One for the scale boys - Multi blade heads

[Sesame]

We were having a chat today at the field and the conversation same around to flybarless and multiblade heads for scale.

first question, if you run a multiblade head do you need to run it on something like a V-bar or G3 unit?

Secondly if for instance you have a 4 blade head, do you run it at half the headspeed of a 2 blade head?(we couldnt decide, the thought was 4 blades give twice the lift???)

Finally if you are running at half the headspeed, do you get twice the duration out of the same lipos(on a leccy model), or does the extra drag of the exta blades negate this?

Im not likely to be using this information at the moment, but was just intrigued by what the answers may be.

[pchristy]

We were having a chat today at the field and the conversation same around to flybarless and multiblade heads for scale.

first question, if you run a multiblade head do you need to run it on something like a V-bar or G3 unit?

Not if you are a *real* pilot!

Secondly if for instance you have a 4 blade head, do you run it at half the headspeed of a 2 blade head?(we couldnt decide, the thought was 4 blades give twice the lift???)

No, because you need to swing smaller blades. The Agusta pictured above uses 50 size mechanics, but 30 size blades.

Finally if you are running at half the headspeed, do you get twice the duration out of the same lipos(on a leccy model), or does the extra drag of the exta blades negate this?

Im not likely to be using this information at the moment, but was just intrigued by what the answers may be.

Most multi-blade heads need to run at a HIGHER head-speed than normal before they "lock".

The JR one fitted to the Agusta is a bit of an exception, being quite stable at normal head-speeds. The gearing on that machine is bog-standard 50 class mechanics.

--
Pete

[Aidi]

First Question: Nope, you do not have to use electronics on the head for multiblades. I flew my 450 with a 5 blade head with no stabalization. Would i recommend it... NO! Its skitty as hell and will bite very quickly from my experience. To really enjoy the flying then i would go electronic.. but i am sure others would say its cheating!!

If FBL electronics had been as cheap as they are now when i had, i would not have sold it!

I found headspeed a bit hit and miss. I cant remember if it was higher HS or lower HS, but when i changed it i would loose tail authority when applying pitch. I think it was when i dropped the HS. I have since heard that a 4 blade tail rotor helps compensate.

On the last point, i would say that you wouldnt see much difference for the exact reasons you stated, plus with a scale heli you would fly well within the limits of the battery due to the cost involved!!!

Hope the small amount of experience i have on this helps somewhat!

Rgds
Aidi

[serpent887]

Whilst the majority of world is watching 22 grown men kick a bag of wind around a park I have great pleasure in taking this time to offer some information .

Heli's fitted with multiblade heads can (and are) flown quite successfully without any form of stabilisation but it is alot easier with one! Maybe you recall my little Augusta 109 which was fitted with a Gaui 365 FBL controller capable of multiblades; also the VBar is equipped to handle the necessary adjustments in phasing and the set-up is quite straightforward.
Although I own an Align 3G, I don't know how the compensation is done there yet but I'm sure Align will tell us when their multiblade heads become available.

The headspeed generally doesn't change although it is unusual to see a scale model with an idle-up headspeed setting! Therefore you should expect such a model flown sedately through the air with a respectable lower headrevs. Also, you may expect to fit longer (or even additional) blades to the tail for some increased authority.

It is a fascinating question and I will look with interest for the others replying to this thread, ultimately scale helis is where I would like to be in the long term

See you soon

[electricsceadu]

be a man
no electronics for me also
2 blade fbl
3 blade (30 size)
4 blade (trex 450)


and the best head to use



one with a flybar



due to the direct connection to the blade grips the head is more responsive to an input
this can be tamed down by rates and expo
its a learning curve to get the head to feel how you want it and it not being over sensitive or to sluggish

and the head speed on the 3 and 4 blade head is higher blade head

[Rotorhead]

Secondly if for instance you have a 4 blade head, do you run it at half the headspeed of a 2 blade head?(we couldnt decide, the thought was 4 blades give twice the lift???)

Lift wise, if you double the lifting area (blade count) the headspeed should be reduced to about 70% of the original headspeed to get the same lift at the same pitch angle, however that may cause a loss of tail rotor effectiveness, so either longer tail blades or more tail blades should compensate for that, or a change in the gearing if possible.

BTW thats going by the lift/drag formulas, that 70ish% comes from needing to multiply V by 0.707 to balance the equasions with the extra blades if all other things remain constant.

F = 0.5 x CL x Air density x Area x (Velocity^2)

For example

6125N = 0.5 x 1 x 1.225 x 1 x (100^2)
6125N = 0.5 x 1 x 1.225 x 2 x (70.7^2)

For the same lift, if area is doubled, CL (pitch/AOA) is the same then you have to reduce V, in practice it may need a bit of tweaking, say for instance the original tip speed was 100m/s if you doubled the blade count, but wanted to hover at the same pitch setting (same CL), your new tip speed should be 70m/s, but like I said it may need a bit of fine tuning.

[alioth]

As an interesting aside, I tried an experiment a few months back with my flybarless 500 (with a GU-365). I tried lowering and lowering the headspeed to see how well/badly it flew (merely by reducing the throttle curve).

I got the throttle curve down to maxing out at 45% power, and that was just about enough power to still be able to hover out of ground effect. I could practically count the blades going round, it sounded like it was doing about 1200 RPM (sounded like a light aircraft's prop at taxi speed). It wasn't exactly responsive (flew like a blancmange) but it did fly, it didn't nod, and the tail rotor had enough authority (presumably due to much lower torque from the motor).

[antsis]

I'm flying a 5 blade head on my Trex600e/MD500 without any electronic assistance except for some mixing in the Tx.

Regarding the HS, I am using 530mm blades and have tried speeds ranging from 1200 to 1700rpm and have not felt any noticable difference in the model's behaviour. Some scale site maintain that the HS should be quite high around the 1700rpm mark, but I have spoke to other experienced pilots who maintain that 1400rpm is a more realistic speed. This makes some sense when you consider the blade area is much more than a conventional two blade setup.

[Rotorhead]

1400 works out to 70% of 2000, which would be normal for a two blade heli, so its not surprising that they suggest that headspeed.

That is, assuming they where talking about going from 2 blades to 4, but even so, if they where talking about 3 or 5 blade heads that wont be too far out either, maybe 100rpm either side.

[pchristy]

The thing is that its not just a question of lift - a far bigger problem is control response.

As has been suggested above, a lot depends on the head design. Some heads are quite stable at normal rpm, but a rigid (or semi-rigid) head will usually have a speed at which it suddenly "locks" in. In most cases this is quite a bit higher than normal, and because of the extra blades, it is difficult to generate enough power to get to the required headspeed. This isn't a problem with turbines, which is why you see quite a few multi-blade turbine set-ups!

The head speed can seem quite alarming - a 4 bladed head appears to be turning twice as fast as a 2-blader, even though they are both doing the same rpm! On the plus side, multi-bladers tend run much smoother!

A few years ago Coolice (IIRC) arrived at Charmouth with a tiny home made 3 or 4 blader for indoor use, and was struggling to get it controllable. I think it was Steve Roberts, who simply cranked the throttle curve right up, and suddenly it just locked and thereafter flew beautifully!

--
Pete