies....eCCPM mixing means that 3 servos work together for the swashplate....so the 3 work together for collective pitch....when cyclic is used the servos interact to give the required swashplate movement....clever stuff...and it means more power to the swashplate...as the servos share the loads.

other mixing includes cyclic/rudder response to throttle mixing.....so as a command is made the throttle can interact to give that little bit more power.....I havent bothered using this function yet...though might do in the future.

eCCPM is electrical mixing....CCPM is the mechanical version....where linkages work off independant servos.....the collective servo would work by itself to raise and lower the swashplate assy....the elevator and aileron servos are seperate too...but ultimately join the swashplate assy.

I have based my answers on 30 size helis that I own....plus a large petrol powered Predator gasser which uses eCCPM.

Rob

[chaos]

Expanding a bit ... I think it helps in understanding this "curve" thing if you understand what's doing the curving. We're talking about points on a graph, and the graph has two axis. On one axis, usually the vertical, we have a percent number. More on that in a moment. The other axis, usually horizontal, is a number which may be from 1 to 5, usually, though it may be 1-3 or 1-14 or.... Anyway, those numbers represent stick position. What stick? We could call it the throttle stick or the collective stick, since it usually changes both, perhaps "throttlective" would be a better name for it, but on some helis there is no collective so throttle is all it has to control main rotor thrust. Just for fun, I'm going to call it the throttlective stick. So, the numbers along the bottom of the graph represent throttlective positions.

Position #1, or point #1, is when the stick is all the way back, towards you, "minimum". With a 5-point system, point #5 is when the stick is all the way up, "maximum". I put that in quotes because it doesn't really have to be maximum throttle or pitch, it can be any value. But it represents the stick being full up or away from you. So, when the stick is full back, you're operating at point #1 and when the stick is full up you're operating at point #5, in a 5-point system. Point #3 in that system would be with the stick in the middle. Point #2 is 1/4 stick and point #4 is 3/4 of the way to full up. At each of these points you can select a percent from 0 to 100. Percent of what?

When you set up the mechanics, one of the things you need to do is to make sure the servos aren't trying to bend anything. End point adjustments are available for this purpose. These adjustments limit the throw of the servo. After adjusting the limits, the range of possible servo positions is set. If we call one end of that range of motion zero, then the other end would 100% of what's possible. If point #3 is set to 50%, for example, then that would put the servo 50% of the way from one end to the other when the stick is at point #3, the center position. At 0% the servo is all the way at one end, and at 100% it's at the other end. Any of the points can be adjusted to put the servo anywhere in the range of 0% to 100% of its range of motion, and recall that this is limited by the endpoint adjustment.

There are two such graphs, one for throttle and one for collective. When you have the stick at point #1 are selecting point #1 on both graphs; and so on. The values set for this point are independent, each graph can be independently set to any value from 0% to 100% at any of the points.

With idle-ups you select another pair of graphs. In the so-called "normal mode" you're working with one pair of graphs, and in "idle-up" mode - sometimes more than one idle-up - you're working with a different pair of graphs. There might be 3 or more pairs of graphs, switch selectable.

When you reset the TX, as you would when programming from scratch, the default throttlective curves are usually set to a straight line. On a 5-point system, point #1=0%, #2=25%, #3=50%, #4=75%, and #5=100%. Both throttle and collective graphs are set that way. That's good for setting up the mechanics. With the stick in the center, selecting point #3, with the value at that point set to 50%, the servo is in the center of its range of motion. Fine. We should set the swashplate at the center of its range of motion and connect it to the servo while adjusting the length of the pushrod so that the meeting point between the servo arm and the pushrod is a 90 degree angle. With the swashplate in this centered position the pitch would be set to zero; assuming you want equal positive and negative pitch. Likewise, for the throttle graph (throttle curve) at point #3 (with that point set at 50%) the engine throttle should be half open and the throttle arm and the servo arm should meet that pusrod at a 90 deg angle. Then you'd want to move the throttlective towards point #1 and make sure the swashplate goes all the way to the max negative pitch end of its range of motion and that the throttle gets cut completely off, without binding anything by trying to overdrive it. The endpoint limits would be set to obtain this limit. Then move the stick full up to point #5 (or whatever the max point is) and again check the limits and set endpoints as necessary.

So now we can select any value (0% to 100%) for any point (1 to 5) on the graph and not do any damage. But we still have to mess with the graphs to be able to get the engine safely started and bring the rotor up to speed without stressing things and get enough rpm with enough pitch to fly the thing. Your preference and skill level enter into the exact setup you use. For any kind of flying you will need to be able to bring the rotor up to speed somewhat gradually. So the throttle point #1 might be around 10% to have enough power to idle, and point #2 might be around 50%, if you want to hover at half-stick, with maybe 65% at point #3 to hover. You might want to lower point #5 to 80% and split the difference at point #4 to 72%, if you're new. As for pitch, it's probably a good idea to not be loading up the system with pitch while bringing the rotor up to speed, so you might set the pitch at points #1 and #2 to maybe +1 degree; whatever percent that requires, which would be a bit over 50% since 50% is zero pitch on the blades (center of swashplate movement). Point #3 on the pitch curve would be whatever it takes to get hovering pitch, somewhere around +6 deg, if you want to hover at half-stick, and the throttle value at point #3 would have to be adjusted to give you the desired rpm on the rotor. If you're new, you might want to limit the collective pitch at points #4 and #5 to not much greater than the value at point #3 in order to "tame" the machine a bit. For learning to hover you would only need to mess with one set of curves, the "normal" curves.

