Meaning of power in Helicopter Control (SLF question)

[ScienceDoc]

Hi,

While not being a pilot I wondered how a helicpoter is controlled.
More precisely I wondered how important power changes are in controlling a helicopter.

Do you set the power to a certain level and than make your control inputs by only changing the blade angles? Or do you also change the power to decrease or increase lift by changing the blade RPMs?

The same question applies to prop driven aircrafts, where the blade pitch can be changed: Do you change the power or the blade pitch in order to change the thrust? And why do you do so?

SD.

[FlyingForFun]

And since you also asked about aeroplanes:

The most common situation in light aeroplanes is to have two levers, one for the throttle, and one for the propellor.

The throttle lever directly controls a throttle butterfly just as it does in a car engine, or in almost any other piston engine.

The prop lever controls the RPM. a Constant Speed Unit (CSU) is used to automatically adjust the blades to an angle which gives the appropriate RPM. A change in either power or airspeed, both of which would change the RPM if the pitch of the prop blades was fixed, instead causes the CSU to adjust the angle of the blades to maintain constant RPM.

In general, an increase in power is accompanied by an increase in RPM and vice versa. In this case, when increasing power, the prop RPM is increased first, then the power. For a decrease, the power is decreased first, then the RPM. This sequence ensures that the engine is not damaged by applying high power at low RPM. (Imagine flooring the accelerator in your car, in top gear, at 10mph.)

There are other setups, though. In the aircraft I fly, the pilot directly controls the pitch of the prop blades, there is no CSU to do this for you. Although there is no problem with applying too much power at low RPMs with this setup (the engine/prop combination has been designed to not break even at full power and full coarse), there is a danger of overspeeding the prop at high power, moderate-to-high airspeed, and with the prop blades set to fine, so this is the main area you need to think about, especially at the top of the climb.

Many turboprops use just one lever, which sets the engine power directly. I believe that these aircraft will use a CSU to maintain a sensible RPM based on the position of this lever.

Hope that's some help.

FFF
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[simfly]

What Chuck says is correct, but just to addd...

ROTOR RPM is the constant, however the more collective pitch you pull the more ENGINE RPM is required to overcome the drag of the increased pitch. This is mostly in the hover, as with forward airspeed (or reverse & sideways if fast enough :eek: ) you get "transational lift" which alters air flow angles (too tired to go into more detail, yawn) and increases lift, therefore less engine power required........

[ScienceDoc]

Thank you very much for the replies.

[teeteringhead]

Up to a point Chuck Yeager ....

what you say is true for piston engined helicopters, in the power on clutch engaged mode.

Turbine powered helos have no solid connection between gas generator (the engine) and the power turbine and rotor. So engine rpm (ggrpm, N1, Ng) has no fixed relationship to rotor rpm or Nr, and will change with power demands....

I guess that's got sciencedoc really confused now...

[ScienceDoc]

Yes, some kind of confused. ;-)

Ok, let's assume that we have a turbine engine driven helicopter or plane.

a) turbine powered helicopter

I want to take off. I open the throttle to let's say 85 %, wait until all the nice engine gauges are ok. Then I change my collective until enough lift is created for takeoff. Now the helicopter takes off and as I am lazy and the engine is providing enough power at the 85% setting, I don't care about adjusting power to get a constant RPM (RPM should decrease due to increased drag, right?). Now I could in principle fly around without changing the power settings, right?

b) turbine powered plane

My nice little imaginery turbine powered plane is having a lever which changes the pitch of the blades. Now could I do the same as above?

c) something piston powered

I understand that in piston engines torque and RPMs are related in a certain way. So I cannot just open the throttles and only change the blade angles, because the engine wouldn't simply be able to output constant RPMs, right? So changing pitch does only provide an easy way for an efficient and machinery friendly engine operation, right?

Some additional thoughts about piston engines:

So wouldn't the ideal piston powered helicopter or plane not have a lever which simply gives us the lift we want to have? You move the throttle to "1 kN lift" and the engine computer looks into a table what RPM and torque is required and simply changes RPM and pitch/collective angle?

[sycamore]

SD, what has been posted so far by others is all good stuff depending on how tech you are. A website which may also help is
www. howstuffworks.com, and look for "transportation"
There are a few points you should note;

Helicopter rotor RPM is kept constant because rotor tip speed can be high, ie around 7-800 fpsec, and at high can reach near critical Mach numbers; this would lead to high stresses and loss of control., hence the need to keep the rotor "governed", usually mechanically in piston engines, and hydro/electronic on jet powered.

Depending on the type of gas turbine engine employed, it can be a free-turbine, or a fixed-shaft turbine.
The former employs a separate turbine next to the engine`s turbine, and this drives the rotor thru` the gearboxes,etc.This turbine does all the "governing" bit, as it immediately senses a change in power-demand from the rotor system, depending on what collective/yaw/cyclic demands are made by the pilot.Now, either electronically or hydromechanically it sends a signal to the engine to speed up, or slow down.Thus the engine itself can accelerate/decel rapidly to give the necessary power output.

The fixed -shaft turbine is different in that the engine and turbines are one unit, linked through the transmission by a clutch, like the piston one ,like your car. So the engines( in both cases, f/t& f/s) can be started and then accelerated to engage the rotors to their governed RRPM. The f/s turbine will, however be running at about 98-99% rpm. When a power demand is made, fuel -flow is increased to maintain rpm by the governor which will maintain RRPM at 99-100%

In the f/t the engine rpm would have increased/dec.following the demands.

The same is true of turbo-prop a/c ; they can have f/t or f/s engin/ prop combinations.

f/s= fixed shaft turbine
f/t =free-turbine

[ScienceDoc]

Ok, thanks guys. I got it. ;-)

[rotornut]

Hey Chuck,

Have you ever heard of "beeping up" RPM? Have you actually flown a 206?

[EMSAV8R]

What I love about pilots is they think like I do. What I hate about pilots is they think like I do. Concerning copter pilots... what I really hate about this crowd (myself included) is how very little financial compensation motivates us to pursue this career as a living!:{