Igor's Terminal Velocity
 

Nineteen years after the Westland Lynx achieved 400 kph. it still retains the world speed record for rotorcraft. A speed achieved by modifying the exhaust to take advantage of the extra available engine power.

It is unlikely that any future helicopter, which is equipped with only a single main rotor, will ever exceed this record. This is because future modifications to the profile, twist and tip of the blades cannot overcome the single-rotor's conflict between advancing tip compression and retreating blade stall.

There has been polite ridicule on this forum for my promotion of lateral symmetry for rotorcraft. However, with little humility, I would like to provocatively state that 'The above speed record will only be exceeded by VTOL craft that have lateral symmetry.'

All the V/STOL craft that are shown on the American Helicopter Society's Wheel have lateral symmetry. Unfortunately, many of these craft are impractical and few have efficient hover capabilities.

The only laterally symmetrical 'pure rotorcraft' configurations are; the Tandem, the Coaxial, the Intermeshing, the Interleaved and the Side-by-side. It should be noted that only the last three have true lateral symmetry, with its inherent advantages.

For these 'pure rotorcraft' to achieve fast forward speeds some modifications must be made, such as;
~ Advancing Blade Concept, with its high rotor rigidity, dissymitry of lift and slowed rotor.
~ Active Blade Twist; including Reverse Velocity Utilization.

Simply and argumentatively put, if 'pure rotorcraft' are going to progress there must be
http://www.unicopter.com/No_Tail_Rotor_Small.gif


http://www.unicopter.com/Temporary/c...department.gif :D

Dave,

Agreed, we've known that for thirty years or more. Sikorsky tried, spent lots and then gave up.

Still ultimately limited by the single rotor, but how about the revival of the "rotodyne" principle? Thrust engines, tip jet start and finish, with autorotation in the middle. Low blade pitch angle in the cruise gave it a good speed range coupled with excellent load lifting capabilities.

The very fine Fairey Rotodyne was killed off, mainly on politics over noise issues, over forty years ago, but is generally recognised to be way in advance of its time.

Surely it should now be possible to design a quieter tip jet. These aircraft even get complimentary blade de-icing into the bargain due to the ducting of compressor air! After this winter in UK, that's a great bonus.

P.s. Doesn't the Fairey Rotodyne still hold the speed record for a rotary aircraft flying between London and Paris?

Here is an excellent link for those not familiar with that aircraft: http://avia.russian.ee/vertigo/fairey_rotodyne-r.html

Shy and Dave,

Dave's premise is right, the single rotor has a fundamental limit at about 250 mph, and a practical limit at about 210 mph (practical means this is all the aircraft will do in commercial service).

The problem is not beating that number. It is maintaining commercial viability while beating that number. Fuel flow, payload, parts count all count in the real equation.

I think there are coaxial configurations that will serve, but they do not involve low offset dual systems where the down sweeping blades are free to flap. That is because the downsweeping blade will flop around and create real problems for the designer as it goes into stall/reverse flow. Wild flapping will cause major problems.

The rotodyne is very ineffecient in hover (with its tip jets running) and the jets create drag at high rotor tip speeds. If that drag can be reduced with better nozzle shapes, it might be possible to create a high speed vtol rotodyne with very brief hover capability. The rotodyne only flew at 165 knots, fast for its day, but no barn-burner!

Included in the equation has to be the price, i.e. bang per buck or knot, in this particular case. Sikorsky didn't come up with a viable answer to the problem, nor has anyone else.

It seems there is a "hole" in the overall performance / versatility / cost equation, somewhere between the top end performance of helis and the highly manouvreable STOL fixed wing aircraft with a reasonable cruise speed, if such an aircraft exists....

The tilt rotor is another way of getting better cruise speed at the expense of true helicopter low speed performance and manoeuvreability. Of course, it has proved to be massively expensive and with it's own set of problems. Looking forward to the day I'll see one in Battersea Heliport - but I'm not holding my breath... especially since they have introduced that 1400 ft restricted area to the north, resulting in the requirement for an even steeper approach and climbout ;)

Something that has intrigued me about the old noisy tip jets that the Rotodyne had. In a similar way that modern high bypass ratio turbofans have quietened down the airliner, it might be possible to design an efficient high flow / low speed outlet propulsion burner integral to a modern rotor blade. Such a blade could possibly have the benefit of both low drag & noise while being "self powered".

