3-bladed R44 or R66

[Hughes500]

Originally Posted by Agile

That is interesting insight, In aerodynamics, induced drag is the overwhelming factor that drive efficiency, Induced drag is generated by the blade tip vortices. As the result a long aspect ratio blade will always be more efficient (akin a high performance glider). Robinson blades are one of the highest aspect ratio blades around, and I contend that it is one the design choice that has driven the success of the R44 especially. Think about it, 4 passenger with 225hp (Astro and Raven I) could only be the result of a higher efficiency rotor. roughly computed:
R44 1089Kg 225hp 4.8Kg/hp
EC120 1715Kg 504hp 3.4Kg/hp
H500 1610Kg 425hp 3.8kg/hp
So even with the benefit of the lightweight turbine engine and a heavier airframe nobody come close to the weight carrying capability of the R44 per hp.

You bring a good subject, the drivers have changed, look at Archer aviation and Joby, their creative design make you think about blades differently, aiming for different drivers (noise)

H 500 D and E models max continuous is 350 Hp and 375 for 5 min take off

[RVDT]

Originally Posted by Agile

That is interesting insight, In aerodynamics, induced drag is the overwhelming factor that drive efficiency, Induced drag is generated by the blade tip vortices. As the result a long aspect ratio blade will always be more efficient (akin a high performance glider). Robinson blades are one of the highest aspect ratio blades around, and I contend that it is one the design choice that has driven the success of the R44 especially. Think about it, 4 passenger with 225hp (Astro and Raven I) could only be the result of a higher efficiency rotor. roughly computed:
R44 1089Kg 225hp 4.8Kg/hp
EC120 1715Kg 504hp 3.4Kg/hp
H500 1610Kg 425hp 3.8kg/hp
So even with the benefit of the lightweight turbine engine and a heavier airframe nobody come close to the weight carrying capability of the R44 per hp.

You bring a good subject, the drivers have changed, look at Archer aviation and Joby, their creative design make you think about blades differently, aiming for different drivers (noise)

You might want to check your numbers -
EC120 1715Kg 402hp 4.26 Kg/hp
H500 1610Kg 375hp 4.29 Kg/hp

Power figures with MGB TOP input.
For sure it is "a" metric but not really a practical one.

[Agile]

Originally Posted by RVDT

You might want to check your numbers -
EC120 1715Kg 402hp 4.26 Kg/hp
H500 1610Kg 375hp 4.29 Kg/hp

Power figures with MGB TOP input.
For sure it is "a" metric but not really a practical one.

Yes, in practice, TOP is subject to other limitations, but the point remains that the R44 rotor design is maximizing the use of 225 HP (or 245hp for the raven Ii) to the point of making it a practical 4 seat heli. I remember Bruno Guimbal was discussing a larger cabri and tying it to a powerplant issue. It seemed to suggest that his core design would not scale up with a piston engine, and turbine engine would out price the business model, lastly other exotic engine, like high performance diesel, were a big market risk. So a 3 blade design if only 10% less efficient keeps you out of the 4 seat market without jumping to a turboshaft.

There has been in the past experiments with ultra long blade design, so long that they would touch the ground when at rest. The rotor was turning so slow that you could follow the blades with your eyes. There is a historical movie about that, I cannot find it back.

[Mee3]

A bike will always have a higher hp/weight ratio than a Lotus roadster. Engineers make their trade-offs, and so do the customers.

[Hot and Hi]

Originally Posted by paco

You don't automatically get GR with more rotor blades, it's whether they have drag dampers or not. Actually, the teetering head is the most susceptible to ground resonance as a design.

Really! Every day I learn something new.

Fact remains that GR avoidance is not taught to Robinson pilots, as it is not a concern on this type.

[Agile]

Originally Posted by Hot and Hi

Really! Every day I learn something new.
Fact remains that GR avoidance is not taught to Robinson pilots, as it is not a concern on this type.

In a 3-blades system a blade will drag independently of the other blades, it will displace the rotor system center of mass and create a lateral oscilation.
GR is when that lateral oscilation (undamped) comes in counter phase with fuselage oscilation.

In a 2-blade system, when a blade wants to drag back the other wants to drag forward, an about equal amount, because they are on oposite azimuth, thus the rotor system center of mass can hardly move I assume.
from a theoretical perspective, if blades we let to drag independently in a teetering head they have the power to displace the center of mass much more, is that why it is more suseptible ?...

[casper64]

Originally Posted by Bell_ringer

3 blades? Why? Do Robbie owners want a 50% greater chance of chopping off the tail?

No, cuz you would actually get rid of this silly teetering rotorhead…

[DavidSmithHeli]

Originally Posted by paco

You don't automatically get GR with more rotor blades, it's whether they have drag dampers or not. Actually, the teetering head is the most susceptible to ground resonance as a design.

this is not correct. Below is a reasonable layman’s explanation of why 2 bladed teetering hubs are not susceptible to GR.

Cant post the direct link but Google this site and read the explanation.

spinningwing
/helicopter/ground-resonance

[paco]

I would hardly call that a reasonable layman's explanation. My comment above was based on one of those old POF texts- I will try to find the reference, but I believe it was Ray Prouty.

[IFMU]

If I remember correctly, the classic Sikorsky Blue Book shows ground resonance happening because of lead-lag making the effective center of mass move off the rotor shaft centerline. That puts a moment on the shaft which makes the aircraft move and the gear responds to that movement. I've also read a bunch of Ray Prouty's work. The teetering rotor is locked rigidly with the blades 180 degrees apart so they don't have the same issue as the articulated rotor. That's also what I was taught in groundschool.

[paco]

And it's correct - teetering blades have reinforced blade roots so they don't need dampers, There is a natural resonance (linked with phase lag) when the blades go all the way to the hub which may be part of his thinking.

I don't know where pylon rock fits in with that.

[Ascend Charlie]

Isn't there also a requirement for the landing gear to be fitted with dampers, or to be on wheels (one tyre to be under-inflated) with oleos that are out of adjustment.

Most teetering head helos are on skids on crosstubes, which are not really susceptible to GR. However, a really ham-fisted pilot might be able to get it. Squirrels (different head) could bounce easily on those blades at the back of the skids.

[paco]

And Squirrels do - a friend of mine got into GR in one on his training course in the US.

[juszs]

Originally Posted by DavidSmithHeli

this is not correct. Below is a reasonable layman’s explanation of why 2 bladed teetering hubs are not susceptible to GR.

Cant post the direct link but Google this site and read the explanation.

spinningwing
/helicopter/ground-resonance

If this really is the real David Smith, can I just say how fantastic it is to see the new CEO of Robinson engaging with the community directly... bravo sir! Drop some more teases please...!

[DavidSmithHeli]

Originally Posted by juszs

If this really is the real David Smith, can I just say how fantastic it is to see the new CEO of Robinson engaging with the community directly... bravo sir! Drop some more teases please...!

Well…stay tuned!

[IFMU]

Originally Posted by paco

I don't know where pylon rock fits in with that.

It fits in because pylon rock drives the lead lag of the blades, which drives imbalance, and then back to mast rock. The relevant part of the blue book:

[paco]

Hence my comments on two bladed rotor systems not having drag dampers - having had it myself on a 212. Thank you for that.

[hesketh786]

2 blade rotor system is much. better