Head Speed

[Rotorhead]

It is a square law, you square your speed, but I was just trying to make it clear how that law works, by giving an example where the speed is doubled.

Like I said, if there is a 2N force at speed x, then at speed 2x there will be a 8N force.

The point is it doesnt matter what x is, what ever it is, when you double it you will get 4x the original drag force.

[Rotorhead]

Play about with that, put any number you want in as the force and just double the speed, you will always get 4x what you put in originally.

Most people would see that if you chose to more than double it then it would more than quadruple (3x speed would work out to 9x the drag force), but I was trying to keep the example clear and easy as can be, and you cant get clearer than an example like; double the speed 4x the drag.

Its a perfect and clear example of why you need constantly more power to make much smaller increases in speed as the speed regieme goes up.

[scallybert]

Well, as a beginner, I'm running a comparatively high head speed on my 500, 100% throttle curve on a 13t pinion. It certainly seems less twitchy than lower head speeds to me. Is it possible that there is a gyroscope effect, and the higher the headspeed, the more this effect shows?

AFAIK, the gyroscopic effects are limited to precession in both the flybar and the rotor blades.

To clarify what I mean by 'precession':
Assume you have a rotor rotating around a vertical axis;
You apply a torque the rotor perpendicular to the axis;
the rotating rotor starts to rotate about a 3rd axis perpndicular to the other two.

In the context of a heli, if you apply a cyclic input that makes the r/h tip pitch up, and the l/h pitch down; the rotor disk will rotate so the front goes down and the rear comes up. (Except there are going to be other factors like the heli & main shaft applying other torques to the rotor.)

That aside, this broadly explains why there's a 90 deg phase difference incorporated int the controls.

So what ? Well, if you pull an equation for precession off Wikipedia (!) Precession - Wikipedia, the free encyclopedia ('Classical (Newtonian)')

you can make some guesses.

What this says is that the rate at which the rota disk tips (eg front goes down, rear up) is given by the torque (from the cyclic input) divided by the Moment of Inertia of the rota (sort of how heavy the blades are) divided by the head speed.

Before you jump to thinking that this means 'twitchiness' decreases with head speed, bear in mind that the torque from the cyclic input will go up with head speed due to increased aerodynamic effect. My aerodynamics are not good, but I think it will increase with head speed squared.

So you have head speed **2 divided by head speed -> so on this basis higher head speed gives more responsiveness. Which is what we've tended to see.

So why might the heli seem better behaved at higher head speeds ?

My guess is that the fly bar get to be more effective. Maybe friction at the linkages becomes less significant. maybe it's a bit like winding up the gain.

Both of these could make the stabilisation from the fly bar 'tighter'.

Which supports the view that in general a high head speed is good. If that makes it too twitchy, you can calm that down trhough cyclic expo or rate.

[Rotorhead]

Correct.

I think whats happening with regards to the heli being better behaved is that while its true that inputs you give the heli are amplified more (heli is more responsive and aerobatic) the flybar and paddles get a bigger responce when they are disturbed from their resting plain (perpendicular to the mast)

Really all you're doing when you apply a control input is you are adjusting that plain, so its no longer at 90 degrees to the mast, that then adjusts all the cyclic blade angles and that causes the heli to roll or pitch its nose up.

More speed gives more responce to your input, but also more responce to any input that you dont put in but wind and turbulance does, the only difference is your input makes the heli want to roll and pitch, whereas the inputs that are caused by the wind and the turbulence are mixed in in the opposite way and counter any unwanted roll or pitch motion.

Also you may notice with some helis that if say you are pointing the heli so that there is a side wind, the heli may want to pull its nose up or push it down, thats also an effect of the precession on the rotor head.

[Brian Gifford]

AFAIK, the gyroscopic effects are limited to precession in both the flybar and the rotor blades.

To clarify what I mean by 'precession':
Assume you have a rotor rotating around a vertical axis;
You apply a torque the rotor perpendicular to the axis;
the rotating rotor starts to rotate about a 3rd axis perpndicular to the other two.

In the context of a heli, if you apply a cyclic input that makes the r/h tip pitch up, and the l/h pitch down; the rotor disk will rotate so the front goes down and the rear comes up. (Except there are going to be other factors like the heli & main shaft applying other torques to the rotor.)

That aside, this broadly explains why there's a 90 deg phase difference incorporated int the controls.

So what ? Well, if you pull an equation for precession off Wikipedia (!) Precession - Wikipedia, the free encyclopedia ('Classical (Newtonian)')

you can make some guesses.

What this says is that the rate at which the rota disk tips (eg front goes down, rear up) is given by the torque (from the cyclic input) divided by the Moment of Inertia of the rota (sort of how heavy the blades are) divided by the head speed.

