Is Tandem the future rotorcraft?

[Graviman]

Another speculative thread, for technical debate... :rolleyes:

Recent discussions with Dave Jackson have brought up some interesting issues. Nobody will dispute the advantages of counterrotation to avoid retreating blade stall in high speed flight, but the planform is the subject of much debate.

My belief is that, after weight, the single biggest constraint to helicopter operation is aircraft width. For a given disk loading coaxial suffers from weight over designs with laterally seperated rotors, but allows a very compact design. This leads to the next question: What about longitudinally seperated rotors?

The CH-47 Chinook (http://www.chinook-helicopter.com/model_comparison/comparison.html) has 3 bladed rotors and an effective hinge offset of 6.3%, but is limited in speed by vibration. For a smaller design, what if the rotor system was replaced by Comanche style rotors with 4 blades and an effective hinge offset of 15%? This design minimises the disk loading for a given width, while maintaining a relatively compact aircraft. The longer fuselage perhaps limits it to transport applications, although interleaving is feasible.

In high speed flight the fuselage would suffer torsion as the retreating blades were unloaded. In principle it could achieve the same speeds as X2 using seperate propellers. The front rotor could have a lower disk loading to ensure VRS required nose-down recovery before rear rotor entered VRS. The only downside i can see is the increased complexity of a distributed powertrain, needing to be fatigue proofed at full power. An upside would be easy packaging of the twin swashplates which will eventually allow variable blade twist in different flight conditions.

Any merit, or have i completely lost the plot? :}

Mart

[Dave_Jackson]

Views of Chinook (http://www.combataircraft.com/aircraft/hch47_vl.jpg), and of specific interest; the Plan View.

It is interesting to note the large amount of fuselage that is located in the downwash from both rotors. In addition, some of the fuselage is subjected to the higher induced velocities coming from the tip end of the blades.

I have always assumed that the main reason that the interleaving configuration (http://www.unicopter.com/1441_small.gif) was never pursued was due to the large amount of thrust that would be wasted on the fuselage. It now appears that the downwash on the Chinook tandem fuselage and that on the Interleaving fuselage will be fairly equal.

In fact, in forward flight and with the inclusion of the Advancing Blade Concept (ABC) (http://www.synchrolite.com/0890.html), the Interleaving must be significantly better then the Tandem. With ABC the retreating blades are providing a very small portion of the thrust and on the Interleaving configuration the fuselage is located under these retreating blades.
______________

Colonel Crosby has said "I don't see the CH-47 even in its growth as meeting that requirement [the Joint Heavy Lift aircraft]"

If Boeing doesn't seriously look into the Interleaving configuration then perhaps the Europeans or the Chinese will.


Dave

[Mr Toad]

Graviman and Dave, may I as an illiterate observe that during BAH's BV234LR introduction we were led to believe by Boeing's TP that a stretched, 4 bladed version was actually built and tested by Boeing. It was supposedly much smoother and faster than our Short Fat Ugly version; however the main customer was the US army who didn't want or need it. I don't know whether this is true or not; if true what happened to the bits? Clearly Boeing were already aware of the problems you mention.

Yes the Chinook has considerable downwash to the fuselage; in flight you could see the passengers vibrating up and down in their seats beside the "airline style" fuselage interior trim because they sat on a "vibration absorbing" floor. So too the external fuel tanks and the cockpit seats. We loved to fly it but nobody wanted to travel in the back...

I don't accept that only the Chinook vibrates badly; ALL of them (ancient or modern) have a critical vibration phase. Eurocopter pilots say that their aircraft vibrate less than most due to the materials employed but it is an inherent problem with any rotorcraft.

Perhaps your suggestion has merit and could be further examined.

[Graviman]

Mr Toad, responding as just a dumb engineer ;) , interesting that Beoing have tried it. I would imagine that the biggest problem is scaling up the blades, so what in an attack heli would be 15% hinge offset might go down to 8% in CH-47. I imagine the vibration is caused by the onset of retreating blade stall, since the vortex shedding generates useful lift for each cycle in heli cruise - so all helis will do it to some extent. To get the most benefit would require additional props to generate cruise thrust.

Dave, i suspect the real reason interleavers have never made either service or market is the width of the aircraft on or near the ground. There may be advantages once airborn, but parked up those rotors are just an annoying waste of space that take up hanger volume or interfere with ground ops. Don't forget the whole point of a heli is to operate away from nice long runways with neatly painted ground boxes.

Mart

[Dave_Jackson]

http://www.unicopter.com/1441-A.gif

Swing the rotor/engine arms backward and inward for additional width reduction.

