My experience is from the scale and F3C viewpoint, so for 3D, there maybe some variations.

Typically when running 3, 4, 5 blades, you would use shorter mains, but longer tails compared to a 2 blade setup. For example, on my MD500 with 600 mechanics, I was running a 5 blade head with 530mm blades, but instead of the usual 86mm tails, I ran 105mm tails. Typical HS was 1500rpm and flight duration with a 6S 5000mA lipos was still 6-7mins. This seem to be the normal situation with the scale models, but I do know some guys who are flying 700 size scale models with 3 blades and using 700mm blades. On my Sylphide I have the 3 blade head and 720mm blades, but have to use 110-115mm tails.

Running the extra blade does mean you can run a lower HS. The F3C pilots are typically hovering at 1300rpm, (compared to 1400-1500rpm), although the current champ testing the new SAB urukay is going as low as 1100rpm. On the other end of the scale, for aerobatics, the typical HS is around 1600-1800rpm again this is for F3C. Gearing will be a factor in order to drop the HS, although this may not be so critical for 3D. I have found the 3 blade head can be more pitch sensitive with only a 50rpm change which is not good for F3C, but ok for 3D.

Although the extra blade will give you extra lift, it will also produce extra drag. Adding more blades will increase the drag to a point where it is fine for scale, but may not necessarily be ok for 3D. I have found the 3 blade head on the Sylphide to be smoother in aerobatics than my Quest with the conventional flybarred head, but there is also the FBL controller to consider.

As for design changes. I think if you went with a shorter boom, you will need to increase the length of the tail blades. Obviously there is a limit to this, so keeping the current boom length or going longer would allow shorter tails.

 

Interestingly, when you quote the drag will slow the speed, does that mean forward movement or colloective movement. Forward speed works well for hurricanes, big loops etc but alot of hard smak within a confined space may do better with more collective pop. Thats what i can see when i watch these heilicopters fly, very much like what you are describing and i think it would be a good change for smak..... But there are always going to be tradeoffs....

 

I did not actually say the extra blade would slow the speed, (although I must admit my Sylphide does not seem as fast as my Quest in FF). It would be able to provide extra lift at a lower headspeed. Adding additional blades increases the drag and loading. For example, going from a 2 x 700mm blade setup to a 4 x 700mm blade setup, the drag would double. However, your lifting capacity would increase, but to run at the same HS with the same batteries would reduce flight times. Ways to reduce this are to use shorter blades, lower the HS and use a smaller pitch range. For optimum efficiency when using a multiblade head, you really want to run semi-symmetrical blades with a flat bottomed section. Of course, this is no good for aerobatics.
There are loads of discussions all over the web on this subject.

 

Thanks @Ant, I think its a whole new ear we may be moving into. Looking at how a multibladed heli pops and hangs, I would think they could be great for aerobatics. Imagine a 3 bladed heli system running at the same RPM as existing 2 bladed systems! the amount of pop would be incredible!

 

You mean the amount of 'pop' that the esc will make from having to drive all of that extra mass!


Sent from my Electronic Appendage

 

Another factor to consider is blade weight. For scale, I have taken sets of normal blades and checked the weight and COG to get the best match. When doing this for a 5 blade head, it amounts to a lot of checking. The weight issue is not so critical for scale and you are going to run a lower HS, so the pay off is similar flight times. With the popularity of FBL and multiheads, blades are being redesigned to work better. We know that FB blades will work in a FBL environment, but not as well as FBL blades. The same applies to multiblade setups. Watching Nick Maxwell with his E700, it looks like his 3 blade head uses the same length blades (as a 2 blade FBL), but he mentions that they are lighter than normal blades.