Dave,

That is quite interesting. It shows where S69 program lessons are being carried forwards. Have a lookout to see whether vertical stabilisers are forward swept for improved autorotation yaw control, and whether there is a flow seperation keel plate to avoid shuffle in hover.

It looks to me like the hover performance has been slightly compromised in order to reduce the high speed reverse air flow circle down thrust. Aerofoils suffer divergence in reverse flow, so this is quite a sensible idea. I wonder whether that interesting planform, which will be slightly more expensive to manufacture, will find it's way into conventional helicopters.

The potential advantage of coaxial is that there is no loss of downwash in the reverse flow region. I realise your arguement is that interleaving avoids downwash over the fuselage, but good aerodynamics should allow the flow to pass without too much transition to turbulance. I wonder whether that blade profile helps here too.

Mart,

Your first two paragraphs are valid.

I should have been clearer about the use of the disks. They were presented to show the aerodynamic interaction of two main rotor disks, and specifically the regions of high and low induced velocities.

As you know, high induced velocities are less efficient than low induced velocities. The fixed wing is far more efficient then the rotor blade simply because the fixed wing generates a fairly constant induced velocity over its surface.


As an aside;

Momentum Theory is a reasonable method of performing an initial assessment of rotor performance. This is because all single rotors are basically the same except for the nuances such as; twist, taper, etc., plus blade count.

However, IMHO, Momentum Theory totally breaks down when it come to the consideration of multiple, aerodynamically interactive main rotors.

This is why I express my disappointment that Igor led rotorcraft away from the aerodynamically superior twin rotors. It should be noted that virtually all of the research and development followed along behind the single rotor.

Rotorcraft have always been inefficient. However, when electric drives start permeating the fields of transportation the rotorcraft industry must look back to the past for it to move forward into the future.


Dave

Well perhaps, Dave. I think Sikorsky have taken the engineering approach most likely to yield a successful project. Moving away from an S69 led design would just be too risky. Like Nick comments, X2 improves Vh by sacrificing some hover performance.

Once X2 does "what it says on the tin", there will be plenty of opportunity for aerodynamic refinements. My own thoughts are that there will be opportunities for a helicopter entrepreneur to introduce active blade twist. ;) Remember successful innovators identify the market gap, and taylor their devlopment to fill it...

At the risk of appearing a tad negative.
 

Me says;
I think Sikorsky has taken the approach most likely to yield http://www.unicopter.com/Temporary/Dollar.jpg


Me says;
Au contraire. My complaint is that they did move away from the S69 design; 30 years ago.
And please, please don't use 'advances in technology' as the reason for the 30 year gap.


Theodore Shadow

Sikorsky need to pay all those nice technicians, engineers and test pilots a salary so that they can pay their mortgage and put food on the table. All those selfish individuals won't work for peanuts, claiming instead that they want some security in their old age. Amazing really. :rolleyes:

Regarding shareholders, well, that's just the way the economy works. In the UK we regularly see some engineering company or another close because "the city" doesn't like the idea of long term investment for a small return. In fairness, i'm impressed Sikorsky continues to function only 12 miles from New York.

But these are not topics for this thread, please!

Concerning the 30 year gap from S69 to X2, i agree it can be seen as a shame. However it has been 35 years since Gene Cernan set the lunar challenge of today to forge man's destiny of tomorrow in Taurus-Littrow, and only now are Moon return missions being planned. In both cases the required market interest, to justify further development investment, just wasn't there.

Let's face it no one else has even tried to push a pure helicopter to 250kts.
But Sikorsky have now picked up where they left off, and frankly should be congratulated for making the effort.

Leishman holds the Minta Martin Chair of Engineering and is a Professor of Aerospace Engineering at the University of Maryland. His new book is showing that he is not an enthusiastic proponent of the tiltwing.

Here is one unflattering statement. ~ "at its maximum gross weight, the V-22 cruises at a true airspeeds closer to 220-250 kts (not 300 kts as superciliously claimed in various sources, including the manufacturer's own publicity materials) at altitudes of above 4,000 feet," ~ Source THTF - page 21

Tha BA609 has done 310kts. So far... Can't wait for the 'B' model!
:}

I thought that would end the discussion!!!
;)

21st, i changed my post to say that no one has pushed a "pure helicopter" to 250kts. And indeed congrats are in order for the BA609.

The V22 is generally cited for tiltrotors, but the discussions are not favourable because of the high disk loading in hover. This means the tiltrotor does not lift as much payload as a comparable helicopter.

The reason X2 has generated such interest is that it has the diskloading of a conventional helicopter, but also overcomes the retreating blade stall limitations. On paper this means X2 derived machines will gave tiltrotors like the BA609 a run for their money.

X2 will open the door for a new generation of helicopters... :ok:

Mart,
You're quite right, many things in aviation require a trade off. Both aircraft will likely perform very well. The tiltrotor will always have the edge on speed, and the X2 will have better lifting capability relative to power available.

X-2
 

The X-2 will be able to autorotate.

John Dixson

Apparently the BA609 will be able to autorotate as well. Unlike the V-22 that would have in the area of a 4-6000fpm ROD, the 609 may be in the area of 3000fpm (double an avg helicopter). So the best thing to do would be to land it like an airplane when possible, and if not, autorotate and expect to walk away with a dinged up aircraft (the penalties of combining very high speed with vertical t.o. and landing capability). The X-2 variants won't have the same limitations (or the same top speed capability). Dual turbine engine flameouts can occur (i.e., fuel contamination, etc.), but what is the occurrence rate?

21,

ROD is only part of the story. You have to arrest that ROD too, even if it is "only" 3000 fpm.

-- IFMU

"Only" 3000 fpm!!?? :eek:

What does the H-V curve for BA609 look like?

I'm curious what the criteria for a tiltrotor are...

Hasn't been released by BAAC yet.

X-2 Autorotation
 

Sorry, should have made my post more specific:

I meant to observe that the X-2 configuration will be able to meet the power off landing requirements.

John Dixson

John,

Re: Autorotation;

From; Department of the Army Historical Summary - FY 1976, on the Coaxial - Sikorsky ~ S-69 (XH-59) ABC

"Flight testing of the XH-59, advancing blade aircraft, resumed during the year. The tests confirmed certain advantages and shortcomings of the advancing blade concept, which is a coaxial counterrotating, hinge-less rotor system. The aircraft control system was quite responsive, noise levels were low, retreating blade stall was virtually eliminated, and hover performance was improved by the absence of a tail rotor. Structural loads in the rotor and control system ranged from low to moderate and indicated potential for substantial weight reduction. Weak directional control power in partial power descents and autorotation was observed. In future testing, improvements in autorotation directional control at low collective settings and high flare angles will be emphasized."


In autorotation, it will be interesting to see how the; Driven, Driving and Stall Regions play out, if the blade profile on above posting #102 is used.

Dave

X-2 Autorotation
 

I recall that issue Dave, and as I remember, there were discussions about how best to dress that situation up, but I don't recall if there was a specific future solution ( for example, the Kamov approach ) that was decided upon. Having a fly-by-wire control system certainly improves the ability to implement whatever option is selected.

John

Ray Prouty
 

If you read Prouty's "More Helicopter Aerodynamics" he has a chapter on coax and synchrocopter configurations. He mentions the S69 and how the rudder was swept aft, like a c172. In autorotation, the airflow was up the hinge line. Prouty makes some comment about how that would be a lesson learned for the next time around. If I can find the Prouty book I could post his statement more accurately, maybe somebody else has it.

-- IFMU