Why Helicopters Are So Slow

The most powerful helicopters in the world max out around at 170 or maybe 180 knots of airspeed. These can be multi-million dollar machines with two, or even three, of the most powerful turbine engines available. Why are the fastest helicopters slower than some of the quickest piston single airplanes?

There is a natural aerodynamic speed limit to a conventional helicopter that is similar to the “sound barrier” in fixed-wing airplanes, but in helicopters the barrier is more solid. The only way to accelerate past the helicopter speed barrier is to adopt alternate technologies such as the tiltrotor or something like Sikorsky’s experimental twin concentric rotor technology demonstrator that is capable of airspeeds well beyond 200 knots indicated.

The fundamental reason that helicopters are airspeed limited is a phenomenon called retreating blade stall. As the main rotor blades complete their trip around the circle, the blades “see” a different angle of attack at each portion of the rotation. The reason is that as the helicopter moves, one side of the rotor disk is advancing into the relative wind of motion, but the other side is retreating from the relative wind.

On one side of the main rotor disk the blade advancing into the relative wind of motion experiences an increasing angle of attack, just like a normal wing increases its “alpha” as airspeed increases. That’s a nice benefit for the helicopter on one side of the rotor disk because it is getting sort of free lift from the forward motion.

But the problem occurs on the other side of the rotor disk where the relative wind of motion is subtracted from the airspeed created by rotation of the blade. On the side of the rotor disk where the blade is retreating from the relative wind the alpha continuously decreases. The rotor system design keeps increasing the pitch of the blade on the retreating side, but at some forward speed the relative wind of motion overcomes the maximum lifting ability of the retreating blade, no matter how much that blade is pitched up. When that happens the helicopter experiences retreating blade stall, just as a conventional wing stalls, and the helicopter rolls toward the retreating blade. At some airspeed the rolling caused by the retreating blade stall becomes uncontrollable and that is what sets the effective maximum airspeed for the helicopter.

An easy way to visualize retreating blade stall is to think of taking off downwind in a conventional airplane. With only a few knots of wind blowing on the tail the takeoff can work. But try it with a 100-knot tailwind and see what happens. That is what the retreating blade in the helicopter main rotor experiences.

So, why not just spin the main rotor faster so the retreating blade moves through the air quicker to defeat the effect of the relative wind? Well, that works to a point, but at some rpm – actually a pretty low rpm compared to propellers – the local airspeed over the outer portions of the rotor nears or even exceeds Mach 1, the speed of sound. When that happens lift creation is disrupted and efficiency goes away. In fact, that “slapping” sound of a Huey that became the trademark audio of the Vietnam War is caused by the blade momentarily reaching Mach 1 during the added load of maneuvering.

There are two pretty obvious ways to overcome the airspeed limitation of the retreating blade. One is the tiltrotor concept, in which the aircraft flies slowly and hovers as a helicopter, but then the rotors rotate 90 degrees to become propellers and pull the aircraft forward using a conventional wing for lift. The other method is to have two main concentric rotors and spin them in opposite directions. With the dual rotor concept there is always an advancing blade on both sides of the rotor disk so the loss of lift from the retreating blade is canceled.

The dual concentric rotor concept has been tried many times over the decades with only limited success. Most often the weight and complexity – not to mention cost – overcomes the airspeed gained. To really increase the airspeed the dual rotor needs a source of forward thrust in supplement that generated by the main rotor, so that is an added complication.

But with new materials and technology it appears that Sikorsky may be on the verge of overcoming the natural obstacles to fast helicopter flight. Sikorsky is using a pusher propeller on the tail for thrust and dual rotors for lift and expects to have an aircraft for the military that can perhaps fly at 250 knots indicated while carrying a reasonable amount of payload. The price will still be high, but undoubtedly less than for a tiltrotor while being less complex than that combination of airplane and helicopter.

Maybe the retreating blade stall airspeed barrier in helicopters is like the sound barrier is to fixed wing airplanes – a technical and cost challenge, but one that can be resolved with the best people and materials. I hope so.

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40 Responses to Why Helicopters Are So Slow

  1. Again ! Thanks,Mac – so simple , I can understand. Jack

  2. Elton Folkerts says:

    Excellent explanation. As a former military helicopter machanic I have tried to explain this myself many times but with limited sucess.

