Now this is where I start to get into trouble! Why? well how you set up the collective pitch/throttle curves of your helicopter depends to a large degree on what sort of flying you are going to do and this is not just modelling its politics! I saw a meeting recently between two well known model heli pilots, one a fine scale pilot, the other shall we say, not unknown for speed. The topic of flying style had not been going long before some very basic Anglo-Saxon terminology was being exchanged. Suffice to say, in the eyes of the other, they were respectively members of the 'Antiquated Flatulence' Brigade, and 'Rectal' School of flying. So I have resigned myself to getting 'flack' pretty much whatever I say here. Since this article is quite a bit heavier going than my previous W3MH offerings I would be interested to hear if this is the sort of thing you want. You can contact me by Email as colin@nhpltd.co.uk (Ed: Please note this was written some time ago...)
Before I get into the actual setting up of the pitch/throttle relationship I think it would be handy to start by looking at what's 'wrong' with having fixed pitch in order to see what we want from our collective pitch systems. I say 'wrong' in quotes because there have been (and still are) some quite respectable fixed pitch designs.
With a fixed pitch helicopter, the lift can only be controlled by changing the rotor RPM. So, to go from the hover to the climb we have to accelerate the rotor. Because of the inertia of the blades we have to increase the torque being transmitted to the main rotor. Now this doesn't instantly increase the revs it just causes them to build steadily so there's a delay between opening the throttle and the heli starting to climb. This sort of delay accompanies all the lift changes of the fixed pitch machine and needs to be compensated by some anticipation on the part of the pilot, especially in descents! The change in the main rotor RPM also means a change in the tail rotor RPM. Now, in going from the hover to the climb the initial increase in torque to the main rotor demands more thrust from the tail rotor and hence more tail rotor pitch. Once the RPM builds up the torque from the motor falls back somewhat and the extra revs make the tail rotor more effective so less tail rotor pitch will be needed. So the transition from hover to climb is accompanied by a tail swing first in one direction and then in the other. This makes control of the tail on fixed pitch machines more 'interesting' for the pilot.
Ed: Hi Colin, why is the helicopter on the right hand side bigger?
Csm: Because it's climbing towards you, dummy....
Ed: Oh........
With a collective pitch model we have the opportunity of trying to maintain a constant rotor RPM. If we achieve this our example of transitioning from the hover to the climb goes like this. The collective pitch of the main rotor blades is increased to create the extra lift while the throttle is opened so the engine provides just the right amount of extra torque to 'pay' for this extra lift. In this way the model settles into a steady climb quickly without any change in head speed or the delays associated with them. There will still need to be an increase in tail rotor pitch to compensate for the extra engine torque. But since the RPM of the tail is constant the efficiency of the tail will not change and maintaining a steady heading will be easier. Another advantage is that, with suitable gearing, we can ensure that the engine is constantly run at its optimum RPM so full power is immediately 'on tap'.
I suppose the next thing we need to know is where the engine power goes. This can be a very involved study and I'll come back to it in later articles but for now I'll just apologise for the minor bits I'm going to miss out. Broadly speaking we can split the power requirement up like this. First we need some power simply to drive the main blades through the air even when they are not producing any lift (This is called Profile Drag Power). Next, to produce lift we need to throw air downwards and this also takes power (called Lift Power or Induced Power). We also need power to drive the tail rotor and, in forward flight, power to push the bodywork through the air (Parasitic Drag Power). Finally, in a climb we need power simply to raise the weight of the helicopter. I don't want to put anyone off by going into the equations but I think some typical figures could be handy. Taking a '30' sized helicopter with a rotor span of 1.25metres and weighing 2.75kg hovering with a head speed of 1750 RPM we get:
| 1. Profile Drag Power = 210 watts (0.28 HP) |
| 2. Induced Power = 90 watts (0.12 HP) |
| 3. Tail Rotor Power = 25 watts (0.03 HP) |
Every 1Metre/second rate of climb will need about a further 25 watts so we need about 325 watts from the engine to hover this helicopter; conversely, a descent will provide about 25 watts for every 1Metre/second rate of fall.
As yet we haven't tied the power to the actual collective pitch. Again, without going into the equations the heli in this example will hover at 1750 RPM at a collective pitch of about 4 degrees. Also, If the collective pitch is reduced to about -4 degrees the rotor speed will be maintained in an autorotation with the power required coming from the falling weight of the helicopter. So at last we have two points that we can use as a guide to the power required at each collective pitch setting. I say 'ideal' because this is only ever going to be a compromise since forward flight will modify the requirements and such things as 'vortex ring effects' also ruin this simple picture.
To complete the picture we need to know something about the engine. According to the manufacturers a typical '30' motor puts out roughly 750 watts (1HP). However the manufacturers usually don't quote the conditions under which they measured the output. I would bet that these figures are often for 30% nitromethane (even nitroglycerine perhaps) and an open exhaust. Anyway, if we knock off a bit for the manufacturers optimism and a bit for losses in the gears etc. we can perhaps rely on 650 watts actually getting to the blades. If we assume that the power output of the motor is proportional to the throttle opening (and that's a pretty big assumption) we can see from these figures is that we are going to need something like one third throttle just to provide the Profile Drag Power needed to get the blades going through the air at the required speed. To provide the induced power for the hover needs about a further sixth of throttle movement taking us to about half throttle at the hover.
