Everyone faces this question from time to time. Like most questions about RC helicopter setup, there is more than one correct answer. I hope this document will help you find a head speed that makes you happy.

First, a word about safe operating limits. Generally speaking, you should not exceed 2000 RPM with a 30-class helicopter, or 1900 RPM with a 60-class helicopter. There are exceptions to every rule, but generally speaking you'll be better off if you stay within these guidelines. When a rotor head fails, the entire helicopter is usually destroyed. Not just damaged, but destroyed beyond repair. In addition, many pieces typically fly in many directions, posing a safety hazard for everyone within 50 yards or more.

Next, a word about exhaust systems. If you're using a tuned pipe, your engine will probably only be happy within a narrow RPM range. This complicates things a bit. If you're using any other sort of muffler, you can probably run the engine happily over a wipe RPM range.

Some answers:

The short answer, if you're running a muffler: If you're using the same size engine your helicopter was designed for, it doesn't matter what heli or blades or fuel or whatever... go with 1700 for starters. Really. I can just about guarantee that 1700 RPM will work just fine.

If you get much slower, the engine is probably out of its powerband. If you get much faster and you may not be able to pull enough pitch to climb reasonably fast without losing head speed. 1700 is a safe place to be.

The short answer, if you're running a tuned pipe: there is no short answer. You just have to find the right RPM range by trial and error, or by comparing notes with someone who is already using the same engine, tuned pipe, tuned pipe length, etc, etc.

The short answer, if you're running a monster motor: If you've done something creative like stuffing an OS 50 into a Raptor 30, or YS80/OS90 into anything at all, then I don't know the "short answer" to this question. Go find someone using the same engine and gear ratio you're using, and see what they're doing. If you're running a monster motor and a tuned pipe, I salute you, but I can't help you. If you find something that works, do let me know, that sounds like a fun combination.

Medium answer for muffler systems: It depends on what kind of flying you do.

For precise hovering or scale flying, 1500 is a good place to start.
For cruising around, the usual range is 1600-1750.
For aggressive aerobatics, the usual range is 1750-1850 for 60s and 1850 to 2000 for 30s.

Medium answer for tuned pipes: You've got to find the RPM at which the tuned pipe resonates with the motor.

As you raise the throttle slowly at the start of a flight, you will probably notice a point at which the motor gains speed faster than you raise the stick - and it might even continue gaining speed if you hold the stick still or even back down a hair. This is happening because the pipe tuning is starting to increase the motor's power. When the speed stabilizes, you have found the lowest speed at which the engine and pipe are likely to run cheerfully. Add around 10% to this RPM and the result is probably a good target head speed.

If you get a lot of overspeeding while flying around, that's the pipe's way to telling you that you're trying to run too slowly - step up the RPM a bit. If you get a lot of underspeeding, you're probably either loading the engine way too much, or you're trying to run it too fast. Try changing your flying style and/or or reducing your rotor speed.

The long answer:

There are a few things to ponder when choosing a rotor speed. They are presented here in no particular order...

1. Lower head speeds typically make for a less responsive helicopter. The collective, cyclic, and tail rotor controls will feel softer. The roll rate will be reduced. This is preferable - to a point - for precise hovering, especially for beginners and contest flyers.
2. Higher head speeds typically make for more a responsive helicopter in every way - more responsive collective, cyclic, and tail rotor control. This is preferable - to a point - for aerobatic flight.
3. Generally speaking, a higher head speed will require a smaller collective pitch range. Conversely, a lower head speed will allow a greater pitch range. For example, full throttle might allow you to pull 9 degrees at 1750 RPM, or 10 degrees at 1600.
4. Higher head speeds waste power overcoming drag. Though you can generate the same lift with less collective pitch, it will typically take more power to generate the same amount of lift at a higher head speed. The 10@1600 setup will probably climb faster than the 9@1750 setup.
5. In some cases, a higher head speed may allow the engine to generate more power, by virtue of the higher crankshaft speed and and nature of the engine's power curve. In some cases, the increase in engine power may be greater than the increase in drag, allowing for a faster rate climb and higher top speed. In other cases, the increase in power may be smaller than the increase in drag, resulting in poorer performance.

The final point is most important because it's the least obvious. If engine power were the same across the RPM range, picking a head speed would be a simple matter of choosing a tradeoff between faster cyclic response (high RPM, lower allowable collective pitch) and faster top speed (lower RPM, greater allowable collective pitch).

But alas, it's not that simple. This point was most clearly illustrated when during the first flights on a YS 61 ST2 that I just purchased. With a hovering head speed of only 1450 RPM, I was only able to pull about 8 degrees of collective pitch, and even then the engine was struggling and the rotor was slowing into neighborhood of 1400 RPM. After raising the hovering RPM to 1650, I found that the engine was able to carry more than 8 degrees of collective pitch without slowing down.

How can you tell if the rotor is staying at a constant speed if the heli is in flight?

By ear, mostly. Keeping the doppler effect in mind, if it sounds constant, it's close enough. At least, that works well enough for me. :-) Initially, I think it might be best to have someone else watch and listen when you're flying, so you can concentrate on flying and they can concentrate on listening.

If you're picky and you have access to a tach, you can have someone watch your head speed in different situations, like hover, fast forward flight or vertical climb, inverted hover and climb, knife-edge stall. I've done this a couple times, it's good for a little fine tuning.

You'll always lose speed if you push it hard enough, especially when you add cyclic stuff to the picture, but IMO if you have the same head speed in a hover and fast forward flight you're 80% of the way there.

What if you don't have a tach?

Set up your heli so you hover at 4-5 degrees, and make sure you have 100% throttle at full stick. Make the pitch curve linear - something like -5 to +2.5 to +10, or -10, 0, +10. Listen to the head speed. Go to full stick (max throttle and collective). If it's slower when it's climbing, reduce your top-end pitch. If it's faster when it's climbing, increase your top-end pitch.

Having chosen and dialed in your head speed with climbout tests, adjust your hovering throttle percentages until you can hover with the same headspeed as your climbouts. From a hover, go to full stick. Did the head speed seem to change? If so, raise or lower your hovering head speed with the throttle curve - remember, you're done with the pitch curve now.

You will get some overspeeding when you descend "elevator-style" with the rotor level. In a flight mode used mostly for upright flight (e.g. the -5, 0, +10 setup I mentioned earlier), you can tune this out by reducing the throttle values at or near low stick. If you're using a "3D" flight mode (e.g. -10, 0, +10), so far as I can tell, a governor or a well-tuned exhaust is the only way to solve this without creating bigger problems. A governor will help by backing off on the throttle as the engine attempts to overspeed. A tuned pipe will help if your head speed is set near the top end of the pipe's tuned RPM, because increases in head speed will cause a decrease in horsepower.