  3. A very well done and exhaustive explanation, my compliments!

  4. Bob Briggs says:

    Mac – you are confusing me with “On one side of the main rotor disk the blade advancing into the relative wind of motion experiences an increasing angle of attack, just like a normal wing increases its “alpha” as airspeed increases.”

    “alpha” is the angle, measured in degrees, between the chord of the wing and the relative wind. see http://en.wikipedia.org/wiki/Angle_of_attack

    Thus a normal wing does not increase its “alpha” as airspeed increases unless the pilot pulls back on the yoke. The thing that does increase as the airspeed increases is the lift of the wing, which is proportional to the square of the velocity of the relative wind for a fixed angle of attack.

    Your statement would be correct if changed as follows:
    “On one side of the main rotor disk the blade advancing into the relative wind of motion experiences an increasing lift, just like a normal wing increases its lift as airspeed increases.”

    I would add that the blades flap to keep the lift constant. Flapping means the advancing blade is rising, thus reducing the angle of attack, and the retreating blade is falling, thus increasing its angle of attack (and lift).

    BTW I highly recommend James R. Chiles’ book “The God Machine” which traces the technological discoveries that enable the modern helicopter. It relegates the technical details to a plethora of footnotes to allow the author to keep the average reader’s interest by describing the personalities involved.
    Regards
    Bob Briggs

    • Martin Randall says:

      You are correct. The advancing blade doesn’t experience an increased AOA. It provides more lift simply because of the increased velocity due to the forward motion of the helicopter. The blade actually (UH-60) flaps up to decrease AOA thus equalizing the lift between the advancing and retreating blades.

      Still a good article Mac and you explained the effects of retreating blade stall in an easy to understand manner.

      Martin Randall
      UH-60 IP

  5. No offense, but this is AERO 101 stuff. I worked a bit with Sikorsky during the early development of the X2. It will go even faster once they get the aero shroud around the rotor. As impressive as the X2 is, the Lockheed AH-56 Cheyenne was nearly as fast in 1969 but with a single rotor. Its tail rotor could be swung to the rear to act as a pusher. The winding down of Vietnam and success of the Cobra killed the Cheyenne, which was 40 years ahead of its time. But what does all this have to do with sport aviation?

    • Jim Patterson says:

      Reference your comment about the AH-64 have a “swing to the rear” tail rotor, all the pictures I saw of the AH-64 showed a regular tail rotor and a pusher prop.

  6. Gary Frieden says:

    What? No mention of the Carter PAV 2+2 prototype and the military’s interest in Jay Carter’s designs, which seem much more cost-effective.

    • Mac says:

      Hi Gary,

      Th Carter aircraft is not a helicopter. The Cartercopter is an impressive machine in many ways, but it is really an autogyro with a fixed wing to supplement lift. Its unique slowed main rotor helps transfer the lifting job to the wing at highe airspeeds, but it is still not a helicopter.
      A true helicopter can hover. It can lift off vertically, and then move in any direction, including backwards. Even more importantly, a helicopter can stop in flight.
      The Carter, as did some autogyros earlier, has a mechanism that can spin up the rotor allowing a near vertical lift off, but then the aircraft must immediately begin to move forward to maintain lift and control.
      Helicopters have all sorts of complications and compromises, but the one single virture that no other aircraft has is that a helicopter can hover.
      Bests,
      Mac Mc

  7. Pingback: Why Helicopters Are So Slow | Aviation Blogs

  8. Pete says:

    Yep, another excellent article by J. Mac! If I didn’t know better, I’d say that J. Mac is also an accomplished licensed A&P Mechanic. If not J. Mac, you should pursue your license. You’d make a great A&P Mechanic, and a great Aviation Technical School instructor!

  9. Bill Berson says:

    Some would argue that the Sikorsky X-2 is not a true helicopter, but rather a compound helicopter. The traditional compound helicopter has a propeller or jet for propulsion in high speed cruise and usually has a small wing for lift so the rotor can be unloaded.
    The X-2 doesn’t have a wing, so it may not be a traditional compound, but I would not classify it as a true helicopter either.

  10. Lawrence Sciortino says:

    Though the Carter Copter has been the subject of much hyperbolic coverage, it is simply a gyro-copter in a nicely faired airframe. With higher than average thrust for its size, it flies faster than other gyro-copters, but does not resemble a helicopter any more than any other gyro-copter does, nor is its recently heralded “vertical takeoff” a new maneuver for gyro-copters.