Before you jump to thinking that this means 'twitchiness' decreases with head speed, bear in mind that the torque from the cyclic input will go up with head speed due to increased aerodynamic effect. My aerodynamics are not good, but I think it will increase with head speed squared.

So you have head speed **2 divided by head speed -> so on this basis higher head speed gives more responsiveness. Which is what we've tended to see.

So why might the heli seem better behaved at higher head speeds ?

My guess is that the fly bar get to be more effective. Maybe friction at the linkages becomes less significant. maybe it's a bit like winding up the gain.

Both of these could make the stabilisation from the fly bar 'tighter'.

Which supports the view that in general a high head speed is good. If that makes it too twitchy, you can calm that down trhough cyclic expo or rate.

Thanks, I think I understand that (possibly)

[swampy]

my head now 'urts but thanks fella's, most interesting

[scallybert]

The more the better...

So, with a couple of provisos, the conclusion I'm heading towards is that, even for beginners, more head speed is better.

The provisos being that if you overdo it, you'll over stress components, and may also reduce flight time (battery capacity, rather than mid-air disintegration...); and also that you may need to adjust collective & cyclic set-up [0] to compensate for the model being more responsive.

In the case of a G5, the manual recommended '3D' 2450 RPM seems fine for basic flying - though I guess there may be a case for building up to that on the very first few flights. (I did.)

[0] eg elev & aile expo; calmed down pitch curve.

My guess that the conventional wisdom of beginners running lower head speeds maybe dates back to times before programmable Txs with expo, etc. Maybe in those days headspeed was the only or main parameter for calming things down.

It may also be worth pointing out that a G5 has a programmable head, allowing the mixing of flybar and 'direct' input to be adjusted. I'm running a (relatively) high head speed with the 'beginner' mix that gives the flybar most authority. This may skew my view of things. I also have flybar weights on my little GAUIs.

[Brian Gifford]

This is a fascinating thread, thank you to all the contributors.

Now, while I can set up my head correctly (I hope!), I don't really understand how the head works - how the flybar calms everything down - if it does?

Is there anyone who fancies having a go at explaining all this in simple language?

[scallybert]

Now, while I can set up my head correctly (I hope!), I don't really understand how the head works - how the flybar calms everything down - if it does?

The flybar is similar to a gyro in a gymbal, in that it will tend to maintain it's orienattion, unless a torque is applied to make it precess.

That torque would come from the flybar paddles.

Assuming there's no input, the flybar 'disc' will pretty much stay horizontal - assuming it started there.

If you hold your fly bar horizontal, and move the heli underneath it, so the flybar seesaws (but the heli is moving not the flybar), the pitch of the main blades will change.

This pitch change will be like a cyclic input that acts to bring the heli level with the flybar.

So the flybar is like a mechanical gyro stabiliser in the pitch and roll axes.

Early Bell helicopters (eg UH-1) had a much simpler flybar with no paddles.

There are several in the Museum of Army Flying, you may also be able to see it in picture.

[swampy]

I'm guessing it would be part gyroscopic/part airfoil of the paddles ? that resisted pitch change in the main blades and calmed down any control input ? the faster the headspeed, the more aurthourity the paddles would have ?

[swampy]

that was a guess btw, for me anyways

[scallybert]

I'm guessing it would be part gyroscopic/part airfoil of the paddles ? that resisted pitch change in the main blades and calmed down any control input ?

My understanding (or guess...) is that the stabilising comes from the flybar acting like a gyro.

The flybar paddles are to 'steer' the flybar. eg so it's not trying to keep the heli level when you're trying to bank/dive/roll/etc.

Here's a link to a Bell UH-1 http://military.discovery.com/techno...ey-625x450.jpg . See the flybar with no paddles ?

[swampy]

I'm finding all this facinating, all the factors that work in unison to make the little buggers fly, so am I on the right track in thinking that the faster the headspeed, the faster the flybar is moving, the more torque the flybar produces to precess the disc Bert ?

[scallybert]

so am I on the right track in thinking that the faster the headspeed, the faster the flybar is moving, the more torque the flybar produces to precess the disc Bert ?

The faster the flybar rotates (headspeed), the greater the aerodynamic force from the paddles, and the faster it precesses. (See previous. Control responsiveness increases with headspeed.)

However, looking at the torque the flybar poduces is probably a red herring.

What you're interested in is the position of the flybar as this is what is mixed to affect the main blade pitch.

[You can see this on the G5 head. The (beginner) mixer position to increased the flybar effect means the main blade pitch changes more for a given flybar movement. A side effect is this means *less* force is provided. But it still works - so posiiton is the issue, not force.]

However, the systems are imperfect. Friction, etc will act to mess things up. So in that respect more 'good' forces are likely to make things better.

My argument is that higher head speed is good for stability partly because it increases the 'good' forces; and also because it increases the effective gain of the flybar. (Like increased tail speed effectively increases the tail gyro gain.)