To fly inverted you might want to hover rightside up at 3/4 stick and hover inverted at 1/4 stick, with 1/2 stick being zero pitch on the blades. For the collective servo, the curve would pretty close to the default straight line value... 50% at center stick, 0% at full back stick, 100% at full up stick, and split the difference with the 1/4 and 3/4 positions. But when you're upside down, hovering with 1/4 stick, you're going to need the same amount of power as hovering rightside up, so the throttle curve value at point #2 would have to be the same as the value at point #4, and point #1 would be the same as point #5. But you can't start the helli this way... well, you shouldn't, because you'd be starting with full power and trying to drive the heli into the ground. So this set of curves would be set into the idle-up mode. The normal mode could be used simply to get the engine started and up to speed. You could set the pitch curve to a flat 50% of all points, zero pitch on the blades. The throttle curve would go from idle at point #1 to maybe 65% at point #3... or whatever value you have point #3 set to in idle-up. This way, you can get the rotor up to speed and at half stick switch to idle-up without the engine speed changing any. And when you land you would switch back to normal mode and then you could bring the throttle back to idle position.

Even just for flying forward, and especially to autorotate, you need to be able to set some negative pitch on the blades. When slowing from forward flight to a hover you will likely need some negative pitch and you want to get it without loosing rotor speed. So you would use a idle-up mode and provide at least some negative pitch at low stick (point #1) with enough throttle to maintain rotor rpm.

Anyway, this lengthy pile of drivel was intended to associate stick positions with positions along a graph, 3 or 5 or more points, where each point can be set to any value from 0 to 100 which represents the percent of servo movement from one end towards the other, with the endpoint settings being the limit. One graph controls the throttle and the other graph controls the collective. One such pair of graphs is selected by the so-called "normal" mode and other pairs of graphs may be selected by "idle-up" modes.

[hawkman]

Mr Chaos has written a book there....good description there!! its quite a lengthy subject in itself.

I used to use something like minus 4 degrees for bottom stick....to be able to do an auto if needed.....I would usually go up high and try this out....I can do one right the way down now....but only if I have to....I do loads on the sim....its an important skill....I would rather a dented heli after a wonky auto than a total pile of wreckage....after 'freezing' and letting the headspeed decay too much....in that state you dont have any time to get the blades turning....anyway...autos are a subject by itself.


The method I used later was to have my pitch range set from -9 bottom stick....0 degree at mid stick and +9 degrees at top stick.

both normal and idle up 1 were set up this way.....its a good method because if you are flying in a breeze...and go downwind and turn to come in and land for example....the heli can balloon about so the natural thing is to drop the collective....to reduce collective....but the power decays too much....which can be dangerous for the heli as the cyclic response can get poor.

With a better negative range of movement and better power the heli descends nicely with a good headspeed.

At the moment I am flying a Predator gasser.....the pitch range is as follows...-4 degrees +5 and +12 top stick.....the Predator has enough power for +12.....the engine is 23cc's !

I figured -4 degrees is enough for an auto.....I have altered the power curve slightly to suit....though I must get a proper tacho to get a better figure.

Rob

[HeliSmelly]

Chaos and Hawkman,

Nice posts! I feel like I ought to pay you guys for those excellent descriptions.
I must say it's really cool to get quality information like that when I'm basically all by myself up here trying to get into flying these things.

tim

[chaos]

Rob, excellent points. I was more geared to a newb when using +1 at bottom stick, so as to avoid the "flat heli" syndrome... where the new pilot slams the stick all the way down in a panic when trying to hover. At +1 it will sorta float down rather than embarking on a journey to the center of the earth.

But before embarking on any forward flight you're on the ball to point out the need for some negative pitch along with enough throttle to hold rpm. This almost requires the use of idle-up, or else the stick is pretty touchy during spool up if a half-stick hover is set up.

I've read sad stories of people forgetting to idle-up and going inverted low and then the heli goes on a dirt-digging expedition. I'm likely to forget so I set mine so they won't even take off in normal mode and I just use that mode to get it wound up.

[chaos]

all by myself up here trying to get into flying these things

Hey Tim, you might be by yourself but the Hawkman and Daz and me and others are with you in spirit.

[hawkman]

Hi Helismelly....I hear what you are saying about being alone....some guys here in the Falklands tried rc helis some years ago....Shuttle nitro powered they were...I think....I remember seeing one in a porch while I was working on this house....I thought 'wow' 'love to own and fly that' but never had the money to do it...years later got into planes...and was using a simulator to help me learn....Helis were a 'dark art' with lots of magic and voodoo about them.

I was intrigued and learned on the simulator...there were two helis...a jetranger and '3d'...the jetranger was really docile so I got good on that...the 3D was impossible for me but I mastered it after many hours.

My first real heli? a Nexus 30 ss...nitro powered...after hovering it for 5 minutes and finding it easy I did my first circuits....I was amazed and thankful to whoever invented RC simulators.

Tips:

read as much as you can on the subject...theres books out there...try Amazon

get DVDs on the subject...I havent done this yet...dont really need to but it would be helpful for any newcomers.

If you dont have a sim then get one and get good on it..really good...but dont get carried away for real flying....take it EASY.

Get a popular heli thats easy to find parts for..nothing worse than getting an obselete model...my Nexus 30 was a lovely heli but parts were getting rare....I replaced it with a Century Hawk...which is a great heli.

Chaos...good point about learning and pitch range....good idea.

I inhibited my Idle ups on the radio....just in case I got silly and hit a switch by accident....just kept it on normal mode.

Flying upside down is not everyones idea of fun....I got fed up with the risk factor involved....one small screw up and its game over.

Rob