I don’t exactly understand the whys and wherefores of helicopter main rotors. For instance, let’s say we have a typical helicopter like a Bell UH-1 or Jet Ranger. Let’s say it has a 2 bladed main rotor with blades 1 foot wide and 50 feet in total rotating diameter (this isn’t the correct term, but I mean something like “rotary wing span”). Why couldn’t one get the same lift and so on out of 4 one-foot-wide blades with a 25 foot span? Or 2 blades that are 2 feet wide and 25 feet in span? I am sure the answer is relatively simple, but there is some logical principle that I am missing.
Because not all parts of a blade are moving at the same speed. The further away from the hub you go, the higher the airspeed of that part of the blade, and the greater the lift it is able to generate. If there were no mechanical problems to worry about (strength, vibration, flutter etc.), the highest lift would come from the largest rotor diameter.
Plus–(hope I’m right)–
The backwash of one blade tends to “get in the way” of the other, so to speak: a lot of blades, like a daisy, means less space between them, and more of a problem. The air just can’t move aside fast enough. (More accurately: can’t flood in to replace the pushed-aside air fast enough.)
I read not long ago that a chopper with a single blade (ie, one “radius”, as opposed to a blade on each side) is perfectly feasible and would be the most efficient of all.
Also note that longer blades can build up angular momentum, thus putting less demand on the engine.
Not to mention the weight penalty you’ll get if you have transmissions for four sets of rotors. And they’d have to be interlocked so that if one or more transmission fails, the remaining ones will drive the rotors. Heavy.
BTW: The rotor system in motion is referred to as the “rotor disc”.
I’d be highly skeptical a one bladed helecopter rotor could be built because of the highly assymetrical load on the hub. It could be balanced for weight but once it was producing lift it would be a different matter. Helecopter rotors have a relatively low speed and would subject the load to a circular oscillation. It’s not like a lot of helecopters don’t vibrate enough now. Mind you I’ve only ridden on H-46s and H-3s A symmetric rotor won’t have that problem.
I have seen single bladed propellors on control line speed models. Mind you the blade length is only a few inches and they are spinning upwards of 25,000 rpm. The oscillation is so fast the airframe can’t react to it.
Johnny, if you’re talking about intersecting rotor disks that’s done all the time. I can think of one with side by side rotors with the hubs only a few feet apart. The H-47 and H-46 don’t stricly have intersecting rotor disks but the rotors are geared so that blades won’t hit in any event. That type of drivetrain is bulky but totally elimnates the need for a tail rotor.
Thank you!
Good answers!
Elvis says (so I read): that it is more the outer parts of the rotor disk that “perform” than the inner. The parts of the rotor closest to the hub are (of course) necessary to connect those further out to the hub.
Scott raises an interesting point: In helicopters, it is this “rotor disc” —a rotating set of blades acting in effect like a disk— that provides the lift, etc. But he also refers to a “daisy”, which makes me think of the lift fan in certian V/STOL aircraft; for instance, the US Marines/Royal Navy version of the X-35 (Joint Strike Fighter). In such a case, it is not the disc itself that lifts, but rather the “downwash” from the disc. Correct? Or is there a difference?
No, actually I was replying to this:
I didn’t take that to mean intermeshing rotors, but just four rotors.
The thing to remember about the CH-46 and CH-47 is that they were designed to carry loads over a wider centre of gravity than single-rotor helicopters. By putting a rotor on each end, the crew have more “latitude” in distributing the load along the “longitude” 
(Sorry, that popped in while I was typing.) Without this requirement, there would be no reason to have dual rotors and they wouldn’t have them. Another thing is that the transmissions are interlinked so that if one fails the other can take over. This requires heavy machinery and lots of power. BTW: The Russians have a coaxial-rotor helicopter, and I think they have a meshing-rotor one. We used to have the Husky, and Kaman (ka-MANN) is making a lifter with meshing rotors.
Designing aircraft is a study in compromise. A dual-rotor helicopter like the CH-46 or CH-47 eliminates the tail rotor because it eliminates the torque effect. But it does so at the penalty of weight, which requires more power, which requires more fuel to make the power; and the penalty of complexity, which increases maintenance costs and probability of a failure. These penalties are acceptable because of the need for a helicopter that has a wide CG range and is able to carry lots of stuff.
It’s awfully convenient to have the rotor’s center of lift pretty much right over the aircraft’s center of gravity. With a single blade, even if the rotor were counterweighted to keep vibration in control, the machine would be constantly trying to flip itself over, and usually succeeding.
The force that pushes the helicopter up is generated by pushing air down. Up to a certain point it is more efficient to push more air accelerating it less, than to accelerate a small amount of air a lot. If you have a smaller diameter propeller you have access to less air and you have to accelerate it more. As you increase speed you begin to lose efficiency in the blade and run into cavitation and other problems. On the opposite side of the coin, if the blade is too long the outer tip also is goig too fast and loses efficiency or the inner part is going too slow.
The number of blades is not that important in the sense that if you have two blades and you increase the number to four, the extra lift generated would be negligible as you are just spreading out the same work among four blades instead of two. Imagine a boat with a prop with blades that cover 360 degrees and now put a second prop on the same shaft. You get no more push because both props are moving the same water. The only way to get more push is to move the water faster or to move more water.
