The answer to Question 13
WHAT ARE THE FACTORS GOVERNING ROTOR SYSTEM EFFICIENCY?
or why does the HELICYCLE hover easily at 10,000 ft?
The run of the mill helicopter enthusiast follows his eyes to the most attractive fuselage and that’s that. A helicopter pilot looks at the mechanics. A designer begins his evaluation by scrutinizing the rotor system. You can hang any shape you want below the rotor. Almost anyone can design a cockpit or a fuselage. It’s too bad this information won’t be discerned until the kit builder tries to fly his creation.
When you purchase a kit helicopter one of your first questions should be, “Please tell me about your rotor system.”
We would have to write a book to cover this subject in any detail. What we’ve decided to do to keep your attention, is to discuss a few primary factors that dictate a world class rotor. Everyone knows that a three combination lock has 999 possibilities. Rotor system design is complex, because it has many more than three variables.
We’ll define only 9 factors and to a limited extent, discuss their effect. The designer carefully evaluates each of these parameters and settles on what he calculates to be the correct numbers and shapes. Flight-testing will confirm the validity of his choices. A super computer with extensive modeling is Bell Helicopter’s answer today, however these tools are still a bit scarce among would be kit helicopter designers.
An air superiority rotor like the one that sits on top of the HELICYCLE© , has the following parameters carefully sized and proportioned.
1. Disc loading
2. Solidity ratio
3. Tip speed
4. Twist - root to tip
5. Air foil shape
6. Air foil aspect ratio
7. Airfoil conformity
8. Blade - weight
9. Amount of tip weight
We’ll define each parameter:
1. Disc Loading
Divide the area of the rotor disc (in square feet) by the gross weight of the machine. A “good” number to begin with is 1 lb. / sq. ft. per blade.
Therefore a five bladed rotor would have a 5 lb. / sq. ft disc loading. Mission requirement & physical size can cause some deviation from this rule of thumb. Excessively high disc loading results in poor autorotation characteristics.
2. Solidity Ratio
This refers to the area of the blade in relation to the rotor disc. This relationship in conjunction with the disc loading can be quite difficult to get right on the first try. For instance, why is the blade chord of the MINI-500 1" wider than the 1,500 lb. gross weight R-22, who got it right?
3. Tip Speed
The speed at the tip of the rotor at flight rpm is usually calculated in ft. / per
second. The correct tip speed is vital for a number of reasons. We’ll mention two.
A. Too low a speed will result in poor rpm recovery in the all
important flare at the end of an autorotation. “Falling through
the flare” is a ticket to disaster.
B. Too high a tip speed results in excessive centrifugal force
build-up which shortens fatigue life.
Tip speeds for a 20" - 25" diameter rotor are commonly 600 to 660 ft. /
second. A high speed rotorcraft slows the retreating blade dramatically. To
permit a greater “advance ratio” the tip speeds of this rotor must be increased to prevent blade stall. I don’t think any Bell test pilots have lived through a blade stall in a Huey Cobra. I have talked to a pilot who did in an early day 3-bladed rotor. Blade stall happens at high speed and the ship is inverted almost instantaneously.
4. Blade Twist
Most commercial helicopters have a twist from root to tip of approximately 7º. This enhances the helicopters ability to hover with a 7% increase in useful load for little or no increase in horsepower. Twist does not enhance autorotational capability. Note that efficient autogiros have untwisted blades. The trick is to build an efficient enough rotor to provide adequate lift in a hover without resorting to twist.
5. Airfoil Shape
An asymmetrical blade shape is necessary for high lift efficiency. The problem with them is that they exhibit a “nose down pitching moment”. Without a trick like a reflex trailing edge, it would be impossible to feather (pitch) this kind of blade in flight. An asymmetrical rotor however can easily make up for the loss in twist efficiency.
6. Airfoil Aspect Ratio
Normal thickness to chord width is 15%. Blades with a lesser thickness have less parasite drag, therefore they are normally more desirable. The HELICYCLE rotor is the first one to have less than a 15% thickness, in
B. J. Schramm’s career.
7. Airfoil Conformity
1. There are two aspects to this parameter.
A. The blade must be manufactured as close as possible to the N.A.C.A. airfoil shape. (Specified by co-ordinates).
B. Blades must be a perfect match with one another.
It has taken 40 years of experimentation with the airfoils and rotor blade methods of manufacturing to create the magic of the HELICYCLE rotor. Getting the blade airfoil conformed exactly to N.A.C.A. co-ordinate numbers in a lesser thickness chord is a Bell Helicopter sized problem. By the time one figures out how to achieve the required tolerances, costs have climbed way out of the kit helicopter price range. Learning how to create this level of technology for ½ the price of other kit helicopter rotors on the market has not been a simple task.
8. Blade Weight
Two bladed rotors should weigh close to 20% of the empty weight of the machine. A heavier rotor would begin to raise the centrifugal force load, shortening fatigue life. A lighter rotor will loose stiffness and it’s kinetic energy will be lessened requiring heavier tip weights to make up for the loss in “flywheel” effect.
9. Tip Weight
To enhance power-off operation we need to pay particular attention to how the rotor rpm responds in the flare and after a throttle chop. Small diameter rotors are notoriously lacking in stored energy. Adding a lot of extra tip weight is the obvious, but not the best solution. An additional 8oz. Of tip weight added to the HELICYCLE rotor increases the centrifugal force over 500 lbs. This increases the stress at the blade root and consequently lowers the fatigue life. Higher amounts of tip weight are dangerous from another perspective. The tip plane path of the rotor is positioned by the pilot using the cyclic control. A heavy mass at the tip of the rotor will resist change in-plane, especially a rapid change. If the pilot moves the stick too rapidly on a rotor of this type he could initiate a condition called “rotor weave.” The tip plane path of the rotor responds only to the physics of the induced motion, and movement of the cyclic is useless. We experienced this phenomena during flight testing in the late 70's and only by the Grace of God were we able to survive it. The amplitude of the weave in this case was over two feet and the event occurred at above 90 mph.
We are happy to report that the HELICYCLE has just the right overall blade weight and the correct match of tip weight to protect the pilot during that all important power off event. Proof of this is seen in two flight situations. First, the HELICYCLE (with proper technique) can be safely and easily throttle chopped from an 8 ft. high skid height. Second, at a low altitude (10' - 20') at 55 - 60 mph., power can be chopped and the ship flared to a very near zero speed touch down. Of course, at lower forward speeds any helicopter will touch down with some forward speed. (Touch down’s above 20 - 25 mph. need to occur on wet grass.)
The balance and relationship between the above 9 factors largely control how lift efficient our rotor system will be and how docile it will be in an unpowered condition.
As we have said many times before, the helicopter is the rotor. An efficient rotor, combined with good handling qualities along with a high power to weight ratio is the correct formula. The HELICYCLE excels in each of these categories. That’s why it’s capable of flight at 10,000 ft. high Leadville, Colorado.
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