  11. Sam Holman says:

    Westlands have a helicopter that can achieve 232 mph as I recall.

  12. Alex Kovnat says:

    A helicopter based on the Sikorski configuration that can fly as fast as a Mooney single piston engine retractible-gear airplane, and with a range of 300 miles or so, would be of benefit to business aviation. But …… where would it land and take off? If the only places where you can land and take off is a small airport such as Mettetal Airport in Plymouth-Canton Township Michigan, then a flying businessperson might as well use said Mooney retractible piston single, or a Cirrus SR-22, etc., depending on his or her needs.

    Every time, as I drive here and there in the business of my life, I see an office building with a flat roof I find myself wondering what would be the problems, the hazards, and the politics (i.e. objections from neighbors, objections from the FAA or from local authorities) of landing a helicopter on said roof and taking off. Many a successful flying businessperson might want to do that to save time.

    A problem with a big, fast helicopter, whether its anything like the Sikorski X2 or the V-22 tiltrotor or for that matter, just a big helicopter of ordinary configuration is this: It might be one thing to land a Robinson R-44 or perhaps the exciting new R-66 on the roof of an office building like those I see in my community. But a really big helo?

    Very willingly will I check this blog to see what others might say about that.

    • Martin Randall says:

      Their are plenty of helipads in large cities where an aircraft like an X-2 or larger can land. If you owned land you could even take-off from you back yard. You would save time right there by not having to drive to an airport. Helicopters have no minimun altitude unlike their fixed-wing counterpats. As long as you don’t do damage to persons or property you’re good to go.

      Still, right now helicopters just do make economic sense when compared to fixed-wing for cross-countries. If the industry can produce an aircraft such as the X-2 or Carter Copter that can transport 4 at high speed, hundreds of miles at the price of a high performance single, then it will sell. Right now that just doesn’t exist.

  13. Manning Stelzer says:

    While this is an very good introduction to the subject of retreating blade stall, there is a small but significant error: I believe that what J. Mac is talking about with reference to the X2 is a “coaxial”, not “concentric”, rotor arrangement. Concentric rotors would be virtually impossible since they would have to be in the same disk.

    Also, the reference to “Westlands have a helicopter that can achieve 232 mph as I recall.” made by SAm Holman is probably referencing the 1986 performance of a specially modified (and lightly loaded) Westland Lynx over a straight line course. That is not really reflective of normal performance of a normally configured Lynx with a payload over a closed course. Even so, the speed it reached was 216 knots (249 mph). By comparison, the X2 reached 250 knots (288 mph) in level flight and 260 knots (299 mph) in a shallow dive. And this was without the drag-reducing shaft fairing.

    Oh, and you may have figured out that I work for Sikorsky (note the spelling, please).

  14. Raymond Tolhurst says:

    Faster helicopters is easy, stop the main rotor in fast forward flight! I have a simple mechanical and torqueless way of doing this but limited finance to build it.

  15. Gordon Arnaut says:

    A nice side benefit of the coaxial rotor arrangement is that a tail rotor and boom is not needed. This eliminates a good number of pieces like the tail rotor gearbox, driveshaft, boom, rotor, etc., saving a good deal of weight and also the power required to drive the rotor.

    The tail rotor is necessary, btw, to counter the torque effect of the main rotor, which is caused by the drag of the rotor blades, and would cause the fuselage to rotate in the opposite direction, in accordance with action-reaction.

    As an engineer on the fixed wing side, I must admit being somewhat puzzled that the coaxial rotor is said to be more expensive, despite its seeming simplicity. Radio controlled model helos are mostly coaxial due to cost.

    • Manning Stelzer says:

      Gordon,

      Perhaps I can explain why the coaxial, at least in the development phase, is expensive:

      * Need a far more complicated Main Gearbox with another set of drive gears

      * Need another swashplate (or two) for the upper rotor cyclic and collective control (Note: commecially available RC coaxials do not have any cyclic control over their upper rotors and relay on changing of the rotor speeds to accomplish yaw and normal collective functionality, greatly simplifying their design. That design is not scalable to real helicopters for many reasons.)

      * Need another Main Rotor Hub and a set of Main Rotor Blades – these are very expensive items, especially blades rigid enough to prevent blade strikes between the blades of the upper rotor and blades of the lower rotor.

      * Etc.

      But despite the above, the coaxial’s dramatic performance increment and smaller footprint (etc.) will more than make up for any increased cost, I believe. Furthermore, as development continues through production, the cost will probably drop a bit.

