rotor blades 2 or 3?

[OuterMarker]

In reading about the beginnings of rotor flight, gyros began for the most part with four blades. At some point 4 blades were reduced to 3 blades. Still later, 3 blades became 2 blades. I would like to understand why. Today, with smaller single a two place gyro's, why are there not 3-bladed gyro's? Reader Digest version wanted here and if you have web sites or books to suggest, I would love to have those leads also.
thanks!
albert

 

[MikeBoyette]

Albert,
Two bladed systems are simple. Bottom line more blades, more complication.

 

[Master Roda]

Cost and complexity are the biggest issues.

You get several advantages with more blades, but the market will only bear simple 2 bladed teetering systems.

More blades require a different type of rotor-head. Most are articulating, and have the added purpose of jump take-off.

Sorry I don't have any links for you.

 

[LGoodhind]

A really good combo of books to look at on this topic are “Aerodynamics of the Helicopter” by Gessow/Meyers in the section that covers blade construction and “Cierva Autogyros” by Brooks discussion of early rotors.

Long and short- From an efficiency standpoint a long and narrow wing beats a short wide one; think gliders. Material science available in the early rotor era required heavy spars and guy wires so long narrow blades were literally impossible to make. These rotors rotated much slower than rotors today meaning an exponentially reduced load on the hub and the blades themselves.

As the weight of the machines went down (engines get lighter, airframe gets lighter, rotor gets lighter) you could reduce the HP required (engines get lighter still, etc) ... in the resulting comparatively featherweight machines the rotors don't need to support the cast iron equivalent components that were available in the 20's/30's.

The teetering rotor head has the additional advantage of automatically providing mechanical compliance (the ability to move in response to a periodic force) for the blades forward and aft because the forces are balanced between the leading and retreating blades symmetrically. Usually compliance is accomplished in 3-to-N blade rotors using drag hinges which, as mentioned previously, complicates the design and the machining required to execute the design.

 

[WaspAir]

The McCulloch J-2 and Air & Space 18A both use fully articulated 3-blade rotor systems, and they're the only standard airworthiness (not experimental) gyros currently flying. The real common factor is that they were not amateur built. The current market is dominated by low-cost build-it-at-home gyro designs, and two blades provides the simplest, quickest, and cheapest way to go.

 

[GyroChuck]

The biggest concern for me is since I'm in a shared hangar. How many 3 bladed gyro's could you fit in a hangar. Compared to 2 bladed gyro's.

Everyone so far brings up good points.

 

[scottessex]

Simplicity............................
To quote the greatest aircraft designer of all time. Kelly Johnson (Lockheed u-2, SR-71 etc.)
"If you don't put it on there, it can't break."

 

[OuterMarker]

So, because of the advancing and retreating blades and a teetering rotor head there is balance and if the blades are properly trimmed, no vibration. With three blades and a teetering rotor head, there is still balance between the advancing and retreating blades and hence, still no vibration? The additional mass would result in a slower rpm, more lift and more drag, which could be unsafe? Or, is the overall blade length shortened to maintain an rpm safe to all attitudes of flight....for me and my gyro 360 rpm is the magic number in level flight. Being a newbie is tough when you don't fully understand issues....I cannot visualize a 3 bladed teetering rotor hub...

 

[scottessex]

I do not think that the 3 blade hub is teetering, but rather articulated, like a helicopter.
More moving parts, lead, lag hinges etc.

 

[C. Beaty]

Tilt head 3-blade rotor.

 

[OuterMarker]

I better get a book and do some reading and looking at drawings on the differences. The early gyro's (1930's) began with a fixed rotor head, hence the reason for small wings and the need for control surfaces. However, at very slow or zero airspeed the controls had no effect and landing could become somewhat uncomfortable (high pucker factor). This is why a rotor head was developed that could pivot about to allow control at very slow speeds and remain under control. The wings were dropped from the designs...however, there was still 3 blades turning. At this point, was this hub articulated? This is pre-jump era of the early gyro's.

 

[Rotor Rooter]

Food for thought:


Existing gyrocopter:
Total gyrocopter cost, including blades; $40,000.00
Blades cost; $5,000.00

Existing gyrocopter with MUCH improved blades:
Improved blades cost $100% more; $10,000.00
However, they can support 10% more weight.
Total gyrocopter cost including improved blades is now $45.000.00

However:
This means that the Gross Weight can now be reduced by 10%
If the original gyrocopter's Empty Weight to Gross Weight ratio was 50/50 then the Empty Weight can be reduced by 20%.

The 20% reduction in Empty Weight means a 20% reduction in the price $45,000.00 * 80% = $ 36,000.00.

Moral to the story:
Buy (make) better blades and save money.


If the above is too far off the mark then the food I'll be eating is crow. hwell:


Dave

 

[Arnie Madsen]

Without it maybe having any practical purpose whatsoever , I still think a small 1 person gyro with 4 rotor blades would look fantastic. Some hobbyist with way too much time on their hands should build one for the show circuit or as a novelty item.

