MR/TR compared with Coaxial Aircraft Design
 

Why is it that we are still designing aircraft around Brother Igor's original concept? As a non-engineer/designer, I'd like to inquire as to why hasn't the coaxial designed aircraft been more successful in both the civilan and military markets?

I know that both the US and Soviet/Russians militarys have dabbled with them, but mostly confine themselves to the MR/TR concept. It just seems that without the power requirements of a TR and the limits of RTB of convential rotorcraft, the coaxial rotorcraft would have a greater power & speed advantage.

What is it that I am missing here? Are there some design limitations to the coaxial system or is it lack of imagination?

I wondered the same thing. However, the X2 is looking to take advantage of the those very things...

http://www.sikorsky.com/details/0,30...TI2088,00.html

(btw, been meaning to ask, does anyone have a link to a discussion or site about coaxial rotors and their benefits/drawbacks etc?)

For a co-axial rotor, a bigger/taller rotor mast is needed. There is extra parasite drag from this. In higher speed flight, a tail rotor can be offloaded by good aerodynamic design of the tail empennage.

A co-ax is good for low speed lifting because there is no "wasted" tail rotor thrust.

Two separate main rotors are even better because there is less airflow interference between them.

As always, design is a compromise.

ShyTorque has given a good overview.

If you want to get into more detail, you may find this page of interest; Aerodynamic - Rotor Disk - Dual Configurations

Some cool projects from this Sikorsky page.

Rigid rotor coax is harder to package, resulting in a heavier rotor head. Cost will be more expensive until demand goes up.

Mart

Thanks ShyTorque, Dave_Jackson & Graviman - there's some good reading there.

I notice on the Sikorsky artist impressions that they show streamlined rotor masts. Presumably to reduce parasite drag.

The height between each disc also seems to have been reduced. Is this due to the "rigid rotor" setup mentioned?

Because he got it right!
 

"Two rotors are like two women in the kitchen. You might think they would do twice as much work, but the efficiency of each is lower-ed by 35 percent." -- Igor Sikorsky
http://www.sikorskyarchives.com/charac1.html
http://www.sikorskyarchives.com/iis55.jpg
-- IFMU

Yes. It is mentioned that the rotors and masts were 50% of the drag on the earlier XH-59A ABC. The president of Sikorsky has said that this drag on the X2 will be slightly above 25%.

Yes. The rotors are very rigid. The Advancing Blade Concept (ABC) puts the majority of the lift on the advancing blades during forward flight. This causes the lower advancing blade tips to rise toward the upper rotor's retreating blades. The maximum allowable clearance between the blade tips of the XH-59A was 13", as I recall.

No. This is not true for the Side-by-side, Interleaving, Intermeshing and Tandem configurations. In fact, it is not even true for the coaxial configuration when the total number of blades is the same on both craft.



Dave

Thanks Dave

So, what is the typical net % gain of a coaxial setup vs. the MR/TR setup?

I see you favour intermeshing for your prototypes. How are things coming along with them, btw?

i4ig;It is dependant on the application.
In the case of the Advancing Blade Concept there is no choice but to use two main rotors. However, there are four potential configurations from which to choose. Each has it pros and cons. As Nick has said many times, there is no such thing as a free lunch.
Dave
PS. The project advances slowly and boringly.

Those silly designers that still rely on a tail rotor - don't they know that coaxes are BETTER?
Igor Sikorsky once said, "There will be times when the theory and the facts do not agree, young gentlemen. In that case, it is my ernest belief that you should respect the facts."

The facts are:

TR's "lose" about 3% of the power that the helo has.

Coaxes have about 10% higher cruise drag due to the very high mast and the two rotor heads.

The bottom rotor of a coax has significantly less efficiency than the top because the scewed flow and disturbed air it operates in.

Self-midair is a big problem with coaxes (thus the high mast to separate the disks)

The ABC/X2 is a very very high offset (very stiff) rotor so that it makes it possible to have a lower mast and less disk separation. This helps solve 2 and 4 above.

TANSTAAFL for sure. 95% of all helos have a single main rotor and a tail rotor/fan. Why? because it works well, probably better than anything else.

Then why the X2/ABC? Speed, which will cost some hover efficiency, but will allow a helo to challenge tiltrotors while still hovering and maneuvering very well at low speed, something tiltrotors will never do well.