      Manning

      • Gordon Arnaut says:

        Thanks, Manning.

        Another observation is the fairly small rotor diameter, which is to be expected with a coaxial arrangement. On the downside, the rotor efficiency of a coaxial will always be less, due to interaction of flow between the two rotors.

        The literature gives an efficiency loss of about 16 to 28 percent based on empirical data. This means it takes more power to hover and climb and is one of the tradeoffs in a craft of this configuration.

  16. Gordon Arnaut says:

    Bob Briggs is correct in his explanation of advancing and retreating blade lift, which is caused by the respective increase and decrease in relative velocity, not a change in the alpha.

    Both the advancing and retreating blades do experience changes in alpha, due to the hinges at the rotor root that allow the blade to flap up and down, as Bob points out.

    I would just add that the aerodynamic force that causes the flapping is dynamic pressure. The advancing blade rises up due to the increasing ram pressure due to its increasing velocity. This rise actually causes the blade to see a decrease in its angle of attack, or alpha, because the rising motion of the blade introduces a downward component to the relative wind. Imagine a downgust on your airplane wing.

    The retreating blade likewise drops down due to the deficiency in dynamic pressure, but this actually increases its alpha, again because the downward motion of the blade introduces an upward component to the wind that that the blade sees—think upgust.

  17. E. John Beaman says:

    Great explanation. lt’s really good to be reading you words again!

  18. James Bear says:

    I wonder what the safe-autorotation envelope looks like for this machine. Presumably you don’t want the two rotors at different speeds during that maneuver, so I’m guessing they’re mechanically linked, even when undriven. Lots of additional friction on what looks like smaller, lighter disks… and, the coaxial efficiency hit may apply here, too. Seems like the pilot of this machine would need some real air velocity and/or a lot of altitude to recover and then sustain flyable/landable rotor RPM after loss of power.

  19. Alex Kovnat says:

    About counter-rotating twin main rotors: For a while I wondered what kind of drive mechanism the X2 had. If you use differential gearing (like any car’s axle differential), you are guaranteed to have equal torque distribution between the two counter-rotating rotors, which means you won’t need a tail rotor. (You will have to have a way to temporarily discombobulate this equillibrium in order to yaw to the right or left).

    But then, you can’t guarantee speed or positional synchronicity between upper and lower rotors, which you might want for the same reason you want the propellers on a twin engine airplane to rotate at the same speed.

    You could use rigid gearing, which would guarantee speed synchronicity. But then you can’t guarantee equal torque distribution. So, how do you get both? The X2, as it turns out, has rigid gearing. To have full control, you need independent control of collective pitch on upper and lower rotors. For this reason the X2 has a fly by wire arrangement. This is of interest to the helicopter engineering community, as a while back one of the most respected helicopter engineers of all time, Ray Prouty, wrote an article in Vertiflite magazine (quarterly publication of the American Helicopter Society) in which he criticized fly-by-wire.

    I’d like to end this with a question: We all know about the problems of the V-22 Osprey. Did the tandem-rotor CH-47 (which also utilizes counter-rotating twin rotors and equal torque distribution to avoid the need for a tail rotor) have a difficult time in its early years with accidents related to its drive mechanism?

    • Manning Stelzer says:

      Alex,

      Pure yaw movements on the X2 are achieved by differential collective pitch of the two rotors (the rotors always turn at the same speed in flight – but in opposite directions, of course). So one rotor creates more drag (and more lift) and the other rotor creates less drage (and less lift. Total lift remains constant and the rotor yaws opposite the rotation direction of the “draggier” rotor.

      Pretty devious, eh?

      Manning

  20. Manning Stelzer says:

    Gordon,

    Something often glossed over about the X2: it has extremely rigid rotor blades with no hinges (in the conventional sense) which are essential to its extraordinary performance. As a result, flapping and lead-lag movements are extremely small. This was necessary for several reasons, primarily to prevent blade strikes between the rotors (which would tend to ruin your day) during high speed flight and maneouvering . If you examine photos of the aircraft at rest you can see that there is no apparent blade sag – a significant departure from the appearance of conventional rotor blades. It is useful to compare this arrangement to that of the Kamov family of coaxial helicopters (Ka-28, Ka-29, Ka-31, Ka-50, etc.) which use more flexible blades – and are therefore much more limited in their top speeds and maneouverability.