Just my 2 cents worth but Hey .... the Canadian Dollar is at par with the American dollar today so my 2 cents could be worth some real money you know!!!

 

[Alan_Cheatham]

I better get a book and do some reading and looking at drawings on the differences. The early gyro's (1930's) began with a fixed rotor head, hence the reason for small wings and the need for control surfaces. However, at very slow or zero airspeed the controls had no effect and landing could become somewhat uncomfortable (high pucker factor). This is why a rotor head was developed that could pivot about to allow control at very slow speeds and remain under control. The wings were dropped from the designs...however, there was still 3 blades turning. At this point, was this hub articulated? This is pre-jump era of the early gyro's.

You may want to search online and download the old patents for the Cierva autogiros, they can be very informative as to how Cierva implemented his tilting head.
.

 

[Arnie Madsen]

Simplicity............................
To quote the greatest aircraft designer of all time. Kelly Johnson (Lockheed u-2, SR-71 etc.)
"If you don't put it on there, it can't break."

.
Perfect Scott.

I have the Kelly Johnson Skunkworks book and his story is one of my favorites. He started with a handful of chosen people in a workshop at Lockheed made out of crates and tarps and built the fastest aircraft in the world.

Fantastic guy who used a unique blend of genius , common sense and practical instincts. He also knew how to pick the right people to work with. Most of his projects were completed ahead of time and under budget.

When military planners called him to meetings for some advanced aircraft proposals he would disappear into the hangar for a couple of weeks, build a prototype and fly it.

Then he would pop back into the still on-going meetings and ask the suits ... "Is this what you had in mind".

I only wish he would have chosen a different title for his book because it got pretty much overlooked in aviation circles. Fantastic reading, excellent management lessons. Great Skunkworks stories. Getting hard to find and not cheap. I have an autographed copy and if anyone wants it they would have to kill me first ...

 

[[email protected]]

Four blades will have the least vibrations and give the gyro-designer more freedom in construction due to short blades and will carry heavier loads.

Disadvantages are: more complicated construction of rotorhead, thus more expensive, gyro needs more space in hangar

existing types:

Magni 4-blade rotor (Magni M16)
Rotortec Clouddancer II (at stage preserial test-flights)

Overall nothing really new though .................

 

[C. Beaty]

Sailplanes have long, skinny wings.

Why not short, broad wings?

Wouldn’t wings half as long but twice as wide do the same job?

The answer, of course is no; the trick is to understand why.

 

[Rotor Rooter]

Sailplanes have long, skinny wings.

Why not short, broad wings?

Wouldn’t wings half as long but twice as wide do the same job?

The answer, of course is no; the trick is to understand why.

Chuck;

The following 'Why' is offered for the fun of a technical argument.
Consider;

• Sailplane wings travel at a relatively low velocity.
• Rotor blades (particularly mechanically power rotor blades) travel at a high velocity.
• Drag = Velocity[squared] * Area
• Due to the velocity being squared, there is an advantage to reducing the speed and increasing the area.

Half way down this page is a PPRuNe discussion with Nick Lappos, plus supporting calculations, on this subject.

Early rotorcraft pioneers, such as Cayley and Cornu, knew things that we don't give them credit for. Perhaps the later helicopters went to narrow blades so that they could keep their weight down by using centrifugal force instead of heavy construction or guy wires.

Composite construction allows today's builders to move back in the direction of wider chords and slower blades.


Dave

 

[RotoPlane]

In my opinion, a 0012 is a far better airfoil at the tip of a 400fps moving blade than say an 8H12. However the same size airfoil at say midway from the hub, sees much less velocity and therefore less Reynolds numbers and lift than the tip. At this area an 8H12 would give more RN and lift than a 0012. But increase the chord and you will increase the RN. If you increase taper the chord from the tip to the root, the RN will be higher at the midpoint of the blade than a rectangular high aspect ratio blade, and so would provide more lift with the same diameter. High aspect ratio wings are great for slow moving, low velocity airflow and not so great with high velocity airflow, like jet fighters.

The need for more blades is diminished if one can provide the required lift using just two…..

 

[Jean Claude]

For a fixed wing, the aspect ratio works similarly low or high speed. It reduces deflection behind the wing and the deflection on the wing itself. Less deflection on the wing itself produces less drag.
For a rotor, if the aspect ratio of the blades is greater, then less drag of the blade. The rotor turns faster. But the blades pass in the deflections more numerous previous. Finally the overall deflection by the disk is the same and the drag to the rotor is the same. Only the r.p.m is increased. The induced drag of the rotor are not reduced.
The change of mass necessary to achieve the same cone angle seems more important to me that the subtle change of Reynolds numer or Mach number.
Jean Claude