I visualise something like a length of trailing edge burner, moving a high volume of gas more slowly, rather than a pure tip jet which, like a turbo-jet moves a little gas very quickly and in the process generating a lot of unwelcome noise. The reverse flow problem might be a problem for high speed cruise if reducing the gas velocity but NOT for a blade that is only powered in the low speed regime, such as the approach, hover and climb.

I'll still be flying ancient technology until I retire though, I'm certain. :E

hi Shy,

you need an ejection speed higher than the blade speed.
IMO if you dilute the airflow on the trailing edge, you will loose efficiency, unles your compressor can provide enough air.
Having larger nozzles doesn't mean more thrust : just less speed higher pressure, but the same airflow (mass/seconds).
the thrust is made by the airmass/section/speed, so you won't have a different thrust. and the blade angular speed will be function of the thrust.
so, maybe locating the nozzle a little in the disk and making it longer may be the better solution (just an opinion).
BTW, the outer enveloppe of an airjet is what produces losses, if you increase it, you increase losses, the rounded shape is the better shape (less circumference) but , IMO, not the quietest.

the noise, from what i'vre read, comes from the airflow speed and vorticity. i also think there are some ways to improve that.


sorry if i am not very clear.

The unloaded-rotor concept, which was used in the Fairey Rotordyne, is currently being reapplied in the Carter Copter, the Piasecki Vectored Thrust Ducted Propeller and Boeing Canard Rotor/Wing X-50A. I believe that this compound configuration of rotor plus wing will never prove to be viable product. The wings represent an undesirable weight and thrust obstruction in hover, while the rotor represents an undesirable weight and drag during forward flight.

IMHO, today's high strength, lightweight materials combined with advanced electrostrictive actuators will allow the wings and the rotor(s) of the compound configuration to be united. The result of this union will be a pair of laterally displaced and significantly advanced rotors.

Nick brings up the valid argument about " The problem is ... maintaining commercial viability. Fuel flow, payload, parts count all count in the real equation." This is where the three lateral rotor configurations should 'shine'. The Side-by-side, the Interleaved and the Intermeshing offer various pros and cons visa-vie each other, however, as a group, they offer a number of significant advantages.

During hover and other slow flight requirements they should outperform all other alternatives, and particularly the V-22 tiltrotor.

In fast forward flight, they may not quite match the cruise speed of the tiltrotor. However, these three configurations can have propellers that are optimized for forward flight, whereas the V-22 proprotors are a compromise between a propeller and a rotor. In addition, these three configurations can substitute turbofans with PTOs for even faster forward speeds, which the tiltrotor cannot do.


Sikorsky's Advancing Blade Concept combined with Active Blade Twist should make this all come true.

Dave,
Sorry to be kind of negative here, but in my opinion, your quest for a speed record with rotorcraft has no real need in this economy. What is needed is a low cost, quiet and crashworthy helicopter for training and personal use.

As for symmetry, think of a car with a flat tire on the left front. It goes thump,thump, thump and pulls to the left (just like a helicopter). If you let the air out of the right side tire that will make it symmetrical and cure the pull to one side but the heavy thump, thump, thump remains. What good is symmetry?

I think absolutely rigid blades as you propose would vibrate even though it appears to be symmetrical, just my opinion.

slowrotor,

Please don't apologize for being negative. Criticism is desired. It instigates the the evaluation of potential defects.

The following are responses to your three concerns. Hopefully, they will satisfy your objections or solicit further criticism.

• Speed: You are no doubt correct in that high speed serves little purpose for very-light rotorcraft. For large rotorcraft however, it probably is very important, since it increases the envelope of usefulness. The effort that is being put into the V-22 supports this belief.
  
• Lateral symmetry: (Only for the non-religious. :O ) Darwin's theory of evolution has shown that in nature, only the successful survive. It must be noted that virtually all 'mobile' forms of life have lateral symmetry, be they; in the water, on the land, or in the air. Since most of mankind's inventions have their foundation in nature, it understandable that man's 'mobile' creations have lateral symmetry also. Again, consider the vessels in the water, the vehicles on the land and the craft in the air. IMHO, the motorcycle sidecar and the single-rotor helicopter are aberrations. :oh:
  
• Rotor Induced Vibration: A valid concern. Research is currently being conducted at universities etc. to try to reduce vibration, and its Siamese-twin, noise. The ultimate successes will be twice as beneficial for twin rotor configurations.

Dave