Of course there’s this dead guy named Bernoulli…
Helicopters don’t actually fly; they just beat the air into submission.
:rolleyes: Next you’ll be saying that helicopters are so ugly, the Earth repels them.
I’ll bet you fly fixed-wing!
(obligatory )
>> Of course there’s this dead guy named Bernoulli
Yep, and still, flying machines generate lift by accelerating air downwards just like boat props generate it by accelerating water backwards. Sorry but there’s no way around that.
If you think a helicopter can hover in the middle of the air and not move the air downwards you are dreaming. The acceleration of the air down is what generates the lift that pushes the chopper up.
And don’t forget that with longer rotor blades, the speed at the tips combined with forward velocity of the aircraft makes breaking the sound barrier with the advancing blade an issue.
Also some blades tend to fly better together than others. This may be due to balance issues or finishes, or wear on one or the other. A new blade and an unrefinished used blade can give you all kinds of headaches when you are trying to get them adjusted properly and flying smoothly to gether. I’m referring to UH-1 semirigid rotor systems of course. Fully articulated systems (H-53 Sikorsky) probably have similar issues but may not be as sensitive as the H-1.
There is a debate over which is more important to developing lift: Bernoulli Effect or downwash. Certainly both occur when air flows over a wing, but which delivers more lift?
I have a little experiment that demonstrates the Bernoulli Effect overcoming downwash. Affic a flat rigid card to a straw, with the end of the straw protruding from the middle. Lay a flat sheet of paper on a flat surface. Position the straw device over the paper so that the rigid card is parallel to the paper. Blow into the straw. The air is deflected off of the paper and is prevented from moving upward by the card. This creates a lateral airflow. The paper will rise off of the table overcoming not only the downwash of the air being blown on it, but also the suction that exists when you have a flat sheet on a flat table. There is no air being deflected downward by the paper that would cause it to rise. Based on that experiment, I would opine that the Bermoulli Effect generates more lift than downwash. Furthermore, if downwash developed more lift than the Bernoulli Effect, then why would aircraft manufacturers go to so much trouble and expense to make curved wings?
As for boat propellers, you’ll notice that they also have an “airfoil” shape. That is, they are curved on the front and flat on the back.
As noted above, the further from the axle a point is, the faster it moves through the air. At 50 ft. from the center, that piece of the blade is rotating twice as fast as a piece 25ft. from the center. Since lift generated varies in proportion to the square of the speed, but only directly with the surface area, the piece at 50 ft. will generate four times as much lift as a similar piece at 25 ft., and twice as much as a piece twice as large at 25 ft.
Of course, this is a gross oversimplification, ignoring the effects of compressibility, reynolds number, the shape of the blades, the movement of the helicopter itself, and the movement of air along the blade, among other things. But then, explaining aerodynamics at any level below postdoctoral seems to involve some degree of oversimplification.
Ever notice that the “c” key and the “x” key are right next to each other? :o
I suspect that the issue of down-wash vs. lift depends to some degree on whether or not you’re in a hover or forward flight. Also, I bet whether or not you’re in ground effect, or not, plays a major factor.
If I may address the OP. This is a nutshell treatment.
There are trade-offs in designing a rotor. You’ve hit on the blade area, trading off number of blades and radius. That’s a good observation. Other things that factor in heavily are rotational speed (rpm) and desired forward speed. Mainly, the thing that answers your question is that the outer portions of the blade are moving faster than the inner. All other things fixed, if you shorten blades and add more blades (just to keep the area the same) you’d lose lift (for fixed blade pitch) because you’re cutting off the parts of blades that are actually working the hardest.
Design compromises look like this: say you want to build a rotor to lift a certain amount. For good hover performance, you’d like a large diameter rotor so you can move lots of air, but not with too much velocity change. To save on blade weight, you’d like to get the tip speed (the product of radius and rotational speed) up to somewhere comfortably under the speed of sound (considering tip speed plus desired forward flight speed) - that way, you can get more work out of smaller, lighter blades without transonic troubles. Higher rotor speed also means lighter transmissions. So your radius will probably be chosen based upon tradeoffs between how much parking/maneuvering space you have, and the other aforementioned issues.
Now you can start thinking about how much chord you need, how much twist to put into your blades, and what kind of airfoil distribution you want. The number of blades will probably be a trade-off on expense, hub type, and vibration (the trend seems that more blades yields more tolerable vibration).
Don’t forget about the effect of blade weight on autorotational characteristics (heavier is generally better). Tip treatments can maybe squeeze a few more knots and a few less decibels out of your design.
I don’t think there’s a debate over which is the greater effect. The debate is over which is the better way to explain it.
You can explain how a free-falling ball accelerates either by saying that gravity speeds it up by 9.8 meters per second every second, and adding up the distance it travels in each fraction of a second. Or, you can explain it as conservation of energy equation. The first is more intuitive, but the second is a nice practical way to do it once you’ve learned the first part. They both are correct.
Bernoulli is basically a conservation of energy equation. It’s very non-intuitive. And lift can be perfectly explained by simple reaction forces. Pushing the air downwards creates lift. It couldn’t create lift if it didn’t push down on something, and the air is the only thing there to push against.