Intro to multi-blade

[RotaryOne]

Two things I want to do; have an MD500 scale body on decent size mechanics and, secondly, go multiblade, partly because an MD500E does not look right without it.

On the first it looks as though I will have to go the T-Rex 600 route (but a 700 would be really nice - there is an MD500 body for it). Nice to dream.
On the latter I thought perhaps I would get, say, a three bladed head to go on a Swift for starters, since I have the spare mechanics and get up to speed (no pun intended) on that. And get going on multi-blade that way.

However, elsewhere:
　
http://www.scalerchelis.com/phpBB2/viewtopic.php?f=7&t=12118&sid=8a8962f85682dd19834b 2487df31db06
there is the comment that rigid side frames are very important, quote,

'Most of us that have multi heads seem to agree that we like the rigid heads better. but then again we are a bunch of whack jobs. But thats just what we found. Stiff side frames make a good base for every scale heli. I added alum block to the inside of my airstar benzin frames to take out some of the flex.'

Which had me wondering whether starting on multi-blade with PVC frames is such a good idea. Would be great from a practical point of view because I already have the basic mechanics in that format.

Comments and suggestions welcome please.

Nigel

[maderdog_1024]

I'm about to fit a trex 600 into a fuz using gf frames, heliartist 3 blade head and 550mm blades.

Why would the frame stiffness make a difference ?

[Rotorhead]

The reason they want to use rigid frames is due to vibration harmonic frequencies caused by having more blades, in a full scale heli with some elasticity in its structure this can build up and flex them so much that they can distort which will cause large changes to the control inputs to the swash, and in extreme cases damage the heli.

So they have to be careful about what rpms they run different numbers of blades at, and rigid heads make this problem worse potentially.

Its mostly the same thing in models, but in general its better to have as much rigidity in the frames in any heli scale or aerobatic.

[macca221]

Multi blade vibration

Hello from the colonies Having flown twin tripple quad and five blade copters i have come to the conclusion that the 3 blade heads creat the most vibration followed by the five blade wheras the two and four blade heads show no dicernable difference.If you go to the five blade head for the 500 d fuesalage make sure you use scale type blades and you should have no problems wheras the three blade head should be avoided like the plauge as the are imposible to balance properly and the vibration is almost uncontrolable

[RotaryOne]

Many thanks, Rotorhead and macca221 for your really helpful and informative replies.

That clearly settles the point. Will not be using Swift for multi-blade. So the next stage is definitely getting the T-Rex 600E and working up from there.

Many thanks again.

Nigel

[Aidi]

When i ventured down the multiblade route a couple of years ago, to ensure i got balanced blades, i called Midlands, explained what i was doing and they very kindly weighed blades prior to sending so as to get 3 sets as close as possible.

Might be worth a call and see if they would do the same for you?

[RotaryOne]

When i ventured down the multiblade route a couple of years ago, to ensure i got balanced blades, i called Midlands, explained what i was doing and they very kindly weighed blades prior to sending so as to get 3 sets as close as possible.

Might be worth a call and see if they would do the same for you?

Many thanks, Aidi. Excellent thought. Will contact Trevor in due course.

You can also get (although not cheap) a dynamic blade balancer. Given that multi-blade heads readily generate more vibration, and part of that is that the more blades you have the more likely it is that some will not be absolutely balanced as well as others, then this whole aspect of vibration - or rather reducing it - become even more important. So would that piece of kit be worthwhile or, would one just be duplicating what Trevor would do for you?

