Four Blade rotor

A very common misconception is that more blades permit a smaller rotor diameter. Not so.

The power required to produce lift, i.e., to change the momentum of the oncoming air depends only upon disc loading. That’s known as induced power and it varies as 1/airspeed.

The power required to drag the rotor blades through the air, profile power, depends upon tip speed, which in turn is directly related to blade area. Tip speed varies as the square root of blade area.

It makes little difference how blade area is achieved; wider chord or more blades. Increasing the chord of seesaw rotors exacerbates the vibration problems.

Scissors motion is only the tip of the iceberg with crisscrossed seesaw rotors. Adding the mass of a second rotor at the center of a single seesaw rotor will always tune the resonant frequency to near 1/rev and ensure large excursions during forward flight (a rotor is a vibrating reed). Blades or hub will ultimately fail.

A possible solution might be teeter bearings that are stiff radially but soft axially and in torsion. The axial compliance might offer sufficient isolation to keep inplane resonance above 1/rev. Sounds like a laminated metal/rubber bushing but not something to be designed “by guess and by gosh.”

4 Blades in flight

4 Blades in flight

Found this on Youtube, looks interesting to me,


YouTube - Rotortec, 4 Bladed Rotor System in Flight........

Hello,

I posted in a different thread before. The two seesaw rotors of this Cloud Dancer 2 are connected with an elastic scissoring joint. However, the other issue Chuck mentions, the additional weight pulling into a different direction at any time, I don't know about. The first, subjective impression of the test flight was, that the rotor provided more lift than expected, even though shorter than the 2-blade one.

Kai.

Was done with a Pit Bull gyro 20 years ago worked OK, Giogemini set up simi ridgid stack up two blade system 90 deg to each other,Made it look like a Pitcarin,

Rotortec 4 baldes

Rotortec 4 baldes

Rotortec is testing more on the 4 Bladed System, this Video I have found under Youtube. Getting more Interesting every day. Great Job,thank you Rotortec..

Steven

http://www.youtube.com/user/FSMike1986#p/a/u/0/I1vnGc0Z5zM

Four blades teeter can not reduce the required diameter. This can reduce the vibration in the stick, but it increases unduly stress at the root.
Jean Claude

Rotortec 4 baldes

Rotortec 4 baldes

Hi Jean,

have you testet yourself a System like this ?? Do you speak from experiance or just from your feelings.
If you can proof your statemant, please supply....

http://www.youtube.com/user/copterhaus#p/a/u/0/maiG9Q00f2E

As seen in the Video, we are the proof to the "pudding" Rotor Diameter 22" feet, Prerotation to 180 RPM, lift off after 20 meters, 33 Deg Cels. 2300 feet MSL.

Why don’t we have biplane sailplanes? With 4 wings, the wingspan would only need to be ½ as great.

Unfortunately, nature doesn’t work that way. Lift depends upon the mass of air that can be accelerated downward per second.

Newton said:

Force (lift) = mass x acceleration.

Energy expended = ½ mass x velocity²

For the same lift with ½ the mass, velocity must be doubled. Energy expended is also doubled.

That’s the reason we don’t have biplane sailplanes.

The complete story is that with lower blade loading, tip speed can be lowered which reduces the power required to drag the rotor blades through the air at several hundred MPH. The power saved can be used to compensate for the increased induced power of a smaller rotor. But no more so than doubling the chord of a 2-blade rotor.

When Ralph Taggart first evolved the Gyrobee, he was using Rotordyne blades and required something like a 26-ft. diameter rotor to get acceptable performance with a Rotax 447. With DW rotors, 23-ft. works fine even though the chord is a bit less. The difference is that DWs have much less profile drag than Rotordynes so the reduced profile power could be traded off for increased induced drag.
**
People have tinkered with crisscrossed seesaw rotors for years. The first to my knowledge was Alf Crowe in New Zealand who installed crisscrossed Bensen wood rotors on his Gyroglider in the 1960s.

An early SRC member, Bob Sheffield from the Gainesville area crisscrossed Hughes 269 rotors on his Bensen in the 1970s.

Ernie Boyette crisscrossed DWs on a gyro 5 or 6 years ago, complete with scissors hinge. They developed trailing edge cracks within a few hours.

Adding mass to the center of a seesaw rotor lowers the inplane resonant frequency. If the resonance falls near to 1/rev, large resonant vibrations occur that can’t be detected from the seat. Strain gauges and a way of recording blade root stress is required.

The Pitbull some time back tried a stacked 4 blade design and liked it ,but the added cost killed the idea,its more to do with $$$$$ than an idea.