So at last we get a likely pitch/throttle 'curve' like this
This gives us a basis for a typical 'normal' pitch/throttle curve that will, in level, upright flight give us a substantially constant head RPM as we climb or descend. There are a variety of other arrangements.
Now, if your a complete beginner who is just at the hopping-about-on-the-ground stage you will have some slightly different priorities and I would suggest you use a somewhat different pitch/throttle arrangement. Why? Well, until you learn the error of your ways, your natural panic reaction will be to shut the throttle sharply. If this also applies say -4 degrees of collective pitch the machine is going to dump itself very firmly onto the ground. In all too many cases this will result in a 'boom strike' (one of the main blades, going at about 200mph, coming into contact with the boom) I would suggest that, initially, you use a pitch range with a low point of say +1 degree. This will reduce the chances of a boom strike until a suitably gentle touch with the throttle can be learned. At the risk of making this sound like a commercial, a few hours spent on a simulator at this stage can save you a lot of hours spent rebuilding your heli. Before progressing to flying circuits its a good idea to get used to having more negative pitch available. If you try flying a circuit without enough negative pitch then extra care will be needed as you lose height since descent is going to be accompanied by a fall in head speed.
Having a reduced head speed at the bottom of the descent will reduce the amount of lift available to arrest the sink and you could just find the ground intervening in the equation. The beginner may also want to restrict the top end of the pitch range somewhat to reduce the maximum rate of climb, however, if you take this to extremes you are in danger of re-inventing the fixed pitch helicopter, complete with the handling! If your Tx allows you could reduce the top of the throttle curve to go with the limited top end pitch.
I guess that's about enough for now. Next time I'll look at alternative pitch/throttle arrangements and tie this in with tail rotor compensation. Unfortunately, I don't think I've been controversial enough this time so let me just leave you with this. So far I have not made any reference to the position of the throttle stick, and that's because, so long as you get the relationship between the throttle and the pitch right, it mattereth not about the stick position. (The distant rumble you can now hear comes from the legions of the 'A.F.' Brigade getting out their quill pens for battle).
Part 4 (Originally published January 1996)
I recently replaced my venerable OS32FH motor in my Concept 30SR with the later model OS32SX. I hasten to add this change was not prompted by the old motor being clapped out. In fact it still looks in great shape after 3 years hard use and I think I would have to beat it to death with a hammer if I wanted to get rid of it!
So far the SX promises to be just as fine a purchase. I have married it up with the Hatori 30HTS-2 tuned pipe and header as I understand Hatori have designed this pipe with the 32SX in mind. I was expecting this combo to require about 5% nitro but so far it has proved to have superb throttling even on straight fuel so for the moment that's what I'm using. I was hoping by now to give you a more detailed report on this motor, however things have conspired against it. Having had a few gentle flights to run it in and get the carb set up I took the machine out to give it a real work-out. This flying session showed quite clearly that the SX has a significant power advantage over its predecessor despite its having the same displacement. This gave me the confidence to bring my tumbles much lower than before in the perhaps misguided belief that the new reserve of power would help me climb out of any trouble I might get in to.
Anyway, I had about four flights in which I pushed my luck more than normal and still had an intact heli at the end. However, immediately after this flying session I discovered a new and novel way to crash a helicopter: put it in the boot (trunk?) of a car and crash the car!! This method has the merit of smashing both the model and the flight box without the need for precision through-the-pits flying! I now have the heli back in one piece, and have removed the 12 volt starter battery from inside the canopy where it was causing a bit of a CG problem. I now find that the PCM receiver suffers from random lock-outs..........
Last time I discussed the ways in which a helicopter uses engine power and applied this to identify a sort of 'ideal' normal pitch/throttle relationship for which the head speed is constant. To obtain this in the descent the throttle is closed progressively as the pitch is reduced (and thus increasing the descent speed) so that by the time the heli is in a full autorotation and the blades are being driven by the falling weight of the helicopter, the engine has been throttled back to a tick-over. Conversely, as pitch is added to climb the throttle is opened to provide the extra power to raise the weight of the helicopter.
Since the throttling of the engine is to some extent an unknown quantity, the only way to fine-tune the pitch/throttle relationship is by flying the machine. Start by getting the head speed how you want it at the hover point. I found an optical tachometer very handy as, to start with, I just could not judge what the head speed was like. I know this kind of 'kit' is expensive (and being mean I made my own tacho) but It all comes back to my Tip Number One For Beginners - Resign Yourself to the Expense (see October's issue). So, if you're starting out, get yourself a tacho, get yourself a good pitch gauge and keep notes on your set-ups so you know how to get things back the same way again after the inevitable 'rotivations'.