[slowrotor]

Is tandem the future?
The multirotor is better for heavy lift for large helos. The multirotor is to complex for smaller low cost aircraft.

Have you seen a small tandem anywhere?
Only the army and a few heavy lift companies have tandems.

[Ian Corrigible]

Have you seen a small tandem anywhere?
There have been handful, including the Rotor-Craft X-2, Jovanovich Jov-3 and Piaggio PD4, but nothing that took the world by storm. Any small tandem is likely to adopt an intermeshing rotor due to size constraints and the lack of a requirement for a high-capacity fuselage.

Always amazes me how timeless the CH-47 design looks nearly half a century after its first flight.

I/C

[Shawn Coyle]

What about a co-axial design?
Nothing beats it for straight up and down lifting - ease of designing for lifting heavy loads, unaffected by wind, all it needs is a thruster to get away from awesome nose down attitudes in the cruise - oh, wait. I've just described the old Sikorsky Advancing Blade Concept....

[quadrirotor]

http://www.lafhelicopters.com/pictures/welcome/2004july.jpg

[Graviman]

Dave, that helps but is still a very complex solution. Remember single rotor helis also have stowing blades, which will still take up less space. Introducing an additional hinge also complicates the drivetrain, and is an additional mode of failure which must be considered. It is feasible, but would struggle to be competetive in cost and weight.

Slowrotor, the reason for multi rotors it is generally the gearbox weight. Since tip speed limits Nr then larger Rr requires a squared torque increase. I am only considering this concept against for example Sikorsky's ABC concept. At the cost of additional gearboxes and driveshafts it allows easier swash plate packaging and some hover performance gains. Compromise either way.

Ian, agreed CH-47 is timeless. The design is just right for it's role.

Shawn, agreed. X2 should be the control for this concept. Pusher/puller props are a must for fuselage/hub drag.

Quadrirotor, LaFlamme looks interesting. Maybe this concept could be seen as a 4-blade rotor version of that machine, with 15% offset, specifically designed for high speed cruise.

I notice their concept for a 3-blade version, but am not sure of effective hinge offset. Since each blade is "pinged" by retreating stall to upset it's 3P mode, 3 blades would vibrate in phase. 5 blades will be the same with 5P mode, so 4 seems to be optimum for keeping resonances out of step. Not sure why we don't see 6 bladed designs out there. Improved materials and techniques will change this (maybe Bug's ring rotor too).

Mart

[Shawn Coyle]

What's the latest on the Laflamme machine - website stops at 2005....

[Deemar]

Shawn, agreed. X2 should be the control for this concept. Pusher/puller props are a must for fuselage/hub drag.


Maybe I'm missing something here, but surely you could get an equivalent reduction in drag by having the rotor mast angle forward somewhat. The helicopter would hover tail low (but this is less of an issue with a counter-rotator as there is no tailrotor), but the fuselage would be horizontal in fast forward flight.

Is there some additional drag from having the disk at a tilt?

Daniel

[Dave_Jackson]

Daniel,

The principle reason for implementing a propulsor is to increase the cruise speed of helicopters. Current rotorcraft have essentially reached the limits of compression on the advancing tip and stall on the retreating tip. In addition, these faster rotorcraft, such as the X2, will have; twin rotors, a wider chord and slower rotor rpm.

It would be interesting, but probably not practical, to see how much a helicopter's cruise speed might be increased by; including twin rotors, increasing the chord, reducing the rotor rpm, pitching the mast angle further forward but not including the propulsor.

Dave

[Graviman]

Daniel,

I thought this too, but the problem is principally the hub drag. To improve rotor efficiency the hub on the X2 has an oversize faired beenie, to minimise the fountain effect (upwash leakage at the hub). This means the hub works best if cruising at a level attitude. Clearly only pusher props will do this.

Apparently Comanche involved a study of ABC vs conventional. Since the ABC concept was without pusher prop it only had a speed advantage of 10kias. The pusher prop X2 also has a faired shaft housing between the hubs, so the whole rotor assy has only a marginal drag increase over a conventional single rotor.

The other benefit of seperate prop is that the prop can be optimised for high speed only. At low speeds the machine can transition like a normal heli, while at high speed MR Nr can be beeped down. The only downsides are cost and weight, although this can be offset by allowing rotor to operate nearer optimum Nr in hover.

Big Sky has gone quiet about X2, so i imagine they have encountered some technical difficulties - that's leading edge engineering folks. Nick sold his soul to the fixed winged one, but will still have much better insight to X2 than the rest of us put together. :ok:

Mart