MR/TR compared with Coaxial Aircraft Design

[bayou06]

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?

[i4iq]

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

http://www.sikorsky.com/details/0,3036,CLI1_DIV69_ETI2088,00.html

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

[ShyTorque]

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.

[Dave_Jackson]

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 (http://www.unicopter.com/B360.html)

[Graviman]

Some cool projects (http://www.sikorsky.com/file/popup/0,9604,1960,00.pdf) from this Sikorsky page (http://www.sikorsky.com/details/0,,cli1_div69_eti2365,00+en-uss_0fsbc.html).

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

Mart

[i4iq]

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?

[IFMU]

Why is it that we are still designing aircraft around Brother Igor's original concept?
"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

[Dave_Jackson]

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


The height between each disc also seems to have been reduced. Is this due to the "rigid rotor" setup mentioned? Yes. The rotors are very rigid. The Advancing Blade Concept (ABC) (http://www.unicopter.com/0890.html) 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.

"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 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.

http://www.unicopter.com/Configurations.gif


Dave

[i4iq]

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?

[Dave_Jackson]

i4ig;So, what is the typical net % gain of a coaxial setup vs. the MR/TR setup?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.
http://www.unicopter.com/NoFreeLunch2.jpg :ooh:
Dave
PS. The project advances slowly and boringly.

[NickLappos]

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.

[IFMU]

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."

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

[bayou06]

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.

[NickLappos]

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.

[Dave_Jackson]

?
Nick sez;
TR's "lose" about 3% of the power that the helo has.
CRAN sezed;
Representatives of the Kamov company have been present at the last two big UK Helicopter technical get-togethers and on both occassions have claimed that the co-axial rotor configuration, when properly designed is in fact more efficient than a single main rotor and tail rotor configuration. To the tune of about 15%!

Western spin versus Eastern spin?

More spin. (http://www.unicopter.com/1119.html) :)
Dave

[i4iq]

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?

[NickLappos]

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.

[Dave_Jackson]

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.

______________________________________


Chose something to bear in mind: 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

[IFMU]

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

[Graviman]

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

[Graviman]

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

Nick, any chance of the titles of those papers you mention? SAE website is useless for searching, and doesn't even show you the abstracts.

Other ABC papers i have found:

Rotor Technology for New Generation Helicopters - David S. Jenney (http://www.sae.org/technical/papers/751106)
Multi Service Applications for Advancing Blade Concept Aircraft - L. G. Knapp (http://www.sae.org/technical/papers/801226)
Status Report on the ABC UtUm Technology Demonstrator Program - Vincent P. Bailey (http://www.sae.org/technical/papers/770954)

I'm planning to order any recommended papers with the S76 Honeywell SPZ7600 ( http://www.sae.org/technical/papers/911008) paper. Any S76s near me would be Aberdeen way.

Mart

[Dave_Jackson]

The forgoing discussion in this thread implies that widening the chord allows for a decrease in the speed of the rotors.

In addition, Wieslaw Z. Stepniewski was recommending twin single-speed slow turning rotors for faster, quieter rotorcraft.

Therefore, these calculations (http://www.unicopter.com/1090.html#Calculations) were done to get a better understanding about the subject. They suggest that a single-speed slow turning rotor will be slightly more efficient in hover than it will in cruise.

The original Coaxial ABC helicopter (http://www.unicopter.com/0891.html) had a two-speed rotors. The tip speed was 650 fps for hover and 450 fps for cruise. The slower speed in cruise was necessary so that the advancing blade tip would not approach the speed of sound.

It will be very interesting to see if the new X2 Coaxial ABC (http://www.unicopter.com/1465.html) implements multi-speed rotors or single-speed rotors.

Dave

Opps!
This post (and most of the following posts) should have been placed on the [Why are Helicopters with the Flettner-System so slow?] thread.

[IFMU]

L = Cl * ((ρ * A * Vfwd ^ 2) / 2)
It will be very interesting to see if the new X2 Coaxial ABC (http://www.unicopter.com/1465.html) implements multi-speed rotors or single-speed rotors.
Dave
From http://www.sikorsky.com/details/0,9602,CLI1_DIV69_ETI2365,00.html:
"The main rotor will be slowed during high speed flight to keep the rotor tips below supersonic speeds."

-- IFMU

[bladepitch]

Nick, Dave and gang
Great post guys, keep the info coming!:ok:

[Dave_Jackson]

IFMU,

Thanks. The incomplete algorithm has been removed.

"The main rotor will be slowed during high speed flight to keep the rotor tips below supersonic speeds."

Strange. :confused:

It makes sense for a compound helicopter, such as the Piasecki (http://www.piasecki.com/index.html), to have a higher RRPM in hover than in cruise since the rotor will off-load some of its lift to the wings during cruise. However, the Advancing Blade Concept does not have wings.

On the X2, The propeller will provide the forward thrust. The rotor is responsible for lift and this lift will not vary much between hover and level forward flight. Therefore, wide chords and slowed rotors should be ideal for all modes of flight.

The single speed should also reduce the weight and complexity of the transmission.

Dave

[IFMU]

IFMU,
The single speed should also reduce the weight and complexity of the transmission.
Dave
Yeah, clutches, synchros, pressure plates, and gearshift levers sure add a lot of weight! :)

-- IFMU

[Graviman]

Dave,

Prouty sums it up nicely for us sad engineering types in his "big red book of helicopters" (Or was it "Helicopter Performance Stability and Control" - i'll put down my crayons and check). He has an equation on page 26 that for ideal twist goes:

Figure_of_Merit =
1 / (1 + ( 1.5*Tip_Speed / (Cl/Cd)*SQRT(2*Disk_Loading/Air_Density) ))

This equation is a rewrite of the equation he actually derives:

Figure_of_Merit = 1 / (1 + ( 1.5*SQRT(3) / (SQRT(Solidarity_Ratio)*(Cl^1.5/Cd)) ))

The results are plotted for ideal twist on page 28 (fig 1.13), and show without doubt that the higher the solidity ratio the higher the figure of merit. I was hoping for Nick's comment on this, but he gave up on me as a lost cause and ran for the hills - i get that a lot. :}

My interpretation of the above info is that a helicopter runs at peak efficiency when the blades are operating just below stall, for maximum Cl/Cd. Using wide chord blades then allows a nice low tip speed, by chosing a low Nr for hover. Forward flight Nr is a compromise on fixed Nr conventional helis, due to the need to avoid retreating blade stall (particularly increasing loading during sharp manouvres :ok: ). Counterrotators do not have this constraint so can reduce Nr to keep advancing blade airspeed ~constant.

Mart

[Graviman]

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.

Just realised the main reason for coaxial to show such a small speed advantage over MR/TR in Comanche is that you need a pusher prop to make any real gains.


Any news on X2? I have never known a technically ambitious project to make the deadlines, but i'm just curious...

Mart