Main blade lengths: Pros and cons?

[jhamel]

I understand that many helicopters can accept a range of blades lengths. 700 size helis can generally accept around 690-710mm blades. 600 size helis can generally accept 600-620mm blades. 500 size helis can generally accept 500-520mm blades.

How do you go about choosing the correct blade length? Does it depend on flying style? Something else?

[Jmann841]

I'll give it a stab...

There is a lot that goes into blade performance. Length is only one contributing factor. Blade length will affect your disk loading which typically gives you a floaty or heavy feel.

Blade chord, rigidity, shape, etc all effect different things as well. SAB Blades are known to be thin chord which will give you a specific feel vs. cyclone blades which have a tapered chord and will feel differently.

I am no expert, but, when it comes to blades there are so many contributing factors, you really have to try it for yourself and see how your machine handles it and how it fits your flying style.

Blade length is typically fairly predictable in terms of floaty vs. heavy feel but some people don't want a floaty heli as they want high headspeed and tons of power.

[jhamel]

Originally Posted by Jmann841

...you really have to try it for yourself and see how your machine handles it and how it fits your flying style.

That's the thing. At $120-$140 per set of 700 size blades, it is not very economical to compare and contrast by merely trial and error. I believe there should be a more scientific method to anticipate what sort of performance one would get from different blades.

[Icanfly]

simply, as long as possible without hitting tail blades. Shorter blades will be more responsive, longer ones more difficult to throttle through higher and lower rpm. Shadowing the tails will cause a loss of tail authority too. Longer ones hold momentum and are an increase in effective area thus more "floaty".

[Jmann841]

Originally Posted by curmudgeon

That's the thing. At $120-$140 per set of 700 size blades, it is not very economical to compare and contrast by merely trial and error. I believe there should be a more scientific method to anticipate what sort of performance one would get from different blades.

The best way is to ask for reviews from other users than, so much of blade "feel" in this hobby is subjective all on it's own as well.

[jhamel]

How about different blade lengths from the same company? Rail blades come in 696mm and 716mm lengths. Why choose one over the other?

[Jmann841]

Originally Posted by curmudgeon

How about different blade lengths from the same company? Rail blades come in 696mm and 716mm lengths. Why choose one over the other?

Disk loading. Larger will feel more floaty, smaller will be quicker on cyclic and less floaty. Depends on your flying style and headspeed.

Generally higher headspeed and smack 3d will prefer less floaty and low headspeed mild 3d and sport will prefer more floaty.

Also depends on the weight of the machine. A lighter 700 may perform better with smaller blades where a heavier more powerful 700 will do better with larger blades

[Icanfly]

feeling compelled to add something else to this it must be said longer blades exert more gyroscopic effect on a heli as well as needing more power to maintain head speed through high stall type maneuvers. They have more area therefor feel "floaty"and the gyroscopic effect means the longer rotor is slower to swing through cyclic changes.

Try a set of 600s on a 700 machine, lots of guys have gone from 600 to 700 blades on an extended 600 heli, rpm loss due to drag becomes a matter of upping the motor hp or living with the limitations. You theoretically could mount 700 blades on a 550 but without a good hp motor flight will be much more tame. Many chassis are built with motor and batteries in mind. Here's where proper gear ratios are quintessentially important to obtain the highest performance level in every aspect of the heli. Shorter blades, lower gearing = high head speed. Higher gear ratio with longer blades means same motor rpm and slower rotor rpm but the motor will optimally be at it's highest hp/efficiency level to power through high drag/stall events.

I have a 300 which at one point had 220mm blades down from 250, it flew the best in terms of racy sporty flight with almost instant cyclic response. When I got a new set of 250mm blades the rotor speed slowed down but the heli had more lift, so the longer blades ate more juice while they added lift. It was a trade off where battery times were almost identical. The shorter blades could never auto rotate, neither do with so little blade area. What is learned on small things grafts up to larger things.

I hope that simplifies everything you ever wanted to know about blade length, lol, haha.

[Jmann841]

Originally Posted by Icanfly

feeling compelled to add something else to this it must be said longer blades exert more gyroscopic effect on a heli as well as needing more power to maintain head speed through high stall type maneuvers. They have more area therefor feel "floaty"and the gyroscopic effect means the longer rotor is slower to swing through cyclic changes.

lol I'll just wait for extrapilot on this one...

[R_Lefebvre]

[extrapilot]

Mate

You have confused a bunch of things. Forgetting some of the technical stuff-

There is no point discussing things like following rate (delay between cyclic input and cyclic motion) if you are not keeping the RRPM constant- since following rate is largely a function of RRPM. Except, in several cases, you talk about changing gearing, and slower RRPM due to drag, etc.

You cannot talk about blades generically. You don’t know the mass, the distribution of mass, the stiffness, the foil, the chord, etc etc.

Autorotation is largely a function of blade loading- where potential energy (height) is used to provide rotational energy to drive the rotor. In many cases, you can increase auto-rotation hang time with a higher machine mass/payload, because you have higher potential energy (yes- it took more energy to get to altitude- not a free lunch).

This doesn’t scale as you suggest- there is a square/cube law, whereby as objects get larger, they get more massive at an even faster rate. There is also the Reynolds number, where small rotors autorotating (at lower RPM) are in a very low Reynolds realm- where air is quite sticky. Typically, you don’t get to a point where it is feasible to autorotate until about a 450-class, and even then, you need some mass, and some blade tip weight, to make it tolerable to those learning the maneuver, and how to fix problems before they become a blade stop, etc.

The OP is not asking about throwing 700-class blades on a 500 chassis. He is asking about general guidelines for implications on 690-710mm blades on a 700, etc.

OP- At the end of the day, you cannot predict what blade will do what with your machine. There are too many variables as regards your rotor’s head design, your target RRPM, the servo performance, the blade type and Lock number, etc. Best bet in my view is to hop into a forum which deals with your model specifically, and see if you can get any direct feedback from users running your motor and your RPM target- see what feedback you get. This is VERY subjective, because you don’t know much about FBL settings, any expo they might have in their TX, how they fly (sport flying to one may be 3D to another), etc.

The very basic rule is that if you want efficient hover, you want longer blades with low chord. If you want performance, you want lots of chord, with blades as long as required for the flight profile.

[Russell Harding]

I agree completely. Too many variables to 'test' or calculate.

What we really need is a quality magazine for RC helicopters! The could easily front the money (and probably get product samples, or pay for something somewhat affordable like blades, but not batteries) and test them all... provided they could find a 'stig' of RC helis that would give an unbiased flight description/review of each.

There is a lot of marketing in any hobby, but without good journalism, we are left to fend for ourselves and use what we read on the forums to help us navigate. I almost always buy a different brand of blade when I crash... I have used about 8 types for stretch 450s (MSH, Align, SAB, Radix V1, Radix V2, RJX, Hobbyking, Spin, and probably a couple others I'm forgetting), but I usually only have two pair at a time to compare against eachother...
Still, it is hard these days because FBL tuning makes worlds of difference and blades seem to add a little character. Sometimes the FBL needs a little tuning between blades, making the comparison even harder.