Choice of head speed is somewhat a matter of personal taste. Higher head speeds give a crisper response to both collective and cyclic commands but also make the machine more responsive to turbulence as well. Personally, I aim for a head speed of about 1850RPM on my Concept 30SR. 60 size machines tend to be run at somewhat lower head speeds of around 1700RPM. Once the hover point is right a full-power vertical climb seems to be the normal way of establishing the top end pitch. If the revs sag significantly in the climb the top end pitch needs to be reduced while a rise in revs indicates that more pitch can be used. Monitoring the head speed in the descent allows the bottom end pitch to be set. It's not too easy judging this as with the motor throttled back you don't have so much 'din' to go on. If, as you arrest the descent smoothly back to the hover you find you are having to push the throttle a long way beyond the hover point and you also find the nose swinging to the left (to the right for anticlockwise rotation helis) then you know the head speed has dropped and you need more negative pitch at bottom stick.
There are several points to bear in mind in doing the setting up. Firstly, the set-up can only be a compromise. Since the induced power (that's the power used in chucking air at the ground) decreases with increasing forward speed of the helicopter, a set-up perfected for vertical climb and descent will be imperfect for forward flight etc. In forward flight, head speed can be maintained with a greater top end pitch than can be employed in the vertical climb. Also, when the helicopter is manoeuvred, 'g' pulled in turns, and cyclic commands applied, the loading on the engine will vary and ruin our careful set-up. Luckily there are several things that make the set-up less critical.
The drag power (the power used simply to drive the blades through the air) increases with the cube of the rotor speed. This means that a 10% increase in rotor speed increases the drag power by about 30%. If there's an excess of engine power tending to increase the rotor speed this will fairly rapidly get 'soaked up' by the extra drag power as the head revs rise. This helps to limit the variations in head speed from imperfections in the pitch/throttle relationship. We can also help stabilise the head speed by running the engine at, or preferably slightly above, it's peak power revs. If we do this then unloading the engine will cause a rise in revs which in turn will take the engine away from its optimum RPM and reduce its output. This of course helps reduce the amount of overspeed. We are more than somewhat dependant on the helicopter manufacturer to select the gearing of the helicopter so that the engine can be run in the right rev range though we can help ourselves by the choice of exhaust system we use as this allows us to shift the power curve of the engine somewhat. This is perhaps one reason why tuned pipe exhausts, with their peakier power curves, are popular among heli pilots, especially since changing the length of the pipe can change the peak power RPM.
For 'normal' upright flight the beginner has little need for more than the one pitch/throttle curve. Even loops can be performed quite happily using the 'normal' curve. However, once further aerobatics (rolls, stall turns, inverted flight etc.) are being contemplated other pitch/throttle curves will be needed. Even the most basic of helicopter transmitters allow for at least one so called 'idle up'.
The name comes from the facility on early basic heli transmitters in which a switch on the set allowed the minimum throttle opening (with the throttle stick at the idle position) to be increased - hence the name 'Idle Up'. At certain times (such as the inverted bit of a roll for example) it is necessary to bring the collective pitch down to say -3 degrees but if this is done with our 'normal' pitch/throttle relationship it will also chop most of the power as well. The use of 'Idle up' allows more power to be kept on at these reduced pitch values and the head speed to be maintained. Modern computer radios typically allow for two such switch selectable pitch/throttle curves to be selected. In the basic heli transmitters of the pre-computer era the idle up switch only affected the relationship between the throttle channel and the stick position while leaving the collective-to-stick position relationship unchanged. More recent computer sets allow each idle up to have its own collective-to-stick and throttle-to-stick relationships.
Every pilot has his own set-ups but a typical set of idle up curves would perhaps go like this
I used to use a set of idle-ups something like those tabled above but now (under the influence of Bob Johnston*) I use a much simplified arrangement in which all my pitch ranges (including throttle hold) go from about -9 to +9 degrees with 0 degrees at mid stick. I use 'normal' only for starting and switch to idle up 2 (much like the one above) for all flying except for the use of throttle hold. I don't however think you could classify this as a 'typical' set-up! (Ed: I don't know, both myself and Jeremy Morcom fly like that too!) For me, the important aspects of this arrangement are:-
1) I don't get any sudden jumps in the collective pitch as I operate the throttle hold or idle up switches, and
2) the idle up 2 gives me inverted handling that is as near as possible to the upright handling.
A topic very closely linked to the pitch/throttle arrangements is that of tail compensation so next time I'll start look at this aspect of the set-up. Oh yes, before I forget, is anyone interested in a 2-door Jaguar XJ6 ? (both doors on the same side - corners best to the right therefore would suit clockwise rotor flyer)
Colin Mill
* For those who have not met Bob Johnston he is easily identified - if you see a heli doing rolling backwards figures of eight, indoors, under a 35 ft ceiling then just take a look in the pilot's ear. If you see a brain on gimbals going round in there you've just found Bob!
(Parts 3 & 4 Originally published December 1995/January 1996)
Copyright Colin Mill and Lance Electronic Publishing 1997