4 balded System

4 balded System

Hi Chuck,

would an elastomeric Damper installed in one of the Blade section get rid of the problem you are discribing.

Steven

In my opinion, the only time that two shacked rotors will not try to swivel beyond 90° from each other….or when both RTV's are in alignment, is only when moving straight down. In forward flight, with the blades locked in position on the hub-bars and the rotor RTV's pointing in different directions….something, sometime is bound to let loose….even with a flex joint between the two rotors. I wonder what the cyclic forces are during crank and bank…..

A viable teeter-bolt gyro rotor will never be able to make it to a true four place aircraft. Change the head to a two bladed articulated type (at least in flapping) rotor with wider blade chords and you may be able to do it. I think a Hughes 500 type head with four blades of generous chord and approx. a 28' rotor may work well….if lifting power is more important than money.

ROTORTEC said:

Do you speak from experiance or just from your feelings.

Click to expand...

Rotortech,
The performance improvement come principaly from the total chord, not from the twin rotor. Less tangential speed of the blades, less power profile. It remains more power to climb. No feeling, just simple calculations.

ROTORTEC said:

As seen in the Video, we are the proof to the "pudding" Rotor Diameter 22" feet, Prerotation to 180 RPM, lift off after 20 meters, 33 Deg Cels. 2300 feet MSL.

Click to expand...

20 meters is about 3 Diameter. Is this what this video shows? What is the mass in this video? 180 r.p.m gives what ratio from r.p.m in flight) ? Thank you.
Jean Claude

Hi Jean,

to get the facts right. Rotor Diameter 22 feet DW, Total Weight 380 kg on Test Day, Temp. 33 Deg.Cels. Elv. 2300 MSL Prerotation up to 180 rpm, Video shows Take off run prox. 20- 30 meters, RPM in flight 330, Engine 135 HP, hope you have more Information now, let me now if you need more.

John

rotormaster said:

Hi Chuck,

would an elastomeric Damper installed in one of the Blade section get rid of the problem you are discribing.

Steven

Click to expand...

Drag hinges would of course solve the resonance problem. The in-plane resonant frequency would then be well below the 1/rev frequency.

If in-plane resonance is tuned to ~60% of 1/rev, dampers can be eliminated without the specter of ground resonance. The Brantly helicopter used drag hinges located ~ 1/4 radius outboard and got by without drag dampers.

Dick DeGraw’s 3-blade rotors use rubber bushings (silentblock bushings from Honda motorcycles) as springs in the drag links to raise in-plane resonance to ~60% of 1/rev and avoid dampers.

Landing gear dampers are still necessary with rotor in-plane resonance tuned to 60% of rotor speed.

`

Sorry to say the Brantly has Dampers on the outer blade sections, Small but the little buggers are there...

ROTORTEC said:

Hi Jean,

to get the facts right. Rotor Diameter 22 feet DW, Total Weight 380 kg on Test Day, Temp. 33 Deg.Cels. Elv. 2300 MSL Prerotation up to 180 rpm, Video shows Take off run prox. 20- 30 meters, RPM in flight 330, Engine 135 HP, hope you have more Information now, let me now if you need more.

John

Click to expand...

John,
Is this the weber with the plantary gear? Due you have thrust numbers for this engine.
Thanks for posting videos that are a closer up of the the Cloud Dancer. How many hours does this aircraft now have on the engine?

I hope your rotor system proves to be a reliable system, Keep posting.

hillberg said:

Sorry to say the Brantly has Dampers on the outer blade sections, Small but the little buggers are there...

Click to expand...

What some people mistakenly perceive to be dampers on a Brantly are rubber bumpers that serve as stops to limit travel.

http://books.google.com/books?id=Q2...page&q=Brantly helicopter drag hinges&f=false

John, one last piece of information: What was the speed of the wind? (we can see the grass move in the foreground)

Hi Jean,

Wind speed that Day, 5 Knots max.

John

@ Mark,

yes the setupis the Weber Engine 135 HP with the Panetrey Drive and on this test we used a 1750 mm Helix Prop. Trust Numbers I will supply to you as soon as we have all Props tested. We have about 60 Hours on the Engine, same on the Redrive. In some Videos you will see closeup shots, hope you will find them.

John

Hi John
Here's what my calculator gives me when I get the data (in blue) for a two-blades rotor ( Cl = 0.58, as DW)) with blade chord 0.36 m :
Results (in red): rpm 330 , lift 3800N at 27m / s and 9.5 ° (disk). This seems very close to your twin rotor with blade 0.18 m,
Do not be surprised inputs with lots of decimal : it is to give imbalances strictly zero