Chinese Weights

[trillian]

That's really cool! He setup a very good rig for measuring everything and it pretty well proves the theory but you can also get an idea how much weight it takes and where it's mounted to make any big difference.

[GravityKills]

you'll be surprised how much force the blade grips alone generate, with all blades removed.
It's an experiment you can do easily in 10 minutes. It was a real eye-opener for me.
This explains a hundred thousand "help-my-tail-is-all-over-the-place" posts: overloaded tail servo, simple as that.

[J-S-Q]

It's pretty surprising how few helis come with chinese weights built in seeing as the theory has been well documented for a very long time. The little know SJM brand have had them in their helis for ages but the likes of Align are only recently onto this one and it doesn't even look like they fully understand how to implement it judging by that video!

[unwind-protect]

You'd have to take a tennis racket in a vacuum and spin it by a point exactly in the middle to see if there is any 'tennis racket effect'

It's pretty hard to separate the two experimentally, short of running it in a vacuum chamber!

I have experimented with Chinese weights on my tail blade grips. There's a surprising amount of centering force generated purely by the blade grips themselves. It's also very easy to "overload" the weight so that the tennis racket effect force pushes the pitch assembly away from centre to the extremes of travel, even with the blades in place. It seems to me that the tennis racket effect is probably the dominant force.

[trillian]

It seems to me that the tennis racket effect is probably the dominant force.

Well, if you have a tailrotor with a circumference of 30 cm (roughly one foot). At 19,000 rpm that's 316 feet per second the blade tip is travelling. That's over 215 miles per hour. So the air resistance will be a significant force.

Basically the minimum air resistance is equal to the energy required to counteract the rotor torque plus any movement against the direction of the torque and minus any movement with the torque and plus the efficiency lost in the process.

[unwind-protect]

Well, if you have a tailrotor with a circumference of 30 cm (roughly one foot). At 19,000 rpm that's 316 feet per second the blade tip is travelling. That's over 215 miles per hour. So the air resistance will be a significant force.

Well, maybe, but ISTM that air resistance isn't necessarily applying force to the pushrod.

I was pretty incredulous that alloy blade grips alone could create any centering force. But I tried it and was surprised. If you have a 450 handy take off the head and give it a go. IIRC (quite possibly I don't) adding the blades didn't actually make a huge difference.

[trillian]

I have just done an improvised weight mod on my GL450. I just used longer screws and a ball link sandwiched between bolts. It's only on one side because there is limited clearance on the side facing the boom.

So far I have just hovered it a bit and the first thing was it had a wag, so I reduced the gain from 78 to 75% and that seems pretty good so far. (It's just a cheapy oem type Align gyro so that's probably the limiting factor).

In any case it requires less gain now, so I take that as a positive.

[acwman]

In any case it requires less gain now, so I take that as a positive.

I think you should find that the servo should have less load on it, and you should be able to increase the gain a few points, rather than having to reduce it it find the benefits of the mod.

[moyesboy]

Well, the benefit of the chinese weights cold go either way. If he servo was running out of puff then reducing the load on it could mean the gyor now moves the servo more for the same output, which increases the gain mechanically.

When the grip is at zero degrees its centre of mass is furthest from the tail shaft. This is because of the position of the redius of gyration of the grip.
Adding the chinese weights makes is so the centre of mass is the same distance from the shaft whatever the incidence angle the grip is set to.
The aerodynamic stability of the tail blades is another thing again. Tail blades seem to be made very stable - try spinning up with the links off - they are very happy to stay at zero pitch. try putting them on backwards and its a different story.
I found fitting the blades backwards (accidentally) on my 600n produced some interesting results and required vastly lower gyro gain but performance was actually fairly good and not noticably worse than with them on the right way round. However given the chance of things failing it would be better to have a failure to a stable zero pitch result than to rely on the servo and linkage working rather hard to keep the blades from going to match pitch ... !

[GravityKills]

in my experience, I have to reduce the gyro gain with chinese weights.
It makes sense to me, since the force opposes servo movement (pulls it back to center). The force is reduced, therefore the servo has more authority.

[acwman]

in my experience, I have to reduce the gyro gain with chinese weights.
It makes sense to me, since the force opposes servo movement (pulls it back to center). The force is reduced, therefore the servo has more authority.

..Maybe I'm wrong.

I did experiment a while back.. but trying to recall from memory what I did with the gain.

Getting old, and my memory just ain't what it used to be

[trillian]

Well, this is not a great gyro to begin with, so it's hard to say what something a bit better would do in the same situation. But basically I can tell from what I have seen in other Chinese weights that I am not over-doing it so even if it just partially counteracts the blade grip inertia then it should be a positive result.