In a standard helicopter, the power going to the tail rotor to counteract main rotor torque is just parasitic, it contributes nothing to lift. Counteracting the torque instead with a second, contra-rotating rotor, a la the Chinook, turns all the power into lift so it seems it should be the better choice for maximizing lift. But the single-rotor design is the overwhelming favorite of heavy lift helicopters. Why is that, what am I missing? Where is the efficiency advantage of the conventional single-rotor design that makes it a more popular choice than the dual rotor for heavy lift?

Less weight in a conventional main/tail rotor design.

It works on the K-Max.

Yes, the single seat, not widely used K-max is an outlier.

I agree that it's simply the weight and size / complexity that keeps it from being more prevalent.

I think if we see a departure from a single main rotor it will probably be to four rotors, this will require the helicopter to be electric because the complexity of the transmission(s) and drivelines.

There's a cost penalty associated with the added complexity. There are reasons why the K-max sees limited use in the external lift space. Still hard to beat a Huey.

The interaction between two rotors does have a performance penalty. There is also a weight and complexity penalty and higher manufacturing and maintenance costs.

There was a dude, Canadian IIRC, at OSH some time in the last 20 years who had a booth for his experimental, V8 powered, dual rotor helo. It looked like a mini Chinook.

I think the motor was amidships with a parallel shaft gearbox (gearbox input and output bores were parallel to each other) up to two driveshafts, one going fore and one going aft.

No idea what happened to it.

I love adventurous engineering attempts like this. I wish they could all succeed.

As a few have alluded to, there is both drive system complexity and aerodynamic instabilities that have to be dealt with, that have traditionally made it an expensive engineering solution.

A few things that they have to address:

1. You have two rotor systems and all the mechanics behind them (smash plates, pitch links, control rods etc.).

2. The two systems have to be interconnected. This is done with a drive shaft usually. If the drive shaft or mechanics fail, you have a big problem, because it’s not as easy as saying autorotate. The rotor discs have to maintain synchronization and speed. In a chinook I think they might have 5 transmissions in total (someone may be able to verify it).

3. Your center of gravity isn’t over a single disk, which means it’s behind or ahead, which means you have longitudinal moments that have to be engineered into the structure to support the weight (what’s easier, lifting 5 pounds straight up or having to lift 5 pounds off the end of a 2 foot stick.)

4. They have historically had lateral and longitudinal flight instability. A lot of this is the interplay of the front and rear rotors down wash onto each other. In different flight configurations they play into each other. This can cause significant pitching up or down of the aircraft in different speed envelopes. With new computer controlled flight systems, this can be more easily corrected and stabilized, such that the pilot can fly the aircraft more like a conventional helicopter.

5. Overall size for equivalent lifting power can be smaller for conventional over tandem systems. You can fold tails, fold rotors back and make a significantly smaller footprint on deck of ship. I haven’t verified, but I would also guess, you might be able to get into a smaller LZ also.

All in all, without the backup of a government or military, it appears that they are expensive to design and engineer for what you gain. A few cool factoids:

1. I believe the chinook can fly up to 200 kts. Incredibly fast for a helicopter
2. The new CH-53 King Stallion’s tail rotor may have more thrust than a Black Hawks main rotor. I think in tests they have been able to lift 36,000#’s

I read that the Chinook is the fastest current helicopter. So no tail rotor must help there.

And also that the mechanism that translates the cyclic and pedals into what the pilot intends is akin to magic.

As already mentioned the K-Max is a bit of an outlier. However the intermeshing rotor system that it utilizes is extremely efficient and very very stable for lifting.

Some interesting things about the K-Max. It uses the same engine as the Super Huey and has a very similar empty weight but will lift 6,000lbs on the hook with comparative ease.

It is much simpler than it looks from the outside. There is 1 engine, 1 drive shaft, 1 gearbox, no hydraulic system, and a simple DC electrical system.

The marines bought the Kaman HTK-1 in the 1950s as a primary trainer but very quickly abandoned because it was too stable and easy to fly and pilots transitioning to other types were not prepared well.

What’s the downside? The intermeshing system (at least the Kaman designed one) has a lot of drag and it is painfully slow going places.

I’ve spent most of my flying career in intermeshing helicopters starting with almost 1,000 hours the H-43B and then over 13,000 in the K-Max. They are really really good for lifting things and not much else…

At OSH ~20 years ago I remember a LSA helo with coaxial rotor system. I could not figure out the transmission/clutch arrangement … and was scratching my head until the designer/builder confirmed it was direct drive - no way to autorotate. Really big brass ones.

In the same timeframe I was driving x-country on US-2 and stumbled upon a firefighting base that was still flying H-43’s. Anyone know if H-43’s are still working fires?

Billings Flying Service operates Army Chinooks, probably 10-12 now. When they get to a fire what they disgorge out of the inside is incredible. Pickup trucks, tools/parts, cooking and sleeping supplies, etc. They can carry many times what a Huey does.

I don't have experience with the current Chinook but have extensive experience with the D-model. The drive line is not horribly complex - two transmissions for the rotors. Yes there are engine gearboxes to get power from the engine to the aft main rotor transmission but this is common in any multi-engine helicopter.

As for longitudinal moments, that's what makes the Chinook so versatile as an external load lifter. Being able to sling big, awkward loads like a Howitzer, and to fly with that load in a stable configuration because it's attached on more than one hook and held so it's pointing into wind is a HUGE advantage.

Of course, being able to lift 26,000lbs on the center hook is kind of nice, too!

While I'm not rotary-wing rated I do have a fair bit of stick time in a diverse set of helicopters. Of them all, the Chinook was the easiest to fly, and by a wide margin. Turn off the AFCS computers and, well, you've got your hands full. Without AFCS I was able to hover but just barely. If one experiences a failure of the dynamic vibration damping hammers things get really rough - those hammers suck up main rotor vibration very well.

The Kaman factory was using an HH-43 as a K-max trainer, i'm not aware of any other HH-43s in service anywhere.