Throttle curves...Why??

Ever since I started flying I've used a throttle curve that follows approximately the the stick position (25% stick, 25% throttle).

While this works 90% of the time, I've noticed that the head speed still varies a lot. I know that because of the rotor/wing loading this will change but it leads me to this question:

In full size helicopters, most notibly the turbine powered ones, the engines are run at a constant speed (near as I can tell from the pilots I've talked to and from what I've seen- I could be wrong though!).

Why can't we do that? Why should we? Or Shouldn't we?

Any thoughts?

[darren_uk]

Hey jhp :-)

Just giving you the info about the full-size helis:

You're right, full size helis operate within a certain engine RPM and rotor RPM range (called "keeping the needles in the green").

I can't remember the JetRanger dials exactly enough, but I can tell you about the Hughes 300 (I've got the tech specs for the JetRanger if you're interested, that is if I haven't bored you to death with the following already!!)

There's one tachometer with two dials: a rotor rpm dial and an engine rpm dial. Always the two are superimposed, unless the engine isn't driving the rotors in which case "the needles are split" (hopefully this will only happen when you're either on the ground starting up or shutting down, or you're in autorotation whilst flying )

There are two green arcs that the rotor rpm (the longer of the two needles) should be kept within. The narrow green arc is for "power applied" (i.e., when the engine is driving the rotors - i.e., when the two needles are superimposed (the short one overlays the long one)). The wider green arc is for autorotation.

Always the rotor rpm must be kept within either of these two green arcs (the narrow one for powered flight, the broader one for autorotations).

The Hughes 300's I've flown don't have governors, so it's up to me to keep the needles in the green. With powered flight, I do this with the throttle. Not as hard as it sounds - once the thing's been set in the hover and then the transition and finally the climb and cruise, it pretty much looks after itself (there's a linkage called a correlator between my collective position and the throttle, so when I raise my collective in a climb, more throttle is fed in; when I lower in a decent, throttle is reduced). However, it isn't perfect, and there's a certain amount of me adjusting the throttle slightly to "keep the needles in the green".

If I'm in autorotation, then I use the collective to control the rotor rpm. Normally it's all the way down. But if I'm flaring or turning, then the rotor rpm will speed up, and if I'm heavy (got a fat passenger for example) then the rotor rpm is at risk of going over the green arc (which is putting too much stress on the blade roots and there's a risk of breakage). So I raise the collective to anticipate the rise in rotor-rpm thus increasing drag, and contain the rotor rpm within the green arc. I then immediately lower the collective again. This last step is important - when you're in auto there's a lot going on (Mayday message, warning the passengers, and flying the thing to the field of your choice whilst at a high decent rate) - so it's very easy to forget all about the raised collective, and before you know it you've dragged the rotor rpm below the green arc which is too low for autorotation, the blades cone up, things are beyond recovery and you plummet to the ground.


ANECDOTE: I was out and about on my own playing around and practicing my field aiming in autos. Anyway, I got lax. I auto'd down to 1000feet and raised the collective to recover, but without putting enough throttle in to bring the engine-rpm back up to rejoin the needles. The rotor rpm needle went south and was approaching the bottom of the green arc (and looked like it was going to overshoot it - i.e., lost too much rotor rpm, the blades will cone up due to insufficent centripetal accelaration, and I fall).

Recover was:
- immediately lower the collective to begin to re-establish auto
- flare the helicopter which has the effect of spinning up the main rotors, i.e. arrest them slowing down
- wind on the throttle to get that engine rpm back up to re-engage the clutch and drive the rotors again.

Recovery was around 600 feet.

(what the normal procedure is to recover from a practive auto, is to roll on the throttle to get the engine driving the rotors again, then raising the collective to recover from the decent. I tried to be clever and roll on the throttle at the same time as raising the collective, and didn't get it right)

TURBINES: They have a governor. It's pretty much turn the throttle to full throttle (called "Flight Idle" for some strange reason) and then just leave it there, with some fine tuning if required via a switch under your thumb to increase or decrease the engine rpm via the governor. Other than that the JetRanger is just point and fly :-) Even the cyclic is hydraulic. <sigh> what a lovely machine.


"Why can't we do that? Why should we? Or Shouldn't we?": We should if we can. The only thing that should be changing is the pitch of the blades. Not the rotor rpm nor (because they're in direct connection via an engaged clutch) the engine rpm. However, the engine and rotor rpms do fluctuate naturally, as long as it's within margins (the green arc if you will).

If things are going too fast we risk:
- overspeeding the engine, or certainly drawing it's active life to an earlier close than we'd prefer
- overspeeding the head: possibly the blades could fly off, or maybe there's hidden damage (fatigued blades or root bolts) that will bite us later on

If things are going too slow we risk:
- The blades coning up (lack of being flung out) which means loss of lift, and the helicopter crashes.

There are governors out there I believe that keep the engine at a constant rpm. I'm not familiar with them though, just heard of them.

[rotorz]

HI dudes.
I use a GV-1 on the vertol it has a fairly low rotor speed and winding up to drive two rotors needs some careful flying with a small engine. It does allow me more precise flying but you do need to remeber to switch it off at landing time otherwise it is not very graceful.
I have tried it for aerobatic flight (GV-1) not the vertol, and because of a slight lag in the control electronics you can hear the rotor over run when you unload it. My preference is to use the idle up throttle options I personally feel more in control of head speed.
Engine size and quality is another issue related to sag for obvious reasons. I have flown a Shuttle & .32 with -10 to +10 degrees of pitch you have to remember to be less wasteful of your available energy particulary when doing silly manouvers close to the ground.

The object is still to get a fairly consistent head speed on my Sceadu 50 it is mostly 1850 rpm for silly stuff and 1350 for general wizzin around.
Happy Landings

You could never bore me with the art and information of full size helicopters! The only thing keeping me grounded is money- I decided I liked helicopters too late in life and couldn't get in the military to do it so now *I* have to pay for all the training.

Ok, more going on than I thought. A governor sounds like the answer I'm after. I know the rpm varies all the time based on the load on the rotors and leaving it at one setting could kill the engine when you unload at altitude (because of wind or what ever).

Hmmmmmmmm

Rotorz- How does that GV-1 work? You can turn it on and off like a heading lock gyro?

[rotorz]

Hi JHP
If I had the do$h I would be doing it in real heli's, I am so envious.
The closest I get is as a passenger, oh well!

Right, There are several makes of governor I have tried and like the GV-1 because of the information it gives. This box of electronics works of a magnet and hall effect switch that sits on the fan so it controls engine speed. This sits between your RX and throttle servo like the gyro for your rudder.

In theory with it switched on you do not need to use any idle up throtle curves as your head speed is set. You can also set it so that it switches off with throttle position for normal flying. There are selectable head speeds via your idle up switch as well.

With no governor fitted, in idle up two position I run 100 - 60 -100 throttle and +11 to -11 degrees of pitch.

Have a look on the Futaba or Ripmax site you can download manuals for those two devices.

Happy landings

[colin]

My normal pitch curve is quite linear, except for the 50% point where it is slightly flat. This is to give me a less twitchy heli at the hover point.

For my idle up settings, I find that the hardest pitch/throttle setting is when the pitch of the blades is at 0 deg. This effectively gives no wind resistance, and the engine noticibly races. My throttle setting at 0 deg is around 25%, but the engine still races. I wonder what the rest of you have for idle up settings?