Re: if 2 bladed propellers more efficiant, why do chinooks have several?

That's a really good question!

I'm a rated helicopter pilot, so I researched that same question many years ago when I first started flying.

You are correct that two blades can be more efficient than several. Actually, one blade would be best, but that introduces a number of practical problems as you can imagine.

There are a number of factors that influence the design of helicopter rotors. They include gross weight, hover performance, top speed, autorotation performance, and to a lesser degree, maintainence costs. The are two important figures aeronautical engineers use to characterize rotors: disk loading and rotor solidity. Disk loading is calculated by dividing the rotor disk area by the gross weight of the helicopter. Rotors with high disk loading require more power during hover than those with lower disk loading. In other words, helicopters with longer blades need less power to hover than those with shorter blades. Solidity is the ratio of blade area (length times chord) to rotor disk area. The gross weight of a helicopter is ultimately limited by the strength of it's blades. Big, heavy helicopters require higher rotor solidity than light helicopters. Solidity can be increased with wider blades (longer chord length) or more blades. More blades are preferred because long chord blades are more susceptible to stall.

The venerable UH-1 "Huey" built by Bell helicopter has two blades with a 44 foot diameter. 44 feet is considered large for a helicopter that has a maximum gross weight around 10,000 pounds. The Huey is a very reliable machine, but it's not particularly fast (around 125 mph max speed) and not particularly maneuverable. The reliability is derived from it's simplicity, but it tends to be sluggish on the controls due to the large, two blade configuration. The Huey was the first military utility helicopter (UH). It was designed in the early 1960's for use in Viet Nam. Back then, reliability and simplicity were most important. For these reasons, Bell helicopter chose a rotor configuration that required less power and yielded excellent autorotation (no power landing) characteristics. I have read that the Huey has enough rotor inertia that a pilot can spool up the rotor to 100%, shut down the engine, rise to a five foot hover, perform a comple 360 degree pedal turn, and land normally. I don't think there is any other helicopter in the world that can do that, not even today. The Huey is a very unique machine, you can always recognize an approaching Huey from the regular thumping of it's slow, two bladed main rotor.

In contrast to the sleek, two bladed simplicity of the Huey, the Sikorsky CH-53 Super Stallion is lumbering behemoth. The CH-53 has seven blades and a gross weight of over 70,000 pounds. The CH-53 is surprisingly fast at over 170 mph and quite maneuverable ("like a bumblebee" according a veteran Navy pilot I once met). The high gross weight, speed and agility is directly related to it's complex, seven bladed rotor. The Boeing Chinook is in between the Huey and the Super Stallion. The Chinook has three blades for each of it's main tandem rotors and a gross weight around 50,000 pounds.

I've never flown a Huey or a Super Stallion, but I have flown the Bell JetRanger and the Hughes 500. The two bladed JetRanger, like it's bigger cousin the Huey, is slow and sluggish. In contrast, the five bladed Hughes is fast and agile. The JetRanger is the most widely used helicopter in the world, largely due to its reliability. The Hughes is considered by many as a pilots helicopter, a real joy to fly.

I hope I have answered your questions fully, if not, don't hesitate to drop me a line at madhu@madhu.com.