    Incidentally, this issue of blade stiffness will present a significant challenge to the development of heavy lift (much larger) versions of the coaxial helicopter. But we believe that a certain amount of scaling-up will be possible.

    Manning

  21. Gordon Arnaut says:

    Thanks for the info Manning.

    Maybe you could help clear something up for me. Not being a rotorcraft guy, I have but one helo book on my shelf, Leishman’s Helicopter Aerodynamics, a useful introductory text.

    Unfortunately not much in there about coaxials, but one thing jumped out at me and that is the forward flight power required—it is just about three times as high as a single rotor. No wonder your helo has a prop in back! (The reference in Leishman quotes the 1954 NACA Technical Note 3236 by Dingeldein).

    Tried to find something on coaxial autorotation, but no luck. So what’s the scoop? With the efficiency hit in hover, climb, and a huge hit in rotor power needed for forward flight, I have to wonder about the autorotation in this machine, as James noted above.

  22. Gordon Arnaut says:

    And come to think of it, if the yaw control is due to differential rotor torque, what happens with no engine and hence no torque? How do you control yaw in autorotation?

  23. Gordon Arnaut says:

    Never mind. I guess that’s what those little rudders are for out back.

  24. Gordon Arnaut says:

    Well I hate to be carrying on a debate with myself, but I just found a paper by Kamov deputy chief designer Eduard Petrosyan that claims a coaxial helo actually has an efficiency ADVANTAGE of about 20 percent.

    This is due to the contra-rotating blades straightening out the swirl imparted by blade rotation, which is lost energy; as well as the power saved in not having a tail rotor. I know the swirl energy in a prop can be quite a bit but I assumed that the much slower swirl velocity in a large helo blade was not much consequence.

    And what about the data from NACA and those “measured” tests to back up the theory about coaxials being less efficient? Mr. Petrosyan states matter of factly that the Kamov machine has a higher hover ceiling of several thousand feet (500 to 1000 m) and a climb rate advantage of almost 1000 fpm (4 to 5 m/s).

    Hmm. Very interesting. Hard to argue with the proven success of the Kamov, which is a highly regarded ship by rotorheads I know. Well Mr. Stelzer? Maybe we naysayers here on this blog should be basting our crow right about now?

  25. Alex Kovnat says:

    Arguments that the Kamov helicopters are more efficient in hover or slow-speed climbing, are indeed of interest if your particular reason for having a helicopter involves a lot of hovering, i.e. in the construction business where you’re lifting something to the top of a building and holding it there until the workmen can secure it in place. But how efficient are the Kamov helicopters at their most efficient cruising speed, or at whatever cruise speed you want, compared to a helicopter utilizing the usual single main rotor + tail rotor?

    At air shows, one finds two really big American helicopters in use by our armed forces for heavy hauling. There’s the CH-47, which uses counter-rotating tandem rotors and the CH-53, which utilizes one main rotor and a tail rotor. Which of the two, considering differences in weight and size, has the greater cruise efficiency and which has the greater hover efficiency?

  26. Gordon Arnaut says:

    Well I downloaded NACA TN3236 (from the Nasa technical reports server), which is referenced in the Leishman book. A nice little report from 1954 wind tunnel tests at Langley on a coaxial and tandem, comparing them to a single-rotor of the same power and disk solidity.

    Bottom line from these tests is that the coaxial does pretty good in hover, actually surpassing the single rotor by a few percent at higher thrust coefficients. Combined with the Kamov data I think that we can say that the coaxial is NOT less efficient in hover. Rereading the section in Leishman again, I think I may have drawn the wrong conclusions, although the author does claim that the coaxial is at a “slight” disadvantage in Figure of Merit, a hover efficiency measure. Mea Culpa.

    In level flight, the coaxial is indeed at a bit of a disadvantage according the the Naca study, needing “up to” 14 percent more power. Now if we consider that getting rid of the tail rotor alone could make up for this, it seems the coaxial is not really at a big disadvantage here either. I have to again admit to misinterpreting the info presented in the Leishman book. By no means is the efficiency hit anything resembling three times more power required, as I first stated.

    Tandem rotor does very well in the hover tests, having “greatly improved hovering efficiency” over the single, and needing 18 percent LESS power. In level flight the tandem rotor is only slightly worse than the single. So a pretty good config, especially for heavy lifting.

