Two blades or three

[thekite]

My friend BW, reg35 engineer, designer,builder and flyer of numerous and noteworthy aircraft has and is designing a two seat traning helicopter
To my surprise he has specified a three bladed rotor sytem. This of course increases the cost substantially over the more usual two bladed system.
"To what advantage?" I queried.
To eliminate he risk of mast bumping, quoth he. Well yes, but at what cost? I wondered.
He is right of course. All teetering rotors have this Achilles Heel, although the lovers of Hillers, B47s JetRangers etc will deny, the fact is, they will all do this awful thing.
But once the INCONVENIENT TRUTH (where have I heard that before) is grasped and handled, the pilot is no worse off than recovering alcoholic.
To wit: yes ther is a problem, but we can deal with it.
Training is the answer. Don't believe me? Then ask Frank Robinson, or fly with his instructors. In knowledge there is power.
My Yak 52 requires two terrified and strong men to get it out of some of the spins that it can be forcd into. So how not to die? DO NOT PUT THE AIRCRAFT INTO THAT PREDICAMENT!! And live a little longer.
Same with mast bumping. Do not put the aircraft into that predicament. And live a little longer, again, as I say.
So since we can fly two-bladed rotors for a lifetime without experiencing mast bumping, providing the training is correct, what other reason could BW for specifying 3 blades. Don't recall Enstroms and H300s as being noticably superior to B206s, Robbos etc.
Welcome some input here.
thekite

[NickLappos]

If your friend thinks the number of blades controls the propensity for mast bumping, he should be designing snowmobiles, not helos. Where did he come by this stunning revelation?

I hope he knows that the rotorhead is where the solution comes from, and not the number of blades.

[Gaseous]

DO NOT PUT THE AIRCRAFT INTO THAT PREDICAMENT!! And live a little longer.
Trouble with that argument is no one mast bumps deliberately.
Re Enstrom and 300, no one ever died mast bumping. They have problems but this isnt one of them.
Forgive my ignorance Nick, but are there any two bladers where mast bumping is not a possibility?

3 bladers need bigger hangars.

[Aesir]

And besides with 3 blades if one blade breakes off then you still have 2 to fly with so it must be safer :\

[TheMonk]

"And besides with 3 blades if one blade breakes off then you still have 2 to fly with so it must be safer :\"
So that extra blade is a spare then. :}
Monk

[NickLappos]

gaseous,
No, but there are three bladed rotorcraft that can mast bump (the V22 Osprey is an example.) It is the teetering head, not the number of blades, of course, that makes mast bumping possible. It is entirely possible to build a two bladed rotor with a flapping hinge, or a three bladed head with a gimble ring that teeters.

Think hinge offset as the way to avoid mast bumping, not number of blades.

[Graviman]

I gather that some guys from Westland are developing a 4-bladed autogyro with high effective hinge offset. Can't find any details, but it may be this chap:

http://www.aer.bris.ac.uk/contact/academic/bunniss1.shtml
http://www.bris.ac.uk/research/newco/spinouts/rwi.html

I'm curious about the control system being used. Maybe we will see a modern CAE makeover of the Lockheed mechanical system (forerunner to SAS). Being in permanent autorotation it also avoids the possibility of rotor stall.

[Gaseous]

Thanks Nick. So if its possible to design a 2 blader that doesnt bump, why are there not any?

[Max_Chat]

I am not sure that I fully understand the "Mast Bumping" thing. I have read that the V22 when carrying out negative G manouvers the rotor hub can come into contact with the mast and then detach

Sorry for my ignorance and, maybe, a stupid question but why and how does this happen ?

[Gomer Pylot]

Gaseous, everything in aircraft design is a compromise. The teetering system is the simplest and most cost-effective way to get everything to work.

[tottigol]

Max, the underslung design refers to the fact that the teetering hinge is located in a higher position than the blades rotating plane.
The whole rotor hub assembly teeters as one, this to allow for flapping motion. At low rotor RPMs or extreme low Gs manouvering, the flapping motion can exceed its static limits and cause the rotor hub tio contact the mast with consequences as severe as severing it.
The typical Bell two blades rotor system is considered a semirigid one.
When you mentioned the V-22 didn't you really mean the R-22? But then you mentioned negative Gs so it has to be the V-22.

[TheMonk]

Tottigols, "When you mentioned the V-22 didn't you really mean the R-22? But then you mentioned negative Gs so it has to be the V-22."

The V22 Osprey, with three blades per hub can get into negative G situation also, per Nick Lappos. The R22 with two blades are notorious for mast bumping.

Monk

[Max_Chat]

Tottigol thanks for the reply. Yes I did mean V22, it came from a V22 technical web site, although it does seem strange that the V22 should use a design that would be at odds with normal military types of flight profile i.e. fast descents into war torn areas of the world. I expect they know what they are doing!