Nick,

You left out the weight of the second main rotor/gears/shafts. I suspect it is a net gain in weight compared to ditching the tail rotor, tail gearbox, and shafting. The price of speed.

-- IFMU

Thank you for the Responses
 

I appreciate you taking the time to respond to my question. It appears the bottom line is, whatever the gain, there must be a cost somewhere. I guess until the payoff is appreciable the coaxial design will remain a distant second to the MR/TR configuration.

The original Comanche/LHX design was derived from a tradeoff contest between a conventional single rotor and an ABC, an effort that I strongly pushed for (in the hopes that an ABC would win). The 10,000 lb class helos that emerged were the same in payload, but the ABC weighed about 500 lbs more empty weight, and cost about 10% more as a result (more complex transmission, controls and rotors. Obviously, since their numbers were all nearly doubled, and they are the expensive bits.

The ABC had a 1/2 g maneuver advantage, and a 10 knot speed advantage (no compounding was added), both great combat virtues, but not valued in the LHX mission scenarios, which were low speed battle.

More spin. :)
Dave

Nice to have the definitive answer!:hmm:

So, what happens to the tip vortices? Is "settling with power" less of an issue for coaxial rotor systems?

Dave and i4iq,

You are confused by the difference between 15% and 3%, and I can see why. The data shows that TRs eat about 3 to 5% of the total power of a single rotor helo, usually 3% in still air. Kamov talks about "efficiency improvement" of 15%, whatever that means. (It is probably the sum of disk loading, lack of TR power, coax planform factors and some spin.)

I published several papers that show the actual HP consumed by the TR, that data is real.

If you chose to read everything everybody puts a "%" symbol to and try to equate them, I have no doubt that you will be confused!

Chose something to bear in mind:

15,000 single rotor helos were built by people who were foolish and knew crap about efficient designs
or
They knew a few more things than Dave presents.

Nick,

To my knowledge, 'Principles of Helicopter Aerodynamics' by Leishman (University of Maryland) is the latest Western book (2000) on rotorcraft aerodynamics. Few people, if any, would dispute his competence.

On page 224 he says "Roughly, the tail rotor consumes up to about 10% of the total aircraft power. This power is completely lost, because unless the tail rotor is canted, as on the UH-60 Blackhawk, it provides no useful lifting force."

In another section of the book he remarks that earlier Western aerodynamist had under estimated the power consumed by a tail rotor.



OK. :)

"What went so wrong and why has there been sixty years of tail-rotor dominance? At the risk of irritating some, I suggest that the Germans, plus the Russians and the Europeans lost the Second World War, economically, whereas the United States was the major winner. The ideas that were prevalent in the United States at that time became the predominant configuration. The majority and the financially endowed went with the single rotor, and 'simply' offset its torque with a horizontal fan. I suspect that the problems confronting the rotor aerodynamists were formidable and the early American developers resisted compounding these problems by having to deal with two main rotors. Unfortunately, this configuration with a tail rotor created a new set of problems that can never be adequately overcome."


Dave

Don't forget about yaw control
 

Prouty talks about coaxial & synchrocopters in one of his books. One of the challenges is that you lose yaw authority in certain regimes. So, you have to add rudders and such, but do you ever get yaw control that is good in all regimes (not counting tail rotor failure) as a conventional rotorcraft?

-- IFMU

Folks, we need some clarification...

Are we discussing pusher or puller TRs, front upward or downward rotation, behind or below MR?

A well designed front upward pusher TR, with MR downwash just catching forward section, would likely achieve 3% loss. A badly designed front downward puller TR, with downwash causing reverse flow, would likely achieve 10% loss. Need to compare apples with apples before upsetting the applecart. :8

----

I'm amazed ABC only showed 10kts advantage over MR/TR. Especially since X2 250kts is so far above anything else. Was this to do with Comanche blade twist being optimised for lower speeds? I imagine the need to optimise twist for high speed flight is why X2 suffers in hover. Prouty Fig 1.13 (Chap 1 page 28 - or even the eqn on page 26) convinced me, admitedly in the absence of actual test data, that wide chord blades and low Nr should improve efficiency.

I've not seen any of the CFD sims, so have no idea how this correlates to reality. Certainly i can understand how the lower rotors operating in turbulence from upper rotors will get nowhere near clean air ideal for blade drag, hence rotor torque.

Mart