    I just wanted to correct those earlier comments I made which were obviously quite off the mark. Even in level flight it appears the coaxial should not be at a significant disadvantage, something that the Kamov experience bears out.

    On the plus side I think I did get the directional control issue right, because Kamov’s Petrosyan mentions that it took them quite a while to solve that. One of the bugaboos being control reversal on the pedals in autorotation. Stomp right, go left…

    Anyway, I’m glad to see Sikorsky expand the envelope. I always thought the coaxial arrangement was rather attractive in its symmetry and there is a neat little ultralight coax called the airscooter, which is like a big flying model because it does not have any blade pitch control and therefore cannot autorotate (hope and pray your engine doesn’t fail and make sure it has a parachute).

    Interesting historical note that Igor Sikorsky’s very first helo back in 1909 was coax, but never did get off the ground. His improved 1910 model, also a coax, apparently did make a couple of tiny hops, but unmanned (wonder how he managed that in the days before radio control?).

  27. Alex Kovnat says:

    Here’s another angle to the single rotor + tail rotor versus coaxial counter-rotating rotors debate: If you go to the AOPA website (www.aopa.org), you will find a special area devoted to helicopters titled Hover Power (http://blog.aopa.org/helicopter/). If you then look at the entry for February 21 2011, titled Photo flights and tail rotors, you will read a discussion on the phenomenon of Loss of Tail Rotor Effectiveness, or LTE. Here’s a quote from said article:

    “Older model Bell 206B Jetrangers have a smaller tail rotor with a symmetrical designed airfoil that makes it more susceptible to what’s referred to as Loss of Tail Rotor Effectiveness (LTE). According to FAA Advisory Circular AC90-95, any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where LTE or an unanticipated right yaw may occur. It also advises of greater susceptibility for LTE in right turns and states the phenomena may occur in varying degrees in all single main-rotor helicopters at airspeeds less than 30 KIAS. ”

    Accordingly, if your particular mission exposes you to an LTE situation, one wonders if a Kamov-type helicopter might be a better choice. I’d rather have the pilots among this group, have the final say on that.

  28. Bob Briggs says:

    Alex -
    Regarding your March 18 comment of roof landing convenience, the James R. Chiles’ book “The God Machine” devotes a whole chapter to the beginning, proliferation and demise of roof landings in populated areas (Ch 13 “Chariot of the Gods”.) Basically, the anti-noise crowd organized, called it a rich man’s nuisance, and convinced the politicians to kill it.

    Bob Briggs

  29. Gary says:

    Mac,

    Your comments regarding the carter copter are true for the current prototype, but there is another version on the drawing board with hover capabilities. This one has a larger gearbox to handle the additional load. It also has two side-mounted propellers instead of a single rear propeller. The pitch on one side propeller is reversed to compensate for torque while hovering. The fuel efficiency is less with the heavier gear box, but is still much better than a conventional fixed wing aircraft since the Carter version has smaller fixed wings. BTW, you did not mention the drag created by the rotor. This is another reason that helicopters are slow. According to the folks at Carter, the drag is a cube function of the rpms of the rotor. When the Carter Copter reaches a high enough speed to gain lift from its small fixed wings, the rotor is no longer needed for lift and is slowed down significantly to reduce drag. It spins just fast enough for the centrifugal force to prevent the blades from peeling back at high speeds.

  30. Søren says:

    Hi folks.

    Im newly educated helicopterpilot, and thus still learning. I’ve read most of the comments, and it comes down to one final question regarding retreating blade stall:

    Is it true that the retreating blade on the x2 actually does stall due to low speed, but it doesnt matter because it does it equally on both sides (due to coaxial)?

    Im not an expert on the matter, but I should think that, that makes unusual high demands of the engineers, calibrating and balancing the blades.

    I’ll just add, I recognize it might be wrong to use the term “retreating blade stall” in this regard.

    I hope I made myself clear, english is not my native language.
    best regards

  31. Mac says:

    Hi Soren,

    I doubt that the retreating blade of the X-2 actually stalls when the helicopter is flying at high speed because the blade is unloaded. As you know, the loading on an airfoil is one of the elements of the stall so with the advancing blade on the opposite side carrying the load the retreating blade wouldn’t stall in the classic sense.

    The rotors on the X-2 are very stiff compared to most helicopters so rigging issues are certainly different, but as you point out, the rigging must be very precise.

    Bests,

    Mac Mc

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