I get the impression that any vibration (in MBHs) will emphasise the more sensitive handling characteristics of not having the Bell-Hiller stabilisation, so if you are going to fly without stabilisation system on an MBH, and many it seems do, then very precise attention needs to be applied to both the head, blades and even ensureing the mainshaft are all absolutely spot on.

copterdoctor gives an excellent demo of 5 blade head on T-Rex 600 with MD500 body and without a stabilisation system

[ame]http://www.youtube.com/watch?v=unxsvFAM4C8[/ame]

FROM the thread :
http://www.scalerchelis.com/phpBB2/viewtopic.php?f=7&t=10968

Nigel

[antsis]

I am using Century 530mm CF blades on my MD500/Trex600e 5 blade setup. I have run the head at speeds from 1200 to 1650rpm and found no discerable difference in the way it behaves. The most important thing to do, which has been mentioned already, is to ensure the blades are balanced.

[Paulice]

the three blade head should be avoided like the plauge as the are imposible to balance properly and the vibration is almost uncontrolable

I don't agree at all, At one of the places i fly the are model Lama's and Alouette 2, with Nitro and turbine not one has any wobble or un balance at all.
the Lama has a 3 blade main and tail and the Alouette 2 has a 3 blade main and 3 or 2 blade tail.

Balance your blades and you will not have a problem with 3,4,5 blade heads.

M-blade do sets of blades as do Vario helicopters.

[pchristy]

I've only got one multiblade head myself - a 4-blader - but my observations of other scale helis indicates that odd numbers of blades are generally smoother then even numbers.

A multiblade head should always be smoother than a 2-blader - if it isn't then something is wrong!

Odd numbers are more difficult to balance, as each blade has to be made the same. With a 4-blader, two matched pairs are sufficient (the blades at 90 degrees don't need to match each other!)

My 4 blader is turbine smooth using two pairs of "off-the-shelf" SABs. I run a fully articulated head, and the mechanics are standard JR metal frame.

The biggest problem is tracking! This is no longer a one-man operation! Multi-blade heads want to wander in the hover, and don't just sit still on their own! Its difficult to hold them steady AND eyeball the tracking, so you need a spotter!

I ended up adding insulating tape to each blade in turn until we found the odd one out!

You can get some odd gyroscopic precession effects if you make sharp turns, but you shouldn't be doing that with a scale model anyway! On my machine they are perfectly manageable - not vicious in any way - but just something of which you need to be aware!

[maderdog_1024]

3 blades are supposed to cause less vibration than a 2 blade system.

[electricsceadu]

i run a three blade hirobo head and it doesn't vibrate
you will only get bad vibrations if the blades and head aren't balanced correctly

even if you get three pairs of blades the same weight
you still need to blance all the blades as one set

[Cuppa]

I run a heli artist trex 500 with x4 blade head and tail i run a 3G fbl unit. 16kv scorpion motor this baby flys realy smooth, head n tail were from mark at sportsmoto look cool n work well, the blades i run were with the head n tail, flexable no head damping. balance the blades in pairs easy! just finnished fiting nav lights w/strobe anti collision made for me by Craig at pegasus mods thanx mate thay are mega! and thay can be swtich'd no/off from my TX. oh yeh side frames are g/f thay come with the body supply'd, i must get some picks up now i've got the tail flip'd to a left output like a real one doh!, my only critisisum of the heli artist Apache AH64 for T Rex 500( tail output on the wrong side!!)

[RotaryOne]

I am using Century 530mm CF blades on my MD500/Trex600e 5 blade setup. I have run the head at speeds from 1200 to 1650rpm and found no discerable difference in the way it behaves. The most important thing to do, which has been mentioned already, is to ensure the blades are balanced.

Many thanks, antsis, that is very helpful.

Nigel

[RotaryOne]

You can get some odd gyroscopic precession effects if you make sharp turns, but you shouldn't be doing that with a scale model anyway! On my machine they are perfectly manageable - not vicious in any way - but just something of which you need to be aware!

Thanks, Peter, for your very helpful input. I recall our conversation at Charmouth, but the last bit (as quoted above)is particularly interesting, given also, if I remember correctly, that you are not using a stabilisation system.

I get the impression that a lot of pilots 'brought up the old way' do not use stabililsation systems, which leads one to think that one of the best approaches is to learn to fly well in the first place.

Regards

Nigel